Jason Klimek, Brigham Young University
Recent outcrop observations have revealed a large mud diapir that measures roughly 90 meters in diameter in the Wilkins Peak Member of the Green River Formation of southwest Wyoming. The diapir is exposed within outcrop of the younger Laney Member of the Green River Formation, locally deforming sands within the Laney Member around the margins. Continued analysis of satellite images revealed the existence of a second, smaller diapir 3.9 kilometers to the east. Mud diapirs and soft sediment deformation structures are common within the Green River Formation, and have been attributed by others to paleoseismic events, but are typically much smaller in scale. Within the diapirs, deformed but well-preserved bedding is common in much of the surface exposure, while other portions of the diapir are characterized by large, matrix supported, angular carbonate clasts. In both expressions, strata are dominated by thinly bedded dolomicrictic mudstones with common siliceous diagenetic veins based on XRF analysis and thin section evaluation. Along the margin of the diapirs, intra-diapiric strata are dominated by highly brecciated facies, suggesting stress was differentially concentrated along the margins of the diapir. Syntectonically deformed, coarse-grained sandstones bordering much of the larger diapir change dip from up to 50o at the interface with the diapir to near-horizontal, regional dip within 30 meters of the contact. This syndepositional, halokinetic deformation of channelized sands, along with the coherency of beds within the diapir and the similar ages of the diapir and host rock strata, indicates that diapirism occurred very soon after deposition of the Wilkins Peak Member. This suggests that this could be a larger sedimentary expression of a hypothesized paleoseismic event responsible for dewatering and deforming Green River strata in the region.