Controls on Shelf-margin Depositional Architecture, Eastern Gulf of Mexico
Dr. Jacob Covault
Research Scientist and co-PI, Quantitative Clastics Laboratory
Bureau of Economic Geology
The University of Texas
A widely used model for shelf-edge to basin-floor depositional architecture comprises a single, deeply incised submarine canyon that transitions to a levee-confined channel system feeding deep-water depositional lobes. This model was developed in the tectonically active southern California Borderland, which is subject to significant terrigenous sediment reworking and dispersal in the littoral zone by longshore currents and poorly developed deltas. However, in some settings, especially petroliferous continental margins like the eastern Gulf of Mexico, shelf-edge deltas can transition directly to multiple levee-confined channel systems rather than a single canyon. Here, we use published ages and a large (nearly 15,000 sq. km) three-dimensional seismic-reflection dataset (dominant frequency ~30 Hz) covering an area east of Mississippi Canyon in the Gulf of Mexico to document the stratigraphic evolution of the Dorsey-Sounder deep-water channel network linked to the Mobile river since ~300 ka (Oxygen Isotope Stage 8). Previous workers focused on the older Fuji and Einstein channel systems, which were also linked to the Mobile river. We mapped a failure scar of the thick (>400 ms TWTT), ~300-150 ka (Oxygen Isotope Stage 8-6) Mobile shelf-edge delta; this scar transitions downstream to a mass-transport complex that is erosionally truncated by the Dorsey-Sounder channel network (channel widths 500-1000 m). These channel systems were abandoned during the Oxygen Isotope Stage 5 highstand. In contrast with the Fuji and Einstein channel systems, mass-wasting processes of the thick Mobile delta reshaped the entire shelf edge and upper slope and established an erosional template for the development of the Dorsey-Sounder channel network. The amount of sediment supply from the Mobile river controlled the initiation and scale of linked shelf-edge delta and deep-water channel systems across the eastern Gulf of Mexico margin. Lobe switching at the shelf edge promoted the development of a broad, arcuate Mobile delta complex feeding multiple levee-confined channel systems; this depositional architecture is common to other shelf margins, such as the Niger, Rio Grande, and Magdalena delta-fan systems.