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Geology

Extensional collapse along the Sevier Desert reflection, northern Sevier Desert basin, western United States

James C. Coogan and Peter G. DeCelles

Geology 1996;24;933-936 doi: 10.1130/0091-7613(1996)024<0933:ECATSD>2.3.CO;2

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Notes

Geological Society of America Downloaded from geology.gsapubs.org on January 29, 2010 Extensional collapse along the Sevier Desert reﬂection, northern Sevier Desert basin, western United States

James C. Coogan Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Peter G. DeCelles

ABSTRACT ern corner of Sevier Desert basin (Fig. 21). Newly released and previously published seismic reflection data from the northern The profile delineates the internal structure Sevier Desert basin provide a complete seismic transect between the tilted western margin of the eastern part of the basin across the of the basin and the eastern breakaway zone. When tied to well and surface age data, the breakaway fault zone mapped by Otton transect delineates a continuum of extensional fault and basin fill geometries that devel- (1995) and Sussman (1995) (Figs. 2b and 3). oped between late Oligocene and Pleistocene time across the basin. A minimum of 18 km A complete transect of the northern Sevier of top-to-the-west normal displacement is estimated across the Sevier Desert from only the Desert basin from the Canyon Range in the most conspicuous growth geometries and offsets across listric normal faults that sole east to the Cricket Mountains block in the downward into the Sevier Desert reflection (SDR). The SDR clearly marks a normal fault west combines this profile with a previously zone beneath the entire basin, where stratal truncations are imaged for 50% of the 39 km published profile from the north-central and length of the reflection east of the Cricket Mountains block. Restoration of extensional northwest Sevier Desert basin (Fig. 2a; displacement along this entire 39 km fault length is necessary to reconstruct the pre- Mitchell and McDonald, 1987). We hope to Oligocene configuration and erosion level of Sevier thrust sheets across the Sevier Desert minimize the inherent ambiguity of seismic area. The SDR normal fault zone underlies the former topographic crest of the Sevier interpretation of the SDR by focusing the orogenic belt, where it accommodated extensional collapse after cessation of regional con- debate on an area of better data quality tractile tectonism. across the basin.

INTRODUCTION a result of deep target acquisition parame- SEISMIC GEOMETRY WITHIN THE Over the past decade, the existence and ters of the COCORP survey and the appar- SEVIER DESERT BASIN regional extent of the Sevier Desert detach- ently complex internal structure of the basin The principal features of the northern ment fault have become widely accepted in that is indicated by modeling of industry Sevier Desert basin seismic geometry in- the structural geology and tectonics commu- profiles (Planke and Smith, 1991). We clude: (1) the SDR, which is divided into nity, largely as a result of seismic reflection present a seismic reflection profile from an western and eastern segments following studies across the Sevier Desert basin. How- area of better data quality in the northeast- Anders and Christie-Blick (1994) and Ham- ever, recent subsurface and surface studies ilton (1994); (2) the Cricket Mountains across the Tertiary–on–lower Paleozoic block, which consists of thrust-faulted Paleo- contact that defines the Sevier Desert re- zoic and Proterozoic strata and forms the flection (SDR) present conflicting interpre- western structural boundary of the basin; tations of this lower bounding surface of the and (3) three subbasins that are bounded by basin. Anders and Christie-Blick (1994) re- west-dipping normal faults (Fig. 2a). The ported that cuttings and core from industry western subbasin includes tilted Cenozoic exploration wells show little evidence for strata above the Cricket Mountains block, as fault-related deformation in the rocks across well as a graben in its eastern part that di- the SDR and concluded that the reflection rectly overlies the SDR and forms the struc- represents an unconformity beneath most tural axis of the basin. The central and east- of the Sevier Desert basin. In contrast, ern subbasins are half grabens containing surface mapping by Otton (1995) and extensional growth strata that are