Geochemical Investigation of Rocks from the Argillaceous Rock Atlas

Argillaceous rocks are of great importance as they make up 60% of the stratigraphic column, can be used to determine past tectonic settings and upper continental crust composition averages, and are source rocks for hydrocarbons. Thirty-six well-documented Phanerozoic shale samples from the Argillaceous Rock Atlas (O'Brien and Slatt, 1990) were analyzed for inorganic elements. The major and trace element were normalized to upper continental crust (UCC) estimates and the rare earth elements were normalized to the Post-Archean Australian Shale (PAAS)composite. Chemical index of alteration (CIA) values are determined to give insight to the past weathering of the sample provenance. Six well-established trace element ratios are used to determine the paleo-redox conditions for each shale at the time of deposition. Results from the ratios are compared to presence/absence of bioturbation, or other known documentation of oxic or anoxic conditions during deposition. Many different discrimination plots were made to analyze the samples including their provenance, tectonic setting, and other geochemical relationships. Many of the shales have admixed carbonate, and less silica and other major elements than estimates of the average upper continental crust (UCC) based on shale composites. Correspondingly the CIA values were strongly influenced by carbonate content. Most shales are enriched in select(Cd, Mo, V, etc.) trace metals presumably because of their organic carbon content. Provenance diagrams consistently showed derivation from rocks of typical upper continental crust (granodiorite). Paleo-redox ratios from individual samples were generally not consistent nor a good predictor of redox conditions in this sample set.