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Geological Society of America Bulletin

doi: 10.1130/0016-7606(1996)1082.3.CO;2 1996;108;1580-1593Geological Society of America Bulletin

James P. Evans and Robert Q. Oaks, Jr. Cache Valley basin, eastern Basin and Range province, United StatesThree-dimensional variations in extensional fault shape and basin form: The

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ABSTRACT

Seismic reflection profiles, drill-hole data, and geologic maps delimitthe form of normal faults and Tertiary sedimentary rocks in the south-ern half of the Cache Valley basin in northern Utah. Dips of faults andsedimentary rocks were estimated from time-migrated reflection pro-files by using the stacking velocities for the data. At the southern end ofthe basin, the East Cache fault zone is listric; it shows 50°W dips nearthe surface and ≈20°W dips at depth. The fault zone has been the siteof ≈5.6 km of net dip slip. Tertiary rocks dip 18°E–25°E and exhibit arollover geometry above the fault zone. No faults are interpreted on thewestern side of the basin. In the central part of the basin in Utah, theEast Cache fault is a single fault that dips at least 45°W near the sur-face and is the site of 4.5–6.4 km of net dip slip. Here, the basin isbounded to the west by the West Cache fault, which is the site of at least1 km of net slip that increases northward to 2 km of net slip. Slip on theEast Cache fault resulted in planar, east-dipping, older Tertiary rocksnear the bottom of the basin. Younger Tertiary strata, with southwest,west, and northwest dips, reflect complex tilting due to slip on the Westand East Cache faults. Anticlines in the Tertiary basin-fill deposits arepresent in the central part of the basin and may reflect changes in nor-mal-fault geometry at depth. Northward, dip slip on the East Cachefault zone decreases to 2.5 km. The basin is broad, shallow, and filledwith nearly flat lying Tertiary rocks. This area, near the north-southmidpoint of the basin, is bounded by the West and East Cache faults,but slip on the West Cache fault appears to diminish northward. Anorth-trending reflection profile tied to both drill-hole data and theeast-trending seismic profiles indicates that the basin is deeper in thesouthern end. The along-strike changes in fault geometry, the amountof fault-slip, the subsurface form of the basin-filling sedimentary rocks,and the form of the basin indicate a complex history of faulting anddeposition during its formation. This study and other recent ones fromthe Basin and Range province indicate that such complexities may betypical of many Tertiary basins in the region.

INTRODUCTION

Analyses of subsurface geology of extensional basins in the Basin andRange province commonly are based on east-trending seismic reflectionprofiles combined with surface geologic data, and rare drill-hole data (e.g.,Anderson et al., 1983; Bohannon et al., 1993; Brocher et al., 1993; Libertyet al., 1994; Thompson et al., 1989). Such studies provide valuable insightinto the form of the basin-bounding faults and the geometry of basin-fillingsedimentary rocks. In this study, we present an analysis of a grid of seismicreflection 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 thesubsurface geometry of the major normal faults in the basin. These dataalso provide evidence of north-south and east-west variations in the dipsand thicknesses of Tertiary basin-fill deposits. We combine the subsurfacedata with surface geologic data to show that the basin exhibits significantalong-strike variations in (1) the amount of slip on the dominant, basin-bounding East Cache fault zone, (2) the thickness and form of basin-fillingstrata, (3) the fault-related depositional history in the basin, and (4) the rel-ative importance of the East and West Cache faults in controlling basingeometry.

