ABSTRACT: Frontier Formation Stratigraphy and Depositional Systems Architecture, Green River Basin, Wyoming: A Record of Sequence Development in a Proximal Foreland Basin

HAMLIN, H. SCOTT, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Frontier Formation terrigenous clastics record early Late Cretaceous foreland-basin sedimentation. Well-log and core analysis established the stratigraphic framework along the Moxa arch, a 150 km long north-trending intrabasin structure where the Frontier contains low-permeability gas reservoirs. Erosional and transgressive surfaces and depositional systems tracts were traced from the Moxa arch to the Rock Springs uplift 60 km east and to Frontier outcrops in the thrust belt 20 km west. Mid-Cretaceous chronostratigraphy is documented in these outcrop sections, allowing comparison of subsurface correlations with interregional sequence stratigraphic frameworks.

The Frontier is a sandstone-rich succession, 30 to 300 m thick, enclosed in marine mudstones. A distal progradational parasequence set (Third and Fourth Frontier) downlaps a condensed section in the underlying Mowry Shale. Coastal plain facies (Fourth and Fifth Benches, Second Frontier) conformably overlie the Third Frontier but are erosionally truncated by a transgressive surface at the base of the Second/Third Bench, a progradational shoreface sandstone that is probably continuous with the outcropping Oyster Ridge Sandstone. A subaerial erosion surface, which probably corresponds to a 90 Ma interregional unconformity, overlies and progressively truncates the Second Bench. Above the unconformity, First Bench lowstand and transgressive systems tracts are conformably overlain by a cond nsed section in the Hilliard Shale. Frontier foreland-basin sequences lack certain components of the passive-margin sequence model.

Tectonic subsidence and sediment supply are primary controls on Frontier stratigraphy. Thrust-plate loading created maximum subsidence (foredeep) along the western margin of the basin, a depositionally proximal terrane. Largely nonmarine facies in the foredeep

thin and become more marine-dominated eastward. Frontier strata also thin southward along the Moxa arch in response to syndepositional uplift. The preponderance of nonmarine facies at the north end suggests that an important source area was northwest.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)