truncated Sussman (1995) indicates that the eastern along the SDR. The growth strata are over- basin margin is a complex normal fault lain in the eastern subbasin by the Leam- zone at the breakaway of the Sevier Desert ington delta complex, which consists of a detachment. This paper combines newly zone of west-dipping clinoforms that under- released and previously published seismic lie surficial deltaic sediments at the mouth reflection data from the Sevier Desert ba- of Leamington Canyon. The delta complex sin in central Utah to illustrate the con- reflections are truncated along the SDR ad- tinuum of extensional fault and basin fill jacent to the breakaway normal fault zone at geometries between the tilted western ba- the eastern basin margin. sin margin and the normal faulted eastern SDR. The SDR is a multicycle, high-am- basin margin. Figure 1. Generalized geologic map of the plitude reflection zone that is tied by wells to Previous seismic studies of the SDR fo- Sevier Desert, Utah, showing location of seis- the Tertiary–lower Paleozoic contact in the mic reflection profiles included in Figure 2 eastern Sevier Desert basin (Mitchell, 1979; cused on the central and southern Sevier (solid lines) and discussed in text (dashed Desert basin, where the deep seismic struc- lines). GSI and PanCan seismic lines are from Anders and Christie-Blick, 1994); it also un- ture was imaged well by the COCORP sur- Mitchell and McDonald (1987); COCORP seis- derlies lower Paleozoic and Proterozoic sed- vey (Fig. 1; Allmendinger et al., 1983, 1986; mic lines are from Von Tish et al. (1985). Bore- imentary rocks of the Cricket Mountains holes: GG—Gulf Gronning 1; AE—Argonaut Von Tish et al., 1985). However, the internal Energy Federal 1; APB—Arco Pavant Butte 1. geometry of the shallow, eastern part of the Thrust faults: CRT—Canyon Range thrust; 1Loose insert: Figure 2 is on a separate sheet basin is poorly imaged in this area, both as PT—Pavant thrust. accompanying this issue.

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Figure 3. Depth section from seismic profile UHR 2. Depth conver- sion interval velocities are indicated in metres per second. Published with permission from P.G.&E. Resources Co. SDR—Sevier Desert reflection. block beneath the western half of the basin well ties a reflection zone with 16Њ–17Њ east- (ϳϪ600 m elevation) from the hanging wall (Fig. 2a; McDonald, 1976; Allmendinger et ward apparent dip on GSI 25 (assuming a of the fault to a probable Oligocene evapo- al., 1983; Smith and Bruhn, 1984). Thrust 3180 m/s average velocity). These reflections rite section in the Argonaut well (ϩ640 m faults that place lower Cambrian and possi- lie immediately above higher amplitude elevation) in the footwall (Fig. 2a). This cor- bly Proterozoic clastic rocks over younger 18Њ–25Њ east-dipping reflections identified as relation requires that the intervening fault Cambrian carbonates were penetrated be- Oligocene by Von Tish et al. (1985) and as accommodated ϳ4.5 km of top-to-the-west neath the southwestern Sevier Desert basin “older Tertiary” by Mitchell and McDonald normal displacement. An additional 0.4 km (Mitchell and McDonald, 1986). These (1987) above the basin-bottom unconform- of displacement is indicated by the offset of faults may correlate with reflective zones ity. They are also truncated upward by an Pliocene basalt reflectors across the anti- near the base of the otherwise seismically angular unconformity beneath 2Њ–5Њ east- thetic east-dipping fault between the wells. transparent block (Planke and Smith, 1991). dipping reflections from 4 Ma basalts dated This displacement must have been trans- As a result, Hamilton (1994) and Anders in the Gulf well (Lindsey et al., 1981), indi- ferred downward and westward to the SDR and Christie-Blick (1994) argued that de- cating that about 11Њ to 15Њ of tilting oc- normal fault zone beneath the Cricket tachment fault interpretations are based on curred between late Oligocene and early Mountain block. the coincidental alignment and miscorrela- Pliocene time. Von Tish et al. (1985) inter- The correlation and significance of the tion of two genetically and spatially separate preted eastward fanning and thickening of tilted and truncated reflections in the west- reflection groups beneath the Sevier Desert: the subbasalt sequence as evidence for ern Sevier Desert basin were recently con- an eastern, shallow reflection group that hanging-wall stratal growth associated