--> Abstract: Incised Valley and Interfluve Drainage Patterns of the Grassy Member (Campanian), Book Cliffs, Utah, by C. J. O'Byrne and S. Flint; #90987 (1993).

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O'BYRNE, CIARAN J., Shell Development Co., Bellaire Research Center, Houston TX; and STEPHEN FLINT, University of Liverpool, Department of Earth Sciences, Liverpool, UK

ABSTRACT: Incised Valley and Interfluve Drainage Patterns of the Grassy Member (Campanian), Book Cliffs, Utah

The late Campanian Grassy Member of the Blackhawk Formation, Mesaverde Group, Utah consists of two unconformity-bounded, high frequency sequences. Each sequence boundary unconformity records a minimum basinward shift in facies tracts of 40 km. The basin margin profile during deposition of the Grassy sequences was a gently inclined ramp, typical of foreland basins. Locally, however, an early phase of Laramide tectonic activity in the form of either Late Palaeozoic salt movement or cover deformation above basement fault structures, modified the contemporary shelf topography and partly controlled sites of valley incision during base level falls. Incised valley deposits overlying sequence boundaries of the Grassy Member are laterally separated by extensive interfluve deposits showing evid nce of subaerial exposure and development of minor, pod-like tributary channel systems. These minor, headward-eroding tributary channels are interpreted to have transported very little bedload and acted primarily as water run-off or bypass channels on the flank of a growing domal structure and drained into the main incised valley system. The channels were 'passively' infilled with very fine sand and silt derived from reworked lowstand shoreline or estuarine shoal deposits during subsequent base level rise. Shallow shelf gradients away from the tectonic or halokinetic structure and rates of base level change determined the degree to which valleys incised. A northeasterly shift in the locus of valley incision associated with successive base level falls is interpreted to reflect growth of t e tectonic structure and northward migration of the adjacent rim syncline with time.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.