Paper No. 185-6
Presentation Time: 9:30 AM
RECONSTRUCTING BAR AND CHANNEL MORPHODYNAMICS FROM ANCIENT RIVER DEPOSITS (Invited Presentation)
HAJEK, Elizabeth, Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, GREENBERG, Evan, Department of Geosciences, Pennsylvania State University, University Park, PA 16802, CHAMBERLIN, Ellen P., Geosciences Department, Denison University, F.W. Olin Science Hall 301, Denison University, Granville, OH 43023 and HELLER, Paul, Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave, Laramie, WY 82071, hajek@psu.edu
Uniquely identifying ancient deposits of meandering rivers is important because the stratigraphic archive holds information about controls on channel- and landscape-scale morphodynamics in meandering systems and also records examples of how these networks have responded to climate, land-cover, and tectonic perturbations. Ancient river deposits with relatively narrow channel sand bodies encased in abundant floodplain mudstone accumulations are often interpreted as representing deposition by meandering rivers. However, these commonly cited attributes do not themselves directly reflect meandering processes, and the scale and dimensions of channel-belt sandstones and the overall preservation mudstone in fluvial deposits can be significantly influenced by sediment supply, basin accommodation, and river-avulsion dynamics.
Here, we use new paleomorphodynamic measures to reconstruct and compare ancient bar and channel characteristics in a range of Mesozoic and Cenozoic river deposits in the western United States. We first identify avulsion-generated channel-belt deposits, then we evaluate individual bar elements specifically focusing on bar preservation, bar persistence (defined as the relative distance a bar deposit migrated with a coherent form), and bar stacking within a given channel belt. Collectively these measures allow us to compare relative avulsion frequency, relative channel migration, relative bar stability, and the prevalence of forced vs. free bars among different deposits.
This work demonstrates that bar and channel deposits vary widely across ancient fluvial systems, even among systems with similar sand-body dimensions, basin-scale architecture, and overall sand:mud ratios. These measurements allow us to compare detailed aspects of channel morphodynamics to long-term avulsion and floodplain-accumulation patterns in an effort to understand how the process of river meandering is linked to larger-scale fluvial landscape dynamics.
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