LOSS SLAKE INDEX OF AN UPPER PENNSYLVANIAN MUDROCK, APPALACHIAN PLATEAU, WEST VIRGINIA

The Loss Slake Index (LSI) of Bryson, et. al. (2012) is an empirical metric with potential as a simple and efficient method for determining weathering durability of mudrocks. Analysis of samples from the vertical profile of a thin (~20 ft) Upper Pennsylvanian “redbed” mudrock at one location on the Appalachian Plateau in West Virginia extends the stratigraphic applicability of the LSI method. The “redbed” examined, part of the Benwood-Ritchie stratigraphic interval of the Monongahela Group, consists of four subunits each of which was tested to determine natural moisture percentage and LSI. Subunit 1 is red clay shale with low fissility, 0.28 meters in thickness. Subunit 2 is like subunit 1 but separated by a very thin layer (0.10 m) of gray clay shale, 0.30 meters in thickness. Subunit 3 is gray clay shale with high fissility, 0.35 meters in thickness. Subunit 4, the thickest at 2.4 meters, is red claystone with abundant pedogenic slickensides. Subunits 1 and 2 had similar moisture contents, 4.68% and 4.66%. Subunit 3 had the highest moisture content at 7.40%, whereas subunit 4 yielded 6.08%. Following the analytical approach of Bryson, et. al., exponential plots of the SDI₁₀/SDI_time ratio vs. cycle time were constructed for each subunit. The curvature of such plots increases with decreasing durability. Plots for subunits 1, 2, and 3 show well-defined trends with R² values of 0.985, 0.782, and 0.907, which is comparable to values reported for the Mid and Lower Paleozoic mudrocks of Kentucky studied by Bryson et. al. Numerically, LSI is the slope of a line going through the origin when plotted on a semi-log scale. For subunits 1, 2, and 3 the LSI is 0.22, 0.24 and 0.69, respectively. The LSI for unit 4 was not determined due to lack of a well-defined trend attributed to abundant pedogenic slickensides common in vertic paleosols. The closely spaced slickensides precluded testing of comparable sample splits at the successively longer time intervals as required by the LSI method. As a result, subunit 4 of the “redbed” did not produce a well-defined trend as indicated by an R² value of 0.17. When plotted on the moisture content vs. LSI graph of Bryson et. al., units 1, 2, and 3 lie closely along the established trend line indicating moisture content is a strong predictor of LSI for the non-slickensided part of this Upper Pennsylvanian “redbed” mudrock.