EDIACARAN-LIKE TEXTURED ORGANIC SURFACES FROM THE BOREAL EARLY TRIASSIC: MICROBIAL MATS THRIVED IN THE ABSENCE OF GRAZING AFTER THE PERMIAN-TRIASSIC MASS EXTINCTION

The marine Lower Triassic strata of the Sverdrup Basin, Arctic Canada, is dominated by a thick succession of hyperpycnite facies in mid-shelf settings that pass downdip into basinal mudrocks. Geochemical proxies for oxygenation (Mo/Al, Th/U and pyrite framboid analysis) indicates that dysoxic conditions prevailed in the Basin for much of the Early Triassic. This inhibited bioturbation and allowed the frequent development of textured organic surfaces (TOS) reminiscent of those from the Ediacaran Period, including wrinkle and pucker structures and bubble texture. The microbial mats responsible for these structures are envisaged to have thrived, in the absence of grazing, within the photic zone. The dysoxic history was punctuated by better-oxygenated phases, which coincide with the loss of TOS. Thus, Permo-Triassic boundary and Griesbachian mudrocks from the deepest-water settings have common benthos and a well-developed, tiered burrow profile dominated by Phycosiphon. TOS are also lacking from Skolithos-burrowed, shoreface sandstones that developed during basin-wide oxygenation in the late Dienerian. The presence of the intense burrowing in the earliest Triassic contradicts the notion that bioturbation was severely suppressed at this time due to extinction losses at the end of the Permian.