Shoulder Formation in the Paradox Basin: A Record of Progressive Diapir Narrowing and Minibasin Expansion

Abstract

Salt shoulders may be an underappreciated constituent of vertical diapirs, but are key to understanding the kinematics of rising diapirs and adjacent subsiding minibasins. Salt shoulders, are abrupt inward steps of the margins of salt diapirs, and represent narrowing of the diapir. Integrating field mapping, observations, and measured sections, we identified salt shoulders on 6 Paradox Basin diapirs. Previous studies misinterpreted the salt shoulders in the Paradox Basin as roof collapse blocks. Most of the diapirs exhibit long histories of progressive shoulder formation that decrease the areal extent of the diapir over time. The diapirs began rising in the Late Paleozoic and continued until the latest Jurassic. Gypsum, Paradox and Onion Creek salt walls have Permian shoulders on their northeastern sides. Gypsum, Moab, Castle, and Sinbad salt walls have Triassic shoulders. Several diapirs exhibit shoulders that formed progressively from the Late Permian through the Latest Jurassic. Shoulders have shaped much of the stratigraphy and structure at the margins of the Paradox Basin diapirs.

Paradox shoulders are buried by stratal wedges that thin onto, and onlap the exposed diapir.,with successive halokinetic sequences that truncate underlying shoulder strata at angular unconformities. Clasts of reworked gypsum and carbonate caprock are common in the basal intervals. Blocks of strata enclosed in caprock indicate karstic dissolution between some episodes of onlapping deposition. Syndepositional folding and growth in synclines is common and interpreted to form through salt removal during shoulder formation, although it is unclear whether removal occurs through dissolution or movement of salt toward the adjacent still-rising parts of the diapir. Faults are present but uncommon, except for radial faults that tend to form at bends in the diapir margin. The shoulders were subsequently folded into the diapir through later salt dissolution. Some shoulders have been displaced on faults through dissolution and collapse of the diapir roof, without losing their identifiable shapes and stratigraphy. The salt shoulders observed in the field, which are not visible on 2D seismic data because of near-surface data quality, serve as outcrop analogues for shoulders visible on seismic data in the Paradox and other salt basins. The salt shoulders represent stratal architectures that may have significant implications for hydrocarbon exploration and development in salt basins.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018