HYDROCARBON-DERIVED CARBONATE BUILDUPS OF THE NORTHERN GULF-OF-MEXICO CONTINENTAL-SLOPE - A REVIEW OF SUBMERSIBLE INVESTIGATIONS

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10-20 m relief above the surrounding seafloor. The authigenic carbonates are characterized by delta(13)C negative values in the range -18 parts per thousand to -55 parts per thousand (PDB) suggesting mixing of seawater carbon with C-13-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are ''diluted'' with biogenic material produced by reefs-bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbon-derived carbonates. At two sites carbonates are mixed with barite.