Authigenic carbonate rocks occur as thin layers and concretionary zones in the Monterey Formation in California and in equivalent strata off southern California and Baja California. Calcium-rich dolomite (49 to 56 mol % CaCO₃) is the dominant carbonate although authigenic calcite also occurs. Sedimentary structures, including laminations and burrows, are common in these carbonate rocks and commonly continue across concretion and layer boundaries. Microtextures run the spectrum from sparsely distributed dolomite crystals in dolomitic mudrocks to dolomites composed completely of interlocking 5 to 10 µm crystals. Dolomite cements and impregnates the host lithology. Dolomitization of existing biogenic carbonate also occurs.

Isotopic and chemical data suggest that these dolomites formed in shallow subsurface zones of high alkalinity spawned by abundant carbon dioxide and methane production during progressive microbial decay of organic matter. Oxygen isotopes range from 23 to 34 ppt SMOW (Monterey dolomites) and from 27 to 35 ppt SMOW (offshore dolomites). Approximate ranges in formation temperatures computed from these values are 17 to 72°C and 10 to 50°C, respectively. Highly variable carbon isotopes, -25 to +21 ppt PDB (Monterey dolomites) and -30 to +16 ppt PDB (offshore dolomites), reflect the isotopic reservoirs in which the carbonates formed. Oxidation of organic matter through microbial reduction of sulfate at shallow burial depths favors light-carbon dolomites; heavy-carbon dolomites pro ably formed below this zone

End_Page 972------------------------------

where HC¹²O_3- is preferentially removed by reduction of CO₂ to methane during methanogenesis. A controlling factor in these reactions is the sedimentation rate which dictates both the preservation of organic matter on the sea floor and the depth distribution of subsurface zones of organic matter decay.

End_of_Article - Last_Page 973------------