three-dimensional variations in extensional fault shape and basin form

7/30/2019 Three-Dimensional Variations in Extensional Fault Shape and Basin Form

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ABSTRACT

Seismic reflection profiles, drill-hole data,and geologic maps delimit

the form of normal faults and Tertiary sedimentary rocks in the south-

ern half of the Cache Valley basin in northern Utah. Dips of faults and

sedimentary rocks were estimated from time-migrated reflection pro-

files by using the stacking velocities for the data. At the southern end of

the basin, the East Cache fault zone is listric; it shows 50W dips near

the surface and 20W dips at depth. The fault zone has been the site

of5.6 km of net dip slip. Tertiary rocks dip 18E25E and exhibit a

rollover geometry above the fault zone. No faults are interpreted on the

western side of the basin. In the central part of the basin in Utah, the

East Cache fault is a single fault that dips at least 45W near the sur-

face and is the site of 4.56.4 km of net dip slip. Here, the basin is

bounded to the west by the West Cache fault, which is the site of at least

1 km of net slip that increases northward to 2 km of net slip. Slip on the

East Cache fault resulted in planar, east-dipping, older Tertiary rocks

near the bottom of the basin. Younger Tertiary strata, with southwest,

west, and northwest dips, reflect complex tilting due to slip on the West

and East Cache faults. Anticlines in the Tertiary basin-fill deposits are

present in the central part of the basin and may reflect changes in nor-

mal-fault geometry at depth. Northward, dip slip on the East Cache

fault zone decreases to 2.5 km. The basin is broad, shallow, and filled

with nearly flat lying Tertiary rocks. This area, near the north-south

midpoint of the basin, is bounded by the West and East Cache faults,

but slip on the West Cache fault appears to diminish northward. A

north-trending reflection profile tied to both drill-hole data and the

east-trending seismic profiles indicates that the basin is deeper in the

southern end. The along-strike changes in fault geometry, the amount

of fault-slip, the subsurface form of the basin-filling sedimentary rocks,

and the form of the basin indicate a complex history of faulting and

deposition during its formation. This study and other recent ones from

the Basin and Range province indicate that such complexities may be

typical of many Tertiary basins in the region.

INTRODUCTION

Analyses of subsurface geology of extensional basins in the Basin and

Range province commonly are based on east-trending seismic reflection

profiles combined with surface geologic data, and rare drill-hole data (e.g.,

Anderson et al., 1983; Bohannon et al., 1993; Brocher et al.,1993; Liberty

et al., 1994; Thompson et al., 1989). Such studies provide valuable insight

into the form of the basin-bounding faults and the geometry of basin-filling

sedimentary rocks. In this study, we present an analysis of a grid of seismic

reflection profiles that cover the southern half of the Cache Valley basin, in

northern Utah (Figs. 1 and 2). These profiles run parallel and perpendicu-

lar to the north trend of the structural basin and thus tightly constrain the

subsurface geometry of the major normal faults in the basin. These data

also provide evidence of north-south and east-west variations in the dips

and thicknesses of Tertiary basin-fill deposits. We combine the subsurface

data with surface geologic data to show that the basin exhibits significant

along-strike variations in (1) the amount of slip on the dominant, basin-

bounding East Cache fault zone, (2) the thickness and form of basin-filling

strata, (3) the fault-related depositional history in the basin, and (4) the rel-

ative importance of the East and West Cache faults in controlling basin

geometry.

As part of exploration for hydrocarbons in Tertiary basin-fill deposits in

northern Utah,Amoco Production Company (Patton and Lent, 1977) and

Placid Oil Company ran 160 km of seismic reflection profiles in the south-

ern part of the Cache Valley basin. The data were collected in 1973 and

1974; they were reprocessed in 1982 by Amoco and acquired in 1982 by

Placid Oil. Data acquisition and processing parameters for the reflection

profiles shown in this paper are summarized in Table 1. Seismic reflection

profiles acquired by Amoco and provided to us are time-migrated sections

with depths assigned to various times on the basis of the stacking veloci-

ties. We use depths from these sections, correlated where possible with sur-

face or subsurface geologic information, to draw cross sections across the

basin. The seismic reflection profile provided by Placid Oil is an unmi-

grated section.

Two drill holes that penetrated the entire Tertiary section provided criti-

cal control on the identification of reflections on the seismic reflection pro-

files. The Amoco Lynn Reese drill hole (sec. 17, T. 12 N., R. 1 E.) pene-

trated 2231 m of Tertiary Salt Lake Formation rocks and 125 m of the

Tertiary Wasatch Formation, and had a total depth of 2487 m in the Or-

dovician Swan Peak Formation. The North American Resources Hauser

Farms drill hole (sec. 10,T. 13 N., R. 1 W.) on the western side of the basin

penetrated 1417 m of Tertiary Salt Lake Formation rocks, which uncon-

formably lie on Cambrian rocks (Brummer, 1991). We tie the drill hole

data to the seismic reflection profiles by correlating the depth to the top of

each unit encountered in the drill hole with reflections on the nearest seis-

mic reflection profile.

