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Presence and Variability of Froude Transcritical and Supercritical Flow Sedimentary Structures in Fluvial Strata

Abstract

Froude transcritical and supercritical or upper flow regime (UFR) sedimentary structures are commonly over looked in fluvial strata, as their presence in the existing fluvial facies models is minor. In many cases they are misinterpreted as cross stratification built by dune migration. In some cases, they are even considered as hummocky (HCS) or swaley cross strata (SCS) built by storm waves. Especially the latter can be a source of misunderstanding and incorrect environmental interpretations. UFR sedimentary structured consist of planar laminations, scour and fill, low angle convex-up laminations, and sigmoidal or humpback cross-laminations. This study aims to provide both quantitative and visual descriptions of the UFR sedimentary structure types, and their internal structural and dimensional variability in fluvial strata, provide recognition criteria and criteria for differentiating these UFR structures from dune cross strata, and from HCS and SCS. The study utilizes field data from the Eocene Wasatch and Green River Formations in the Uinta Basin and the Cretaceous Williams Fork Fm. in the Piceance Basin, and compares to literature descriptions and experimental results. Data were collected by measuring stratigraphic sections, documenting lateral extent of the sedimentary structures, sketching, and taking photographs. These sections were then entered into Microsoft Excel and further quantitatively analyzed. Results indicate that UFR sedimentary structures are common in fluvial strata of flashy or seasonal river systems, where deposition dominantly occurs from high-intensity floods. The UFR structures commonly occur in thick packages indicating high deposition rates. The study highlights that high deposition rates are required to preserve large thicknesses of UFR deposits that otherwise have a very low preservation potential. Other indicators of high deposition rates include aggradational nature of the bedforms and gradational or internally graded nature of the planar laminations. The results help to characterize the deposition that occurs from highly seasonal river systems, such as in monsoonal and subtropical climates.