14. SITE 7861 Shipboard Scientific Party

Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 125

14. SITE 7861

Shipboard Scientific Party2

HOLE 786A

Date occupied: 2 April 1989

Date departed: 4 April 1989

Time on hole: 2 days, 6 hr, 45 min

Position: 31°52.48'N, 141°13.58'E

Bottom felt (rig floor; m; drill-pipe measurement): 3069.5

Distance between rig floor and sea level (m): 11.40

Water depth (drill-pipe measurement from sea level; m): 3058.1

Total depth (rig floor; m): 3236.00

Penetration (m): 166.50

Number of cores: 19

Total length of cored section (m): 166.50

Total core recovered (m): 84.95

Core recovery (%): 51

Oldest sediment cored:Depth (mbsf): 115.71Nature: nannofossil marlEarliest age: middle Eocene

Hard rock:Depth (mbsf): 124.90Nature: metabasalt

HOLE 786B

Date occupied: 4 April 1989

Date departed: 16 April 1989

Time on hole: 12 days, 2 hr, 45 min

Position: 31°52.45'N, 141°13.59'E

Bottom felt (rig floor; m, drill-pipe measurement): 3082.4

Distance between rig floor and sea level (m): 11.40

Water depth (drill-pipe measurement from sea level; m): 3071.0

Total depth (rig floor; m): 3911.00

Penetration (m): 828.60

Number of cores: 72

Total length of cored section (m): 666.10

Total core recovered (m): 190.07

Core recovery (%): 28.5

Oldest sediment cored:Nature: sedimentary breccias, sandstones, siltstones, and clay-

stones interbedded with igneous rocksEarliest age: early Eocene

Hard rock:Depth (mbsf): 162.5

1 Fryer, P., Pearce, J. A., Stokking, L. B., et al., 1990. Proc. ODP, Init.Repts., 125: College Station, TX (Ocean Drilling Program).

Shipboard Scientific Party is as given in the list of participants precedingthe contents.

Nature: pillowed and massive flows, breccias, dikes and sills ofpicrites, high-magnesian basalts, boninites, basalts, andesites,dacites, and rhyolites

Principal results: Site 786 (proposed Site BON-6C) is located in thecenter of the Izu-Bonin forearc basin about 120 nmi east of theactive volcano Myojin Sho.

The stratigraphic section recovered at Site 786 has beenassigned to four lithologic units (I through IV). Units I throughIII are defined only in Hole 786A, in which the sedimentarysequence at the site was recovered. Unit IV is defined in bothHoles 786A and 786B. Unit I, from 0 to 83.46 meters belowseafloor (mbsf), consists of nannofossil marls and clays and is ofearly Pleistocene to middle Miocene age. Unit II (83.46-103.25mbsf) is late Oligocene to middle Eocene age, nannofossil marland nannofossil-rich clay. Also present is a volcanogenic com-ponent containing vitric ash and mineral fragments. Unit III(103.25-124.90 mbsf) is a sequence of volcaniclastic breccia, ofmiddle Eocene age. Unit IV (124.9-166.5 mbsf in Hole 786A and162.5-826.6 mbsf in Hole 786B) consists of volcaniclastic andsedimentary breccias, vitric siltstones and sandstones, lavas,dikes, and pyroclastic flows.

The rocks in the bottom of Hole 786A and the top of Hole 786Bare considered to be the same material. Sedimentation rates werehighest in the Pliocene (8.6 m/m.y.) and decreased (to 4.2 m/m.y.)between the latest Miocene and middle Miocene. A hiatus existsbetween the middle Miocene and the late Oligocene. The igneousbasement has mainly massive and brecciated flows, ash flows, andintercalated sediment in an upper part and pillow lavas and dikesor sills below. Rock types include high-magnesian basalts, bon-inites, basalts, andesites, dacites, and rhyolites. There are severalprominent shear zones and hydrothermal breccias within thesequence. Studies of physical properties show that the averagebulk densities are 1.65 g/cm3 in the sediments and 1.8 to 2.1 g/cm3

in the volcanic sequences; the average grain density of 2.65 g/cm3

varied little between the sediments and the volcanic rocks. Pre-liminary interpretation of the paleomagnetic data indicates littletranslation since the late Miocene.

BACKGROUND AND SCIENTIFIC OBJECTIVES

Site 786 is located on the outer half of the Izu-Boninforearc, about 120 km east of the active arc volcano of MyojinSho and about 70 km west of the axis of the Izu-Bonin Trench(Fig. 1). This site was originally designated as a contingencysite to augment or fulfill the objectives of Sites 782 and 785.Thus, the objectives of this site are the same as those of Sites782 and 785, namely to determine the following:

1. The stratigraphy of the forearc and hence the temporalvariations in sedimentation, depositional environment, paleo-ceanography, and intensity and chemistry of the volcanism inthe active arc;

2. The uplift/subsidence history of the outer half of theforearc to provide information about forearc flexure and basindevelopment as well as about the extent of any verticaltectonic activity that may have taken place since formation ofthe forearc terrane;

3. The nature of igneous basement forming the forearc, toanswer questions concerning the nature of volcanism in the

313

SITE 786

33°N

31

140°E " ^ = y Torishima 141° 142°

Figure 1. Location of Site 786 within the Izu-Bonin forearc. Contours are in kilometers below sea level.

initial stages of subduction, the origin of boninites, and formationof the 200-km-wide arc-type forearc crust; and

4. The microstructural deformation and the large-scalerotation and translation of the forearc terrane since theEocene.

Details of the approach for achieving these objectives aredescribed in the "Background and Scientific Objectives"section, "Site 782" chapter (this volume).

OPERATIONS

Transit to Site 786 (proposed Site BON-6C)

Time was available to drill Site 786 (proposed Site BON-6C) because the pumice layer at Site 785 could not bepenetrated. The 64-nmi transit to Site 786 was made in 10.5 hrand included a 4.75-hr site survey. A beacon was deployed at2015UTC, 1 April 1989, to establish Site 786.

Hole 786AA standard advanced hydraulic piston corer/extended core

barrel (APC/XCB) bottom-hole assembly (BHA) with a lockableflapper valve, nonmagnetic drill collar, and soft-formation bitwas assembled and deployed. Three APC water cores were

taken to a depth of 29 mbsf (from the bottom depth indicated bythe precision depth recorder). The BHA was raised 7.5 m andanother APC shot recovered 9.7 m of core. Hole 786A wasofficially spudded at 0815UTC, 2 April, and the mud line wasestablished at 3058.1 m below sea level (mbsl).

From the first three APC core barrels we recovered 29.6 mof core for a recovery rate of 103%. The XCB was thendeployed from Cores 125-786A-4X to 125-786A-19X (28.7-166.5 mbsf, total depth), where 55.3 m of core was recoveredfor a recovery rate of 40.1%. A total of 166.5 m was cored inHole 786A in 19 runs that recovered 85 m of core for a totalrecovery of 51% (Table 1). Heat flow was measured at 28.7and 76.8 mbsf. Upon reaching basement, a round trip wasmade so that Hole 786B could be drilled into basement usingthe rotary core barrel (RCB). Hole 786A officially ended at0300UTC, 4 April, when the bit was back on deck.

Hole 786BA standard RCB BHA was prepared with a mechanical bit

release and a medium-formation bit. The vessel was offset 30m to the south as the BHA was tripped in. Hole 786B wasofficially spudded when the seafloor was tagged at a depth of3071.0 mbsl at 0845UTC, 4 April. The hole was washed to adepth of 162.6 mbsf before coring began.

314

SITE 786

Table 1. Coring summary for Site 786. Table 1 (continued).

Date(April

no. 1989)Core Time

(UTC)Depth(mbsf)

Lengthcored(m)

Lengthrecovered

(m)Recovery Core

Date(April1989)

Time(UTC)

Depth(mbsf)

Lengthcored(m)

Lengthrecovered

(m)Recovery

125-786-A-1H2H3H4X5X6X7X8X9X10X11X12X13X14X15X16X17X18X19X

Coring totals

125-786-B-1R2R3R4R5R6R7R8R9R10R11R12R13R14R15R16R17R18R19R20R21R22R23R24R25R26R27R28R29R30R31R32R33R34R35R36R37R38R39R40R41R42R43R44R45R46R47R48R49R50R51R52R53R54R55R56R

0830093010301350144515351700175522152300000501450430063009101140145017152035

00300300050006000753092510301150134515301720191021302315010003000430060008000910104512101325145517001840201822150100041507300905104015151720194000000245050008000940134516502015221501000300054507450920110013001500170520300130

0.0-9.79.7-19.219.2-28.728.7-38.238.2-47.647.6-57.157.1-67.167.1-76.876.8-86.486.4-96.096.0-105.7105.7-115.3115.3-124.9124.9-134.6134.6-144.3144.3-154.0154.0-155.0155.0-164.0164.0-166.5

162.5-169.5169.5-179.2179.2-189.0189.0-198.6198.6-208.2208.2-217.8217.8-227.6227.6-237.1237.1-246.6246.6-256.3256.3-266.0266.0-275.7275.7-285.4285.4-295.0295.0-304.6304.6-314.3314.3-323.9323.9-333.6333.6-343.3343.3-353.0353.0-362.6362.6-372.3372.3-381.9381.9-391.5391.5-401.2401.2-410.8410.8-420.4420.4-430.1430.1-439.7439.7-449.4449.4-459.0459.0-468.7468.7-478.3478.3-488.0488.0-497.7497.7-507.4507.4-517.1517.1-526.8526.8-536.3536.3-545.9545.9-555.6555.6-565.0565.0-574.6574.6-584.3584.3-593.9593.9-603.5603.5-613.2613.2-622.9622.9-632.5632.5-642.2642.2-651.9651.9-661.6661.6-671.2671.2-680.8680.8-690.1690.1-699.8

9.79.59.59.59.49.5

10.09.79.69.69.79.69.69.79.79.71.09.02.5

166.5

7.09.79.89.69.69.69.89.59.59.79.79.79.79.69.69.79.69.79.79.79.69.79.69.69.79.69.69.79.69.79.69.79.69.79.79.79.79.79.59.69.79.49.69.79.69.69.79.79.69.79.79.79.69.69.39.7

9.739.949.962.185.476.707.511.168.618.458.962.270.410.410.250.630.471.360.48

84.95

0.700.620.870.821.832.700.200.742.161.701.532.212.490.882.552.060.900.860.760.962.103.880.121.810.771.031.851.170.642.952.321.940.954.634.040.852.681.193.003.634.354.402.151.351.253.001.272.574.671.801.382.831.685.544.037.64

100.0104.0105.022.958.270.575.111.989.788.092.423.64.34.22.66.5

47.015.119.2

51.0

10.06.48.98.5

19.028.12.07.8

22.717.515.822.825.79.2

26.521.29.48.97.89.9

21.940.0

1.318.87.9

10.719.312.06.7

30.424.120.09.9

47.741.6

8.827.612.231.637.844.846.822.413.913.031.213.126.548.618.514.229.217.557.743.378.7

125-786-A-57R58R59R60R61R62R63R64R65R66R67R68R69R70R71R72R

0700103513502000013006001025131015301800003002150730114517502200

699.8-709.4709.4-714.4714.4-719.0719.0-728.7728.7-738.4738.4-748.0748.0-757.7757.7-767.4767.4-777.1777.1-786.7786.7-796.3796.3-797.3797.3-806.9806.9-815.6815.6-823.6823.6-828.6

9.65.04.69.79.79.69.79.79.79.69.61.09.68.78.05.0

9.535.495.358.327.803.511.633.522.793.260.760.608.134.404.562.42

99.3110.094.585.880.436.516.836.328.733.97.9

60.084.750.657.048.4

Coring totals 66.1 190.07 28.5

The RCB was deployed 72 times, coring 666.1 m andrecovering 190.1 m of core for a recovery rate of 28.5%.Total depth was 828.6 mbsf, when coring was halted so thatthe remainder of the time allotted to Leg 125 could be usedfor logging.

The BHA was pulled to 3172.6 mbsl, and the logging linewas prepared. Six suites of logging tools were used in Hole786B. The first suite of logging tools, consisting of the dualinduction tool (DIT), the digital sonic tool (SDT), and thenatural gamma-ray spectrometry tool (NGT), was run from3520.0 mbsl (the depth of a bridge that the tools could notpenetrate) to the top of the hole. The second suite, consist-ing of the lithodensity tool (LDT), the compensated neutrontool (CNT), and the NGT, was run to 3458.1 mbsl. Anotherbridge encountered at that depth could not be penetrated bythese tools, so the hole was logged from there to the top.

The Schlumberger equipment was rigged down, and thepipe was run in the hole to clear the bridges. The hole wasclear from 3516.6 to 3808.6 mbsl, and the BHA was raised to3519.6 mbsl so that the lower section of the hole could belogged.

The third logging suite, consisting of the DIT, LDT,CNT, and SDT, was deployed to 3892.0 mbsl, and the holewas logged from that point upward to 3535.2 mbsl. Thefourth suite, consisting of the induced gamma-ray spectrom-etry tool (GST), the aluminum clay tool (ACT), and theNGT, was deployed to 3892.0 mbsl, and the hole then waslogged to the mud line. The borehole televiewer (BHTV)was deployed for the fifth logging run, but failed downhole.During the sixth logging run, the spare BHTV was deployed,and we logged 80 m of the hole before logging time expired.

The BHTV was pulled out of the hole and the loggingequipment was rigged down. The BHA was pulled to thedrill floor at 0800UTC, 16 April, officially ending Hole 786B.The BHA was then inspected and secured in the derrick.With the BHA back on deck, the vessel was under way at1130UTC, 16 April, and the occupation of Site 786 officiallyended.

Site 786 to TokyoA survey was performed over Site 786 before steaming

north to Tokyo. At 1430UTC, 16 April, the seismic gear wasretrieved and the transit began. The 252-nmi transit to Tokyowas made in 35.5 hr. The anchor was dropped at 2300UTC, 17April 1989, officially ending Leg 125.

315

SITE 786

LITHOSTRATIGRAPHY

A downhole stratigraphy of nannofossil marls, sedimentarybreccias interlayered with nannofossil marls, and igneousrocks defines four lithologic units at Holes 786A and 786B(Table 2). Lithologic Units I through III are lower Pleistoceneto middle Eocene sediments. Lithologic Unit IV is earlyEocene age volcanic basement present in both holes, withdrilling-produced chips and pebbles at Hole 786A and theentire stratigraphic section recovered at Hole 786B. Hole786B is 30 m south of Hole 786A.

Unit ISections 125-786A-1H-1, 0 cm, to 125-786A-9X-5, 66 cm; depth,

0.0-83.46 mbsf.Age: early Pleistocene to middle Miocene.

Lithologic Unit I is a succession of lower Pleistocenethrough middle Miocene nannofossil marls and clays with apersistent volcanogenic component. Upper and middlePleistocene strata are missing, on the basis of an earlyPleistocene assemblage of nannofossils in the uppermostsample (125-786A-1H-1, 2 cm). Sediment color varies fromvery pale brown (10YR 7/3), light brownish gray (2.5Y 6/2),gray (10YR 6/1), and dark grayish brown (2.5Y 4/2) to darkgray (5Y 4/1 and 10YR 4/1). Nannofossils are the dominantbiogenic component (present in amounts up to 56%; in onesample up to 70%) forming nannofossil marl (in one sampleforming a thin—<l-cm-thick—calcareous ooze), glass-richnannofossil marl, and nannofossil-rich clay. Foraminifers(trace-30%) and micrite (trace-40%) are also present. Ra-diolarians (trace-4%), diatoms (trace-3%), and sponge spi-cules (trace-5%) form the biosiliceous component. Terrige-nous detritus is predominantly clay (5%-72%) with somequartz (trace-1%).

Volcaniclastics are ubiquitous as dispersed vitric particlesor pumice fragments and in ash layers (Table 3). Locally,concentrations in pelagic sediments are sufficient to defineglass-rich nannofossil marls and glass-rich clays, such as inCores 125-786A-4X and 125-786A-5X. Volcanogenic materialis primarily vitric debris (trace-57%) with lesser amounts ofpyroxene and feldspar (trace-5%). Pumice is disseminatedthroughout lithologic Unit I as subrounded to rounded, gran-ule-sized, light brownish gray (2.5Y 6/2) particles. Scatteredlarger pumice clasts, up to 3 cm in diameter, occurring

downhole from about Core 125-786A-3H are brown (7.5YR5/4), dark gray (5YR 5/1), and white (10YR 8/1). Volcaniclasticlayers of predominantly vitric detritus (60%-100%) are com-mon throughout Unit I (see Table 3) and probably representboth pyroclastic air-fall and epiclastic deposits.

Sedimentary structures are rare. A few volcaniclasticlayers have normal size grading, and others have poorlydefined laminae. A 55-cm-thick layer of feldspar-rich, fora-miniferal vitric sand in Section 125-786A-1H-5, 60-115 cm,contains vague laminae. Nonclastic sediment sequences areextensively burrowed; thus, possible sedimentary structuresare obliterated. A few synsedimentary normal faults arepresent, with one prominent example truncating a burrowfilled with black (N 4/0) clay (Section 125-786A-2H-7, 28cm).

Carbonate-cemented pelagic sediment occurs at two levelsin Unit I (Sections 125-786A-1H-6, 140 cm, and 125-786A-6X-4,0-10 cm) as indurated and blocky pieces of black (N 4/0)lithic fragments. A pebble of chlorite-rich, vitric, silty clay-stone is present in the top 2 cm of Core 125-786A-6X. Thelithic fragment is gray (5Y 5/1), with a discoid and roundedshape (3.5 × 3 × 2 cm in size), contains clasts of granule-sizedchloritized pumice as well as sand-sized altered igneous rocks,and is cut by numerous chlorite veins. Microfossil assem-blages suggest an age of middle Miocene, slightly older than orequivalent to the age of the host sediment (see "Biostratigra-phy" section, this chapter).

The basal sediment in lithologic Unit I is a black (10YR 2/1)vitric ash composed of sideromelane and manganese oxide(both in about equal proportions) with trace amounts ofcalcareous nannofossils. Ferromanganese oxides are presentboth as disseminated silt-sized particles and as coatings withlaminated-botryoidal textures on the edges of blocky sedimentclasts. This 10-cm-thick deposit in Section 125-786A-9X,56-66 cm, represents a hiatus separating middle Miocene andupper Oligocene biozones (see "Biostratigraphy" and "Sed-iment-Accumulation Rates" sections, this chapter).

Unit IISections 125-786A-9X-5, 66 cm, to 125-786A-11X-5, 125 cm;

depth, 83.46-103.25 mbsf.Age: late Oligocene to middle Eocene.

A distinctive color change to reddish brown (10YR 5/4)nannofossil marls marks the change to lithologic Unit IL Gray

Table 2. Lithologic units recovered at Site 786.

Lithologicunit

I

II

III

IV

Cores

786A-lH-l,0cm, to786A-9X-5, 66 cm

786A-9X-5, 66 cm,to786A-HX-5,125 cm

786A-11X-5, 125 cm,to 786A-13X-CC,44 cm

786A-14X-1, 0 cm,to 786A-19X-1, 15cm

786B-1R-1, 0 cm, to786B-72R-2, 138cm

Depth (mbsf)

0.0-83.46

83.46-103.25

103.25-124.90

124.90-166.50

162.50-826.60

Dominant lithology

Nannofossil marl,nannofossil-rich clay,glass-rich clay

Nannofossil marl,nannofossil-rich clay

Volcaniclastic breccia

Igneous rocks

Volcaniclastic andsedimentary breccias,lava and pyroclasticflows, vitric siltstoneand sandstone

Stratigraphic age

lower Pleistoceneto middleMiocene

upper Oligoceneto upperEocene

middle Eocene

middle Eocene(?)

middle Eocene

316

SITE 786

Table 3. Volcaniclastic layers in lithologic Unit I, Hole 786A.

