Development of middle Jurassic microbial buildups in the Bighorn Basin of northern Wyoming

by Ploynoi, Manwika.

This research examines the development of microbial buildups in the Middle Jurassic Lower Sundance Formation in the Bighorn Basin of northwestern Wyoming. Previous studies of Jurassic microbial research have focused on development along the Tethys Sea. This research examines buildups along the western side of the North America continent, in present day Wyoming. Microbial buildups were studied through outcrop measurements, hand sample and petrographic analysis. In addition, the stable carbon and oxygen isotopic composition were run to explain paleoclimate and chemical composition of seawater during the Middle Jurassic. The studied outcrop is located in northern Wyoming near Cody along the east side of Cedar Mountain. The microbial outcrop has an excellent semi-circular, exposure of microbial buildups at the location. The buildups are about 2.27 meters in height, 2.9 meters in width and 30 cm in thickness. The microbial buildups are composed of several thrombolitic heads. The microbial buildups are associated with the Gypsum Spring and Lower Sundance Formation. The microbial units were deposited along a contact between these two Formations. The lithology of the Gypsum Spring is dominated by massive white gypsums at the lower unit and red shale at the upper unit. These deposited evaporites represent hypersaline condition with seasonally arid and warm climate over an extended period. The lithology of the Lower Sundance Formation is dominated by basal oolitic grainstone at the contact between itself and the Gypsum Spring Formation. The Lower Sundance Formation is also dominated by green shale and carbonate rocks with highly abundant in fossil especially ostracods, crinoids, and pelecypods. Based on petrographic analysis, the microbial buildups have thrombolitic or clotted characteristic features. Thrombolite structures are produced by sediment trapping, binding and/or precipitation as a result of the growth and metabolic activity of microorganisms, principally cyanobacteria. The thrombolite or bindstone is composed of highly clotted-looking algae such as Girvanella (Cyanobacteria), Cayeuxia (Chlorophyta), Solenopora (Rhodophyta) and other encrusting microorganisms such as foraminifera, ostracods, bryozoan, and cyanobacteria. The isotopic composition values of belemnites can be used as a proxy for chemical composition of marine water because belemnites are believed to have not fractionated. As a result, belemnite calcite constitutes the best standard for the geochemistry of Jurassic seawater and provides a reasonable iv approximation of sea water chemistry during deposition of a rock unit. The carbon isotopic values of belemnite from the Sundance Formation are 2.11 to 2.54 ‰, and oxygen isotopic values are -2.34 to - 2.36‰. The carbon isotopic values of carbonate rock samples including microbial thrombolite from the Lower Sundance Formation range from 1.5 to 2.5 ‰, and an oxygen isotopic value range from -5.5 to -9 ‰. Overall, the isotopic values of carbonate rocks from the Lower Sundance Formation of the study section are more negative than the standard for seawater as represented by the belemnite isotopic values. The result of the entire stratigraphic column has been altered post-depositionally. Negative values suggest that rocks have been diagenetically altered by meteoric water since deposition during the Jurassic. As a result, carbon and oxygen isotopes can not be used to determine original sea water chemistry. However isotopic composition values from the belemnite can be used to determine paleotemperature. The paleotemperature of the Middle Jurassic (Late Bathonian to Early Callovian) seawater based on the belemnite samples from Wyoming is calculated to about 15-17oC. v