STRATIGRAPHY AND SEDIMENTOLOGY OF THE UPPER JURASSIC MORRISON FORMATION, DILLON, MONTANA

purple and gray-green mudrocks near Dillon, Montana, have been mapped traditionally as the Up-per Jurassic Morrison Formation. There are very few studies of the Mesozoic strata in this area and questions exist as to whether this unit actually is the Morrison Formation, or whether it is similarly variegated mudrocks of other formations at nearly the same stratigraphic interval. This study compares the stratigraphy, sedimentology, and paleon-tology of rocks near Dillon to the Upper Jurassic Morrison Formation elsewhere in the northern portion of the West-ern Interior Basin. The average thickness of the unit in question is approximately 24 m. It is composed of four lithofacies: 1) interbedded gray-green sandy siltstone showing weak pedogenic modification; 2) red calcareous mud-stone on which moderately-to well-developed paleosols formed; 3) intraformational mudclast conglomerates; and 4) black claystone. These lithofacies are interpreted as distal floodplain deposits of a mud-dominated alluvial system. The paleosols are characterized by mottled coloration, carbonate nodules, clay slickensides, and abundant carbonate rhizoliths and rhizocretions. Dinosaur-bone fragments and the Late Jurassic gastropod Viviparus reesidei were also present. The stratigraphy, sedimentology and paleontology compare best with the Morrison Formation of northern Wyoming and south-central Montana