Core, section,interval (cm)

125-786A-1H-1, 62-771H-2, 3-51H-2, 5-181H-4, 12-161H-4, 23.5-251H-5, 62-1111H-6, 77-781H-7, 5-82H-1, 10-142H-1, 29-302H-1, 802H-2, 99-1132H-2, 143-1462H-3, 145-1462H-4, 36-402H-4, 84-852H-4, 93-962H-4, 141-1452H-5, 28-302H-5, 122-1252H-6, 14-173H-1, 41-443H-1, 54-583H-1, 87-903H-2, 100-1033H-3, 116-1203H-3, 116-1203H-5, 79-80.53H-6, 102-1123H-CC, 10-173H-CC, 214X-1, 60-615X-1, 39-415X-1, 74-765X-1, 74-765X-1, 78-795X-1, 106-1075X-2, 14-156X-1, 21-256X-1, 93-946X-2, 37-466X-2, 102-1036X-3, 126.5-1286X-5, 8-96X-5, 22-276X-5,36-407X-1, 138-1407X-2, 125-1277X-3, 3-47X-3, 15.5-167X-5, 9-107X-5, 14-157X-5, 18-247X-5, 64-667X-6,27-289X-3, 43-449X-3, 85-899X-3, 128-1319X-4, 23-269X-5,56-66

Depth oftop ofsection(mbsf)

01.51.54.54.567.599.79.79.7

11.211.212.714.214.214.214.215.715.717.219.219.219.220.722.222.225.226.728.8928.8928.738.238.238.238.238.239.747.647.649.149.150.653.653.653.657.158.660.160.163.163.163.163.164.2979.879.879.881.382.8

True depth(mbsf)

0.62-0.771.53-1.551.55-1.684.62-4.66

4.735-4.756.62-7.118.27-8.289.05-9.089.80-9.949.99-10.0

10.512.19-12.3312.63-12.6614.10-14.1414.56-14.6015.04-15.0515.13-15.1615.60-15.6515.98-16.0016.92-16.9517.34-17.3719.61-19.6419.74-19.7820.07-20.1021.70-21.7323.36-23.4023.36-23.4025.99-26.00527.72-27.8228.99-29.06

29.129.30-29.3138.59-38.6138.94-38.9638.94-38.9638.98-38.9939.26-39.2739.84-39.8547.81-47.8548.53-48.5449.91-49.9250.12-50.13

51.865-51.8853.68-53.6953.82-53.8753.98-54.058.48-58.5059.85-59.8760.13-60.14

60.255-60.2663.19-63.2063.24-63.2563.28-63.3463.74-63.7664.56-64.5780.23-80.2480.65-80.6981.08-81.1181.53-81.5683.36-83.46

Volcaniclasticlayer

123456789

101112131415161718192021222324252626272829303132333334353637383940414243444546474849505152535455565758

and greenish gray colors similar to those of lithologic Unit I (5GY5/1 and 5Y 5/1) reappear downcore (at Section 125-786A-9X-CC,14.5 cm) in nannofossil marls and sandy nannofossil oozes.These are followed again (Section 125-786A-10X-4, 70 cm) bynannofossil marls and nannofossil-rich clays, which have thesame reddish brown color noted at the top of Unit II. Thissuccession of gray and red nannofossil marls and oozes, whichdefines lithologic Unit II, spans the upper Oligocene to middleEocene section. Each color change is in close proximity to anunconformity of minor duration (see "Biostratigraphy" and"Sediment-Accumulation Rates" sections).

Sedimentary components are similar to those noted for UnitI. Biogenic components are dominantly nannofossils (2%-60%),with lesser amounts of foraminifers (2%-10%), micrite (1%-35%), and radiolarians (trace-2%). Clays are present in amountsvarying between 10% and 50%, opaque minerals between 3%and 15%, and chlorite and serpentine in trace amounts. Volca-nogenic components are usually vitric particles (35%-98%) withfeldspar (l%-7%) and pyroxene (l%-10%), disseminated in thesediment as detritus or pumice. Volcaniclastic layers are notpresent. Pumice colors provide a variegated contrast to thepelagic sediment and range from light gray (10YR 4/1), lightbrownish gray (2.5Y 6/2), white (10YR 8/1), brown (7.5YR 5/4),dark gray (5YR 5/1), and orange to dark yellowish brown (10YR5/4) and black (N 4/0).

Extensive mottling reflects bioturbation; no sedimentarystructures are present. A fault with apparent normal movement(see "Structural Studies" section, this chapter) in Section 125-786A-11X-4, 100 cm, marks an abrupt sediment color changefrom light reddish brown and reddish brown (5YR 6/3 and 5YR5/3) to light yellowish brown (2.5YR 6/4). This continues as thedominant background color for about 0.5 m downhole, and thenchanges between Sections 125-786A-11X-4 and 125-786A-11X-5to white (2.5Y 8/2) and light gray (2.5Y 7/2). General sedimenttypes (nannofossil oozes and nannofossil marls) and sedimentarycomponents do not change across the fault. Dispersed through-out the sediment below the fault are rounded clasts, 1 to 3 cm indiameter, of glass-rich silt that have a distinctive bright, lightgreenish gray color (10Y 6/6).

Unit IIISections 125-786A-11X-5, 125 cm, to 125-786A-13X-CC, 44 cm;

depth, 103.25-124.90 mbsf.Age: middle Eocene.

Increased clast content and an abrupt color change of thematrix sediment to dark brown (2.5YR 4/2) at Section 125-786A-11X-5, 125 cm, define the volcaniclastic breccia that is thedominant lithology of Unit III in Hole 786A; breccia with thiscolor and clast content was not recovered at Hole 786B. Angularto rounded clasts of vitric ash and dacite are green (5GY 6/4),greenish gray (10Y 6/6), dark greenish gray (5BG 4/1), white (N8/0), dark yellowish brown (10YR 6/8), grayish brown (10YR5/2), and black (10YR 2/1). The black color is caused bymanganese oxides disseminated within the ash. Clast diametersaverage about 1 cm, with rare larger clasts reaching 4 cm, suchas in a moderately well-sorted breccia in Sections 125-786A-11X-6 and 125-786A-11X-CC. Dacite fragments up to 6 cm indiameter form a poorly sorted breccia in Sections 125-786A-12X-1 and 125-786A-12X-2. Clasts float in a matrix of white(2.5Y 8/2) and light gray (2.5Y 7/2) nannofossil marl.

Sedimentary components in the matrix are predominantlynannofossils (up to 65%, locally forming nannofossil oozes);additional components include foraminifers (l%-10%), mic-rite (l%-35%), radiolarians (trace-2%), clay (10%-50%), andopaque mineral grains (3%-15%).

No sedimentary structures occur in this unit for interpret-ing depositional and post-depositional processes.

Unit IVSections 125-786A-14X-1, 0 cm, to 125-786A-19X-1, 15 cm; depth,

124.90-166.50 mbsf. Age: middle Eocene(?).Sections 125-786B-1R-1, 0 cm, to 125-786B-72R-2, 138 cm; depth,

162.50-826.60 mbsf.Age: middle Eocene.

Hole 786A

Lithologic Unit IV at Hole 786A is a mixture of drilling chipsand drilling-produced pebbles of igneous rocks and vein materialfrom these rocks, as well as an assortment of volcaniclastic and

317

SITE 786

sedimentary rocks typical of the clast lithologies in lithologicUnit III. The determination of biostratigraphic age was notattempted because of the uncertain stratigraphic position of thechips and pebbles; however, a middle Eocene age may beassumed for lithologic Unit IV based on the oldest age obtainedfrom similar lithologies in the overlying lithologic unit.

Hole 786B

Sedimentary breccias, sandstones, siltstones, and clay-stones interbedded within a666.1-m-thick succession of lavaflows, pyroclastic flows, pillow flows, and dikes constitutelithologic Unit IV at Hole 786B. Deposition during themiddle to late Eocene is inferred for this unit based upon alimited nannofossil assemblage. Sedimentary rocks formnearly one-half of the stratigraphic sequence if all thevolcaniclastic rocks are considered sedimentary. If theserocks are treated as igneous because they may be autobrec-ciated interflow deposits, then sedimentary rocks representless than 10% of the stratigraphic succession. In order ofabundance, rock types are volcaniclastic breccias, volcani-clastic conglomerates, vitric siltstones, vitric clayey silt-stones, and vitric sandstones. The addition of various min-eralogic components forms jasper and chlorite siltstones,chlorite vitric sands, chlorite-orthopyroxene-glass-richsandstones, feldspar-rich vitric silty sandstones and feld-spar, and glass-rich silty clays.

The dominant detrital component is volcanogenic, includ-ing glass (20%-100%), pyroxene (l%-30%), feldspar (1%-15%), igneous rock fragments (trace-10%), and amphibole(trace-5%). Sideromelane and palagonite are common glasstypes in interpillow deposits. Pyroxenes (predominantlyorthopyroxene) form thick sequences of sand as the matrixwithin breccias and conglomerates (e.g., in Cores 125-786B-7R to 125-786B-12R). Another detrital component isclay (10%-53%). Clasts in volcaniclastic breccias and con-glomerates vary in size from 2 mm (granule size) up to 6 cm(the diameter of the core barrel and thus the limit ofmegascopic resolution of clasts from flow/sedimentary lay-ers), have shapes that are angular to sub rounded, andconsist of fragments of vitric and lithic volcanic rocks suchas pillow rinds.

Minerals of presumed diagenetic origin, perhaps in connec-tion with hydrothermal fluid circulation, include opaque min-erals (l%-50%), chlorite (4%-97%), chert and jasper (2%-44%), zeolites (trace-2%), and sepiolite (trace-100%). Pyrite,chlorite, chert, and jasper are found in the lower part of thesection (Cores 125-786B-27R to 125-786B-72R for chlorite and125-786B-61R and 125-786B-62R for chert and jasper). Chlo-rite sands with vitric and pyroxene grains exhibit cross-bedding and laminae structures (Core 125-786B-27R); it is notclear whether the chlorite here represents a detrital fraction orthe post-depositional alteration of detrital vitric grains. Zeo-lites are found only in the upper part of the middle of thestratigraphic section (Core 125-786B-22R). Section 125-786B-22R-1, 28-42 cm, also includes a spectacular 14-cm-thicksepiolite layer of moderate pink (5R 6/4) to grayish pink (5R8/2) that may have a hydrothermal origin.

Biogenic components such as nannofossils (trace-5%),radiolarians (trace), and a spicule (trace) are rare. Micrite ispresent in trace amounts to 4% and may be of either biogenicor authigenic origin.

A marine sedimentary environment is indicated by thesebiogenic components as well as by small structures interpretedas burrows in Section 125-786B-34R-1. The provenance ofclastic particles is assumed to be nearby igneous rocks on thebasis of (1) the large size (sand or ash size, gravel or block size)and shapes (usually angular to subangular) of grains and clasts,

(2) the high proportion of clasts compared to fine-grained detrituscreating poorly sorted deposits, (3) the similarity in lithologybetween igneous flows/dikes and the volcanogenic detritus, and(4) a predominance of easily abraded, vitric grains. The matrix-supported clast textures of the volcaniclastic breccias and con-glomerates suggest either mass-flow or particle-by-particle dep-osition. Existing criteria are inadequate to differentiate betweenthe two processes in the absence of obvious contacts betweenthese coarse-grained deposits. Sedimentary structures indicativeof depositional processes for sandstones and siltstones arescarce, but normal size grading, syndepositional slump struc-tures, cross-bedding, and laminae demonstrate mass-flow andredepositional mechanisms.

Correlation Between Holes 786A and 786B

Lithologic Unit IV is thought to be correlative betweenHoles 786A and 786B in stratigraphic position and age because(1) of the proximity of the two holes, (2) cores containing thisunit were cut at about the same sub-bottom depth, (3) therecovered igneous rocks appear similar in hand specimen andsmear slide, and (4) biostratigraphic criteria indicate that bothunits may be middle Eocene in age. Hole 786B is 30 m southof Hole 786A. The depth of the bottom of Core 125-786A-19Xat the base of the hole is 166.50 mbsf; the depth of the top ofCore 125-786B-1R is 162.50 mbsf. Rock types from lithologicUnit IV at both sites are dark gray and black, vesicular,olivine- and orthopyroxene-bearing andesites with vesiclescommonly filled by a bluish zeolite.

BIOSTRATIGRAPHYEvidence from calcareous nannofossils, foraminifers, and

diatoms assigns an early Pleistocene age to the uppermostsediments in Hole 786A. Nannofossils date the lowermostsediments to the middle Eocene. The sediments interbeddedwith the lava flows in Hole 786B are assigned an Eocene agebased on their nannofossil assemblages. The biostratigraphyfrom this site is summarized in Figure 2, with the ageboundaries based on nannofossil data.

Calcareous Nannofossils

Hole 786A

Pleistocene through middle Eocene nannofossil assem-blages from Hole 786A are abundant; their preservation isgood to moderate.

The uppermost Sample 125-786A-1H-1, 2 cm, is earlyPleistocene (Subzone CN14a) in age on the basis of theco-occurrence of Pseudoemiliania lacunosa, Calcidiscus mac-intyrei, Gephyrocapsa oceanica, Gephyrocapsa caribbean-ica, and Helicosphaera sellii; thus, upper and middle Pleis-tocene sediments are missing.

Sample 125-786A-1H-1, 150 cm, is late Pliocene (SubzoneCN13b) based on the occurrence of G. caribbeanica and theabsence of G. oceanica. Sample 125-786A-1H-2, 96 cm, hasbeen assigned to Subzone CN12b by the presence of Dis-coaster brouweri and Discoaster surculus and the absence ofDiscoaster tamalis. The occurrences of D. tamalis togetherwith Ceratolithus separatus in Samples 125-786A-1H-4, 92cm, to 125-786A-2H-CC allow us to assign these sediments toSubzone CN12a (late Pliocene).

Sample 125-786A-3H-CC contains Reticulofenestra pseu-doumbilica, Ceratolithus acutus, and Sphenolithus spp. and istherefore early Pliocene in age (Subzone CNlOb).

Samples 125-786A-4X-CC and 125-786A-5X-CC are datedas late Miocene (Zones CN9b and CN9a, respectively) by theoccurrence of Discoaster berggrenii and Discoaster quinquer-amus in both and also in the former sample of Amaurolithus

318

SITE 786

0

50 —

100 —

150 —

Cor

e

1H

2H

3H

4×

5X

6X

7X

8X

9X

10X

11X

12X

13X

14X

15X

16x: i 7 × ,18x

19x

Calcareousnannofossils

zones\ CN14a\ CN13b\ CN12b

CN12a

CN10b

CN9b

CN9a

CN5

CN4

CP19CP18 N

CP15CP14b

CP14a

Planktonicforaminifers

zones

N19-22

N17B-19

N17AN16

N12-17A

N4A-5

P9-11

Diatomzones

N. reinholdiiN. jouseae

T. convexa

N. mioceπica

N. porteri

C.coscinodiscus

Age

early\ Pleistocene /

latePliocene

earlyPliocene

lateMiocene

middleMiocene

late Oligoceneearlv Oliqocene

late Eocene

middleEocene

Figure 2. Summary of biostratigraphic data from Hole 786A.

delicαtus, Amαurolithus tricorniculαtus, and Discoαsterblαckstockαe. Discoαster kugleri and Discoαster exilis arepresent in Samples 125-786A-6X-CC and 125-786A-7X-CC,which are, therefore, middle Miocene in age (Zone CN5).Samples 125-786A-8X-CC to 125-786A-9X-5, 40 cm, are alsodated as middle Miocene, but the occurrence of Sphenolithusheteromorphus, D. exilis, and C. mαcintyrei places thesewithin Zone CN4. The occurrence of specimens of R.pseudoumbilicα more than 7 mm in size indicates that this isthe higher part of Zone CN4. Lower Miocene sediments aremissing (Zones CN1 through CN3); a hiatus with a duration ofapproximately 7.2 m.y. thus is present between the middleMiocene and late Oligocene. A similar hiatus exists betweenthe middle Miocene and the late Oligocene in Hole 782A. Thehiatus at Site 786 coincides with a volcanic ash interval atSection 125-786A-9X-5, 56-66 cm.

Abundant, moderately preserved to well-preserved nanno-fossil assemblages are present from Samples 125-786A-9X-5,80 cm, to 125-786A-10X-6, 51 cm.

Late Oligocene nannofossil assemblages are found inSamples 125-786A-9X-5, 80 cm, to 125-786A-10X-3, 142 cm;they are characterized by Dictyococcites bisectus, Cyclicαr-golithus αbisectus, Helicosphαerα recta, Sphenolithus cipe-roensis, and Sphenolithus distentus, which assigns the as-semblages to Zone CP19. Samples 125-786A-10X-4, 42 cm,to 125-786A-10X-5, 95 cm, are dated as early Oligocene(Zone CP 18) by the appearance of Sphenolithus predisten-tus. Sample 125-786A-10X-6, 37 cm, is dated by the occur-rence of Ericsonia formosa, Reticulofenstra umbilica, andcommon Ericsonia subdisticha as early Oligocene (SubzoneCP16a). A hiatus may occur between Subzones CP18 andCP16a.

Sample 125-786A-10X-CC is near the Oligocene/Eoceneboundary. The assemblage contains species usually found inthe late Eocene, but the CP15 marker species Discoastersaipanensis and Discoaster barbadiensis are missing. The lastoccurrence of these marker species is about 0.5 m.y. beforethe Oligocene/Eocene boundary. Sample 125-786A-10X-CCis, therefore, latest Eocene in age and is assigned to the baseof Subzone CP16a.

The presence of D. barbadiensis, D. saipanensis, Cribro-centrum reticulatum, Chiasmolithus oamaruensis, andIsthmolithus recurvus dates Sample 125-786A-11X-1, 20 cm,to the late Eocene (Subzone CP15b). The C. reticulatumextinction approximates the CP15a/15b boundary; therefore,a short hiatus may be within Subzone CP 15b. Samples125-786A-11X-1, 140 cm, to 125-786A-11X-2, 133 cm, areassigned to the late Eocene (Subzone CPI5a) by the occur-rence of a late Eocene association and the absence ofChiasmolithus grandis, but the genus Chiasmolithus is usu-ally rare in samples.

Middle Eocene nannofossil assemblages (Subzone CP14b)(based on the appearance of C. grandis together with D.saipanensis, D. barbadiensis, C. reticulatum, S. predistentus,and R. umbilica) occur in the interval from Samples 125-786A-11X-3, 38 cm, to 125-786A-11X-CC. Nannofossil assem-blages from Samples 125-786A-12X-2, 35 cm, to 125-786A-19X-CC are assigned to Subzone CP14a by the occurrence ofChiasmolithus solitus, Sphenolithus furcatolithoides, and/?e-ticulofenestra dictyoda, in association in Sample 125-786 A-12X-2, 35 cm, with D. saipanensis. The absolute age isbetween 42.2 and 42.8 m.y., according to Berggren et al.(1985a).

Hole 786B

Rare, poorly to moderately preserved calcareous nanno-fossils are present in sediments interbedded with the lavaflows in Hole 786B.

Samples 125-786B-22R-1, 50 cm, 125-786B-27R-2, 28 cm,and 125-786B-40R-1, 35 cm, are middle to late Eocene in age(CP14-15 Zone), based on the presence of the marker speciesDiscoaster barbadiensis, Discoaster saipanensis, and Reticu-lofenestra umbilica and the species Cyclicargolithus flori-danus and Discoaster deflanderi.

Nannofossil assemblages were also found in Samples 125-786B-40R-1, 35 cm, 125-786B-41R-3, 60 cm, 125-786B-49R-1,50 cm, 125-786B-51R-2, 10 cm, and 125-786B-56R-CC. Thepresence of Ericsonia formosa, Dictyococcites bisectus, andCyclicargolithus floridanus confines these samples to themiddle to late Eocene, although no marker species werefound.

Early Eocene nannofossil assemblages are present in Sam-ples 125-786B-61R-6, 26 cm, and 125-786B-63R-1, 92 cm,based on the co-occurrence of D. barbadiensis, Discoasterdistinct us, and Sphenolithus anarrhopus.

Foraminifers

The Cenozoic planktonic foraminiferal stratigraphy forHole 786A is summarized in Table 4. The relatively poorpreservation and moderate to low abundance of planktonicforaminifers in most of the cores do not allow an accuratebiostratigraphic subdivision of Hole 786A. The presence orabsence of the species listed was used to define the age of asample, but no age determinations using the absence ofspecies were attempted for poorly preserved samples.

The planktonic foraminifers from the sedimentary se-quence in Hole 786A were deposited between the Quater-

319

SITE 786

Table 4. Summary of foraminiferal data, Hole 786A.


