6. SITE 719: BENGAL FAN1

Shipboard Scientific Party2

HOLE 719A

Date occupied: 2 August 1987

Date departed: 6 August 1987

Time on hole: 3.9 days

Position: 0°57.646'S, 81° 23.967'E

Water depth (sea level, m, bottom felt): 4736.8

Penetration (m): 460.2

Number of cores: 49

Total length of cored section (m): 460.2

Total core recovered (m): 185.53

Core recovery (%): 40.3

Deepest sedimentary unit cored:Depth sub-bottom (m): 357.0Nature: silty and silty mud turbiditesAge: late MioceneMeasured vertical sound velocity (km/s): 1.75

HOLE 719B

Date occupied: 6 August 1987

Date departed: 9 August 1987

Time on hole: 2.4 days

Position: 0°57.646'S, 81°23.967'E

Water depth (sea level, m, bottom felt): 4736.8

Penetration (m): 465.6

Number of cores: 0

Total length of cored section (m): 0

Deepest sedimentary unit cored: none

SITE SUMMARYSite 719 is located in the central Indian Ocean approximately

800 km south of Sri Lanka and 200 km northwest of the AfanazyNikitin seamount group (Fig. 1). It lies at the distal end of theBengal Fan within a large region of intraplate deformation,which affects both the basement and the overlying sediments.Site 719 is located near the center of one of the tilted faultblocks that make up the tectonic fabric of the region. It was de-signed as a companion site to Site 717, located 3.2 km to thenorthwest on the northern (lower) edge of the same block. Site717 cored a complete syn-deformation sequence in the axis of asynclinal structure developed in the sediments. Site 719 is lo-cated part way up the fault block, in an area where the syn-de-formation sedimentary sequence is attenuated, to determine thetime of deformation and the history of motion on the block

1 Cochran, J. R., Stow, D.A.V., et al., 1988. Proc. ODP, Irtit. Repts., 116:College Station, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in the list of Participants preceding thecontents.

through a comparison of the sedimentary record at the twosites.

Site 719 consists of two holes. Hole 719A, consisting of oneAPC core, penetrating 4.2 m and recovering 4.27 m (101%),and 48 XCB cores, penetrating 456 m and recovering 181.26 m(39.8%), obtained a section through the syn-deformational sed-iments. Hole 719B was drilled to 465.6 m and was utilized forthree successful logging runs.

Principal drilling results. The stratigraphic section recovered at Site 719ranges from late Quaternary to late Miocene in age and has been di-vided into five main lithologic units. The dominant lithologies andages of the stratigraphic sequence are as follows.

Unit I (0.0-4.0 mbsf): Clay and calcareous clay and mud of Hol-ocene to latest Pleistocene age.

Unit II (4.0-135.0 mbsf): Micaceous silt and silty mud turbiditeswith thin intervening muds of late Pleistocene age.

Unit III (135.0-207.0 mbsf): Mud turbidites and biogenic mudturbidites with thin interbedded pelagic clays of Pleistocene to earlyPliocene age.

Subunit IVA (207.0-240.0 mbsf): Silt turbidites with thin mudsand mud turbidites of early Pliocene age.

Subunit IVB (240.0-357.0 mbsf): Mud turbidites with interbed-ded pelagic clays of early Pliocene and late Miocene age.

Subunit V (357.0-460.2 mbsf): Silt and silty mud turbidites withrare intervals of pelagic clay and organic rich mud turbidites of lateMiocene age.

Sedimentation at Site 719 has been almost exclusively throughfan deposition processes, and the section consists primarily of asequence of turbidites. A thin layer of mud and calcareous mud(4 m) overlies a sequence of silty turbidites that accumulatedvery rapidly during the late Pleistocene. Units III and IV to-gether represent a section of mainly muddy turbidites that accu-mulated during the late Miocene and Pliocene. It is within theseunits that most of the attenuation of the sedimentary sectionbetween Sites 717 and 719 occurs. The lowest unit consists of amonotonous sequence of micaceous silt and silty mud turbiditeswith thin interbedded clays. The seismic horizon that marks theonset of deformation occurs within Unit V and does not repre-sent a lithologic boundary.

The sedimentary section at Site 719 corresponds very closelyto the section obtained at Site 717, and in many instances dis-tinctive individual turbidites and turbidite sequences can be cor-related between the two sites. Attenuation of the section be-tween Sites 717 and 719 appears to have occurred through acombination of pinch outs and thinning of individual beds. Thelimiting factor in determining the deformation history will proba-bly prove to be the age resolution that can be obtained. Bio-stratigraphic control at all of Site 719 is provided primarilythrough nannofossils as the other microfossil groups are eitherpoorly preserved or absent. Preliminary shipboard studies sug-gest that the rotation of the fault block has occurred throughoutthe period of intraplate deformation at a fairly constant rate.

BACKGROUND AND OBJECTIVES

BackgroundSite 719 is the third of three closely-spaced sites located near

1°S, 81°24'E in the central Indian Ocean and is designed as a

155

10°S90°

Figure 1. Location map for Leg 116 sites. (A) Setting of the Leg 116 sites relative to tectonic elements of the eastern Indian Ocean. Heavysolid line represents mid-ocean ridge axes, dashed lines are transform plate boundaries and saw-tooth pattern shows convergent plate bound-aries. (B) Geologic setting of Leg 116 sites within the north central Indian Ocean. Bathymetry is machine-contoured DBDB10 gridded data at500-m intervals. Location of other DSDP drill sites within the region is also shown.

156

SITE 719

companion site to Site 717, located 3.2 km to the north-north-west. The sites are located at the distal end of the Bengal Fanapproximately 800 km south of Sri Lanka and 200 km north-west of the Afanazy Nikitin Seamount Group. They are within alarge area of intraplate deformation that affects both the oce-anic crust and the thick pile of overlying sediments.

Both deposition and deformation in this part of the centralIndian Ocean have been affected by the collision between theIndian and Eurasian Plates. The continental portions of theseplates began to interact during the Eocene at Anomaly 22 time(approximately 53 Ma) with a soft collision, probably betweencontinental India and an island arc lying seaward of Asia (Cur-ray et al., 1982). This collision resulted in a reduction of thespreading half-rate from 8 cm/yr to about 4 cm/yr (Sclater andFisher, 1974). A regional unconformity observed on seismic pro-files in the proximal- and mid-fan areas (Curray and Moore,1971) has been dated to this time by extrapolation from DSDPSite 217 (Moore et al., 1974).

During the Oligocene, the spreading half-rate decreased fur-ther to 2.5 cm/yr, the direction of spreading on the southeastIndian Ridge changed from N-S to NE-SW (Sclater et al., 1976),and sedimentation in the region now covered by the Bengal Fanprobably increased (Powell and Conaghan, 1973). The hard con-tinent-continent collision may have occurred at that time, al-though other estimates are as late as upper Miocene (Powell andConaghan, 1973; Gansser, 1964, 1981). A second major regionalunconformity in the area of the Bengal Fan has also been datedas late Miocene by drilling at DSDP Site 218 (von der Borch,Sclater, et al., 1974). The unconformity marking the onset ofdeformation in the distal fan (Unconformity "A", Fig. 2) is in-ferred by extrapolation to be the same event (Weissel et al.,1980).

Rapid terrigenous sedimentation on an incipient Bengal Fanbegan in the Eocene as a response to the first intraplate collisionand has continued to the present, building the world's largestsubmarine fan. The Bengal Fan is over 2500 km long and ismore than 16 km thick under the northern Bay of Bengal (Cur-ray et al., 1982). In the area of the Leg 116 sites, the sedimentsare about 1.5 to 2 km (2 s of two-way traveltime) thick under thedistal part of the fan.

This enormous volume of sediments is assumed to have beenderived principally from denudation of the uplifting HimalayanMountains and then supplied to the delta front via the Ganges-Brahmaputra river systems. Sediments are funneled very effi-ciently to the fan via a delta-front trough, the "Swatch of NoGround." This trough is presently connected to only one activefan channel, but has been effectively cut off from rapid sedi-ment supply since the most recent rise in sea level, probablyabout 7,000-10,000 yr ago (Emmel and Curray, 1983). The ac-tive fan channel appears to terminate some 50-100 km north ofthe Leg 116 sites and no channel- or levee-type features havebeen observed on seismic records in the vicinity of the Leg 116area.

Increased resistance to subduction, and shortening across theHimalayas since the continent-continent collision, combinedwith continued spreading on the southeast Indian Ridge, hasplaced the central Indian Ocean under a large N-S compressivestress regime (Stein and Okal, 1978). As a result, the oceaniccrust and most of the overlying sediments have been deformedinto long-wavelength (100-300 km) undulations with peak-to-trough amplitudes of 1 to 3 km over a large area. This areaextends from the Chagos-Laccadive Ridge in the west to theNinety East Ridge and from 5° N to 10° S latitude (Weissel etal., 1980; Geller et al., 1983).

Superimposed on the long-wavelength undulations are faultedand rotated blocks, 5-20 km wide, interpreted as showing a re-

verse sense of displacement on the faults separating them (Gel-ler et al., 1983), consistent with N-S compression. The top ofthe oceanic crust is seen to be offset across these faults by up to0.5 s (approximately 0.5 km) on seismic reflection records (Fig.2). The lower portion of the sedimentary sequence is character-ized by reflectors which are parallel to the basement, and thesequence is assumed to predate the deformation. A prominentunconformity (Unconformity "A", Fig. 2) separates the prede-formation sequence from an upper syn-deformation sequence,which thins toward the top of the blocks and shows pinchingout of individual reflectors.

ObjectivesSite 719 is located in 4737 m of water, near the center of

same fault block on which Site 717 is situated. Site 717 was situ-ated in the thickest part of the sedimentary section between ad-jacent block highs in a location where the seismic reflection re-cords suggested that sedimentation has been nearly continuous(Fig. 2). Site 719 is located part way up the fault block where thesyn-deformation sediments have thinned from 0.70 s at Site 717to 0.52 s. A well-defined unconformity (Unconformity "A") isdeveloped separating pre- and syn-deformation sediments andin addition the section appears to have thinned through thepinching out of reflectors. The principal objectives of Site 719,were as follows:

1. To determine the sedimentary sequence within an area af-fected by rotation of the fault block and by comparison withSite 717 to determine the history of motion and uplift;

2. To characterize the main lithofacies and determine the de-positional processes responsible;

3. To establish the provenance of the sediments on the distalfan and to assess to what extent compositional variations reflectthe uplift history of the Himalayas;

4. To characterize the nature of early diagenesis in deep-wa-ter fan sediments; and

5. To measure the physical and hydrological properties ofsediments in an actively deforming area.

OPERATIONSJOIDES Resolution moved the 7 km from Sites 718 to 719 in

dynamic positioning mode with 4500 m of drill pipe suspendedunder the ship. This was more time efficient than carrying out afull round trip, and a seismic line had already been run over thesite during the short survey between the two previous sites. Thebeacon was dropped at 0°57.6'S, 81°23.95'E at 2045 hr on 2August.

The bit was positioned just above the seafloor with the aid ofthe TV camera and the APC core barrel fired to spud Hole719A. The mud line was established at 4747.3 m below the rigfloor and one 4.2-m core was brought on deck at 2300 hr on 2August. The rest of the hole was cored continuously to a depthof 460.2 mbsf using the XCB system. The last core was broughton deck at 0415 hr on 6 August, giving an average core recoveryof 40.3% that was good in the mud lithologies and poor in thesilts. Details of the coring operation are given in Table 1.

A round trip was made to avoid using a bit-release assembly,and a second hole was drilled after moving the ship 10 m. Hole719B was then drilled ahead to 466 mbsf and conditioned foruse as a dedicated logging hole. Four successful logging runswere made, using three separate tool strings, from the bottom ofthe hole to the seafloor. During logging operations the drill pipewas positioned at 143 mbsf. Following completion of the log-ging, pipe was pulled as the ship returned to Site 718 in dynamicpositioning mode.

157

SITE 719

N

717Fault Fault

Approx. 5 km

Figure 2. North-south seismic reflection profile across the Leg 116 drill sites. Locations of Sites 717, 718, and 719 are shown. Also noted are the loca-tions of the faults bounding the tilted blocks and prominent unconformities "A" and "B".

LITHOSTRATIGRAPHY

LithofaciesThe stratigraphic sequence recovered at Site 719 consists pre-

dominantly of interbedded clays, silty clays, and silts, with lesseramounts of sandy silts and calcareous clay, ranging in age fromQuaternary to late Miocene. The sediments can be divided intoseven distinct facies on the basis of texture, sedimentary and bi-ogenic structures, color, organic carbon content, and the pres-ence or absence of calcareous microfossils.

Facies 1. Silt and Silt-Mud Turbidites (Fl)

Facies 1 consists of sharp-based upward-fining beds of dark-gray silt or, less commonly, sandy silt, which grade upwardthrough silty clay to clay. Internally the beds may be massive orrarely show a faint horizontal lamination in the basal few centi-meters. The beds of this facies range from 5 to 95 cm thick, av-eraging 40 cm. Silt fractions are very micaceous (5-15%), gener-ally quartz-rich (50-70%) and often slightly calcareous (2-10%calcite). Bioturbation and calcareous microfossils are rare.

Facies 2. Organic-poor Mud JUrbidites (F2)

Facies 2 consists of sharp-based, 5- to 80-cm thick (average33 cm) beds of light gray to gray, silty clays which pass upwardinto clay. Typically silt comprises only 10% of this facies. Thebases of beds are often marked by 0.5- to 3.0-mm thick quartz-rich basal silt laminae. The silt fraction consists mainly of quartzwith mica (less than 10%) and feldspar (traces). Plant debrisranges from zero to 1 % and calcareous microfossils are largelyabsent. Bioturbation is common in the upper portion of bedsand may extend downward through much of the bed. In somecases bioturbation is so extensive that even the bottom bound-ary appears bioturbated and gradational. The true turbidite na-ture of these beds becomes doubtful.

Facies 3. Organic-rich Mud Turbidites (F3)

Facies 3 is characterized by large amounts (2-15%, avg. 5%)of plant debris and consists of very dark-gray to black, sharp-based, upward-fining beds 5- to 50-cm thick (avg. 17 cm) of

silty clay (Fig. 3). The beds generally commence with clayey silts(less than 80% silt) and pass upward into clay. Typically the ba-sal few centimeters exhibit a horizontal lamination that passesupward into a faint wispy lamination and finally a massive in-terval. The tops of the beds are commonly bioturbated and, aswith Facies 2, this bioturbation may extend downwards throughmuch of the bed. In some cases the bioturbation appears tohave been caused by organisms burrowing upward from the un-derlying unit (Fig. 4). These beds are less clearly of true turbi-dite origin. Pyrite, or an iron sulfide precursor, (after wood,burrows, and as concretions) is ubiquitous (see below).

Facies 4. Biogenic Mud JUrbidites (F4)This facies consists of sharp-based, upward fining, olive-gray

calcareous silty clays (with a silt content less than 10%) andclay. Beds of this facies are found only in lithologic Unit III ex-cept for a single example in Unit IV, and range in thickness from18 to 110 cm (avg. 58 cm). There is a tendency for bed thicknessto increase upsection. A thin (less than 3 cm) basal calcareoussilt layer may occur. Rarely, a faint horizontal lamination ispresent, particularly at the base, but most commonly the bedsappear massive. In contrast to the other facies (with the excep-tion of the basal silts of Facies 6) calcareous microfossils (mainlynannofossils, but with many foraminifers) are ubiquitous, mak-ing up to 20% of the sediment. In addition, unidentified calcar-eous silt and clay-sized material account for 20-30% of the sed-iment. The tops of the beds are commonly bioturbated.

In Core 116-719A-18X a 6-cm laminated white chalk was ob-served within lithologic Unit III that may correlate with a simi-lar chalky horizon at Site 717.

Facies 5. Pelagic Clays (F5)

This facies consists of gradationally based beds of clay 5-10cm thick, varying in color from a mottled green to whitish gray.Moderate to intense bioturbation characterizes the facies. Com-mon trace fossils include chondrites, planolites, and zoophycos.Ferromanganiferous(?) chemical fronts characterize the gray claysof this facies. Calcareous microfossils are rare to absent; thosethat remain show evidence of extreme dissolution.

158

SITE 719

Table 1. Coring summary for Site 719.

Coreno.

