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Sedimentological and Petrographic Characteristics of a Mudstone-Dominated Succession Within a Sequence Stratigraphic Context: The Upper Cretaceous (Lower-Middle Turonian) Tununk Shale, South-Central Utah


The upper Cretaceous Tununk Shale Member of the Mancos Shale Formation accumulated in storm-dominated shelf environments along the western margin of the Western Interior Seaway. Three stratigraphic sections were measured near Hanksville, Utah. Within each section, sedimentological data including lithology, grain-size, bedding thickness, sedimentary structures, and bioturbation features were documented, and relatively unweathered samples were collected at an average spacing of 1 m. Detailed sedimentological and petrographic analysis (optical microscopy and SEM) was conducted on more than 200 polished slabs, 12 thin sections, and 30 ion-milled samples, in order to examine facies characteristics and variations within the Tununk Shale. The Tununk Shale consists of at least 45 parasequences, which can be grouped into 11 parasequence sets, and 4 sequences (S1 to S4 from bottom to top). Detailed petrographic studies indicate the Tununk Shale consists of a wide range of components, including various detrital grains, composite particles (e.g. lithified shale clasts, rip-up intraclasts, and fecal pellets), clay minerals, fossil debris, and miscellaneous grains (foraminifera tests, phosphate, etc.). The Tununk Shale can be divided into a lower calcareous interval (S1 to S3) and an upper non-calcareous interval (S4). Sequence 1 records the greatest water depth during deposition of the Tununk Shale. The relative amount of calcareous particles including foram tests and fecal pellets gradually decreases from S1 to S3, and to non-calcareous S4, reflecting the overall regression of the 2nd-order Greenhorn sea-level cycle. Higher-frequency 3rd- and 4th-order sea-level cycles driven by global-eustasy and climatic cyclicity, respectively are recognized within the Tununk Shale. Upward within each parasequence the silt & sand content and indications of storm/wave reworking increase, and calcareous content decreases. Silt- to sand-sized shale clasts can be commonly identified in thin sections and under SEM throughout the Tununk Shale, and are interpreted to be mainly derived from altered volcanic fragments from hinterland. The relative amount of shale clasts tends to increase as relative sea-level decreases. Cyclic variations of sedimentological and petrographic features within the Tununk Shale are produced by the dispersal and deposition of a variety of allochthonous and autochthonous grains/particles, under the influence of hierarchical sea-level fluctuations and climate cycles.