Hydrocarbon exploration and production on the Louisiana continental slope have produced data derived from high-resolution geohazards surveys which indicate that enormous volumes of authigenic carbonates are present. Geochemical analyses of these carbonates show a wide range of mineralogies as well as 6 13C values (?-25 to -55 ? PDB) indicating an origin from microbial oxidation of hydrocarbons. Broad variation in 6 13C values is thought to pnmari1y reflect various parent hydrocarbons ranging from biogenic methane to thermogenic gas and crude oil.

Recent research submersible work in two areas known for extensive carbonate seafloor, Green Canyon Blocks 52-53 and 184-185, has confirmed considerable variability of seafloor features related to authigenic carbonate production. Authigenic carbonates range from small nodules in unconsolidated slope sediments to massive buildups in excess of 60 ft (-20 m) vertical relief. A complete spectrum of intermediate forms between these extremes was observed during submersible operations, suggesting a wide range of seafloor conditions and geotechnical and/or engineering properties.

In addition to the fact that these carbonates are geologically important because they represent salt dome cap rocks in early stages of development, slope carbonates present substantial challenges for planning drilling/production platform placement, and for pipeline routing. Although high resolution seismic and side-scan sonar data have been used to identify areas of carbonate seafloor, their signatures are frequently similar to mud mounds and other soft-bottom features. Positive identification of carbonate hardgrounds, their relief, and their thicknesses may be difficult using typical remotely sensed hazard survey data. Observations from the submersible, for example, suggest that the topographic variability of carbonate buildups over shallow subsurface salt diapirs is much more dramatic and complex than analysis of seismic and side-scan records has previously indicated.


A project sponsored by NOM National Undersea Research Center (Wilmington, North Carolina) provided the opportunity to investigate "rough seafloor" areas that typically occur above shallow salt diapirs of the Louisiana continental slope. A Pisces II research submersible (Fig. 1) was used to make direct observations of these areas and collect rocks, and sediments related to specific seafloor features. Although other researchers have used submersibles to study Louisiana's slope, their objectives have been focused primarily on identification of oil and gas seeps as well as the chemosynthetic communities that frequently are associated with hydrocarbon vents [1,2,3,4]. The project on which this paper is based was designed to develop an understanding of the geological and geochemical products of seep areas, primarily the authigenic carbonates.

Although previous studies of the slope [5,6,7] recognized its bathymetric and structural complexity as a product of salt tectonics, the vast areas of carbonate-rich sediments and seafloor buildups which are also largely localized by saltrelated features have generally escaped detailed investigation. However, as the oil and gas industry moves into deeper water these areas must receive more attention. In response to this need a number of recent studies [8,9,10,11] have emphasized the widespread abundance of carbonate sediments and buildups on the Louisiana slope.

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