Control of Upstream Variables on Incised-Valley Morphology: A Look at Late Quaternary Systems Along the Northern Gulf of Mexico Margin

Christopher Robin Mattheus¹ and Antonio B. Rodriguez^2
1 University of Alabama, Tuscaloosa, AL
² The University of North Carolina, Institute of Marine Sciences, Morehead City, NC

It has been widely documented that base-level fall associated with the last lowstand shifted fluvial profiles into disequilibrium, causing channels to incise valleys across continental shelves. However, the interplay between upstream (climate and drainage basin characteristics) and downstream (difference between coastal plain and shelf gradients, magnitude and rate of sea-level fall) controls on incised-valley dimension is not well constrained. To address this, cross-sectional dimension was derived for eight Oxygen-Isotope Stage 2 incised-valley systems from core and seismic data at locations close to the highstand shoreline of the previous sequence. Studied systems, located along the northern Gulf of Mexico margin, are distinguished by drainage basins that vary in size by three orders of magnitude, cover a margin presently characterized by steep climate gradients, and extend across a continental shelf that varies along strike in width (80-160 km) and gradient (1.0-2.0 m/km). Incision depths vary for systems extending across similar coastal-plain and shelf gradients but characterized by different drainage-basin dimensions, suggesting significant control of upstream variables on incised-valley morphology. Linear relationships between drainage-basin area and incised-valley cross-sectional area and width indicate the applicability of the accepted, empirically-derived correlation between discharge and cross-sectional channel area to incised-valley systems when compared at a similar location. The level of variability across the northern Gulf of Mexico margin is not unique; therefore, comparison of incised-valley dimension between different systems across a margin should yield insight into the size of their respective drainage basins and/or climate gradients. Although base-level fall promotes incision, upstream variables control incised-valley dimension.