Widening of Salt Diapirs in the Absence of Extension: Evidence From the Megaflap of Gypsum Valley Diapir, Paradox Basin

Abstract

Many salt diapirs are known to be initiated by, and widen during, regional extension. However, field exposures of strata flanking the southeast portion of the NW-SE trending Gypsum Valley salt wall (Paradox Basin, Colorado), combined with subsurface data, demonstrate that passive diapirs can also widen in the absence of extension. At a regional scale, the diapir is flanked by minibasins with depocenters that shift toward the diapir over time. At a more local scale, Jurassic Morrison Fm. and younger units form an anticline over the northeastern edge of the diapir; deeper strata are rarely exposed, but seismic and well data show minor (max ~25 degrees) dip away from the diapir and relatively subtle thickness changes. In contrast, the southwestern side is dominated by a panel of near-vertical Pennsylvanian Honaker Trail Fm. and uppermost Paradox Fm. black shales adjacent to Paradox Fm. gypsum. The panel is overlain by a wedge of thinning Permian Cutler Group and Triassic and Jurassic strata, all capped by a gently dipping mid-Jurassic unconformity, with increasing angular truncation toward the diapir and Jurassic Entrada and younger strata just above. The vertical panel is interpreted as a megaflap, with seismic and well data showing that it extends ~2.6 km up the side of the diapir over a lateral distance of ~5 km and thins from 350 m in its basinal position to 200 m beneath the unconformity. There is negligible bed-lengthening in the exposed upper part of the megaflap, which is consistent with its length being close to the 3 km width of the diapir at this location. Moreover, conglomerates within the Honaker Trail Fm. in the megaflap contain clasts of Paradox Fm. units, demonstrating the presence of a exposed diapir shortly after evaporite deposition. The observations are combined with cross-section restoration to illustrate the evolution of the diapir. It was initiated as a narrow diapir during Honaker Trail time at the northeastern edge of an inflated pillow, now the northeastern edge of the salt wall. Whereas the northeastern flank subsided vertically with only minor upturn near the diapir, the relative subsidence of the southwestern flank was accompanied by the roof of the inflated salt rolling through a monoclinal hinge from subhorizontal to near-vertical attitudes, thereby forming the megaflap. As the roof rolled over the edge of the diapir, the diapir progressively widened without any regional extension.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015