2 Nephi 5-8. Laman Lemuel 2 Nephi 5 The division - 2 Nephi 5:1-6.
PALEOSEISMOLOGY OF THE NEPHI SEGMENT OF THE … · Figure 4. Scarp profiles from the Willow Creek...
Transcript of PALEOSEISMOLOGY OF THE NEPHI SEGMENT OF THE … · Figure 4. Scarp profiles from the Willow Creek...
Figure 4. Scarp profiles from the Willow Creek site, Nephi segment of the Wasatch
fault zone. Only the fault-scarp portion of the profiles are shown. Profile locations
are shown on figures 3 and 5.
0
2
4
6
8
10
12
14
0
0
2
4
6
8
10
12
0
0
2
4
6
8
10
12
0 40
0
2
4
6
8
10
0 5 10 15 20 25 30
5 10 15 20 25 30 35
405 10 15 20 25 30 35
40
5 10 15 20 25 30 35
Profile m83-34
Profile m83-36
Profile m83-33
Profile m83-35
Distance along profile (m)
Distance along profile (m)
Hei
gh
t (m
)
Distance along profile (m)
Hei
gh
t (m
)
Distance along profile (m)
Hei
gh
t (m
)H
eig
ht
(m)
Lower surface
Lower surface
Lower surface
Lower surface
Willow Creek fan
Willow Creek fan
Willow Creek fan
Fan from side canyon
Lower surface is youngerthan upper surface, thusSO is minimum value
Lower surface is younger than uppersurface, thus SO is minimum value
Lower surface is youngerthan upper surface, thusSO is minimum value
Lower surface is youngerthan upper surface, thusSO is minimum value
SO = 6.0 m
SO = 5.2 m
SO = 5.8 mSH = 6.8 m
SO = 6.7 mSH = 8.7 m
SH = 6.3 m
SH = 6.9 m
SO (surface offsets)are 5.2–6.7 m
(minimum)
SUMMARY
SH (scarp heights)are 6.3–8.7 m
Figure 3. Index map of the Willow Creek site showing fault scarp, trenches, and
other cultural and geographic features.
NN
0
100 m
100 m
429700mE
429600mE
429900mE 430000mE429800mE
4406000mN
4405900mN
4405800mN
4405700mN
4405600mN
4405500mN
4405400mN
4405300mN
4406100mN
Figure 5Figure 5
Mt. Nebo WSA
Mona Irrigation Co. canal
Willow Creek FS Road (gravel)
Will
ow C
reek
WCN trenchWCN trench
m83-36m83-36
m83-34m83-34
m83-37m83-37
m83-38m83-38
U.S
. Fo
rest
S
ervi
ce L
and
U.S
. Fo
rest
Ser
vice
Lan
d (
excl
usi
on
)
Mt. Nebo WSA
m83-39 m83-39
m83-40m83-40
WCStrenchWCS
trench
m83-33m83-33
UTM Coordinates (m E)
UT
M C
oo
rdin
ates
(m
N)
m83-35m83-35
Mt. Nebo WSAMt. Nebo WSA
058
057
055
056
054
053052
051050
059
WPT m E m N
059 429997 4405355058 429994 4405408057 429926 4405489056 429927 4405520055 429930 4405570054 429932 4405806053 429913 4405869052 429866 4405900051 429853 4406025050 429782 4406034
GPS STATION LOCATIONS
NAD 27, UTM zone 12, Nephi 7.5' quad
EXPLANATION
Young alluvial fanTrace of active fault–Bar and ball on downthrown sideGPS Station and way point (WPT) numberLocation of trenchLocation of scarp profile and profile no.CanalsStreamsRoads (unimproved)Forest boundary fence
052
429600mE
63-3
8m
elifo
rP
53-3
8m
elifo
rP
33-38m
eliforP
43-38m
eliforP
N
Relative Elevation (m)
91 - 93
93 - 95
95 - 97
97 - 99
99 - 101
101 - 103
103 - 105
105 - 107
107 - 109
109 - 111
111 - 113
EXPLANATION
Fault
Fence
Unsurveyed areas:
Borrow pit
Area of dense trees
Willow
Creek N
orth
Will
ow C
reek
Sou
th
Survey base station (100 N, 100 E)
Other surveyingstationsDiversion canal
Spoil pile
Trench wall (logged) and excavation
Forest ServiceRoad
Contour Interval: 0.5 m,
labels at 2 m intervals
Fault scarpprofile
wolli
W
keerC
daoR
tseroF
ecivreS
atadyevrusfotimiLLimit of survey data
Limit of survey data
