PALEOENVIRONMENTAL AND STRATIGRAPHIC IMPLICATIONS OF AUTHIGENIC CLAY DISTRIBUTIONS IN MORRISON FORMATION DEPOSITS, BIGHORN BASIN, WYOMING

Authigenic clays resulting from volcanic ejecta have been used in Morrison Formation deposits to support arguments for large alkaline-saline lakes and correlate complex terrestrial and aquatic units. Although it has been assumed that a change in clay mineralogy in the uppermost Morrison Formation is ubiquitous enough to be useful for stratigraphic correlation, distributions of clay minerals near Thermopolis, Wyoming, suggest this is not the case. In the study area near Thermopolis, black, organic-rich mudstone facies in the upper sequences and red mudstone facies in the lowermost sequence have a high percentage of illite. Olive-green and purple mottled mudstone facies contain mixed layer smectite-illite clays with various percentages of smectite and illite. Increase in volcanic input and corresponding changes in hydrology indicated by independent geological and paleontological data may explain the authigenic clay distributions in the study area. Clay mineralogy patterns in vertical stratigraphic successions at the top of the Morrison indicate an increase in volcanic airfall ash input in an increasingly alkaline aqueous environment during the latest Jurassic. Authigenic clays from mudstones appear to be related to environmental differences and are not laterally continuous enough to correlate stratigraphic units. More actualistic and comparative geochemical and sedimentological studies need to be conducted before patterns and changes in clay mineralogy can be used for paleoenvironmental interpretations in continental deposits like the Morrison Formation.