Climatic and tectonic significance of the sedimentology, provenance, and stable carbon isotope geochemistry of continental mudrocks of the Cloverly Formation (Lower Cretaceous) in Wyoming.

摘要

Continental sediments of the Cloverly Formation (Lower Cretaceous) in Wyoming can be subdivided into three depositional systems: perennial to intermittent alluvial, intermittent to ephemeral alluvial, and playa. Organic-rich mudstones, blocky mudstones, and skeletal limestones of the Cloverly Formation, and associated chert-bearing sandstones and conglomerates, represent deposition by perennial to intermittent alluvial systems. Organic-rich mudstones contain abundant wood fragments and other vascular plant remains, consistent with deposition in oxbow lakes, abandoned channels, and on floodplains. Intraformational conglomerates, ripple cross-laminated sandstones, bioturbated limestones, blocky mudstones, and bioturbated mudstones of the Cloverly Formation were deposited by intermittent to ephemeral alluvial systems. Caliche nodules and slickensides in blocky and bioturbated mudstones indicate seasonal to dry conditions. Laminated mudstones, tuffaceous siltstones, vuggy limestones, diamictites, and bioturbated mudstones of the Cloverly Formation were deposited in a playa environment. Vuggy limestones, layers of spheroidal barite and chalcedony, and gypsum beds attest to high alkaline conditions.; Stable carbon isotopic compositions of terrestrial organic matter in organic-rich mudstones of the Cloverly Formation range from −20.8 to −24.3 per mil relative to the Peedee Belemnite (PDB) standard, with an average value of −22.9 per mil (n = 56). Vascular plants in the Early Cretaceous are inferred to use the same carbon fixation pathway as modern C3 plants, but the measured carbon isotopic compositions from the Cloverly Formation (δ 13C of −20.8 to −24.3 per mil) are significantly different than values for modern C3 plants (δ13C of −24 to −30 per mil). The enrichment of 13C in terrestrial organic matter in organic-rich mudstones from the Cloverly Formation is inferred to result from isotopically heavier CO₂ in the Early Cretaceous atmosphere, which is consistent with enhanced input of volcanogenic CO ₂.; The areal abundance and distribution of varying depositional systems within the Cloverly Formation changes systematically across the overfilled portion of the Early Cretaceous Cordilleran foreland basin. The vertical and eastward progression of perennial to intermittent alluvial, intermittent to ephemeral alluvial, and lastly, playa depositional systems within the Cloverly Formation of Wyoming resulted from an intensifying rain shadow induced by the uplift of the Sevier Mountains.