--> A Sedimentologic-Petrographic Study of Parasequences in the Late Cretaceous Tununk Shale of the Henry Mountain Region of SW Utah – Implications for Shale Deposition in the Western Interior Seaway

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A Sedimentologic-Petrographic Study of Parasequences in the Late Cretaceous Tununk Shale of the Henry Mountain Region of SW Utah – Implications for Shale Deposition in the Western Interior Seaway

Abstract

The Tununk Shale, a lateral western equivalent of the lower Mancos Shale, is well exposed on the eastern flank of the San Rafael Swell near Hanksville, Utah. In the context of a regional sequence stratigraphic study, the aim of this study is an inventory of sedimentary and diagenetic features for selected parasequences (PS) in order to understand processes active during PS development. Marine flooding surfaces are marked by a sharp based winnowed and well sorted silty residual of detrital quartz, feldspar and altered volcanic rock fragments, fossil debris, and mud rip-up clasts. Pore spaces are filled with calcite spar. This basal lag is overlain by bioturbated silty mudstones with starved ripple laminae of coarse silt to fine sand, suggesting combined flow conditions. Sharp-based graded beds of mm to cm scale with bioturbated tops are likely event beds, probably related to storm wave activity. Toward the PS top fine sand increases markedly, wave ripples are common, and large metazoan burrows become abundant. In addition, mafic minerals were observed in the sand fraction, as well as authigenic chlorite development in pore spaces. SEM petrography was conducted on large diameter (12.5 mm) ion milled samples, and showed that siltstones as well as more “muddy” intervals contain abundant fine grained aggregates, such as altered volcanic glass and volcanic rock fragments, deformed mudstone rip-up clasts, and fine grained sedimentary rock fragments lacking post-depositional compaction. These aggregates dominate the grain population of siltstone beds, and also appear dominant in intervals of “muddy” appearance. From a physical sedimentology perspective, much of the Tununk appears deposited as bedload transported siltstones, albeit siltstones with “atypical” grains. The mudstone rip-up clasts probably reflect contemporaneous erosion of the Cretaceous seafloor, reworking material that had arrived in the form of fall-out from hypopycnal plumes and bedload mud floccules moved by bottom currents. Volcanic debris probably arrived as ash falls (later reworked) and fluvial transported material. Uncompacted fine grained sedimentary rock fragments suggest input of river borne material from shale outcrops in the hinterland. Overall, these features suggests that sand and silt size shale and volcanic debris was carried to the basin by rivers and redistributed by storm induced currents, and that primary shales were subject to intermittent erosion and supplied mud rip-up clasts.