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Depositional Controls on the Origin of Clay Minerals in the Mancos Shale (Turonian Age), 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, samples from the fine-grained sediments (shales and bentonites) of the Mancos Shale Formation (Turonian age) exposed in south-central Utah were analyzed by X-ray powder diffraction methods. Using combined field observations, sedimentologic facies analysis, and petrographic examinations (optical and electron optical techniques), the origin of clay minerals present in this offshore mudstone succession can be determined. In the study area, the Mancos Shale was deposited in environments ranging from prodelta/lower shoreface to outer shelf during the Early to Middle Turonian. Clay minerals in the shales include a dominant amount of mixed-layered illite/smectite (I/S) with up to 20% illite-type layers and minor amounts of kaolinite and chlorite. Clay minerals in the bentonites contain a dominant amount (> 80%) of smectite and minor amounts of biotite and kaolinite. The almost pure smectite in bentonite samples indicates no alteration of the bentonites by depth diagenesis, and presumably, no alteration of the shales. Integrated with the depositional model, the mixed-layer clays in shales were probably derived from 1) alteration of volcanic debris on land; and 2) erosion of older smectitic mudstones that have previously been buried (i.e., recycled sediments). Most kaolinite and chlorite in shales are derived from 1) in situ alteration of unstable detritus (feldspars, micas, volcanic rock fragments, etc.); and 2) deposition from solutions in the intergranular pores of sediments during early diagenesis. Compared to the shelf mudstones, the prodelta/lower shoreface deposits contain a significantly higher amount of authigenic kaolinite and chlorite as pore-filling cements. The changes in the relative abundance of different types of clay minerals in sediments deposited in different depositional environments are probably caused by variations in sources of sediment supply and physio-chemical conditions across the ancient shelf. 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.