Stratigraphy and depositional environments of the Late Cretaceous (Late Turonian) Codell sandstone and Juana Lopez members of the Carlile shale, southeast Colorado

Abstract

Late Cretaceous formations in the Rocky Mountains are of significant importance to petroleum exploration throughout the region. Recent exploration in the Codell Sandstone Member of Carlile Shale, Smoky Hill and Fort Hays Limestone members of the Niobrara and Greenhorn formations has rejuvenated interest in not only their petroleum potential, but also stratigraphy, petrography, biostratigraphy, geochemistry and other disciplines within the geosciences. Throughout most of northern, central and southeastern Colorado and eastern Kansas, the Codell Sandstone represents the final regressive deposit of the Greenhorn cyclothem and the Fort Hays Limestone represents the basal sequence of the Niobrara cyclothem. The intermediate, unconformity bound Juana Lopez Member of the Carlile Shale has been interpreted as a palimpsest deposit, and generally disregarded as a significant stratigraphic interval within the region. Southeast Colorado contains some of the world's best outcrops of Late Cretaceous strata and has been the focus of research for nearly a century. The purpose of this research was to better understand the impact and regional extent of the late Turonian unconformities on the Codell Sandstone and Juana Lopez members, and the implications on previous interpretations of the depositional environments and Late Cretaceous paleogeography. Analysis of outcrop, core, subsurface well logs and support from hand-held X-ray fluorescence geochemistry data indicates that significant faulting, topographic highs and subaqueous erosion removed the Codell Sandstone differentially throughout the region, creating a depositional remnant prior to and during deposition of the Juana Lopez and the basal Fort Hays Limestone. The Juana Lopez is a fossiliferous grainstone to calcareous sandstone, deposited in a sediment starved, high-energy, current winnowed, shallow marine shelf environment during the basal transgression of the Niobrara cyclothem.