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Description
Near Bowman Lake, the greenschist facies Carboniferous Peale chert forms a moderately E-dipping, NW-trending homocline that is the record of the transition from the Devonian and Early Mississippian Antler Orogeny to the Permian and Triassic Sonoman Orogeny. In ascending stratigraphic order it consists of the ~105 m thick black and white lithofacies, the ~16 m thick mottled lithofacies, and the ~30 m zebra stripe lithofacies. Chemical analyses of 20 samples indicate a small negative Ce anomaly in the black and white lithofacies, both positive and negative Ce anomalies in the mottled lithofacies, and a small positive Ce anomaly in the zebra stripe lithofacies. These relationships suggest a continental margin depositional environment, an interpretation that is supported by plotting the data on the Fe2O3/TiO2 vs. Al2O3/(Al2O3+Fe2O3) and LanCen vs. Al2O3/(Al2O3+Fe2O3) depositional setting discrimination diagrams of R.W. Murray. The ratio of Si/(Si+Al+Fe) reaches maximum values within the mottled and upper part of the black and white lithofacies. Minimum values of this ratio occur in the zebra stripe and lower part of the black and white lithofacies. Values of Th/Sc, Th/U, Zr, and Cr are greater in the zebra stripe and lower black and white lithofacies than they are in the mottled and upper black and white lithofacies. Based on these observations, the greatest terrigenous influence on the composition of samples analyzed from the Peale chert was apparently within the zebra stripe and lower part of the black and white lithofacies. In contrast, an increase in biogenic input appears to have occurred within the mottled and upper part of the black and white lithofacies. Most samples analyzed during this study have moderate negative Eu anomalies, and all exhibit chondrite-normalized distribution patterns that parallel that of sediment derived from North American upper continental crust (NAUCC). However, on a La-Th-Sc ternary diagram samples from the zebra stripe and lower part of the black and white lithofacies cluster more toward the NAUCC field than do specimens from the mottled and the upper black and white lithofacies, which plot toward a more mafic composition. These observations suggest a greater oceanic influence for selliment within the mottled and upper part of the black and white lithofacies than in the zebra stripe and lower black and white lithofacies. Data presented in this thesis suggest that the composition of the lower Peale was strongly influenced by terrigenous matter. It is likely that the source of this material was the Antler orogenic highlands, a positive palaeogeographic feature rimming the margin of southwestern North America during the Mississippian. In contrast, a stronger biogenic input is evident during deposition of the mottled and the upper part of the black and white lithofacies. This relationship may indicate that the Antler highlands were worn down to below average storm. wave base. Following deposition of the mottled facies, an increase in the abundance of terrigenous matter in the upper Peale represents a harbinger to the Sonoman orogeny.
