Evaluating natural and induced bias in detrital zircon geochronology from Jurassic-Cretaceous strata in the Black Hills, South Dakota, USA

In this study, I attempt to address the influence of natural and induced biasing in dating sandstones. Natural bias is how a sample’s composition might be influenced based on different flow conditions for ancient river systems and ancient marine systems due to the natural sorting of grains in a system. To test this, three geologic units in the Black Hills were picked for this study. The Sundance formation: an ancient shallow marine system, the Lakota formation: an ancient fluvial system, and the Fall River formation: an ancient fluvial system along a coastal plain. I test for induced bias by running in parallel test on two sets of 300 grams from each sample. The first set was physically broken using a jaw crusher and disk mill, which breaks a sample by crushing it down to a fine sand. The second set was processed using a SelFrag machine, which breaks a sample along grain boundaries using high voltage. We also collected grain size data for each set after it has been broken down to a fine sand to test for differences in the separation process as well as correlating grain size to the age of the grain. Our results suggest that for our experiments, natural and induced biasing are not present. We also find, similar to previous studies, that the number of grains (n) dated greatly influences how statistical metrics are measured.