As part of exploration for hydrocarbons in Tertiary basin-fill deposits innorthern Utah, Amoco Production Company (Patton and Lent, 1977) andPlacid 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 and1974; they were reprocessed in 1982 by Amoco and acquired in 1982 byPlacid Oil. Data acquisition and processing parameters for the reflectionprofiles shown in this paper are summarized in Table 1. Seismic reflectionprofiles acquired by Amoco and provided to us are time-migrated sectionswith 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 thebasin. 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 theTertiary Wasatch Formation, and had a total depth of 2487 m in the Or-dovician Swan Peak Formation. The North American Resources HauserFarms drill hole (sec. 10, T. 13 N., R. 1 W.) on the western side of the basinpenetrated 1417 m of Tertiary Salt Lake Formation rocks, which uncon-formably lie on Cambrian rocks (Brummer, 1991). We tie the drill holedata to the seismic reflection profiles by correlating the depth to the top ofeach 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-grabensthat form the extensional corridor between the Wasatch and Teton normalfaults (Fig. 1). These normal faults are the loci of Pleistocene and Holoceneseismicity (Smith and Sbar, 1974; McCalpin and Forman, 1991; Piety etal., 1992) and cut Proterozoic through Cretaceous rocks that underwentcontractional deformation during the Sevier orogeny. Seismic reflection

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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. 1580–1593; 14 figures; 3 tables.


http://gsabulletin.gsapubs.org/

CACHE VALLEY BASIN, EASTERN BASIN AND RANGE PROVINCE

Geological Society of America Bulletin, December 1996 1581

profiles across Tertiary basins that developed in the fold-and-thrust beltshow that the normal faults are commonly listric and sole into underlyingmajor thrust faults (Royse et al., 1975; Coogan and Royse, 1990; West,1992). Basins 10–30 km wide formed in the hanging walls of the normalfaults and filled with Tertiary sedimentary deposits (Fig. 1).

The Cache Valley basin is bounded on its eastern side by the East Cachefault zone, which is 70 km long (McCalpin, 1989), and uplifts folded Prot-erozoic and Paleozoic rocks in its footwall. It splays into two major strandsin 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 sideby the West Cache fault, which is ≈20 km long and has been active duringthe Quaternary (Oviatt, 1986a).

Tertiary rocks of the area consist of the Eocene Wasatch Formation andthe Miocene–Pliocene 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 Valleybasin. It consists of mudstones, conglomerates, and minor limestones de-posited in paleovalleys and across a relatively broad surface in northernUtah (Oaks and Runnells, 1992). We infer that the Wasatch Formation wascontinuous 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 least245 m thick locally in the Bear River Range (Oaks and Runnells, 1992).

The Salt Lake Formation consists of conglomerates, tuffaceous sand-stones and siltstones, and limestones (Williams, 1962) that are 2231 m thickin the deepest drill hole in the Cache Valley basin (Brummer, 1991). Thick-nesses and stratigraphic relationships in the Salt Lake Formation are highlyvariable across northern Utah and southern Idaho because basal parts of theSalt Lake Formation probably were deposited across a broad area, butyounger parts of the formation record localized deposition in individualbasins that formed during the development of modern Basin and Rangestructures (Platt, 1988). Late Tertiary and Quaternary lacustrine and fluvialdeposits of the Bonneville and older lake cycles overlie the Salt Lake For-mation 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 difficultto date owing to poor exposures and the uncertain age of the Salt Lake For-mation. Hintze (1988) and Parry and Bruhn (1986) suggested that exten-sion began in the region at ca. 17 Ma. Bryant et al. (1989) proposed that fel-sic volcanic rocks intercalated with the Salt Lake Formation in the SaltLake City area record the onset of extension at 21 Ma and that later rapidsubsidence of basins coincident with faulting began at ca. 11 Ma.

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

Figure 1. Regional setting of the Cache Valleybasin in the northern Wasatch to Teton corridor.Narrow basins (shaded regions) form above-nor-mal faults that are superposed on Sevier foldsand thrusts. Onset of normal faulting rangedfrom ca. 17 Ma for the Wasatch fault to 2–4 Mafor the Grey’s River fault. See Figure 2 for lati-tude and longitude of the study area.



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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 acomplex basin with several east-trending structural highs and lows. Oneeast-trending gravity high, across the central part of the basin in Utah, maycoincide 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 deepin the south and gradually becomes shallower northward.