ata
dy
evrus
fo
timiL
ata
dy
ev r
usf
oti
miL
Base camp
230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20
21
01
22
02
23
03
24
04
25
05
26
06
27
07
28
08
29
09
20
1
00
1
21
1
01
1
22
1
02
1
23
1
03
1
93
91
93
9595
95
97
97
97
97
95
99
99
99
99
99
103
103 103
103
101
101
101
101
101
105
105
105
105
107
107
107
107
109
109
109109
109111
230 220 210 200 190 180 170 160 150 140 130
NORTHING (meters)
120 110 100 90 80 70 60 50 40 30 20
NORTHING (meters)
)sr
ete
m(G
NIT
SA
E
)sr
ete
m(G
NIT
SA
E
0 41
04 1
Suggested citation: Machette, M.N., Crone, A.J. Personius, S.F., Mahan, S.A., Dart, R.L., Lidke, D.J., and Olig, S.S., 2007,
Paleoseismology of the Nephi segment of the Wasatch fault zone, Juab County, Utah—Preliminary results from two
large exploratory trenches at Willow Creek: U.S. Geological Survey Scientific Investigations Map 2966, 2 sheets.
Willow Creek trench site
Figure 3
Red Canyon trench site
North Creek trench site
Santaquintrench site
Figure 2. Index map for the Nephi segment showing location of paleose ismic trench sites
(marked by asterisks). Map and shaded-relief digitial elevation model (DEM) created by Chris
DuRoss (UGS, November 2006).
Nephi
Santaquin
Payson
JUA
BV
ALL
EY
0 10 205km
*
*
*
*
PROVOSEGMENT
NEPHISEGMENT
Northernstrand
Southernstrand
40°
39°
45’
39° 45’
EXPLANATION
Fault; bar and ball on downdropped side
Trench site*
Hansel V alley fault
Towanta F lat graben
B ear R iverfault
E as tern B earLake fault,
s outhern section
Fish S pringsfault
Hogsback fault,s outhern section
J oes Valleyfault
Morganfault
North P romontory fault
Oquirrhfault
S outhern OquirrhMountains fault
S trawberryfault
S ugarvillea rea faults
Great S
alt Lake
fault
Wa
sa
tc
hfa
ult
zo
ne
WFZ
E. Cache fault,Logan s ection
0 10050 km
B righam C itys egment
Webers egment
S LC
Fayettesegment
WVF Z
Nephis egment
Levans egment
P rovos egment
UTAH
J ames P eaksection
Index Map (Fig. 2)
E. Cache fault,
INTRODUCTIONThe Nephi segment is one of the most prominent, yet least understood parts of the Wasatch fault zone (WFZ) in Utah
(fig. 1) (see Machette and others, 1992a, 1992b). Initial studies of this segment of the WFZ by Woodward-Clyde geolo-
gists (Hanson and others, 1981, 1982; Schwartz and others, 1983, 1984) reported evidence for multiple Holocene
surface ruptures at North Creek with only broad bounds of the timing of each event. Machette (USGS) mapped the
surficial geology along the segment in 1983–84 (see Harty and others, 1997), and Michael Jackson (Jackson, 1991)
trenched the southern part of the segment at Red Creek and reconfirmed evidence of three Holocene earthquakes. How-
ever, Jackson’s results did little to refine the original, now two-decade-old chronology. With designation of the Mt. Nebo
Wilderness area, the possibility of further trenching on the segment seemed problematic. However, the Utah Geological
Survey (UGS) recently re-evaluated the segment’s paleoseismic record for seismic hazards (Lund, 2005), and DuRoss and
Bruhn (2005) estimated new slip rates based on the scarp’s morphology. As a result, the UGS identified the Nephi
segment as Utahs’ highest priority for further paleoseismic research (Lund, 2005), and the hunt was on for a new trench-
ing site.
