EVALUATING NATURAL AND INDUCED BIAS IN DETRITAL ZIRCON GEOCHRONOLOGY FROM JURASSIC-CRETACEOUS STRATA IN THE BLACK HILLS, SOUTH DAKOTA, USA

The influence of natural and induced bias are unsolved problems in detrital zircon U-Pb geochronology. This project evaluates natural bias that may result from hydraulic sorting in a depositional environment and induced bias that may result from sample disaggregation during mineral separation. In Mesozoic strata from the Black Hills, South Dakota, the Hulett Member of the Jurassic Sundance Formation represents shallow marine deposition. Within a short section we sampled beds with different sedimentary structures representing varying hydrodynamic conditions. Multiple samples from different positions within two ancient barforms were sampled from the Fuson Member of the Cretaceous Lakota Formation. In the Cretaceous Fall River Formation, we sampled the base and top of a fluvial channel and its associated crevasse splay deposits. Within each stratigraphic unit samples are compared using statistical approaches to test for hydrodynamic sorting. Our preliminary results reveal a high cross-correlation value within the nonmarine Fall River and Lakota formations, suggesting the age data among the samples are similar. In contrast, in the marine Sundance Formation, the cross-correlation value is generally lower. We are testing for induced bias by running in parallel two aliquots of 300 grams from each sample. The first aliquot was mechanically disaggregated using a jaw crusher and disk mill and the second aliquot was processed using a SelFrag Electopulse Disaggregator, which disaggregates along grain boundaries using high voltage electrical fragmentation. We will test for induced bias by evaluating the differences between the age data for each mineral separation method. Our preliminary results reveal a high cross-correlation value within the coarse-grained Fall River and Lakota formations, suggesting that the age data between aliquots of the same sample are similar. Comparatively, in the finer-grained Sundance Formation, the cross-correlation value is generally lower. Our results may indicate that coarser-grained, nonmarine systems fractionate zircon grains less than finer-grained, marine systems, and that the mineral separation process may introduce a greater bias in finer-grained samples possibly due to differences in the degree of disaggregation in different grain sizes.