Channel processes and products in subaerial and submarine environments across the Gulf of Mexico




Swartz, John Marshall

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This dissertation characterizes the geomorphology and associated stratigraphy of channel systems across the northwestern Gulf of Mexico. I focus primarily on fluvial processes and products found within the coastal zone and alluvial plain, and also on a submarine channel network strongly influenced by fluvial behavior. I first use high-resolution bathymetry data combined with academic and industry geophysical data to detail the evolution of a linked shelf-edge delta / slope fan formed by the ancestral Rio Grande in the western Gulf of Mexico. The most recent period of deposition and margin growth was associated with the evolution of a submarine channel network, but one where the uppermost, high-gradient channel segments were strongly depositional and the lower, low-gradient channels with evidence of increasing sediment bypass. Chapter three then examines the controls on drainage network formation within a terrestrial environment, namely the coastal plain from the Rio Grande to the Mississippi delta. I show that floodplain channel network position and geometry is initially controlled by the position of alluvial ridges formed by avulsion of the largest coastal rivers. This provides a new and unique model of how drainage basins evolve in depositional environments. In chapter four I study the morphodynamics of the lower Rio Grande over the last hundred years and illustrate an intermittent river that does not adjust either its channel cross sectional geometry or lateral mobility as it approaches the coast, in contrast to more commonly studied perennial rivers. Finally, the fifth chapter seeks to capture the adjustment of a coastal river to sea-level rise as captured by offshore stratigraphy formed during the Holocene transgression. I provide evidence that the river channel did not simply flood back as commonly proposed but rather shifted from lateral migration to vertical aggradation as it continued to feed delta growth farther downstream. This dissertation provides a variety of studies spanning traditional domains and disciplines and brings together archival historical data, field surveys, and modern aerial and marine geophysical techniques. This work helps illustrate the complex dynamics of depositional environments and provides future research directions for fluvial sedimentology and landscape evolution.


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