--> Abstract: High Frequency Sequence Stratigraphic Controls on Stratal Architecture of an Upper Pennsylvanian “Regressive Limestone” (Bethany Falls Limestone), Midcontinent, USA, by Graham Butler and Peter Holterhoff; #90089 (2009)

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High Frequency Sequence Stratigraphic Controls on Stratal Architecture of an Upper Pennsylvanian “Regressive Limestone” (Bethany Falls Limestone), Midcontinent, USA

Graham Butler and Peter Holterhoff
Department of Geosciences, Texas Tech University

The Bethany Falls Limestone (BFL) is the highstand – falling stage carbonate member of the Swope high frequency sequence as developed on the northern platform of the Mid-continent Basin. It is underlain by the condensed, maximum flooding Hushpuckney Shale and is overlain by the lowstand Galesburg Shale. The Swope sequence is Early Missourian in age and is a significant hydrocarbon reservoir unit in western Kansas. Although the BFL is often considered a uniform shallowing – upward carbonate system, we hypothesize that traceable flooding and erosion surfaces can be recognized within the BFL and that these surfaces define basinward – stepping carbonate clinothems. Recognizing this internal architecture is critical for understanding the potential controls on the deposition and diagenesis of oolite facies developed across the region with the BFL.

Flooding surfaces within the BFL are recognized by mudrock (clayrich shale) partings within the carbonate succession with associated distinctive conodont biofacies. Although not without some ambiguity, the conodont faunas collected from these mudrocks aid in correlation and provide some confidence in mapping these surfaces across the region. Preliminary correlations indicate that basinward stepping clinothem packages can be recognized within the BFL. The conodont abundances and species occurrences also shed light on the depositional environments of the flooding surfaces. Proximal occurrences of the mudrock partings contain a lower abundance, lower diversity fauna compared to more distal locations along a clinoform profile. Surprising faunas of deepwater species and high abundances in the lower BFL are noted at some locations. These occurrences appear to coincide with structural highs and may represent a transitional facies with the underlying condensed horizon.

Distinctive lithofacies offsets and internal exposure surfaces also indicate the presence of high frequency sequence boundaries with the BFL. These offsets are mappable and, like the flooding surfaces, also define clinothem packages within the BFL. The distribution of carbonate lithologies within these stratal packages is consistent with basinward progradation of facies throughout clinothem deposition. Using this combined knowledge it is possible to identify high frequency sequence boundaries (HFSB) within this forced regressive package.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009