with tested by Anders et al. (1995). They argued marks the basal Tertiary unconformity normal slip along the SDR. The seismic data that the 18Њ–25Њ east-dipping reflection se- (eastern SDR, Fig. 2a), and a deeper reflec- also show clear offset and tilting of the Plio- quence identified by Von Tish et al. (1985) tion group composed of probable Sevier age cene reflections across normal fault zones may image lower Paleozoic strata below the thrust faults that underlie Paleozoic and above and east of the Cricket Mountains basin-bottom unconformity, rather than late Precambrian rocks of the Cricket Moun- block, demonstrating that tilting of the basin Oligocene and older Tertiary strata above tains block as well as the House Range to sediments above the block continued after the unconformity. This correlation permits the west (western SDR, Fig. 2a). early Pliocene time and that the tilting was the truncation and some of the tilting of Geometries of Late Oligocene Strata in associated with displacement across discrete these reflections to be related to Sevier the Western Subbasin. The record of Ceno- normal faults. thrusting, rather than Cenozoic extension. zoic tilting of the Cricket Mountains block Reflections from the dated upper Oligo- The contrasting interpretations are partly presents the greatest challenge to interpre- cene strata are truncated downward against based on the seismic correlation to the Gulf tations of the SDR as an unconformity. The the SDR east of the Cricket Mountains well, which was drilled in 1957, prior to the Proterozoic and lower Paleozoic rocks of block. The sequence below the Pliocene ba- introduction of acoustic logging. We corre- the Cricket Mountains block are largely seis- salt reflections is east-dipping but poorly late the base of the Pliocene basalt in the mically transparent above the western SDR, imaged east of the Gulf well on GSI 25 and Gulf well at 1020 m depth to the base of the which Anders and Christie-Blick (1994) and PanCan 1. However, the basin-bottom, up- very high amplitude reflections that lie near Hamilton (1994) considered to image Sevier- per Oligocene, and younger reflection 0.9 s two-way-time on the GSI 25 dip line. age thrust faults. However, well-dated east- groups are clearly truncated against the This tie is justified because the basalts form dipping late Oligocene strata form a coher- SDR on the GSI 15 (Fig. 2a), PanCan 8, the highest amplitude reflections within the ent reflection sequence above the block, COCORP 1, and GSI 7 dip lines based on basin fill and they are correlated along the demonstrating that it was tilted well after strike correlation along COCORP 5 and basin axis to the acoustic log response in the Early Cretaceous–early Eocene thrusting in GSI 8 (Fig. 1). In addition, strike line GSI 8 ARCO #1 Pavant Butte well to the south the region (Lawton, 1985; Royse, 1993; De- images both northward and southward lat- (Fig. 1; Planke and Smith, 1991). We then Celles et al., 1995), but coeval with later Ce- eral truncation of reflections that lie be- derived average velocities from wells along nozoic regional extension (Wernicke, 1992; tween the Pliocene basalts and the basin the basin axis for clastic intervals that cor- Constenius, 1996). bottom sequence identified by Von Tish et respond to the subbasalt depths in the Gulf Age limits for tilting of the Cricket Moun- al. (1985). Because the SDR is known to be well. The resulting interval velocity range of tains block come from palynological and fis- underlain by lower Paleozoic strata, the 3595 to 3960 m/s places the 2458 m total sion track dating of core from the Gulf #1 truncation establishes the SDR as a late depth of the Gulf well between about 1.6 Gronning well that is projected 9.5 km along Oligocene or younger normal fault contact and 1.7 s. This tie lies immediately above the strike into Figure 2a. Lindsey et al. (1981) beneath the basin axis. basin-bottom sequence of Von Tish et al. dated strata near the base of the well as late The eastern boundary fault to the western (1985), but well below the 1.4 s top of the Oligocene (26–28 Ma). The well directly ties subbasin merges downward into the SDR. 16Њ–17Њ dipping reflection interval on GSI seismic dip section GSI 25 and the oblique Lindsey et al. (1981) tentatively correlated 25 that is truncated against the SDR on ad- PanCan 1 section (Fig. 1). The base of the the upper Oligocene strata in the Gulf well jacent strike and dip lines (Fig. 2a).