GEOLOGIC SETTING

The Cache Valley basin is at the southern end of a series of half-grabens

that form the extensional corridor between the Wasatch and Teton normal

faults (Fig. 1). These normal faults are the loci of Pleistocene and Holocene

seismicity (Smith and Sbar, 1974; McCalpin and Forman, 1991; Piety et

al., 1992) and cut Proterozoic through Cretaceous rocks that underwent

contractional deformation during the Sevier orogeny. Seismic reflection

1580

Three-dimensional variations in extensional fault shape and basin form:

The Cache Valley basin, eastern Basin and Range province, United States

James P. EvansRobert Q. Oaks Jr. } Department of Geology, Utah State University, Logan, Utah 84322-4505

GSA Bulletin; December 1996; v. 108; no. 12; p. 15801593; 14 figures; 3 tables.


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CACHE VALLEY BASIN, EASTERN BASIN AND RANGE PROVINCE

Geological Society of America Bulletin, December 1996 1

profiles across Tertiary basins that developed in the fold-and-thrust belt

show that the normal faults are commonly listric and sole into underlying

major thrust faults (Royse et al., 1975; Coogan and Royse, 1990; West,

1992). Basins 1030 km wide formed in the hanging walls of the normal

faults and filled with Tertiary sedimentary deposits (Fig. 1).

The Cache Valley basin is bounded on its eastern side by the East Cache

fault zone, which is 70 km long (McCalpin, 1989), and uplifts folded Prot-

erozoic and Paleozoic rocks in its footwall. It splays into two major strands

in the northern and southern part of the study area (McCalpin, 1989;

Fig. 2). The Cache Valley basin is bounded along a part of its western side

by the West Cache fault, which is 20 km long and has been active during

the Quaternary (Oviatt, 1986a).Tertiary rocks of the area consist of the Eocene Wasatch Formation and

the MiocenePliocene Salt Lake Formation. The Wasatch Formation is ex-

posed at high elevations in the Bear River Range east of the basin and lo-

cally along the east flank of the Wellsville Range west of the Cache Valley

basin. It consists of mudstones, conglomerates, and minor limestones de-

posited in paleovalleys and across a relatively broad surface in northern

Utah (Oaks and Runnells, 1992). We infer that the Wasatch Formation was

continuous across the study area before the onset of large-magnitude ex-

tension. The Wasatch Formation is 110 m thick in the Lynn Reese drill hole

in the central part of the Cache Valley basin (Brummer, 1991) and at le

245 m thick locally in the Bear River Range (Oaks and Runnells, 199

The Salt Lake Formation consists of conglomerates, tuffaceous sa

stones and siltstones,and limestones (Williams,1962) that are 2231 m th

in the deepest drill hole in the Cache Valley basin (Brummer, 1991). Thi

nesses and stratigraphic relationships in the Salt Lake Formation are hig

variable across northern Utah and southern Idaho because basal parts of

Salt Lake Formation probably were deposited across a broad area,

younger parts of the formation record localized deposition in individ

basins that formed during the development of modern Basin and Ran

structures (Platt, 1988). Late Tertiary and Quaternary lacustrine and fluv

deposits of the Bonneville and older lake cycles overlie the Salt Lake Fmation in the basin and are as much as 335 m thick (Brummer, 1991).

The onset of Basin and Range extension in the Cache Valley is diffic

to date owing to poor exposures and the uncertain age of the Salt Lake F

mation. Hintze (1988) and Parry and Bruhn (1986) suggested that ext

sion began in the region at ca. 17 Ma. Bryant et al. (1989) proposed that

sic volcanic rocks intercalated with the Salt Lake Formation in the S

Lake City area record the onset of extension at 21 Ma and that later ra

subsidence of basins coincident with faulting began at ca. 11 Ma.

Williams (1948, 1958) suggested that the Cache Valley basin is bro

Figure 1. Regional setting of the Cache Val

basin in the northern Wasatch to Teton corrid

Narrow basins (shaded regions) form above-n

mal faults that are superposed on Sevier fo

and thrusts. Onset of normal faulting rang

from ca. 17 Ma for the Wasatch fault to 24 M

for the Greys River fault. See Figure 2 for la

tude and longitude of the study area.


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EVANS AND OAKS

1582 Geological Society of America Bulletin, December 1996

shallow, and flat bottomed. In contrast, gravity data (Peterson and Oriel,

1970; Mabey 1985,1987; Zoback, 1983; Cook et al., 1989) documented a

complex basin with several east-trending structural highs and lows. One

east-trending gravity high, across the central part of the basin in Utah, may

coincide with a segment boundary of the East Cache fault zone (Zoback,

1983). The gravity data of Cook et al. (1989) suggest that the basin is deep

in the south and gradually becomes shallower northward.