125-786A-1H-6, 140-1421H-6, 143-1441H-CC

2H-CC

3H-CC4X-CC

5X-CC

6X-CC

7X-CC8X-CC9X-CC

10X-6, 51-53

10X-CC

11X-1, 8-1011X-CC

12X-CC

Abundance

RareRareCommon

Rare

RareRare

Rare

Rare

RareRareAbundant

Common

Common

FewFew

Rare

Preservation

PoorPoorModerate

Poor

PoorModerate

Poor

Poor

PoorPoorModerate

Poor

Moderate

PoorPoor

Poor

Diagnostic species present

Globorotaüa crassaformisGloborotalia tosaensisGloborotaüa tosaensis

Globorotalia crassaformisNeogloboquadrina pachydermaGloborotalia crassaformisGloborotalia plesiotumidaSphaeroidinellopsis praedehiscensGloborotalia lenguaensisGlobigerinoides cf. kennettiOrbulina universaSphaeroidinellopsis seminulinaGloborotalia cf. lenguaensisGloborotalia cf. lenguaensisGlobigerinoides primordiusGlogoquadrina praedehiscensCatapsydrax dissimilisPseudohastigerina micraCribrohantkenina inflataHantkenina alabamensisCribrohantkenina inflataMorozovella aragonensisAcarinina bullbrookiMorozovella aragonensisAcarinina bullbrooki

Diagnostic speciesabsent

Globorotaliatruncatulinoides

—

——

—

—

——

—

—

——

—

Nannofossil zone

upper N19-22N21-lower N22N21

upper N19-21

upper N19-22N17B-lower N19

N16-17A

N9-21

N12-17AN12-17AN4A-5

P13-upper P19

P16-17

P16-17P9-11

P9-11

Age

middle early Pliocene-Pleistocenelate Pliocene-earliest Pleistocenelate Pliocene

middle early Pliocene-late Pliocene

middle early Pliocene-Pleistocenemiddle late Miocene-earliest Pliocene

early late Miocene-middle late Miocene

early middle Miocene-late Pliocene

middle middle Miocene-middle late Miocenemiddle middle Miocene-middle late Miocenelatest Oligocene-earliest Miocene

middle Eocene-early Oligocene

latest Eocene

latest Eocenelatest early Eocene-early middle Eocene

latest early Eocene-early middle Eocene

nary (lower Zone P22) and the latest early Eocene (ZonesPI6 and PI7). Evidence from the sparse planktonic foramin-iferal assemblages suggests that hiatuses may have existedin the late early Miocene to middle Miocene and the lateearly Oligocene. The relatively poor preservation and lowabundance of planktonic foraminifers in Samples 125-786A-1H-6, 51-53 cm, to 125-786A-12X-CC correlate with thelarge amounts of volcaniclastic material in these samples.

DiatomsRare to common and poorly to well-preserved diatom

assemblages are present in the first seven cores of Hole 786A.The samples below Sample 125-786A-7X-CC contain no dia-toms.

The uppermost Sample 125-786A-1H-1, 0 cm, is assignedto the Nitzschia reinholdii A Zone (Pliocene/Pleistoceneboundary) by the co-occurrence of N. reinholdii, Pseudoeuno-tia doliolus, and Rhizosolenia praebergonii.

The presence of Nitzschia jouseae in Sample 125-786A-1H-CC dates this sample as late Pliocene (top of N. jouseaeZone).

Zonal marker species were not found in Samples 125-786A-2H-CC and 125-786A-3H-CC. However, Sample 125-786A-3H-CC is tentatively placed in the early PlioceneThalassiosira convexa Zone by the absence of N. jouseae.

Nitzschia miocenica is present in Sample 125-786A-4X-CCand Nitzschia ported was found in Sample 125-786A-5X-CC,dating this interval as late Miocene.

Zonal markers were not found in Sample 125-786A-6X-CC,but Sample 125-786A-7X-CC contains Denticulopsis punctataf. hustedtii, which allows us to assign this sample to themiddle Miocene Craspedodiscus coscinodiscus Zone.

IGNEOUS AND METAMORPHIC PETROLOGYFrom the two holes drilled at Site 786, taken together, we

penetrated and recovered strata from 0 to 828 mbsf. In Hole786A, sediments were recovered from 0 to 100.5 mbsf(Sections 125-786A-1X to 125-786A-11X-5, 100 cm;"Lithostratigraphy" section, this chapter), and a sedimen-

tary breccia marks the transition to basement dominated byigneous strata of boninitic affinity. Predominantly igneousrocks were recovered in this hole from 124.9 to 166.5 mbsf(Sections 125-786A-11X-5, 100 cm, to 125-786A-19X-CC).Hole 786B is offset 30 m toward the south with respect to thepreceding hole, and cores were recovered from 162.5 to828.6 mbsf (Sections 125-786B-1R to 125-786B-72R-2),stratigraphically below the interval penetrated in Hole 786A.The cores of Hole 786B contain lava flows, breccias domi-nated by igneous clasts, rare sedimentary intervals, and verylow-grade metamorphic lavas crosscut by dikes. Biostrati-graphic studies ("Biostratigraphy" section) constrain theage of the top of the basement to middle Eocene, and severalsedimentary intercalations within the basement also givemiddle Eocene ages. The deepest sedimentary intercalationdated is in Core 125-786B-61R; local faults and a lack ofbiostratigraphic data leave the ages below this depth ambig-uous.

The igneous and sedimentary sequence in Holes 786Aand 786B has been divided into four major lithologic units.The first three units form a lower Pleistocene-middleEocene sedimentary sequence ("Lithostratigraphy" sec-tion). The fourth major unit, basement, is divided into 30subunits, and is the subject of this section. Intercalatedsedimentary strata in lithologic Unit IV are described in the"Lithostratigraphy" section. Unit divisions and rock namesare based on macroscopic criteria as well as on major-element chemical analyses. The distinction between boniniteseries rocks and andesite was not obvious in hand specimen.A nomenclature presented in the "Igneous and Metamor-phic Geochemistry" section (this chapter) was developedusing the MgO and SiO2 contents of the rocks. Rock namesbased on these compositional criteria are given precedencebut have not been rigidly applied throughout. Common rocknames synonymous with, and commonly more specific than,our compositional nomenclature are used in the hand spec-imen and thin-section descriptions when they do not conflictwith the chemical data, or when no analysis is available.Within these limits, the rock names used in unit descrip-

320

SITE 786

tions, from analyses, thin section, and hand specimen, areinternally consistent.

The following approach is used in the assignment oflithologic units. Multiple volcanic strata of similar characterare assigned to a single unit. If two volcanic units areseparated by strata of clear sedimentary affinities, the latterare assigned to a separate unit; breccias adjacent to thesesediments are assigned to this same unit.

A summary of the petrographic characteristics of the rocktypes encountered at Site 786 is given in Table 5. Thecharacteristics of the individual units are discussed in thefollowing. The basement lithostratigraphy is illustrated inFigure 3.

Unit 1

Sections 125-786A-1IX-5, 100 cm, to 125-786A-19X-CC and125-786B-1R-1, 0 cm, to 125-786B-5R-2, 25 cm.

The top of the igneous sequence from Sections 125-786A-11X-5, 100 cm, through 125-786A-12X-CC is formedby a breccia made up of clasts of basaltic andesite in a matrixof marl. The clasts contain 5%-20% fresh phenocrystsconsisting of two pyroxenes with olivine or Plagioclase.Some olivine phenocrysts contain chromium-spinel andhave reaction rims of orthopyroxene. Most of the ground-mass has been altered to a low-grade metamorphic assem-blage, but unaltered patches are locally preserved. Clastsincrease in size (from a few millimeters to 6 cm) andabundance with depth.

The poor recovery precludes a comprehensive lithologicstudy of the remainder of this unit. Cores 125-786A-13X-CCthrough 125-786A-19X-CC are mainly cuttings and clasts inclay of high-magnesian basalts, boninites, and picrites. Ortho-pyroxene is the dominant phenocryst in these samples, withsubordinate clinopyroxene and olivine and rare Plagioclase ina glassy groundmass; these are typical petrographic charac-teristics of rocks of the boninite series (Meijer, 1980). Unit 1lithologies continue in the interval from Sections 125-786B-1Rthrough 125-786B-5R-2, 25 cm, as oligomictic breccias con-taining boninite clasts. These contain mainly phenocrysts oforthopyroxene with subordinate clinopyroxene and Plagio-clase, although the latter can be locally predominant. Glass inthe groundmass of the clasts has been largely devitrified orreplaced by clay.

Unit 2

Sections 125-786B-5R-2, 25 cm, to 125-786B-8R-1, 0 cm.

The top of Unit 2 comprises a flow of intermingled andesiteand basalt overlying basalt. The basalt contains 5% to 7%phenocrysts, made up of olivine, which is replaced by acarbonate/iron oxide assemblage, and minor clinopyroxeneand traces of orthopyroxene and Plagioclase. The glassygroundmass is variably altered (10%-50%) to clay. The baseof the unit is formed by a relatively fresh andesite.

Unit 3


Unit 3 is formed by oligomictic breccias containing asmall number of exotic fragments. The breccias at the top ofthe unit contain andesite clasts petrographically similar tothose of the overlying flow of Unit 2 (Sections 125-786B-8R-1 through 125-786B-11R-1, 90 cm). The andesite contains

Table 5. Petrology of the rock types at Site 786.

BasaltPhenocrysts:

Groundmass:

Comments:

High-magnesianbasaltPhenocrysts:

Groundmass:

Comments:

PicritePhenocrysts:Groundmass:

BoninitePhenocrysts:

Groundmass:

Comments:

AndesitePhenocrysts:

Groundmass:

Comments:

DacitePhenocrysts:

Groundmass:

Comments:

RhyolitePhenocrysts:Groundmass:

Comments:

Clinopyroxene (3%-5%) + Plagioclase (trace-5%) +orthopyroxene (0%-2%)

Glass (60%-70%) + Plagioclase (5%-10%) +clinopyroxene (3%-5%) + opaque minerals (trace-1%)

Hyalo-ophitic; Sample 125-786B-40R-2, 54-56 cm,contains 15%—20% olivine

Clinopyroxene (5%-8%) + orthopyroxene (4%-7%) +olivine (0%-3%) + spinel (0%-trace)

Glass (50%-70%) + Plagioclase (10%-20%) +clinopyroxene (2%-7%) + orthopyroxene (0%-5%) +opaque minerals (0%-2%)

Spinel occurs as inclusions in olivine phenocrysts;glomerophyric texture (olivine + orthopyroxene +clinopyroxene); orthopyroxene has clinopyroxene (100)exsolution lamellae and a clinopyroxene reaction rim

NoneClinopyroxene (25%-30%) + orthopyroxene (l%-2%) +

glass (60%-70%) + opaque minerals (trace-1%)

Orthopyroxene (2%-7%) + clinopyroxene (3%-5%) +Plagioclase (5%-7%)

Glass (70%-80%) + Plagioclase (5%-10%) +clinopyroxene (l%-2%) + orthopyroxene (trace-1%)+ opaque minerals (0%-trace)

Orthopyroxene (2V = -80° to -85°; bronzite) has some(100) clinopyroxene exsolution lamellae and a reactionrim of clinopyroxene

Plagioclase (10%-20%) + clinopyroxene (2%-6%) +orthopyroxene (0%-2%) + opaque minerals(0%-trace)

Glass (50%-60%) + Plagioclase (10%-20%) + opaqueminerals (l%-2%)

Can be further classified as two-pyroxene(clinopyroxene + orthopyroxene) andesites;glomerocrysts are common

Plagioclase (5%-15%) + clinopyroxene (2%-8%) +orthopyroxene (trace)

Glass (60%-70%) + Plagioclase (10%-20%) + opaqueminerals (l%-2%)

Porphyritic (partly glomerophyric) and hyalopilitic

Plagioclase (2%-3%)Glass (40%-50%) + Plagioclase + quartz + opaque

mineralsPredominantly massive rhyolite without flow texture

8% to 30% phenocrysts of Plagioclase and clinopyroxeneand minor orthopyroxene in a black glassy groundmass, withPlagioclase and minor clinopyroxene. Glass is partly tolargely replaced by clays. Alteration rims of clasts appearsimilar to the sandy matrix of the breccia, indicating in-situbrecciation. Exotic fragments of pumice and basalt arepresent in the lower part of the andesite breccia (Sections125-786B-10R-1 through 125-786B-11R-1, 90 cm). High-magnesian basalts and boninite, classified on the basis ofbulk-rock chemistry, are present in Sections 125-786B-11R-1, 90 cm, through 125-786B-14R-1, 0 cm. Basalt clastscontain phenocrysts of two pyroxenes and Plagioclase in afine-grained to glassy groundmass. Clasts alter to materialsimilar to the matrix of the breccia. Boninite clasts alsocontain phenocrysts of two pyroxenes and Plagioclase, butclinopyroxene is subordinate. Sections 125-786B-14R-1through 125-786B-17R-1, 115 cm, are made up of andesite-dacite breccias. Microdiorite fragments, possibly represent-

321

SITE 786

Hole 786A

1 8

1H

2H

3H

4X

50 —

100 —

5X

6X

7X

8X

9X

10X

11X

12X

13X

14X

15X

AA

Age

early Pleistocene

late Pliocene

early Pliocene

late Miocene

late Miocene

middle Miocene

middle Miocene

late Oligocene

early Oligoceneearly Oligocene

late Eocene

middle Eocene

middle Eocene

Lithology

II. •^•

/

* •

^ ̂* //

-

\— Λ

I' // =

*• • * ;

-

= / /

•fi A

\ II

\ ^

w

—

Lithologicdescription

Nannofossil marl,nannofossil-richclay, glass-richclay

Alteredvolcaniclasticbreccia

Drilling chips ofassorted breccias,flows, vein material,and sediments

Lithunit

IV

Key for preservation for 7A and 7B

AA= very altered: groundmass and phenocrysts are pervasively altered, only phenocrysttraces remaining.

A altered: groundmass and olivines altered, other phenocrysts largely fresh, somefresh glass.

F = fresh: some alteration of groundmass and phenocrysts, olivine altered.

FF= very fresh: no signs of alteration, vesicles may contain secondary mineralization.

Figure 3. Lithostratigraphy and core recovery at Site 786. A. Hole 786A. B. Hole 786B.

322

SITE 786

B Hole 786B

200 —

250 —

Q.

O

300 —

350 —

400 —

2R

3R

6R

7R

9R

10R

16R

17R

18R

Q Basalt flowHigh-Mgbasalt flow

2-pyroxene

andesite flows

2-pyroxeneandesite breccia

High-Mg basaltbreccia

C High-Mg basalt-boninite breccia

A-AA j

25R

Lithology

£2: Basalt flow

'smgggtds


Lith.unit

Boninite breccia

Dacite breccia

3-bearingandesite flow

c Andesite brecciaFlow-banded dacite

Sandstone-sepioliteDacite breccia

Sparsely phyricdacite flows

Dacite brecciaGlass-rich chlorite sand

Other comments

Magma mingling

Microdiorite fragmentAltered pillow rinds

Microdiorite fragment

Normal faulting,synsedimentaryslumping

Synsedimentarynormal faulting

Figure 3 (continued).

ing cumulates, were recovered in Samples 125-786B-15R-1,0-3 cm, and 125-786B-17R-1, 0-3 cm. Alteration throughoutthis unit is highly variable.

Unit 4


Unit 4 consists of two flows of variably altered andesite.The top flow occupies Sections 125-786B-17R-1 through 125-

yyw\yyyysmm

m

Pillows and hyaloclastite

Breccia

Andesitic or basaltic flows

Andesitic or basaltic dikes

Boninitic flows

Rhyolitic flows

Boninitic dikes

Rhyolitic dikes

Stockwork

Sand, sandstone

786B-19R-1, and the lower flow, Sections 125-786B-20R-1through 125-786B-21R-1, 0 cm.

Unit5


Unit 5 is a brecciated andesite having minor alteration tochlorite and clays and grading downward into highly brecci-

323

SITE 786

Hole 786B (continued)

450

500 —

.o

£x:Q.

a

550 —

600 —

650 —

ü

27R

45R

46R

A-AA

LithologyLithologic

description

Boninite-andesiteflows

Boninite flows

Boninite flows

Rhyolite flow

Redbed

Andesite flowAndesitebreccia

High-Mgbasalt flow

Dacite flow

Boninite flows

Andesite flows

Andesite breccia_ Conjugate

Basalt flow faultsBreccia and vitricsandstone

Andesite flow

Andesite flowSlickensidessubhorizontal

Andesite breccia

High-Mg basaltboninite-breccia

Lith.unit

11

15

18

19

Other comments

Olivine-bearing

Mineralized

Olivine-bearing

Olivine-bearing


ated and altered andesite. A thin veneer of sediment forms theboundary to the underlying unit.

Unit 6


Unit 6 comprises a flow-banded aphyric dacite. The bound-ary between Units 6 and 7 is placed at the glassy contact oftwo cooling units, although the incorporation of Section

>w_VTΛ>^Λ<ΛJ


Breccia



Boninitic flows

Rhyolitic flows

Boninitic dikes

Rhyolitic dikes

Stockwork

Sand, sandstone

125-786B-21R-2, 0-23 cm, into this dacite unit cannot be madewith certainty without chemical analyses.

Unit 7


Unit 7 comprises glassy aphyric basalt having minor,variable alteration. The lower contact between basalt and the

324

SITE 786

Hole 786B (continued)

700 •

'750 —

α.ΦQ

800-J

54R

55R

60R

61R

62R

63R

64m

65R

66R

67R

68F

69 Rl

7 0 R |

F-AA

F-AA

AA

Lithology

^ S £ •

rsf•

./» ̂ — ^ ^ N ^

1 1 1 1


Boninite breccia

High-anglefaulting

Sandstone

Boninite-andesitepillows andhyaloclastite

Faulting

Rhyolite dike

breccia and sandHigh-Mg basalt,

boninite, andhyaloclastite

Shear zone D u c 1 i l e s n e a r :

Shear zone c a ; t a c , a s t j C S

Metaandesite andmetarhyolite flows

Metarhyolite dike

Metaandesite flow

Metaandesite flowor dike

High-Mg basaltdike

MetaandesiteBoninite dike

Metaandesite

Metaandesitestockwork

High-Mg basalt

Lith.unit

19

20

25

26

27

28

Other comments

Copper traces

Fe-Mn sediment

Shearing

VesicularNonvesicular

Vesicular

Olivine-bearing

Mineralized

Heavily mineralized

*è0A


Breccia



Boninitic flows

Rhyolitic flows

Boninitic dikes

Rhyolitic dikes

Stockwork

Sand, sandstone


underlying sediments of Unit 8 is marked by a thin (12 cm)band of sepiolite.

Unit 8

Sections 125-786-22R-1, 27 cm, to 125-786B-22R-3, 124 cm.

Unit 8 consists of vitric siltstones and sandstones interca-lated with a polymictic breccia that contains clasts of pumiceand volcanic fragments in a matrix of sand and silt.

Unit 9

Sections 125-786B-23R-1, 0 cm, to 125-786B-27R-1, 110 cm.Unit 9 is made up of dacite flows with a Plagioclase,

two-pyroxene, rhyolite breccia at the base (below Section125-786B-26R-1, 115 cm). The dacite has been variablyaltered into clays, and vesicles are filled with a white zeolite.

Unit 10

Sections 125-786B-27R-1, 110 cm, to 125-786B-27R-2, 0 cm.Unit 10 is a thin, glass-rich chloritic sandstone, fining

upward, with cross-bedding and lamination at the base. Itcontains a few clasts.

Unit 11


Unit 11 comprises a series of very phyric (30%) boninites.The boninites contain two pyroxenes and Plagioclase witholivine increasingly present toward the bottom of the unit.Clinopyroxene and Plagioclase are the dominant phenocrystphases. Alteration has been concentrated along veins andfractures that contain carbonate and possible serpentine. InSections 786B-31R-1 through 125-786B-32R-1, 73 cm, olivinehas been altered to hematite and carbonate, and glass has beenlargely altered to clay.

Unit 12


Unit 12 is a rhyolite flow, variably altered to clays andchlorite.

Unit 13

Sections 125-786B-33R-1, 0 cm, to 125-786B-33R-1, 35 cm.Unit 13 consists of olivine-bearing, basaltic andesite that

exhibits pervasive hematite alteration localized around olivine

325

SITE 786

and also disseminated within the groundmass; carbonate-filledveins are common.

Unit 14


The top of Unit 14 consists of a meta-andesite having pyritemineralization, underlain by a brecciated, flow-banded andes-ite. The lower part of the unit (Section 125-786B-34R-1,53-140 cm) is made up of a feldspar-rich, vitric sandstone andsiltstone with graded bedding and a possibly baked uppercontact. These sediments in turn overlie brecciated andesitehaving flow banding and pyrite mineralization. Brecciatedzones have experienced pervasive alteration to a low-grademetamorphic assemblage.

Unit 15


The top of Unit 15 (Sections 125-786B-34R-1, 145 cm,through 125-786B-34R-4, 101 cm) has been formed by flow(s)of sparsely phyric basalt with phenocrysts of Plagioclase,clinopyroxene, and rare orthopyroxene and olivine. Phenoc-ryst contents are higher toward the lower part of this interval.The basalt groundmass has been pervasively altered to alow-grade mineral assemblage, and traces of pyrite occurthroughout. Sparsely phyric dacite flows with Plagioclase andclinopyroxene phenocrysts are present in Sections 125-786B-34R-4, 101 cm, through 125-786B-37R-1, 80 cm, and olivine-bearing boninite is present in Sections 125-786B-37R-1, 80 cm,through 125-786B-37R-3, 30 cm. Andesite makes up the lowerpart of the unit. Dacites and andesites are markedly lessaltered than the more mafic members of this unit.

Unit 16Sections 125-786B-39R-2, 0 cm, to 125-786B-40R-1, 75 cm.Unit 16 comprises a series of andesite breccias. Pyrite is

disseminated throughout the unit and concentrated alongveins that contain zeolites. The groundmass is slightly tomoderately chloritized.

