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Stratigraphic Architecture of Exhumed Fluvial Channel-Belts: Anatomy of an Avulsion

Abstract

River systems play a significant role in the evolution of landscapes and deposition of clastics on Earth’s surface, and fluvial deposits in the subsurface act as important hydrocarbon reservoirs and aquifers. Lithologic heterogeneity within fluvial reservoirs has been shown to strongly affect flow properties and compartmentalization. Fluvial morphodynamics and the spatio-temporal evolution of fluvial channel-belts act as fundamental controls on the distribution of facies within fluvial reservoirs. Therefore, insight into fluvial processes such as avulsion, reoccupation, aggradation/superelevation, and lateral migration is essential for predicting lithologic heterogeneity within these reservoirs. However, characterization of fluvial morphodynamics and planform system evolution is often a non-trivial task due to difficulty in accurately reconstructing stratigraphic surfaces and channel-belt dimensions from 2-D outcrop exposures. Exhumed fluvial channel-belt deposits of the Cedar Mountain Formation located in eastern Utah, USA provide an opportunity to analyze ancient fluvial stratigraphy in three dimensions. The exhumed channel-belts create over 15 m of relief and consist of multiple stacked channel-filling deposits with thicknesses averaging 3.1 m. Our study focuses on the intersection of two such exhumed deposits that we interpret as the site of a paleo-avulsion, based on observed scour surfaces, vertical and lateral changes in facies, and variations in measured paleocurrent direction across the site. Our analysis utilizes 3-D outcrop models derived from high-resolution Unmanned Aerial Vehicle (UAV) photogrammetry of the exhumed channel-filling deposits. UAV-derived visible and topographic basemaps enable continuous, three-dimensional mapping of the sedimentology of this site, including the bounding surfaces of dune cross-sets, bar cross-sets, and channel-filling deposits, as well as paleocurrent measurements. These data permit examination of the avulsion process (including the role of channel aggradation, superelevation, and reoccupation) and promote detailed characterization of the spatio-temporal evolution of fluvial channel-belts, with emphasis on the facies and 3-D stratigraphic architecture of the resultant deposits. Our study has implications for establishing recognition criteria for avulsions in fluvial channel-belt deposits and for understanding distribution of heterogeneity in subsurface fluvial reservoirs, specifically near points of avulsion.