1H2X3X4X5X6X7X8X9X

10X11X12X13X14X15X16X17X18X19X20X21X22X23X24X25X26X27X28X29X30X31X32X33X34X35X36X37X38X39X40X41X42X43X44X45X46X47X48X49X

DateAug. 1987

02030303030303030303030303030304040404040404040404040404050505050505050505050505050505050505060606

Time

2300031504250540062507500845121013051410171018001900224023550125031009401110124014051515163517501900202021402250000501300240040005200640075009051045115513101420153517001825195021002240003002150415

Sub-bottomtop

(mbsf)

0.04.2

13.723.232.742.251.761.270.780.289.799.2

108.7118.2127.7137.2146.7156.2165.7175.2184.7194.2203.7213.2222.7232.2241.7251.2260.7270.2279.7289.2298.7208.2317.7327.2336.7346.2355.7365.2374.7384.2393.7403.2412.7422.2431.7441.2450.7

Sub-bottombottom(mbsf)

4.213.723.232.742.251.761.270.780.289.799.2

108.7118.2127.7137.2146.7156.2165.7175.2184.7194.2203.7213.2222.7232.2241.7251.2260.7270.2279.7289.2298.7308.2317.7327.2336.7346.2355.7365.2374.7384.2393.7403.2412.7422.2431.7441.2450.7460.2

Meterscored(m)

4.29.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.59.5

460.2

Metersrecovered

(m)

4.271.530.100.720.860.640.670.940.060.141.351.301.142.630.74

10.009.979.964.409.544.109.143.301.341.071.193.30

10.144.259.959.354.052.649.575.288.828.748.101.830.650.471.450.980.930.940.851.149.971.03

185.53

Percentrecovery

101.016.1

1.17.69.16.77.19.90.61.5

14.213.712.027.77.8

105.2105.0105.046.3

100.043.196.234.714.111.212.534.7

106.744.7

105.098.442.627.8

101.055.692.892.085.219.26.85.0

15.210.39.89.99.0

12.0105.010.8

Fades 6. Pelagic Calcareous Clays (F6)

Facies 6 consists of light- to dark-gray calcareous biotur-bated clays (50-85 cm thick) with approximately 5% silt. It ischaracterized by an abundant fauna of nannofossils (up to 30%)and foraminifers (up to 5%). Bioturbation is common and in-tense.

Facies 7. Structureless Muds (F7)

Facies 7 consists of beds (10-60 cm thick) of dark- to light-gray silty clay and clay with indistinct bed boundaries. The bedstend to be bioturbated throughout. Ferro-manganiferous(?)chemical fronts are absent. Calcareous microfossils are largelyabsent but occasionally occur in trace amounts.

These facies are very similar to those of Site 717. There is atendency for both silt and mud turbidite facies (Facies 1-3) to bethinner at Site 719 than at Site 717. The biogenic mud turbiditesshow a similar trend.

Lithologic UnitsOn the basis of the distribution of the facies identified, the

sedimentary sequence penetrated at Site 719 has been dividedinto five lithologic units, which correspond with units identifiedin nearby Site 717 (Fig. 5).

Unit I. Cores 116-719A-1H; 0 to 4 mbsf

Unit I is 4.0 m thick and is composed of interbedded struc-tureless muds (F7), organic-poor mud turbidites (F2), and cal-careous clays (F6). This unit was piston cored and the recoverywas 100%.

Unit II. Cores 116-7 19A-2X to 116-719A-15X; 4.0 to135 mbsf

Unit II consists of 131.0 m of predominantly Facies 1 siltand silt-mud turbidites, with thin inter beds of structureless muds(F7). Recovery for this unit was very poor (approximately 10%).

159

SITE 719

30

40

50

60

cm

30

40

Figure 3. Photograph of organic-rich mud turbidite cycle (Section 116-719A-16X-6).

Unit HI. Cores 116-719A-16X to 116-719A-23X; 135 to207mbsf

Unit III is 72 m thick and is characterized by the presence ofbiogenic mud turbidites (F4), which decrease in abundance withdepth through the unit. Inter bedded with the biogenic mud tur-bidites are organic-poor (F2) and organic-rich mud turbidites

50 •—

Figure 4. Photograph of organic-rich mud turbidite cycle showing bio-turbation at base (Section 116-719A-36X-6).

(F3), together with structureless muds (F7), calcareous clays (F6),and pelagic clays (F5). Both Facies F5 and F6 tend to occur asthin caps on the mud turbidite facies. Recovery was approxi-mately 84%.

Unit IV. Cores 116-719A-23X to 116-719A-39X; 207 to357mbsf

At Site 717 this unit was subdivided into four subunits on thebasis of the presence, abundance, and thickness of silt and silt-

160

SITE 719

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IVB-D

Figure 5. Lithostratigraphy of Site 719.

mud turbidites. At Site 719 Subunits IVB, IVC, and IVD can-not be differentiated so are considered here as a single compos-ite subunit, IVB-D.

Subunit IVA. Cores 116-719A-23X to U6-719A-26X; 207 to240 mbsf

Subunit IVA is 33 m thick and is characterized by an abun-dance of silt turbidites of Facies 1, although thin structurelessmuds (F7) and mud turbidites of Facies 2 and 3 also occur. Re-covery was approximately 11%.

Subunit IVB/D. Cores 116-719A-27X to 1I6-719A-39X; 240to 357 mbsf

Subunits IVB/D total 117 m thick and consist of interbed-ded organic-rich (F3) and organic-poor (F2) mud turbidites,structureless muds (F7), and pelagic clays (F5), differing fromUnit III in that the biogenic mud turbidites (F4) are largely ab-

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Legend

Pelagic calcareous (F6) clay

Mud turbiditesMuds and pelagic clays(F 2, 3, 5 dominant)

Biogenic (F4) mud turbidites

Silt and silt mud turbidites(Fl dominant)

sent and that rare silt-mud turbidites (Fl) occur. Recovery wasapproximately 73%.

Unit V. Cores 116-719A-39X to 116-719A-49X; 357 to460.2 mbsf

This unit is a monotonous sequence (103.2 m thick) of pre-dominantly silt and silt-mud turbidites (Fl) with rare, thin inter-vals of pelagic calcareous clay (F6), structureless mud (F7), andorganic-rich mud turbidites (F3). Recovery was approximately18%.

Maximum Quartz Grain SizeMaximum grain size was determined at approximately 10-m

intervals down Hole 719. In each case the coarsest and/or thick-est lithology in the individual core (generally from the base ofthe coarsest turbidite) was chosen for smear-slide analysis and

161

SITE 719

the largest equant quartz grain measured. Results of the analy-sis are shown graphically in Figure 6.

Analysis reveals that the maximum grain size range is from30 to 565 µm (coarse silt to coarse sand on the WentworthScale). As at Site 717 the coarsest maximum grain size corre-sponds to the sandy silt and silt turbidites of Facies 1 withinlithologic Units II and V. Maximum grain size in Unit IVa onlyreaches 240 µm (fine sand). However, the background valuesthroughout the sequence show that coarse silt-sized materialwas able to reach this most distal fan setting via turbidity cur-rents throughout the interval from the late Miocene to the Qua-ternary sampled at Site 719.

Carbonate ContentResults of analysis of carbonate content at Site 719 are shown

in Figure 7. Carbonate values for most of the core are low (0-10%), but at several horizons values of 30-50% are seen. Mostof these very high values probably correspond to the biogenicmud turbidite facies (F4), reflecting their high biogenic calcitecontent. The high value near the seabed corresponds with pe-lagic calcareous clays of Facies 6. The peak at approximately320-m depth corresponds with a thin whitish colored biogenicturbidite that appears to correlate with a similar thin calcareousturbidite at Site 717 (475 mbsf).

100 —

200 —

300 —

400 —

500200 400

Grain size (µm)6 0 0

Figure 6. Graph of maximum grain size vs. depth, Site 719.

5000 20

CaC03 (%)

Figure 7. Graph of carbonate content vs. depth, Site 719.

Pyrite OccurrencePyrite occurs most commonly as scattered concretions or

nodules, 6-20 mm in length, in gray to very dark-gray and blackclays; however, it also occurs less frequently in greenish or blu-ish gray clays of Units III and IVB-D (Cores 116-719A-17X to-719A-23X, 116-719A-27X to -719A-29X, and 116-719A-34X to-719A-38X). Vertically oriented tube fillings, which were com-mon at both Sites 717 and 718, are also observed at Site 719 inX-radiographs taken of the very dark-gray clays. The fillings are1-2 mm in diameter and approximately 5 mm in length. Thethin layers or lenses of pyrite-rich and plant debris-rich silt ob-served at Site 717 appear to be absent here. Pyritized bioclasts(sponge spicules, foraminifers, radiolarians, fine plant debris)scattered or occasionally agglutinated in large (20-40 mm) con-cretions, are abundant (up to 20%) in the silty facies.

BIOSTRATIGRAPHY

DiatomsIsolated or sporadic specimens of diatoms were recovered in

11 of the 91 samples processed and examined from 49 cores ofthis site. They were found in some of the green clays and, lessfrequently, in calcareous oozes and chalks. Diatoms were sparsein all of the samples due to tremendous dilution by terrigenousor carbonate material. Recovered valves were, however, gener-ally very well preserved and easily indentifiable.

Only in the first core (Samples 116-719A-1H-1, 74-76 cm;116-719A-1H-2, 68-72 cm and 136-138 cm) was the recovereddiatom assemblage moderately diverse; up to 29 taxa were en-countered. All three samples contain the same assemblage ofoceanic planktonic diatoms. The last sample also contains fairlynumerous neritic and some freshwater diatoms. Fairly abundantphytoliths also indicate influx of fossils of terrestrial origin.

162

SITE 719

Only a few diatoms with stratigraphic significance were re-covered in the samples from the first core: Actinocyclus ellipti-cus, Coscinodiscus nodulifer, Hemidiscus cuneiformis, Nitzschiamarina, and Thalassiosira oestripii. Following Burckle (1972)and Barron (1983), Cenozoic stratigraphic distribution of thesediatoms in low latitudes is as follows: A. ellipticus, 14 Ma toRecent; C. nodulifer, approximately 13.5 Ma to Recent; H. cu-neiformis, approximately 12.5 Ma to sub fossil; N. marina, 8Ma to Recent; T. oestrupii, 5 Ma to Recent. Pseudoeunotia doli-olus was not encountered. The mid-Pleistocene to the latest Mi-ocene species, Nitzschia reinholdii, was present here, apparentlyredeposited, because the calcareous nannofossils in these sam-ples indicate an age younger than 0.65 Ma, which is the last oc-currence of N. reinholdii.

In Sample 116-719A-18X-4, 53-55 cm, isolated and well-pre-served specimens of Stephanopyxis appendiculata were recov-ered (stratigraphic range from 2 Ma to early Oligocene).

In all the other samples (Cores 116-719A-21X, -719A-22X,-719A-28X, -719A-29X, -719A-31X, and -719A-36X) diatomswere coated with pyrite. Nevertheless, preservation was very goodand identification was possible if a sufficiently large fragmentor whole valve was recovered. The most diverse assemblage wasrecovered from Section 116-719A-31X, CC, which contained sev-eral pelagic diatoms, Actinocyclus divisus among them. Actino-cyclus divisus is known in the Indian Ocean from sediments noolder than earliest Pliocene.

The deepest sample containing diatoms (116-719A-36X-5,111-113 cm), also pyritized, yielded much older taxa. Stepha-nopyxis antiqua and Goniothecium decoratum, which was un-doubtedly redeposited.

Radiolarians

Well-preserved radiolarian assemblages at Site 719 are mostlylimited to Core 116-719A-1H, similar to Sites 717 and 718. Sam-ples 116-719A-1H-1, 74-76 cm, -719A-1H-2, 68-72 cm, and -719A-1H-2, 136-138 cm, contain well-preserved abundant as-semblages belonging to the Buccinosphaera invaginata Zone ofQuaternary age (Nigrini, 1971). They are typically representedby a radiolarian fauna of more than 50 taxa, and are markedlysimilar to the biocoenosis from the modern tropical oceans. Thesamples may be considered of Holocene age.

The diversified fauna in Sample 116-719A-1H-2, 68-72 cmincludes Acanthosphaera actinota, Amphispyris costata, Antho-cyrtidium ophirense, A. zanguebaricum, Acrosphaera spinosa,A. murrayana, Acanthodesmia vinculata, Botryostrobus aqui-lonaris, Botryocyrtis scutum, Buccinosphaera invaginata, Cen-trobotrys gravida, Collosphaera tuberosa, Carpocanarium pa-pillosum, Carpocanistrum sp. D, Clathrocanium diadema, Cor-nutella profunda, Dictyocoryne profunda (the last includesHymeniastrum euclidis), D. truncatum, Didymocyrtis tetratha-lamus tetrathalamus, Druppatractus acquilonius, Euchitonia ele-gans, Eucyrtidium accuminatum, E. hexagonatum, Heliodiscusasteriscus, Hexacontium axotrias, Hexalonche hystricina, Lam-procyclas maritalis, Lamprocyrtis nigriniae, L. hannai, Lit hop-era bacca, Otosphera polymorpha, Peripyramis circumtexta,Phormosticoartus corbula, Pterocanium praetextum praetextum,P. trilobum, Pterocorys campanula, P. zancleus, Siphono-sphaera polysiphonia, Spongaster tetras tetras, S. pentas, Stylo-chamydium asteriscus, Tetrapyle octacantha, Theocorythium tra-chelium trachelium, Theopilium tricostatum, and several addi-tional species. The Samples 116-719A-1H-1, 74-76 cm and-719A-1H-2, 136-138 cm contain less diversified radiolarian fau-nas (about half the number of taxa in the fraction > 63 µm)than the above list due to a slightly increased level of dissolu-tion; they may also be assigned to the B. invaginata Zone. Be-sides the Quaternary species, the Sample 116-719A-1H-2, 136-138 cm contains Eucyrtidium diaphanes (Oligocene to early Mi-

ocene) and Siphostichoartus corona (middle to late Miocene),indicating redeposition.

Below Core 116-719A-1H, preserved radiolarians are pyrit-ized and found only in the following samples: Samples 116-719A-31X-3, 113-115 cm, -719A-31X, CC, and -719A-38X-5,78-80 cm. Among these are many Liosphaerids and Actinom-mids. One specimen each of Anthocyrtidium ehrenbergi ehren-bergi and Lamprocyrtis sp. were found in Sample 116-719A-38X-5, 78-80 cm. A. ehrenbergi ehrenbergi is rare in the earlyMiocene and common in the late Miocene (Nigrini and Moore,1979). It is tentatively concluded that the sample is of Mioceneage pending detailed shore-based analysis of the Liosphaeridsand Actinommids with scanning electron microscopy (as trans-mission light microscopy is generally not definitive for pyritizedradiolarians). The remainder of the samples with pyritized radi-olarians did not yield any useful information concerning age as-signment.

Holocene productivity of Site 719 must be high, based onmorphology of several radiolarian species, which agrees withthe result from Site 718. For example, skeletons of D. profundahave well-developed patagium, an auxiliary spongy skeletal mesh-work, which is known to occur in highly productive geographi-cal areas such as the Panama Basin.

Silicoflagellates

Silicoflagellates are almost completely lacking in the 110 sam-ples examined from this site. Only Sample 116-719A-1H-2, 136-138 cm contains several specimens of Dictyocha messanensismessanensis, a subspecies commonly found in the modern trop-ical oceans.

Foraminifers

Site 719 yielded fewer foraminifers than the previous Leg 116Sites, and consequently provided less biostratigraphic informa-tion than the previous sites.

A Holocene age was recognized in Samples 116-719A-1H-1,120-122 cm and -719A-1H-2, 69-71 cm by the presence of Bol-liella adamsi.

A Pleistocene age was documented downward to Core 116-719A-16X, based on the occurrence of Globorotalia truncatuli-noides in Section 116-719A-14X, CC and Sample 116-719A-16X-4, 120-128 cm.

From Core 116-719A-17X down to Core 116-719A-20X,Sphaeroidinella dehiscens persistently occurs but with no otherindex species. Therefore, the age of these cores can be con-strained only to the early Pliocene-Pleistocene interval.

From Core 116-719A-22X downhole Sphaeroidinellopsis se-minulina enters in the foraminiferal record. Its range, which isextended from late Miocene to early Pliocene, overlaps that ofSphaeroidinella dehiscens in the early Pliocene. However, thetwo species have not been found associated at any level of thissite. Therefore, a late Miocene age assignment is consideredmost reasonable.

