Origin and Implications of Diapir-Flanking Caprock Associated with the Salt Sediment Interface at Castle Valley Salt Diapir, Utah

Erosional exposures of the elongate, NW-trending, Castle Valley (CV) salt wall allow detailed outcrop analysis of strata directly adjacent to the diapir, revealing diapiric influence on sedimentary deposition, diagenesis, and structure. Excellent exposures of the salt-sediment interface are present along a 4 km transect on the SW flank of the CV salt wall. Here, strata directly adjacent to the diapir consist of arkoses of the fluvial Permian Cutler Formation, separated from the diapir by a 30 m thick zone of carbonate rocks. The carbonate is composed of microcrystalline dolomite that has been highly altered and contains local zones of extensive brecciation, hematite nodules, and dedolomite. Abundant pore space within the dolomitic breccias commonly contains asphalt linings. The Cutler and the carbonate beds are overturned adjacent to the diapir. Rip-up clasts of the carbonate can be found in the lowermost Cutler beds with sedimentary structures that provide confident bed tops. At the welded NW end of the CV salt wall, similar carbonate was found adjacent to and as rip-up clasts within younger sequences in the Early Triassic Moenkopi and Late Triassic Chinle formations.

The carbonate is interpreted as forming diagenetically as caprock, in a horizontal position, on top of the CV salt wall and was later rotated off to a flanking position as part of the halokinetic drape-fold process. Caprock rotation is confirmed by rip-up clasts of caprock in the lowermost Cutler halokinetic sequences indicating that the caprock was formed and eroded from the top of the diapir in Permian time. Brecciation of the dolomitic caprock is the result of evaporite solution collapse and dedolomitization. Occurrences of caprock associated with younger stratigraphic units suggest that it may be common for thin slivers of caprock to rotate off a diapir periodically as part of a halokinetic sequence. This study is the first to document caprock in a diapir-flanking position and directly associated with a halokinetic sequence. Results will provide new insight into the genesis, occurrence, and thickness of flanking caprock adjacent to salt diapirs. Some of the irregular geometries and brecciated lithologies documented at the salt-sediment interface in other salt basins may be explained by the presence of caprock assemblages. Due to the size and proximity to salt, these carbonate horizons may be unresolvable on seismic datasets, but should be considered when drilling near-salt.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California