Unit 17


Unit 17 consists of orthopyroxene-olivine-phyric basalt,pervasively altered to chlorite and carbonates and cut by veinsfilled with chlorite, carbonate, and iron oxide.

Unit 18


The top of Unit 18 is an oligomictic breccia having a matrixcomponent that increases in proportion upward. This overliesa laminated vitric sand, and then a polymictic breccia, alsowith a matrix component that increases upward and thengrades into a vitric sand. Clasts in both breccias are andesites.The base of the unit has been formed by an andesite flow.

Unit 19


Various boninite and andesite breccias and flows make upthis unit. The top of Unit 19 is marked by the appearance ofrare olivine phenocrysts in the andesite flows. Pervasivelyaltered meta-andesite having a low-grade metamorphic as-

semblage in Section 125-786B-44R-1 forms the transition tothe underlying brecciated, olivine-bearing andesite of Sec-tions 125-786B-44R-2, 10 cm, through 125-786B-50R-1.Breccia clasts in Section 125-786B-51R-1 have the compo-sition of high-magnesian basalts, but the phenocrysts ofPlagioclase (10%-20%) and two pyroxenes (5%—10% each)suggest a close similarity to the andesites. Breccias continuethrough Section 125-786B-56R-6 with clasts that appear tobe boninitic on the basis of the high proportion of orthopy-roxene, in the mode of Section 125-786B-55R-6. The bon-inite breccia of Sections 125-786B-55R-3 through 125-786B-56R-1 contains a trace of native copper. Chlorite and hema-tite alteration is pervasive. Veins and fractures arecharacteristically absent, except in the copper-bearing inter-val.

Unit 20


A fining-upward sandstone with basal graded bedding andlocal clasts forms Unit 20.

Unit 21


Unit 21 consists of multiple pillow lavas and hyaloclastitesof bronzite-boninite and andesite with red sediments betweenpillows. White and green zeolite and iron oxide fill veins andvesicles. At the base of the unit, a fault breccia forms thecontact with Unit 22.

Unit 22

Sections 125-786B-61R-4, 42 cm, to 125-786B-61R-6, 40 cm,and 125-786B-61R-6, 65 cm, to 125-786B-62R-1, 0 cm.

Unit 22 is a rhyolite flow or (more probably) dike or sill,which is locally oxidized to a dark red color and contains glassaltered to chlorite.

Unit 23


Unit 23 is a shear zone consisting of breccia and sandstone.

Unit 24


Unit 24 consists of altered boninite or andesite pillows andhyaloclastites, locally high-magnesian basalt in composition,with red sediment between the pillows. Fine filaments of redferromanganoan sediment are particularly prominent in thehyaloclastites. Clay and chlorite alteration of glass is perva-sive. Zeolite and sparse carbonate fill veins.

Unit 25

Sections 125-786B-62R-3, 85 cm, to 125-786B-63R-2, 44 cm, and125-786B-63R-2, 78 cm, to 125-786B-63R-2, 92 cm.

Unit 25 consists of Cataclastic meta-andesites and sand-stone in a shear zone. The andesites are largely altered to alow-grade metamorphic assemblage. Orthopyroxene pheno-crysts have been largely altered to a chlorite/carbonate assem-blage, but some unaltered cores remain. Clinopyroxene isvariably altered.

326

SITE 786

Unit 26

Sections 125-786B-63R-2, 44 cm, to 125-786B-63R-2, 78 cm,and 125-786B-63R-2, 92 cm, to 125-786B-67R-1, 0 cm.

Metadacite and rhyolite flows of low metamorphic gradeform this unit. Orthopyroxene has been replaced by chlorite,carbonate, and iron oxide; clinopyroxene has been variablyaltered to chlorite, and Plagioclase is albitized. Vesicles arefilled with zeolite, carbonate, and chlorite and locally containsulfide minerals.

Unit 27


Dikes of basalt and boninite composition make up Unit 27.The dikes show little evidence of chilled margins and havebeen locally altered to chlorite and carbonate. Olivine hasbeen completely replaced by a chlorite/carbonate assemblage,and clinopyroxene has been variably chloritized. Veins con-tain carbonate and sulfide minerals.

Unit 28


Unit 28 consists of meta-two-pyroxene-andesite, probablyof boninitic composition, with abundant chlorite and carbon-ates along fractures and extensive sulfide mineralization. Oneboninite dike, apparently 30 cm thick, also forms part of thisunit.

Unit 29


Unit 29 is an argillized, mineralized, andesite stockwork.Andesite has been completely altered to clays, epidote, chlo-rite, and quartz. Extensive sulfide mineralization is present.

Unit 30


Unit 30 contains fresh, avesicular boninitic glass, possiblyintruded as a dike, in a meta-andesite massive flow or dike.

IGNEOUS AND METAMORPHIC GEOCHEMISTRY

Lithologies drilled at Holes 786A and 786B include massiveflows, pyroclastic flows and interbedded sediments, massiveflows and intrusive sheets, pillow lavas and hyaloclastiteflows, and eventually multiple composite intrusive sheets (see"Igneous and Metamorphic Petrology" section, this chapter).Although the metamorphic grade reaches the upper green-schist facies (albite-chlorite-epidote-quartz assemblage) at thebase of Hole 786B, most of the extrusive rocks analyzedcontain abundant fresh glass. Alteration is therefore thoughtto have had only a small effect on the concentrations of majorelements. A total of 47 samples collected from Site 786 wasanalyzed on board the ship for abundances of both major andtrace elements by XRF spectrometry (see "ExplanatoryNotes" chapter, this volume). Abundances of major and traceelements are presented in Table 6.

Major-Element GeochemistryThe rock types from Site 786 show a large variation in SiO2

and MgO (Fig. 4) and hence have been classified on this basis.This classification arbitrarily subdivides the rocks into high- andlow-magnesian groups. Although the rock types range frompicrites (low SiO2, >12 wt% MgO) to rhyolites (>70 wt% SiO2,<2 wt% MgO), the majority of the samples fall into the high-

magnesian basalt, boninite, and andesite classification fields. Thedispersion on a covariation diagram of magnesium (between 18.5and 0 wt% MgO) against silica (between 48.5 and 74.5 wt% SiO2)forms a crescent shape that converges toward both the picriteand the andesite fields. Because the suite of rocks crosses theline of 63 wt% SiO2 into the dacite and rhyolite fields, the rockscan be inferred to have been derived from parental magmas thatwere saturated with respect to silica.

Nearly all the rock types at Site 786 contain up to 20 modal%of Plagioclase phenocrysts or xenocrysts (see "Igneous andMetamorphic Petrology" section, this chapter). Such large pro-portions of Plagioclase account for the apparent enrichment inaluminum (Fig. 5) and may have been caused by Plagioclaseaccumulation or by a shift in the co-tectic toward Plagioclase byan increased water content of the magma. The wide scatter ofpoints on an aluminum-magnesium diagram (Fig. 5) suggests,however, that the modal percentage of Plagioclase is indepen-dent of the magnesium content of the rock. This scatter may bethe result of magma mixing processes (see "Igneous and Meta-morphic Petrology" section, this chapter).

Despite the large variations in MgO, SiO2, and A12O3, allthe rocks from Site 786 are characterized by extremely lowabundances of these elements and a small range of TiO2 (from0.1 to 0.3 wt%). In some rocks, TiO2 is an order of magnitudeless than normal mid-ocean ridge basalt (N-MORB), whichhas an average TiO2 content of 1.5 wt%. This suggests that allthe rocks may have been derived from a source that wasextremely depleted in the incompatible trace elements.

Trace-Element GeochemistryThe characteristically depleted nature of the rocks from

Site 786 is reflected by their low abundances of high-field-strength (HFS) elements in general and yttrium and titaniumin particular. However, zirconium is variably depleted and thezirconium/yttrium ratio is thus highly variable, ranging fromless than 2 to more than 6. The most primitive rocks are alsothose having the highest abundances of chromium, whichranges from about 220 to 1330 ppm. The accumulation ofchromium-rich spinel is probably responsible for abundancesof chromium in excess of 1000 ppm; these rocks also havevariable amounts of orthopyroxene and olivine phenocrysts(see "Igneous and Metamorphic Petrology" section, thischapter). Two of the factors involved in the petrogenetichistory of the rocks are illustrated in the chromium-yttriumdiagram in Figure 6. Source effects, either variable depletionor partial melting, generate a subhorizontal array of datapoints, whereas fractional crystallization produces a subver-tical trend. Although large variations in yttrium occur in theprimitive rock types, the fractionated rocks exhibit a rela-tively narrow range of yttrium abundances. The former maybe evidence for a range of potential parent magmas related tovariable source fertility or variations in partial melting. Thelatter may be evidence for the generation of the more evolvedmagmas from a single, homogeneous parental magma. For themajority of primitive rocks at Site 786, the abundance ofyttrium is always less than one-half that for N-MORB, whichhas an average of about 30 ppm yttrium. The HFS-element-depleted nature of the rocks from Site 786 is summarized byN-MORB normalized diagrams for a representative boninite,high-magnesian basalt, and primitive andesite (Figs. 7A-7C).Cerium and phosphorus are near or below their detectionlimits, and therefore are not included. However, niobium,zirconium, and possibly yttrium may be variably enrichedwith respect to titanium. The large-ion-lithophile (LIL) ele-ments are enriched in all three representative samples, buttheir abundances are only slightly higher than those in N-MORB. If the enrichment in LIL elements is derived from

327

SITE 786

Table 6. Geochemical data, Holes 786A and 786B.

Sample(interval in cm):

Rock type:a

786A-6X-5,24-27

Andesite

Major elements (wt%)SiO2

TiO2

A12O3

Fe2O3 (total)MnOMgOCaONa2OK2OP2O5LOITotalMg#

52.270.82

16.1212.230.204.109.651.030.34nd0.84

98.140.37

Trace elements (ppm)NbZrYSrRbZnCuNiCrVTICeBa

0.64024

1816.1

1131661823

4285520

2627

786A-12X-1,140-142Basalt

51.430.29

17.278.340.095.048.201.831.38nd2.92

96.840.51

2.446

6.1165

14.1924085

320356

18401036

786A-13X-CC,27-29

High-Mgbasalt

52.450.16

12.157.920.01

12.785.501.310.83nd5.79

98.990.74

2.3275.9

10013.16352

268613143

13203

23

786A-16X-CC,25-28

High-Mgbasalt

51.000.21

14.627.810.10

11.538.061.570.32nd2.41

97.650.72

0.833

9.4147

2.5fi837

190506226

168036

786A-17X-CC,4-6

Picrite

48.680.13

11.439.230.19

14.386.581.740.70nd6.16

99.230.73

0.82210.993

9.573

160215471194

114058

786A-19X-CC,17-20Picrite

47.530.22

10.708.970.16

18.845.761.020.41nd5.21

98.860.78

1.4218.7

647.2

7123

5221338

1951620

nd11

786A-19X-1,0-30

High-Mgbasalt

49.960.20

14.227.670.10

10.827.931.320.50nd

4.6697.39

0.71

—

786B-1R-1,61-64

Boninite

60.110.14

12.397.030.117,806.092.050.51nd2.19

98.480.66

2.144

7.0174

6.2556799

374177

13201237

786B-2R-1,58-61

Boninite

60.880.15

12.376.80.117.056.072.080.55nd2.05

98.100.64

1.8452.2

1787.4

537285

345185

13201439

786B-3R-1,94-97

Boninite

56.680.15

13.097.380.118.545.311.460.80nd3.78

98.280.67

2.247

4.6153236184

103385100

13809

21

786B-5R-2,70-72

Boninite

50.230.28

14.408.550.117.67

10.891.571.250.013.40

97.360.61

0.83412.8

1476.4

6363

170333237

2280nd18

fluids that were driven off from the slab during subductiondewatering, then this component is not as great as might havebeen expected for West Pacific subduction-related magmas.

SEDIMENT/FLUID GEOCHEMISTRY

Sediment Geochemistry

Sediments from Site 786 were analyzed on board the ship forinorganic carbon and for total carbon, nitrogen, and sulfur usingthe techniques described in the "Explanatory Notes" chapter(this volume). The organic carbon content was then calculatedby difference. These results are presented in Table 7 and inFigure 8. The CaCO3 content of the sediments varies from lessthan 1 to 40 wt%, except for a single sample from 102.50 mbsfthat contains 67.2 wt%. The organic carbon content decreasesfrom 0.41 wt% near the seafloor to less than 0.05 wt% at about100 mbsf. The single deep sample, from 154.27 mbsf, contains0.21 wt%. Nitrogen concentrations generally decrease withorganic carbon content and with depth, from 0.22 wt% near theseafloor to less than 0.01 wt% at 14 mbsf.

20

O10 —

1 1

Picrite

- *\

Basalt

1

1 1

β«.High-Mg basalt

•

* / Boninite

• *Andesite j

» Dacitei 1 . •

1

-

JRhyoIüδ

40 50 60SiO2 (wt%)

70 80

Figure 4. Rock classification based on variation in MgO with SiO2.The arbitrary separation between high- and low-MgO groups is usedto provide a consistent framework of classification.

Fluid Geochemistry

The sediments at Site 786 have uniformly lower concentra-tions of methane than any of the other eight sites investigatedduring Leg 125. The concentrations, as measured for 5-cm3

sediment headspace samples, vary from 8 to 11 µL/L (Table8). Methane was the only hydrocarbon detected.

Four interstitial-water samples were obtained from 5.98 to81.25 mbsf in Hole 786A (Table 9). These samples are com-pared with those from Sites 782 through 785 and with surfaceseawater in Figure 9. At Site 786, pH varies from 7.6 to 7.9,within the normal range of 7.5 to 8.5 for interstitial watersfrom deep-sea sediments. Alkalinity and ammonia reach max-imum concentrations that are smaller and occur at shallower

17.5

15.0 —

12.5

10.010

MgO (wt%)

Figure 5. Rocks from Site 786 exhibit a wide variation in MgO vs.A12O3, which may represent variations in the modal% of Plagioclasephenocrysts or xenocrysts in the rocks, but not all plagioclase-phyrictypes have high A12O3. Solid symbols = rocks with Plagioclase greaterthan 10 modal%.

328

SITE 786

Table 6 (continued).

786B-6R-2,128-130

High-Mgbasalt

51.450.28

14.128.570.17

10.929.321.110.25nd1.90

98.080.69

1.13310.7

1362.9

6X69

223501232

2220159

786B-8R-1,45-47

Andesite

54.370.23

17.487.760.123.987.692.370.65nd2.71

97.340.47

0.95116.1

2278.5

736841

8236

19808

17

786B-9R-1,10-15

Andesite

56.090.24

17.077.430.113.837.522.070.57nd1.97

96.880.47

1.65310.4

2266.6

7210140

8239

19201640

786B-11R-1,122-126

High-Mgbasalt

53.640.14

12.897.620.14

10.886.501.190.42nd2.80

97.640.71

1.739

5.5170

5.87140

159471174

13801617

786B-12R-2,14-16

High-Mgbasalt

54.260.16

13.708.280.15

11.547.171.280.31nd2.16

98.510.71

1.6415.6

1854.0

7638

164485153

15003

25

786B-12R-2,120-124Boninite

59.240.12

12.787.110.128.386.721.560.440.031.53

97.990.67

2.137

6.5166

6.35374

116425187

12001337

786B-16R-1,137-141Dacite

61.830.25

14.936.740.091.615.492.860.72nd2.48

96.990.29

1.55710

2177.2

6591

96

2191860

1535

786B-19R-1,91-94

Andesite

58.130.21

16.257.490.123.577.142.000.71nd2.34

97.960.45

1.948

8.7214

7.772783335

2261860

1331

786B-21R-1,26-29

Andesite

56.850.27

16.957.700.101.856.562.800.850.042.91

96.880.29

0.95821

23611.18662185

212100

1227

786B-21R-1,129-131Dacite

66.910.19

12.623.490.070.312.302.902.05nd6.50

97.300.13

2.07611.8

148364968

60

241380

1352

786B-21R-2,72-76Basalt

50.620.15

11.808.140.145.78

13.331.190.39nd5.75

97.270.55

1.229

8.3150

7.85040

188889185

13801320

786B-22R-3,61-64Dacite

66.140.17

12.133.290.060.111.963.582.380.027.97

97.060.05

1.57613.6

162415126

1nd54

13201058

depths than at the other sites, and alkalinity decreases morequickly with depth below its maximum at about 6 mbsf. Thesechanges are consistent with a minimal amount of bacterialsulfate reduction, as indicated by the slight decrease in sulfatewith depth. The steep decrease in alkalinity is accompanied bya decrease in silica with depth below 25 mbsf. Like alkalinity,magnesium, potassium, and calcium change more rapidly withdepth at Site 786 than at the other sites (Fig. 9). These changesin alkalinity, silica, magnesium, potassium, and calcium prob-ably result from reactions between interstitial water andvolcanic material within and below the sediments.

Chlorinity increases by about 2% with depth at Site 786(Fig. 9), as it does at the other sites. This increase probablyresults from the increased chlorinity of ocean bottom waterduring the Pleistocene glaciations, as proposed by McDuff(1985).

10,000 e

1oO

1,000 -

100 --

10 -

Source depletion 'and partial melting

: l v %

=~z

E2

• tn

• I

•

••

i

Jllll 1

1m

ill i i i

Fracti

r

linn

anal

\ S• -

11 -Is

i i i i i i i

10Yttrium (ppm)

100

Figure 6. Chromium-yttrium co-variation diagram for volcanic rocksfrom Site 786. The relatively large variation in yttrium at highchromium indicates a range of primitive liquid compositions thatreflects variable source characteristics. The vertical array of datapoints is a function of crystal fractionation.

A 10 FT 1

Sr K Rb Ba Nb Ce

er

Sr K Rb Ba Nb Ce Zr Ti

Figure 7. Multi-element N-MORB-normalized diagrams. A. Repre-sentative high-Mg basalt (Sample 125-786B-5R-2, 70-72 cm). B.Representative primitive andesite (Sample 125-786B-40R-2, 54-57cm). C. Representative boninite (Sample 125-786B-11R-1, 122-126cm).

329

SITE 786


Sample 786B-24R(interval in cm):

Rock type:a

Major elements (wt%)SiO2

TiOjA12O3Fe2O3 (total)MnOMgOCaONa2OK,OISO,LOITotalMg#

Trace elements (ppm)NbZrYSrRbZnCuNiCrVTICcBa

9-12Dacite

66.160.26

14.855.180.070.894.463.4111.820.020.44

96.520.23

1.471)13.7

2087.3

4554

78

1851920

784

786B-26R-1,68-70Dacite

66.240.26

14.594.740.060.553.813.591.14nd

0.3196.760.17

1.87511.8

19516.14022

20

119I860

1073

786B-30R-1,29-31

Boninite

56.030.09

12.206.980.096.529.831.710.87nd

4.2398. (M

0.62

1.631

5.7195235129

126311132960

1543

786B-30R-1,119-121Boninite

58.520.14

12.946.390.095.638.072.070.070nd

3.4397.940.60

1.642

7.6178

15.85113

127444142

12601045

786B-31R-2,84-86

Boninite

57.710.10

13.096.470.108.028.111.570.51nd1.67

97.360.68

1.6345.7

21110.26320

131381184

10201132

786B-32R-2,86-88

Rhyolite

71.040.23

12.853.850.02nd2.883.351.35nd1.15

96.72nd

1.47412.4

173433920

2nd46

16806

77

786B-34R-3,45-47

High-Mgbasalt

52.130.14

11.617.460.129.00

11.051.420.58nd5.44

98.340.68

1.429

6.814115.35561

277769160

13207

11

786B-35R-1,73-76Dacite

62.480.20

14.865.740.043.255.493.020.52nd1.72

97.280.49

1.65610.1

2137.7

48M2650

1521680

059

786B-35R-2,122-126Dacite

64.800.23

14.725.390.042.365.002.930.69nd1.23

97.270.43

1.659

9.5201

11.44833

47134

16201453

786B-37R-1,95-98

Boninite

56.510.20

14.807.230.086.537.771.960.47nd2.29

97.820.61

1.638

9.318613.46820

149393185

1620nd34

786B-37R-3,45-49

Andesite

57.750.20

14.758.050.102.655.052.141.45nd5.39

97.500.36

1.444

9.1293

29619414nd

224

162012

108

786B-38R-1,6-8

Andesite

58.980.21

15.937.010.092.096.682.051.03nd2.94

97.000.34

1.7499.2

22913.9626612nd

2591740

1354

STRUCTURAL STUDIES

Normal faults have been observed within the soft sedi-ments of Hole 786A in Sections 125-786A-1H-6, 125-786A-2H-4, and 125-786A-2H-7. In Sections 125-786A-2H-4 and125-786A-2H-7, the faults cut graded ash layers. In Section125-786A-2H-7, one of the offset parts of the ash layer ismissing. Although the total amount of displacement along thefault cannot be determined, it is at least greater than 25 cm.The faults can be related to local disaggregation processesduring normal de watering of the sediments. They may alsoindicate regional slope instability, as is probably the case forthe set of faults visible in Section 125-786A-1H-6, which mayrepresent a major tectonic feature. The site is near a majorfault that has a vertical displacement of a few hundred metersand is evident in seismic sections. A fault crosscuts a volcanicpebble in Section 125-786A-11X-4 at 100 cm. This fault alsorepresents the upper limit of the sediments that containvolcanic pebbles and must therefore be regarded as a majorbreak in the sequence recovered from this hole.

