Pacific Section AAPG Convention

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Cement Paragenesis Of Septarian Concretions Of The Cretaceous Holz Shale


Septarian concretions, concretions with a carbonate-composed body and septarian veins (calcite cement-filled interior fractures), from the Upper Cretaceous Holz Shale Member of the Ladd Formation were collected near the junction of Silverado Canyon (Santa Ana Mountains, California) to investigate its formation. The abundance of thin-shelled bivalves, Inoceramus, suggests a primary source of carbon. A geochemical analysis will be administered to the concretion to characterize the origins of carbonate cements of the body and veins. Each sample will be measured against a 915b concentration standard to measure the carbon isotope and the inorganic abundance of each concretion. The CRCP90 isotope will be used to further constrain isotope analysis and ensure proper analytical protocol. The results will be analyzed to determine the possibility of a trend in carbon enrichment with respect to the growth of the concretion, recording this growth from the center outward. This method will also provide a measured amount of the inorganic carbon abundances to determine if microbes were involved and give insight into whether the environment was organic-rich and therefore a significant source of carbon. The isotopic analysis will provide the amount of carbon and calcite contributed from pore fluids to form the concretions and will confirm that the concretions were formed in an organic-rich sediment that provided significant amounts of dissolved inorganic carbon (DIC) for concretion formation.