POLYGENETIC SPELEOGENESIS AND KARST AQUIFER EVOLUTION IN THE SOUTHWESTERN HIGHLAND RIM OF TENNESSEE, USA

Karst aquifers of the Highland Rim of Tennessee are thought to be products of epigene speleogenesis where acidic meteoric water fortified with soil CO₂ is the dissolutional agent in self-organization of permeability structures. Classic karst features like sinking/loosing streams, sinkholes, caves and springs abound. Morphological characteristics of local caves indicate sinking stream and sinkhole recharge with carbonic acid dissolution occurring along fractures and/or bedding planes. However, in the southwest Highland Rim, surface karst is not as well developed as its spatial counterparts. Karst features are subtle but the character of the overall landscape is fluvial in nature though simple caves do exist. This is due to absence of the cave-rich upper Mississippian carbonate sequences common to Tennessee karst areas save for the Fort Payne Formation which outcrops on the surface. Recent surveyed discoveries, geological mapping, hydrogeological tracing and morphometric analyses reveal a complex, two-tiered karst system, now the longest in Tennessee. The upper tier is formed in association with a disconformity between Lower Mississippian Ft. Payne Formation and the Devonian Chattanooga Shale. The uppermost section of the maze is characterized by relict, mostly dry, densely-spaced maze (106 km surveyed) with abundant gypsum and other exotic mineralogies. Though lower sections of the maze are damp to wet there is no obvious hydrogeologic connection to the surface thus hinting at hypogene processes involving acid which is not derived from surface processes. Situated 25 meters below the maze is a kilometer of active stream conduit formed in Silurian limestones with linear morphology terminating at both ends by water-filled passages (sumps). Recharge is likely from a near-by losing river. Several large springs resurge on the river farther downstream. The maze and stream passages display a combination of hypogene and epigene features and challenge the model of homogeneous karstic processes that are either epigene or hypogene end members in speleogenesis but instead suggests that karstification is a polygenetic process.