Detailed descriptions and genetic interpretations of all thestructural post-depositional features encountered in the vol-canic sequence, the volcaniclastic breccias and sandstones,and the interbedded pelagic sediments of Hole 786B are toocomplex to be presented here. We summarize the mostimportant features as follows:

Core 125-786B-21R: Welded tuffs having a vertical foliationthat probably results from flow banding. The attitude ofbanding may be primary or may indicate post-depositionaltectonic tilting.

Core 125-786B-22R: The following observations documenta major shear zone:

1. Analysis of relationships of C-S planes in a stronglysheared and flattened pink sepiolite layer in Section 125-786B-22R-1 suggests a normal sense of shear.

2. Primary bedding in sedimentary rocks in Section 125-786B-22R-1 has high-angle (60°-70°) dips. Numerous synsed-imentary faults and slump structures in the sediments indicatethat syn- to post-depositional tectonic activity and relatedslope instability had taken place before the major tilting.

3. Welded tuffs in Section 125-786B-22R-2 commonly ex-hibit vertically oriented flow-banding and syn-consolidation

extensional microfaulting. This last observation is consistentwith the hypothesis of emplacement along a preexisting slope.

Core 125-786B-27R: Extensional syn-depositional faultsand related cross-lamination are in the graded, coarse, greenvolcanic sandstones in Section 125-786B-27R-1, 50-70 cm.

Core 125-786B-34R: Numerous fractures and calcite-filledveins are visible in Sections 125-786B-34R-2 and 125-786B-34R-3. Dips vary from 45° to 80°. Generally, no displacementtook place along the fractures. The veins show multipleorientations.

Core 125-786B-40R: Two fault planes dipping 55° and 65°with slickensided surfaces are observed at 30 cm in Section125-786B-40R-2. The sense of movement is normal along onefault but is reversed along the other. At 95 cm in the samesection, a fault surface dips at 62°, although clear evidence ofdisplacement is not present.

Core 125-786B-41R: Section 125-786B-41R-3 containswelded tuffs with flow lamination and depositional layeringdipping at 40° to 50°. "Hot" deformation criteria, such asplastic folding of glass laminae, are common.

Core 125-786B-43R: In Section 125-786B-43R-1 a strong,subhorizontal striation is visible along a plane dipping at 70°.This is the only indicator of strike-slip movement observed atSite 786.

Core 125-786B-56R: Three fault surfaces occur in Section125-786B-56R-4 between 30 and 80 cm. They show striationrakes on a slickensided surface indicating normal and reversemovements. Fault dips range from 65° to 75°.

Cores 125-786B-58R and 125-786B-59R: Detailed analysisof vesicle distribution within the glassy rims of the boniniticpillow lavas of these two sections suggests a general tilting(40°-50°?) of this volcanic pile (see "Igneous and Metamor-phic Petrology" section, this chapter).

Core 125-786B-61R: This core is characterized by tectonicdiscontinuities within the sequence of andesitic rocks. Faults,fault breccias and Cataclastic rocks, powdered rocks fromfault gouges, faint pressure-dissolution cleavages, locallyabundant veining, and additional high-angle dips of flowbanding and flow contacts (up to 60°) are in Sections 125-786B-61R-4 to 125-786B-61R-6. In Section 125-786B-61R-4,steps on a fault plane dipping at 65° indicate normal move-ment.

330

SITE 786


786B-40R-2,54-57Basalt

47.230.22

12.068.050.14

11.4011.900.890.26nd5.46

97.600.71

1.6278.9

1365.6

5727

374650193

18002

14

786B-49R-4,32-37

Andesite

58.340.23

15.037.100.133.627.292.040.73nd2.89

97.390.47

1.44510.8

18510.158

3993

2451860

946

786B-51R-1,51-55

High-Mgbasalt

53.250.15

12.748.490.17

12.367.781.170.260.082.01

98.430.72

0.53717.4

1663.7

7041

195

15602022

786B-61R-5,56-58

Rhyolite

72.300.24

13.123.260.05nd2.493.351.11nd1.84

96.94nd

1.47714.1

12118.14556

30

351680

1391

786B-61R-5,81-84

Rhyolite

71.020.26

12.593.360.04nd1.773.851.14nd1.43

95.52nd

2.18214.1

12214.6521860

411800

1193

786B-62R-3,40-42

High-Mgbasalt

53.470.14

12.058.440.13

13.045.272.180.67nd4.33

99.200.73

2.234

7.8110

6.56360

3061032

166

546

786B-63R-2,72-74

Rhyolite

74.290.17

12.411.920.01nd0.452.514.98nd0.27

97.01nd

2.47812.747514163

10

221440

15585

786B-65R-1,17-19

Andesite

54.310.35

17.037.370.-O85.416.512.060.48nd3.23

93.590.58

1.44411.5

1633.1

65602211

267

1046

786B-66R-1,88-90Dacite

68.950.18

13.414.070.080.822.262.223.12nd2.36

95.100.26

1.77411.2

101349650

93331

144019

332

786B-67R-1,63-65

High-Mgbasalt

53.000.16

12.467.540.14

13.086.701.280.21nd2.56

94.570.75

1.228

6.9137

2.55856

296626202

13803

34

786B-69R-1,65-67

Boninite

62.350.16

11.756.780.099.233.982.050.06nd4.31

96.410.70

2.144

8143

075

177212748156

12601039

786B-70R-1,68-73

Boninite

58.440.15

11.436.110.137.404.952.27

nd4.66

96.740.68

1.645

9.2147

1.57770

202764148

13209

52

Note: nd = not detected.a Classified according to Figure 4.

Core 125-786B-63R: Section 125-786B-63R-1 contains Cat-aclastic and sheared andesites. Ductile shearing in the intervalbetween 30 and 70 cm is indicated by progressive bending andstretching of andesite breccias. At 66 and 85 cm, a weak,anastomosing, strongly dipping foliation can be seen withinthe sheared breccias. C-S-like relationships between thecleavage planes are consistent with a normal sense of shear.Both chloritized and primary magmatic minerals are deformedwithin the shear zone. Slickensided fragments from a faultgouge were recovered from between 95 and 113 cm in thissection, and the last sample from this section is a Cataclasticandesite having a strongly dipping foliation defined both bypervasive veining and by alignment of broken fragments.

Core 125-786B-69R: The seven sections of this core arecharacterized by the presence of anastomosing veins andfractures related to hydrothermal brecciation and mineraliza-tion. Abundant veins filled with pyrite, quartz, and carbonatedip from 0° to 80° and define a conjugate fracture set. Despitelocal shearing near vein intersections, no displacement alongveins was observed. Fracturing is associated with pods filledwith calcite and pyrite. The size and frequency of the podsincrease downhole. This fractured zone may represent theupper part or an upper extension of the andesitic stockworkencountered in Core 125-786B-71R.

Core 125-786B-71R: This core contains pervasively frac-tured volcanic rocks and argillitized and mineralized cataclas-ites. Pyrite is found in association with quartz and carbonatesin anastomosing veins a few centimeters wide. In Section125-786B-71R-1 the rocks have fractured along variably ori-ented planes without noticeable displacement. Such featurescharacterize hydraulic fracturing, which is controlled by fluidoverpressure. These rocks represent an andesitic stockwork,commonly encountered in arc environments. Sections 125-786B-71R-2 and 125-786B-71R-3 contain intensively argilli-tized and mineralized fault zones and breccias. The brecciasare composed of angular clasts of strongly hydrothermallyaltered andesites of various colors, ranging from light green towhite. The breccias may represent the result of fluid move-ment along major fractures during the hydrothermal processesor may be fault breccias. At this time, these Cataclasticfeatures at Site 786 cannot be related to a particular structuralsetting or tectonic processes such as normal, reverse, ortranscurrent faulting. Two intervals, probably of fault gouge,

at 100 and 130 cm in Section 125-786A-71R-2, contain smallpieces of slickensided meta-andesite mixed with drill mud.

PALEOMAGNETISM

Magnetic Remanence

Two holes were drilled at Site 786. At Hole 786A, APC/XCB coring penetrated to 166.5 mbsf. There was good recov-ery of the sediments overlying the volcanic rocks, and manyintervals of normal and reversed polarity were identified. Nosediments were recovered in Hole 786B. The broken anddiscontinuous nature of the volcanic rocks in this hole madeany magnetostratigraphy impossible.

The natural remanent magnetization of the archive half ofeach core from Hole 786A was measured using the cryogenicmagnetometer. Magnetic intensities range from 1 to 300 mA/m(Fig. 10). Most cores have values of between 10 and 100 mA/m.Intensity does not change systematically downhole. When thecores were demagnetized at the 10-mT level and remeasured,intensities were reduced to about 10% to 30% of their initialvalues. Discrete samples were also taken from all of the cores inHole 786A. These samples were measured at levels of 0, 5, 10,and 15 mT. In general, the polarities obtained from thesesamples agree with the whole-core data (Table 10). Data fromwhole cores and discrete samples in combination with biostrati-graphic data ("Biostratigraphy" section, this chapter) enabled usto construct a tentative magnetostratigraphy for the sediments inHole 786 A (Fig. 11). Only data from the first 55 m of the hole areshown in Figure 11; below this point, correlations are impossiblebecause of poor recovery and sparse biostratigraphic data. Thecorrelations shown are uncertain because of the sparse biostrati-graphic data and probable incomplete cleaning of spuriousmagnetization.

Past research on drill cores from the Philippine Sea Plate(Bleil, 1982) suggests that the Philippine Sea Plate has movednorthward with time. Inclination data from Hole 786A wereseparated into 25-m sections, grouped into 10° intervals, andplotted on histograms (Fig. 12). The data from the intervalsbetween 0 and 25 mbsf and between 25 and 50 mbsf show goodbimodal distributions with peaks around +45° and —45°,which suggests that there has been little or no translation sincethe late Miocene. In the intervals below this, the data arescattered and not bimodally distributed. However, there are

331

SITE 786

Table 7. Total nitrogen, carbon, inorganic carbon, organic carbon,and carbonate carbon in sediments at Site 786.


125-786 A-1H-1, 84-861H-2, 72-741H-3, 76-781H-4, 71-731H-5, 90-921H-6, 51-531H-7, 21-232H-1, 67-692H-2, 73-752H-3, 78-802H-4, 65-672H-5, 73-752H-6, 88-902H-7, 22-243H-1, 75-773H-2, 64-663H-3, 76-783H-4, 75-773H-5, 62-643H-6, 58-603H-7, 44-464X-1, 32-344X-2, 5-74X-CC, 15-175X-1, 54-565X-2, 54-565X-3, 101-1035X-4, 37-396X-1, 59-616X-2, 59-616X-3, 59-616X-4, 59-616X-5, 16-187X-1, 33-357X-2, 33-357X-3, 33-357X-4, 33-357X-5, 33-357X-CC, 4-68X-1, 48-509X-1, 83-859X-2, 97-999X-3, 93-959X-4, 80-829X-5, 59-609X-5, 98-1009X-6, 25-2710X-1, 89-9010X-2, 82-8410X-3, 72-7410X-4, 82-8410X-5, 82-8410X-6, 25-2711X-1, 80-8211X-2, 83-8511X-3, 78-8011X-4, 84-8611X-5, 50-5212X-1, 42-4417X-CC, 27-29

Depth(mbsf)

0.842.223.765.216.908.019.21

10.3711.9313.4814.8516.4318.0818.9219.9521.3422.9624.4525.8227.2828.6429.0230.2530.6138.7440.2442.2143.0748.1949.6951.1952.6953.7657.4358.9360.4361.9363.4364.3367.5877.6379.2780.7382.1083.3983.7884.5587.2988.7290.1291.7293.2294.1596.8098.3399.78

101.34102.50106.12154.27

Totalnitrogen(wt%)

0.22

0.23

0.19

0.22

0.15

0.17

0.12

0.12

0.14

0.18

0.11

0.09nd

Totalcarbon(wt%)

3.22

3.05

3.58

3.25

3.01

1.61

1.24

3.05

(1.42

2.87

4.04

3.270.34

Inorganiccarbon(wt%)

4.282.812.292.700.272.101.212.921.292.641.722.323.534.452.683.402.513.672.910.083.723.812.883.022.492.932.080.351.281.330.501.440.022.341.041.781.640.951.280.860.122.922.962.720.092.994.002.122.763.383.574.522.634.724.042.943.818.073.380.13

Organiccarbon(wt%)

0.41

0.41

0.18

0.37

0.08

0.28

0.20

0.13

0.33

0.11

0.00

0.000.21

CaCO3

(wt%)

35.723.419.122.52.2

17.510.124.310.822.014.319.329.437.122.328.320.930.624.2

0.731.031.724.025.220.724.417.32.9

10.711.14.2

12.00.2

19.58.7

14.813.77.9

10.77.21.0

24.324.722.70.7

24.933.317.723.028.229.737.721.939.333.724.531.767.228.2

1.1

Note: nd = not detected. Total sulfur was below the detection limit of 0.06wt% in all samples analyzed for total carbon, nitrogen, and sulfur.

many more data points between +15° and —15°, which sug-gests that these rocks may have acquired their magnetizationat a lower latitude than their present position. Additionalresearch will be necessary to determine more accurately thetranslational history for these rocks.

Magnetic Susceptibility

Whole-core magnetic susceptibilities were measured usingthe multisensor track (MST). A downhole plot of these data ispresented in Figure 13. The sediments typically have valuesbetween 0 and 19 × I0"3 S1 units. There are identifiable peaksdownhole, but these peaks do not correlate with any majorchanges in the physical properties of the rocks (see "PhysicalProperties" section, this chapter).

0 20 40 60CaCO3(%)

0.2 0.4organic' '°l

0 0.12 0.24Total nitrogen (wt%)

Figure 8. Weight percent of calcium carbonate, total organic carbon,and total nitrogen in sediments at Hole 786A.

SEDIMENT-ACCUMULATION RATES

Estimates of the sediment-accumulation rate for Hole 786Aare based on calcareous nannofossils (Table 11), which areused to construct the age vs. depth curve shown in Figure 14.

Sedimentation rates for the middle Eocene through Oli-gocene are estimated to vary between 1.4 and 0.8 m/m.y. withtwo possible hiatuses. The hiatuses ("Biostratigraphy" sec-tion) are thought to have occurred in the late Eocene and inthe early Oligocene. A major hiatus occurred in the earlyMiocene (7.2 m.y.; "Biostratigraphy" section, this chapter).In the middle and earliest late Miocene, sedimentation ratesincreased to 4.2 m/m.y., followed by a further increase to 8.6

Table 8. Results of headspace-gas analy-ses of cores at Site 786.


125-786A-1H-5, 0-52H-3, 0-33H-5, 0-35X-3, 0-36X-4, 0-17X-3, 0-19X-3, 0-112X-2, 0-1

Depth(mbsf)

6.0312.7225.2241.2252.1160.1179.81

107.21

Methane(µL/L)a

98

101011111010

0.80.70.90.90.90.90.90.9

a Microliters of methane per liter of wet sedi-ment, assuming a sample volume of 4.2 cm3

of sediment.b Micromoles of methane per liter of intersti-

tial water, assuming a porosity of 50%.

332

SITE 786

Table 9. Composition of interstitial waters from cores at Site 786.

Sample numberCore, section,interval (cm)

Depth(mbsf)

Volume(mL)

Squeezetempera-ture (°C)

Squeezepressure

(psi) pH

Salinity

(R.I., %) (calcium,Chlorinity Alkalinity Sulfate Sodium Potassium(mmol/kg) (meq/kg) (mmol/kg) (mmol/kg) (mmol/kg)

Surface seawater (17 March 1989)1W-1 1H-4, 145-150 5.98 551W-2 3H-4, 145-150 25.18 85IW-3 6X-3, 140-150 52.05 43IW-4 9X-3, 140-150 81.25 37

3234

35,00040,00036,00036,000

8.257.737.827.617.86

35.635.435.535.135.1

34.6534.8134.8535.0035.00

539.2541.9550.7551.7552.6

2.6892.9211.1401.3661.209

27.7827.0223.8124.3424.03

462.0464.2466.7466.0468.4

10.1110.059.787.847.92

m/m.y. through the Pliocene. Middle and upper Pleistocenesediments are missing from the succession.

PHYSICAL PROPERTIES

The excellent recovery in Holes 786A and 786B allowed usto make a highly detailed reconstruction of the physicalproperties of the lithologic sequence at this site. Thermalconductivity, GRAPE bulk densities, index properties (bulkand grain densities, porosity, water content, and void ratio),and electrical resistivity were measured in the sedimentarysections encountered in Hole 786A. The preponderance ofvoids in the cores precluded the use of the P-wave logger forthese holes. Thermal conductivity, GRAPE bulk density (ondiscrete samples), index properties, and compressional-wavevelocities of discrete samples were measured for the igneousrocks recovered from Hole 786B. The physical-property mea-surements are listed in Tables 12 through 14 and plotted inFigures 15 through 19.

Hole 786A

Thermal-conductivity measurements in the sedimentaryrocks are scattered about 1 W/mK and show little variation

with depth (Table 13 and Fig. 15). Extremely low values (0.127and 0.171 W/mK) were recorded in cores from the base of thehole, where the probe did not couple well with the sedimen-tary cores.

Wet-bulk densities measured by the GRAPE sensor on theMST show little variation with depth throughout the sedimentarysection (Fig. 16). Densities of sedimentary cores cluster at 1.6 to1.7 g/cm3 for the uppermost 100 mbsf. Poor recovery in theinterval from 110 to 140 mbsf prevented measurement ofGRAPE densities. Rocks from the deepest part of the hole showconsiderable scatter in bulk density, which may well be relatedto the brecciated nature of the rocks.

Index properties were measured on discrete samples through-out the hole (Table 12). Bulk densities of sedimentary coresdetermined from discrete sample measurements are similar tothose measured by the GRAPE sensor, and the values aregrouped about 1.6 to 1.7 g/cm3 and show no significant depthvariation (Fig. 16). The bulk densities of samples of igneous rockfrom the base of the hole are higher (1.8 to 2.1 g/cm3). Graindensity also shows little variation with depth in both sedimentsand igneous rocks and ranges from 2.6 to 2.7 g/cm3 for bothsediment and hard-rock samples.

fssw o782 • 783 Δ784 786

100

200

300

400 I • I • I I • I

T T

I • I • I

0 1 2 3 4

Alkalinity (meq/kg) pH

10 0 2 4 6 8 10 0 100 200 300 12 18 24 10 30 50

Silica (µmol/kg) Ammonia (µmol/kg) Sulfate (mmol/kg) Ca2 +(mmol/kg)

10 30 50

M g 2 + (mmol/kg)

Figure 9. Composition of interstitial waters from sediments at Site 786 (inverted triangles) compared with those from Sites 782 (squares), 783(diamonds), 784 (triangles), and 785 (X's) and surface seawater collected 22 February and 4 and 17 March 1989 (crosses).

333

SITE 786


Calcium(mmol/kg)

Magnesium(mmol/kg)

Bromide(mmol/kg)

Silica(µmol/kg)

Ammonia(µmol/kg)

10.1814.0820.6924.4425.00

52.4848.2440.7739.0637.79

0.8300.8500.8660.8990.915

0385540467374

0<12415937

Where possible, electrical resistivities were measured forevery second section of sedimentary material (Table 13).Resistivity may increase with depth, but the data are scattered(Fig. 17).

Hole 786BThermal conductivity was measured once per section using

the half-slab technique (Table 14). Conductivities ranged from1.0 to 1.8 W/mK from 150 to 750 mbsf (Fig. 18), but the datashow considerable scatter. Below 750 mbsf, the rocks displaygenerally higher thermal-conductivity values (up to 2.5 W/mK).This increase in thermal conductivity corresponds to the bound-ary between breccias and dikes (see "Igneous and MetamorphicPetrology" section, this chapter).

Discrete measurements of GRAPE density were performedon whole-round core sections, generally twice per section.Density increases with depth from 2.1 to 2.5 g/cm3 over the800 m cored (Fig. 18). These data are in good agreement withlogging results (see "Downhole Measurements" section, thischapter). Rocks from higher levels in the hole show morescatter than those from deeper levels. The decreased scatter in

the data is probably caused by increasing regularity downholeof the core diameter, which allowed us to collect data ofhigher quality. Index properties do not correlate well withGRAPE and downhole measurements (Fig. 19). Neither bulkdensity nor grain density shows significant downhole varia-tion. Bulk densities increase from 2.3 to 2.5 g/cm3, but exhibita large degree of scatter, probably related to sampling ofnonrepresentative rocks. Low bulk densities are recorded forsamples from shear zones, where the rocks have significantlyhigher water contents (see Table 12). Grain densities arenearly constant at 2.7 g/cm3.

