DATING THE EXTENT AND TIMING OF PALEOFLOOD SEQUENCES FROM THE BLACK HILLS AREA, SOUTH DAKOTA, USING OSL GEOCHRONOLOGY

A multifaceted investigation is underway in the Black Hills region of South Dakota to improve the understanding of the frequency and magnitude of exceptional Holocene flood events, such as the lethal 1972 flood in the area of Rapid City. The main components of this study involve stratigraphic analysis and geochronology of fine-grained slackwater flood deposits along with hydraulic modeling for estimation of discharges associated with the paleoflood deposits. Geochronology for these deposits has been primarily obtained by radiocarbon. However, several sites either could not be dated with radiocarbon or had ambiguous results, leading us to utilize optically stimulated luminescence (OSL) dating on sediment layers found within the caves and rock shelters along French Creek, Box Elder Creek, Spring Creek, and Rapid Creek drainages. Although the caves have formed primarily in the Madison limestone, a peripheral outcrop ring around the Black Hills, the flood sediment contains mica, quartz, K-feldspar and other accessory minerals derived from the Precambrian metamorphic and Tertiary intrusive rocks that comprise the core of the Black Hills. The difference in sediment mineralogy between locally-derived materials (i.e., rockfall or slopewash) allowed identification of the flood deposits, and the quartz found in the flood sediments facilitated OSL analyses. OSL analyses were carried out using a masking disc mount, assuring the smallest aliquot amount possible at 1 mm. Single aliquot analyses were performed. OSL dating of the 1972 flood layer produced results that are too old, perhaps owing to the peak discharge of flood occurring after darkness had fallen. OSL dating of other documented flood sediment produced ages at 200, 1100, and 3520 years, which tentatively match other observations or radiocarbon results. Several other key deposits will need to be further sampled and studied before the OSL results can be put into their clearest context. Leading edge techniques were used to obtain the final equivalent doses, and data will be shown and discussed for each site.