Drowned shelf-edge deltas, barrier islands and related features along the outer continental shelf north of the head of De Soto Canyon, NE Gulf of Mexico
Introduction
This is the third of three studies along the NE Gulf of Mexico mid and outer continental shelf and upper slope (Gardner et al., 2002, Gardner et al., 2005) that provides highly accurate base maps for future studies based on data from state-of-the-art high-resolution multibeam sonars (MBES). The total mapped area extends from east of the Mississippi River delta off Louisiana to an area SE of the Apalachicola River Delta off Florida (Fig. 1). The focus of this paper is the area immediately north of De Soto Canyon. Previous bathymetric maps of the De Soto Canyon area were produced in the 1970s (Ballard and Uchupi, 1970) and again in the 1990s (McBride and Byrnes, 1995) from single-beam sounding data collected by the U.S. Coast and Geodetic Survey during the 1940s through the 1960s. These maps are published as contour maps or wire-mesh models generated from contours and only provide a gross generalization of the bathymetry. In contrast, today's MBES systems can collect tens of millions of hydrographic-quality soundings per day, as well as simultaneously collected co-registered acoustic backscatter, that can reveal 100% of the seafloor with a level of accuracy that was unheard of as recently as the 1980s. The principle objective of this paper is to describe in detail the geomorphology of the mid and outer shelf and upper slope along the northern head of De Soto Canyon south of Alabama and the Florida panhandle. The lack of available ground truth and age dating allows only speculations and potential constraints on interpretations for the processes that formed and modified the landscape. The study lacks ground truth because very little, if any, ground truth has been collected in this area of the margin. Speculations on the sediment facies are based solely on empirical data modeled for the acoustic remote sensing frequency.
A Kongsberg EM3000 MBES was used to map the area. An overview of high-resolution MBES systems can be found in Hughes Clarke et al. (1996) and Hughes Clarke (2000) and details of the EM3000 used for the mapping can be found in Gardner et al. (2002). The accuracy of individual depth soundings is ∼ 5 cm RMS independent of water depth. Navigation for the cruise was with an inertial-aided DGPS system that provided spatial accuracies of ± 0.5 m. The 300-kHz EM3000 MBES not only generates hydrographic-quality bathymetry but also collects semi-quantitative seafloor acoustic backscatter data that can be displayed in a sidescan-sonar-like image. When properly calibrated and ground truthed, the acoustic backscatter values can be used to gain insights into the spatial distribution of the geological character of the seafloor.
Section snippets
Study area
The study area is located 25 to 50 km south of the present coastline and along the upper rim of the head of De Soto Canyon, a broad depression with a steep northern flank and a gentle southern flank (Fig. 1). General descriptions of the morphology of the area are provided by Martin (1978), Bryant et al. (1991), Coleman et al. (1991), McBride and Byrnes (1995), McBride et al. (1996), and McBride et al. (1999). The continental shelf in this area ranges in width from 50 to 80 km and the narrowest
Discussion
This study describes the physiography of the midshelf to upper slope immediately north of De Soto Canyon as revealed by high-resolution multibeam echosounding. The physiography reflects a complex interplay of processes modulated by variations in local climate and eustatic sea level. The entire NE Gulf of Mexico mid and outer shelf is covered with a blanket of sediment that has been transported from the N and NE and has partially or totally buried many features. Evidence for the transport
Conclusions
The final segment of a 400 km long, 5 to 10 km wide corridor along the mid and outer continental shelf and uppermost continental slope of the NE Gulf of Mexico has been completely mapped with high-resolution multibeam sonar. The landscape immediately north of the De Soto Canyon is dominated by four drowned shelf-edge delta complexes and two shelf-edge deltas that most likely formed during periods when eustatic sea level was at stasis or undergoing a very slow regression. Long, narrow, barrier
Acknowledgments
We acknowledge the superb ship handling of the officers and crew of the RV Moana Wave and the support and diligence of the C&C Technologies crew that collected the data. All of their efforts are greatly appreciated. We thank the Minerals Management Service and the USGS for funding the cruise and data processing. Also not forgotten are our shipboard companions who helped with the tedious data processing, including Semme Dijkstra, James Glynn, and Chris Glynn. Joel Johnson, Kurt Schwehr and
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