ABSTRACT: Fluvial and Estuarine Reservoir Geometry within Alluvial Transgressive Systems Tracts: Examples from the Kaiparowits Plateau of Southern Utah

SHANLEY, KEITH W., Shell Development Company, Houston, TX, and PETER J. MCCABE, U.S. Geological Survey, Denver, CO

Incised valleys, related to relative sea level falls, have been recognized in lower Coniacian and lower Santonian nonmarine strata in the Kaiparowits Plateau of southern Utah. Strata overlying these sequence boundaries comprise transgressive systems tracts that consist dominantly of alluvial strata. Basal deposits of the alluvial transgressive systems tract are dominated by abbreviated (top truncated) fining-upwards units that consist of coarse-grained to pebbly, multistory, high net/gross sheet sandstones that were deposited by low to moderate sinuosity rivers. Individual stories range from less than 1 m at the base to 5-8 m at the top of the sheet sandstones. Width/thickness ratios are commonly greater than 50:1. Terrace deposits associated with the valley incision have been recogni ed; however, they are often difficult to separate from fluvial deposits that accumulated during the subsequent base-level rise. Potential reservoir sandstones are well interconnected, reflecting the low rates of base-level rise common to the early part of transgressive systems tracts.

Fluvial strata become progressively finer grained and more heterogeneous in the middle and upper part of the transgressive systems tract, in part reflecting the increased rates of relative sea level rise. Channel sandstones are more isolated and contain thicker cross-bed sets and more completely preserved fining-upward units. Thick rooted mudstones, carbonaceous shales, and thin discontinuous coal seams and also common. The top of the systems tract is gradational with overlying highstand alluvial deposits; however, it contains widespread evidence of tidal processes including double mud-drapes, sigmoidal bedding, wavey and lenticular bedding, synaeresis cracks, Teredolites, and Arenicolites, and inclined heterolithic bedding. While these tidally influenced fluvial strata are rarely of eservoir

quality, they provide time significant units for chronostratigraphic correlation and are equivalent to maximum flooding surfaces in marine strata.

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