SR- AND U-ISOTOPE STUDIES OF KARST LANDSCAPE EVOLUTION AT BUFFALO NATIONAL RIVER, NORTHERN ARKANSAS, USA

Landscape evolution of the Buffalo National River (BNR) region in northern Arkansas is strongly affected by karst processes resulting in abundant caves, sinkholes, losing streams and springs, as well as spectacular cliffs along river cutbanks that can reach heights of 150 m or more. Current geomorphic studies are aimed at understanding the development of this terrain using a variety of approaches. Here, we present Sr- and U-isotopic results on ground- and surface-water samples as well as U-Th disequilibrium ages of tufas and speleothems that are useful for constraining incision rates.

Radiogenic isotopes of Sr (⁸⁷Sr/⁸⁶Sr) and U (²³⁴U/²³⁸U) are natural tracers of hydrologic flow and water/rock interaction. Values from regional springs and local seeps show distinct differences between Mississippian and Ordovician aquifers, and between different geographic areas. Differences are attributed to path length for ²³⁴U/²³⁸U and lithology of recharge areas for ⁸⁷Sr/⁸⁶Sr. At Big Bluff, tufa ~30 m above the modern channel that formed from seepage out of Ordovician limestone has U-Th disequilibrium ages less than 5 ka and initial ²³⁴U/²³⁸U activity ratios (IAR) greater than 4.5. Tufa ~110 m above the channel formed from seepage out of Mississippian limestone is older (150–170 ka) and has lower IAR (<1.5) reflecting distinct water sources. Differences in ages and elevations can be used to limit long-term vertical incision rates to less than ~0.7 mm/yr. Another approach uses deposits in Fitton Cave within BNR. Clastic sediments dated by cosmogenic nuclides indicate speleogenesis was active between 740 and 2,200 ka. Subsequent incision abandoned older deposits, and some became capped with calcite reflecting vadose flow on stabilized surfaces. Maximum U-Th ages at several sites range from 400–700 ka. Assuming that the stream profile through the cave is maintained over time, the difference of 5–10 m between elevations of the modern cave stream and earlier depositional horizons yields incision rates of 0.01–0.02 mm/yr. Those rates are consistent with values estimated from other karst environments in the Appalachian Mountains. Younger speleothems are being used to assess hydrologic and environmental responses to glacial climate cycles in the southern Ozarks.