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Along-Strike Variability of the Fluvial Castlegate Sandstone, Wasatch Plateau, Utah: Depositional Trend and Sequence Stratigraphic Architecture

Abstract

There are no published high-resolution attempts to resolve the stratigraphy of the Upper Cretaceous Castlegate Sandstone by correlating from the well-studied and depositional-dip-oriented Book Cliffs to the depositional-strike-oriented and basin-proximal Wasatch Plateau. In this study, both tectonic and eustatic signals augmented the establishing of a sequence stratigraphic framework for the highly-amalgamated, fluvial Castlegate Sandstone along the northern Wasatch Plateau. The study results indicate that its basal third-order (low-frequency) sequence boundary, which separates it from the Blackhawk Formation below, has a prominent erosional relief of nearly 30 m. The lower Castlegate Sandstone, which onlaps onto this basal sequence boundary, tapers abruptly southward as correlated from north and the type-section. Another (third-order) unconformity separates the Castlegate Sandstone from the overlying conglomeritic Price River Formation. Higher frequency sequence stratigraphic surfaces were identified within the Castlegate Sandstone, and, for the first time, the surface coeval to the base of the Sego Sandstone (a Member of the Castlegate Sandstone) has been correlated along the Wasatch plateau. This high-frequency surface that divides the Castlegate Sandstone into lower and upper units was commonly found down-cutting into an organic-rich siltstone and displaying Teredolites longissimus burrows, which is indicative of marginal-marine setting. Channelized sandbodies of the Castlegate Sandstone show pervasive basal erosions with mud rip-up clasts. Sedimentary structures include predominately trough cross-bedding with subordinate ripple cross-lamination, parallel lamination, convolution, and sole marks. Using dune-set thickness thus dune height as a proxy for paleo-river water depth, the study results indicate that channels were deeper and were filled by coarser grained sediments in the southern part of the study area. Closer to the type section, thin overbank deposits are better preserved, and finer sands filled shallower channels.