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

Coogan and DeCelles (1996) provided a welcome addition to the debate on the Sevier Desert reflection. The evidence and arguments presented on the nature of this subsurface feature merit particular scrutiny, as they bear directly on a firstorder issue in tectonics: the mechanical paradox of low-angle normal faults. Field geologists have argued that in some cases such faults must have moved at dips of 20° or less; tectonophysicists maintain that such interpretations are inconsistent with our present knowledge of rock mechanics, and seismologists have yet to record a single earthquake that can be related unequivocally to slip on a low-angle normal fault. If, as Coogan and DeCelles (1996) and others have argued, the seismically imaged Sevier Desert reflection of west-central Utah is a rooted detachment fault with as much as 39 km of top-to-the-west slip, the seismic-reflection geometry effectively requires normal-sense slip on a surface dipping 11°. We believe, however, that geometry can also support alternative interpretations. As Coogan and DeCelles recognized, a key to the distinction between detachment and nondetachment interpretations is the geometric relation at the western margin of the basin, between east-tilted Proterozoic and Paleozoic rocks of the Cricket Mountains block and overlying Tertiary sediments of the Sevier Desert basin. Eastward fanning of Tertiary strata above this block would be consistent with gradual tilting above a rooted detachment fault (as inferred by Coogan and DeCelles, 1996); in contrast, the absence of fanning or of significant stratal dip in Tertiary sediments would be consistent with a combination of alternative basin-forming mechanisms, including regional subsidence, offset along high-angle normal faults, and the development of erosional topography at the margin of the late Mesozoic–early Tertiary orogen. Our own analysis of regional seismic data within the basin suggests that in this critical area the evidence is at best equivocal, but that it tends to support the second view. We do not believe that the Cricket Mountains block was appreciably tilted during sedimentation in the Sevier Desert basin. Central to the interpretation of Coogan and DeCelles (1996) is reflection geometry evident in profile GSI 25 in the vicinity of the Gulf Oil Gronning #1 well, and specifically a panel of reflections that between 1.4 s and 1.7 s twoway travel time dip between 16° and 17° to the east and appear to terminate downward against the inferred detachment fault (Fig. la; location shown in their Fig. 1). If the reflections were primary, similar geometry might be expected on other profiles in the same area. Curiously, however, the dipping reflections are virtually absent on profile Pan Canadian 1 (Fig. lb), which directly crosses the Gronning #1 well and intersects GSI 25 at about 50° (location shown in Fig. 1 of Coogan and DeCelles, 1996). We examined the recovered core from near the bottom of the Gronning #1 well (between 2107 m and 2448 m ) and found that dips in cross-stratified sandstone range from 3° to 14°, with no discernible downhole trend; dips in siltstone range from 5° to 8° (average, 6°), markedly less than the 16°–17° dip estimated by Coogan and DeCelles. Although no vertical deviation data are available for the well, we note that in profile Pan Canadian 1, reflections at the same level in the vicinity of the Gronning #1 well dip gently eastward at about 4° (Ain Fig. lb, assuming a 3180 m/s average velocity and correcting for apparent dip to the same azimuth as GSI 25), consistent with the estimate obtained from the core. We suggest, therefore, that the dipping reflections may not be primary, but may instead be multiples related to dense, layered basalts higher in the succession (0.7 to 0.9 s two-way travel time at the Gronning #1 well). The prominent reflection at A in Figure lb can be traced westward to its intersection with a reflection that dips at about 25° east (labeled B in Fig. 1), and which Von Tish et al. (1985) erroneously interpreted as Oligocene due to a miscorrelation with the Gronning #1 well (Anders et al., 1995). In marked contrast to the stratal fanning inferred by Coogan and DeCelles, this low-angle onlapping geometry can be seen on all appropriately oriented seismic sections in the southern part of the basin (where no tilted basaltic rocks are present higher in the succession). We suggest that the onlap surface is the unconformable base of the Tertiary basin. The existence of high-angle normal faults that appear to sole or terminate downward into the Sevier Desert reflection and evidence for stratal thickening toward such faults are indeed consistent with the presence of a rooted detachment fault that was active during sedimentation (Coogan and DeCelles, 1996). However, the same geometry is also consistent with the widespread presence within the basin of lacustrine evaporites as much as 1.5 km thick (Argonaut Energy Federal #1 well; Mitchell, 1979; Gary Mitchell, personal commun., 1997). Salt structures associated with prominent velocity pull-ups, and illustrated in Figure 2a of Coogan and DeCelles (1996), are prima facie evidence for salt mobility. Coogan and DeCelles adroitly summarize the circumstantial evidence for a detachment at the Paleozoic-Tertiary contact, but we do not believe that they pay sufficient attention to data from the contact itself. No evidence for deformation has yet been found at this surface in any industry well in the basin (Anders and Christie-Blick, 1994). This includes the 10-m-thick unit at the Paleozoic–Tertiary contact in the Argonaut Federal #1 well (Mitchell, 1979), which Coogan and DeCelles characterize as a “possible fault breccia.” We have examined samples, and concur with Mitchell’s (1979) conclusion that it is a depositional conglomerate.