RESULTS

Five east-trending cross sections (Figs. 3, 6, 8, 13, and 14) and onenorth-trending cross section (Fig. 12 below) were constructed along linesthat correspond closely to the locations of seismic reflection profiles(Figs. 4, 5, 7, 9, 10, and 12). Each cross section extends beyond the seismicreflection profiles and depicts the bedrock geology of the adjacent rangesfrom Evans (1991) and subsurface interpretations of Kendrick (1994).Thus, the sections depict the nature of Tertiary extension superimposed onSevier folds and thrusts of the region. The north-trending seismic reflectionprofiles (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 ofthe Tertiary rocks. This reflector is well displayed on the profiles, and itslocation at depth on one of the east-trending and one of the north-trendingprofiles 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 ofthe basin, but quality was poor in the northern part. We discuss the basinand 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 15°E–20°E toward the East Cache fault zone and may flatten slightlyeastward near the fault. The reflection profile CVC-101 (Fig. 4A) crossesthe fault zone, but because the section is unmigrated, fault geometry andbasin fill cannot be easily resolved (see also Smith and Bruhn, 1984). Com-plexities due to the lack of the migration include “bow tie” or apparentcrosscutting reflections and the appearance of the shallow-dipping part ofthe inferred normal fault cutting the steeper part. We interpret the closelyspaced 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 determinedby the downdip projection of the surface expression of the base of the se-quence. The fault exhibits a distinct listric form with rollover geometrieson its hanging wall (Fig. 4). There is no evidence from either map data

Figure 2. Generalized geologic mapof the Cache Valley Basin and sur-rounding area. Paleogene rocks consistof the Wasatch Formation that was de-posited across thrust sheets. Neogenerocks of the Salt Lake Formation con-sist of conglomerates, siltstones, sand-stones, limestones, and tuffaceous sedi-mentary units deposited in and alongthe flanks of extensional basins. Cross-section locations are shown by boldlines, with corresponding figure num-bers. Seismic reflection profiles areshown by dashed lines. Drill holes dis-cussed in the text or on cross sectionsare indicated by drill-hole symbols.HF—Hauser Farms drill hole, LR—Lynn Reese drill hole, SV—Utah SteamVenture, ECF—East Cache fault,WCF—West Cache fault, WF—Wa-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 10–56 13 12 60Southern November 26, 1973 Vibroseis 201 100 48 traces on 9 track digital 4 6 10–56 7 12Southern February 26, 1974 Vibroseis 134 67 48 traces on 9 track digital 4 5 16–56 7 12Central April 22, 1976 Dynamite 134 67 48 traces on 9 track digital 2 6 18/36 124 60





(Williams, 1958; Oaks, unpub. data) or from the reflection profiles that afault 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 3were 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 estimatedas ≈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 wasused 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 inferredfrom the unmigrated profile (Fig. 4) and a migrated profile 3 km to thenorth (Fig. 5). The curved surface that may represent the East Cache faultin 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 qualitythan that of Figure 4, but may also show the general form of the fault. Onthe basis of the location of the surface trace of the East Cache fault and thesubsurface form inferred from the reflection profiles, the fault dips≈45°W–60°W near the surface and flattens to dips of 30°W.

The dip and geometry of the West Cache fault at depth likewise cannotbe precisely determined in this area. The reflection profile ends ≈1.5 kmeast of the inferred surface trace of the West Cache fault. Projection of re-flectors representing the base of the Tertiary rocks west to the West Cachefault and projection of the base of Tertiary rocks in the footwall of the WestCache fault suggest a minimum of 915 m of net slip along it and a dip thatexceeds 55°E.

