Downloaded from geology.gsapubs.org on January 29, 2010 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 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. 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. Geology; October 1996; v. 24; no. 10; p. 933–936; 3 figures; 1 insert. 933 Downloaded from geology.gsapubs.org on January 29, 2010 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 168–178 east- (;2600 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 (1640 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 188–258 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 ;4.5 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 28–58 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 118 to 158 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.