Where Have the Mahogany Oil-Shale Beds Gone? Possible Evidence of Large-Scale Slumping at Sand Wash, Uinta Basin, Utah

David Keighley and Michael Vanden Berg
University of New Brunswick

The Mahogany Oil Shale Zone (MOSZ) of the Green River Formation (GRF) contains numerous beds of high-oil-yield shale that form part of the largest such resource in the world. The zone can be correlated across the Uinta Basin and adjacent Piceance Creek Basin, forming one of the most reliable marker beds in the GRF. Several previous authors have noted that some of the oil shale beds contain small-scale recumbent folds or convolute bedding, indicating instability in the pre-lithified organic layers. Preliminary work in the Sand Wash area of the Uinta Basin indicates much larger scale instability, with the MOSZ being folded, brecciated, or completely absent from the succession. Underlying argillaceous beds of up to 15 m thickness contain recumbent folds, brecciation, dewatering structures and structureless beds. Seismicity may be invoked as the trigger for the liquefaction or fluidization of poorly lithified sub-MOSZ beds, which resulted in the foundering and slumping of the overlying, more competent bed that was the precursor of the oil-shale. Alternatively, further west, interbedding of shoreface sandstone within the MOSZ simply may point to an interpretation of overloading by these sands on underlying unstable beds.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013