Compressional-wave velocities were measured in two di-rections in each index-property sample. The A velocity ismeasured parallel to the split surface of the core, and the Bvelocity is measured perpendicular to the split surface. Thesamples measured generally show little seismic anisotropy(Table 14). Velocities range from 3000 to nearly 6000 m/s ineach direction, and do not have coherent downhole trends(Fig. 19). The large velocity range is caused by the fracturesand microcracks in the samples, which significantly lowercompressional-wave velocities.

DOWNHOLE MEASUREMENTSHole 786B was logged in five different runs. The first log

string consisted of the dual induction tool (DIT), the/digitalsonic tool (SDT), the natural gamma-ray spectrometry tool(NGT), and the temperature logging tool (TLT) and was runfrom 449 mbsf up to the end of the drill string. The secondstring—consisting of the lithodensity tool (LDT), the compen-sated neutron tool (CNT), NGT, and TLT—was run from adepth of 387 mbsf up to the end of the drill string. These toolswere then combined into the Quad string (consisting of the

t ssw o782 • 783 784 786

100

r 200

300

400 i . i r5 7 9 11 13

Potassium (mmol/kg)


460 480 500 33.4 34.4 35.4 33 34 35 36 37 0.8 0.9 1 539 544 549 554

Sodium (mmol/kg) Salinity as sum Salinity by R. I. (‰) Bromide (mmol/kg) Chlorinity (mmol/kg)of salts (‰)

334

SITE 786

20

40

60

Q

80

100

120

^.$>-•

ΛV . ‰ .

'- - Mi

0.1 1 10 100 1000Intensity (mA/m)

Figure 10. A downhole plot of magnetic intensity for Hole 786A.

DIT, LDT, CNT, SDT, and TLT). This third string was usedto log uphole from 821 to 464 mbsf. The fourth run was ageochemical string, consisting of the induced gamma-rayspectrometry tool (GST), the aluminum clay tool (ACT), theNGT, and the TLT, which was run from 821 to 0 mbsf. Thefifth logging run was the borehole televiewer (BHTV). Afterone failure, the tool was run again; 100 m (469-569 mbsf) ofthe hole was logged before time expired.

Logging results are presented at the end of this chapter.Because of various hole problems, not all logs are continuousthroughout the entire hole.

Both lithologic Units I and II, with low percentages of SiO2

and those of CaCO3 of approximately 5% to 20%, are difficultto distinguish in the logging data. Lithologic Unit III is clearlydifiFerentiated from Unit II by the much higher MgO content(—10%), slight decrease in SiO2, and sharp decrease in CaCO3

in Unit III. The sediment/basement interface at 162.5 mbsf ismuch less distinct. The only logs that show a significantchange at this depth are the bulk density log, which increasesto 2.3 g/cm3, and the porosity log, which decreases to 30%.

Based on the petrology, basement at Hole 786B has beendivided into 30 units, some of which are heterogeneous inchemical composition. This large number of units precludes adetailed, unit-by-unit description of the logs here. A moredetailed analysis of these logs will appear in the Leg 125Scientific Results volume. In general, the MgO, SiO2 (upondetailed examination), A12O3, and perhaps sulfur logs are themost useful for distinguishing lithology.

The gamma-ray logs (CGR and SGR) show little variationdownhole and average about 20 APL units. Certain units standout with slight variations that are a few API units higher orlower. The most striking feature observed in the gamma-ray logsis an increase of 55 to 70 API units between 752 and 783 mbsf,which is basement Unit 26 (metadacite and rhyolite flows). Onedike within the unit has much lower gamma-ray values.

The resistivity logs show a certain amount of variation fromunit to unit that is largely controlled by the amount of fracturingand brecciation. Resistivity is about 10 ohm-m in the upper partof the hole and slowly increases to 20 ohm-m at the bottom.Certain intervals, such as from 308 to 358 mbsf, have lowervalues of 3 to 7 ohm-m. Local variability also changes through-out the hole, with the interval from 468 to 593 mbsf and the zonebelow 733 mbsf showing increased local variability.

The caliper log shows that the hole is somewhat rugose in theupper part (down to 336 mbsf) with diameters of 0.36 to 0.41 m.The hole is narrower and smoother (from 0.30 m down to 0.25 m)in the lower part of the hole, at 478 mbsf. Data from the intervalfrom 336 to 478 mbsf are unusable.

Bulk density ranges from 2.1 to 2.2 g/cm3 at the top of thelogged interval and 2.4 g/cm3 at the bottom. Density generallyincreases with depth, but individual units may show increasesor decreases about the trend. Variability can also be enhancedlocally, especially at unit boundaries.

Porosity is about 50% at the top of the logged interval anddecreases to about 30% at the bottom of the hole. The porosityabruptly decreases between 735 and 740 mbsf, with one smallarea of increased porosity in an interval with remarkablecorrespondence to Units 22 (rhyolite dike/sill), 23 (shearzone), and 24 (altered boninite-andesite pillows). The porosityabruptly decreases again at 752 mbsf, in Unit 26 (see thepreceding discussion of gamma-ray logs).

The photoelectric effect is 2.9 barns/e at the top of thelogged section and 3.2 barns/e at the bottom of the section.

Thorium is low and variable throughout most of the holeand ranges between 0.3 and 1 ppm. Thorium gradually in-creases to 3 ppm in the interval from 728 to 778 mbsf. Valuesdecrease below that depth.

Uranium values are also low and variable throughout thehole, generally in the range of 0.25 to 0.5 ppm. Values up to2.0 ppm occur in the interval from 533 to 538 mbsf, which ispart of Unit 16, an andesitic breccia.

Potassium values vary somewhat in most of the hole,ranging from 0.5% to 1.0%. The major exception is withinUnit 26 between 752 and 783 mbsf, where values rangebetween 2.5% and 4.0%.

A discussion of the elemental logs of MgO, SiO2, A12O3,sulfur, CaO, and Fe2O3 will be presented in the Leg 125Scientific Results volume.

Borehole Televiewer

The ultrasonic borehole televiewer (BHTV) was used tolog the interval from approximately 469 to 569 mbsf. Both thehigh-frequency (1.4 MHz) and low-frequency (400 mHz)transducers were used in logging. The low-frequency trans-ducers gave the best data. Preliminary analysis of the BHTVdata indicates breakouts along the N10°E-S10°W direction,which shows the direction of minimum compressive stress.The direction of maximum compressive stress is at rightangles to this and is aligned roughly along the orientation ofthe motion of the Pacific Plate.

SUMMARY AND CONCLUSIONS

Site 786 (proposed Site BON-6C) is located in the center ofthe Izu-Bonin forearc basin about 120 nmi east of the activevolcano Myojin Sho. The objectives of this site were to study (1)the uplift and subsidence history of the forearc basin at this

335

SITE 786

Table 10. Remanence data for the discrete samples taken from Hole 786A.

Core, section,top of

interval (cm)

125-786A-1H-1, 1421H-2, 1441H-3, 1401H-4, 1361H-5, 1451H-6, 1121H-7, 272H-1, 702H-2, 782H-3, 692H-4, 732H-5, 762H-6, 702H-7, 333H-1, 703H-2, 703H-3, 703H-4, 703H-5, 703H-6, 703H-7, 384X-1, 695X-1, 665X-2, 665X-3, 665X-4, 506X-1, 876X-2, 686X-3, 686X-4, 677X-1, 597X-2, 597X-3, 597X-4, 597X-5, 598X-1, 559X-1, 699X-1, 699X-2, 889X-3, 309X-4, 859X-5, 469X-6, 3610X-1, 8010X-2, 7710X-3, 6310X-4, 8910X-5, 8910X-6, 1711X-1, 4311X-2, 9611X-3, 4711X-4, 9211X-5, 90

N = normal; R

Natural remanent magnetization

Inclination(degrees)

-46.135.25640.632.7

-0.20.5

47.9-35.6

47.857.153.933.821.332.331.760.150.643.740-4.147.440.630.1

- 1 510.742.237.8423.6

-33.3-11.9-12.5-45.8-38.9

38.215.115.19

-38.6- 8

3.2-7.538.212.7

- 2 2- 2- 916.222.6-2.917.431.231.8

= reversed.

Declination(degrees)

184.3339.729.6

350.810.8

157.1167.5186.527.4

193189.3176.5359.934053.3

356.965.425.131.625.2

2275.1

27.9337.1242.89161.9185.1236.2214.669.1

175.7181.7165339.732.4

193.2295.4295.4

188.6

308312.4325.3337.2227.7

10.2220.5346.4

1.8353.6133.2128.7163.1266.1

Intensity(mA/m)

49.2176.621.548.374.85118

113.2115.628.637.173.9

5.60.4

28.9156.529.317.821.48.20.1

58.324.228.216.337.663.255.355.623.814.817.240.462.533.144.8

1551553222.96.5

43.728.589.6

207.7176.3222.7

9.132.1

24592.614.94.15.5

Remanent magnetization (15 mT)

Inclination(degrees)

-12.4-4.2

-35.9-32.7-28.3-28.6

42.951.7

-44.455.658.15237.614.449.338.751.835.738.240.7

-27.753.348.45130.561.248.53942.7-3.553.711.82.1

25.4-23.7

39.630.330.32.5

-30.754.7

-32.345.750.560.844.9- 1

7.248.547.355.838.42443.4

Declination(degrees)

175.8163.3178.7179170.9181103.9191.1

7.1180.1197.9175.1359344

2630.323.726.213.532.1

223.35.3

26.7328.9220.7166.6182.6251.622152.2

156.8158.2152.217.430.1

211.4131.6131.692.270.7

21988.3

2739.2

323.99.4

135.8165.311.9

336.3166.6120.2102255.8

Intensity(mA/m)

716.222.658.335.796.721.999.671.920.528.551.74.30.1

35.786.326.114.613.58.80.03

51.319.822.55.2

3440.234.63920.28.96.5

26.939.414.625.912.312.316.513.17.25.58.7

32.516.821.47.84.59.169.27.25.63.2

Polarity

N?RRRRRNNRNNNN7NNNNNNRNNNNNNNNRNN?NRNNN7RNRNNNN77NNNNNN

locality; (2) the temporal record of sedimentation, depositionalenvironment, paleoceanography, and the intensity and nature ofarc volcanism; (3) the nature and age of the igneous basement tothe forearc basin, and (4) the regional stress field of the forearc,microstructural deformation, and large-scale rotation and trans-lation of the forearc. Site selection was based on multichannelseismic records and a short shipboard seismic survey; the sitechosen was 5 nmi east of the intersection of line 5 (0510UTC) ofFred H. Moore 3505 survey and Conrad line 38 (1125UTC).Occupation of the site began at 2015UTC, 1 April, and ended at0800UTC on 16 April 1989.

Hole 786A (31°52.48'N, 141°13.58'E) was spudded with theAPC in a water depth of 3058.1 m at 0815UTC on 2 April.Recovery of 103% was achieved for the 28.7 m drilled before

changing to the XCB and drilling an additional 137.8 m with40.1% recovery, for a total of 166.5 m drilled with 51.0%recovery. Hole 786B (31°52.45'N, 141°13.59'E), in a water depthof 3071.0 m, was washed through 162.5 m of the sedimentarycolumn before recovery of igneous basement began at 162.5mbsf. Drilling continued to a total depth of 828.6 m using thebionic bit M84F, which showed no signs of failure after 118.4 hrof use when drilling was stopped to begin logging.

Four suites of logging tools were run in Hole 786B. Thefirst suite, consisting of the DIT, SDT, and NGT, was runfrom 3520.0 mbsl up to the end of the drill string. The secondsuite, consisting of the LDT, CNT, and NGT, was run froma depth of 3458.1 mbsl up to the end of the drill string. Thethird logging suite, consisting of the DIT, LDT, CNT, and

336

SITE 786

5 5 -

Whole-core datasuggest intervalis normal

No recovery

Figure 11. The magnetostratigraphy of Hole 786A for the interval from 0 to 55 mbsf correlated with the polarity time scale of Berggren et al.(1985b).

337

SITE 786

irem

ents

mea

sLro

flu

mbe

60

50

40

30

20

10

0

i i i i i

| I^^MlBHlBBl11 1 1 1 1 1 1 1

0-25 mbsf

J U I I • U • U I L H L I I I11 1 1 1

l•Ull^•á•i•-85 -75 -65 -55 -45 -35 - 2 5 - 1 5 -5 5 15 25 35 45 55 65 75 85

Inclination (degrees)

30

| 20

.QE

-85 -75 -65 -55 -45 -35 -25 - 1 5 - 5 5 15 25 35 45 55 65 75 85Inclination (degrees)



Figure 12. Inclination distribution as a function of depth, Hole 786A.

SDT, was used to log the hole from 3892 up to 3535.2 mbsl.The fourth suite, consisting of the GST, ACT, and NGT,was run from a depth of 3892.0 mbsl to the mud line. TheBHTV was deployed for the fifth logging run and logged 80m of Hole 786B after one failure before logging time expired.

The stratigraphic section recovered at Site 786 is assignedto lithologic Units I through IV (Fig. 3). Units I through IIIare defined only in Hole 786A, in which the sedimentarysequence was recovered at Site 786. Unit IV is defined inboth holes. Unit I (0-83.46 mbsf) comprises 83.46 m ofnannofossil marls and clays and is of early Pleistocene to

middle Miocene age. It is made up predominantly of nanno-fossils (up to 70%), with lesser amounts of micrite (trace-40%), foraminifers (trace-30%), clay (5%-7%), sponge spi-cules (trace-5%), radiolarians (trace-4%), diatoms (trace-3%), and quartz (trace-2%).

Unit II (83.46-103.25 mbsf) is an upper Oligocene tomiddle Eocene nannofossil marl and nannofossil-rich clay.The unit consists of nannofossils (2%-60%), with lesseramounts of foraminifers (2%-10%), micrite (l%-35%), andradiolarians (trace-2%). Unit II also contains clays (10%-50%) and opaque minerals (3%-15%) with traces of chlorite

338

SITE 786

Age (Ma)

2000

Volume magnetic susceptibility (4π × 10 SI)

Figure 13. A downhole plot of susceptibility for Hole 786B.

Table 11. Biostratigraphic data used toplot the age vs. depth curve for Site786 (Fig. 14).

Biostratigraphic eventa

Calcareous nannofossilsFO G. oceanicaF0 D. surculusLO D. tamalisLO Sphenolithus spp.LO A. delicatusLO D. quinqueramusFO A. delicatusFO D. quinqueramusF0 D. hamatusLO D. kugleriLO S. heteromorphusLO D. bisect usF0 S. ciperoensisF0 S. distent usLO E. formosaF0 /. recurvusLO C. grandisF0 S. predistentusF 0 D. saipanensis

Depth(mbsf)

0.23.96.9

24.528.731.841.447.649.557.168.985.192.394.895.797.7

102.9105.7107.5

Age(Ma)

1.72.42.63.43.65.66.58.3

10.010.814.423.730.134.135.137.839.642.242.8

100— 11X —

Figure 14. Age vs. depth curve for Hole 786A and estimated sedimen-tation rates.

and serpentine. Vitric ash and mineral fragments are alsopresent.

Unit HI (103.25-124.90 mbsf) is a sequence of volcani-clastic breccia, of middle Eocene age. The sedimentarycomponents of the breccia matrix are nannofossils (up to65%), clay (10%-50%), micrite (l%-35%), foraminifers(l%-10%), radiolarians (trace-2%), and opaque minerals

a FO = first occurrence; LO = lastoccurrence.

Unit IV (124.9-166.5 mbsf in Hole 786A and 162.5-826.6mbsf in Hole 786B) consists of volcaniclastic and sedimentarybreccias, vitric siltstones and sandstones, lavas, dikes, andpyroclastic flows. Vitric fragments, glass (20%-100%), pyrox-ene (l%-30%), feldspar (1%-15%), lithic fragments (trace-10%), and amphibole (trace-5%) dominate the detrital com-ponent of the sediments; clays (10%—53%) are also present.

Dating is based on calcareous nannofossil, foraminifer, anddiatom bio stratigraphy. The rocks in the bottom of Hole 786Aand the top of Hole 786B are considered to be the samematerial. Sedimentation rates are highest for the Pliocene (8.6m/m.y.) and drop (to 4.2 m/m.y.) between the latest Mioceneand middle Miocene. A hiatus exists between the middleMiocene and the late Oligocene.

The igneous basement (Fig. 3) consists mainly of massiveand brecciated flows, ash flows, and intercalated sediment inits upper part, and pillow lavas and dikes or sills in its lowerpart. Rock types include high-magnesian basalts, boninites,basalts, andesites, dacites, and rhyolites. There are severalprominent shear zones and hydrothermal breccias within thesequence.

Studies of physical properties show that average bulk densi-ties are 1.65 g/cm3 in the sediments and 1.8-2.1 g/cm3 in the

339

SITE 786

the volcanic sequences; the average grain density of 2.65 g/cm3

varied little between the sediments and the volcanic rocks.Preliminary interpretation of the paleomagnetic data shows

many intervals of normal and reversed polarity. The two inter-vals between 0 and 50 mbsf show good bimodal distributionswith peaks around +45° and —45°, which indicates little transla-tion since the late Miocene. Between 160 and 240 mbsf there isno bimodal distribution, although the many more data pointsbetween +15° and —15° suggest translation from a lower latitude.

The principal achievement at this site is the deep penetrationof an Eocene volcanic edifice, which will provide (1) a record ofthe construction and structure of the forearc volcanic basementand (2) the basis for the understanding of early arc magmatism ingeneral and boninite petrogenesis in particular.

REFERENCES

Berggren, W. A., Kent, D. V., and Flynn, J. J., 1985a. Jurassic toPaleogene: part 2. Paleogene geochronology and chronostratigra-

phy. In Snelling, N. J. (Ed.), The Chronology of the GeologicalRecord: Geol. Soc. London Mem., 10:141-198.

Berggren, W. A., Kent, D. V., Flynn, J. J., and Van Couvering, J.,1985b. Cenozoic geochronology. Geol. Soc. Am. Bull., 96:1407-1418.

Bleil, IL, 1982. Paleomagnetism of Deep Sea Drilling Project Leg 60sediments and igneous rocks from the Mariana region. In Hus-song, D. M., Uyeda, S., et al., Init. Repts. DSDP, 60: Washington(U.S. Govt. Printing Office), 855-873.

McDuff, R. E., 1985. The chemistry of interstitial waters, Deep SeaDrilling Project Leg 86. In Heath, G. R., Burckle, L. H., et al.,Init. Repts. DSDP, 86: Washington (U.S. Govt. Printing Office),675-687. Meijer, A., 1980. Primitive arc volcanism and a boniniteseries: examples from western Pacific island arcs. In Hayes, D. E.(Ed.), The Tectonic and Geologic Evolution of Southeast AsianSeas and Islands, Part I. Am. Geophys. Union Geophys. Monogr.Ser., 23:269-282.

Ms 125A-114

NOTE: All core description forms ("barrel sheets") and core photographs have beenprinted on coated paper and bound as Section, near the back of the book, begin-ning on page 383.

340

Table 12. Index properties for Holes 786A and 786B. Table 12 (continued).

