Elsevier

Geomorphology

Volume 89, Issues 3–4, 15 September 2007, Pages 370-390
Geomorphology

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

https://doi.org/10.1016/j.geomorph.2007.01.005Get rights and content

Abstract

Four drowned shelf-edge delta complexes, two drowned shelf deltas, three drowned barrier islands, large areas of “hardground”, and fields of bedforms on the mid and outer continental shelf and uppermost slope north of the head of De Soto Canyon, NE Gulf of Mexico were mapped with high-resolution multibeam echosounder. Deltas formed not during the last eustatic low stand, but during one or more interstadials when eustatic sea levels were only 60 to 80 m below present sea level. The barrier islands and deltas must have been cemented prior to rapid falls of eustatic sea level that occurred during global glaciations. Cementation is necessary to have preserved the barrier islands from erosion and subsequent destruction by the rapid sea-level rise during the last deglaciation. The preservation of the relict bathymetry is so good that features that superficially resemble trough blowouts are found in association with one of the relict barrier islands. Asymmetric bedforms on the midshelf in water depths of 50 to 60 m indicate transport directions to the SW but asymmetric bedforms in water depths of the upper slope between 100 and 120 m on the S and SE flanks of the drowned shelf-edge deltas indicate a different current direction, a separate flow that is a continuation of a SW-flowing current that was previously found on the upper slope off NW Florida.

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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