Geochemistry and shock petrography of the Crow Creek Member, South Dakota, USA:Ejecta from the 74 Ma Manson impact structure

Abstract

The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale Formation of eastern South Dakota and northeastern Nebraska, but it is the only unit that contains shock-metamorphosed minerals. The shocked minerals represent impact ejecta from the 74-Ma Manson impact structure (MIS). This study was aimed at determining the bulk chemical compositions and analysis of planar deformation features (PDFs) of shocked quartz; for the basal and marly units of the Crow Creek Member. We studied samples from the Gregory 84-21 core, Iroquois core and Wakonda lime quarry. Contents of siderophile elements are generally high, but due to uncertainties in the determination of Ir and uncertainties in compositional sources for Cr, Co, and Ni, we could not confirm an extraterrestrial component in the Crow Creek Member. We recovered several shocked quartz grains from basal-unit samples, mainly from the Gregory 84-21 core, and results of PDF measurements indicate shock pressures of at least 15 GPa. All the samples are composed chiefly of SiO2 (29-58 wt%), Al2O3 (6-14 wt%), and CaO (7-30 wt%). When compared to the composition of North American Shale Composite, the samples are significantly enriched in CaO, P2O5, Mn, Sr, Y, U, Cr, and Ni. The contents of rare earth elements (REE), high field strength elements (HFSE), Cr, Co, Sc, and their ratios and chemical weathering trends, reflect both felsic and basic sources for the Crow Creek Member, an inference, which is consistent with the lithological compositions in the environs of the MIS. The high chemical indices of alteration and weathering (CIA and CIW: 7599), coupled with the Al2O3-(CaO*+Na2O)-K2O (A-CN-K) ratios, indicate that the Crow Creek Member and source rocks had undergone high degrees of chemical weathering. The expected ejecta thicknesses at the sampled locations (409 to 219 km from Manson) were calculated to range from about 1.9 to 12.2 cm (for the present-day crater radius of Manson), or 0.4 to 2.4 cm (for the estimated transient cavity radius). The trend agrees with the observed thicknesses of the basal unit of the Crow Creek Member, but the actually observed thicknesses are larger than the calculated ones, indicating that not all of the basal unit comprises impact ejecta.