THE ORIGIN OF CLAY MINERALS IN THE UPPER CRETACEOUS TUNUNK SHALE MEMBER OF THE MANCOS SHALE FORMATION, HENRY MOUNTAINS REGION, SOUTH-CENTRAL UTAH

Clay mineralogy has been extensively used as a provenance indicator for characterizing the intensity of continental weathering and reconstructing the Late Cretaceous climate (e.g., wet vs. dry cycles). In this study, clay samples from shales and bentonites in the Tununk Shale Member of the Mancos Shale Formation (Turonian age) were analyzed by X-ray powder diffraction methods. Integrated with combined field observations, sedimentologic facies analysis, and petrographic examinations (optical and electron optical techniques), the origin of clay minerals present in this thick shelf mudstone succession can be determined.

The Tununk Shale was deposited on a storm-dominated shelf along the western margin of the Western Interior Seaway (WIS) during the Early to Middle Turonian. In the Tununk Shale, clays in the bentonites contain a dominant amount (>80%) of pure smectite and minor amounts of biotite and kaolinite (up to 20%). Clays in the shales consist dominantly of mixed-layered illite/smectite (I/S) with up to 20% illite-type layers, small amounts of kaolinite and mica, and sometimes trace amount of chlorite. The almost pure smectite in bentonite samples indicates no alteration of the bentonites by depth diagenesis, and presumably, no alteration of the shales. Therefore, the mixed-layer I/S in shales must have developed before rather than after deposition. Integrated with petrographic examinations, possible sources for the mixed-layer clays in the shales include 1) weathering of volcanic debris inland, and 2) erosion of older smectitic mudstones that have previously been buried. Most kaolinite present in both bentonites and shales is interpreted to be the alteration product of volcanic materials (e.g., biotite and feldspars).

Clay minerals in the Tununk Shale were derived from various sources, including erosion of the Sevier orogenic belt, volcanic debris altered on land, alteration of wind-borne volcanic ashes, and recycled sediments from outside and inside of the seaway. When developing the link between clay mineralogy and the intensity of continental weathering (and associated climate), an integrated analysis of clay mineralogy and petrography in a well-developed depositional framework is required to sort out the clay minerals of detrital origin (weathering products) from other sources.