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Geochemical Variation in Strata Bridging the Green River Fm. — Uinta Fm. Boundary in the Northeast Uinta Basin, Uintah County, Utah: Provenance and Sequence-Stratigraphic Implications

Harcourt, Nicola M.1; Keighley, David G.1
1 Geology, University of New Brunswick, Fredericton, NB, Canada.

In the northeastern Uinta Basin, the contact between the middle Eocene basal Uinta Fm. sandstone and underlying shale of the Green River Fm. is complicated by deformation at various scales. At the meter scale, mudstone may be isoclinally folded, sandstone may be dewatered, and the contact punctuated by large flame structures. At the decameter scale (~30 m), domal and diapiric mudstone structures are visible. The localized nature of the deformation indicates their syndepositional origin as soft-sediment deformation features, but their presence complicates lithofacies interpretations and, consequently, sequence-stratigraphic modeling.

Locally, deformed domal mudstone may be overlain by dipping, parallel, 0.5 - 2.0m-thick sandstone bedsets that are partly dewatered with no distinct internal structure, which could be interpreted as large-scale, lake highstand, delta-front foresets that deformed and dewatered as they prograded. Elsewhere, horizontal bedsets of similar scale preserve asymmetric ripples and cross-beds, paleocurrent readings of which indicate flow from a multitude of directions. These latter beds, truncated by large-scale diapiric mudstone, alternatively suggest flat-lying sheetflood deposition with subsequent intrusion, and localized bed tilting, by fluid mud. Consequently, our current favoured hypothesis is that the sandstone bodies reflect numerous ephemeral-delta sheetfloods, which in a sequence-stratigraphic context would relate to a lake lowstand setting, potentially reworking basin-margin highstand alluvial terraces. With this interpretation, the basal Uinta sandstone forms part of a Lowstand Systems Tract above a major sequence boundary.

In an attempt to confirm these interpretations, XRD and ICP-MS analyses are being undertaken on samples from the upper Green River Fm. and lower Uinta Fm. In total, ~350 samples were collected from two 300m logged sections at Buck Canyon in the west and Cowboy Canyon in the east, with an additional 100m midway section at Bitter Creek and six other intervening locations. Resulting data will provide information regarding clay mineral phases (potentially indicating early diagenetic conditions) and provenance of heavy minerals, at the same time establishing a chemostratigraphic profile for the succession.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009