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

KHANDAKER, NAZRUL I., Lafayette College, Easton, PA, and CARL F. VONDRA, Iowa State University, Ames, IA

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. Fifty shale and mudstone samples were collected from the nonmarine Aptian to Albian Cloverly Formation, marine Albian Shell Creek Shale, marine Cenomanian Mowry Shale, transitional marine to marine Turonian Frontier Formation, and marine Coniacian Cody Shale. The mineralogical composition of the less than two micron-size fraction was determined. Mine al constituents in marine and nonmarine samples are almost identical and their provenance is presumed to be the same. However, the individual proportions of minerals vary in both the marine and nonmarine groups. Plagioclase feldspar, calcite, kaolinite, and illite proved to be very significant discriminators for separating marine from nonmarine samples at a significant level of 0.05. A similar pattern of enrichment was documented from the marine Cretaceous Pierre Shale of northern Great Plains. 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 regi n 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 confidently to tectonism rather than different provenance. This geochemical classification can be utilized for stratigraphic correlation of the several lenticular sandstone members encased within the various Cretaceous units and offers an excellent opportunity for future oil and gas exploration in this region.