MICROBIAL SAND CHIPS - A NON-ACTUALISTIC SEDIMENTARY STRUCTURE

Microbial mats show a marked decline with the transition from the Prec ambrian 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 m icrobial mat is indicated (1) by refractory remains of organic matter in the laminated parent sediment, (2) by the concentration of early di agenetic pyrite associated with sandy intraclasts in quartzites indica ting a concentration of organic matter in the clast, and (3) by plasti c deformation of the clasts during the erosive event. As bioturbation spread into most aquatic depositional environments during the Phaneroz oic, the formation of microbial sand chips became restricted to habita ts 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 Pr ecambrian sandy subaqueous sediments is non-actualistic.