Three-dimensional anatomy of a Cretaceous river avulsion

Abstract River avulsions are fundamental mechanisms underlying the construction of fluvial stratigraphy. Although avulsions occur in three dimensions (3-D), interpretations of avulsion deposits in the terrestrial stratigraphic record remain largely based on two-dimensional (2-D) lithologic stacking patterns with little spatial context of the avulsion node location. Moreover, though the genesis of intersecting fluvial ridges observed on Mars is commonly attributed to avulsion, little is known about the history or stratigraphic architecture of these intersections nor the duration of the martian fluvial activity they represent. Here, we analyze a fluvial ridge intersection in the Cretaceous Cedar Mountain Formation of eastern Utah, USA, that we interpret to reflect the evolution of a river channel avulsion node exposed in 3-D. Through integration of digital outcrop models and field characterization, we document recognition criteria for avulsion nodes and reconstruct pre-, intra-, and post-avulsion channel evolution at the node, identifying evidence of channel-bed aggradation and pre-avulsion superelevation. Using a new 3-D stratigraphic modeling approach, we simulate the avulsion and analyze the resultant synthetic stratigraphy in parallel with outcrop exposures to link avulsion processes to stratigraphic products. Our results promote recognition and process-based interpretation of ancient river avulsion deposits observed in cross section and plan view on Earth and Mars and constrain the period of fluvial activity represented by the intersection to the order of centuries rather than more significant geologic time, as has been hypothesized previously.