Assessing Early Triassic paleoceanographic conditions via unusual sedimentary fabrics and features

Abstract

Analysis of the Lower Triassic sedimentary rock record has revealed a variety of unusual sedimentary fabrics and features, many of which were more common in the Proterozoic than the Phanerozoic. These so-called “anachronistic facies” developed across Lower Triassic carbonate platforms from shallow subtidal environments to deep, basinal environments and include features associated with the enhanced precipitation of calcium carbonate (e.g., synsedimentary seafloor cements), microbialites, and fabrics that formed due to a reduction in the intensity and depth of bioturbation (e.g., vermicular limestones). Three interacting factors controlled the development of anachronistic facies during the Early Triassic including: 1) environmental conditions (i.e., the unusual chemistry of Early Triassic oceans); 2) biotic factors (e.g., bioturbation and grazing pressures); and, 3) actualistic sedimentologic processes (e.g., waves and currents, sedimentation). Because these three factors typically acted in concert with each other, it is important to consider the role of each when using anachronistic facies to reconstruct Early Triassic paleoceanographic conditions. Overall, anachronistic facies are indicative of enhanced calcium carbonate precipitation during the Early Triassic, but do not necessarily indicate the former presence of anoxic or suboxic conditions where they were deposited. The influence of seawater chemistry on the deposition of anachronistic facies was most pronounced in deeper water environments, where anomalous paleoceanographic conditions heightened calcium carbonate precipitation and caused a decrease in bioturbation and grazing pressures via the upwelling of anoxic, alkaline waters. The degree of environmental influence lessened shorewards, where biotic factors and actualistic processes were more important, and slight modification of seawater by microbial activity, shallow burial, or CO2-degassing was necessary for calcium carbonate precipitation to occur. Anachronistic facies are a useful tool for reconstructing paleoceanographic conditions and examining shifts in the distribution of environmental stress during the recovery from the Permian–Triassic mass extinction, but only when the biologic, sedimentologic, and geochemical factors that led to their growth are carefully considered.