Key questions about the paleoseismic history of the Nephi segment focused on:
(1) Time of the most recent surface-rupturing earthquake (P1) was known to be younger than 750–1,110 yr from charcoal
in a faulted debris flow, but was considered to be considerably younger (300–400 yr old) based on the fault scarp’s
youthful morphology;
(2) The penultimate (P2) earthquake was either >1,650 or >3,640 yr based on conflicting radiocarbon dates from charcoal
and organics in the buried A horizon on fault-scarp colluvium deposited after P2;
(3) An earlier (P3) earthquake was inferred from a fluvial terrace inset into the uplifted block at North Creek; and
(4) The entire faulting history at North Creek was known to be <4,000±400 yr (or <4,580 yr) based on two ages from
charcoal in fan-head deposits adjacent to the trench (Bucknam, 1978). (All above ages are based on uncorrected
radiocarbon dates; that is, in 14C yr B.P.).
The purpose of this report is to publish detailed stratigraphic, structural, and chronologic data required to interpret
the paleoseismic history of the Willow Creek site. Such interpretations, which require discussing the entire Nephi segment
and its relation to the WFZ, will be made in subsequent publications with reference to this report .
WILLOW CREEK TRENCH SITEIn 2004, we identified a small parcel of U.S. Forest Service land at the mouth of Willow Creek (about 5 km west of
Mona, Utah) that was excluded from the Mount Nebo Wilderness area owing to previously established egress to the forest.
At the Willow Creek site, which is near the middle of the southern strand of the Nephi segment (see index map, fig. 2), the
WFZ has vertically displaced alluvial-fan deposits >6–7 m, forming large, steep, multiple-event scarps (see fig. 4). Both
DuRoss and Bruhn (2005) and Machette (field notes, 1984) had surveyed topographic profiles in the area. In May 2005,
we dug two 4- to 5-m-deep backhoe trenches at the Willow Creek site, identified three colluvial wedges in each trench, and
collected samples of charcoal and A-horizon organic material for AMS (acceleration mass spectrometry) radiocarbon
dating, and sampled fine-grained eolian and colluvial sediment for luminescence dating (table 1). The new radiocarbon
dates (table 2) have been calendar corrected using Calib v. 5.0.2 (http://radiocarbon.pa.qub.ac.uk/calib/). These AMS
dates are from charcoal and herbaceous materials (twigs) that were separated by Kathy Puseman of Paleo Research
Institute, Golden, Colo.
The trenches yielded a stratigraphic assemblage composed of moderately coarse grained fluvial and debris-flow
deposits and discrete colluvial wedges associated with three faulting events (P1, P2, and P3). About one-half of the net
vertical displacement is accommodated by monoclinal tilting of fan deposits on the hanging-wall block, possibly related to
massive ductile landslide deposits that are present beneath Willow Canyon. The top of these landslide deposits are
exposed as small islands surrounded by Holocene alluvium of the Willow Creek fan, west of our study area.
WILLOW CREEK NORTH TRENCH (WCN)The Willow Creek north trench site (WCN) is about 150 m north of the Willow Creek (U.S. Forest Service) road, and
north of an east-west jog in the U.S. Forest Service boundary fence (see fig. 5). This trench crossed a 8.7-m-high scarp
(profile m83-36, figs. 4 and 5) on colluvium and fan alluvium deposited on the north margin of the valley of Willow Creek.
The alluvial package exposed in the WCN trench comprises alternating fluvial, debris-flow, and loess deposits (fig. 6, table
3), all parallel bedded but slightly finer grained than the fan-head alluvium in the Willow Creek South trench (WCS).
Although deposits on both sides of the fault appeared correlative, dating shows that the hanging-wall deposits are substan-
tially younger that those on the footwall. We mapped three discrete colluvial wedges, each related to a major surface-
faulting earthquake. As with the south trench, tilting accommodated about one-half of the net vertical tectonic throw.
