Cold-Seep Carbonates of the Louisiana Continental Slope-to-Basin Floor
Recent research (Brooks et al., 1984, 1987; Roberts et al., 1987, 1989) directed toward improving our understanding of hydrocarbon seeps and their impacts on Louisiana’s continental slope has led to the realization that authigenic carbonates are extremely abundant in locations where hydrocarbon seeps occur. These carbonates range from nodules formed in near-surface sediments, to massive reef-like buildups of yet unknown age that commonly have 20 m or more relief above the surrounding seafloor. The common link between these seep-related authigenic carbonates is their 13C-depleted isotopic signatures. Extreme depletion in the 13C isotope (δ 13C values generally occur within the range −30 and −59‰ PDB) indicates a direct link with seeping hydrocarbons (Hovland et al., 1987).
Roberts et al. (1989, 1990), Brooks et al. (1987), and MacDonald et al. (1990) have focused a multiyear research effort on hydrocarbon seeps on the Louisiana continental slope to determine their impacts on slope geology and biology. Until spring 1990, direct observation and sampling of hard-bottom areas of the modern slope was limited to a depth of 1000 m by the maximum diving depth of research submersibles utilized (Pisces II and Johnson-Sea-Link). However, in April 1990, dives made with DSV ALVIN (Brooks et al., 1990) extended observation and sampling to the lower slope and basin floor (Fig. 7.1). These dives confirmed that seeps in water depths of nearly 3000 m possess many of the same characteristics of their shallower counterparts, especially the occurrence of authigenic carbonate substrates and buildups.
It is now apparent that the process of hydrocarbon seepage to the modern seafloor creates a special environment that not only attracts a unique benthic community of chemosynthetic organisms but also, through the interplay between members of these communities, particularly the bacteria and seep products (hydrocarbons and hydrogen sulfide), sets the stage for the production of vast volumes of authigenic carbonate (Roberts et al., 1990). Seeps and the areas of sea-floor lithification associated with them are largely concentrated above and on the flanks of shallow-seated salt diapirs. Vertical migration of salt through the sediment column creates a complex network of faults along which subsurface fluids and gases are transported to the modern seafloor. A number of sea-floor responses to seep-related processes have been described in scientific literature (Kennicutt et al., 1985; Hovland et al., 1987; Roberts et al., 1987, 1989; Brooks et al., 1990). This chapter focuses on details of the authigenic carbonates from initial petrology and geochemistry studies.
KeywordsAuthigenic Carbonate Salt Diapir Biogenic Methane Seep Site Hydrocarbon Seep
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- Aharon, P., H.H. Roberts, R. Sassen, and J. Larkin, 1990. Stable isotopes and petrology of chimneys and calcareous fauna from hydrocarbon vents on the Gulf of Mexico slope. Geological Society of America, Abstract Program, v. 22, p. A208.Google Scholar
- Aharon, R, H.H. Roberts, R. Sassen, C. Wheeler, and J. Fang, 1989. An isotope study of cold vent deposits from the continental slope, Gulf of Mexico: Progress report. Geological Society of America, Abstract Program, v. 21, p. A273.Google Scholar
- Berner, R.A., 1971. Principles of Chemistry Sedimentology, McGraw-Hill Co., New York, 240 p.Google Scholar
- Brooks, J.M., P.A. Wisenburg, H.H. Roberts, R.S. Carney, I.R. MacDonald, C.R. Fisher, N.L. Guinasso, Jr., W.W. Sager, S.J. McDonald, R.A. Burke, Jr., P. Aharon, and T.J. Bright, 1990. Salt, seeps, and symbiosis in the Gulf of Mexico: A preliminary report of deep water discoveries using DSV ALVIN. American Geophysical Union Transactions (EOS), v. 71, p. 1772–1773.CrossRefGoogle Scholar
- Brooks, J.M., B.H. Cox, W.R. Bryant, M.C. Kennicutt II, R.G. Mann, and T.J. McDonald, 1986. Association of gas hydrates and oil seepage in the Gulf of Mexico. In: Leythaceuser, D. and J. Rullkotter (eds.), Advances in Organic Geochemistry, 1985. Organic Geochemistry, v. 10, p. 221–234.Google Scholar
- Chafetz, H.S. and R.L. Folk, 1984. Travertines: depositional morphology and the bacterially constructed constituents. Journal of Sedimentary Petrology, v. 54, p. 289–316.Google Scholar
- Colombo, V, E Gazzarrini, R. Gonfiantini, G. Sironi, and E. Tongiorgi, 1966. Measurements of 13C /12C isotope ratios on Italian natural gases and their geological interpretation. In: Hobson, G.D. and M.C. Louis (eds.), Advances in Organic Geochemistry, Pergamon Press, Oxford, p. 279–292.Google Scholar
- Galimov, E.M., 1980. 13C /12C in kerogen. In: Durand, B. (ed.), Kerogen, Insoluble Organic Matter from Sedimentary Rocks, Technical Paper, Paris, p. 271–299.Google Scholar
- Graber, E.R., P. Aharon, and H. Roberts, 1990. Pore water carbonate chemistry from hydrocarbon vents on the Gulf of Mexico slope. Geological Society of America Abstract Program, v. 22, p. A208.Google Scholar
- Griffin, G., 1971. Interpretation of X-ray diffraction data. In: Caver, R.E. (ed.), Procedures in Sedimentary Petrology, John Wiley and Sons, New York, p. 511–569.Google Scholar
- Hovland, M., M.R. Talbot, H. Quale, S. Olanssen, and L. Aasberg, 1987. Methane-related carbonate cements in pockmarks of the North Sea. Journal of Sedimentary Petrology, v. 57, p. 881–892.Google Scholar
- Neurauter, T.W. and H.H. Roberts, 1992. Seismic and visual observation of seep-related structures on the continental slope, Northern Gulf of Mexico. 24th Annual Offshore Technology Conference, OTC #6850, p. 355–362.Google Scholar
- Rezak, R., T.J. Bright, and D.W. McGrail, 1985. Reefs and banks of the northwestern Gulf of Mexico: Their geological, biological, and physical dynamics. John Wiley and Sons, New York, 259 p.Google Scholar
- Rice, D.D., 1980. Chemical and isotopic evidence of the origins of natural gases in offshore Gulf of Mexico. Gulf Coast Association, Geological Society Transactions, v. 30, p. 203–213.Google Scholar
- Roberts, H.H., R. Sassen, and P. Aharon, 1987. Carbonates of the Louisiana continental slope. Proceedings, 19th Annual Offshore Zbchnology Conference, OTC #5463, p. 373–382.Google Scholar
- Roberts, H.H., R. Sassen, R. Carney, and P. Aharon, 1989. Carbonate buildups on the continental slope off Louisiana. Proceedings, Offshore Technology Conference, OTC #5953, p. 655–662.Google Scholar
- Roberts, H.H. and T. Neurauter, 1990. Direct observations of a large active mud vent on the Louisiana continental slope. Transactions, Gulf Coast Association of Geological Societies, v. 40, p. 735.Google Scholar
- Schoell, H.P., 1982. Applications of isotope analysis to petroleum and natural gas research. Spectra, v. 8, p. 32–41.Google Scholar