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Condensed Sequence Stratigraphy in Wave-Influenced Estuarine Deposits of the Middle Cretaceous Dakota Sandstone, Henry Mountains, Southeastern Utah

Antia, Jonathan 1; Fielding, Christopher 1; Joeckel, Robert M.1
1 University of Nebraska, Lincoln, Lincoln, NE.

The Dakota Sandstone consists of up to 38 meters (10 m on average) of the middle Cretaceous (Middle Cenomanian to Early Turonian) section in outcrops around the Henry Mountains in southeastern Utah. The unit is underlain by the Jurassic Morrison Formation and overlain by the lower Mancos Shale, and is hydrocarbon-prospective in the nearby Uinta and San Juan Basins. Little research on the unit has taken place in this area compared to the adjacent Book Cliffs and Kaiparowits Plateau, where multiple, Milankovitch-scale cycles of deposition have been identified. Outcrop sections were logged and correlated along a 50 mile long outcrop belt to characterize the unit between the towns of Hanksville and Ticaboo in southeastern Utah. This area preserves a condensed record where limited accommodation resulted in a laterally and vertically complex array of depositional facies. Overall, the unit can be subdivided into 1) fluvial conglomerates at the base, gradationally overlain by 2) trough cross-bedded sandstones and inclined heterolithically stratified carbonaceous mudstones and coals interpreted as estuarine deposits, erosionally overlain by 3) planar cross-bedded sandstones of interpreted open marine origin. Bimodal or multimodal paleocurrents and sparse paired mud-drapes in Unit 2, along with diminutive and low diversity trace fossil assemblages indicate that this unit was deposited in tidally influenced estuarine and coastal plain environments. Dominance of an archetypal Skolithos Ichnofacies assemblage in Unit 3 supports open marine conditions. Unit 3 also contains multiple intervals of oyster shell pseudo-coquina.

The overall tripartite subdivision of the Dakota Sandstone in this area is comparable to depositional models for modern wave-influenced estuaries, similar to those on the Atlantic and Gulf coasts of North America, the coast of New South Wales, Australia, and the Mono estuary in Benin, Central Africa. This study is also important because it provides data on a well-exposed ancient wave-influenced estuarine depositional system that can be used to better understand hydrocarbon accumulations in the Dakota Sandstone in nearby basins.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009