TROPIC SHALE AND TUNUNK MEMBER OF MANCOS SHALE, SOUTHERN UTAH, USA: A NATURAL LABORATORY FOR SHALE SEDIMENTOLOGY AND STRATIGRAPHY

The Tropic and Tununk record the Cenomanian-Turonian transgressive-regressive cycle, higher-frequency sequences, parasequences and event beds in prodelta to offshore marine paleoenvironments along a 200 km transect in southern Utah. Moderate subsidence and sediment supply in a bight of the mid-Cretaceous seaway allowed preservation of a relatively continuous record of sedimentation from the westernmost exposure of US Western Interior Cretaceous marine mudstones near Cedar City to excellent marine shale outcrops in the Grand Staircase National Monument and Henry basin. Extreme lateral shifts in paleoshoreline and offshore facies belt locations during the transgressive-regressive cycle led to vertical stacking of distinct depositional environments and mudrock lithofacies. An excellent stratigraphic framework developed by generations of workers has allowed pinpointing of the Cretaceous maximum marine flood within a stack of early Turonian shoreline sandstones near Cedar City, and the equivalent maximum flood bedset in offshore marine mudrocks. Early Turonian rhythmic bedsets deposited during the early highstand systems tract correlate bed-by-bed with limestone-shale couplets in Colorado and Kansas that have been interpreted to represent Milankovich climate cycles. Middle Turonian storm deposits encased in marine mudrocks and interleaved with regionally-extensive bentonite marker beds pass laterally from ammonite or bivalve shell beds to siltstone- and very fine sandstone-filled gutter casts to sandstone beds. There was a temporal shift in tempestite paleocurrent orientation. Middle Turonian storm currents in the eastern Kaiparowits Plateau and Henry basin shifted from an offshore, easterly trend to a more shore-parallel, southerly trend. Concretion type commonly changes laterally from simple limestone concretions offshore to septarian limestone concretions in the distal prodelta to cone-in-cone limestone concretions in proximal prodelta paleoenvironments. A sequence-keyed mudrock facies model has been developed, and mudrock lithofacies are distinctly different in the transgressive, early highstand and late highstand systems tracts. Accessibility of good outcrops on public lands, public-domain core and subsurface well logs invite more detailed studies of these rocks.