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Open-File Report 2011–1173

Prepared in cooperation with the U.S. Army Corps of Engineers

The Shallow Stratigraphy and Sand Resources Offshore of the Mississippi Barrier Islands

By David Twichell,1 Elizabeth Pendleton,1 Wayne Baldwin,1 David Foster,1 James Flocks,2 Kyle Kelso,2 Nancy DeWitt,2 William Pfeiffer,2 Arnell Forde,2 Jason Krick,3 and John Baehr

1U.S. Geological Survey, Woods Hole, MA
2U.S. Geological Survey, St. Petersburg, FL
3U.S. Army Corps of Engineers, Mobile, AL

ABSTRACT

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Coastal Mississippi is protected by a series of barrier islands ranging in length from 10-25 kilometers that are less than 2 kilometers wide. The majority of these islands comprise the Gulf Islands National Seashore (GUIS), an ecologically diverse shoreline that provides habitat for wildlife including migratory birds and endangered animals. The majority of GUIS is submerged, and aquatic environments include dynamic tidal inlets, ebb-tide deltas, and seagrass beds. The islands are in a state of decline, with land areas severely reduced during the past century by storms, sea-level rise, and human alteration. Morton (2008) estimates that since the mid-1800s up to 64 percent of island surface area has been lost. Heavy damage was inflicted in 2005 by Hurricane Katrina, which passed by as a Category 3 storm and battered the islands with winds of more than 160 kilometers per hour and a storm surge up to 9 meters.

Since 2007, the U.S. Geological Survey (USGS), in collaboration with the National Park Service, has been mapping the seafloor and substrate around the islands as part of the USGS Northern Gulf of Mexico Ecosystem Change and Hazard Susceptibility project. The purpose of these investigations is to characterize the near-surface stratigraphy and identify the influence it may have on island evolution and fate. In 2009, this effort provided the basis for a collaborative effort with the U.S. Army Corps of Engineers (USACE) to expand the investigation outside of GUIS boundaries as part of the Mississippi Coastal Improvement Project (MsCIP). The MsCIP program consists of structural, nonstructural, and environmental project elements to restore portions of coastal Mississippi and GUIS affected by storm impact. The project includes the placement of sand along the islands, both on the present beaches and within the littoral zone, to mitigate shoreline erosion and breaching. This action requires the location and assessment of offshore sand or sediment deposits that can provide suitable material for shoreline renourishment. The geophysical and sample information collected by the USGS during geologic investigations provides this information.

As part of the MsCIP program, in March 2010 the USGS mapped approximately 300 square kilometers of seafloor around GUIS. Interferometric swath bathymetry, sidescan sonar, and Chirp sub-bottom profiling were used to characterize seafloor elevations, texture, and the underlying stratigraphy. On the basis of this information, potential sediment resources were identified. The most promising offshore deposits for beach restoration include shoals, lowstand valley fill, tidal delta deposits, abandoned barrier deposits, and dredge spoil. Of these, lowstand valley fill deposits and dredge spoil are less desirable; lowstand deposits are buried under a 2- to 4-meter blanket of mud, and dredge spoil volume is small. A relict tidal delta and submerged shoals are the most desirable deposits; the tidal delta contains a large volume of material still exposed on the seafloor, and parts of submerged shoals have modest volume and thin mud cover.

First posted August 4, 2011

Revised March 13, 2014

For additional information contact:
Director
U.S. Geological Survey
Woods Hole Coastal and Marine Science Center
384 Woods Hole Road
Quissett Campus
Woods Hole, MA 02543
(508)548-8700

http://woodshole.er.usgs.gov/

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Suggested citation:

Twichell, David, Pendleton, Elizabeth, Baldwin, Wayne, Foster, David, Flocks, James, Kelso, Kyle, DeWitt, Nancy, Pfeiffer, William, Forde, Arnell, Krick, Jason, and Baehr, John, 2011, The shallow stratigraphy and sand resources offshore of the Mississippi Barrier Islands (ver. 1.1, March 2014): U. S. Geological Survey Open-File Report 2011-1173, at https://pubs.usgs.gov/of/2011/1173/. (Supersedes ver. 1.0 released August 4, 2011.)



Contents

Abstract

Introduction

Coastal Setting and Regional Geology

Methods

Offshore Horn and Ship Islands (Study Area 1)

Bathymetry

Sidescan Sonar

Chirp Seismic-Reflection Profiles

Offshore Petit Bois Pass and Ship Island Pass adjacent to Cat Island (Study Areas 2 and 3)

Bathymetry

Sidescan Sonar

Chirp Seismic-Reflection Profiles

Results

Physical Setting, Study Area 1 (Offshore Ship Island)

Physical Setting, Study Area 2 (Offshore Petit Bois Pass)

Physical Setting, Study Area 3 (Ship Island Pass at Cat Island)

Discussion

Geologic Evolution of the Mississippi Inner Shelf

Western Mississippi Barrier Islands (Study Areas 1 and 3)

Eastern Mississippi Barrier Islands (Study Area 2)

Barrier Island Development

Potential Sand Resources

Study Area 1 (offshore Ship Island)

Study Area 2 (Offshore Petit Bois Pass)

Study Area 3 (Ship Island Pass at Cat Island)

Conclusions

Acknowledgments

References Cited


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