RESULTS

Five east-trending cross sections (Figs. 3, 6, 8, 13, and 14) and one

north-trending cross section (Fig. 12 below) were constructed along lines

that correspond closely to the locations of seismic reflection profiles

(Figs. 4, 5, 7, 9, 10, and 12). Each cross section extends beyond the seismic

reflection profiles and depicts the bedrock geology of the adjacent ranges

from Evans (1991) and subsurface interpretations of Kendrick (1994).

Thus, the sections depict the nature of Tertiary extension superimposed on

Sevier folds and thrusts of the region. The north-trending seismic reflection

profiles (shown as one section in Fig. 12 below) are used to tie the east-

trending sections together and to identify the key reflection at the base of

the Tertiary rocks. This reflector is well displayed on the profiles, and its

location at depth on one of the east-trending and one of the north-trending

profiles is confirmed by the Lynn Reese drill hole. The same reflection is

taken to be the base of the Tertiary section in the other profiles. The qual-

ity of the reflection profiles was good in the southern and central parts of

the basin, but quality was poor in the northern part. We discuss the basin

and fault geometries from south to north.

Southern Cache Valley Basin in Utah

The southern end of the Cache Valley basin is a deep, asymmetric half-

graben bounded on the east by two strands of the East Cache fault zone

(Fig. 3). Reflections that we interpret as Tertiary basin-fill deposits (Fig. 4)dip 15E20E toward the East Cache fault zone and may flatten slightly

eastward near the fault. The reflection profile CVC-101 (Fig. 4A) crosses

the fault zone, but because the section is unmigrated, fault geometry and

basin fill cannot be easily resolved (see also Smith and Bruhn, 1984). Com-

plexities due to the lack of the migration include bow tie or apparent

crosscutting reflections and the appearance of the shallow-dipping part of

the inferred normal fault cutting the steeper part. We interpret the closely

spaced east-dipping reflections to represent the Tertiary basin fill (Fig. 4)

and the complex east-and-west-dipping reflections to be the result of com-

plex refractions near the fault. The base of the Tertiary rocks is determined

by the downdip projection of the surface expression of the base of the se-

quence. The fault exhibits a distinct listric form with rollover geometries

on its hanging wall (Fig. 4). There is no evidence from either map data

Figure 2. Generalized geologic map

of the Cache Valley Basin and sur-

rounding area. Paleogene rocks consist

of the Wasatch Formation that was de-

posited across thrust sheets. Neogene

rocks of the Salt Lake Formation con-

sist of conglomerates, siltstones, sand-stones, limestones, and tuffaceous sedi-

mentary units deposited in and along

the flanks of extensional basins. Cross-

section locations are shown by bold

lines, with corresponding figure num-

bers. Seismic reflection profiles are

shown by dashed lines. Drill holes dis-

cussed in the text or on cross sections

are indicated by drill-hole symbols.

HFHauser Farms drill hole, LR

Lynn Reese drill hole, SVUtah Steam

Venture, ECFEast Cache fault,

WCFWest Cache fault, WFWa-

satch fault.

TABLE 1. DATA ACQUISITION SUMMARY FOR CACHE COUNTY, UTAH, PROFILES SHOWN IN THIS PAPER

Location Date Source Source Station Recording Sample True Sweep Sweep Fold Filtersin acquired type spacing spacing system interval recording (Hz) duration Low High Rejectcounty (m) (m) (ms) time (s)

(s)

Southern June 1982 Dynamite 67 33 96 channel digital 2 6 12 128 72Southern November 27, 1973 Vibroseis 201 100 48 traces on 9 track digital 4 6 1056 13 12 60Southern November 26, 1973 Vibroseis 201 100 48 traces on 9 track digital 4 6 1056 7 12Southern February 26, 1974 Vibroseis 134 67 48 traces on 9 track digital 4 5 1656 7 12Central April 22, 1976 Dynamite 134 67 48 traces on 9 track digital 2 6 18/36 124 60


4/14



(Williams, 1958; Oaks, unpub. data) or from the reflection profiles that a

fault of any appreciable displacement bounds the western part of the basin.

Basin fill may be as much as 1800 m thick in this region (Figs. 3 and 4).

This thickness of the units and the geometry of the fault shown in Figure 3

were found by converting time to depth with velocities used to time-mi-

grate the nearby Amoco profiles (Table 2).

The normal-slip component along the East Cache fault zone is estimated

as 5.6 km at the southern end of the valley. To estimate slip, a hanging-

wall cutoff for the base of the Wasatch Formation in the seismic profile was

used along with a footwall cutoff of the base of these Tertiary rocks pro-

jected updip 4 km from the nearest outcrop of Wasatch Formation in theBear River Range (Dover, 1995).