In the profile shown in Figure 7, the double reflector that we interpret tobe the base of the Wasatch Formation, and strata 300–400 m above the baseof the Tertiary, dip uniformly ≈10°E. Dips on the double reflector becomeshallower upsection in the basin fill, and incline gently west at a depth of≈1200 m. We interpret the reflection profile (Fig. 7) to show the influenceof the West Cache fault on the sedimentary history of the area. The east-

Figure 3. Cross section with no vertical exaggeration at the southern end of the Cache Valley basin. Data from seismic reflection profileCVC-101 (Fig. 4) has been placed in the context of regional geology. Flattening at depth of the East Cache fault is suggested by seismic reflec-tion profile (Fig. 4). Interpretation of thrusts under Bear River Range from Kendrick (1994). Geometry of Wasatch fault and basin above theWasatch fault in the Utah Steam Joint Venture drill hole from Jensen and King (1995). Abbreviations of ages of units: Pzu—undifferentiatedPaleozoic, pC—Proterozoic, C—Cambrian, O—Ordovician, S—Silurian, D—Devonian, M—Mississippian, IP—Pennsylvanian, T—Tertiary,Q—Quaternary.



EVANS AND OAKS


Figure 4. (A) East-trending, unmigrated seismic profile CVC-101 (Placid Oil), of the southern Cache fault (A–A′) with either a sag-type shapeto 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 ourinterpretation of the data.

Inferred position ofthe East Cache fault

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ward sag at the base of the Tertiary section clearly indicates an active faulton the east side of the basin, whereas the wedge of sedimentary rocks be-tween reflections 1 and 2 and the slight westward dip at the top of thewedge indicates that activity on the West Cache fault may have tilted thestrata to the west.

Changes and complexities from south to north in fault history and re-lated basin shape clearly continue northward in the cross section and re-flection profiles in the central part of the study area (Figs. 8 and 9). Thevery good quality of the reflection data (Fig. 9) enables us to determine adetailed history of this part of the basin. Estimates of slip on the East Cache

Figure 5. East-trending, time-migrated seismic reflection profile 1 ofAmoco from the southern Cache Val-ley. Prominent east-dipping band of re-flections (labeled T) may be the base ofthe Tertiary section. The possible listricform of the East Cache fault is sug-gested from the reflectors marked byarrows.

TABLE 2.VELOCITY STRUCTURE USED TO MIGRATE SEISMIC PROFILE CVC-101

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

0–0.30 20120.30–0.60 21650.60–1.2 24691.2–1.5 30791.5–3.0 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 re-flections on the seismic reflection profile 2 (Fig. 7) mark the base of the Tertiary rocks. Dips decrease upsection. Subsurface geology under theBear River Range adapted from Kendrick (1994) and that under the western end of the section adapted from Jensen and King (1995). Abbre-viations of ages of units same as in Figure 3.



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 Cachefault 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 withmodest amounts of displacement.

Fault geometries in this section (Fig. 9) are constrained as before. Theminimum fault dip found by connecting the surface trace of the East Cachefault with the easternmost reflector inferred to represent basal Tertiaryrocks gives a fault dip of at least 45°W. The dip of the West Cache faultmay 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 ≈50°E–55°E (Fig. 9).

Figure 7. (A) East-trending time-migrated seismic reflection profile 2. Fanning dips and a possible antiformal structure (labeled A) are clearlyseen in reflections that we interpret as representing Tertiary basin-fill deposits. Base of Tertiary marked by T. (B) Line drawing of the seismicreflection 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.

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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 ofrocks that dip gently 10°E–20°E (Fig. 9), whereas reflections representingthe younger Tertiary section dip 2°W–5°W. There is an angular discord-ance of as much as 10°–15° between the lower and upper parts of the Ter-tiary section (Fig. 10B). The west dips of the younger strata extend to thetop of the reflection profile, or ≈300 m below ground surface. Reflectionsin the upper part of the basin also exhibit several bends and terminationsthat may indicate the existence of synthetic and antithetic faults.

