Geochemistry of the Cretaceous Shales and Its Implication on Regional Tectonics and Correlation: Wyoming and Montana

Abstract

The Cretaceous strata of north-central Wyoming and south-central Montana consist of shale, mudstone, and sandstone with minor conglomerate, coal, and bentonite. The primary objective in this paper is to utilize the mineralogical and geochemical data of the critical and dominant lithofacies group comprised of shale and mudstone to reevaluate regional tectonics, provenance aspects, and correlation of the Cretaceous foreland basin sediments. Relative abundances of nickel, zinc, lanthanum, and cerium in the marine samples can be attributed largely to the organic-rich nature and presence of mineral phase apatite in the marine group. The abundance of these trace elements is presumably related to the proximity of local bedrock source region and distinctive clay mineral suite in the marine sediments. Low concentrations of zirconium, titanium, and chromium in nonmarine Cloverly mudrock indicate an alkaline to peralkaline original sediment (ash) composition as opposed to trachytic to dacitic component for the marine group. Furthermore, the marine sequences in the Cretaceous foreland basin are related to thrust load-induced flexure. Overall geochemical behavior of the analyzed samples seems to be related more to confidently to tectonism rather than different provenance. This geochemical classification can be utilized for stratigraphic correlation of the several lenticular sandstone members encased withinmore » the various Cretaceous units and offers an excellent opportunity for future oil and gas exploration in this region.« less