Aside from the scanty planktonic record, the occurrence ofthe benthic foraminifer Hyalinea balthica in Samples 116-719A-16X-4, 120-128 cm, -719A-31X-3, 113-115 cm, and -719A-37X-5, 87-89 cm should be noted. These samples are from bio-genic turbidites which, according to their foraminiferal content,are considered to be of shelf-slope provenance. The presence ofH. balthica is consistent with such an assemblage. Taking intoaccount the most recent literature concerning the distribution ofthis species, the lowest occurrence ofH. balthica is documentedat Zone N21 (late Pliocene) in Eureka core material from theGulf of Mexico (van Morkhoven, et al., 1986) However, accord-ing to Bandy (1968, cited in Boltovskoy, 1977) H. balthica hasbeen found even in Miocene strata in the Philippine Islands. Ifthe correlation with the most recent biostratigraphic scales can

163

SITE 719

be confirmed, then the occurrence of H. balthica in the above-mentioned samples of Site 719 is consistent with calcareous nan-nofossil stratigraphy.

Calcareous NannofossilsThe calcareous nannofossil record at Site 719 is, in most re-

spects, very similar to Site 717 and, to a lesser degree, to Site718. The most significant difference is that the various biohori-zons are more closely spaced at Site 719 than at Site 717, whichreflects a lower sedimentation rate. Nannofossils appear to bemore abundant at Site 719; however, this may be an incorrectsurmise, an expression of cummulative experience in recogniz-ing promising fossiliferous horizons among the generally unfa-vorable lithologies of the numerous turbidites. As at Sites 717and 718, the zonal designation of Okada and Bukry (1980) isused, and the numerical chronology is based on the Berggren etal. (1985) compilation.

Sample 116-719A-1H-1, 71-73 cm is the only one that yieldedthe late Pleistocene marker Emiliania huxleyi. This level is, there-fore, clearly late Pleistocene (Zone CN15), i.e., younger than0.28 Ma. The inconsistent occurrence of Emiliania huxleyi atthe top of each of the three sites may indicate that this ubiqui-tous late Pleistocene-Holocene species has been dissolved fromthe assemblages in which it really should be present. Apparentlythis species is more susceptible to dissolution than other mem-bers of the assemblage. Zone CN14b is identified from this levelto the next lower datum, which is the highest occurrence ofPseudoemiliania lacunosa in Section 116-719A-4X, CC, fromwhich level down this second marker species is present consis-tently although usually in low abundance. The age of this da-tum is 0.47 Ma and it marks the top of Zone CN14a.

In Sample 116-719A-17X-4, 40-42 cm, small species of Ge-phyrocapsa dominate to the virtual exclusion of large species ofGephyrocapsa. This and the absence of Helicosphaera sellii andof Calcidiscus tropicus identifies this level as belonging to thesmall Gephyrocapsa acme interval, which extends from approxi-mately 0.93 to 1.27 Ma and is roughly equivalent to Zone CN13b.This same zone was identified in the next lower sample, 116-719A-17X-5, 87-89 cm, but the next lower sample below, at 116-719A-18X-1, 40-42 cm, yielded an assemblage including Disco-aster brouweri. Late Pliocene assemblages continue downwardto 116-719A-22X-CC, except for the early to mid-Pliocene as-semblage at 116-719A-19X, CC, which probably corresponds tovery similar displaced assemblages at 116-717C-35X, CC and116-718-13X-2, 126-128 cm.

The next lower datum is the highest occurrence of Spheno-lithus abies and of Reticulofenestra pseudoumbilica, placed atCore 116-719A-22X, CC, below which level both of these spe-cies are present consistently and in appropriate proportion inthe assemblage. Both also are present, but in disproportionatelysmall numbers, in assemblages above this level, which is to beexpected of these two durable species. The age of this datum is3.5 Ma and it marks the mid-Pliocene. The interval immediatelyabove this datum is assigned to Zone CN12a/b. Immediatelybelow the datum is Zone CN 11, which extends to Sample 116-719A-29X-2, 99-101 cm, the highest occurrence of Ceratolithusarmatus. The lowest occurrence of the latter is in Sample 116-719A-30X-4, 106-108 cm, the interval being assigned to ZoneCNlOb, with an age of 5.3 Ma at its base. Zone CNlOa extendsfrom this level to the highest occurrence of Discoaster quinque-ramus in Sample 116-719A-31X-3, 113-115 cm, which is heretaken to correspond to the Miocene/Pliocene boundary and hasbeen assigned an age of 5.6 Ma. Zone CN9 extends from thislevel to the bottom of the cored interval, although one addi-tional datum, the lowest occurrence of Amaurolithus amplificusin Sample 116-719A-43X-1, 16-18 cm can be recognized. Thislast datum has an age of 5.9 Ma.

The succession of biohorizons at the three sites is remarkablysimilar, given the limitations of extremely poor fossil recovery.Dilution by massive detrital sedimentation, redeposition by tur-bidites, and deposition below the regional CCD, all probablywork against preservation of calcareous fossils. It should comeas no surprise that paleontological correlation and lithologicalcorrelation correspond very closely, as is to be expected on a dis-tal fan. The chronological framework established by the abovecan be used with confidence for further studies.

Sediment Accumulation RateThe depths and ages of the various biohorizons (datum lev-

els) for Site 719 are given on Table 2. A graphic presentation ofthe data is given in Figure 8 as a sediment accumulation ratecurve, along with similar curves for Sites 717 and 718 for com-parison. The sediment accumulation rate was quite high duringthe late Pleistocene-higher than at Site 718 but less than at Site717. All three curves show a sharp reduction in sediment accu-mulation from about 1 Ma down to about 2 Ma, which is actu-ally a hiatus, a period of little or no sediment accumulationduring most of the first half of the Pleistocene. From the Plio-cene/Pleistocene boundary downward, sedimentation rate in-creases progressively and, seemingly, very sharply in the latestMiocene. However, not too much faith should be placed in thedetails of any of the three curves because redeposition and poorfossil recovery combine to render biostratigraphic control some-what unreliable.

ORGANIC GEOCHEMISTRYThe organic geochemical program at Site 719 consisted of

measurements of hydrocarbon gases, organic carbon, and Rock-Eval pyrolysis as at the previous sites. This work is thus an ex-tension of that described in "Organic Geochemistry" sectionsof the Site 717 and 718 chapters.

Hydrocarbons

Vacutainer GasesAlthough Site 719 is located only 3.2 km south of Site 717,

no measureable Vacutainer gases were present at any depth,which is in contrast to the routine recovery of gases from Site717. The difference in average organic carbon concentration be-tween the two sites is not great (0.96% at Site 717, 0.80% at Site719), but isolated high values (up to 7.5%) were observed at Site717. This may account for the different results obtained.

Extracted GasesAnalyses of headspace gas samples are given in Table 3 and

Figure 9. Methane (Q) concentrations range from 0 to 6500ppm, reaching broad maxima in the middle of lithologic UnitII, throughout Unit IV, and in the lower part of Unit V. Valuesin Unit HI and at the top of Unit V are significantly lower.

Table 2. Depths andages of Neogene nanno-fossil biohorizons atSite 719.

Depth (mbsf)

37.7153.1168.5168.8203.7277.3285.3385.7

Age (m.y.)

0.470.932.22.473.55.35.65.9

164

SITE 719

200

_ 400

8-600

800

1000

Ik ' I

—

—

i

• i

VA7̂19 \

\ \

\

7 1 7 \

I

1 1

\ 718

\

1

I

—

—

\ —

Table 3. Headspace gases from Hole 719A.

10Age (Ma)

15 20

Figure 8. Sediment accumulation rates at Sites 717, 718, and 719.

Methane/ethane^!^) ratios are plotted with depth in Figure9 and closely mirror the absolute concentrations of Q . The C2values remain below 5 ppm and show no increase with depth,indicating immature and nonmarine organic matter.

Carbon

Physical property and squeeze-cake samples were used fordeterminations of organic carbon. Table 4 and Figure 9 showthe organic carbon concentration with depth. As a whole, or-ganic carbon contents range from 0.13% to 2.17% (avg. 0.8%)and are less than that at Site 717. In lithologic Units II and V,organic carbon contents are less than in other units. This obser-vation is similar to that at Site 717, except in Unit V, which hasslightly lower organic carbon values than those in Unit V at Site717. The concentrations of organic carbon and Q do not changein parallel and in some parts seem to be directly opposed. Thisis presumed to be due to differences in bacterial activity.

Rock-Eval

The squeeze-cake samples have been analyzed by Rock-Evalpyrolysis. Rock-Eval results are given in Table 4 for 15 samplesfrom Site 719. Hydrogen Index (HI) and Oxygen Index (OI) val-ues are plotted on a van Kreveln-like diagram (Fig. 10). The HIand OI values range from 0 to 74 and from 78 to 554, respec-tively, and fall in the range for Type III Kerogens derived fromterrestial wood. Tmax values range from 231 to 435°C, which in-dicates that organic matter is immature with regard to petro-leum generation as was found at Site 717. These results for Sites717,718, and 719 suggest that a major part of the organic mate-rial in the sediment encountered on Leg 116 was transportedfrom the Ganges-Brahmaputra River and delta plain.

INORGANIC GEOCHEMISTRYSite 719 is located about 3.2 km south of Site 717. At Site

719 one should expect about the same lithology with some thin-

Core-Section(Interval, cm)

Hole 719A

1H-2, 0-52X-1, 118-1234X-1, 0-55X-1, 0-56X-1, 0-57X-1, 0-58X-1, 0-511X-1, 0-512X-1, 0-513X-1, 0-515X-1, 0-516X-5, 0-517X-5, 0-518X-1, 0-519X-3, 0-520X-5, 0-521X-2, 0-522X-5, 0-523X-2, 0-524X-1, 0-525X-1, 0-526X-1, 0-527X-2, 0-528X-3, 0-529X-3, 0-53OX-5, 0-531X-5, 0-532X-3, 0-533X-2, 65-7034X-5, 0-535X-2, 0-536X-6, 0-537X-5, 0-538X-5, 0-539X-1, 0-541X-1, 0-542X-1, 0-543X-1, 0-544X-1, 0-546X-1, 0-547X-1, 65-7048X-5, 0-549X-1, 56-61

Depth(mbsf)

1.505.38

23.2032.7042.2051.7061.2089.7099.20

108.70127.70143.20152.70156.20168.70181.20186.20200.20205.20213.20222.70232.20243.20254.20263.70276.20285.70292.20300.85314.20319.20334.70342.70352.20355.70374.70384.20393.70403.20422.20432.35447.20451.26

Ci(ppm)

3.87.1

1361.01890.04271.05494.05982.05521.06525.04302.02140.04705.01641.0549.0205.0

74.015.1

2016.03097.02878.04592.0

148.04683.04923.04381.05029.05312.04785.04782.05056.04673.03244.04413.04571.03531.0

98.0516.0

1486.03510.04730.05063.04818.05003.0

c2(ppm)

1.01.61.21.60.61.71.01.43.72.61.51.61.12.23.02.83.81.42.62.81.31.02.01.90.61.92.41.71.62.14.70.71.72.81.71.11.02.41.72.54.63.14.9

C i / C 2

3.84.4

1134.01181.07228.02616.05982.03943.01764.01655.01427.02941.01492.0250.068.026.04.0

1440.01191.01028.03532.0

148.02341.02591.07301.02647.02213.02815.02989.02408.0

994.04634.02596.01632.02077.0

89.0516.0619.0

2065.01900.01101.01554.0

102.0

ning of layers due to the deformation as seen from the singlechannel seismic line of R/V CONRAD Cruise 2706. The chem-istry of interstitial waters (IW) at Site 719 should also be com-parable with that of Site 717.

Sampling and MethodsFluid sampling was carried out mostly at depth intervals of

about 30 m. One successful in-situ pore-water sampling offeredthe opportunity to check possible large deviations in composi-tions due to the sampling procedure and the squeezing effects.As already noticed at the two previous sites, the interstitial wa-ter (IW) recovery was poor in the clay-rich layers (8-10 cm3) andlarger in the silty layers (15-70 cm3). The analytical methodsand procedures employed for IW are the same as those for Site717 and are described in the "Explanatory Notes" chapter.

Results

Chloride (Cl) and Sodium plus Potassium (Na + K)

The concentration of Cl as well as Na + K show large varia-tions ranging over 10% (Fig. 11). The overall variations of Clvs. depth are similar to that of Site 717. The extremes are largerthan at Site 717, and there is a slight depth difference in their lo-cation (about 40-50 m between Sites 719 and 717) at depthsgreater than 125 mbsf, the depths at Site 719 being shallower.

165

2000 4000C| (ppm)

6000 2000 4000

C,/C26000 8000 0.0 0.5 1 .0 1.5

TOC (wt %)2.0 2.5

Figure 9. Concentrations with depth of (A) Cu (B) Q/C2 ratios in gases extracted from sediments by the headspace procedure, and (C) organic car-bon, from Hole 719A.

Na + K variations in the upper 125 mbsf are quite differentfrom those found at Site 717. The lower values obtained at Site719 may be due to more extensive K and Na uptake in clay min-erals. At depths larger than 125 mbsf, one finds the same pat-tern of Na + K vs. depth variations as at Site 717. As for the ex-tremes of Cl, extremes of Na + K are larger than at Site 717.Also the depth profile seems to be shifted 40-50 m upward ascompared to 717 at depths greater than 125 mbsf.

The (Na + K)/Cl ratios decrease in clay-rich layers and arelarger in silt, although always lower than in seawater. Thegeneral trend is toward decrease of (Na + K)/C1 with increasingdepth. This corresponds to the effect of clay diagenesis with up-take of K and Na.

Calcium (Ca)

The Ca vs. depth profile for Hole 719A (Fig. 11) has thesame features as that of Site 717. The general trend is toward de-crease of Ca in the upper 150 mbsf, probably due to depositionof carbonate, and gradual increase of Ca at lower depths. Someshifts are found in the depths of characteristic features:

1. A first minimum is found at 20 mbsf at both locations;2. A small maximum followed by a sharp minimum are

shifted up 30 m compared to Site 717;3. A the last IW sample (about 455 mbsf) shows a decrease

of Ca which might be compared to that found at 505 mbsf atSite 717.

However, at Site 719 this last variation corresponds to a siltlayer in lithologic Unit V and is located under a clay-rich layer,whereas at Site 717 it is located in a silt layer interbedded in the

clays of lithologic Unit IVC. Because this feature of Ca seems tobe quite characteristic of what is otherwise a quite smooth Cavs. depth curve, its location is not in agreement with the litho-stratigraphic observations.

Magnesium (Mg)The general features of the Mg vs. depth curve (Fig. 11) com-

pare well with those of Site 717. However, the decrease of Mg ismore accentuated at Site 719. This may correspond to a moreextensive clay diagenesis and is in agreement with the larger var-iations of (Na + K)/Cl.

The depth profile of Mg is the same as at Site 717 in the first50 mbsf. Between 50 and 125 mbsf the two sites give quite dif-ferent results as Mg presents a small maximum and not a mini-mum as in Hole 717C. Such a maximum may be related to theMg-NH4 exchange in clay minerals and/or inhibition of Mg de-position with dolomite as suggested at Site 717. This suggestsfuture careful studies of clay mineralogy compositions and fine-grained carbonates to decipher this early difference in diagene-sis. Below 140 mbsf the variations of Mg with depth are nearlylinear with local variations due to lithology (clay vs. silt). Wecan estimate dMg/dz = -59 mmol/(L × km) (dashed line,Fig. 11), which is much larger than at Site 717 where dMg/dz =- 28 mmol(L × km). This should correspond to more extensiveuptake of Mg by clay minerals as well as deposition with car-bonates.

Alkalinity (Alk) and Calcium Carbonate SaturationAs for the elements discussed above, the Alk values from

Hole 719A (Fig. 11) are comparable with those at Site 717.There is a shift in the characteristic high values found at 140-

166

SITE 719

Table 4. Rock-Eval data summary from Hole 719A.