The angular discordance in the Tertiary basin fill (Fig. 10) may reflectmultiple stages of basin development (Fig. 11) resulting from alternatingdominance 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 Paleozoicrocks. Later slip along the West Cache fault resulted in western rotation ofthe earlier basin-fill deposits and Paleozoic rocks; this motion resulted inshallower dips of older units, compared to the steeper dips of correlativeunits in the Wellsville Mountains, directly to the west. It also resulted inwestward dips of younger Tertiary rocks deposited during slip on the WestCache 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 theresult of a higher slip rate on the West Cache fault relative to that on theEast Cache fault. Finally, the west-dipping sequences may be the result oflarge 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 EastCache fault (Figs. 6–9). These structures may represent rollover of sedi-mentary units above the normal fault or deformation of hanging-wall stratain response to variations in fault geometry at depth (Dula, 1991; Rowan andKligfield, 1989; Groshong, 1989; Xiao and Suppe, 1992; Schlische, 1995;Janecke, 1996). Unfortunately, the reflection profile does not extend farenough eastward to apply any of these geometric models to infer what thefault shape at depth may be.

A north-trending seismic reflection profile with ties to the Lynn Reesedrill hole (Fig. 12A) traverses part of the Cache Valley basin in Utah. We

interpret that a relatively continuous and strong reflection in profiles in thesouth and central parts of the basin in Utah correlates with the base of Ter-tiary rocks encountered in the Lynn Reese drill hole (Fig. 12B). This cor-relation with the reflections on the profile is made on the basis of drill-holecuttings, velocity logs, and drilling reports. The data show that the base ofthe Tertiary strata slopes gently northward near the area of Figure 8, al-though reflections in the overlying Tertiary strata exhibit overall dips to thesoutheast. The data suggest that the Tertiary rocks could be as much as3200 m thick in the central part of the basin in Utah.

The southward deepening of the base of the Tertiary basin coincides witha southward dip of the Paleozoic rocks, resulting in much younger Paleo-zoic rocks at the basin bottom in the southern part of the basin (Fig. 12B).Bedrock geology west of the basin also shows a marked southward young-ing of the Tertiary subcrop pattern (Oviatt, 1986a, 1986b), and this transi-tion is associated with a transverse zone identified by Zoback (1983) andMabey (1987) by geophysical signatures. This south-dipping homoclinemay indicate structure in the Paleozoic rocks associated with the Sevier de-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 theTertiary rocks (Fig. 12). The northward rise of the base of the Tertiary stratamay be due to the interplay among fault dip, variations in the amount ofslip along the strike of the normal faults, and amounts and rates of deposi-tion of the basin-fill deposits.

Northern Cache Valley Basin in Utah

The quality of the seismic reflection profiles acquired across the north-ern part of the basin is very poor. From the profiles, we were able to inferthe location of the base of Tertiary rocks, but no internal structure of basin-fill deposits or fault geometry could be determined. From the scant insightsprovided by the profiles and from map data, we infer that the northern partof the basin in Utah is a broad, flat-bottomed basin with a thin sequence ofgently east-dipping Tertiary rocks (Figs. 12 and 13). Normal slip on theEast Cache fault zone is 4.5–6 km (Fig. 12), and slip becomes distributednorthward on several fault strands. The eastern splay dies out northward

Figure 8. Central cross section with no vertical exaggeration, based on seismic reflection profile shown in Figure 9. The minimum dip of theEast Cache fault is 45°W, and the West Cache fault dips east. Basin structure indicated by lines representing prominent reflectors, with a pro-nounced unconformity between upper west-dipping strata and lower east-dipping strata. Abbreviations of ages of units same as in Figure 3.Subsurface geology under Bear River Range conjecture based on Kendrick (1994).



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Base of Tertiary ?

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Figure 9. (A) East- and northwest-trending, time-migrated seismic reflection profile 3. Deep east-dipping reflections inferred to be Paleozoicrocks 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 atleast 45°W. (B) Line drawing of seismic profile shown in A. Prominent reflections 1–4 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

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into Proterozoic and Cambrian rocks of the Bear River Range (Brummer,1991), whereas the western splay appears to increase in displacementnorthward and exhibits evidence for recent activity (McCalpin, 1989).