The shape of the East Cache fault zone shown in Figure 3 is inferred

from the unmigrated profile (Fig. 4) and a migrated profile 3 km to the

north (Fig. 5). The curved surface that may represent the East Cache fault

in Figure 4 (see also Smith and Bruhn,1984) was converted to depth by us-

ing the velocity structure in Table 2. The reflection profile shown in Fig-

ure 5 (acquired 3 km north of the line shown in Fig. 4) is of poorer quality

than that of Figure 4, but may also show the general form of the fault. On

the basis of the location of the surface trace of the East Cache fault and the

subsurface form inferred from the reflection profiles, the fault dips

45W60W near the surface and flattens to dips of 30W.

The dip and geometry of the West Cache fault at depth likewise can

be precisely determined in this area. The reflection profile ends 1.5

east of the inferred surface trace of the West Cache fault. Projection of

flectors representing the base of the Tertiary rocks west to the West Ca

fault and projection of the base of Tertiary rocks in the footwall of the W

Cache fault suggest a minimum of 915 m of net slip along it and a dip t

exceeds 55E.

In the profile shown in Figure 7, the double reflector that we interpre

be the base of the Wasatch Formation, and strata 300400 m above the b

of the Tertiary, dip uniformly 10E. Dips on the double reflector beco

shallower upsection in the basin fill, and incline gently west at a depth

1200 m. We interpret the reflection profile (Fig. 7) to show the influe

of the West Cache fault on the sedimentary history of the area. The ea

Figure 3. Cross section with no vertical exaggeration at the southern end of the Cache Valley basin. Data from seismic reflection pro

CVC-101 (Fig. 4) has been placed in the context of regional geology. Flattening at depth of the East Cache fault is suggested by seismic refl

tion profile (Fig. 4). Interpretation of thrusts under Bear River Range from Kendrick (1994). Geometry of Wasatch fault and basin above

Wasatch fault in the Utah Steam Joint Venture drill hole from Jensen and King (1995). Abbreviations of ages of units: Pzuundifferentia

Paleozoic, pCProterozoic, CCambrian, OOrdovician, SSilurian, DDevonian, MMississippian, IPPennsylvanian, TTertia

QQuaternary.


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EVANS AND OAKS


Figure 4. (A) East-trending, unmigrated seismic profile CVC-101 (Placid Oil), of the southern Cache fault (AA) with either a sag-type shape

to Tertiary basin-fill deposits or a steep dip to at least 3 s in two-way traveltime. (B) Line drawing of the seismic reflection profile indicating our

interpretation of the data.

Inferred position ofthe East Cache fault

Two-waytraveltime(sec)

0

1

2

3

Base of Tertiary rocks ?

Paleozoic rocks ?

A

B


6/14



ward sag at the base of the Tertiary section clearly indicates an active fa

on the east side of the basin, whereas the wedge of sedimentary rocks

tween reflections 1 and 2 and the slight westward dip at the top of

wedge indicates that activity on the West Cache fault may have tilted

strata to the west.

Changes and complexities from south to north in fault history and

lated basin shape clearly continue northward in the cross section and

flection profiles in the central part of the study area (Figs. 8 and 9). T

very good quality of the reflection data (Fig. 9) enables us to determin

detailed history of this part of the basin. Estimates of slip on the East Ca

Figure 5. East-trending, tim

migrated seismic reflection profile 1

Amoco from the southern Cache V

ley. Prominent east-dipping band of

flections (labeled T) may be the base

the Tertiary section. The possible list

form of the East Cache fault is su

gested from the reflectors marked

arrows.

TABLE 2.VELOCITY STRUCTURE USEDTO MIGRATE SEISMIC PROFILE CVC-101

Time Two-wayinterval velocity(s) (m/s)

00.30 20120.300.60 21650.601.2 24691.21.5 30791.53.0 4329

Note:Profile shown in Figure 4. Data areapproximately the same velocity structure usedin profile VX-5 from 5 km north of

CVC-101 line.

Figure 6. Cross section with no vertical exaggeration across the study area in the south-central part of the Cache Valley basin. Prominent

flections on the seismic reflection profile 2 (Fig. 7) mark the base of the Tertiary rocks. Dips decrease upsection. Subsurface geology under

Bear River Range adapted from Kendrick (1994) and that under the western end of the section adapted from Jensen and King (1995). Abb

viations of ages of units same as in Figure 3.


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EVANS AND OAKS


fault are complicated by the presence of a thrust fault and a shallowly dip-

ping normal fault along the western margin of the range (Lowe and Gal-

loway, 1993). The dip-slip estimates provided here are based on projec-

tions of the base of the Cambrian contact across the fault. The West Cache

fault appears to die out northward (Oviatt, 1986a, 1986b; Figs. 2, 12, 13),

and the western margin of the basin is marked by several normal faults with

modest amounts of displacement.

