DATING THE STE. GENEVIEVE LIMESTONE IN WEST-CENTRAL INDIANA USING SR-ISOTOPE STRATIGRAPHY

A section of Mississippian limestone is exposed in quarry walls at the DePauw Nature Park in Greencastle, Indiana, but its precise age is not known. This research project aimed to date the exposed wall using 87Sr/86Sr isotopic ratios to determine the absolute age. Ten samples of brachiopods and ooids were collected at various levels along the stratigraphic column. Isotope ratios were measured in these samples at ALS Scandinavia using a Neptune Plus MC-ICP-MS. Those ratios were then placed on a LOESS seawater Sr-isotope curve (McArthur, 2020) to determine the absolute age range for each sample. Whereas several samples were contaminated by matrix or had recrystallized, placing them completely out of stratigraphic order on the LOESS curve, three of the samples yielded valid results, bracketing a section of the quarry wall to 332 –333.2 Ma. All three valid samples fall on the LOESS curve in stratigraphic order and are clustered together in a way that is to be expected with their stratigraphic proximity within the sampling area. Thin sections of each valid sample showed no signs of recrystallization. Additionally, two of the valid samples sampled from superjacent beds yielded identical ratios with small standard deviations, decreasing the chance of both samples being contaminated. Our new absolute ages constrain the age of the basal Ste. Genevieve Limestone in Indiana. With additional sampling and dating along the quarry wall, we could calculate the sedimentation rate and the full range of ages for the Ste. Genevieve Limestone exposed in the Nature Park quarry.