LITHOLOGY AS AN EROSIONAL CONTROL ON A FLUVIOKARST SYSTEM, COMPARING PROFILES OF LIMESTONE AND SANDSTONE BEDDED STREAMS

Fluviokarst systems are common in areas where there is contact between carbonate and non-carbonate rocks. Lithological differences between the rock types control erosional resistance that serve as a key component in the development of fluviokarst. Carbonates are susceptible to both physical and chemical weathering, while non-carbonate rocks, i.e. siliciclastic rocks, are weathered by physical processes due to low solubility. This work examines the role of lithology in the development of a fluviokarst system where sandstones overlie limestones. Using the stream profile for a river in a topographic steady-state, stream power can be calculated using: dz/dx=(U/K)^(1/n)·A(x)^-(m/n), where z is elevation, x is horizontal distance, U is the rate of uplift, K is the erodibility coefficient, A is drainage area, and m and n are exponents related to hydrologic conditions. An integral method transforms the equation such that elevation, instead of slope, is the dependent variable and the spatial integral of drainage area is the independent variable. The integral method allows for the comparison of the steepness index (U/K) along the main stem to those along the tributaries and for the analysis of equilibrium within a single stream or for an entire watershed. Using the integral method, the role of lithology on the development of a fluviokarst system in eastern Kentucky was examined. The system in Carter Caves State Resort Park has developed in limestone overlain by a sandstone caprock; we examine whether the system is in equilibrium and if lithology has a role in the state of equilibrium. Results indicate that the sandstone stream segments are in greater equilibrium than the limestone segments. Disequilibrium can be caused by a number of factors including uplift, changes in base level, climate, and variation in lithology. As the only dissimilarity among the controlling factors in our study area is lithology, we interpret that to be the cause of disequilibrium in the Horn Hollow. With a mean of 0.03, the steepness index of the limestone segments is statistically higher than the mean steepness index (0.01) of the sandstone segments (t(51) = -10. 10, p < 0.01). Because the U of the rock units is the same, the K of the sandstone must be higher than the limestone, suggesting the sandstone is eroding at a faster rate and/or the limestone is more resistant.