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Microfacies of the Cotham Marble; a tubestone carbonate microbialite from the Upper Triassic, southwestern U.K.

Yadira Ibarra, Frank A. Corsetti, Sarah E. Greene and David J. Bottjer
Microfacies of the Cotham Marble; a tubestone carbonate microbialite from the Upper Triassic, southwestern U.K.
Palaios (January 2014) 29 (1): 1-15


A remarkably aerially extensive ( approximately 2,000 km (super 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 approximately 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 ( approximately 1-3 cm diameter), characteristic of "tubestone" microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils ( approximately 2 mu m diameter and approximately 10 mu m in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions ( approximately 200 mu m 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 (sub 2) and relatively warmer conditions associated with the events of the end-Triassic mass extinction.

ISSN: 0883-1351
Serial Title: Palaios
Serial Volume: 29
Serial Issue: 1
Title: Microfacies of the Cotham Marble; a tubestone carbonate microbialite from the Upper Triassic, southwestern U.K.
Affiliation: University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States
Pages: 1-15
Published: 201401
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 72
Accession Number: 2014-051913
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: NSF Grant EAR-1338329
Illustration Description: illus. incl. 1 table, geol. sketch map
N50°42'00" - N51°40'00", W02°58'00" - W02°00'00"
Secondary Affiliation: University of Bristol, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 201428
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