Low-latitude Stratigraphic Response to Gondwanan Glaciation Apparent in the Stratigraphic Architecture of the Amsden and Tensleep Formations (Mississippian-Pennsylvanian, Northern Wyoming)

Sebastien Blanchard, Christopher R. Fielding, and Tracy D. Frank
Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Bessey Hall, Lincoln, NE 68583
([email protected])

During the Pennsylvanian, Gondwanan ice sheets waxed and waned dynamically. We investigate how changes in southern hemisphere glacial conditions influenced the stratigraphic architecture of the Amsden and Tensleep Formations in northern Wyoming, USA, and its impact on the distribution, quality and internal heterogeneity of potential petroleum reservoirs. To evaluate changes in facies distribution and stratigraphic style in these formations, we measured five sections from outcrops in the eastern Bighorn Basin. The Amsden Formation, which unconformably overlies carbonates of the Madison Formation, commences with a fine sandstone member overlain by up to 27 m of red siltstones interpreted as floodplain deposits. The limited range of facies indicates little variation in depositional environment. The upper Amsden is characterized by dolomite beds of inferred Serpukhovian-Bashkirian age. The overlying Tensleep Formation consists of alternations between eolian sandstone bodies, sabkha deposits and shallow marine siltstone. The latter are major bounding surfaces that indicate repeated flooding of a coastal erg. This increased range of facies indicates a change from low to high-amplitude sea-level change. Moscovian fusulinids suggests that these facies variations correspond to a major phase of Pennsylvanian glaciation (i.e. C4 glacial event of eastern Australia). The low permeability marine siltstones can be traced across tens of kilometers and may now serve as barriers to vertical hydrocarbon flow between eolian bodies. Additional potential barriers include lateral facies changes from eolian dunes to sabkha and interdune deposits. These preliminary results provide new insight into the paleotropical response to the waxing and waning of Gondwanan glaciers and its impact on reservoir architecture.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013