Core Section(Interval, cm)

Hole 719A

1H-2, 145-1502X-1, 46-505X-1, 43-536X-1, 47-507X-1, 53-5810X-CC (0-4)11X-1, 110-12012X-1, 74-7613X-1, 36-4014X-1, 140-15016X-6, 138-14217X-4, 140-15018X-6, 90-9419X-3, 81-8520X-4, 140-15021X-3, 35-3722X-6, 91-9523X-1, 140-15024X-1, 66-7025X-1, 52-5626X-1, 77-8728X-6, 60-6429X-2, 140-15030X-6, 103-10731X-6, 23-2732X-2, 140-15033X-2, 59-6234X-6, 85-8935X-2, 140-15036X-4, 90-9437X-6, 39-4338X-4, 140-15039X-CC (17-21)42X-1, 93-10344X-1, 37-4147X-1, 70-7548X-4, 6-1049X-1, 61-66

Depth(mbsf)

2.954.66

33.1342.6752.2380.2090.8099.94

109.06119.60146.08152.60164.60169.51181.10188.05202.61205.10213.86223.22232.97259.30263.60278.73287.43292.10300.79316.55320.60332.60344.59352.10357.22385.13403.57432.40445.76451.31

'max

385

344

231

303

350

316

392

389

308

399

435

361

353

249

272

Si

0.19

0.05

0.03

0.06

0.11

0.05

0.25

0.16

0.05

0.13

0.06

0.08

0.06

0.01

0.02

s2

0.55

0.02

0.00

0.06

0.25

0.08

1.07

0.46

0.07

0.58

0.16

0.35

0.26

0.00

0.01

s3

3.74

1.21

0.63

2.89

3.57

1.94

1.90

3.78

0.88

3.04

0.31

0.83

1.77

0.60

1.00

oc

2.170.540.370.490.120.500.480.490.550.890.390.350.251.661.450.631.932.160.540.360.361.651.820.260.260.400.911.860.840.200.400.620.160.610.390.220.290.13

HI

25

5

0

19

28

23

74

21

19

32

40

42

42

0

OI

172

327

525

602

401

554

131

175

244

167

78

99

274

98

155 mbsf at Site 719 and at 155-185 mbsf at Site 717. This is ashift similar to those for Ca and Mg profiles in the same depthrange.

The variations of the Ion Activity Product (IAP) of CaCO3(Fig. 12) are less scattered than at Site 717. Also, there is novalue above the calcite solubility product. The overall IAP val-ues are close to saturation with 3%-Mg calcite. The comparisonof Ca, Alk, and IAP depth profiles suggests there may be car-bonate deposition in the upper 300 mbsf.

Sulfate (SO4)

The depth profile of SO4 from Hole 719A (Fig. 13) shows thesame features and the same corresponding SO4 values as that ofSite 717. After a sharp decrease in the upper 30 mbsf, a similarsecondary maximum was found near 60-100 mbsf which corre-lates with that of Ca. Below 125 mbsf the SO4 values are nearlyconstant, with local variations which may be due to the oxida-tion of pyrite following coring operations. Below 125 mbsf theSO4 values are about 1 mmol/L, which is 1.5 mmol/L less thanat Site 717. This difference reflects slightly more reducing condi-tions and more extensive microbial degradation of organic mat-ter at Site 719.

Silica (SiO2)

As shown in Figure 13, the SiO2 values decrease in the upper30 mbsf of Hole 719A after the classical maximum near the sea-floor. There is a maximum of SiO2 near 160 mbsf which corre-

lates with that of Alk. It is probably due to dissolution of sili-ceous tests.

Phosphate (PO4) and Ammonia (NH4)In the upper 200 mbsf the values of PO4 and NH4 are high

and correspond to extensive degradation of organic matter. Maxi-mum values of PO4 and NH4 are obtained near 100 mbsf,slightly above those of Alk and SiO2 (150-160 mbsf). Below 200mbsf PO4 is low (2000 mmol/L) and NH4 is quite variable, inthe range 0.9-1.5 mmol/L.

In-Situ Sample

Cl, Ca, Alk, IAP (CaCO3), NH4, and PO4 values are higherin the in-situ sample than in the IW sampled 1.5 to 9.5 m be-low it (the uncertainty in depth is due to poor core recovery),whereas Na + K, SO4, and SiO2 values are lower. Differences areabout 10% for Ca and Alk, 5% for Mg, and less than 1% forCl. However, due to the large variations in compositions withdepth observed in the depth range of the in-situ sampling, suchdifferences should be expected. Thus we can consider that thesampling of IW and the in-situ pore water give concordant re-sults.

DiscussionThe results obtained at Site 719 are quite clear and their pre-

liminary interpretation supplements those acquired at Site 717.The suggested diagenetic effects (carbonate deposition, dissolved

167

SITE 719

1000

800

600

4 0 0

200

' 1I

—I11

1 /

1 f1 1

1 - 1

l'l

"Tj1 ,I ii I

"'I

. ^―L

1

•

"1A

I

—

—

-

in

HI100 200

Oxygen index3 0 0

Figure 10. Hydrogen and Oxygen Indexes obtained from Rock-Eval py-rolysis of sediments from Hole 719A and plotted on a van Krevelen-likediagram.

ions-clay exchanges) will be studied in detail in shore-based lab-oratories for further confirmation.

As expected, the concentrations of the dissolved elements inthe IW samples are nearly the same in equivalent sedimentarylayers sampled some 3.2 km apart. This indicates the role oflithological control as well as diagenetic control on the concen-tration of dissolved elements in IW. Hence the differences, suchas that found for Mg, which seem to be more affected by dia-genesis at Site 719, are also significant and should be investi-gated further.

It is worth noticing that the Ca profiles are similar in the re-spective depth ranges of 320-455 and 370-505 mbsf at Sites 719and 717. This is not in complete agreement with the preliminarydepth assignement of Lithologic Units IVC and V between Sites717 and 719. It is, however, not so contradictory to the assignedages of the sediments at the same depths.

Conclusions

The depth profiles of the major dissolved elements from in-terstitial waters collected at Site 719 are very similar to those ob-tained at Site 717, with some shifts in depths. Hence, most ofthe conclusions given for Site 717 can be applied at Site 719:

1. There is a differentiation in values due to variation in li-thology (i.e., clay vs. silt);

2. The effect of clay diagenesis on Cl and Na + K decreasesin the clay-rich layers and increases in the silt layers;

3. There is an exchange of Ca and Mg in clays; and,4. Deposition of carbonates (probably Mg-rich) occurs at

least in the upper 300 mbsf.

On the basis of (Na + K)/Cl, Mg, Alk, and nutrients, the ef-fects of clay diagenesis and microbial degradation of organicmatter are more extensive at Site 719 than at Site 717.

PALEOMAGNETISM

Remanent Magnetic Intensity

The natural remanent magnetization (NRM) of the archivehalf of each core from Hole 719A was measured at 5-cm inter-vals. Although the range of NRM intensities was similar to thatobserved at Sites 717 and 718, the mean intensity, 18.2 mA/m,is considerably smaller. As a result, at this site only eight sec-tions (i.e., 7%) were too strongly magnetized to be measuredwith the cryogenic magnetometer. The histogram of the NRMintensities (Fig. 14) shows a skewed distribution similar to thatobserved at Site 718. All sections were demagnetized using analternating field of 9 mT, and their remanence was remeasured.After alternating field (AF) demagnetization, all sections couldbe measured with the cryogenic magnetometer. The mean inten-sity of the demagnetized cores, 3.7 mA/m, was much smallerthan that observed at Site 718 but very similar to the value ob-tained at Site 717 (2.5 mA/m).

The remanent intensities after demagnetization were used tocalculate the logarithmic mean values for the lithologic Units IIthrough V (See "Lithostratigraphy" section). These values arecompared with those from Sites 717 and 718 in Table 5. Becauseof their proximity and the fact that both sites were located onthe same fault block, it was anticipated that the values for Sites717 and 719 would be very similar. In the case of Units II and V,however, the values were significantly lower at Site 719, althoughthe recovery was poor for both units and the values obtainedmay not be representative. Unit III, on the other hand, ap-peared to be much more strongly magnetized at Site 719 than atSite 717 and was closer to Site 718 in intensity (Table 5). Of thevarious units, only the mean intensities for Unit IV at Sites 717and 719 were comparable. The intensity of Unit IVA agreed re-markably well, though for Site 719 the mean value was based ononly 37 measurements. Although somewhat more strongly mag-netized at Site 719 than at Site 717, the mean intensity of thelower part of Unit IV was nevertheless much closer to that ofthe same unit at Site 717 than at Site 718.

With the limitations imposed by the poor recovery, it wasdifficult to directly compare many of the units from site to site.Unit III, however, was an exception as there was good recoverythroughout this depth interval. On the basis of the values ateach of the three sites, it appeared that a significant part (about60-70 m) of the upper, more weakly magnetized portion of thisunit is absent at Sites 718 and 719.

Comparison of the remanent intensities with the volume mag-netic susceptibility measurements yielded several relatively goodcorrelations. For example, between depths 140 and 205 mbsfboth parameters showed large intensity values suggesting thatferromagnetic grains were responsible for both signals. As notedat previous sites, however, there were several zones of high-inten-sity remanence (e.g., 40-90 mbsf) where the susceptibility waslow or moderate and vice versa. Correlations of the remanentintensity patterns between Site 717 and 719 were attempted butfound to be too uncertain to be used without further constraints.

Magnetic Inclination

In a similar manner to both Sites 717 and 718, the NRMmagnetic inclinations at Site 719 were biased toward large nega-tive inclinations reflecting the coherent vertical overprint thathas been ascribed to the drill pipe or core barrel. In the case ofSite 719, however, the negative bias appeared to have been al-most completely removed by the demagnetization process. InFigure 15, histograms of inclination values before and after AFdemagnetization indicate a change in the peak value from about-45° to about -5°. The positive and negative values of incli-nation were analyzed separately to determine the amplitude ofany residual inclination bias in the demagnetized values. The

168

SITE 719

100

~ 200maE

300

400

500

© in-situsample

530 550 430Chloride <mmoi/L)

460 490 0 10 20 30 40 50 0 6 12 18NatK (mmol/U Calcium (mmoi/LO Magnesium (mmol/U Alkalinity <mmoi/L)

Figure 11. Plot of Cl, Na + K, Ca, Mg, and alkalinity concentration vs. depth for Hole 719A. For Mg the dashed line shows estimated linear trend.For alkalinity the dashed line shows trend of values from below 200 mbsf.

100

200

1S•a 300

400

500

3× M<j Calcite/•-equilibrium

Calcite"equilibrium

500

' Araqoniteequilibrium

© in -situsample

1000

Figure 12. Plot of calcium carbonate ion activity product (IAP) vs.depth for Hole 719A. The 3%-Mg calcite, calcite, and aragonite equilib-rium curves have been corrected for pressure and temperature effects atin-situ conditions.

mean values of positive and negative inclinations obtained were24.5° and -20.1°, respectively, and imply a small positive biasof about 2°. In view of the large standard deviations of thesemean values (about 20°) and the obvious negative nature of theoverprint, the +2° bias was considered insignificant. However,though it may be assumed that the negative overprint has beeneffectively removed from the core as a whole, it must be remem-bered that individual cores were affected by the overprint to agreater or lesser degree. The overall mean inclination was ap-proximately 22°, somewhat steeper than expected at this site formaterial less than 6 Ma. Adjustments to the observed value ofmagnetic inclination, based upon the dip of the sedimentarylayers and the influence of far-sided long-term nondipole ef-fects, are discussed in the Site 718 chapter. As rocks older than6 Ma were not sampled at Site 719, the maximum expected incli-nation for an axial dipole is about 6°, taking into account platetectonic drift. It was difficult to account for the observed valuesolely in terms of the factors mentioned above; it appears likelythat there are other effects, possibly related to the biscuiting ofthe core material by the drilling procedure, that also contributesystematically to this disparity.

A study of variations in inclination with depth was carriedout by determining the mean inclination for each successivegroup of 800 measurements. Core recovery at this site was sopoor in the upper 140 mbsf that it is meaningless to calculatemean inclinations based upon uniform depth intervals. Not sur-prisingly, the analysis failed to show any appreciable inclinationincrease, as only a modest increase was expected and the uncer-tainty in each mean value was large.

The relatively poor recovery and the use of the XCB forCores 116-719A-2X through -719A-49X made any attempt atmagnetostratigraphy fruitless. A somewhat cursory look at themagnetic polarity based upon the inclination values yielded twozones where the field seems to be dominantly normal or domi-nantly reverse. The first was between 140 and 160 mbsf wherethe core material is normally magnetized, the second was be-

169

SITE 719

100

C 200

400

300 -

400 -

50010 20

SO4 <mmol/L)30 0 200 400 600 800

SiO2 <µmol/L)

Figure 13. Plots of SO4 and SiO2 concentration vs. depth for Hole719A.

tween 308 and 332 mbsf and indicated reverse magnetization.Comparison of these zones with the biostratigraphic control(see "Biostratigraphy" section) suggested that the first may cor-respond with the Olduvai normal polarity period (Chron 2),while the second may coincide with the reversed interval be-tween 4.79 and 5.41 Ma (Chron 3r). Such correlations, however,are extremely tentative, as the variations in inclination valuesare large and the core recovery poor.

Magnetic DeclinationA histogram of the magnetic declination values before and

after AF demagnetization is shown in Figure 16. The distribu-tion of NRM declination values was fairly flat, reflecting an al-most random distribution of values. This is in marked contrastto Site 718 where the values were not randomly distributed buthad a major peak near 180°-200°. Unlike Site 718 where the AFdemagnetization reduced the azimuthal bias, in the case of Site719 the AF values were more highly peaked than the NRM val-ues with a maximum between 60° and 80° suggesting that theoverprint in this case may have been due to the demagnetizationprocess itself. If so, then the preferred orientation may havebeen the result of an anhysteretic remanence produced by thedemagnetizing coils in the cryogenic magnetometer. In supportof such an interpretation, there is also a peak in the histogramof the AF demagnetized declination values for Site 718 that liesbetween 80° and 100°.

Magnetic SusceptibilityVolume magnetic susceptibility measurements were made at

5-cm intervals on 1.5-m whole core sections from Hole 719A. Inall, 3,427 readings were recorded from these cores. The suscepti-bility values displayed a similar range to those found in coresfrom Sites 717 and 718. For Hole 719A cores, the lowest re-corded susceptibility was 9.0 × 10~6 cgs, in Core 116-719A-16X, whereas the highest was 5.7 × 10~4 cgs, in Core 116-719A-31X.

z 200 —

0 1Log (intensity)

Figure 14. Histogram of logarithmic intensity of magnetic remanencefor (A) NRM, and (B) 9-mT AF demagnetization measurements forHole 719A.

The character of the susceptibility record from Hole 719Acores was also similar to that of Hole 717C cores. Althoughpoor recovery in the silt layers at Site 719 made comparison dif-ficult, the susceptibility values in these layers appeared mod-erately high and scattered as at Site 717. Better recovery wasachieved in the mud layers of Hole 719A. They displayed a rela-tively low background susceptibility of about 1.0-3.5 × 10~5

cgs punctuated by peaks, often in excess of 1.0 × 10~4 cgs, thatwere well-correlated with dark-gray mud turbidites. These char-acteristics were also typical of similar mud layers in Hole 717Ccores.

Table 5. Logarithmic mean remanent inten-sity (mA/m) after 9-mT AF demagnetiza-tion of sediment units encountered on Leg116.

Unit

IIIIIIV

V

I-V

Subunit

allABCallAB

Site 717

20.11.31.24.51.00.67.4——2.5

Site 718

24.56.7

18.2———6.46.06.77.4

Site 719

13.55.02.75.02.7*

—3.3—_

3.7

* includes B thru D

170

SITE 719

400

200 —

400

-90 -60 -30 0 30inclination (degrees)

60 90

Figure 15. Histogram of magnetic inclinations for (A) NRM, and (B)9-mT AF demagnetization measurements for Hole 719A. Dashed linesin (B) indicate arithmetic mean values of positive and negative inclina-tions.

z 200 —

90 180 270Declination (degrees)

360

Figure 16. Histogram of magnetic declinations for (A) NRM, and (B)9-mT AF demagnetization measurements for Hole 719A. Horizontaldashed lines indicate distribution of declinations expected for a randomdistribution of values.

On a large scale, the Hole 719A susceptibility measurementsshowed a pattern of low and high values similar to those ofHole 717C. Between 140-210 and 250-360 mbsf the recoveredsediments gave many large susceptibility spikes (Fig. 17). Addi-tionally, high-amplitude peaks were also measured in the lastcore of Hole 719A. Each of these three pulses of high-suscepti-bility value corresponded to a section of gray mud, whereas theintervening lower values corresponded to silty sediments. Theshallowest susceptibility pulse encompassed Unit III, and thepulse below it, Units IVB-IVD. The high-susceptibility valuesfound in cores taken near the bottom of the hole were mainlyfrom a gray mud layer in Unit V. Given lithologic (see "Litho-stratigraphy" section) and biostratigraphic (see "Biostratigra-phy" section) constraints, the susceptibility pulses from Hole719A were correlated with those measured in Hole 717C as shownin Figure 18. The high values between 140-210 and 250-360mbsf in Hole 719A cores probably correspond to the high valuesfound between 250-320 and 390-480 mbsf in Hole 717C cores.Furthermore, the high-susceptibility values observed in the lastcores of Hole 719A may correspond to those measured begin-ning at 570 mbsf in Hole 717C cores.

