Abstract: Coastal Sedimentation Model, Tensleep Sandstone (Pennsylvanian), Central Wyoming

Mitchell W. Reynolds, Thomas S. Ahlbrandt, James E. Fox, Paul W. Lambert

Along the south and north flanks of the Granite Mountains, central Wyoming, the Tensleep Sandstone (Pennsylvanian) consists of terrigenous clastic and carbonate rocks that accumulated on a coast and shelf of low topographic relief in a series of environments ranging from marine shoreface to eolian dune. These environments have been interpreted in drill core and outcrop from criteria including sedimentary structures such as bedding, mass-flow structures, and extent and type of bioturbation, vertical and lateral stratigraphic succession, grain size, and fauna.

The basal part of the formation contains strata of four depositional cycles, each cycle beginning with accumulation of terrigenous sand in a foreshore environment and terminating with carbonate deposition on an intermittently exposed shoal. Each cycle abruptly succeeds the underlying one with sediment at the top of one cycle reworked by wave and biologic activity early during deposition of the next cycle. The middle, and thickest, part of the Tensleep consists of sandstone deposited in shoreface and foreshore environments. Six major depositional cycles, several with minor internal fluctuations, are represented by the strata. Deposition in each cycle graded from shoreface upward to foreshore environments. South of the Granite Mountains, the upper part of the formation contains sandstone of eolian-dune origin that grades laterally at Lost Soldier into sandstone deposited in intertidal and lower foreshore environments. Equivalent rocks north of the Granite Mountains accumulated in an intertidal setting.

Within the area studied, Tensleep strata of all depositional environments are petroleum reservoirs. Anticlinal closure, fractures, and distribution of mineral cements in the strata, rather than sedimentary facies, control the occurrence of petroleum.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana