Paleotopography and Sea-level Controls on Facies Distribution and Stratal Architecture in the Westerville Limestone Member (Upper Pennsylvanian) NE Kansas and NW Missouri
Fairchild, Justin M.
University of Kansas
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Oolitic grainstone facies form important reservoirs in the subsurface around the world. The Westerville Limestone Member (Pennsylvanian) is an oolitic grainstone-rich reservoir analog deposited during high-amplitude glacioeustatic fluctuations and exposed in a 510 km2 area near Kansas City in Kansas and Missouri. The Westerville Limestone Member consists of eight lithofacies: bioclastic packstone; bioclastic grainstone; oolitic grainstone; oncolitic packstone; fossiliferous siliciclastic-mudstone; peloidal packstone; microbial boundstone; and coarse-grained packstone. Interpreted deposition of the Westerville Limestone Member (2-6 m thick) is divided into three intervals, W1, W2, and W3, each separated by a marine or subaerial truncation surface. Interval W1 is composed mostly of bioclastic packstone of relatively uniform thickness (~1 m) throughout the field area. Deposition occurred in normal marine water during a relative rise in sea level and did not fill accommodation. A marine truncation surface altered local paleotopography and created subtle (dm-scale) relief during a relative fall in sea level and prior to deposition of Interval W2. Interval W2 is composed of grainstones (oolitic and bioclastic) that were deposited during a relative fall in sea level. The oolitic grainstone facies are preserved within paleotopographic low areas. A subaerial exposure surface truncates the unit, and the tracing of this surface and interpretation of depositional depths for facies are used to calculate a relative fall in sea level of at least 15.5 meters. After exposure, a relative rise of at least 11.5 meters is interpreted before Interval W3. Initial W3 deposits are composed of oolitic grainstone facies that locally build constructional relief on the flank of a regional paleotopographic high and represent deposition during highstand at an intermediate ramp position. During a relative fall in sea level of at least 8.2 meters, accommodation became limited, oolitic grainstone is deposited on the flanks of local paleohighs, a thin layer of microbial boundstone drapes paleotopography, and oncolitic packstone and fossiliferous siliciclastic-mudstone onlap paleotopography and fill local paleolows. Deposition of stratigraphically higher supratidal peloidal packstone fills accommodation, and unit thickness requires a relative rise in sea level of at least 3 meters. Exposure features are present along the uppermost surface of the Westerville Limestone Member and indicate a period of subaerial exposure after a relative fall in sea level of at least 2.3 meters. Results of this study indicate that fluctuations in relative sea level interacting with subtle paleotopography can result in significant facies heterogeneity. Oolitic grainstones are typically thought to form constructional relief on high areas in shallow water. This study indicates, however, that oolitic grainstone deposits may accumulate in paleotopographic lows, especially during falling stages of sea level. The interpreted depositional history of the Westerville Limestone Member and associated units demonstrates that smaller scale variations in the rate and direction of fluctuations in sea level can have a significant effect on sequence heterogeneity during a large scale fall in sea level.
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