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.
AAPG Search and Discover Article #90067©2007 AAPG Mid-Continent Section Meeting, Wichita, Kansas