934 GEOLOGY, October 1996 Downloaded from geology.gsapubs.org on January 29, 2010 Extensional Growth Geometries, Central City Formation, which may lie within a lat- Range and the Quaternary alluvial deposits and Eastern Subbasins. Two subbasins with eral rider block in the footwall of the subbasin. of the basin margin (Fig. 3). Nevertheless, growth and truncation geometries lie east of Stratigraphic growth and truncation in the the footwall rocks are cataclastically de- the western subbasin. The central subbasin central and eastern subbasins require a nor- formed adjacent to the seismic tie (Fool is separated from the western subbasin by an mal fault boundary at the base of the Sevier Creek locality of Otton, 1995), which is char- intrabasin high that was penetrated by the Desert basin along the SDR. The amount of acteristic of footwall deformation where the Argonaut Federal #1 well (Fig. 2a; Mitchell, top-to-the-west extensional displacement basin-bounding normal fault is exposed to 1979). The Argonaut well penetrated an up- associated with the growth and tilting in the the south (Otton, 1995). Likewise, mapping per 780-m-thick clastic section that is prob- two eastern half grabens alone (9–9.5 km) and microstructural analysis by Sussman ably partly correlative to Pliocene-Pleisto- implies that the SDR must be a zone where (1995) 2.0 km to the south indicates that the cene strata in the upper part of the Gulf extensional slip is accommodated between buried fault lies along a trend of en echelon Gronning well. The middle 1570 m of the subbasins. normal faults that cut Cretaceous thrust well consists of halite with interbedded an- Tilted and Truncated Leamington Delta structures within the Canyon Range and hydrite and shale that yielded Tertiary paly- Complex. The easternmost element within that locally form the basin margin contact. nomorphs (Mitchell, 1979). Lindsey et al. the seismic transect of the Sevier Desert ba- Salt and Subbasin Development. The in- (1981) postulated that the halite-bearing sin is a complex of west-dipping reflections trabasin highs between the subbasins in Fig- section is Oligocene. The evaporite interval that merge downward with a shallowly east- ure 2a are east-facing monoclines truncated forms a seismically transparent zone above dipping basal reflection on seismic line westward by normal faults. Thick evaporite 2350 m depth, where the well ties the SDR. UHR 2 (Figs. 2b and 3). The stratal geom- zones in the monocline cores are evident A 10-m-thick conglomerate (Mitchell, 1979) etry, stratigraphic position, and location of from penetration by the Argonaut well and or possible fault breccia lies on top of Paleo- this reflection package indicate that it is a from velocity pull-ups beneath both the Ar- zoic carbonate rocks across the SDR. The tilted Pliocene-Pleistocene delta complex gonaut high and the high between the cen- east flank of the evaporite-cored intrabasin that formed at the mouth of Leamington tral and eastern subbasins. The basinwide high is overlain by eastward-fanning reflec- Canyon, where the Sevier River enters the distribution of evaporites indicates that they tors between 0.7 and 1.5 s. The eastward Sevier Desert basin. The west-dipping clino- remained relatively immobile with respect divergence (10Њ–12Њ in depth section) and forms within this sequence merge westward to their original stratigraphic position, and truncation of these reflectors against both with flat-lying reflections at the top of the that their present geometry is largely the re- the SDR and the east margin of the subbasin Sevier Desert basin that correlate through- sult of slip on underlying curved normal are best explained by sedimentary growth out the basin to the largely lacustrine Plio- faults. Palinspastic restoration of the mono- above a west-dipping listric normal fault that cene-Pleistocene deposits that have been clines as fault-related folds is easily accom- soles downward into the SDR. The age of dated in several wells (Mitchell and Mc- plished through removal of the tilting and these growth strata is unknown, but they Donald, 1987). In addition, the clinoform translation recorded by the growth strata may correlate to the Miocene Oak City For- complex is overlain by surficial Pleistocene along their eastern flanks. mation, which displays a similar dip profile deltaic deposits of the Sevier River asso- Salt diapirism and withdrawal have been adjacent to surface normal faults. ciated with the Little Valley (ϳ140 ka) and proposed as mechanisms for intrabasin up- Otton (1995) reported that surface dips