Fault geometries in this section (Fig. 9) are constrained as before. The

minimum fault dip found by connecting the surface trace of the East Cache

fault with the easternmost reflector inferred to represent basal Tertiary

rocks gives a fault dip of at least 45W. The dip of the West Cache fault

may be constrained by gaps in reflectors at the western edge of the section,

near the region where the profile crosses the West Cache fault, and may in-

dicate a fault dipping 50E55E (Fig. 9).

Figure 7. (A) East-trending time-migrated seismic reflection profile 2. Fanning dips and a possible antiformal structure (labeled A) are clearly

seen in reflections that we interpret as representing Tertiary basin-fill deposits. Base of Tertiary marked by T. (B) Line drawing of the seismic

reflection profile shown above. Prominent reflections 1 and 2 used in the interpretation are shown. We interpret the rocks bounded by reflec-

tion 2 and the base of the Tertiary as showing the influence of the East Cache faults, whereas the sequence bounded by reflections 1 and 2 indi-

cate rotation due to the West Cache fault.

0

-1

-2

-3

-4

-5

Depth

(km)

-3

-2

-1

0


Base of Tertiary rocks (?)

Reflection 1Reflection 2

A

B


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Reflections that we interpret to be from older Tertiary units and under-

lying lower Paleozoic rocks make up an eastward-thickening wedge of

rocks that dip gently 10E20E (Fig.9), whereas reflections representing

the younger Tertiary section dip 2W5W. There is an angular discord-

ance of as much as 1015 between the lower and upper parts of the Ter-

tiary section (Fig. 10B). The west dips of the younger strata extend to the

top of the reflection profile, or 300 m below ground surface. Reflections

in the upper part of the basin also exhibit several bends and terminations

that may indicate the existence of synthetic and antithetic faults.

The angular discordance in the Tertiary basin fill (Fig. 10) may reflect

multiple stages of basin development (Fig. 11) resulting from alternating

dominance of the East and West Cache faults. This interpretation requiresthat early slip along the East Cache fault resulted in east-dipping older Ter-

tiary sedimentary units and eastward rotation of underlying Paleozoic

rocks. Later slip along the West Cache fault resulted in western rotation of

the earlier basin-fill deposits and Paleozoic rocks; this motion resulted in

shallower dips of older units, compared to the steeper dips of correlative

units in the Wellsville Mountains, directly to the west. It also resulted in

westward dips of younger Tertiary rocks deposited during slip on the West

Cache fault. This double tilting could also result from changes in relative,

long-term slip rates on the two faults, wherein the west tilting could be the

result of a higher slip rate on the West Cache fault relative to that on the

East Cache fault. Finally, the west-dipping sequences may be the result of

large sediment influx from the east into the basin.

The two reflection profiles from the central part of the basin also have re-

flections that appear to define anticlines in the hanging wall of the East

Cache fault (Figs. 69). These structures may represent rollover of sedi-

mentary units above the normal fault or deformation of hanging-wall strata

in response to variations in fault geometry at depth (Dula, 1991; Rowan and

Kligfield, 1989; Groshong, 1989; Xiao and Suppe, 1992; Schlische, 1995;

Janecke, 1996). Unfortunately, the reflection profile does not extend far

enough eastward to apply any of these geometric models to infer what the

fault shape at depth may be.

A north-trending seismic reflection profile with ties to the Lynn Reese

drill hole (Fig. 12A) traverses part of the Cache Valley basin in Utah. We

interpret that a relatively continuous and strong reflection in profiles in

south and central parts of the basin in Utah correlates with the base of T

tiary rocks encountered in the Lynn Reese drill hole (Fig. 12B). This c

relation with the reflections on the profile is made on the basis of drill-h

cuttings, velocity logs, and drilling reports. The data show that the base

the Tertiary strata slopes gently northward near the area of Figure 8,

though reflections in the overlying Tertiary strata exhibit overall dips to

southeast. The data suggest that the Tertiary rocks could be as much

3200 m thick in the central part of the basin in Utah.

The southward deepening of the base of the Tertiary basin coincides w

a southward dip of the Paleozoic rocks, resulting in much younger Pal

zoic rocks at the basin bottom in the southern part of the basin (Fig. 12Bedrock geology west of the basin also shows a marked southward you

ing of the Tertiary subcrop pattern (Oviatt, 1986a, 1986b), and this tra

tion is associated with a transverse zone identified by Zoback (1983) a

Mabey (1987) by geophysical signatures. This south-dipping homocl

may indicate structure in the Paleozoic rocks associated with the Sevier

formation in the area, such as a lateral ramp on a deep-level thrust.

