Geochemical investigation of the late Ordovician Bighorn Dolomite of western Wyoming

Abstract

The Bighorn Dolomite of the Central Rockies and Great Plains is a Late Ordovician (Katian) dolostone that formed prior to the Late Ordovician glaciation and mass extinction. This study is primarily concerned with the Steamboat Point Member (~70 m thick), which contains cm-scale mottled fabrics passing through four distinct facies. I complete a geochemical survey, examining the relationship between facies changes and geochemical trends, in order to correlate the facies chemostratigraphically and construct a story about the changing depositional environments. Methods include facies observations and geochemical analyses of core C520 from Sheridan County Wyoming, including isotopic analysis of carbon, oxygen, and strontium; determination of concentrations of iron, manganese, and strontium; and facies interpretations. Carbon isotope, conodont, and facies changes data reveals the Steamboat Point Member contains the Fairview carbon isotope excursion of the Katian Stage. Trends in trace metals, when combined with inferred sea level, indicate dispersion of metals via ocean circulation. The introduction of burrowing organisms accompanies sea level rise, suggesting a eustatic driver for species migration. Textural and geochemical observations support a relative lack of diagenesis among carbon and oxygen; however, strontium shows evidence for significant post-depositional augmentation. Scanning electron microscopy (SEM) reveals abundant micrometer-scale hydrocarbon droplets throughout the dolomite crystals. The Steamboat Point Member experienced sea level rise and fall, which overprinted and affected the geochemistry, facies, and ecology. The climate oscillation of the Late Ordovician occurred a few million years after the Steamboat Point Member was deposited, linking eustatic instability to mass extinctions and glaciation.