A new type of shelf margin deposit: rigid microbial sheets and unconsolidated grainstones riddled with meter-scale cavities

Middle Cambrian microbial limestone contains a network of unusual, predomin antly horizontal cavities up to 2 m in length and 0. 5 m in height. The mic robialite experienced rapid syndepositional lithification, but adjacent gra instone sediments remained unlithified during deposition. This juxtapositio n contributed to sediment instability, resulting in fracturing and brecciat ion of the lithified microbialite while unconsolidated grainstones underwen t slumping and injection into some cavities. Remaining space within the cav ities was colonized by a series of encrustations: thin crusts (2-8 mm) of l aminated algal mats, followed by several generations of calcified Renalcis- like cyanobacteria up to 45 mm thick. Remaining void space was partially fi lled by internal sediment, and then sequentially occluded by banded radiaxi al fibrous calcite, herringbone calcite, and finally saddle dolomite cement s. The radiaxial and herringbone calcite cements precipitated from porewate rs derived from seawater that became anoxic through the breakdown of organi c matter in the microbialite. Noteworthy is the presence of herringbone cal cite cement, not as a seafloor precipitate, but as an early cavity fill. We propose that the unusual bedding-parallel fractures were caused by gravity collapse along a shallow platform margin. Coeval foreslope sediments show syndepositional slumping, faulting, and mass flow deposits. These redeposit ed sediments contain boulders of microbialite and grainstone of platform ma rgin provenance. (C) 1999 Elsevier Science B.V. All rights reserved.