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The Miocene Monterey Formation contains thin organic-rich laminations that mimic benthic mats forming today in coastal upwelling regimes of the eastern Pacific. Modern mats are comprised dominantly of filamentous sulfur-oxidizing bacteria of the family Beggiatoaceae. The characteristic spongy texture imparted to the sediments by these bacterial mats is most readily recognized using scanning electron microscopy (SEM). The spongy network becomes progressively compressed with increasing compaction and diagenesis, but is still distinguishable.
Rock-Eval pyrolysis of modern mats reveals high S1 peaks and unusually high oxygen indices. Both parameters decrease gradually with diagenesis, as evidenced by mat-laminated Monterey Formation samples of different thermal grades. In addition, anomalously light nitrogen isotopic signatures of modern mats may be retained. The ^dgrN15 values of mat-laminated Monterey Formation samples, although diagenetically enriched in 15N, are still lighter than would be expected for algally dominated organic matter in marine low-oxygen environments.
On the basis of organic richness and high hydrogen indices of mat-laminated samples, and coincidence of mat-laminated rocks and oil in some fields in the San Joaquin Valley of California, the organic-matter assemblage contained within filamentous bacterial mats is suggested to be a significant contributor to petroleum formation. Additional evidence for this conclusion is provided by similar investigations of subtidal cyanobacterial mats in carbonate rocks such as the Green River Formation oil shales.
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