The north-trending cross section reveals some relief at the base of

Tertiary rocks (Fig. 12). The northward rise of the base of the Tertiary str

may be due to the interplay among fault dip, variations in the amoun

slip along the strike of the normal faults, and amounts and rates of depo

tion of the basin-fill deposits.

Northern Cache Valley Basin in Utah

The quality of the seismic reflection profiles acquired across the nor

ern part of the basin is very poor. From the profiles, we were able to in

the location of the base of Tertiary rocks,but no internal structure of ba

fill deposits or fault geometry could be determined. From the scant insig

provided by the profiles and from map data, we infer that the northern p

of the basin in Utah is a broad, flat-bottomed basin with a thin sequenc

gently east-dipping Tertiary rocks (Figs. 12 and 13). Normal slip on

East Cache fault zone is 4.56 km (Fig. 12), and slip becomes distribu

northward on several fault strands. The eastern splay dies out northw

Figure 8. Central cross section with no vertical exaggeration, based on seismic reflection profile shown in Figure 9. The minimum dip of

East Cache fault is 45W, and the West Cache fault dips east. Basin structure indicated by lines representing prominent reflectors, with a p

nounced unconformity between upper west-dipping strata and lower east-dipping strata. Abbreviations of ages of units same as in Figur

Subsurface geology under Bear River Range conjecture based on Kendrick (1994).


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EVANS AND OAKS


Base of Tertiary ?

West Cache fault ?

-1

-2

-3


1

2

3

4

-2

-3

-4

-5

-6

Depth(km)

-1

0 0

Figure 9. (A) East- and northwest-trending, time-migrated seismic reflection profile 3. Deep east-dipping reflections inferred to be Paleozoic

rocks on the basis of data from Lynn Reese drill hole to north. Easternmost reflections at depth constrain the dip of East Cache fault to be at

least 45W. (B) Line drawing of seismic profile shown in A. Prominent reflections 14 may be unconformities that mark change from east-dip-

ping and west-dipping sequences; the base of Tertiary rocks may be above reflector 4.

A

B


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into Proterozoic and Cambrian rocks of the Bear River Range (Brummer,

1991), whereas the western splay appears to increase in displacement

northward and exhibits evidence for recent activity (McCalpin, 1989).

The form and thickness of the basin fill is constrained by the reflection

profiles, by regional gravity data (Peterson and Oriel, 1970; Cook et al.,

1989) and by the Hauser Farms drill hole that encountered 1400 m of Ter-

tiary sedimentary rocks before reaching Cambrian carbonate units (Brum-

mer, 1991). The gravity data from the region and the northward projection

of the reflection on the north-trending seismic line also support a Tertiary

basin fill of1.5 km and suggest that the basin bottom is nearly flat.

DISCUSSION

The Cache Valley basin is a narrow, deep half-graben above a single

west-dipping, listric normal fault at its southern end. In the central part, the

basin is a doubly-tilted graben bounded on both sides by normal faults,

with east-to-west variations in dips and thicknesses of Tertiary units. At the

northern end, the basin is broad, shallow, and flat bottomed (Table 3).

Where constrained by the reflection data, the subsurface of the East Cache

fault zone is listric. Maps of the southwestern part of the Cache Valley

basin (Williams, 1958) show few normal-fault traces, which suggests that

the West Cache fault dies out or splits into a series of poorly exposed splays

southward. The northern two cross sections (Figs. 13 and 14) show that to-

tal slip is distributed across a series of range-bounding normal faults and

that net slip along these faults decreases northward.The cross sections based on seismic reflection profiles clearly demon-

strate a complex architecture of the basin not revealed by surface geologic

data. The seismic data also show that single, across-strike sections, or

widely spaced sections, may not reveal the along-strike complexity of

basins in extending regions (see also Liberty et al., 1994; Bohannon et al.,

1993; Brocher et al., 1993; Russell and Snelson, 1994). Basin width, sedi-

mentary basin-fill thickness and geometry, fault slip, and fault geometry all

vary significantly from north to south. This nonuniformity may be caused

by (1) along-strike changes in fault slip, (2) development of fault segments

as the basin-bounding faults grew, (3) along-strike variations in fault geo

etry, (4) interactions between timing of slip and deposition,or (5) devel

ment of subbasins or younger basins superimposed on older basins.

The narrow and deep trough of Tertiary rocks above a west-dipping fa

at the southern end of the basin is supported by the data presented here a

by the gravity signal of the area. Cook et al. (1989) showed that a narr

gravity low extending from the south-central part of the basin toward

Figure 10. True-scale, no vertical exaggeration depictions of the se-

quences of rocks in seismic profiles 2 and 3 (Figs. 7 and 9). Depths

from the time-migrated reflection profiles are plotted with third-order

polynomial fit to data. (A) In 2, base of the Tertiary dips 9E, and re-

flector 2 dips 5E. Reflector 1 dips 2W. (B) In 3, West Cache fault

dips 40E, and the reflector at the base of the Tertiary dips 10E.