If these correlations are valid, they have several interestingimplications. Both of the upper susceptibility pulses from Hole719A were approximately the same width as measured in Hole717C, yet they were found closer together and shallower in Hole719A (Fig. 18). This suggests that these particular sedimentaryunits have remained intact whereas those surrounding them have

been either eroded or not deposited. It appears that the less-magnetic upper part of Unit III observed in Hole 717C is notpresent in Hole 719A, a conclusion that also fits the remanentmagnetic intensity data discussed above. Additionally, these re-sults suggest that only about half of Unit IV between 310-400mbsf, sampled in Hole 717C, is found in Hole 719A. However,the distance between the deeper susceptibility pulses observed inHole 719A cores were the same as those observed in Hole 717C,suggesting that this section is intact in Hole 719A cores.

Although the correlation of large-scale susceptibilty featuresbetween Sites 717 and 719 is good, the correlation of fine struc-ture is not obvious. The individual susceptibility peaks, causedby turbidites, could not be traced from one hole to the otherwith any confidence. A comparison of the upper susceptibilitypulses in both holes showed that the susceptibility peaks fromHole 719A were noticeably narrower and decidedly more nu-merous than those from Hole 717C. Within this portion of thecore, four times as many peaks with amplitudes in excess of 1.0× 10~4 cgs were found in the former as in the latter. The rela-tive narrowness of the Site 719 turbidite layers compared to Site717 might be expected if Site 719 were located updip from Site717 due to vertical tectonics. However, the greater number ofturbidite layers found in Hole 719A is then unexpected and dif-ficult to explain. One possibility that must be considered is thatthe correlation between holes made on the basis of the broad-scale susceptibility features was in error. These differences werenot found when comparing the second susceptibility peak down-

171

SITE 719

100

200

CD

Q

300

400 h

200 400 600

k (10"6 cgs)

800

Figure 17. Whole-core volume magnetic susceptibility (k) plotted vs.depth for Hole 719A.

hole. In this section of each hole the susceptibility peaks were ofsimilar width and number. However, in this case as before, itwas difficult to make a one-to-one correlation of susceptibilitypeaks from hole to hole. With more detailed lithologic and agecorrelations, this task might prove tractable.

PHYSICAL PROPERTIES

Core 116-719A-1H was obtained by the Advanced Piston Core(APC). Cores 116-719A-2X through 116-719A-49X were recov-ered by the Extended Core Barrel (XCB) operations. Physicalproperties measured routinely on all cores recovered at Site 719include GRAPE density and thermal conductivity from full-round core sections, vane shear strength, compressional wavevelocity, and index properties from split sections. The methodsused are described in the "Explanatory Notes" chapter. Resultsof physical properties measurements at Site 719 are summarizedin Tables 6 to 9.

Index Properties

Index properties determinations included wet-bulk density,water content (related to wet weight), porosity, and grain density(Table 6). The measurements were mainly made on fine-graineddeposits because silty to sandy sediments were too disturbed bydrilling to determine index properties. The scatter of the datapoints are attributed to disturbance by drilling, variations insediment composition, and measurement errors.

Porosity

The porosity of the sea-floor sediments is approximately 80%(Fig. 19). The porosity decreases within the upper 100 m nearlyexponentially to a value of 55%. Below that depth the decreaseis almost linear with a rate of 2.86% per 100 m. The porosity atthe bottom of the hole averages 45%. As previously noted atSites 717 and 718, the porosity values show a range of scatter-ing, especially in the compositionally more complex lithologicunits.

Wet-Bulk Density

Wet-bulk density (Fig. 19) increases from 1.37 g/cm3 at theseafloor to about 1.75 g/cm3 at 100 mbsf with a rate of 0.38 g/cm3 per 100 m. Below this depth, the rate of increase decreasesto 0.07 g/cm per 100 m. The wet-bulk density at the bottom ofthe hole averages 2.00 g/cm3.

Water Content

The wet-water content decreases nearly exponentially from asea-floor value of about 60% to about 30% at 100 mbsf (Fig.19). Below that depth, the rate of decrease is nearly linear with arate of 1.43% per 100 m. The water content averages 25% at thebottom of the hole.

Grain Density

The grain density has averages ranging between 2.75 and2.85 g/cm3 (Fig. 19). Grain densities between 2.60 and 2.90 g/cm3 are considered to be reliable and to be related to variationsin mineralogy and organic carbon content.

Compressional Wave Velocity

Λ> Sonic velocity measurements range from 1.514 km/s at themud line to about 1.7-1.8 km/s at the bottom of Hole 719A(Table 7; Fig. 19). Because of poor recovery and drilling distur-bances, only two sonic velocity measurements were done in lith-ologic Units I and II (interval 0-135 mbsf). Lithologic Unit III(135-207 mbsf) shows homogeneous velocities ranging around1.6 km/s with a slight increase with depth (0.016 km/s per100 m). Lithologic Unit IVA (207-240 mbsf) again correspondsto a gap on the sonic velocity dataset. Lithologic Unit IVBshows a less homogeneous dataset than Unit III but a highergradient (0.077 km/s per 100 m) can be observed in this unit.The few data collected in lithologic Unit V have an average of1.7 km/s.

Drilling disturbances affect the sonic velocity measurements:values on the same core and same lithology range from typically1.55 km/s in Sections 1 to about 1.65 km/s in Sections 6 and/orcore catchers.

A systematic anisotropy (up to 8%-10%) was observed onrelatively well-consolidated samples, the horizontal velocities be-ing always higher than the vertical velocities.

Thermal Conductivity

The thermal conductivity measurements were routinely per-formed on each section from the mud line to the bottom of thehole (Fig. 19). Thermal conductivities measured at Site 719 wereused for evaluation of the heat flow in connection with tempera-

172

SITE 719

6000 200 400 600 800 1000

k (1Q•6 cgs)

0 200 400 600 800 1000k (10"6 cgs)

Figure 18. Possible correlation of magnetic susceptibility values from Hole 719A (left) with 717C (right).Hatchured bands show suggested correlations of high-susceptibility sections of core.

ture measurements in Hole 719A (see "Heat Flow" section, thischapter). The conductivities measured range from 1.95 × 10~3

cal/(°C × cm × s) at the top of the hole to maximum values ofabout 5.2 × 10~3 cal/(°C × cm × s). The thermal conductiv-ity dataset, characterized by highly variable values, appears dif-ficult to interpret. A mean thermal conductivity of 3.34 × 10~3

cal/(°C × cm × s) was calculated for Site 719.

Undrained Shear Strength

The undrained shear strength was measured in Hole 719Adown to 349.8 mbsf, below which core material was too dis-turbed to make valid measurements. No measurements were car-ried out between 4.9 and 120.2 mbsf because of low core recov-ery and the large amount of drilling disturbance. Values ob-tained for undrained shear strength are presented in Table 9 andare plotted vs. depth in Figure 19.

The undrained shear-strength vs. depth profile shows fourmajor trends within the tested sections of Hole 719A.

1. Depths 2.24-120.21 mbsf: the shear strength increasesslightly within the upper interval from 9.5 kPa at 3.79 mbsf to49.7 kPa at 120.21 mbsf.

2. Depths 138.38-200.03 mbsf: the shear strength for this in-terval shows considerable scatter. The range of the shear-strengthdata range between 125.3 kPa (141.05 mbsf) and 397.2 kPa(166.7 mbsf). This interval corresponds to lithologic Unit III.The boundary between Units II and III is marked by a sharpincrease of shear strength from 49.7 kPa at 120.21 mbsf to132.4 kPa at 138.38 mbsf. Erosion may provide an explanationfor this sharp transition.

3. Depths 202.54-251.68 mbsf: the shear-strength data forthis interval are very low and consistent compared to the upperand lower regions. The values range from 208.1 kPa (204.8 mbsf)to 262.4 kPa (206.17 mbsf). This interval corresponds to the siltyturbidites of lithologic Unit IVA.

4. Depths 252.96-349.8 mbsf: the shear-strength data are veryscattered within this interval. The values vary between 122.9 kPa

173

SITE 719

Table 6. Index physical properties from Hole 719A.

Core

1H1H1H1H2X5X6X7X8X

10X11X12X13X14X16X16X16X16X16X16X16X17X17X17X17X17X17X17X18X18X18X18X18X18X18X18X19X19X19X19X20X20X20X20X20X20X20X21X21X21X22X22X22X22X22X22X23X23X24X25X27X27X28X28X28X

Sec./CC

123

CC11111

CC1112123456712345671234567

CC123

CC1234567123123456121112123

Top(cm)

786060

1462447

1121

88743646

110258060

101138449980

11245903612

130140

1470

13490324

801058135

1461301467070

12431699035

11033

1361338491

105906652

116554021

108

Bot.(cm)

8264646

502850

3174

90764050

114298464

10514248

10384

11649944016

134144

1874

13894368

841098540

1501341507474

12835739237

11437

1401378895

109947056

120594425

112

Depth(mbsf)

0.782.103.604.104.66

32.9442.6751.7161.3280.2190.5899.94

109.06120.16138.30138.95141.00142.30144.21146.08146.64147.69149.00150.82151.65153.60154.56155.82157.50159.10159.34161.40163.54164.60165.52165.77166.50168.25169.51170.04176.66178.00179.66180.40181.90183.94184.51185.39187.10188.05195.30196.03198.56200.03201.04202.61204.75206.10213.86223.22242.86243.75251.60252.91255.28

Watercontent

(%)

59.6962.0235.8143.9745.6232.1629.0431.6633.5030.3528.3645.8026.7731.6725.5333.9734.6134.0634.1233.5730.0134.9935.0234.0735.2235.2729.5230.8033.5733.7631.9032.7129.5034.6831.0327.8131.5033.6532.7729.6429.7735.8332.4628.8932.4631.5227.7631.1333.0735.4327.9831.3232.7529.7131.8133.7833.7326.5428.2827.3828.7627.3028.8027.9928.21

Porosity

w80.1881.0161.6668.2069.6657.0252.5956.1757.5354.4252.1570.1850.1555.6448.3957.6557.9158.1658.8057.4553.1060.8356.6357.4159.9660.6353.4055.5857.3155.9155.2757.2625.5052.4653.4750.2360.1456.3656.1952.9453.2558.2255.0252.2553.1050.6951.5254.8154.3560.0851.6553.2456.4453.1756.3456.6358.5549.9852.2651.6252.6651.5052.4251.7752.41

Bulkdensity(g/cm3)

1.381.361.681.621.591.841.881.831.791.861.882.481.931.831.961.771.751.751.781.781.851.751.741.771.761.761.861.861.791.741.781.801.851.781.801.891.801.751.811.851.841.731.781.851.791.791.911.821.771.751.901.811.791.871.831.781.781.931.891.911.881.911.881.921.89

Dry bulkdensity(g/cm3)

0.550.511.080.910.871.251.331.251.191.291.351.351.411.251.461.171.151.151.171.191.301.141.131.161.141.141.311.291.191.151.221.211.311.161.241.371.231.161.221.301.301.111.201.321.211.221.381.251.191.131.371.241.211.311.251.181.181.421.361.391.341.391.341.381.36

Graindensity(g/cm3)

2.752.622.922.762.772.842.752.802.722.782.792.822.792.742.782.682.632.732.792.712.682.922.452.642.792.862.782.852.692.522.672.790.872.102.592.663.332.582.672.712.722.532.582.732.392.262.802.722.442.782.792.532.702.722.802.592.812.812.822.872.792.872.762.802.84

174

SITE 719

Table 6 (continued).

Core

28X28X28X28X29X29X29X30X30X30X30X30X30X30X31X31X31X31X31X31X32X32X32X33X33X34X34X34X34X34X34X34X35X35X35X35X36X36X36X36X36X36X37X37X37X37X37X37X38X38X38X38X38X38X39X42X44X47X48X48X48X48X48X48X

Sec./CC

456712312345671234561231212345671234123456123456123456

CC

cc11123456

Top(cm)

431156020

126804565

105185352

1034762

105125

154

231469153

1245980

10685636185

1120108124

1148100104539990

13010783

14558394890

11490

13322171

3754

13785956

7730

Bot.(cm)

471196424

132844869

109225756

1075166

109129

198

271509557

1276284

110896765897

124112128

515010410857

10394

13411184

15061435294

11894

13726213

4156

1418999108134

Depth(mbsf)

256.13258.35259.30260.40261.96263.00264.15270.85272.75273.38275.23276.72278.73279.67280.32282.25283.95284.35285.74287.43290.66291.61292.73299.94300.79309.00310.76312.05313.33314.81316.55317.21318.90320.28321.94322.21328.68329.70331.24332.23334.19335.60338.00339.27340.53342.65343.28344.59346.68348.60350.34351.60353.53353.92357.22385.24403.57432.24442.57443.55445.15445.76447.97449.00

Watercontent

(%)

32.5128.4332.7724.9548.6730.256.30

28.7034.2429.1033.3633.3123.7031.9726.5633.3529.8526.1028.8026.3334.4733.4225.0631.1926.6029.1128.2425.1932.0027.7430.2824.9433.5230.5528.4226.7035.2730.8324.7531.5631.1925.8431.0831.5125.8823.5729.3423.4530.4025.5833.7331.5835.4527.5921.6024.2122.6523.5125.0228.9924.0528.4723.6524.49

Porosity(%)

57.6952.7257.6947.7365.0554.7320.1352.7158.9552.4757.6058.1946.7653.4349.6858.0553.4949.5151.5449.9760.1357.1448.1555.2349.8653.1652.5847.8656.1551.3054.6948.0558.3654.2252.2850.1359.4954.8847.4156.0154.9849.1654.8355.2548.8846.6753.2345.6054.2748.8358.5455.6458.6652.7843.7747.0744.9245.9248.3953.2747.0453.1643.0747.24

Bulkdensity(g/cm3)

:1.81.92

1.821.971.991.88.88

1.881.801.881.801.832.061.841.931.791.861.971.881.981.811.801.97.84.94.88.92.98.81.91.87.98.77.83

1.891.921.751.832.591.831.441.981.831.821.941.991.872.001.861.951.791.851.761.932.091.992.031.991.971.881.991.922.022.00

Dry bulkdensity(g/cm3)

1.221.381.221.471.021.311.761.341.191.331.201.221.571.251.421.201.311.451.341.461.191.201.481.271.421.331.381.481.231.381.311.491.171.271.351.411.131.261.951.260.991.471.261.241.441.521.321.531.291.451.191.271.131.401.641.511.571.531.471.341.511.381.541.51

Graindensity(g/cm3)

2.872.852.842.791.972.833.822.812.792.732.752.822.872.472.772.802.742.822.672.832.912.692.822.762.782.802.862.772.762.782.822.822.822.732.802.802.732.772.782.802.732.822.732.722.782.882.782.782.762.822.812.762.622.982.872.832.832.802.852.832.852.892.482.80

175

100

200

8-

300

400

500

X

f.m*

jKλ

h .

%

. 1 1

i.^ *

•5

%-

1*

• •••36

#••M

.1.1

•

• V

I . I .

t

20 60 100 1.0 2.0 aθ 0 40 80 2.0 2.75 3£> 1400 1600 1800 0 2 4 6 0 200 400 600 0 2000 4000 OX) 02 OA OJ6Porosity Wet-bulk Water Grain P-wave Thermal Undrained Overburden Cg/po

(%) density Content density velocity conductivity shear strength pressure ratio(g/cm3) {% wet wt) (g/cm3) (mis) (i0-3Cal/oC cm 8) (kPa) (kPa)

Figure 19. Porosity, bulk density, water content, grain density, sonic velocity, thermal conductivity, shear strength, effective overburden pressure, and the ratio of undrained shear strength cu toeffective overburden pressure p0 vs. depth at Hole 719A. In the last plot, sediments with a ratio of less than 0.2 are underconsolidated.

SITE 719

Table 7. Sonic velocity from Hole 719A. Table 7 (continued).

Core

2X5X

16X16X16X16X16X16X17X17X17X17X17X17X17X18X18X18X18X18X18X18X18X19X19X19X19X20X20X20X20X20X20X20X21X21X21X22X22X22X22X22X22X23X23X24X25X27X28X28X28X28X28X28X

Sec.