Bonneville (ϳ20–Ͻ10 ka) lake systems lift and associated stratal growth in the within the Oak City Formation locally flat- (Oviatt et al., 1994). Sevier Desert basin (Mitchell, 1979; Anders ten eastward toward the west-dipping nor- The most tectonically significant features and Christie-Blick, 1994). On the scale of mal fault zone along the eastern margin of of seismic line UHR 2 are the eastward tilt- the entire basin, however, sedimentary the Sevier Desert basin. Although poorly ing and truncation of the delta complex growth prisms and associated normal faults determined, a Miocene age for the growth against the SDR and the surface tie of the are not flanked by the parallel trends of strata of the central subbasin is consistent reflection to the breakaway fault zone thickened salt that would be expected in a with the similar tectonic setting and stratal mapped by Otton (1995) and Sussman coupled salt-withdrawal/sedimentary growth geometry of the Oak City Formation at the (1995) along the west side of the Canyon system. The trend and areal extent of thick surface, as well as with its subsurface strati- Range. The basal surface of the delta com- salt zones are approximated by residual graphic position above the proposed Oligo- plex dips ϳ4Њ east toward the SDR, and it is pull-up features that are superimposed on cene salt interval in the Argonaut well and truncated ϳ800 m beneath the modern the otherwise smooth structure of the SDR below the known Pliocene-Pleistocene clas- 1400 m elevation at which the Sevier River that is mapped using a basinwide average tic section that caps the basin. cuts through Proterozoic rocks in Leaming- velocity-depth function. The salt velocity is The eastern subbasin is imaged by seismic ton Canyon. About 3.5 km of normal dis- ϳ1200 m/s greater than the average basin line UHR 2 (Fig. 2b). It is separated from placement along the SDR is necessary for velocity. Such mapping by Planke and Smith the central subbasin by a largely transparent the vertical offset of the basal reflection of (1991; their Fig. 7) shows that domes, basins, intrabasin high. Below 0.7 s, the western the delta complex from the threshold eleva- and noses of probable pull-up origin along part of this subbasin displays internal stratal tion. The present geometry of the Leaming- the SDR are limited to the areas immedi- growth similar to that of the central subba- ton delta complex is therefore best ex- ately surrounding the Argonaut and APB sin. The 9Њ–13Њ divergence of these strata in plained by downward displacement and wells that encountered the thickest net salt depth section (Fig. 3) is consistent with their eastward tilting above a gently curved, listric intervals in the basin. In contrast, the position above a listric normal fault zone normal fault along the SDR. Syndeposi- bounding fault, Pliocene-level syncline, and along the SDR, which truncates these re- tional slip on the fault must have accommo- Tertiary thicknesses through the basin axis flections to the east. As in the central sub- dated this eastward-thickening wedge of follow a persistent north-south trend with basin, growth strata of the eastern subbasin sediment in the hanging wall. The bounding little variation adjacent to the thick salt ar- may be correlative to the Miocene Oak City normal fault is buried where the SDR eas (Mitchell and McDonald, 1987; Planke Formation. These growth strata lie 2 km projects beneath the topographic break be- and Smith, 1991). northwest of surface outcrops of the Oak tween Proterozoic strata of the Canyon Combined with cross-sectional thick-

GEOLOGY, October 1996 935 Downloaded from geology.gsapubs.org on January 29, 2010 nesses, the areal distributions of salt and between the Cricket-Drum Mountains block Coney, P. J., and Harms, T., 1984, Cordilleran metamor- phic core complexes: Cenozoic extensional relics of growth strata indicate that the volume of and the Canyon Range was uplifted and Mesozoic compression: Geology, v. 12, p. 550–554. growth strata is much larger than the current eroded after Paleocene time, and subse- Constenius, K. N., 1996, Late Paleogene extensional collapse of the Cordilleran fold and thrust belt: Geo- volume of evaporites within the basin. An quently replaced by an Oligocene and logical Society of America Bulletin, v. 108, p. 20–39. otherwise undocumented period of salt re- younger basin through an unknown subsi- DeCelles, P. G., Lawton, T. F., and Mitra, G., 1995, Thrust timing, growth of structural culminations, moval from the presently closed basin sys- dence mechanism, or (2) some portion of and