Reflectors higher in the section dip 3W5W.

Figure 11. Schematic illustration of the evolution of the Cache V

ley basin along the section line in Figure 8. Basin evolution based

geometry of the Tertiary basin-fill deposits in the reflection profile

Dashed lines indicate the extent of reflections from the west. (A) Ea

slip on the East Cache fault results in east-tilted, eastward-thicken

Tertiary basin fill (EC1). (B) Subsequent slip on the West Cache fa

results in the westward-thickening sequence (WC1). A clastic wed

(CW) shed from the footwall of the East Cache fault is permissib

based on the updip projection of the base of WC1. This clastic wed

may be similar to coarse conglomerate exposed along the western ed

of the Bear River Range (Adamson et al., 1955; Lowe and Gallow

1993). (C) Renewed or accelerated slip on the East Cache fault resu

in another eastward-thickening wedge of sediment (EC2). (D) R

newed or accelerated slip on the West Cache fault results in a seco

wedge of westward-thickening sediment (WC2). (E) The pres

geometry of the basin from the reflection profile.

A

B


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EVANS AND OAKS


1.0

2.0

3.0


-1

-2

-3

-4

-5

-6

Amoco Lynn Reese

Depth(km)

Tsl

Ordovician rocks

Base of Tertiary rocks

Line of section, Fig. 6and reflection profile, Fig. 7

Line of Section, Fig. 8and reflection profile, Fig. 9

Silurian - Pennsylvanian rocks

Quaternary deposits

5 km

41 37'41 45'

South North

A

B


12/14



southern tip of the basin may indicate that the southern Cache Valley is a

compound basin, with a MioceneRecent basin superimposed on an

older, pre-Miocene structural low. Evidence for north-trending, pre-

Miocene normal faults bounding narrow grabens in the Bear River Range

(Oaks and Runnells, 1992), possibly early Tertiary ages for ash-fall u

in the southern part of the basin (Williams, 1964; K. Smith, personal c

mun.), and the evidence for pre-Miocene extension 80 km south of

Cache Valley (Bryant et al., 1989) all support the hypothesis that the so

ern Cache Valley basin may consist of an earlier Tertiary basin that

modified by Basin and Range extension.

The large amount of slip inferred on the East Cache fault at the sout

end of the basin must decrease abruptly southward or be transferred to

recognized faults, as the fault has not been mapped across the struct

high between Ogden and Cache Valley (Nelson and Sullivan, 19

Williams, 1958). This abrupt change in fault displacement is at odds

models for slip distributions on normal faults, in which slip is though

taper toward the ends of the faults (Cowie and Scholz, 1992; Scholz, 1

Dawers et al., 1993). The lack of southward-tapering displacements

also support the model of some preBasin and Range faulting at the so

Figure 12. (A) North-striking seismic reflection profile 4 through the

Cache Valley basin. Tertiary basin-fill deposits thicken southward, and

reflections from Paleozoic rocks are deeper in southern part of the sec-

tion. (B) Line drawing of the reflection profile shown in Figure 12A.

The thickness of the Tertiary strata increases southward from 1.2 to

3 km. The reflection that we infer to be due to the base of the Tertiary

section is penetrated by the Lynn Reese drill hole.

Figure 13. Cross section in the nor

ern part of the basin shows that

basin is shallower and flatter than

the southern part.

Figure 14. Northernmost cross section in the study area, based on seismic reflection profiles acquired by Amoco and profile acquired

Placid Oil. Cache Valley basin at this latitude is a broad, flat-bottomed structure with one horst block. To the north are several horsts cove

by lower Paleozoic rocks (Peterson and Oriel, 1970; Williams, 1948).

TABLE 3.SUMMARY OF CACHE VALLEY BASIN STRUCTURE

Figure East Cache fault West Cache fault Basin Structure Maximum Width/Component Component width of basin fill thickness depth

Dip Dip slip Dip Dip slip at surface of Tertiary

() (km) () (km) (km) sedimentaryunits(m)

14 >45 3.6 Not present 22 Flat-bottomed basin 1400 15.713 >45 4.56.0 Unknown Hundreds 21 Slight east tilting of a flat-bottomed basin 2000 10.58 >45 5.4 minimum 45+ 300400 20 Doubly tilted prisms, anticline present 3900 5.16 >30 4.56.4 >55 915 12.6 East-tilted prism, some late west tilt, anticline present 2700 4.63 2050 5.6 Not present 12 East-tilted prism, curved bedding, one master listric fault 2800 4.2

Note:Section in Figure 3 crosses the southern Cache Valley. Other sections cross the northern part of the valley. See Figure 2 for locations.


