MICROFACIES OF THE COTHAM MARBLE: A TUBESTONE CARBONATE MICROBIALITE FROM THE UPPER TRIASSIC, SOUTHWESTERN U.K.

摘要

A remarkably aerially extensive (,2,000 km~2) unit of carbonate microbialites occurs in many Triassic-Jurassic boundary interval outcrops of the southwestern United Kingdom and captures petrographic evidence that could link them to the end-Triassic extinction event. The bioherms-known regionally as the Cotham Marble-occur as discrete,20-cm-thick, decimeter- to meter-scale mounds, and display at least five growth phases that alternate between laminated and dendritic mesofabrics. Cross sections parallel to bedding through the dendritic phases expose a reticulate dendritic framework separated by polygonal spaces (,1-3 cm diameter), characteristic of "tubestone" microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils (,2 mm diameter and,10 mm in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions (,200 mm in diameter) within the dendrolites (previously interpreted as serpulid worm tubes) likely resulted from the production of gas bubbles within rapidly lithifying mats or are a two-dimensional artifact of evenly spaced three-dimensional branching within the mats. The fill between the dendrolites of the first layer contains abundant phycoma clusters of the green algal prasinophyte Tasmanites, commonly considered a "disaster taxon." The cyclic phases represent episodic and laterally extensive environmental change within shallow water coastal environments during a marine transgression. Collectively, the presence of microbial micrite in a shallow marine setting, the marked lateral extent of the bioherms, and the abundance of Tasmanites suggest the Cotham Marble microbialites formed during the high pCO_2 and relatively warmer conditions associated with the events of the end- Triassic mass extinction.