1123456712345671234567

CC123

CC123456712312345612112123456

Top(cm)

4624258060

101138449980

11245903612

130140

1470

1349032

4105808135

1301461467070

12431699035

11033

1361338491

105906652554021

10843

11560

Bot.(cm)

5028298464

10514248

10384

11649944016

134144

1874

1389436

8109848540

1341501507474

12835739237

11437

1401378895

109947056594425

11247

11964

Depth(mbsf)

4.6632.94

138.95141.00142.30144.21146.08146.64147.69149.00150.82151.65153.60154.56155.82157.50159.10159.34161.40163.54164.60165.52165.77166.75168.00169.51170.04176.50178.16179.66180.40181.90183.94184.51185.39187.10188.05195.30196.03198.56200.03201.04202.61204.75206.10213.86223.22243.75251.60252.91255.28256.13258.35259.30

Sonicvelocity(m/s)

1514.81500.91596.41587.11569.11583.81583.81605.41593.01567.21600.91575.81576.21607.21576.51598.31586.11602.51599.81599.61586.81595.81655.51605.61576.21615.41632.21623.21600.51604.31584.81617.61614.51663.01591.21604.31584.51597.51595.11599.51613.31608.21596.41587.11610.61568.01533.91600.61613.11598.61612.61617.41644.81623.3

Core

28X29X29X29X30X30X30X30X30X30X30X31X31X31X31X32X32X33X33X34X34X34X34X34X34X34X35X35X35X35X36X36X36X36X36X36X37X37X37X37X37X37X38X38X38X38X38X39X44X48X48X48X48X

Sec.

71231234567124623121234567123412345612345613456

CC12356

Top(cm)

20126804565

105185352

1034762

10515239153

1245980

10685636185

1120108124

1146100104539990

13010783

145583948

11490

13322173785957730

Bot.(cm)

24132844869

109225756

1075166

10919279557

1276284

110896765897

124112128

515010410857

10394

13411184

152614352

11894

13726214189998134

Depth(mbs0

260.40261.96263.00264.15270.85272.75273.38275.23276.72278.73279.67280.32282.25284.35287.43291.61292.73299.94300.79309.00310.76312.05313.33314.81316.55317.21318.90320.28321.94322.21328.66329.70331.24332.23334.19335.60338.00339.27340.53342.65343.28344.59346.68350.34351.60353.53353.92357.22403.57443.55445.15447.97449.00

Sonicvelocity

(m/s)

1645.41627.41628.01612.91593.41608.21605.91622.01611.41677.01626.51611.41558.41614.71621.51658.61626.51639.81699.31653.71628.51677.81693.91672.51710.21637.71724.81655.21642.61780.41646.01660.71685.61674.61682.81731.21659.01673.41734.91690.61644.61680.91657.31686.31758.21654.41672.71803.41637.01649.91627.51740.91753.1

and 543.8 kPa at 270 and 349.8 mbsf, respectively. This intervalcorresponds to lithologic Unit IV.

The variations of shear-strength values within individual unitsappear to be facies controlled.

State of Consolidation

Methods used to estimate the state of consolidation of thesediments are described by Skempton (1970) relating undrainedshear strength (cu) to effective overburden pressure (Po).

Po was calculated for every depth interval (Fig. 19) as de-scribed, for instance, by Richards (1962).

The relation of undrained shear strength vs. effective over-burden pressure (cu/P0) is shown in Figure 19. The cu/P0 rela-tionship shows that most of the sediments at Site 719 are slightlyunderconsolidated, with the exception of the upper 5 m of the

sediments and the interval 138-200 mbsf. The sediments withinthese intervals are slightly overconsolidated or, rather, normallyconsolidated.

Overconsolidation of near-surface sediments could be causedby early cementation because apparent overconsolidation seemsto be a widespread phenomenon in pelagic deep-sea sediments(Einsele, 1982). Removal of overburden or slumping may pro-vide an explanation for the high cu/Po values within lithologicUnit III. The conditions that would lead to underconsolidationare high rates of deposition, low permeabilities, and the pres-ence of gas in the pore space of the sediments. The conditionsof underconsolidation displayed by Site 719 sediments are at-tributed to high rates of sedimentation and low permeabilitiesof clays.

HEAT FLOWThe region of intraplate deformation in the central Indian

Ocean is characterized by heat-flow measurements that show a

177

SITE 719

Table 8. Thermal conductivity measurements fromHole 719A.

Table 8 (continued).

Core

1H1H1H2X4X5X6X7X7X

11X11X12X12X13X13X13X13X14X14X15X16X16X16X16X16X16X16X17X17X17X17X17X17X17X18X18X18X18X18X .18X19X19X19X19X20X20X20X20X20X20X20X20X21X21X21X21X22X22X22X22X22X22X22X22X23X23X24X

Sec.

1231111111111111112112345671234567134566

CC12312345566112212345566121

Top(cm)

11562607020203939392071207112274371

1006040

11062

11062

1106221

11162

11162

1116240606060

10763

11429

11062678787878760

11360

11361

10043

1246060606030743080

1006060

Bot.(cm)

11663617121214040402172217213284472

1016141

11163

11163

1116322

11263

11263

1126341616161

10864

11530

11163688888888861

11461

11462

10144

1256161616131753181

1016161

Depth(mbsf)

1.152.123.604.90

23.4032.9042.5952.0952.0989.9090.4199.4099.91

108.82108.97109.13109.41119.20120.30128.10138.30139.32141.30142.32144.30145.32146.41147.81148.82150.81151.82153.81154.82156.10156.80159.80161.30163.27164.33164.84165.29166.80167.82169.37176.07177.57179.07180.57181.80182.33183.30183.83185.31185.70186.63187.44194.80196.30197.80199.30200.50200.94202.00202.50204.70205.80213.80

Thermalconductivity(10~ 3 Cal/

(°C × cm × s)

2.45051.95123.08672.84175.01993.49804.14944.83593.76353.47143.73603.30622.87902.86302.14623.40264.13004.50654.04634.24793.76902.86602.70402.97102.65202.58702.75002.71102.62102.74802.65203.11402.95903.06403.03664.36063.01573.21403.40702.94902.90902.90602.76302.87902.73913.34933.15092.82082.94242.96073.39602.93102.78093.02523.22853.46543.35113.80232.96502.94282.78202.85423.13922.86922.72602.81463.8690

Core

25X26X27X27X28X28X28X28X29X29X29X30X30X30X30X30X30X30X31X31X31X31X32X32X32X33X33X34X34X35X35X35X35X36X36X36X36X36X36X37X37X37X37X37X37X38X38X38X38X38X38X39X40X41X42X43X44X45X46X47X48X48X48X48X48X48X48X

Sec.

1112123412312345671235123122312341234561234561234561111

1234567

Top(cm)

4060

11570

11560

11560

1156060

11060

11060

1106030

11062

1101101108257

1106262

11060606020

11011011011011062

11360

113604747474790909015701414705040403040

11060

11060

1106030

Bot.(cm)

4161

11671

11661

11661

1166161

11161

11161

1116131

11163

1111111118358

1116363

11161616121

11111111111111163

11461

114614848484891919116711515715141413141

11161

11161

1116131

Depth(mbsf)

223.10232.80242.85243.90252.35253.30255.35256.30261.85262.80264.30271.30272.30274.30275.30277.30278.30279.50280.80281.82283.80286.80290.30291.52292.77299.80300.82310.32312.30318.30319.80321.30322.40328.30329.80331.30332.80334.30335.32337.83338.80340.83341.80343.17344.67346.67348.17350.10351.60353.10353.85356.40365.34374.84384.90394.20403.60413.10422.50432.10442.30443.30445.30446.30448.30449.30450.50

Thermalconductivity(10 ~ 3 Cal/

(°C × cm × s)

4.36963.84403.02093.49984.33663.86903.22953.82123.90803.20913.07213.58762.70513.16142.80142.51173.41282.86594.03303.48803.08002.72902.93504.57403.35702.70863.58264.24613.48073.73263.05243.69893.10582.82423.06983.81533.50003.73823.00122.90603.91303.66743.75793.02603.03073.11773.83612.93872.95663.72772.85373.77104.41003.48655.23383.38194.53664.09545.17552.93313.52833.15284.09353.66793.12443.98334.0089

great deal of variation, but average higher than theoreticallypredicted for seafloor of its age (Weissel et al., 1980; Geller etal., 1983). The heat-flow measurements made during the sitesurvey confirmed that both of these observations, made on a re-gional scale from scattered data points, also are found on a lo-

cal scale within the region investigated on Leg 116 (Fig. 20).Measurements ranged from 44 to 166 mW/m2 with a mean of83.7 mW/m2 and a standard deviation of 21.7 mW/m2.

The presence of such large variations over a short distancepoints to the influence of water flow in the subsurface. This wasconfirmed by temperature measurements at Site 718, which is

178

SITE 719

Table 9. Shear-strength measurements fromHole 719A.

0°50'S F ' ' ' r

Core

1H1H2X

14X16X16X16X16X16X16X16X17X17X17X17X17X17X18X18X18X19X19X19X20X20X20X21X21C22X22X22X22X22X23X23X27X27X28X28X28X28X29X30X30X30X3 OX30X31X31X31X31X31X31X32X32X33X34X34X34X34X34X34X36X36X36X37X38X38X

Sec.

2312123456712345634713

CC3461

CC133461212123431245612345612112344635

CC213

Top(cm)

74797051

118228567

10813540

10487

11860954189

12343

10086307020766515

10229

12813384

11094

112404826

113455762

1006050

10169

112120909091

12495208515

1172085

1101381618504560

Bot.(cm)

75807152

119238668

10913641

10588

11961964290

12444

10187317121776616

10330

12913485

11195

113414927

114465863

1016151

10270

113121919192

12596218616

1182186

111139

1719514661

Depth(mbsf)

2.243.794.90

120.21138.38138.92141.05142.37144.28146.05146.60147.74149.07150.88151.80153.65154.61160.09161.93165.63166.70169.56169.99178.90179.90183.46185.35188.35195.22197.49198.48200.03202.54204.80206.14242.82243.60251.68252.96255.33256.15264.27270.82272.70275.30276.70278.71280.39282.32283.90285.10286.60288.11290.44291.65298.90309.05309.85312.37312.90313.55316.80331.58333.36335.81338.70346.65349.80

Shearstrength(kPa)

11.89.5

16.649.7

132.4245.9125.3245.9257.7236.4234.1260.1210.4253.0219.9274.3215.2149.0224.6312.1397.2283.7357.0248.3231.7127.7229.3212.8144.2245.9373.6264.8238.8208.1262.4248.3227.0224.6262.4253.0364.1375.9122.9437.4401.9385.4366.5255.3375.9364.1231.7328.6293.2229.3345.2432.7373.6437.4236.4401.9281.4394.8267.2253.0293.2210.4449.2543.8

located on a local heat-flow high (Fig. 20). Temperature mea-surements at that site showed a temperature inversion at depth.This observation can be explained by an upward flow of warmwater along the fault to the north of Site 718. This water ap-pears to spread laterally through the permeable silty turbidites

Uplifted• block

Surface fault

l

81°15'E 81°27'E

Figure 20. Site survey for Leg 116 carried out in 1986 on board RobertD. Conrad. Heat flow stations are shown by small crosses with value inmW/m2. Locations of Leg 116 sites are shown by large dots. Alsoshown are seismic reflection lines (heavy solid lines).

of lithologic Unit II. At the same time, cooler sea water is mov-ing downward through permeable horizons within the underly-ing clayey turbidites which appear on seismic records to cropout a few km to the south of Site 718.

Downhole temperature measurements were taken in the sedi-ment at the bottom of Hole 719A preceding Cores 116-719A-8X(61.2 mbsf), -719A-11X (89.7 mbsf), -719A-14X (118.2 mbsf),and -719A-18X (156.2 mbsf). The measurements were made withthe Uyeda/Kinoshita temperature probe described in the "Ex-planatory Notes" chapter. The first three temperature runs weresuccessful, yielding good-quality data. However, on the fourthrun, the temperature probe fell off of the wire line during re-trieval and fell 4000 m back into the hole, slightly damaging theinstrument and resulting in garbled data.

Plots of temperature vs. time are shown for the three success-ful runs in Figure 21. The temperature vs. time curves have acharacteristic shape. The lowering of the probe through the wa-ter is marked by a rapid cooling followed by a relatively constanttemperature as it is held at the mud line prior to insertion. Inser-tion is marked by a sharp rise in temperature caused by fric-tional heating followed by a gradual cooling as the probe coolsto the temperature of the sediments. Pullout may be accompa-nied by another frictional spike and the temperature profile ob-

179

SITE 719

48 96Measurement number

Figure 21. Barnes-Uyeda Probe temperature vs. time record for down-hole temperature measurements. Bars under temperature readings showportion of record used to extrapolate to the equilibrium temperature.(A) Depth 61.2 mbsf preceeding Core 116-719A-8X. (B) 89.7 mbsf pre-ceeding Core 116-719A-11X. (C) 118.2 mbsf preceeding Core 116-719A-14X.

tained during lowering is mirrored while the instrument is raisedto the surface. The portion of the plot during which the probewas inserted into the sediments is marked on each plot. Thetemperature does not return completely to equilibrium duringthe time that the probe is in the sediments, but the temperaturevs. time values can be extrapolated to find the asymptotically-approached equilibrium temperature.

The temperature measurements, extrapolated to equilibrium,are listed in Table 10 and plotted against depth in Figure 22. Thepoints appear to lie on a straight line which can be fit with aslope of 27.67 °C/km (standard error = 0.06). Thermal con-ductivity measurements in the upper 150 m of sediment havegiven average values of 3.2 × 10~3 cal/(°C × cm × s) or 1.34W/(m × °C) at all three Leg 116 sites. This thermal conductiv-ity inplies a heat flow of 37.1 mW/m2. The temperature gradi-ent determined at Site 719 is considerably smaller than that de-termined at Site 717 (39.5 °C/km). The best fitting line to theSite 719 data also predicts a temperature of 2.8 °C at the sea-floor, which is more than 1 °C higher than the observed temper-ature of about 1.7 °C.

However, if the temperature measurements from Site 717 areplotted along with the measurements from Site 719 (Fig. 23), itcan be seen that there is no great difference in the temperatureswith depth at the two sites and all of the measurements appearto fall on a single trend. If a straight line is fit to all five mea-surements, a slope of 34.08 °C/km (standard error = 0.21) isfound with a projected sea-bottom temperature of 2.23 °C. Thebest fitting line when the sea-bottom temperature of 1.7 °C isincluded in the fit has a slope of 37.93 °C (standard error =0.23) and a sea-bottom temperature of 1.84 °C.

It can be argued that the temperature gradient is nearly thesame beneath Sites 717 and 719 and is in the range of 34 to 38°C/km. The resulting heat flow through the fault block underSites 717 and 719 is thus in the range of 45.6 to 50.9 mW/m2.This is very close to, although slightly less than, the theoreticalheat flow for 78-Ma crust, which is 53.5 mW/m2 (Parsons andSclater, 1977).

The fact that the temperature gradient in the upper 150 m ofsediment is the same at Sites 717 and 719 and appears to be lin-ear with depth would tend to suggest that hydrothermal circula-tion is not occurring at shallow depth above the fault block onwhich these two sites are located. This is in marked contrast tothe block immediately to the south, where downhole tempera-tures at Site 718 gave direct evidence for vigorous hydrothermalcirculation. However, the tip of the fault block on which Sites717 and 719 is located is completely buried by the upper siltyturbidite layer and thus the lower units are not directly exposedto sea water as they appear to be on the southern fault block.Therefore, it is probable that such a hydrothermal system wouldnot be developed under the northern block. If this is the case, itwould also be expected that surface heat-flow measurementswould give relatively constant values across the northern block.However, the surface heat-flow pattern is not known across theblock containing Sites 717 and 719 as the site survey of heatflow stopped just north of the fault separating the two blocks(Fig. 20). As a result there is no data other than the downholetemperature measurements on the nature of the thermal regimein the sediments around Sites 717 and 719.

SEISMIC STRATIGRAPHYSite 719 was chosen on the basis of single-channel seismic re-

flection data acquired with a water-gun source during a site sur-vey conducted by R/V Robert D. Conrad on cruise C2706. The

Table 10. Downhole temperaturemeasurements at Site 719.

Depth TemperatureMeasurement (mbsp) (°C)

719A-8X719A-11X719A-14X

61.289.7

118.2

4.525.246.10

180

SITE 719

g

40

80

120

i An

1 X 1

— \

ABX t

— \

A11X V

— A 1

I . I .

1

—

-

—

A"Temperature <βC)

Figure 22. Plot of temperature vs. depth for Hole 719A. Solid line is ge-otherm determined by a least-squares fit to a straight line.

