Journal of Conference Abstracts

Microbialite Sheets Riddled with Submarine Cavities, The Middle Cambrian Ledger Formation, Central Appalachians, USA

Carol de Wet (c_dewet@acad.fandm.edu)¹, J. A. D. Dickson (jadd1@esc.cam.ac.uk)² & Rachel Wood (rw43@esc.cam.ac.uk)²

¹ Dept. of Geosciences, Franklin & Marshall College, Lancaster, PA, USA

² Dept. of Earth Sciences, Downing St, University of Cambridge, Cambridge,, UK

Middle Cambrian shelf margin carbonates typically consist of grainstone or reefal facies, but here we report a new type of depositional facies; carbonates that formed as an undulating, submarine, microbial plain, interbedded with shallow water oolite and peloidal grainstone. The Ledger Formation microbial limestone was deposited in shallow subtidal conditions along the eastern Laurentian shelf margin, adjacent to a sharp shelf break. Coeval foreslope sediments show syndepositional slumping, faulting, and mass flow deposits. These redeposited sediments contain boulders of Ledger Formation microbialite and grainstone of platform margin provenance. Cobble and boulder-sized fragments of microbialite contain relict high concentrations of magnesium, showing Mg²⁺ enrichment relative to in situ microbialite.

The microbialite experienced rapid syn-depositional lithification, as evidenced by meter-scale cavities within it containing microbial communities, internal sediments and multiple generations of fibrous cement, including herringbone calcite cement. The cavities are unlike those previously reported from shelf margin reefal deposits. For instead of vertical fissures, the cavitites within the microbialite are horizontal, or bedding parallel. The cavities form a network of predominantly horizontal cavities up to 2 m in length and 0. 5 m in height. The juxtaposition of unlithified grainstone and rigid microbialite resulted in sediment instability, which coupled with the depositional setting, led to the formation of submarine slumps and open fractures. We interpret these cavities as forming through previously undocumented shelf margin processes.

Remaining space within the fractures was colonized by a series of encrustations: usually by thin crusts (2-8 mm) of laminated cyanobacteria, followed by several generations of calcified Ephiphyton-like cyanobacteria up to 45 mm thick. Remaining void space was partially filled by internal sediment, and then sequentially occluded by banded radiaxial fibrous, herringbone calcite, and finally saddle dolomite cements. The radiaxial and herringbone calcite cements precipitated from porewaters derived from seawater that became anoxic through the breakdown of organic matter in the microbialite. The herringbone calcite cement shows consistent depleted carbon isotopic values relative to radiaxial fibrous cements, even those within the same cavity. This suggests that the herringbone calcite growth form is linked to unusual porewater conditions. In this instance we interpret that the herringbone calcite reflects near-anoxic porewater Eh because of large quanitities of decaying microbial material.

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