THE ROLE OF VARIABLE PALEOTOPOGRAPHY IN FACIES DISTRIBUTION AND STRATAL ARCHITECTURE OF THE TRANSGRESSIVE MERRIAM LIMESTONE (UPPER PENNSYLVANIAN), NE KANSAS

Transgressive limestones of Midcontinent Pennsylvanian cylcothemic strata, which are commonly thin units, may display significant thickness variations due to variable paleotopography. The Merriam Limestone in NE Kansas was deposited during marine transgression over irregular topography of the Bonner Springs Shale that formed from subaerial exposure and erosion. Additional erosion during transgression created large-scale channels (100's of meters wide, 3-5 meters deep). Smaller channel-like features (10's of meters wide, 1-2 meters deep) are developed locally. Filling and draping of paleotopography during sea-level rise created vertical and lateral facies variations within Merriam strata, and an increased thickness in lows (up to 5 m thick) compared to topographically high areas (<1 m to 1.5 m thick).

Initial deposits in large- and small-scale channels consist of argillaceous skeletal wackestones/packstones with re-worked Bonner Springs clasts. These deposits grade upward to cleaner wackestones/packstones with interbedded skeletal packstones/grainstones indicating fluctuating energy conditions and cessation of siliciclastic deposition during continued transgression. Small-scale channels are characterized by a variety of stratal geometries at the margins, including thinning and convergence, lap-out, and overlap. At the margins of large-scale channels lower Merriam strata are characterized by thinning and draping of the margins. In contrast to channel and channel-margin areas, lower Merriam strata on surrounding highs consist solely of shallow water coated-grain packstones and grainstones. Uppermost Merriam strata maintain consistent thickness and facies character (skeletal packstone, phylloid-algal wackestone) throughout the study area, indicating relief had been largely filled.

As shown in this study, thicker transgressive deposits in paleo-lows may show complex facies associations and geometries. This variability could lead to highly heterogeneous reservoir or aquifer systems in the subsurface. Other transgressive deposits in Midcontinent cylothems may exhibit similar characteristics as a result of deposition over variable paleotopography.