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Abstract: Non-Marine and Marine Trace Fossil Assemblages- Indicators of Key Sequence Stratigraphic Surfaces Within the Cretaceous Blackhawk Formation, Eastern Utah

WACH, GRANT D., Texaco Exploration and Production Department; and KEITH W. SHANLEY, Amoco Exploration and Production.

Analysis of trace fossil assemblages from subsurface (Shell Development Company, Core Hole No. 2.) and nearby outcrop data from Thompson Canyon, Grand County, Utah were employed to decipher the sedimentary processes, depositional environments and sequence stratigraphic surfaces of the Upper Cretaceous sediments of the Desert and Castlegate members of the Blackhawk Formation. In most instances examination of the biogenic sedimentary structures and determination of the ichnofacies assemblages provides a higher order of stratigraphic resolution than physical sedimentary structures alone. Traces fossils were more readily identified from the core compared to the outcrop, but the outcrop exposures provide a greater opportunity to examine lateral facies relationships. Depositional environments and sedimentary processes were determined from physical and biogenic sedimentary structures, integrated with the lithofacies and trace fossil assemblages.

Ichnofacies realtionships help to characterize diastems and other stratal discontinuities including Glossifungites surfaces within the core and outcrop. These can assist in delineating surfaces such as flooding surfaces and sequence boundaries that have significant correlation value. Parasequences and sequence boundaries within the Desert and Castlegate members were defined.

Discrete ichnofacies assemblages are defined for each depositional setting. Sediments representing marine environments range from distal offshore, through lower and upper shoreface. In the marine realm distinctions can be made between storm and fair-weather trace fossil assemblages based on the behavioral response, of the individual trace fossils to changes in the oxygenation of bottom waters, energy conditions, rate of sedimentation and the effect these have on the nature of the substrate.

Non-marine depositional environments range from tidally-influenced channel fill, abandoned channel fill and floodplain or marsh deposits. An unconformity marks the base of the non-marine sediments and is interpreted as a sequence boundary. Along this sequence boundary non-marine sediments are juxtaposed upon the marine sediments indicating a significant basinward shift in facies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah