Microbial sand chips—a non-actualistic sedimentary structure

Abstract

Microbial mats show a marked decline with the transition from the Precambrian to the Phanerozoic, most likely due to the advent of effective benthic grazing organisms. Hitherto most aquatic sediments up to sand grade were commonly coagulated by organic material allowing reworking to form intraclasts (new term: microbial sand chips). Presence of a microbial mat is indicated (1) by refractory remains of organic matter in the laminated parent sediment, (2) by the concentration of early diagenetic pyrite associated with sandy intraclasts in quartzites indicating a concentration of organic matter in the clast, and (3) by plastic deformation of the clasts during the erosive event. As bioturbation spread into most aquatic depositional environments during the Phanerozoic, the formation of microbial sand chips became restricted to habitats where bioturbation and grazing are limited by environmental stress, and where the preservation potential for microbial sand chips is very low. Therefore the widespread formation of microbial sand chips in Precambrian sandy subaqueous sediments is non-actualistic.