α

100 —

160

Temperature (°C)

Figure 23. Plot of temperature vs. depth for Holes 717C (triangles) and719A (dots). Dashed line is geotherm determined by a least-squares fitto a straight line. Solid line is geotherm obtained when sea bottom watertemperature is included as a datum point.

Conrad seismic line was filtered using a time varying band-passfilter and single-channel migration was employed to improve theresolution and to more precisely locate the fault surfaces. Theprocessing greatly improved the amount of detail that could beobserved in the acoustic stratigraphy and removed most of thediffractions associated with the faults (Fig. 24).

The site chosen was located at time 1418Z 11 July 1986 onthe Conrad seismic record (Fig. 2; see Fig. 20 for track chart).The site is located near the center of one of the tilted faultblocks that make up the tectonic fabric of the area of intraplatedeformation in the central Indian Ocean. It is designed as acompanion site to Site 717, 3.2 km to the north-northwest on

the northern (lower) edge of the same fault block. Site 717drilled a complete syn-deformation sedimentary sequence in theaxis of a synclinal structure developed in the sediments. Site 719is located part way up the fault block in an area where the sedi-mentary section is attenuated, to determine the time of defor-mation and the history of motion on the faults. The sedimentthickness at Site 719 is 1.82 s (two-way traveltime), compared to1.96 s at Site 717.

The seismic stratigraphic section in the region studied by Leg116 can be divided into two first-order acoustic units separatedby a prominent unconformity labeled "A" on Figures 2 and 24.The lower unit is consistently 1.15 to 1.35 s thick and reflectorswithin it are parallel and follow the basement. This unit appearsto represent the pre-deformation sedimentary sequence. Drillingat Site 717 suggested that Unconformity "A" lies within a se-quence of upper Miocene silty turbidites. It was assigned an ageof about 7.5 Ma by interpolation between paleontologic hori-zons. This identification was confirmed at Site 718, althoughdating there suggests a slightly older age. Unconformity "A"thus probably corresponds to the Upper Unconformity noted byCurray and Moore (1972) farther up the fan, which was as-signed an upper Miocene age based drilling results from Site 218of DSDP Leg 22 (Moore et al., 1974). Unconformity "A" isfound at a depth of 0.71 s (two-way traveltime) at Site 717 andat 0.52 s at Site 719.

The upper acoustic unit, above Unconformity "A," recordsa complex history of the motion on the fault block. It includesan upper, highly layered sequence which is essentially flat lyingand laps up onto the uplifted sediments at the tip of the tiltedblock. Further down the block, this sequence overlies and trun-cates inclined reflectors within the underlying sediments (Un-conformity "B" on Figs. 2 and 24). The syn-depositional sedi-ments between Unconformities "A" and "B" are characterizedby numerous reflectors which are tilted up to the south (higherside of the tilted block) and which commonly terminate by pinch-ing out against underlying reflectors. It is within this intervalthat most of the thinning of the sediments over the top of thetilted block occurs. Unconformity "B" is at a depth of 0.19 s atSite 717 and 0.17 s at Site 719, thus the section between the twounconformities thins from 0.50 s at Site 717 to 0.34 s at Site719.

JOIDES Resolution approached Site 719 from Site 718 in dy-namic positioning (DP) mode with 4500 m of drill pipe sus-pended from the ship. Thus no seismic line was acquired duringthe final approach. However the location of Site 719 was di-rectly beneath the Conrad site survey line which was positionedwith GPS navigation. This line is shown in Figure 24.

A synthetic seismogram was calculated using acoustic im-pedance values calculated from bulk density and compressionalwave velocity obtained from the wireline logs (see "Logging"section). The synthetic program utilizes a 1-D convolution of adigitized Conrad air-gun source signature with the log-derivedimpedance profile. The program includes the effects of internalmultiples but not sea-floor multiples. The calculated acousticimpediance profile and synthetic seismogram are shown withthe lithologic column in Figure 25, and the synthetic seismo-gram is compared with the observed siesmic record in Figure 26.

The upper portion of the section consists of a thin upperlayer (4.0 m) of mud turbidites and pelagic calcareous muds(lithologic Unit I) over a thicker (131 m) sequence of micaceoussilt turbidites (lithologic Unit II). The silts abruptly overlie a se-quence of mud turbidites interbedded with pelagic clays (litho-logic Unit III) at 135 m. The synthetic seismogram implies thatthe lithologic boundary between Units II and III corresponds tothe upper of two continuous reflectors marking unconformity"B" at 170 ms depth in the sesmic section (Figs. 25 and 26).

The mud turbidite and pelagic clay sequence (lithologic UnitsIII and IV) extends from 135 to 357 m. In the reference section

181

SITE 719

— f . .,. ^ "* -

Fault Fault

Figure 24. Single-channel seismic line across the Leg 116 sites collected during the Conrad survey. A lithologic column is shown for each site locationwith depths converted to two-way travel time. The total length of the seismic line is 18.5 km.

drilled at Site 717, this sequence contains two intervals of siltyturbidites (lithologic Units IVA and IVC). Lithologic Unit IVAis also recognized at Site 719, extending from 207 to 240 mbsf.The base of this layer is characterized by a small reflector in thesynthetic seismogram, which is also found in the seismic sec-tion. The synthetic peak has a characteristic wide peak (Fig. 25)also found in the observed section (Fig. 26). The lower silty unit(lithologic Unit IVC) was not recognized in the lithologic col-umn. However, it appears to be present in the log data as a thin(10-m) zone of high acoustic impedance at 300 mbsf, corre-sponding to a zone of poor recovery. The logs show a sharp in-crease in both density and velocity at that depth. This creates aprominent reflector in the synthetic seismogram. This syntheticreflector corresponds to a reflector in the observed section thatdoes not have a large amplitude at the location of Site 719 butincreases rapidly to the north toward Site 717, where the litho-logic Unit is better developed.

The top of lithologic Unit V at 357 mbsf is marked by asharp increase in the acoustic impedance, which creates a corre-sponding reflector (Fig. 25). This reflector does not correspondto the Unconformity "A", but rather to a reflector labled "Z"located about 0.09 s above. Unconformity "A" corresponds to adouble-peaked reflector at the bottom of the synthetic seismo-gram about 90 m below the top of Unit V.

LOGGINGThree Schlumberger wireline logging tool strings were used

in Hole 719B with the objective of providing a continuous re-cord of in-situ physical and chemical properties of the sedi-ments. These logs will provide a correlatable record of the litho-stratigraphic as well as the seismic stratigraphic sections vs. truedepth.

The first tool string was the seismic stratigraphic combina-tion consisting of three separate instruments: (a) a digital long-spacing sonic log (LSS) measuring interval transit time of soundwaves in microseconds per foot, (b) a phasor dual induction log(DIT) measuring the bulk resistivity of the formation in ohm-meters, and (c) a natural gamma spectral log (NGT) measuringthe absolute magnitude of gamma radioactivity of the forma-tion and the relative content of the primary radioactive elements

potassium (% weight) and uranium and thorium (parts per mil-lion).

The second tool string, the geochemical combination, con-sisted of three separate instruments: (a) a gamma spectroscopylog (GST), which measures various elemental yields in the sedi-ments using gamma-ray spectra emitted by formation nuclei af-ter bombardment by 14-Mev neutrons; (b) an aluminum activa-tion clay log (AACT), which measures the weight percent of alu-minum via neutron capture and subsequent emission of 1.78-Mevgamma rays; and (c) an NGT log to aid in correlation betweenthe two logging runs.

The third tool string, the lithoporosity combination, was threeinstruments: (a) a lithodensity log (LDT) measuring bulk den-sity (RHOB in units of g/cm3) and photoelectric absorptioncross section of 661 Kev gamma rays (PEF in units of barns/electron), (b) a compensated neutron-porosity log (CNT) mea-suring the hydrogen content of the formation and convertingthis into porosity (NPHI in percent using the Schlumberger lime-stone porosity scale), and (c) an NGT log to correlate with theother logs.

A more complete description of the logging measurementscan be found in the "Explanatory Notes" chapter. The originallogging data are displayed at the end of this chapter, along withcorrelations to core locations and recovery. The "Total" and"Computed" gamma-ray curves represent the total activity andthe total activity minus the contribution from uranium. The"yield" curves from the geochemical tools (AACT and GST)represent relative counts from the specified elements, with thetotal count sum normalized to 1.0.

Measurements

Seismic Stratigraphic Combination

This tool string provided excellent-quality data from the bot-tom of the drill pipe at 87 to 450 mbsf. Although a borehole di-ameter measurement was not available, the sonic transit timecurve indicated that the borehole was much more uniform andsmaller in diameter than Hole 718C. Small washouts may haveoccurred in the intervals 104-119, 134-135, 158-162, and 187-189 mbsf.

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SITE 719

Lith.unit

Syntheticseismogram

Acousticimpedance

O.O5 —

0.10 —

0.15 —

= 0.20 —

* 0.25 —

0.30 —

0.35 —

0.40 —

0.45

IVA

IVB-D

100

150

200

250

300

350

400

450

Figure 25. Synthetic seismogram for Site 719 compared with the litho-logic column. The acoustic impedance profile was determined from sonicvelocity and bulk-density logs. The two-way traveltime scale is linearand measured from the top of the logged interval (90 mbsf).

Sonic transit time and resistivity measurements showed a gen-erally increasing trend downhole with relatively featureless in-tervals alternating with intervals of greater variability. Both ofthese measurements respond to porosity, and this general trendcan be attributed to compaction of sediments with depth. Theintervals of greater variability indicate areas of greater porosityvariation. Transit time varies from approximately 185 µs/ft atthe top of the logged interval to 150 µs/ft near the bottom. Thiscorresponds to a sonic velocity of 1643 m/s at the top to 2027m/s at the bottom. The maximum observed velocity was 2251m/s at 381 mbsf.

Natural gamma-ray emissions show large variability and char-acter throughout the hole and range from a high of 110 APIunits to a low 20 of API.

Geochemical Combination

This tool string provided good-quality data from the bottomof the pipe to the bottom of the hole and was run twice for im-proved resolution of the measurements. The AACT and GSTcurves shown at the end of the chapter are the result of com-

puter processing of the merged spectra from the two passes. Thelogs of relative abundance of sulfur, iron, calcium, silicon, tita-nium, and gadolinium are of good quality and will be useful inlithological identification. Chlorine and hydrogen readings areaffected by borehole fluids and must be carefully examined be-fore they are employed in any analysis.

Relative abundances as calculated by the Schlumberger log-ging computer can give good qualitative estimates of the com-ponent elements of the formation. Readings of negative relativeabundances are an effect of the normalization of total abun-dances to 1.0 and should not be interpreted as true formationpercentages.

Weight percentage of the elements can be found by correct-ing the bulk-density reading for porosity and bringing apparentgrain density and atomic weight into the calculation. The com-putation can then be compared to clay mineralogy analysis ofcore samples. This data will also be invaluable in electrofaciesanalysis.

Silicon has a fairly constant relative abundance throughoutthe log, generally varying about 0.1. Maximum silicon readingis 0.15 with a minimum of 0.06. Calcium is present throughmost of the hole but is absent or reduced in three intervals, 89-122, 162-182, and 369 mbsf to the bottom of the hole. Calciumvaries from -0.02 to 0.06 and anticorrelates with silicon. Ironis also present throughout the log about an approximate meanof 0.15. Sulfur generally correlates with iron variation with theexception of a 1-m interval at 216 mbsf. Sulfur varies between-0.05 and +0.05 (Fig. 27). The wet-weight percent of alumi-num averages approximately 7% with a minimum of 4% and amaximum of 12%.

A detailed geochemical analysis of the GST data may showsystematic differences in clay mineralogy in the units and will becompared to laboratory analysis.

Lithoporosity CombinationThis tool string also provided logs of good quality over the

entire logged interval.The character of bulk density (RHOB) and neutron poros-

ity (NPHI) readings closely correlates with sonic transit time(DELTA T) measurements reflecting the close relationships be-tween bulk density, porosity, and transit time. This will allow agood determination of grain density as porosity is well con-strained. NPHI, RHOB, and photoelectric cross-section read-ings may be erroneous in the intervals of large hole diameter be-cause accurate measurements of these formation parameters re-quires close contact between the tool and the borehole wall.

NPHI is not a good measure of true porosity in clays as thepresence of bound water in clays appears as porosity to theCNT instrument. This fact may be useful in these sequences be-cause differences in porosity computed from RHOB, NPHI,and DELTA T may provide a clue to the sedimentary structurescausing this response.

Photoelectric cross-section (PEF) measurements vary between1.8 and 3.5 but generally are fairly constant between 2.5 and 3.PEF does not show a lot of useful detail throughout but mayaid in lithological identification as it is sensitive to the iron con-tent of clay minerals and can distinguish between calcite and do-lomite.

Good-quality measurements of all parameters will allow adetailed comparison of log response to the chemical and physi-cal structures observed in core samples. Sonic velocity data andbulk-density measurements have enabled a good synthetic seis-mogram to be computed (see "Seismic Stratigraphy" section).

A comparison of shaling upward and downward trends ob-served in NGT data with the appropriate cores will allow identi-fication of the facies sequences responsible for these distinctivepatterns.

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SITE 719

6.0-

Figure 26. Single-channel seismic reflection record obtained by the Conrad at Site 719, along with the synthetic seismogram. Unconformities "A" and"B" and reflector "Z" discussed in text are noted to the left of the profile.

Wireline Unit CharacteristicsUnit boundaries as picked from the wireline logs are chosen

on the basis of distinct changes in the physical parameters mea-sured in the borehole. As these are different criteria than thoseused by sedimentologists, the unit boundaries indicated on logsmay not be in agreement with lithologic boundaries chosen onthe basis of facies observed in cores. Wireline depth determina-tion was based on a continuous measurement of the line length,whereas the drillers depth is based on an incremental measure-ment of pipe lengths (chaining). These and other reasons, in-cluding differences in stretch in the line and the pipe, can causesmall discrepancies in the depths of lithologic units and wirelineunits.

Unit II

The lower third of lithologic Unit II was observed in this holefrom the bottom of the drill string at 87 mbsf, to 138 mbsf. Twoshaling-upwards sequences are observed on the NGT log. (Foran explanation of the term "sharing upwards", see "Logging"section, Site 718 chapter. These sequences are characterized by asharp decrease at the base and a gradual incremental increase ingamma emissions upwards. The bulk density (RHOB) also de-creases sharply at the base to near 1.5 g/cm3 and then increasesrapidly to close to 2 g/cm3. Neutron porosity and interval tran-sit time indicate higher porosity at the base of these intervalswhich decreases upward.

Values of RHOB recorded through the washout in this inter-val may be erroneously low. There is a broad interval from 91 to102 mbsf of high gamma emissions (100-110 API) at the top ofthe logged interval registering RHOB of 1.9 g/cm3.

Between these shaling-upward sequences is what appears tobe a 5-m uniform interval of lower NGT readings (40 API) at115-120 mbsf. Lower iron and aluminum readings indicate a de-

0.03

c/)

-0.050.01 0.25

Iron concentration

Figure 27. Relative concentrations of sulfur vs. iron over entire loggedinterval as measured by the geochemical combination.

crease in clay content. There is a washout indicated on the soniccurves through this interval and measurements will require cor-rections when borehole diameter is available. Core recovery ispoor in this unit so few direct comparisons can be made. Loganalysis will enable a reconstruction of the major facies in theunit.

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SITE 719

There is a 1-m thick radioactive marker at 127 mbsf. This in-terval is not in a recovered interval but may correlate with simi-lar features observed in Hole 718.

The low-density readings near the bottom of the pipe areprobably not valid as the tool will lose contact with the boreholewall as the top of the string enters the pipe.

Unit III

The top of wireline Unit III at 137 mbsf was selected on thelogs on the basis of the increasing RHOB and decreasing NPHIreadings that depart from the measurements of these parame-ters in the rest of Unit III below. This trend can also be observedin sonic transit time.

Unit III is characterized by alternating shaling-upward andshaling-downward sequences on the NGT curves, some of whichhave sharp bottom boundaries or inflection points. Other se-quences grade more gradually into each other. These observa-tions generally agree with NGT measurements in Unit III inHole 718C. In a shaling-upward interval, RHOB decreases up-ward. The converse is observed over a shaling-downward inter-val. NPHI measurements indicate the porosity structure in thesesequences alternate rapidly but generally follow a trend thatconfirms the RHOB response with porosity decreasing upwardsin a shaling-upward sequence. Good core recovery in this unitwill allow a direct comparison of the cores to the logs.

