SEDIMENTOLOGY AND GEOCHEMISTRY OF THE VIRGINIA SOLITE QUARRY, DANVILLE/DAN RIVER BASIN (NEWARK SUPERGROUP): A NEW "SHALLOW LAKE" INTERPRETATION FOR FOSSIL-BEARING, LAMINATED, LACUSTRINE SEDIMENTS

The early Mesozoic Newark Supergroup is an exquisite succession of rift basin sediments that provide critical information about global environmental change and biotic turnover across the Triassic-Jurassic boundary. The Virginia Solite Quarry (in the Danville/Dan River Basin) preserves a Late Triassic Lagerstätte of vertebrates and invertebrates, including a 34mm thick “insect layer” notable for detailed soft tissue impressions of Tanytrachelos as well as complete insects with fine details (e.g., microtrichiae). Unique details of depositional and diagenetic conditions are recorded by these sediments. Fine-scale stratigraphic and chemostratigraphic analyses of a 332mm thick section reveal a change from black siliciclastic layers (Unit 1) to dark brown dolomitic laminites (Unit 2 and “insect layer”), separated by a 1-3mm thick boundary dolostone layer that is traceable for >200m. Various lines of evidence indicate that the dolomitic laminites are primary: the presence of microspar, the chemical variation between dolomite crystal cores and rims, and low porosity and permeability that would preclude large-scale replacement. The dolomite is interlayered with carbonaceous laminations, suggesting microbial mediation of precipitation. The “insect layer” was the most oxygenated in the succession, suggesting shallow conditions. Therefore, these sediments indicate that paleo-Lake Danville was a shallow, saline, alkaline rift lake capable of primary dolomite precipitation. High F levels in the “insect layer”, as well as sufficient Mg and Ca to precipitate dolomite, are attributed to lake-margin groundwater seeps. The toxicity of these seeps (rather than significant water depth) excluded bioturbation and/or predation and aided fossil preservation. This interpretation of laminated lacustrine sediments is in contrast to previous depositional models that suggest that detailed fossil preservation correlates to anoxic deep lake conditions (up to 200m), and may instead suggest that preservation of complete insects indicates shallow lacustrine conditions. Furthermore, these deposits (and the fossils therein) have impressively survived significant diagenesis, including quartz dissolution and coarsening of dolomite.