Abyssal Seep Site Cementation: West Florida Escarpment

A. C. Neumann, C. K. Paull, R. Commeau, J. Commeau, J. Chanton, C. Martens, M. Gardemal, W. Trumbull, W. Showers

The deepest submarine cements known so far occur along the 3,300 m deep base of the Florida escarpment and are associated with methane-bearing brine seeps, which emanate there. These deep Holocene carbonates, which occur as surficial and buried crusts, burrow fillings, and friable horizons, were sampled via ALVIN. The carbonates form irregular halos extending up to 20 m from seeps colonized by chemosynthetic fauna. Mussels, gastropods, and clams, the carbonate components of the community, produce a shell hash that is locally cemented by coarsely crystalline low-Mg calcite. Halos of palisade calcite are reminiscent of ancient examples of marine cements. Also present are carbonate hemipelagics cemented by micrite into crusts and burrow fillings. The degree of cementation va ies from pervasive to light. Slabs of cemented crust up to 30 cm thick contrast with typical shallow crusts and exhibit irregular tops and smooth bottoms indicating different chemical gradients and pathways. Bulk ^dgr¹³C values of the carbonates are low, ranging from -2.4 to -48.5^pmil (PDB) and implicating as the carbonate source the biogenic methane that occurs in high concentrations at the seeps. The interaction of methane and sulfate in these cement reactions is still unclear. The presence of course mollusk-fragment hardgrounds overlying an eroded limestone and covered by hemipelagics, if encountered elsewhere, could be mistaken for a much shallower setting. The erosion of limestone scarps and the concurrent development of deep hardgrounds containing a fossil chemosyntheti fauna at the unconformity is a scenario that needs to be included in the growing list of limestone facies interpretations.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.