AAPG ACE 2018

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Three-Dimentional Geometry and Reconstruction of a Backwater Trunk-Tributary, in the Cretaceous Ferron Sandstone, Utah

Abstract

Incised tributary valleys are identifiable in planform view due to their distinct morphology; however no model yet exists to distinguish trunk valley deposits from those of their associated tributary deposits. Paleocurrent data and planform mapping of the youngest nonmarine sequence of the Turonian Ferron Notom Delta in Central Utah reveals an incised tributary valley, that feeds a previously mapped trunk valley. Using drone photography and photogrammetry modelling, 32 measured sections were correlated over a 2 km2 area to reconstruct the tributary valley fill architecture and produce orthopane images free of the parallax normally associated with outcrop imagery. High resolution GPS modelling was used to map the valley floor and shows up to 15 m of erosional relief. Thin sections were taken from point bar deposits to estimate porosity and permeability of a 2.5 m deep tributary channel with a bankfull paleodischarge of 11.6 m3/s. Paleohydraulic estimates, lithology, channel dimensions and reservoir quality were compared between trunk and tributary valley fills. The trunk valley channel discharge was calculated to be 2 orders of magnitude greater than that of the tributary while median grainsize shows little difference.

Environmental reconstruction of backwater conditions was investigated using micropaleontology. The relative abundance of fluvial Thecamoebian and marine Foraminifera populations within floodplain shale deposits was used as a proxy to determine the incursion of tidal water and backwater influence within the system over time.