Bulk Grain Water Bulk Grain WaterCore, section, Depth density density Porosity content Void Core, section, Depth density density Porosity content Voidinterval (cm) (mbsf) (g/cm3) (g/cm3) (%) (%) ratio interval (cm) (mbsf) (g/cm3) (g/cm3) (%) (%) ratio

125-786A- 10R-2,68-69 248.73 2.46 2.68 13.5 5.8 0.131H-1, 84-86 0.84 1.65 2.43 56.2 36.2 0.56 11R-1, 22-24 256.52 2.24 2.79 31.7 15 0.321H-2,72-74 2.22 1.73 2.76 59.8 36.7 0.6 12R-1,49-51 266.49 2.24 2.77 30.4 14.4 0.31H-3,76-78 3.76 1.62 2.53 61.3 40.3 0.61 12R-1,49-51 266.49 2.24 2.77 30.4 14.4 0.31H-4, 71-73 5.21 1.65 2.66 62.3 40.1 0.62 12R-2, 14-16 267.57 2.23 2.57 22.2 10.6 0.221H-5,90-92 6.9 1.51 2.52 67.8 47.6 0.68 13R-1,85-87 276.55 2.46 2.75 16.8 7.2 0.171H-6,51-53 8.01 1.41 2.72 56.7 42.8 0.57 13R-2,70-72 277.83 2.39 2.78 22.1 9.8 0.221H-7, 21-23 9.21 1.64 2.54 59.9 38.8 0.6 14R-1, 35-37 285.75 2.4 2.69 18 8 0.182H-1,67-69 10.37 1.59 2.64 65.6 43.9 0.66 15R-1, 35-37 295.35 2.19 2.65 28.2 13.7 0.282H-2,73-75 11.93 1.65 2.64 62.3 40.2 0.62 15R-2, 128-130 297.75 2.11 2.61 32.1 16.2 0.322H-3,78-80 13.48 1.66 2.76 64.3 41.2 0.64 16R-1,78-80 305.38 2.21 2.73 30.7 14.8 0.312H-4, 65-67 14.85 1.63 2.71 64.6 42.1 0.65 16R-2, 14-16 306.21 2.3 2.47 11.8 5.4 0.122H-5,73-75 16.43 1.6 2.6 63.9 42.3 0.64 17R-1,67-69 314.97 2.33 2.73 24 10.9 0.242H-6, 88-90 18.08 1.6 2.64 65 43.2 0.65 18R-1,61-63 324.51 2.34 2.6 16.5 7.5 0.172H-7, 22-24 18.92 1.63 2.72 65.7 42.8 0.66 19R-1,27-28 333.87 2.36 2.57 14.1 6.4 0.143H-1, 75-77 19.95 1.69 2.71 61.1 38.5 0.61 20R-1,71-73 344.01 2.29 2.55 17.1 7.9 0.173H-2,64-66 21.34 1.66 2.73 63.1 40.3 0.63 21R-1, 125-127 354.25 2.22 2.43 14.6 7 0.153H-3,76-78 22.96 1.74 2.73 53.4 32.5 0.53 21R-2,86-88 355.3 2.5 2.66 10.2 4.3 0.13H-4, 75-77 24.45 1.64 2.66 63 40.8 0.63 22R-1, 31-33 362.91 1.86 2.52 44.7 25.6 0.453H-5,62-64 25.82 1.63 2.67 63.7 41.6 0.64 22R-1,70-72 363.3 2.05 2.56 33.6 17.4 0.343H-6, 58-60 27.28 1.61 2.62 64 42.3 0.64 22R-2,62-64 364.71 1.97 2.29 23.7 12.8 0.243H-7,44-46 28.64 1.6 2.73 66.9 44.4 0.67 22R-3,30-32 365.85 1.96 2.12 14.6 7.9 0.154X-2,5-7 30.25 1.56 2.71 58.6 46.5 0.69 24R-1, 10-12 382 2.33 2.52 12.5 5.7 0.124X-CC, 15-17 30.61 1.59 2.58 64.4 43.1 0.64 24R-2, 13-15 383.53 2.38 2.54 10.7 4.8 0.115X-1,54-56 38.74 1.55 2.73 68.6 46.9 0.69 25R-1,68-70 392.18 2.41 2.67 14.9 6.6 0.155X-3, 101-103 42.21 1.66 2.82 64.9 41.5 0.65 26R-1,65-67 401.85 2.33 2.63 18.7 8.5 0.195X-4, 37-39 43.07 1.69 2.8 63.3 39.8 0.63 27R-1,89-91 411.69 2.16 2.73 34.1 16.8 0.345X-4, 59-61 43.29 1.73 2.98 64.6 39.7 0.65 27R-2,40-42 412.7 2.25 2.94 36.3 17.2 0.366X-1, 59-61 48.19 1.7 2.72 60.8 38.1 0.61 28R-1,98-100 421.38 2.43 2.56 8.6 3.8 0.096X-2,59-61 49.69 1.62 2.63 63.5 41.7 0.63 29R-1,22-24 430.32 2.39 2.77 21.7 9.6 0.226X-3,59-61 51.19 1.65 2.76 64.6 41.7 0.65 30R-1, 82-84 440.52 2.48 2.7 13.3 5.7 0.136X-5, 16-18 53.76 1.63 2.61 62.4 40.6 0.62 30R-2,70-72 441.9 2.45 2.71 15.6 6.8 0.167X-1, 33-35 57.43 1.77 2.82 59.1 35.6 0.59 3OR-3,29-31 442.99 2.51 2.61 6 2.5 0.067X-2, 33-35 58.93 1.7 2.83 62.9 39.3 0.63 31R-1, 106-108 450.46 2.55 2.63 4.7 2 0.057X-3, 33-35 60.43 2.15 2.85 77.1 38.2 0.77 31R-2,84-86 451.68 2.64 2.73 5.1 2 0.057X-4, 33-35 61.93 1.79 2.76 56.7 33.7 0.57 32R-1, 119-121 460.19 2.53 2.62 5.8 2.4 0.067X-5, 33-35 63.43 1.84 2.82 54.8 31.6 0.55 32R-2,86-88 461.36 2.34 2.55 13.7 6.2 0.147X-CC, 4-6 64.33 1.79 2.7 54.9 32.6 0.55 33R-1, 18-20 468.88 2.55 2.62 4.2 1.7 0.048X-1,48-50 67.58 1.74 2.71 57.6 35.1 0.58 34R-1, 128-130 479.58 2.14 2.3 12.7 6.3 0.138X-CC, 6-8 68.18 1.75 2.75 58.2 35.3 0.58 34R-2,73-75 480.53 2.6 2.71 6.5 2.6 0.069X-1,83-85 77.63 1.55 2.69 69 47.3 0.69 34R-3,45-47 481.65 2.49 2.64 9.3 4 0.099X-2,97-99 79.27 1.6 2.79 68.1 45.3 0.68 34R-4,90-91 483.6 2.55 2.7 9 3.7 0.099X-3,93-95 80.73 1.57 2.66 66.8 45 0.67 35R-1, 126-128 489.26 2.36 2.46 6.7 3 0.079X-4,80-82 82.1 1.5 2.65 71.1 50.3 0.71 35R-2,41-43 489.91 2.19 2.46 18.9 9.2 0.199X-5,98-100 83.78 1.79 2.98 61.2 36.3 0.61 35R-3,60-62 491.6 2.15 2.28 10.9 5.4 0.119X-6, 25-27 84.55 1.79 2.77 56.7 33.7 0.57 35R-4, 35-37 492.85 2.04 2.24 16.9 8.8 0.1710X-1,88-90 87.28 1.85 2.88 55.8 32 0.56 36R-1,63-65 498.33 2.1 2.27 14.1 7.1 0.1410X-2,82-84 88.72 1.67 2.52 57.4 36.5 0.57 37R-1,69-71 508.09 2.4 2.68 17.3 7.6 0.1710X-3,72-74 90.12 1.71 2.69 58.9 36.5 0.59 37R-2, 82-84 509.63 2.44 2.52 5.7 2.5 0.0610X-5, 82-84 93.22 1.58 2.51 63.4 42.7 0.63 37R-3, 14-16 510.45 2.53 2.65 6.9 2.9 0.0710X-6, 25-27 94.15 1.79 2.71 54.9 32.5 0.55 38R-1,79-81 517.89 2.4 2.48 5.8 2.6 0.0611X-1, 80-82 96.8 1.78 2.79 57.9 34.7 0.58 39R-1,48-50 527.28 2.3 2.34 17.7 8.2 0.1811X-2,83-85 98.33 1.74 2.78 59.8 36.5 0.6 39R-2,65-67 528.95 2.33 2.36 17.3 7.9 0.1711X-3,78-80 99.78 1.38 2.74 79.6 61.1 0.8 39R-3,27-29 530.03 2.43 2.62 11.9 5.2 0.1211X-4, 84-86 101.34 1.58 2.61 65.3 43.9 0.65 40R-1, 102-104 537.32 2.59 1.82 89.3 36.6 0.8911X-5, 50-52 102.5 1.64 2.81 65.8 42.5 0.66 40R-2,46-48 538.03 2.71 2.78 3.9 1.5 0.0411X-6, 63-65 104.13 1.92 2.94 53.3 29.5 0.53 40R-3,61-63 539.35 2.7 2.78 4.8 1.9 0.0512X-1,42-44 106.12 1.62 2.66 64 41.9 0.64 40R-4,25-27 540.49 2.53 2.68 9.9 4.1 0.114X-1, 16-18 125.06 2.05 2.77 41.4 21.4 0.41 41R-1,61-63 546.51 2.39 2.86 25.7 11.4 0.2615X-CC, 9-11 134.69 1.85 2.66 50.2 28.8 0.5 41R-2, 17-19 547.42 2.33 2.66 20 9.1 0.216X-1,9-11 144.39 1.54 2.09 52.3 36.1 0.52 41R-3,9-11 548.84 2.08 2.93 45.1 23.1 0.4516X-CC, 19-21 144.69 2.1 2.86 41.7 21.1 0.42 41R-4, 23-25 550.48 2.47 2.71 14.5 6.2 0.1417X-CC, 27-29 154.27 1.96 2.72 44.9 24.3 0.45 42R-1,67-69 556.27 2.55 2.75 11.8 4.9 0.1218X-1,65-67 155.65 2.03 2.73 41.4 21.6 0.41 42R-2,67-69 557.77 2.56 2.65 5.6 2.3 0.06

125-786B- 43R-3, 124-126 559.78 2.48 2.59 7.4 3.2 0.071R-1,61-64 163.11 2.44 2.63 11.4 5 0.11 42R-4, 13-15 560.17 2.61 2.73 7.2 2.9 0.072R-1,58-60 170.08 2.51 2.61 6 2.5 0.06 43R-1,30-32 565.3 2.42 2.58 10.2 4.5 0.13R-1,94-97 180.14 2.47 2.86 21.7 9.4 0.22 43R-2,32-34 566.62 2.34 2.61 16.9 7.7 0.174R-1, 110-112 190.1 2.7 2.72 1.5 0.6 0.02 44R-1, 30-32 574.9 2.68 2.77 5.5 2.2 0.055R-1, 1-3 198.61 2.27 2.95 34 15.9 0.34 45R-1, 53-55 584.83 2.39 2.69 17.8 7.9 0.185R-2,53-55 200.63 2.4 2.55 9.5 4.2 0.1 46R-1,59-61 594.49 2.58 2.88 16.4 6.7 0.166R-1,38-40 208.58 2.33 2.69 21.6 9.8 0.22 46R-2, 59-71 596.09 2.41 2.72 18.5 8.1 0.186R-2,43-45 210.13 2.51 2.73 12.7 5.4 0.13 47R-1, 101-103 604.51 2.44 2.75 18.2 7.9 0.188R-1,45-47 228.05 2.25 2.73 28.5 13.5 0.29 48R-1,73-75 613.93 2.28 2.65 23.1 10.8 0.239R-1, 10-12 237.2 2.45 2.65 12.9 5.6 0.13 48R-2,50-52 615.09 2.43 2.8 20.8 9.1 0.219R-2, 128-129 239.84 2.07 3 47.4 24.3 0.47 49R-1,32-34 623.22 2.45 2.57 8.1 3.5 0.08

SITE 786

Table 12 (continued). Table 12 (continued).


49R-1, 82-8449R-3, 62-6449R-4, 50-5250R-1, 24-2650R-2, 31-3351R-1, 49-5151R-2, 6-852R-1, 81-8352R-2, 143-14553R-1, 48-5053R-2, 55-5754R-1, 107-10954R-2, 85-8754R-3, 65-6754R-4, 59-6154R-5, 44-4655R-1, 49-5155R-2, 85-8755R-3, 90-9256R-1, 124-12656R-2, 107-10956R-3, 58-6056R-4, 65-6756R-5, 79-8157R-1, 110-11257R-2, 89-9157R-3, 65-6757R-4, 66-6857R-5, 50-5257R-6, 86-8857R-7, 70-7258R-1, 77-7958R-2, 127-12958R-3, 83-8558R-4, 19-2159R-1, 68-7059R-2, 86-8859R-3, 127-12959R-4, 35-3760R-1, 72-7460R-2, 115-11760R-3, 94-9660R-4, 106-10860R-5, 49-5160R-6, 47-4961R-1, 74-7661R-2, 29-3161R-3, 10-1261R-4, 82-8461R-5, 56-5861R-6, 53-5562R-1, 34-3662R-2, 42-4462R-3, 107-10963R-1, 83-8563R-2, 74-7664R-1, 136-13864R-2, 90-9264R-3, 56-5865R-1, 17-1965R-2, 37-3965R-3, 52-5466R-1, 88-9066R-2, 80-8266R-3, 34-3667R-1, 35-3768R-1, 9-1169R-1, 78-8069R-2, 24-2669R-3, 130-13269R-4, 51-5369R-5, 63-6569R-6, 75-7769R-7, 30-3370R-1, 5-770R-2, 5-7

Depth(mbsf)

623.72625.88627.18632.74634.31642.69643.76652.71654.83662.08663.65672.27673.49674.79676.23677.26681.29683.04684.53691.34692.67693.56695.06696.7700.9702.1703.14704.6705.75707.42708.76710.17712.17713.05713.84715.08716.5718.37718.93719.72721.45722.69724.31725.16726.48729.44730.49731.8733.83735.07736.26738.74740.19742.12748.83750.24759.06760.04761.16767.57769.27770.89777.98779.34780.3787.05796.39798.08798.93801.1801.81803.09804.66805.63806.95808.45

Bulkdensity(g/cm3)

2.642.332.392.42.532.472.432.442.482.692.712.372.582.422.532.642.562.452.322.311.932.261.992.192.362.342.532.672.362.192.182.32.32.472.242.362.312.292.352.372.532.432.322.412.312.342.382.352.452.481.952.562.392.422.462.52.482.352.522.662.22.222.312.482.412.442.452.552.342.372.462.482.372.432.452.45

Graindensity(g/cm3)

2.762.672.72.952.812.752.712.972.612.932.892.762.662.672.742.732.652.662.442.481.152.492.692.362.632.962.953.072.632.832.492.492.532.882.452.542.532.52.532.582.622.532.422.522.422.512.592.562.572.622.562.652.582.762.112.72.612.52.62.732.582.372.52.562.532.552.542.612.662.562.692.592.622.542.562.58

Porosity(%)

62.820.818.728.916.116.417.629

8

12.89.7

22.75.9

15.712.25.35.5

138.9

11.89415.642.612.816.73221.919.71735.621.113.416.122.314.711.914.215.61213.75.56.37.177.6

10.512.4147.68.8

40.25.6

12.319.745.810.98.7

10.45.14.3

25.111.213.35.28.27.164.1

19.712.414.37.3

15.88.97.38.3

Watercontent

(%)

25.39.58.3

12.86.87.17.7

12.63.4

5.13.8

10.22.46.95.12.22.35.64.15.4

51.77.3

22.86.27.5

14.59.27.87.6

17.310.36.27.49.66.95.36.57.25.46.12.32.73.33.13.54.85.56.33.33.8

21.92.35.58.7

19.84.63.74.72.21.7

12.15.46.12.23.63.12.61.795.56.23.17.13.93.13.6

Voidratio

0.630.210.190.290.160.160.180.290.080.130.10.230.060.160.120.050.060.130.090.120.940.160.430.130.170.320.220.20.170.360.210.130.160.220.150.120.140.160.120.140.050.060.070.070.080.10.120.140.080.090.40.060.120.20.460.110.090.10.050.040.250.110.130.050.080.070.060.040.20.120.140.070.160.090.070.08


70R-3, 117-11970R-4, 49-5171R-1, 115-11771R-2, 106-10871R-3, 107-10971R-4, 71-7371R-5, 16-1872R-1, 87-8972R-2, 80-82

Depth(mbsf)

811811.68816.75818.16819.59820.65821.6824.47825.9

Bulkdensity(g/cm3)

2.482.462.232.332.212.532.512.552.32

Graindensity(g/cm3)

2.692.722.52.582.542.642.592.622.5

Porosity(%)

12.615.618.817.421.574.94.4

12.3

Watercontent

(%)

5.46.88.98

10.32.92.11.85.6

Voidratio

0.130.160.190.170.220.070.050.040.12

342

SITE 786

Table 13. Physical properties for Hole 786A. Table 14. Physical properties for Hole 786B.


125-786A-1H-1, 751H-2, 751H-3, 751H-4, 751H-5, 751H-6, 751H-CC, 102H-1, 752H-2, 752H-3, 752H-4, 752H-5, 752H-6, 752H-7, 272H-CC, 223H-1, 753H-2, 753H-4, 753H-5, 753H-6, 753H-7, 343H-CC, 85X-1, 684X-2, 194X-CC, 225X-15X-1, 695X-2, 695X-26X-16X-1, 506X-2, 506X-36X-3, 506X-4, 407X-17X-1, 657X-37X-3, 657X-57X-5, 658X-18X-1, 659X-1, 659X-19X-3, 659X-39X-59X-5, 659X-510X-110X-1, 7510X-3, 7510X-310X-4, 7510X-5, 7510X-510X-6, 2911X-1,7511X-1, 7511X-3, 7511X-5, 7511X-6, 5612X-1, 6312X-2, 2012X-CC, 2415X-CC, 1016X-CC, 35

Depth(mbsf)

0.752.253.755.256.758.259.63

10.4511.9513.4514.9516.4517.9518.9719.519.9521.4524.4525.9527.4528.5428.9729.3830.3930.6838.438.8940.3941.347.948.149.6

0.851.152.557.1357.7560.260.7563.3563.7567.2567.7577.4577.880.4582.2683.183.4583.686.787.1590.1590.2691.6593.1593.294.1996.7598.2599.75

102.75104.06106.33107.4107.82134.7144.85

Thermalconductivity

(W/mK)

1.0041.0790.9710.9650.7780.9990.770.9590.9340.8940.9140.97

(

.01

.035

.028

.026).991.042.036.069

0.9350.9331.0190.9541.006

1.0570.997

0.7880.962

1.0370.873

1.039

1.066

1.099

0.9940.715

0.915

0.987

0.9470.921

0.9970.838

1.0191.0370.930.8341.0170.8541.0010.9440.1270.8220.171

Resistivity(ohms)

23.3

27.923.5

25.9

38.1

24.5

27.7

34.1

17.5

2418.5

4051.5

42.5

28

Formationfactor

2.72

3.262.74

3.02

4.45

2.86

3.23

3.98

2.04

2.82.16

4.676.01

4.96

3.27


125-786B-1R-1, 60-622R-1, 56-583R-1, 89-914R-1, 52-544R-1, 110-1125R-1, 1-35R-2, 53-556R-1, 38-406R-2, 43-458R-1, 45-479R-1, 10-1210R-1, 110-11210R-2, 16-1811R-1, 22-2411R-1, 60-6212R-1, 46-4812R-2, 14-1613R-1, 82-8412R-2, 66-6814R-1, 31-3315R-1, 33-3515R-2, 126-12816R-1, 75-7716R-2, 12-1417R-1, 64-6618R-1, 61-6319R-1, 10-1220R-1, 77-7921R-1, 97-9912R-1, 125-12722R-2, 62-6422R-2, 107-10922R-3, 30-3222R-3, 46-4824R-1, 10-1224R-2, 11-1325R-1, 63-6526R-1, 61-6327R-1, 86-8827R-2, 37-3928R-1, 96-9829R-1, 17-1830R-2, 65-6730R-1, 75-773OR-2, 70-7230R-3, 21-2331R-1, 102-10431R-2, 78-8032R-1, 119-12034R-1, 64-6634R-2, 80-8234R-3, 108-11034R-4, 78-8035R-1, 115-11735R-2, 24-2635R-2, 41-4335R-3, 57-5935R-4, 31-3336R-1, 60-6237R-1, 67-6937R-2, 82-8437R-3, 11-1338R-1, 74-7639R-1, 44-4639R-2, 64-6639R-3, 25-2740R-1, 98-10040R-1, 120-12440R-2, 41-4340R-2, 46-4840R-3, 54-5640R-4, 25-27

Depth(mbsf)

163.1170.06180.09189.52190.1198.61200.63208.58210.13228.05237.2247.7248.21256.52256.9266.46267.57276.52277.79285.71295.33297.73305.35306.19314.94324.51333.7344.07353.97354.25364.71365.16365.85366.01382383.51392.13401.81411.66412.67421.36430.27441.85440.45441.9442.91450.42451.62460.19478.94480.6482.28483.48489.15489.74489.91491.57492.81498.3508.07509.63510.42517.84527.24528.94530.01537.28537.5537.98538.03539.28540.49

Thermalconductivity

(W/mK)

1.171.6431.1941.194

1.353

1.2151.359

1.0811.2431.040.7760.9751.1011.2351.472

1.041.0421.28

0.957

0.949

1.7281.5293.9280.9161.1761.6921.1621.3751.412

1.4681.3451.619

1.6711.5741.7611.691.5052.332

1.6191.7321.7161.403

1.5331.0721.2631.0181.1921.332

1.34

0.924

Compressional-wavevelocity

A direction B direction(km/s)

5.275.593.77

5.643.254.43.643.963.844.323.78

3.68

3.323.755.254.655.284.093.173.744.23.244.45

4.24.114.563.72

3.75

4.194.943.983.73.23

4.483.59

3.734.114.484.434.54.39

3.673.863.753.673.964.334.214.873.994.044.69

4.06

4.254.734.03

(km/s)

5.015.34

5.823.334.53.453.963.444.343.63

3.58

3.363.624.794.345.263.773.143.764.033.614.19

4.214.053.743.97

3.74

4.444.664.173.993.28

4.53.51

3.914.164.434.374.514.42

3.673.583.753.733.74.274.274.964.024.14.43

4.1

4.344.733.97

343

SITE 786

Table 14 (continued). Table 14 (continued).


