Facies Re-Interpretations of Sunnyside Delta Sandstone in Nine Mile Canyon, Uinta Basin, Utah: the Perils of Model-Driven Geology

David Keighley and Stephen Flint
University of New Brunswick

Numerous workers over the years have studied the interbedded carbonate, shale, and sandstone interval that forms part of the middle Green River Formation in Nine Mile Canyon of the Uinta Basin, Utah. The fine-grained, generally low-calcareous sandstones in question contain internal bounding surfaces that have variable dip angle and direction together with abundant asymmetric and less common symmetric cross lamination. They interbed on a 10 to 20 m scale with coarsening upward carbonate beds (oolitic, oncolitic and fossiliferous grainstone, algal mats, micrite) and an oil shale, and truncate calcareous grey shale and red-maroon paleosols. Most workers have interpreted the sandstone to be mostly fluvio-deltaic (at various or unspecified positions on the lacustrine delta top), and to include in-channel bars. Recently, detailed study of two such sandstones along one, ~1.5km-long, canyon wall has resulted in their re-interpretation as a terminal mouth bar deposit lakeward of a river-dominated delta. This latter suggestion is rejected based on numerous pieces of evidence. The lower sandbody is exposed on more than one canyon wall and so can be confidently mapped over 2.5 km of outcrop as a low sinuosity NW flowing channel sandstone with down-stream accreting bars and side bars adjacent to ~5 m deep channel margins. The upper sandbody is more sheetlike but across 9 km of canyon walls locally downcuts through mature red paleosols. Neither observation is consistent with a terminal bar lakeward of a river-dominated delta. Also, the carbonate interbeds are indicative of a high energy shoreface which equates with models showing the E Uinta basin to be in an exposed down-fetch position. Any rivers emptying into a lake at such an exposed location would have mouth bars extensively reworked. As such, if mouth bars were to be present, those associated with a wave-dominated, not river-dominated, delta would be more appropriate and consistent with Walther's Law.

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