Evaluating the signatures of fluvial, wave, and tidal processes on deltaic deposits in the Cretaceous Western Interior Seaway

Abstract:

Characteristic differences in delta morphology suggest different balances of wave, tidal, and fluvial processes on different coastlines. These insights have been used to interpret stratigraphic deposits and reconstruct paleogeography of ancient fluvial-deltaic environments. This work is often accomplished through combined efforts to map the architecture of ancient delta deposits and document the abundance and distribution of sedimentary structures that indicate sediment transport by uni- or bi-directional currents or waves. Statistical tools for quantifying sedimentation patterns in ancient deposits provide an opportunity to directly compare architectural patterns among ancient delta deposits. In order to evaluate the degree to which interpreted influence of waves or tides is manifested as a change in delta architecture, we compare detailed stratigraphic measurements from ancient deposits. Well-studied Cretaceous deltaic deposits in the Western Interior Seaway (USA), including the Ferron and Sego formations, have been interpreted as reflecting wave- and tide-influenced conditions, respectively. Using measurements made in the field and from terrestrial lidar outcrop scans, we compare the compensation index and paleohydraulic reconstructions from distributary channel deposits in order to quantify sedimentary architecture in each unit. Preliminary results suggest that the scaling and architecture of topset, foreset, and prodelta deposits in each of these units may have been broadly similar, despite distinct differences in the abundance of wave- and tidal-indicators present within each unit. This indicates that the presence of traditionally diagnostic deposits, particularly sedimentary structures indicative of fluvial, tidal, or wave processes, may not accurately reflect the balance of these processes acting on paleo-shorelines.