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The Middle Trassic Grenzbitumenzone is a 16 m thick sequence of interbedded organic matter-rich dolomites and black shales. The dolomites contain up to 10 wt% organic carbon, while the black shales can reach 40 wt%. Geochemical analyses of trace metals and paleontological data indicate deposition in a silled shallow marine basin (30-100 m deep) at very low sedimentation rates (2 to 5 m/my) under anoxic conditions. Sedimentary structures suggest that dolomites formed from periodic turbiditic transport of calcium carbonate mud, diluting a consistent organic matter-siliciclastic background. Carbon isotopes in the dolomites range from -1.4 to -5.6%o (PDB), indicating formation in the sulfate reduction zone of organic matter diagenesis. No evidence of methanogenetic activity suggests dolomite formation occurred before sulfate depletion in pore waters. The study shows that slow sedimentation rates and high organic matter supply favored early diagenetic dolomitization. This slow rate allowed magnesium and sulfate to diffuse into pore waters, while organic matter decomposition increased alkalinity, leading to dolomite supersaturation and the replacement of calcium carbonate. Organic geochemical data classify the immature organic matter as type II, with a significant bacterial contribution indicated by high hopane concentrations. A model is proposed where sedimentation and organic matter preservation are influenced by carbonate mud tur
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Geochemical and microbial controls on dolomite formation in anoxic environments, Stefano M. Bernasconi
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- 1994
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