Preserving Fluvial Sediments in Highly Mobile, Limited Accommodation Systems: Lessons from the Powder River, Montana

Fluvial stratigraphy records many details of the depositional system's sediment supply, accommodation, and transport processes throughout the period of deposition. However, interpreting fluvial stratigraphy can be extremely complicated, especially in ancient deposits where age control is limited and the dominant transport processes can be hard to determine (e.g. meandering vs. braided). Modern fluvial systems offer a critical opportunity to directly observe the connection between active processes and the resulting stratigraphy in a wide variety of systems. One such system is the Powder River in southeastern Montana USA, which is a meandering river with high sediment supply, limited accommodation, and high rates of lateral channel migration. Here, we investigate how the physical processes of the system control how sediment is preserved across a ~30 km stretch of the basin. In order to characterize the physical processes active in the system, we use the repeat cross section surveys that have been done by the U.S. Geological Survey within this stretch of the river; additionally, we use discharge records from USGS stream gages upstream and downstream from the study reach. In order to characterize the lateral stratigraphic architecture, we mapped lateral accretion packages using lidar and aerial imagery. We then characterized the vertical stratigraphic architecture with a series of trenches, hand auger cores, and exposed bank material. We find that the stratigraphic architecture is strongly correlated to the vertical and lateral distance from the channel during deposition and the relative size of the discharge during a given event. The preservation potential is likewise strongly correlated to the age of the deposit and the distance from the active channel. Our work demonstrates how we can use modern observations of process and stratigraphy to better develop models of how sediment is preserved across fluvial systems.