The form and thickness of the basin fill is constrained by the reflectionprofiles, 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 projectionof the reflection on the north-trending seismic line also support a Tertiarybasin fill of ≈1.5 km and suggest that the basin bottom is nearly flat.

DISCUSSION

The Cache Valley basin is a narrow, deep half-graben above a singlewest-dipping, listric normal fault at its southern end. In the central part, thebasin 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 thenorthern end, the basin is broad, shallow, and flat bottomed (Table 3).Where constrained by the reflection data, the subsurface of the East Cachefault zone is listric. Maps of the southwestern part of the Cache Valleybasin (Williams, 1958) show few normal-fault traces, which suggests thatthe West Cache fault dies out or splits into a series of poorly exposed splayssouthward. The northern two cross sections (Figs. 13 and 14) show that to-tal slip is distributed across a series of range-bounding normal faults andthat 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 geologicdata. The seismic data also show that single, across-strike sections, orwidely spaced sections, may not reveal the along-strike complexity ofbasins 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 allvary significantly from north to south. This nonuniformity may be causedby (1) along-strike changes in fault slip, (2) development of fault segments

as the basin-bounding faults grew, (3) along-strike variations in fault geom-etry, (4) interactions between timing of slip and deposition, or (5) develop-ment of subbasins or younger basins superimposed on older basins.

The narrow and deep trough of Tertiary rocks above a west-dipping faultat the southern end of the basin is supported by the data presented here andby the gravity signal of the area. Cook et al. (1989) showed that a narrowgravity low extending from the south-central part of the basin toward the

Figure 10. True-scale, no vertical exaggeration depictions of the se-quences of rocks in seismic profiles 2 and 3 (Figs. 7 and 9). Depthsfrom the time-migrated reflection profiles are plotted with third-orderpolynomial fit to data. (A) In 2, base of the Tertiary dips ≈9°E, and re-flector 2 dips ≈5°E. Reflector 1 dips ≈2°W. (B) In 3, West Cache faultdips ≈40°E, and the reflector at the base of the Tertiary dips ≈10°E.Reflectors higher in the section dip 3°W–5°W.

Figure 11. Schematic illustration of the evolution of the Cache Val-ley basin along the section line in Figure 8. Basin evolution based ongeometry of the Tertiary basin-fill deposits in the reflection profile 3.Dashed lines indicate the extent of reflections from the west. (A) Earlyslip on the East Cache fault results in east-tilted, eastward-thickeningTertiary basin fill (EC1). (B) Subsequent slip on the West Cache faultresults in the westward-thickening sequence (WC1). A clastic wedge(CW) shed from the footwall of the East Cache fault is permissiblebased on the updip projection of the base of WC1. This clastic wedgemay be similar to coarse conglomerate exposed along the western edgeof the Bear River Range (Adamson et al., 1955; Lowe and Galloway,1993). (C) Renewed or accelerated slip on the East Cache fault resultsin another eastward-thickening wedge of sediment (EC2). (D) Re-newed or accelerated slip on the West Cache fault results in a secondwedge of westward-thickening sediment (WC2). (E) The presentgeometry of the basin from the reflection profile.

A

B



EVANS AND OAKS


1.0

2.0

3.0

Tw

o-w

aytr

avel

tim

e(s

ec)

-1

-2

-3

-4

-5

-6

Amoco Lynn Reese

Dep

th(k

m)

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





southern tip of the basin may indicate that the southern Cache Valley is a“compound” basin, with a Miocene–Recent basin superimposed on anolder, 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 unitsin the southern part of the basin (Williams, 1964; K. Smith, personal com-mun.), and the evidence for pre-Miocene extension ≈80 km south of theCache Valley (Bryant et al., 1989) all support the hypothesis that the south-ern Cache Valley basin may consist of an earlier Tertiary basin that wasmodified by Basin and Range extension.

