DIAGENESIS IN TERTIARY STRATA AND THE FORMATION OF CHALCEDONY VEIN HORIZONS: BADLANDS NATIONAL PARK, SOUTH DAKOTA

Tertiary strata found in Badlands National Park in southwest South Dakota contain stratabound chalcedony vein horizons within the Brule and Chadron Formations of the White River Group. The stratabound chalcedony vein horizons occur at multiple stratigraphic levels within both the Chadron and Brule Formations. These veins and accompanying clastic dikes are thought to have formed due to diagenetic processes. Mapping and sampling the abundant clastic dikes and tiered chalcedony horizons in order to characterize the complex diagenetic interactions in the park was the focus of an undergraduate research expedition in the summer of 2009. Clay, sandstone and chalcedony samples were collected and chalcedony vein and clastic dike orientations were taken from selected localities inside the park. Vertical sample suites that traverse the stratabound chalcedony vein horizons, and a vertical suite of samples from a clastic dike and from adjacent sediments were collected to document the diagenetic changes associated with the formation of these features.

XRD analysis was performed on clay mounts taken from inside and outside chalcedony horizons with hopes of identifying any mineralogical differences and diagenetic signatures. Initial XRD results from a vein horizon in the middle of the Brule Formation appear different from an earlier study on older strata at the Chadron-Brule contact sampled in the nearby Buffalo Gap National Grasslands, where the proportions of smectite and illite differed inside and outside the vein bearing horizon (Schuler- UNO Senior Thesis, 2008). The different signatures suggest multiple mechanisms may be operating to generate the chalcedony veins.

Analysis of grain mounts has identified the existence of unaltered volcanic ash (in the form of glass shards) in some of the siltstone and claystone samples. Well preserved volcanic ash is puzzling, given that diagenesis could be expected to have altered the glass. Observed stratigraphic differences in glass preservation and instances of devitrification also point to a complex diagenetic history.

Future work on this project includes further clay XRD analysis, petrographic analysis of thin sections, a comparison of mineralogies within and outside of clastic dikes and further analysis of other vein horizons.