Sedimentation, pedogenesis, and paleodrainage straddling the Neogene-Quaternary boundary : a perspective from an infilled sinkhole lake, the Gray Fossil Site, northeastern, TN, USA

Pedogenesis, soil weathering and paleohydrology play important roles in our paleoenvironmental interpretations. Thin section and clay mineral analyses, combined with extensive field descriptions, were conducted on paleosols and fluvial gravels deposited within a Pliocene paleo-sinkhole that existed in northeastern Tennessee at the Gray Fossil Site (GFS). Low-chroma soil colors and abundant redoximorphic features occur in the paleosols in the lower portion of the soil profiles. In contrast, the upper portions of the soil profiles have high-chroma soil colors and very few redoximorphic features. Volcanic and metamorphic (schistose and gneissic) quartz grains, as well as various forms of chert were petrographically identified in the silt- and sand-size fractions. XRD analyses revealed that the dominant clay, in both the pedogenic (1μm) fractions, was kaolinite. Each palesol represents a period of geomorphic stability following fluvial gravel deposition. The paleosols had experienced redoximorphy early in their pedogenic history; this is evident because Fe-Mn coatings filled soil macropores prior to the accumulation of translocated clay. The upward transition from mostly reducing conditions to an overall oxidizing environment may represent a climate shift from warner, wetter conditions in the later part of the Pliocene to the cooler, drier conditions of the Pleistocene and Holocene. Volcanic and metamorphic quartz grains, unlike chert grains, are not derived from the local carbonate bedrock and are interpreted as extrabasinal grains derived from teh Neoproterozoic Mt. Rogers and Grandfather Mountain Formations >50km away from the GFS, despite extreme entrenchment of most Appalachian fluvial systems. The dominantly kaolinitic clay mineralogy is attributed to both climate and the direct weathering of potassium feldspar-rich, felsic volcanic extrabasinal grains. The sediments and paleosols filling the paleo-sinkhole lake at the Gray Fossil Site appear to contain a record that begins in teh Neogene and continues into the Holocene.