WILLOW CREEK SOUTH TRENCH (WCS)The Willow Creek south trench (WCS) is about 55 m north of the Willow Creek (U.S. Forest Service) road (fig. 5) and
crossed a 6.3- to 6.9-m-high scarp (profiles m83-33, m83-34, and m83-35; figs. 4 and 5) on fan-head alluvium of Willow
Creek. We suspected that the alluvium is middle Holocene in age, based on a 4,580 14C yr B.P. date from charcoal
(Bucknam, 1978) collected from alluvial deposits at North Creek, 6 km to the north (fig. 2). The alluvial package exposed
in the WCS trench comprises alternating fluvial, debris-flow, and loess deposits, all parallel bedded and largely conform-
able (fig. 7, table 4). The trench exposed correlative deposits on both sides of the fault, as demonstrated by dating. Not
finding correlative deposits is a common problem in paleoseismic investigations, often resulting in minimum estimates of
fault displacement and incomplete paleoseismic records.
We mapped three discrete colluvial wedges, each related to a major surface-faulting earthquake. The oldest wedge
(Cw3) is the smallest, but fills a large fissure along the main fault. This suggests the event P3 may have caused both broad
tilting in the footwall block and brittle faulting. We mapped some minor synthetic faults, but no antithetic faults or grabens
were found in the trench. As a result, about one-half of the net vertical tectonic throw in WCS is related to tilting rather
than brittle faulting.
RELATIVE TIMING OF EVENTSCurrently, our conclusions about the absolute timing of surface faulting events at Willow Creek are limited. The
luminescence age estimates (table 1) and radiocarbon dates (table 2) provide broad timing constraints. Two dates from the
WCS trench tightly bracket the most recent faulting event (P1) at between 270 and 315 calendar yr ago (see results of
radiocarbon dating, table 2). Event P2 is probably older than 1,227±54 calendar yr since this is the age of organic
material collected from the A horizon of the soil formed on the P2 colluvium (unit Cw2 in the WCN trench, fig. 7). The
oldest recorded event (P3) in the Willow Creek trenches must be much older than P2 (>1,227 calendar yr), but younger
than faulted deposits in the footwall, which are dated at about 2,500 to 3,150 calendar yr (dates generalized from tables 1
and 2).
Combining our new dates with those previously published for the North Creek and Red Canyon sites, our analyses
suggest new and interesting interpretations. Our preliminary interpretations of the timing of major surface-faulting events
at Willow Creek are as follows:
P1—probably about 300 yr ago,
P2—probably about 1,230 yr ago, and
P3—less than 2,320 yr ago.
These times are much younger than those from the North Creek and Red Canyon sites and will have to be reconciled
in light of our new data. In addition, new studies at the Santaquin site (fig. 2) should reveal timing for the northern strand
for the Nephi segment of the WFZ.
Figure 1. Major Quaternary faults in northern Utah. Red box shows location of Nephi segment of Wasatch fault zone
(fig. 2). Faults from Black and others (2003); shaded-relief digitial elevation model (DEM) created by Chris DuRoss
(UGS, November 2006). Abbreviations: WFZ, Wasatch fault zone; WVFZ, West Valley fault zone; SLC, Salt Lake City.
110°
42°
41°
40°
39°
112°114°
Figure 5. Detailed topographic map of the Willow Creek site showing trenches, profile locations and other features.
PALEOSEISMOLOGY OF THE NEPHI SEGMENT OF THE WASATCH FAULT ZONE, JUAB COUNTY, UTAH—PRELIMINARY RESULTS FROM TWO LARGE EXPLORATORY TRENCHES AT WILLOW CREEK
ByMichael N. Machette1, Anthony J. Crone1, Stephen F. Personius1, Shannon A. Mahan1, Richard L. Dart1, David J. Lidke1, and Susan S. Olig2
1U.S. Geological Survey, Denver, Colorado, and 2URS Corporation, Oakland, California2007
SCIENTIFIC INVESTIGATIONS MAP 2966 SHEET 1 OF 2 Version 1.0
U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY
Manuscript approved for publication April 1, 2007
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For sale by U.S. Geological Survey Information Services, Box 25286, Federal Denver, Denver CO 80225
1-888-ASK-USGS
A PDF for this map is available at http://pubs.usgs.gov/sim/2007/2966