WERE PROTEROZOIC SHELF EDGES INHERENTLY UNSTABLE DUE TO LACK OF BIOTURBATION ?

Conventional wisdom states that Precambrian marine sediments were mechanically firm and stable because they were well stratified due to a lack of bioturbation. However, the pronounced anisotropy in grain size, sorting and permeability also allowed the rapid saturation of the pore volume by gases and liquids, derived from the decay of ubiquitous buried microbial organic matter. In comparison to Phanerozoic sediments, overpressure in Precambrian sediments may have built up quickly and at unusually shallow depths, thereby reducing internal shear strength.

This hypothesis can be tested at various localities, throughout the Precambrian, and at several scales: On hand-sample scale, Precambrian shelf sediments commonly display microbial mat textures whose mechanical behaviour can occasionally be inferred from their response to soft-sedimentary deformation or sedimentation processes. Hand sample examination commonly indicates a “sticky” behaviour of these biomats and a “leathery” strength, immobilizing grains, oversteepening ripple foresets, and trapping gas bubbles.

On outcrop scale, stacked microbially-influenced strata occasionally show large-scale fluid-escape structures that indicate overpressure development and seal failure at depths of 2-15 m. Despite the low confining pressures, dewatering at these shallow depths was already retarded. This suggests a high degree of very early and effective permeability compartmentalization, e.g. in individual “pressure cells” lined by microbial-mat bounding surfaces. The mechanical behaviour of these sediments may have resembled water beds.

Finally, on a basin scale, the pronounced sedimentary anisotropy and common overpressure buildup in Precambrian high-productivity shelf and shelf-edge sediments may have facilitated the initiation of sediment displacement, increased their frequency, and contributed to high runout distances of olistostromes and slide sheets. The shelf-margin architecture of Precambrian passive margins should therefore be examined for the presence of a higher degree of mass wasting than in their Phanerozoic counterparts.