SIMPLE INTERPRETATION OF LACUSTRINE TRIASSIC-JURASSIC KONSERVAT LAGERSATTEN OF THE RIFT BASINS OF EASTERN NORTH AMERICA WITH COMMENTS ON THE YANLIAO AND JEHOL BIOTAS OF CHINA

Lakes need only be small in area relative to their depth to result in turbulent stratification leading to chemical stratification and anoxic bottom waters, if productivity is adequate. Rift lakes often meet these criteria (e.g., Lake Tanganyika and Malawi) and the Triassic-Jurassic rift lakes of eastern North America are also examples. A lack of macroscopic benthos below the chemocline as well as virtually no currents means that larger organisms such as insects and fish that sink into the hypolimnion are preserved without scavenging, sometime with soft-tissue preservation. The results are classic lacustrine Konservat Lagersatten, the best example of which is the “Solite” assemblage from North Carolina and Virginia. However, there are many other examples. There are never evaporitic minerals directly in the layers with diverse biota. The highest density of continental or nearshore biota occur unsurprisingly in the strata closest to shore which in lakes of fluctuating depth on precessional timescales occur just as the chemocline transgresses the lake floor bottom or when it regresses past the same. In the gently sloping floors and margins of eastern US rifts little time was spent at any one place during the transgression and regression of the chemocline and hence the thickness of the microlaminated strata with abundant nearshore fauna tends to be on the scale of a centimeter or two. But in other places in the world where the relief was more dramatic, lending itself even more easily to turbulent and chemical stratification, such as the Late Jurassic and Early Cretaceous lakes producing the Yanlaio and Jehol biotas, respectively, nearshore conditions may be persistent. Therefore, the probability of entraining feathered, non-avian dinosaurs or other remarkable creatures, is correspondingly higher. However, in all of these cases special explanations such as saline springs or pyroclastic entombment, in the absence of extraordinary evidence, do not falsify the simple stratified lake model.