13/14

EVANS AND OAKS


ern part of the basin.

The along-strike variations in basin shape, especially between the cen-

tral and southern parts of the basin in Utah, may also show the conse-

quences of long-term heterogeneous behavior of basin-bounding faults

changes in fault slip, fault geometry, and depositional patterns along strike.

Fault segmentation recognized on the basis of geomorphology (Machette,

1987; McCalpin, 1989), structures in the footwall bedrock of normal faults

in the Basin and Range (Susong et al., 1990; Janecke, 1993), and gravity

data (Wheeler and Krystinik, 1987) imply that segments may persistthrough several million years (Scholz, 1990; Cowie and Scholz, 1992).

Earlier onset of slip, a faster slip rate, or concentration of slip along a sin-

gle fault for the southern part of the East Cache fault zone could have re-

sulted in a deeper basin in the south and a shallower basin farther north,

where later onset of slip, a slower slip rate,or slip distributed along a num-

ber of fault strands making up the central segment may have dominated.

One major transition zone, between the northern and southern parts of the

basin in Utah, coincides with a transverse structure of the Cache Valley basin

inferred by Zoback (1983) and with a segment boundary of the East Cache

fault defined by McCalpin (1989). The thinning and apparent folding of Ter-

tiary basin-fill deposits in this area may have been caused by the develop-

ment of a branch in the East Cache fault zone. This splaying may also be co-

incident with changes in the total amount of slip on the fault, creating a

deeper basin toward the south. It is also possible that an east-striking, down-to-the-south normal fault may have caused this intrabasin high. Such subdi-

vision of Tertiary basins in the Great Basin may be common (Effinoff and

Pinezich, 1981; Liberty et al., 1994; Bohannon et al., 1993) and may reflect

complexities of inherited structure or of growth processes of normal faults

and normal-faultbounded basins (Russell and Snelson, 1994).

Numerous interpretations of other normal faults in the northern Wasatch

to Teton corridor (Arabasz and Julander, 1986; Coogan and Royse, 1990;

Evans, 1991; Sprinkel, 1979; Webel, 1987; West 1992, 1993) show that

many of the normal faults probably curve with depth and in some cases co-

incide with older thrust structures of the Sevier orogeny. The seismic re-

flection data in the Cache Valley do not clearly show the relationships be-

tween normal faults and thrust faults, nor the dips on the deeper parts of the

normal faults. The East Cache fault cuts small-displacement thrusts along

the western margin of the Bear River Range (Dover, 1995; Evans et al.,1996), but the subsurface relationships to larger thrusts cannot be confi-

dently inferred. The listric geometry that we infer for the southern East

Cache fault suggests that the fault lies within the Willard thrust sheet and

does not sole into the basal thrust. It may be related to a structurally higher-

level thrust not well expressed at the surface or in the reflection profiles.

CONCLUSIONS

The interpretations presented here indicate that the normal faults of the

region are not simple, planar structures (cf. Westaway, 1989a, 1989b).

Evaluation of extensional kinematics and seismicity of the Basin and

Range province should account for a variety of fault shapes and displace-

ments, both along and perpendicular to the normal faults of the region

(Effinoff and Pinezich, 1981; Bohannon et al., 1993; Liberty et al., 1994).Our work also provides new evidence for a complex history and geometry

of what appears at the surface to be a relatively simple hanging-wall basin

at the eastern margin of the Basin and Range province. These data indicate

that the geometry of normal faults, slip rates and total amounts of slip, the

geometry and distribution of sedimentary units,and the geologic history of

Basin and Range basins cannot be inferred from single cross sections.

ACKNOWLEDGMENTS

This work was partially supported by the Utah Geological Survey. We

thank Amoco Production Company, Placid Oil Company, Chevron, Exxon,

and Texaco for access to seismic reflection profiles, and we are grateful for

the assistance of Bob Cook,Roger Dickinson, Clyde Kelley, Don Roberts,

Tad Schirmer, and Doug Sprinkel. We give special thanks to T. L. Patton

III for assistance in getting Amoco profiles released for publication. We

thank Amoco Oil Co. for permission to publish the seismic reflection pro-files. The interpretations presented here are solely ours and not those of

Amoco or Placid or its employees. Mark C. Hall compiled most of a

1:100 000 geologic map used in this study. J. Coogan provided unpub-

lished maps of the area east of Bear Lake and several cross sections. We

thank James P. McCalpin for providing assistance in understanding the

neotectonics of the region, Susanne U. Janecke for her insights into exten-

sion and for a critical review of this paper, James Pechmann for his thor-

ough review of an earlier manuscript that was the basis of this paper,Bul-

letin reviewers Robert Bohannon and Joe Kruger, and associate editor

Steven Reynolds, for many useful comments on the original manuscript.

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