Dynamics of Bar Preservation in Ancient Fluvial Stratigraphy

River bars migrate, expand, amalgamate, and rework themselves in response to local and reach-scale water and sediment-supply dynamics in channel belts. The stratigraphic architecture of channel fills preserves a composite, time-integrated record of channel morphodynamic feedbacks on ancient landscapes. In braided rivers the interaction of these dynamics at multiple scales complicates channel deposit interpretation limiting our ability to (1) reconstruct landscape history; (2) predict subsurface fluid reservoir properties; and (3) leverage braided river deposits to reconstruct the sensitivity of braided river systems to changing landscape forcing.

We use field observations and digital-outcrop observations to describe of the architecture and preservation of bar deposits in ancient braided systems and use these measurements to reconstruct paleo-channel morphodynamics in the Jurassic Salt Wash Member of the Morrison Formation (Ute Lands, Utah, USA), deposited under flashy discharge conditions, and the Cretaceous Castlegate Sandstone (Ute Lands, Utah, USA), deposited in more perennial flow conditions. We compare these results to scaled synthetic stratigraphic cross-sections derived from an unperturbed braided river simulated using the Reynolds-Averaged-Navier-Stokes solver NAYS2DH.

We find that the rate of bar turnover relative to the timing and frequency of thread abandonment and avulsion is the primary control on bar preservation and assess the degree to which discharge variability enhances or impedes bar preservation. This approach to reconstructing paleo-bar morphodynamics provides a new pathway for reconstructing changes in river planform, style, and broader channel-floodplain interactions from ancient deposits and predicting subsurface reservoir characteristics.