FORMATION AND PRESERVATION OF MICROBIAL TEXTURES IN SILICICLASTIC SEDIMENTS

Late Ediacaran and early Paleozoic sandstones and siltstones preserve a host of unusual sedimentary structures and fossils of soft-bodied organisms. The exceptional preservation of microbial textures and macroscopic fossils in these siliciclastic rocks invite questions about the interplay between biological changes and environmental conditions at the time. We address these questions by lab experiments which explore the preservation of aggregate- and mat-forming photosynthetic filamentous cyanobacteria on siliciclastic sand. The roles of trapping and mineral precipitation around cyanobacteria are evaluated by growing cyanobacterial mats and aggregates in gently agitated solutions, with siliciclastic sand substrate that is either in direct contact with the aggregates or separated by dialysis bags, and in the presence of different silica concentrations. The sediment contains mostly quartz sand, which is immobile under our experimental conditions, and 0.03% w/w of smaller grains which can become suspended and are enriched in micaceous minerals and clays. We find that the sheaths of ~ 0.5 mm thin filamentous cyanobacteria can become rigid tubular structures coated by 0.1-2 micrometer size aluminosilicate minerals within five days to one month after the inoculation, i.e, during microbial growth and before microbes and their textures decay. Most mineral grains coating the sheaths are trapped from the suspension. Additional coating is derived from the precipitation of aluminosilicates in the presence of high, 0.1-0.4 mM concentrations of silica. Thus, successful preservation of cyanobacterial mats and textures in sandstones and siltstones requires the presence of suspended clay-sized minerals, high concentrations of silica in the solution and the absence of extensive mechanical disturbance (e.g., grazing or bioturbation). These constraints may account for the exceptional preservation of microbial mats and textures in some Ediacaran and Paleozoic sandstones and siltstones.