Sonic velocity throughout this unit is fairly constant at 1690m/s. RHOB is also relatively stable, varying around a value of1.7 g/cm3. NPHI readings average 58%.

Geochemically, the transition from Unit II to Unit III is sub-tle, with a slight decrease in potassium and an increase in theneutron-capture cross section.

Unit IV

The top of Unit IV can be picked from the logs at 214 mbsf.Features on three of the curves mark this boundary. The intervaltransit time increases and becomes less variable above the transi-tion; RHOB and NPHI also indicate the boundary by a sharptransition.

NGT character in Unit IV is similar to that in Hole 718C,but the magnitude of the radiation level is not appreciably lowerin this unit compared to other units as it was in Hole 718C, indi-cating that the large borehole in 718C was shielding the toolfrom formation radioactivity.

NGT curves show shaling-upward sequences with sharp bot-tom boundaries and increasing gamma emissions upward. Rela-tively less uranium content is observed in this unit with respectto thorium and potassium as indicated by the tight tracking ofthe SGR and CGR curves. Pronounced cycling between near100 and 60 API is observed in some intervals with wavelengthsof close to 1 m. Shaling-downward sequences are not pronouncedin this unit but are observed in the intervals 250-257, 329-332,and 342-347 mbsf. The latter two correspond to Cores 116-719A-36X and -718A-37X.

Transit time and RHOB measurements show considerablevariation at the top of the unit in the interval 214-237 mbsf.NPHI shows two sequences of decreasing porosity. Below this,the interval 237-267 mbsf has less activity on all curves apartfrom a 2-m section centered at 248 mbsf. This section is charac-terized by a drop in the NGT, a bulk density of 1.4 g/cm3, and aPEF of 2.2. In the rest of the interval, RHOB varies between 1.7and 1.9 g/cm3. NPHI varies about a mean of 55%. PEF readsclose to 2.7 throughout.

The interval 267-357 mbsf has more variation on all curves.A distinctive 3-m interval centered at 305 mbsf has a bulk den-sity of 2.15 g/cm3 and sonic velocity of 2100 m/s (145 µs/ft).PEF measures 3. This may be a silty interval.

The GST and AACT measurements indicate a slight decreasein aluminum and an increase in silicon throughout the unit,along with an abrupt decrease in the neutron-capture cross sec-tion.

Unit V

The transition from Unit IV to Unit V at 357 mbsf is charac-terized on the logs by a decrease in sonic transit time from near2030 to 1790 m/s (170 to 150 µs/ft) accompanied by an increasein RHOB from 1.8 to 2.15 g/cm3 and a sharp drop in NPHIfrom 55% to 35%.

Unit V is very similar in character to the same unit in Hole718C and shows shaling-upward sequences with abrupt bottomboundaries mixed with shaling-downward trends. Poor core re-covery will make the log analysis of this unit important. Mea-surements of all parameters show greater variability in this unitthan above. Sonic transit time measurements vary from a low of1740 m/s to a high of 2260 m/s (135 to 175 µs/ft); RHOB from1.3 to 2.25 g/cm3; NPHI from 38% to 58%.

The geochemical measurements provide a clear indication ofthe transition from clay-rich to silt-rich sediments across theUnit IV/Unit V boundary at 357 mbsf. The iron yield increasesfrom an average of 0.10 to 0.16, and the neutron-capture crosssection drops from 32 capture units to 28 capture units. Thewet-weight percents of aluminium and potassium remain ap-proximately constant through both units, but the combinationof lower porosity and increase in mica content in Unit V proba-bly accounts for this. The marked increase in thorium in Unit Vis also a reflection of lower porosity and increased mica content.

SUMMARY AND CONCLUSIONSTwo holes were drilled at Site 719 on the distal Bengal Fan in

a water depth of 4737 m. Hole 719A was cored continuouslyfrom the seafloor to a depth of 460.2 mbsf, using one APC coreand then the XCB system. Core recovery averaged 40% and, asat the two previous sites, was good in the mud sections and poorin the presumed loose silts and sandy silts. The second hole(Hole 719B) was immediately adjacent to the first and waswashed down to 466 mbsf for wireline logging. Three successfullogging runs were completed in good hole conditions and with-out technical problems.

Site 719 was drilled as a companion hole to Site 717 on thesame fault block and is located 3.2 km further south and partway up the block (Fig. 2). Here, the syn-deformation sectionhas thinned from 0.70 s at Site 717 to 0.52 s, whereas the prede-formation section below reflector "A" remains constant at 1.2to 1.3 s thick. The main objective at Site 719 was to determinethe history of motion and uplift on the block by comparison ofthe sedimentary record with that at the reference Site 717.

The recovered stratigraphic section ranges from late Pleisto-cene to late Miocene in age and has been divided into five mainlithologic units (Fig. 5). The section is in all respects very similarto that at Site 717. Unit I (0-4.0 mbsf) represents the topmostcalcareous clays and mud turbidites of late Quaternary age thatare present at all three sites, although the Holocene section ismostly missing, presumably due to disturbance by the initialcore penetration. Unit II (4-135 mbsf) is dominated by mica-ceous silt turbidites in the 10% of section recovered. It is as-sumed that the nonrecovered intervals are of a similar lithologyand, by extrapolation between widely-spaced age control, thatsediment accumulation rates were well in excess of 135 m/m.y.The base of this Unit corresponds to seismic reflector "B",which at Site 719 gives the appearence of being an erosional sur-face (Fig. 24). Units III and IV are a 222-m thick section ofmainly light- and dark-grey mud turbidites and thin interbeddedpelagic clays, which are calcareous only in the upper 20-30 m.

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SITE 719

Unit III is characterized by distinctive greenish colored biogenicturbidites, and Unit IV by an increased proportion of silty tur-bidites. The average accumulation rate was between 40 and 50m/m.y. Slightly over 100 m of Unit V gray micaceous silts werepenetrated before drilling was stopped just below a 10-m thickgreenish clay horizon that correlates with a similar interval nearthe top of Unit V at Site 717. Seismic reflector "Z" marks thetop of the silty section and reflector "A" is about 90 m deepernear the bottom of the hole (Fig. 24).

The biostratigraphic control at Site 719 is based largely onnannofossils as most of the other microfossil groups are eitherpoorly preserved or absent in all but the uppermost unit. Theseafloor at the site apparently lay beneath the carbonate com-pensation depth for much of its early history and then beneaththe lysocline for the past 2-3 m.y. Siliceous fossils have proba-bly also been removed by dissolution. Most of the fossils usedfor dating, therefore, have been resedimented by turbidity cur-rents, but the general absence of mixing between nannofossilzones indicates that resedimentation took place shortly after de-positon. A consistent feature of the biostratigraphic record atall three sites is the presence of a much-condensed sequence inthe sedimentary record between late Pleistocene and Pliocenethat occurs near the top of, but within Unit III.

The geochemical data from interstitial pore waters are re-markably similar to those from the equivalent sections at Site717. The downhole trends in Ca, Mg, Na + K, and alkalinity in-dicate a significant exchange with the sediments during earlydiagenesis. From the inorganic carbonate content measurementsand from observations on smear slides, it would appear thatcarbonate is being precipitated preferentially in the silty beds.Total organic carbon contents range up to 2.5%, tend to beslightly higher in the muddy units, and show a distinct cyclicityof high and low values at a spacing of between 20 and 40 m.Rock-Eval measurements show that the organic carbon consistsexclusively of vitrinites and inertinites derived from land plants.Biogenic gas contents are low throughout.

Whereas all attempts at deducing magnetostratigraphy hadbeen abandoned for Site 719 due to the problems of nonori-ented cores and equatorial location encountered at the two pre-vious sites, the data on magnetic susceptibility and intensityproved very interesting. In general, these data show the samecorrelation between sites as that determined from the lithostra-tigraphy, but with some discrepancy in the upper part of UnitIII. The magnetic studies imply significant erosion (about 50 m)at the top of this interval.

A good suite of physical property measurements was ob-tained at Site 719 from both the shipboard analysis of core ma-terial and from the three successful logging runs. In particular,the gamma-ray, sonic, and density logs show interesting varia-tions in character within, as well as between, lithologic units.The siltier units show higher sonic velocities than the muddyunits, but also show an increased gamma reading. It is not yetclear what is causing this particular response as both clay andorganic carbon contents are higher in the muds.

The temperature gradient through the upper 150 m of sedi-ment at Site 719 is very similar to that at Site 717. When plottedtogether, temperature data from the two sites give a linear gradi-ent of between 34 and 38 °C/km. These observations suggestthat hydrothermal circulation is not active in the upper sedi-mentary section of the fault block containing Sites 717 and 719,in contrast to the results found at Site 718. This is believed to bedue to the fact that the tip of the fault block to the south, onwhich Site 718 is situated, is exposed to the seafloor, allowingcold water influx from the surface, whereas the block beneathSites 717 and 719 is completely buried by the upper units.

In summary, the principal results of drilling at Site 719 de-pend upon its correlation to and comparison with the referenceSite 717.

1. The general lithostratigraphic correlation between all threesites is good (Fig. 28). The main lithologic units can be readilycorrelated and a number of marker horizons can also be recog-nized within the units. These correlations are supported by thebiostratigraphic data and, for the most part, by the other ship-board analyses. However, certain discrepancies in detail remainto be resolved by further work, for example, the degree of ero-sion at the top of Unit III.

2. As expected, the main difference in the thickness of unitsoccurs within the syn-deformation sequence. There is an ap-proximate 30% reduction in this upper section between Sites717 and 719. Most of the reduction (160 m) takes place withinUnits III and IV, much less within Unit II (15 m), and no ob-servable difference is noted in the topmost part of Unit V.

3. Detailed bed-to-bed correlation indicates that this reduc-tion has taken place in part by the thinning of individual turbi-dite beds, in part by the pinching-out of beds, and in part byerosion at the top of Unit HI. It appears that this process hasbeen relatively constant through most of the period since theonset of fault-block rotation at about 7.5 Ma, suggesting thatmovement on the fault has also been gradual. The apparentblanketing of the uplifting fault block by the silts of Unit II sug-gests that, since the mid-Pleistocene, the rate of sediment accu-mulation outstripped motion on the fault.

4. The existence of a fault block with a significant elevationabove the surrounding seafloor has had a profound effect onthe deposition of turbidites on this part of the fan. Many tur-bidity currents clearly passed over the obstacle but depositedthinner beds higher up the flank. In some cases the currentspassed into a nondepositional or erosional mode; whereas otherminor ones may have been deflected completely.

5. The highly condensed late Pleistocene to late Pliocenesection that occurs within the top of Unit III is not yet fully un-derstood and likely relates to a period when turbidity currentseither eroded or simply did not deposit material at this part ofthe fan. This was presumably due to the relatively high elevationof the tip of the fault block on which the two sites are located.Alternatively, an intensified bottom circulation system coupledwith a period of very low input of fan sediments may beresponsible.

REFERENCES

Barron, J. A., 1983. Latest Oligocene through early middle Miocenediatom biostratigraphy of the eastern tropical Pacific. Mar. Micro-paleontol., 7:487-516.

Berggren, W. A., Kent, D. V., and van Couvering, J. A., 1985. The Neo-gene: Part 2, Neogene geochronology and chronostratigraphy. InSnelling, N. J., (Ed), The Chronology of the Geological Record,Geol. Soc. Mem. 10:211-260.

Boltovskoy, E., 1977. Neogene deep water benthonic foraminifera of theIndian Ocean. In Heirtzler, J. R., Bolli, H. M., Davies, T. A., Saun-ders, J. B., and Sclater, J. G., Indian Ocean Geology and Biostratig-raphy: Washington (American Geophysical Union), 599-616.

Burkle, L. H., 1972. Late Cenozoic planktonic diatom zones from theeastern equatorial Pacific. Nova Hedwigia Beihefte, 39:217-246.

Curray, J. R., and Moore, D. G., 1971. Growth of the Bengal deep-seafan and denudation of the Himalayas. Geol. Soc. Am. Bull., 82:563-572.

Curray, J. R., Emmel, F. J., Moore, D. G., and Raitt, R. W., 1982.Structure, tectonics and geological history of the northeastern In-dian Ocean. In Nairn, A.E.M., and Stehli, F. G. (Eds.), The OceanBasins and Margins, (Vol. 6) The Indian Ocean: New York (PlenumPress), 399-450.

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Einsele, G., 1982. Mass physical properties of Pliocene to QuaternarySediments in the Gulf of California, Deep Sea Drilling Project Leg64. In Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64, Pt.2: Washington (U.S. Govt. Printing Office), 529-542.

Emmel, F. G., and Curray, J. R., 1983. The Bengal deep sea fan, north-eastern Indian Ocean, Geo-marine Letters, 3:119-124.

Gansser, A., 1964. The Geology of the Himalaya: London (Wiley Inter-science).

Gansser, A., 1981. The geodynamic history of the Himalaya. In Gupta,H. K., and Delany, F. M. (Eds.), Zagros—Hindu Kush—HimalayaGeodynamic Evolution: Washington (American Geophysical Un-ion), Geodynamics Series, 3:111-121.

Geller, C. A., Weissel, J. K., and Anderson, R. N., 1983. Heat transferand intraplate deformation in the central Indian Ocean. J. Geophys.Res., 88:1018-1032.

Moore, D. G., Curray, J. R., Raitt, R. W, and Emmel, F. J., 1974.Stratigraphic-seismic section correlation and implications to Bengalfan history. In von der Borch, C. C, Sclater, J. G., et al., Init.Repts. DSDP, 22: Washington (U.S. Govt. Printing Office), 403-412.

Nigrini, C , 1971. Radiolarian zones in the Quaternary of the equatorialPacific Ocean. In Funnell, B. M., Riedel, W. R. (Ed.). TheMicropa-leontology of Oceans: 443-461.

Nigrini, C. and Moore, T. C , 1979. A guide to modern Radiolaria.Spec. Publ. 16, Cushman Foundation.

Okada, H. and Bukry, D., 1980. Supplementary modification and in-troduction of code numbers to the low latitude coccolith biostrati-graphic zonation (Bukry, 1973, 1975). Mar. Micropaleontol., 5:321-325.

Parsons, B., and Sclater, J. G., 1977. An analysis of the variation ofocean floor bathymetry and heat flow with age. J. Geophys. Res.,82:803-827.

Powell, C. M. A., and Conaghan, P. J., 1973. Plate tectonics and theHimalayas. Earth Planet. Sci. Lett., 20:1-12.

Richards, A. F , 1962. Investigations of deep sea sediment cores II:Mass physical properties. U.S. Navy Hydrographic Office, Tech. Rep.106.

Sclater, J. G., and Fisher, R. L., 1974. The evolution of the east centralIndian Ocean with emphasis on the tectonic setting of the NinetyEast Ridge, Geol. Soc. Am. Bull., 85:683-702.

Sclater, J. G., Luyendyk, B. P., and Meinke. L., 1976. Magnetic linea-tions in the southern part of the Central Indian Basin. Geol. Soc.Am. Bull., 87:371-378.

Stein, S., and Okal, E. A., 1978. Seismicity and tectonics of the NinetyEast Ridge area: Evidence for internal deformation of the Indianplate. / . Geophys. Res., 83:2233-2245.

van Morkhoven, F.P.C.M., Berggren, W. A., and Edwards, A. S., 1986.Cenozoic Cosmopolitan Deep-Water Benthic Foraminifera. Bull.Centres Rech. Explor.—Prod. Elf-Aquitaine, Mem. 11; Pau, 1-421.

von der Borch, C. C , Sclater, J. G., et al., 1974. Init. Repts. DSDP, 22:Washington (U.S. Govt. Printing Office).

Weissel, J. K., Anderson, R. N., and Geller, C. A., 1980. Deformationof the Indo-Australian plate. Nature, 287:284-291.

Ms 116A-107

Site 717Age Lithology

Site 719Lithology

Site 718Lithology Age

Legend: dominant lithology

Gray silt and mud turbidites

1000 - 1

l÷÷^-l Mud turbiditesSilt horizons within muds

— — Green biogenic turbidites—1—w "White" biogenic turbidites

Figure 28. Lithostratigraphic correlation between Sites 717, 718, and 719.

187

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SHALLOW RESISTIVITY

COMPUTED

21

22

23

27

28

29

301

31

321

33

34

35

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SITE 719

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NOTE: All core description forms ("barrel sheets") and core photo-graphs have been printed on coated paper and bound as Section 3,near the back of the book, starting on page 213.

196