Origin of the Deformed Basal Uinta Formation in Eastern Utah (Uinta Basin): Progradational Delta Clinoforms of a Lake Highstand or Ephemeral Fluvial Sheetfloods of a Lake Lowstand?

Harcourt, Nicola and David Keighley
University of New Brunswick, Fredericton, NB

     The early to middle Eocene stratigraphic succession in the Uinta Basin can be roughly divided into a lower coarse grained unit (Wasatch Formation), medial fine grained interval (Green River Formation), and upper, mixed fine-coarse grained and locally evaporitic unit (Uinta Formation). Lithofacies interpretations are of an alluvial intermontane basin containing a major lake of fluctuating size and depth.
     The presence of saline facies in the upper part of the succession in the west of the basin has generally lead to the conclusion that 'Lake Uinta' gradually dried out as the basin was infilled. However, in the east it was thought possible that there existed inclined lake-delta-front foresets on a decameter-scale. This interpretation would indicate that there was gradual coarse-clastic infilling of a deep lake that preceded the onset of drier conditions.
     Interpretation of basal Uinta Formation strata in the east, and the nature of its contact with underlying shale and oil shale of the Green River Formation, is complicated by deformation on various scales. The localized nature of the deformations indicates their syndepositional origin as soft-sediment deformation features. At the meter scale, beds may be dewatered and large flame structures punctuate the contact. At the decameter scale, domal and diapiric mudstone structures can be viewed.
     Currently, the working hypothesis is that the decameter scale "foresets" are actually related to the large-scale domal structures, and the beds were originally flat-lying sheetflood deposits now partly dewatered and tilted after loading and intrusion. This suggests that the large Uinta Lake at the time of oil shale deposition subsequently experienced a major base-level fall before any of the observed coarser grained units were deposited.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah