REWORKED LOESS AND RED CLAYS IN MISSOURI CAVES: A COMPARATIVE COMPOSITIONAL STUDY

Cave sediments commonly contain evidence of landscape evolution, past climate, biologic activity, and other important geologic phenomena. However, the source, age, and mode of deposition of many of these materials remain uncertain. Chemical characterization by XRF analysis of two prevalent “types” of Missouri cave sediment confirms the presence of reworked loess in caves and shows the variability of oft-discussed Ozark “unctuous” red cave clay. Reworked loess in caves is relatively homogeneous chemically and very similar to local surface loess, although small systematic differences distinguish the two populations (lower Fe, Ni, Al, Ga, and V, and higher Si, Zr, Nb, and Ti in cave reworked loess, plus differences in degree of element variability). Such differences are consistent with the weathering of some minerals and the concentration of resistate phases like quartz, rutile, and especially zircon. Loess-derived cave sediments are common near large rivers, where thick surficial loess deposits also exist. Red cave clay is considerably more variable in composition and, for most elements, indistinguishable chemically from local bedrock residuum. Several samples of red cave clay and surface bedrock residuum that are associated with the same geologic unit are similar chemically. On appropriate compositional diagrams, chemical trends extrapolate to specific mineral end-members (quartz, dolomite, calcite, possibly illite). Loess-based cave and surface sediments can be distinguished easily from red cave clay and bedrock residuum based on their higher Na and Sr contents (likely present in albite, confirmed by plagioclase XRD peaks), higher Ba/Rb and K/Ga, and generally higher Si, Ca/Mg, Zr and lower V, Fe, Al, Ni, and Zn/Fe. A puzzling consistency in the Ga/Al of all of the analyzed samples, despite presumed differences in mineralogic hosts for these elements (clays versus feldspars), remains unexplained. Samples having intermediate field characteristics commonly exhibit intermediate chemistry, and may represent mixed sediment types. Emerging results include the first chemical characterization of Missouri reworked cave loess, insights into its similarities and differences as compared to local surface loess, and a new understanding of Ozark “unctuous” red cave clay.