STRUCTURAL CONTROLS ON KARST DEVELOPMENT AND GROUNDWATER FLOW, REDSTONE ARSENAL, HUNTSVILLE, ALABAMA

Redstone Arsenal covers 154 km2 in Huntsville, Alabama and contains 424 identified springs, 1886 mapped sinkholes, a highly evolved epikarst, solution cavities in ~70% of bedrock boreholes, and 26 mapped caves. It is situated on the south flank of the Nashville Dome where geologic structure is usually assumed to consist of gently southward dipping beds of Mississippian-age carbonates overlying the Chattanooga Shale. Five distinct hydrogeologic regimes have been identified, and a generalized network of subsurface conduits is inferred from a structural-stratigraphic model. Recently the Army completed nearly 50 km of reflection seismic surveys, nine dye traces, and 32 deep coreholes, documenting significantly more complex structure than previously imagined, with block faulting superimposed on the regional dip. Faulting apparently played a significant role in development of shallow karst aquifers, as shown by dye tracing, and may also have facilitated deep karst development. Drilling revealed transmissive solutional voids up 6 cm thick below the Tennessee River baselevel in the lower Tuscumbia limestone and Fort Payne formations. These strata host natural hydrocarbons that are probably related to block faulting. Groundwater in deep strata are rich in Na-SO4, grading into Na-Cl water downward toward the Chattanooga Shale. Pyrite and gypsum infilling in deep cores, H2S and methane at depth, and the distinct water chemistries may suggest a hypogenic origin for the deep karst development. The faulting may be responsible for the juxtaposition of all of these conditions, and for the karst and caves as well.