The large amount of slip inferred on the East Cache fault at the southernend of the basin must decrease abruptly southward or be transferred to un-recognized faults, as the fault has not been mapped across the structuralhigh between Ogden and Cache Valley (Nelson and Sullivan, 1987;Williams, 1958). This abrupt change in fault displacement is at odds withmodels for slip distributions on normal faults, in which slip is thought totaper toward the ends of the faults (Cowie and Scholz, 1992; Scholz, 1990;Dawers et al., 1993). The lack of southward-tapering displacements mayalso support the model of some pre–Basin and Range faulting at the south-

Figure 12. (A) North-striking seismic reflection profile 4 through theCache Valley basin. Tertiary basin-fill deposits thicken southward, andreflections 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 Tertiarysection is penetrated by the Lynn Reese drill hole.

Figure 13. Cross section in the north-ern part of the basin shows that thebasin is shallower and flatter than inthe southern part.

Figure 14. Northernmost cross section in the study area, based on seismic reflection profiles acquired by Amoco and profile acquired byPlacid Oil. Cache Valley basin at this latitude is a broad, flat-bottomed structure with one horst block. To the north are several horsts coveredby 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) sedimentary

units(m)

14 >45 3.6 Not present 22 Flat-bottomed basin 1400 15.713 >45 4.5–6.0 Unknown Hundreds 21 Slight east tilting of a flat-bottomed basin ≈2000 10.58 >45 5.4 minimum 45+ 300–400 20 Doubly tilted prisms, anticline present ≈3900 5.16 >30 4.5–6.4 >55 915 12.6 East-tilted prism, some late west tilt, anticline present 2700 4.63 20–50 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.



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 faultsin the Basin and Range (Susong et al., 1990; Janecke, 1993), and gravitydata (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 thebasin in Utah, coincides with a transverse structure of the Cache Valley basininferred by Zoback (1983) and with a segment boundary of the East Cachefault 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 adeeper 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 andPinezich, 1981; Liberty et al., 1994; Bohannon et al., 1993) and may reflectcomplexities of inherited structure or of growth processes of normal faultsand normal-fault–bounded basins (Russell and Snelson, 1994).

Numerous interpretations of other normal faults in the northern Wasatchto Teton corridor (Arabasz and Julander, 1986; Coogan and Royse, 1990;Evans, 1991; Sprinkel, 1979; Webel, 1987; West 1992, 1993) show thatmany 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 thenormal faults. The East Cache fault cuts small-displacement thrusts alongthe 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 EastCache fault suggests that the fault lies within the Willard thrust sheet anddoes 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 theregion are not simple, planar structures (cf. Westaway, 1989a, 1989b).Evaluation of extensional kinematics and seismicity of the Basin andRange 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 geometryof what appears at the surface to be a relatively simple hanging-wall basinat the eastern margin of the Basin and Range province. These data indicatethat the geometry of normal faults, slip rates and total amounts of slip, thegeometry and distribution of sedimentary units, and the geologic history ofBasin and Range basins cannot be inferred from single cross sections.

ACKNOWLEDGMENTS

This work was partially supported by the Utah Geological Survey. Wethank Amoco Production Company, Placid Oil Company, Chevron, Exxon,and Texaco for access to seismic reflection profiles, and we are grateful forthe assistance of Bob Cook, Roger Dickinson, Clyde Kelley, Don Roberts,Tad Schirmer, and Doug Sprinkel. We give special thanks to T. L. PattonIII for assistance in getting Amoco profiles released for publication. Wethank Amoco Oil Co. for permission to publish the seismic reflection pro-files. The interpretations presented here are solely ours and not those ofAmoco or Placid or its employees. Mark C. Hall compiled most of a1:100 000 geologic map used in this study. J. Coogan provided unpub-lished maps of the area east of Bear Lake and several cross sections. Wethank James P. McCalpin for providing assistance in understanding theneotectonics 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 editorSteven Reynolds, for many useful comments on the original manuscript.

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