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Sequence Stratigraphic Analysis of the Turonian Tununk Shale Member, Henry Mountains Region, Utah: Implications for a Depositional Model of Shelfal Mudstones in Epicontinental Seas

Abstract

Lower-Middle Turonian strata of the Tununk Shale Member of the Mancos Shale Formation were deposited along the western margin of the Cretaceous Western Interior Seaway during the Greenhorn second-order sea level cycle over a time span of about 2.5 million years. Detailed stratigraphic and sedimentological analysis of the Tununk Shale near Hanksville, south-central Utah has revealed a wide range of sedimentary and biogenic features that indicate deposition within storm-dominated shelf environments in a shallow epeiric sea. The Tununk Shale in this area shows a gradual northward thickening and fining trend, with total thickness ranging from 170 to 220 m. Paleocurrent data and paleogeographic reconstructions indicate that sediment supply for the Tununk Shale dominantly came from north of the study area and was carried southward by longshore-currents. The lateral variations in total thickness and grain size are interpreted as due to reduced accommodation (i.e. shallower water depth, more sediment reworking) in the southern area due to the proximity to the forebuldge. At least 45 parasequences can be identified within the Tununk Shale. Each parasequence is characterized by a coarsening-upward trend with an upward increase in the silt & sand content and indications of storm-wave reworking. Based on vertical changes in facies characteristics and parasequence stacking patterns, the 45 parasequences can be grouped in to 11 parasequence sets, and 4 sequences, indicating that there are higher-frequency (3rd-order) relative sea-level cycles superimposed on the second-order Greenhorn cycle. These 3rd-order relative sea-level cycles are dominantly controlled by glacio-eustasy based on the rather close correspondence with global sea-level cycles. The average duration of parasequence-scale facies variations within the Tununk Shale is on the order of 104-105 years, suggesting the influence of Milankovitch-scale climatic cyclicity. In conclusion, deposition and distribution of the Tununk Shale in this area are largely controlled by the tectonically-driven second-order Greenhorn cycle and development of accommodation and depocenter. Vertical variations in facies characteristics within the Tununk Shale are interpreted to reflect the transgression and regression of a storm-dominated shelf mud blanket governed by the combined effects of third- and fourth-order glacio-eustasy and climate change cycles, respectively.