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ABSTRACT: Offshore Sand Resources for Coastal Erosion Control in Louisiana

Karen E. Ramsey, Shea Penland, John R. Suter, Randy A. McBride, Jeffress Williams

An inventory of existing geophysical data supplemented by more than 15,000 km of high-resolution seismic profiles and 400 vibracores collected cooperatively by the Louisiana Geological Survey and U.S. Geological Survey since 1981 indicates that a wide range of aggregate minerals occurs on the continental shelf in a variety of depositional settings. The distribution of these deposits is controlled by the geometry of the pre-existing fluvial and deltaic channel systems and the stratigraphic signature of the Holocene Transgression across these features. The geology of coastal and offshore Louisiana is tied to the depositional history of the Mississippi River. Offshore of the delta plain, five types of aggregate sources can be identified: inner shelf shoals, submerged barrier islands, tidal inlets, distributary channels, and barrier platforms. This paper describes the geology of offshore Louisiana, the available geophysical data sets, and the distribution of aggregate mineral resources.

On the continental shelf of the Mississippi River delta plain, two extensive seismic survey grids have been developed by the Louisiana Geological Survey and U.S. Geological Survey. These seismic surveys represent the best regional coverage in offshore Louisiana and include: (1) south-central Louisiana shelf collected in 1982-1989 (Louisiana Geological Survey and U.S. Geological Survey) and (2) southeast Louisiana shelf collected in 1981-1989 (Louisiana Geological Survey and U.S. Geological Survey).

Offshore of the southeastern Louisiana delta plain, one finds submerged barrier islands, distributaries, shelf reefs, inner shelf shoals, tidal inlets, beach ridge plains, and barrier platforms. West to east along the delta plain lies Trinity Shoal, an ancient submerged barrier island associated with the late Holocene delta plain. This feature is 30 km long, 10 km wide, and 8 m thick. The texture of Trinity Shoal is very fine sand and is estimated to contain some 2,000,000,000 m3 of material. Between Marsh Island and Sandy Point, the LGS documented 55 nearshore sand resource targets ranging in size from 2 km2 to greater than 400 km2 with estimated volumes of available sand varying from less than 2,000,000 m3 to greater than 1,600,000,000 m3. The most prospective resources found are the huge sand bodies of Ship Shoal and associated distributaries, Cat Island Pass tidal channels and associated tidal deltas, and Barataria Pass/Grand Terre tidal channels and associated tidal deltas. East of the mouth of the Mississippi River are the Chandeleur Islands, where LGS identified seven major sand resource targets, truncated barrier-spit and tidal inlet deposits, submerged beach ridges, and distributaries associated with abandoned St. Bernard delta complexes.

Abundant sand resources can be found in offshore Louisiana. Many of the sand bodies contain heavy minerals, but their concentration and distribution is unknown. Other potential sand resources not yet adequately explored include Sabine Bank, the Outer Shoal, and the St. Bernard shoals. The utilization of these resources either as borrow material for beach nourishment or for industrial use will depend upon the economics of dredging at their offshore locations and the potential environmental effects resulting from altering existing continental shelf geomorphological and biological conditions.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990