Interpreting Paleo-Avulsion Dynamics from Multistory Sand Bodies

Abstract

Abstract Channel avulsion plays a significant role in building alluvial stratigraphy, including fluvial sand bodies with through-going erosion surfaces, called multistory sand bodies (MSBs); consequently, MSB characteristics may be useful for interpreting paleo-avulsion dynamics from fluvial deposits. Here, using a combination of literature review, geometric modeling, and field observations, we explore the degree to which MSB characteristics may be useful for interpreting paleo-avulsion dynamics from ancient deposits. We use published interpretations of MSB origins to identify characteristics that are exclusively associated with avulsion-related MSBs and find that vertical story stacking, irregular MSB bounding surfaces, and floodplain deposits that correlate with individual stories are attributes uniquely associated with avulsion-origin MSBs. To evaluate how avulsion patterns may affect MSB properties, we use a 2D geometric model to build synthetic stratigraphy and compare characteristics for MSBs formed under different avulsion patterns at a range of model-aggradation rates. Avulsion patterns include random (equal probability of a channel relocating anywhere across the model domain), compensational (channels avulse to the lowest elevation in the model domain at each time step), and clustered (where channel relocation is restricted to a zone near a previously occupied location). Model results show that at moderate to high aggradation rates, MSBs formed under clustered avulsion patterns can be differentiated from those formed via random and compensational avulsion patterns. Specifically, MSBs formed by clustered avulsion patterns have many stories and commonly contain remnant floodplain deposits. We apply these insights to the well-exposed lower Williams Fork Formation (Upper Cretaceous, Piceance Basin, Colorado, USA) to demonstrate how they can be used in practice. Fifty-four percent of lower Williams Fork MSBs contain clear evidence for avulsion reoccupation, 32% have characteristics suggesting intra-channel-belt origins, and the remaining 20% have ambiguous evidence or insufficient exposure to permit confident process interpretations. Overall this study demonstrates that despite inherent challenges in eliciting information about avulsion dynamics from the stratigraphic record, some MSB characteristics can be interpreted as clear signals of paleo-avulsion processes.