THE POTENTIAL OF LAKE-MARGIN SEDIMENTS TO PRESERVE DYNAMIC LACUSTRINE CONDITIONS: LESSONS FROM RIFT BASIN LAKES

Lake-margin environments, especially on the shores of closed-basin or rift lakes, are often dynamic settings that can record a wide variety of sedimentological features, fossils, and clues to the paleoenvironmental past. Despite the fact that these environments may be limited in area, when accurately identified in the rock record, they can preserve important geological elements that are crucial to the interpretation of the overall lacustrine system. The research presented here explores several examples. Proper identification of lake-margin sediments is critical so that they are not merely lumped together with (often more widespread) lacustrine deposits. Research at Olduvai Gorge in East Africa indicates that discrete sedimentological differences delineate lacustrine deposits from lake-margin sediments. Proper identification of the lake-margin exposures produced a regional picture of, in this instance, freshwater availability that was utilized by Pleistocene hominins. Ongoing work at the Solite Quarry, in southwestern Virginia, has revealed a thin (32mm thick) bed of Triassic lake-margin sediments that preserve exquisite fossils of insects, plants, and soft parts of aquatic dinosaurs. Geochemical analysis of the fossil-bearing rocks indicate that the superb preservation of these fossils was due to lake margin toxicity (specifically, high F content from lake-margin seeps), which excluded predators as well as bioturbation. Lastly, carbonates found in lake-margin environments (e.g., closed basin lakes in Nevada, rift basin lakes in East Africa) can be dynamic recorders of fluctuating aquatic conditions, if and when the carbonates are formed in equilibrium with the ambient environment. Newly described lacustrine carbonates called “phytocretions” form above the sediment surface in standing water (often on lake margins where evaporation stimulates calcite precipitation). Their stable isotopic signatures accurately record rapid (annual?) environmental changes as the aquatic conditions fluctuate. Thus, lake-margin environments, when properly identified in the geologic record, can provide exquisite preservation of fossils (body and trace), faithfully record changing environmental conditions, and provide an important glimpse into localized paleolandscape features.