Facies Analysis of the Morrison Formation (Upper Jurassic) in Kansas Based on Petrophysical Logs and Drill-Cuttings Records

John H. Doveton. Kansas Geological Survey, University of Kansas, Lawrence, Kansas 66047  [email protected]

Morrison Formation (Upper Jurassic) outcrops in Colorado are world-famous for their dinosaur remains. Detailed field studies by the Morrison Extinct Ecosystem Project have concluded that the formation was deposited on an inland plain under arid to semi-arid conditions with the formation of evaporites, windblown sands, ephemeral stream clastics, and lacustrine limestones. The eastward extension of the Morrison into Kansas is confined entirely to the subsurface and almost all information is restricted to drill-cuttings and limited log suites. Analysis of the nuclear logs shows distinctive electrofacies that can be matched with outcrop lithologies. Particularly intriguing is the lacustrine limestone facies, which is often highly radioactive.  Drill-cuttings show the Kansas Morrison to be composed of sandy shales, with green and variegated shales, pink chalcedonic chert, chalky limestone, very fine grained sandstones, anhydrite, and gypsum. The occurrences of these lithologic components in wells across western Kansas were coded and their aggregate patterns analyzed by latent class analysis and mapped. Striking regional components on the map can be readily interpreted within a simple paleogeographic and depositional model. The subcrop edge on the eastern and southeastern edge is probably close to the basin margin and has a dominantly northeast-southwest strike. Sandstones occur at the eastern margin with possible trends of dispersal along northwestward-trending axes. To the northwest, these sandstones are associated with anhydrite and flanked by localized areas of anydrite, anhydritic limestone, and limestone. An environmental interpretation that is consistent with detailed outcrop studies in eastern Colorado suggests that rivers transported sand eroded from sources in the southeast to the northwest across an alluvial plain, which had a patchwork of ephemeral streams, playas, evaporitic ponds, and freshwater lakes.