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Thrombolite Variability in Cambrian Carbonates, Southern Great Basin, USA

Harwood, Cara L.*1; Sumner, Dawn Y.1
(1) Geology Department, University of California, Davis, Davis, CA.

Thrombolites are a common component of carbonate buildups through Earth history. They are defined as composed of a clotted microbial framework, but they show substantial variability in texture. In spite of this variability, they are typically treated as a uniform group of biosedimentary structures. Interpreting environmental and evolutionary changes from thrombolites is difficult without a good descriptive framework for this variability. Results from Cambrian carbonates of the Great Basin reveal that thrombolites are more variable than has been widely appreciated and may represent varying origins. Thrombolites from the Carrara, Bonanza King, Highland Peak, and Nopah Formations were characterized in stratigraphic context, outcrop and thin section. Thrombolites were also reconstructed and visualized in a 3D environment to further explore their internal structure and porosity. Five classes of Cambrian thrombolites emerged based on the geometry, connectivity, and arrangement of mesoclots, and relative volume of framework vs. internal cavities: bushy, digitate, mottled, grainy, and dense thrombolites. Bushy thrombolites are defined by branching mesoclots with abundant cavity space. Digitate thrombolites consist of sub-parallel to radiating columns with a labyrinthine geometry. Mottled thrombolites consist of polylobate and coalesced micritic mesoclots and are texturally similar to highly bioturbated mudstone. Grainy thrombolites consist of micritic and grainy polylobate mesoclots. Dense thrombolites are micritic with indistinct mesoclots. Many thrombolite mounds contain multiple intergrading classes, and different classes co-occur in adjacent mounds. In addition, some thrombolites textures are intermediate between the mottled form and bioturbated sediments. This variability demonstrates that grouping them simply as “thrombolites” ignores their textural and morphological differences, as well as the varying conditions under which they likely formed. Furthermore, variations in texture and gradations between thrombolite types suggest that thrombolites reflect environmental processes, microbial growth, and metazoan activity in multiple ways. Ongoing research will evaluate the origins of these different classes of thrombolites. A refined understanding of the variability and what causes it will improve interpretation of thrombolite environments and ecosystems, and will improve evaluation of the reservoir potential of thrombolite boundstones.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California