synorogenic sedimentation in the type Sevier tem is needed to accommodate the salt with- the Canyon Range thrust sheet has been orogenic belt, western United States: Geology, dropped down along high-angle normal v. 23, p. 699–702. drawal hypothesis. However, the ϳ825 km Hamilton, W. B., 1994, “Sevier Desert detachment,” of published seismic data within the basin faults at the margins of the Sevier Desert Utah—A nonexistent structure: Geological Society show no evidence of piercement diapirs or basin. The former hypothesis is not sup- of America Abstracts with Programs, v. 26, no. 2, p. 57. salt walls for tectonic removal of salt from ported by post-Paleocene provenance and Lawton, T. F., 1985, Style and timing of frontal struc- deep stratigraphic levels (Smith and Bruhn, structural relations on the basin margins; tures, thrust belt, central Utah: American Associ- ation of Petroleum Geologists Bulletin, v. 69, 1984; Von Tish et al., 1985; Mitchell and the latter can be ruled out because seismic p. 1145–1159. McDonald, 1987). In addition, lithologic de- reflections below the SDR are not offset Lindsey, D. A., Glanzman, R. K., Naeser, C. W., and Nichols, D. J., 1981, Upper Oligocene evaporites in scriptions of wells drilled through evaporite across the east and west basin margins basin fill of the Sevier Desert region, western Utah: zones (McDonald, 1976; Mitchell, 1979; (Fig. 2a), and because the Paleozoic rocks American Association of Petroleum Geologists Bulletin, v. 62, p. 251–260. Mitchell and McDonald, 1987) record no below the SDR are underlain by 1635 Ma McDonald, R. E., 1976, Tertiary tectonics and sedimen- collapse breccia or cap rock residuum above crystalline basement rather than Proterozo- tary rocks along the transition: Basin and Range province to plateau and thrust belt province, Utah, or within the evaporite intervals for salt re- ic strata of the Canyon Range thrust sheet in Hill, J. G., ed., Symposium on geology of the moval by dissolution. Finally, thick salt sec- (Allmendinger and Royse, 1995). Cordilleran hingeline: Denver, Colorado, Rocky Mountain Association of Geologists, p. 281–317. tions in wells that tie seismic lines across the We conclude that seismic reflection data Mitchell, G. C., 1979, Stratigraphy and regional implica- southern Sevier Desert basin are character- and borehole and surface age constraints de- tions of the Argonaut Energy No. 1 Federal, Mil- lard County, Utah, in Newman, G. W., and Goode, ized by gently dipping, subparallel reflec- lineate a spatial and temporal continuum of H. D., eds., Basin and Range symposium and Great tions within, above, and below the gross salt extensional fault and basin fill geometries Basin field conference: Denver, Colorado, Rocky intervals (e.g., Fig. 3 of Planke and Smith, that developed between late Oligocene and Mountain Association of Geologists, p. 503–514. Mitchell, G. C., and McDonald, R. E., 1986, History of 1991), which indicates that the salt remained Pleistocene time across the northern Sevier Cenozoic extension in central Sevier Desert, west- relatively immobile within its original strati- Desert basin. This extensional domain, as central Utah, from COCORP seismic reflection data: Discussion: American Association of Petro- graphic position. well as the Snake Range extensional domain leum Geologists Bulletin, v. 70, p. 1015–1021. farther west, represents extensional collapse Mitchell, G. C., and McDonald, R. E., 1987, Subsurface Tertiary strata, origin, depositional model, and hy- DISCUSSION AND CONCLUSIONS of the topographic crest of the Sevier oro- drocarbon exploration potential of the Sevier The 18 km of extensional displacement genic belt after the cessation of contractile Desert basin, west central Utah, in Kopp, R. S., and Cohenour, R. E., eds., Cenozoic geology of western above the SDR that is estimated from fault tectonism, which fits a pattern of initial Ce- Utah—sites for precious metal and hydrocarbon offsets and stratal growth within the Sevier nozoic extension established throughout in accumulations: Utah Geological Association Pub- lication 16, p. 533–556. Desert basin is only a minimum estimate de- the North American Cordillera (Coney and Otton, J. K., 1995, Western frontal fault of the Canyon rived from the most conspicuous seismic Harms, 1984; Constenius, 1996). Range: Is it the breakaway zone of the Sevier features in Oligocene and younger strata. Desert detachment?: Geology, v. 23, p. 547–550. ACKNOWLEDGMENTS Oviatt, C. G., McCoy, W. D., and Nash, W. P., 1994, Most recent studies of thrust belt structure Supported by National Science Foundation grant Sequence stratigraphy of lacustrine deposits: A in central Utah regard the SDR as a low- EAR-9316700. We thank Bruce Baum, John Wingert, Quaternary example from the Bonneville Basin, and Steve Swanson of P.G.