Relating the Fluvial Avulsion Process to Channel-belt Stratigraphic Architecture: An example from the Cretaceous Cedar Mountain Formation, Utah

Fluvial processes such as lateral migration and aggradation produce deposits with unique stratigraphic architectures that document the spatio-temporal evolution of river channels. Channel avulsion, the process by which the main flow of a river is relocated to a different position on the floodplain, is common in modern fluvial systems. The deposits associated with avulsions therefore comprise an important portion of the alluvial stratigraphic record. Despite this, characterization of the avulsion node and the resulting channel-belt architecture has not been achieved in outcrop, as few well-exposed 3-D examples of avulsion deposits exist worldwide. The Cretaceous Cedar Mountain Formation of eastern Utah, USA provides 3-D exposures of fluvial stratigraphy. Cedar Mountain deposits consist of several meters of coarse-grained channel material underlain by thicker (>10 m) intervals of fine-grained floodplain facies and interspersed thin-bedded channel deposits. Due to topographic inversion, the coarse-grained deposits now form ridges with 5-25 m of relief outcropping over an area ~40 km². Our study focuses on the intersection of two ridges that we interpret as the site of a paleo-avulsion based on stratigraphic relationships, vertical and lateral facies changes, and spatial variability in paleocurrents. Specifically, we characterize the facies and 3-D stratigraphic architecture at the location of the interpreted avulsion node, as well as the channel-belt deposits upstream and downstream of this point. Our analysis utilizes 3-D outcrop models derived from Uncrewed Aerial Vehicle (UAV) stereophotogrammetry. UAV-derived basemaps and orthorectified photopanels enable three-dimensional mapping of erosional and bar-scale accretion surfaces. Field data include ~300 paleocurrent and accretion strata orientations and 10 stratigraphic sections. Integration of field data and outcrop models enable detailed characterization of the spatio-temporal evolution of fluvial channel-belt deposits near the proposed avulsion node. We use stratigraphic forward models to produce realizations of synthetic stratigraphy for comparison with field data and outcrop model interpretations. Our study has implications for establishing recognition criteria for fluvial avulsions in outcrop and the subsurface.