SALINE, ALKALINE, AND TOXIC— A RECIPE FOR PERFECT PRESERVATION

It is widely thought that the detailed preservation of soft parts correlates directly to water depth and/or anoxia, since the preservation of Lagerstätten often coincides with anoxic marine (e.g., Burgess Shale) or lake (e.g., Green River Formation) environments. Here we present an instance of a Lagerstätte that has not only been preserved in a shallow, oxygenated environment, but has also survived significant diagenesis. The Solite Quarry, near Cascade, VA, exposes several hundred meters of the largely lacustrine upper member of the Cow Branch Formation (Norian), and is considered to be among the richest, most productive Triassic terrestrial sequences in the world. A 332-mm thick microsection includes the “insect layer”, a rich yet thin bed (~3 cm) containing exquisite preservation of macrophyte plant fossils, soft tissues of the aquatic tetrapod Tanytrachelos ahynis, and complete insects. Traditionally, this Lagerstätte was interpreted as an anoxic deep lake deposit, but we present results that indicate a shallow, oxygenated lake margin instead. Below the insect layer, lithology changes from siliciclastic-dominated layers to (primary) dolomite-siliciclastic laminites and a thin, yet laterally continuous, boundary dolostone layer separates this change. This sedimentary shift occurred as a shallow, saline, alkaline rift lake transgressed over a (fresher?) lake margin environment. The absence of bioturbation and predation is not explained by significant water depth. Instead, high concentrations of F, Mg, and Ca in the insect layer sediments suggest toxic bottom waters, perhaps introduced by lake margin seeps that produced the boundary dolostone. Therefore, the Solite Lagerstätte formed as fossils were preserved in a shallow oxygenated, yet toxic, lake margin environment. Modern rift lakes indicate that this unorthodox interpretation is indeed possible. Whole insects are found on the margin of Lake Natron (Tanzania), where caustic saline, alkaline lake- and spring-waters preclude bioturbation and exclude predators. Lastly, the Solite sediments show evidence of diagenetic coarsening of dolomite as well as dissolution of quartz. So while the initial conditions of preservation of the fossils are remarkable, it is equally impressive that they have survived extensive diagenesis.