&E. Resources Company for Utah: Geological Society of America Bulletin, angle, large-displacement detachment fault data access and permission to publish seismic line v. 106, p. 133–144. that structurally inverted preexisting thrust UHR-2; G. Mitra, A. Sussman, F. Royse, and L. Hintze Planke, S., and Smith, R. B., 1991, Cenozoic extension and evolution of the Sevier Desert basin, from seis- sheets(Allmendinger,1992;Royse,1993;All- for discussions and reviews of the manuscript; K. Con- stenius, B. Currie, and G. Mohapatra for discussions of mic reflection, gravity, and well log data: Tectonics, mendinger and Royse, 1995; DeCelles et al., regional relationships; and M. H. Anders, J. M. Bartley, v. 10, p. 345–365. and B. P. Wernicke for constructive reviews. Royse, F., Jr., 1993, Case of the phantom foredeep: Early 1995). About 39 km of normal slip across the Cretaceous in western Utah: Geology, v. 21, SDR is required to restore presently ex- p. 133–136. REFERENCES CITED Smith, R. B., and Bruhn, R. L., 1984, Intraplate exten- tended parts of the stratigraphically distinct Allmendinger, R. W., 1992, Thrust and fold tectonics of sional tectonics of the eastern Basin-Range: Infer- Canyon Range and Pavant thrust sheets the western United States exclusive of accreted ter- ences on structural style from seismic reflection ranes, in Burchfiel, B. C., Lipman, P. W., and data, regional tectonics, and thermal-mechanical across the Sevier Desert basin based on cor- Zoback, M. L., eds., The Cordilleran orogen: Con- models of brittle-ductile deformation: Journal of relation from surface exposures and drilled terminous U.S.: Boulder, Colorado, Geological So- Geophysical Research, v. 89, p. 5733–5762. ciety of America, Geology of North America, Sussman, A. J., 1995, Geometry, deformation history, intervals in the Canyon, Cricket, and Drum v. G-3, p. 583–607. and kinematics in the footwall of the Canyon Range mountains. In addition, the Cretaceous un- Allmendinger, R. W., and Royse, F., Jr., 1995, Is the thrust, central Utah [Master’s thesis]: Rochester, Sevier Desert reflection of west-central Utah a nor- New York, University of Rochester, 119 p. roofing history of the thrust belt can only be mal fault?: Comment: Geology, v. 23, p. 669–670. Von Tish, D. B., Allmendinger, R. W., and Sharp, J. W., explained by restoration of this normal dis- Allmendinger, R. W., Sharp, J. W., Von Tish, D., Serpa, 1985, History of Cenozoic extension in central D., Brown, L., Kaufman, S., Oliver, J., and Smith, Sevier Desert, west-central Utah, from COCORP placement. Foreland basin conglomerates R. B., 1983, Cenozoic and Mesozoic structure of the seismic reflection data: Reply: American Associa- of Cretaceous-Paleocene age were derived eastern Basin and Range Province, Utah, from tion of Petroleum Geologists Bulletin, v. 69, COCORP seismic reflection data: Geology, v. 11, from lower Paleozoic and Proterozoic car- p. 1077–1087. p. 532–536. Wernicke, B., 1992, Cenozoic extensional tectonics of the bonates and quartzites of the Canyon Range Allmendinger, R. W., Farmer, H., Hauser, D., Sharp, U.S. Cordillera, in Burchfiel, B. C., Lipman, P. W., J. W., Von Tish, D., Oliver, J., and Kaufman, S., thrust sheet from a source terrain directly and Zoback, M. L., eds., The Cordilleran orogen: 1986, Phanerozoic tectonics of the Basin and Conterminous U.S.: Boulder, Colorado, Geological west of exposures in the Canyon Range. The Range–Colorado Plateau transition from Society of America, Geology of North America, COCORP data and geologic data: A review: Amer- v. G-3, p. 553–581. unroofing history precludes conglomerate ican Geophysical Union Geodynamics Series, v. 14, clast derivation from the Paleozoic carbon- p. 257–267. Anders, M. H., and Christie-Blick, N., 1994, Is the Sevier Manuscript received April 8, 1996 ate rocks that presently underlie the SDR Desert reflection of west-central Utah a normal Revised manuscript received July 2, 1996 (Lawton, 1985; DeCelles et al., 1995). If the fault?: Geology, v. 22, p. 771–774. Manuscript accepted July 19, 1996 Anders, M. H., Christie-Blick, N., and Wills, S., 1995, Is SDR is not a detachment fault, then either the Sevier Desert reflection of west-central Utah a (1) the entire Canyon Range thrust sheet normal fault?: Reply: Geology, v. 23, p. 670.

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