The Sedimentary Record of Braided River Form and Mobility

Open Access
- Author:
- Alpheus, Safiya
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 03, 2024
- Committee Members:
- Roberto Fernández, Outside Unit & Field Member
Anastasia Piliouras, Major Field Member
Elizabeth Hajek, Chair & Dissertation Advisor
Roman DiBiase, Major Field Member
Donald Fisher, Program Head/Chair - Keywords:
- braided rivers
river mobility
channel avulsion
bar
fluvial stratigraphy - Abstract:
- The fluvial stratigraphic record integrates snapshots of ancient river activity on planetary surfaces. It is an archive that can be leveraged to infer paleo-flow conditions of ancient rivers and constrain the history of climatic, tectonic, eustatic, and land cover changes on Earth. In braided rivers, channel and bar movements influence the shape and architecture of fluvial deposits as channels and bars migrate, bifurcate, amalgamate, and deform spontaneously, and in response to local-, reach-, and catchment-scale conditions. Presently our ability to reconstruct details of flow conditions and channel movements from braided river deposits is challenged by the wide range of temporal and spatial variability associated with natural river networks, gaps in the preservation and sampling of ancient river deposits, and limitations in extending engineering-scale relationships over geologic time. To address these gaps, I combined numerical modeling, remote sensing of modern braided rivers, and field-based observations of ancient braided-river deposits to explore how channel form and mobility are reflected in the architecture and preservation of braided channel deposits. First, I used numerical modeling to explore how processes like channel and bar migration and channel avulsion control the preservation of bar deposits within braided channel sediments and developed heuristic expectations for bar preservation in braided channel deposits. This work demonstrates that: (1) bar preservation in braided river stratigraphy is highly variable in both space and time, with no correlation to channel widening or braided river form; (2) the timing of thread abandonment and channel avulsion relative to bar turnover fundamentally controls bar preservation in braided stratigraphy; and (3) regardless of preservation, bar package thickness approximates the formative flow depth of a paleochannel. Next, I used time-series of Landsat satellite images of 20 active braided river reaches to document channel-bed turnover characteristics across a range of climate zones and water- and sediment-supply regimes. I used multivariate statistical approaches to identify groupings of variables that describe categories of braided channel mobility behavior. These results revealed distinct differences in braided river mobility that manifest as a function of the combined effects of slope, proportion of bedload sediment flux, channel network form, and discharge regime in these rivers. Finally, I applied the insights from numerical models and modern river systems to a comparative study of ancient braided fluvial systems. I mapped trends in bar preservation across six distinct localities in four different formations (Jurassic Salt Wash Member, Morrison Formation, Ute and Southern Paiute Lands, Utah, USA; Cretaceous Castlegate Sandstone, Ute Lands, Utah, USA; Paleocene-Eocene Molina Member of the Wasatch Formation, Ute Lands, Colorado, USA; and the MioceneOligocene Huesca Fluvial System, Spain) and assessed stratal geometries and facies relationships to estimate the proportion of mid-channel to bank-attached bars across these deposits, developing a new proxy for reconstructing paleo-braiding intensity. Preservation trends across these systems show no unique relationship with reconstructed braiding intensity, sediment supply, and basin climate, but demonstrate how differences in river mobility patterns observed in modern rivers can influence preservation patterns in ancient stratigraphy. Finally, these results highlight how differences in sediment partitioning across fluvial landscapes influence avulsion and preservation dynamics within channel belts and sedimentary-basin fills. Collectively, this work establishes expectations for variability in the form, mobility, preservation, and architecture of braided rivers and their deposits. Despite this variability, differences among systems can be determined through consistent sampling across multiple observation scales (e.g., bedform, bar, channel belt). These insights underscore the need for comprehensive comparisons among systems that account for the impact and roles of multiple variables in determining how channels move and deposit sediment and inform how channel and bar mobility dynamics can be reconstructed from the sedimentary record. These outcomes provide stronger predictive frameworks for characterizing subsurface fluid flow in ancient river deposits and assessing channel mobility, flood hazards, and sediment and nutrient flux distribution patterns in modern braided rivers.