41R-1, 61-6341R-2, 17-1941R-2, 122-12441R-3, 9-1141R-4, 23-2542R-1, 15-1742R-1, 67-6942R-2, 67-6942R-3, 90-9242R-3, 124-12642R-4, 13-1543R-1, 30-3243R-1, 38-4043R-2, 34-3644R-1, 30-3245R-1, 53-5546R-1, 56-5846R-2, 65-6747R-1, 98-10048R-1, 67-6948R-2, 45-4749R-1, 32-3449R-2, 77-7949R-3, 60-6249R-4, 50-5250R-1, 24-2650R-2, 31-3351R-1, 49-5151R-1, 59-6151R-2, 6-852R-1, 81-8352R-2, 140-14553R-1, 128-13054R-1, 75-7754R-1, 107-10954R-2, 64-6654R-2, 85-8754R-3, 60-6254R-4, 55-5754R-5, 44-4655R-1, 45-4755R-2, 85-8755R-3, 87-8956R-1, 114-11656R-1, 124-12656R-2, 100-10256R-2, 107-10956R-3, 56-5856R-4, 60-6256R-5, 73-7557R-1, 1-357R-1, 12-1457R-1, 107-10957R-2, 83-8557R-3, 57-5957R-4, 59-6157R-4, 65-6757R-5, 51-5357R-6, 53-5557R-6, 86-8857R-7, 85-8758R-1, 77-7958R-1, 107-10958R-2, 53-5558R-2, 127-12958R-3, 47-4958R-3, 83-85

58R-4, 19-2158R-4, 89-9159R-1, 68-7079R-1, 92-9459R-2, 93-9559R-3, 34-3659R-4, 35-37

Depth(mbsf)

546.51547.42548.47548.84550.48555.75556.27557.77559.44559.78560.17565.3565.38566.64574.9584.83594.46596.05604.48613.87615.04623.22624.72625.86627.18632.74634.31642.69642.79643.76652.71654.83662.88671.95672.27673.28673.49674.74676.19677.26681.25683.04684.5691.24691.34692.6692.67693.54695.01696.64699.81699.92700.87702.04703.06704.53704.59705.76707.09707.42708.91710.17710.47711.43712.17712.69713.05

713.84714.54715.08715.32716.57717.44718.93

Thermalconductivity

(W/mK)

1.194

0.816

1.824

1.9111.57

0.8531.21.0461.1821.1351.191.0421.168

1.235

1.2572.248

1.164

1.1311.343

1.379

1.3321.399

1.373

1.4581.1451.1781.198

1.2731.2611.3161.223

1.1861.093

1.204

1.1251.15

1.278

1.432

1.2491.3531.328



4.064.05

2.664.54

5.064.56

4.374.334.05

3.64.464.344.434.814.473.653.763.464.813.744.574.144.384.43

4.273.944.85

3.9

5.113.914.65.185.154.294.08

4.16

3.563.792.753.93

3.13.144.06

3.523.7

3.24

4.08

3.94

3.14

3.56

4.5

3.65

4.24

(km/s)

4.023.89

2.74.53

5.034.44

4.54.47

3.64.354.274.424.124.623.643.723.444.713.874.514.034.474.54

4.193.874.88

3.81

5.283.824.615.355.274.253.99

4.32

3.623.652.723.84

3.193.23.92

3.63

3.34

4.1

3.75

3.58

4.24

3.84

4.12


60R-1, 44-4660R-1, 72-7460R-2, 37-3960R-2, 115-11760R-3, 69-7160R-3, 94-9660R-4, 91-9360R-4, 106-10860R-5, 35-3760R-5, 49-5160R-6, 47-4960R-6, 130-13261R-1, 68-7061R-2, 26-2861R-3, 1-361R-4, 71-7361R-4, 82-8461R-5,54-5661R-6, 53-5562R-1, 31-3362R-2, 31-3362R-3, 104-10663R-1, 20-2262R-1, 83-8563R-2, 70-7264R-1, 126-12864R-1, 145-14764R-2, 93-9564R-3, 56-5865R-1, 5-765R-1, 17-1965R-2, 37-3965R-2, 68-7065R-3, 52-5465R-3, 38-4066R-1, 88-9066R-1, 95-9766R-2, 70-7266R-2, 80-8266R-3, 34-3666R-3, 43-4567R-1, 33-3568R-1, 1-369R-1, 69-7169R-1, 78-8069R-2, 22-2469R-3, 25-2769R-3, 130-13269R-4, 35-3769R-4, 51-5369R-5, 52-5469R-5, 63-6569R-6, 71-7369R-7, 33-3570R-2, 64-6670R-3, 60-6270R-4, 23-2571R-1, 103-10571R-2, 106-10871R-2, 126-12871R-3, 107-10971R-3, 130-13271R-4, 43-4571R-4, 71-7371R-5, 10-1272R-1, 80-8272R-2, 76-78

Depth(mbsf)

719.44719.72720.67721.45722.44722.69724.16724.31725.02725.16526.48727.31729.38730.46731.71733.72733.83735.05736.26738.71740.08742.09748.2748.83750.2758.96759.97760.07761.16767.45767.57769.27769.58770.89770.75777.98778.05779.24779.34780.3780.39787.03796.31797.99798.08798.91800.05801.01801.65801.81802.98803.09804.62805.66809.04810.43811.42816.63818.16818.36819.59819.82820.37820.65821.54824.4825.86

Thermalconductivity

(W/mK)

1.213

1.143

1.292

1.316

1.399

1.3341.3041.5422.0532.053

1.85

1.8581.7571.3321.253

1.7632.423

2.265

1.759

2.151

2.147

2.1772.488

2.3582.2521.8621.613

1.6761.97

2.07

2.173

1.5741.6131.7871.7062.1982.198

2.415

2.1772.212

2.2361.761



4.03

5.3

4.93

4.39

4.434.33

4.23.953.45

4.414.412.564.473.743.5

4.384.66

4.844.654.71

4.714.12

4.11

4.01

4.784.39

4.174.99

3.993.34

3.86

4.2

4.244.18

3.4

3.21

4.534.834.783.72

(km/s)

3.89

5.27

4.71

4.44

4.444.33

4.44.113.5

4.534.45

4.513.663.45

4.494.73

4.844.684.77

4.74.1

4.09

3.99

4.74.45

4.094.93

3.933.43

3.69

3.51

4.174.2

3.49

3.25

4.444.754.753.62

344

SITE 786

0.8 1.0Thermal conductivity (W/mK)

1.2

Figure 15. Thermal conductivity vs. depth for Hole 786A (the largedegree of scatter in the data from the base of the hole is probablycaused by measurement of irregularly shaped hard rocks).

345

SITE 786

50 —

100

150 —

- t u.

••

. 1 . 1 .

- > •

•

•

•

_ _•• •

. 1 . 1 .

•9

_ _• •• .•

_ _• •

. 1 .2001.0 1.5 2.0 2.5 1.0 1.5 2.0 2.5 2.25 2.75 3.25

GRAPE density (g/cm3) Bulk density (g/cm3) Grain density (g/cm3)

Figure 16. Bulk density (determined by GRAPE), bulk densisty (determined by discretesamples), and grain density vs. depth for Hole 786A.

346

SITE 786

20 30 40Resistivity (ohms)

Figure 17. Resistivity vs. depth for Hole 786A.

50 60

1

-

—

-

-

-

1

1 '

• Φ

:

ΦΦ

ΦΦ Φ

Φ

•

ΦΦ

Φ

• * .

• « .Φ Φ

Φ % |

•'IΦΦΦ

• Φ»• V• .-.

Φ Φ

1:Φ

•»

^ .. :*•Φ

1 .

l '

-

—

-Φ

Φ

Φ

Φ Φ

• I '

"I. "

1 .

100r—

200 \—

300

< 400 k -

S 500

600 r—

700 r—

800 \—

9000 1 2 3 1 2 3

Thermal conductivity (W/mK) Bulk density (g/cm3)

Figure 18. Thermal conductivity and bulk density (determined by2-min GRAPE counts) vs. depth for hard rocks from Hole 786B.

-

Φ

Φ

••

—

-

Φ

1

1 '

Φ Φ '••

X -

• _:•: .ΦΦΦ Φ

••••y —ΦΦ#. . . -••• .• —••• %

\rΦ ΦΦ

Φ Φ Φ

V Φ...••" —

Φ Φ

v. -

1

347

SITE 786

u

100

200

300

400

500

600

700

800

900

1 I ' I

— —

•— * *

#, —• *•

•— . —•• *• •*• • *

.*•• ••••* β

•

. 1 . 1

i 1 i 1 i

— —

j.

- • : . " • -

• .••••— . -. —

— ". —•-

.j• # •#

— \ —^- **

. 1 . 1 .

l

. —

• #

- :.

•j

V

. .

1.8 2.2 2.6 2.0 2.5 3.0 3.5 2 4 6 2 4 6

Bulk density (g/cm3) Grain density (g/cm3) A direction (km/s) B direction (km/s)Compressional-wave velocity

Figure 19. Bulk density, grain density, and compressional-wave velocities measured on the Hamilton Frame apparatusvs. depth for Hole 786B. The velocities in the orthogonal directions are nearly equivalent and show a high degree ofscatter, which is related to microcrack effects.

348

SITE 786

Summary Log for Site 786

UJ

oo

>

CO

VE

UJ

J ELO

V0

R(m

H ^Q_ ( O

UJ —

o cc

0

0

SPECTRALGAMMA RAY

TOTALAPI units

API units

100

100

0.2

O2

0.2

SHALLOW RESISTIVITYohm-m

DEEP RESISTIVITY

ohm-mFOCUSED RESISTIVITY

ohm-m

2OOθ|

120001

120001

TRANSIT TIME

200 us/ft 50

BE

LO

Hid

:

α

?

00

R(

<

12

13 3200

15

16

183250

D A T A R E C O R D E D OPEN H O L E

- 1 0 0

— 150

349

SITE 786

Summary Log for Site 786 (continued)

SPECTRALGAMMA RAY

TOTAL SHALLOW RESISTIVITYIC) API units IOO|O2

Lu tt: |m o L.Oh

X r-J I U

Lu —

o tr

19

27

28

30

31

32

35

3300

3350

3400

ohm-m 20001DEEP RESISTIVITY

öhm~ m " ' ~ ~2~θ"0ë~|FOCUSED RESISTIVITY I TRANSIT TIME

10.2 ohm-m<

LU Lu20001200 us/ft 50| Q CΛ

- 200

-250

- 3 0 0

350

SITE 786


O (T

SPECTRALGAMMA RAY

TOTAL SHALLOW RESISTIVITYJO API units lööl 0.2I COMPUTED II0 API units 100102


öhTn~ m " " ~ Yo~OO*

1361

137

138

139

141

142

144

146

147

148

149

150

151

152

153

3450H

3500-

3550"

FOCUSED RESISTIVITY

10.2 ohm-m 2000

TRANSIT TIME ;

200 us/ft 501

Q- <LU LUo CΛ

-

m

- 4 0 0

- 4 5 0

- 5 0 0

351

SITE 786


SPECTRALGAMMA RAY

TOTAL SHALLOW RESISTIVITYo J I 0 A P I units 10010.2

S o L COMPUTED^ _ 3 Ib ApfürTirs iδölö 2"

W O H C 1K O 0- toO UJ Lu _0 <r o tr

54

55

56

57

58

59

601

61

62 I

63

64

65

66

67

69

70 I

71

72

3600—

3650-

3700-

3750-


ohm" m" " ~ ~2Oθö|FOCUSED RESISTIVITY I TRANSIT TIME

10.2 ohm-m 20001200 us/ft 50a. <UJ LUo en

in

ml

- 5 5 0

- 6 0 0

- 6 5 0

352

SITE 786


SPECTRALGAMMA RAY

TOTALδ J K) API units 10010.2

K £ 0 L COMPUTEDLU O

SHALLOW RESISTIVITY

Cf fy Λ^

O LU LU —

ohm-m 2 0 0 0 1DEEP RESISTIVITY

öh~πwn ~2θ"θO|FOCUSED RESISTIVITY I TRANSIT T IME

3800

3850

0.2 ohm-m 2000|200 us/ft 5010- <LU LU

α CΛ

3900

- 7 0 0

-750

-800

353


SPECTRALGAMMA RAY

$ _ I COMPUTED I I POTASSIUM I 5 - gθ£ Iδ---GAF>rünTti•lδöl NEUTRON PHOTOELECTRIC l 0 Weight % ^ ^

£ £o I TOTAL GAMMA RAY I POROSITY I EFFECT I THORIUM I m g£ X 3 Pδ GAPI units IOOlK)0"~ ' " V " ' ' " 0 1 0 bαrns7er~~i"OT-~2~" " p p m " ~~3l x ü !

£ 8 a.11" I CALIPER I BULK DENSITY DENSITY CORR. I URANIUM I t<S K o |5~~ in. 1811.5 g/cm3 5 K 5 gX^3 . s| 5 ppm ~^Tl ow

I

2

3

4

7

8

| D A T A R E C O R D E D O P E N H O L E — 100

13 3200"ifT— \ \—T^—! ( p^ j~*' .-£ p~

18 I |^ f I I V I J { I ? P^ *?Men iLJ | \ \ \ I i ( i i t—i *4* f-

SITE 786


SPECTRALGAMMA RAY

COMPUTEDo ~§ 15 GAPf ünTts ~ TOO I

POTASSIUM

NEUTRON PHOTOELECTRIC l 0 W β i g h t % 5 | A*£ w g 1 TOTAL GAMMA RAY | POROSITY EFFECT I THORIUM.

jjj X 3 fö GAP I units lOOMOO~~ ' " V " " "~ OFO bön\l7ë~~\0\~Z~' "ppm"" "~3

K 8 α:1"" I CALIPER I BULK DENSITY I DENSITY CORR. I URANIUM I ^ <2 er o 18 in. lβ| 1.5 g/cm^ 5|-.5 g/cm3 5|5 ppm -TI QCO

19

23

24

25

3300—

28

3350-

30

31

32

33

34

35

3400-

1

\L

e

4

<

2 '-*-"-,

-200

— 250

— 300

355

SITE 786


I I Iδ

SPECTRALGAMMA RAY

COMPUTED"~G~APFimits~~lδöl NEUTRON PHOTOELECTRIC

POTASSIUMI0 weight %

CALIPER BULK DENSITY DENSITY CORR. URANIUM1811.5 g/cm3 5 -.5 g/cm3 5 5 ppm

O TOTAL GAMMA RAY POROSITY EFFECT THORIUM I mg3 [0 GAPI units lOOllO~O~" ' " V " " " "ö lö bàwl7e:"\0\-2" ""ppm"" "~3| i π J

-IQ - <

37

38

40

41

43

45

46

47

49

50

3450-

3500-

3550-

- 3 5 0

r )

1

4- 4 0 0

- 4 5 0

- 5 0 0

356


SPECTRALGAMMA RAY

£~ I COMPUTED I I POTASSIUM I 5 - go l δ---GAPrüniti-lδöl N E U T R 0 N PHOTOELECTRIC l θ w e i ^ h t % ^ §S£o TOTAL GAMMA RAY I POROSITY I EFFECT I THORIUM I mg

£ X 3 10 GAPI units lOOllOO "V"" "~δiδ b ö r n ^ ~ T ö µ f ppm"" ""5| ICIJK ° i" I CALIPER I BULK DENSITY 1 DENSITY CORR. I URANIUM | CL<8 K o f β ~ in. 1811.5 g7cm3 5|75 g/cm^ .5|5 ppm ~~H ow

,Mjβoo- a / I I 1 1 |λ I i ^ > I—j i;.t?

TI • - H T H — ^ ^ — 1 ^ - t -^"•

«π - ICI I I TI '/ pj ;] j•̂ J/

_M 37oo- -t ̂ ^ V N 1 ;|4H• / i I 3

6 9• j^ fc. f3 ~tY ~ frt~•f~-| ' 18 " π l " Ml ) I s -65°Λ •I 1 j >•l I | ] f | .

T .it -tzxzzjizzlzj^L

SITE 786


SPECTRALGAMMA RAY

COMPUTED. I' ~G~APf units ~ T o o l

POTASSIUM

NEUTRON PHOTOELECTRICfθ weight %O-5

£ θ I TOTAL GAMMA RAY I POROSITY EFFECT THORIUM> X 3 I0" GAPI units IOθl|θ"θ"~ " V " " ö l δ bòrnlJi"\0\-Z pp"m 3|

Lu O h-LuOC O Q-O LU LU

5l 3^

8CALIPER

in . 18BULK

1.5 g,DENSITY

'cm 3 .5|DENSITY CORR.

-.5 g/cm3 5 5URANIUM

ppm -1Q-<LULU

3800

3850

3900

- 7 0 0

- 7 5 0

- 8 0 0

358

SITE 786


CAPTUREICROSS SECTIONI0 cu.

ALUMINUM0 wt.%

40

20

CALCIUM

SILICON.5

YIELD

YIELD0

IRON.5

SULFUR-.1

YIELD

YIELD

0

.3

HYDROGEN"5CHLORINE0

YIELD J

YIELD1

II

XU.

£<U) LUQco

I I

12

14

15

17

18

3100

3150

3200

3250

359

SITE 786


is3

CAPTURE[CROSS SECTION! CALCIUM YIELD I IRON YIELD J HYDROGEN YIELD J1 0 " " α ü . "" ' " 4 0 1 - " "4l"5 ö1~5 01

SILICON YIELD | SULFUR YIELD I CHLORINE YIELDALUMINUM

19

20

27

28

29

30

31

32

33

34

35

3300

3350

3400

wt.% 20I.5 OFT 3|0

-ice

Q-<UJ LUO

- 2 0 0

- 2 5 0

-300

360

SITE 786


*?3 ~ CAPTURE

£ m θ I CROSS SECTION! CALCIUM YIELD IRON YIELD J HYDROGEN YIELDw X 3 I 0 " ' " αù. " " " "401-.T "* 4T5 OT5 "

u OO ÜJo oc

ALUMINUM SILICON YIELD

J HYD_ROGEN YIELD JòTs~~ "öi

SULFUR YIELD I CHLORINE YIELD

§1

x§

I I QCΛ

36

37

38

39

40

41

42

44

45

46

47

48

49

50

51

{)f

\f

}i\x4

j

t•

;

*

> •

^ !

%

s

/\

<r

4.

* *

Φ*

^f—

f

>

0

; :

J*

•«

« :

<

• e

«<

J

1

•

1

X5*

-"•

p

•=?

f> *

A*

<

<

<

<

>

s%̂

*. ^ >.>•"•e.it"

*i* '

i

fi.:-•j.

<

^.•‰

<

<

<

f1>>

r

>

*

%

1

LU

Q.

Luj

O "

I «

z *LU

o

<

•<,<?

V

!»

>

Λ

V-

• »• •*"

/

= :

'•v

Λ>s

si;

•

i

<

<

<

<<

<<

m

7

\

I

i

>

fi

>r

*

*

to

* )

»

/

f

/

M

<

,4

)

J

*

\

Φ

\

- 3 5 0

- 4 0 0

-450

-500

361

SITE 786


SS oO Luü tr

54

55

56

57

5β1

591

6θ|

61

62 I

63

64

65

66

67

371

72

m O

3650

3700

3750

C A P T U R E)S_S__ __SE_C

c.u.ALUMINUM

UAPIUKt[CROSS SECTIONl|0 c"ü. "40I~•

CALCIUM YIELD I IRON YIELD I HYDROGEN YIELD

SILICON YIELD

I HYDROGEN YIELD J

"ore" "oiSULFUR YIELD I CHLORINE YIELD

wt.% 20I.5 Ol-.l .3 0

X u .

Lülxl

3600 - -=*

- 5 5 0

-600

-650

362

SITE 786


CAPTURE

mICR.OSS SECT|ON JI0 c.u. 401-

CALCIUM YIELD IRON YIELD I HYDROGEN YIELD~ O

ALUMINUM0 wt.% 201.5

SILICON YIELD

I HYDROGEN YIELD J"OT5 "01

SULFUR YIELD I CHLORINE YIELDOl-.l

3800-

3850-

3900-

I f

c

er

;sr

. - *

s_

•B:

I

-M-

41

£ h

<

A 1

K8Q-<LU LuQ

I -.\' 1

- 7 0 0

— 750

- 8 0 0

363

14. SITE 7861 Shipboard Scientific Party

Documents

Transcript of 14. SITE 7861 Shipboard Scientific Party