MODERN PROCESSES ON THE SHORES OF SALINE, ALKALINE LAKE NATRON EXPLAIN PALEO LAKE-MARGIN FEATURES (Invited Presentation)

Lake Natron is a saline, alkaline lake in northern Tanzania/southern Kenya that sits in the eastern Gregory branch of the East African Rift. On the southern shore, a Pleistocene volcanic ash preserves over 360 human footprints and several associated animal trackways. Regional volcanoes (both modern and ancient) are the main source of sediments to the lakeshore in addition to evaporite minerals (e.g., trona). In order to reconstruct the paleoenvironment, one needs only to look to the modern lakeshore of Lake Natron to explain features seen in the Pleistocene ash. On the modern lake-margin, footprints are of various shapes and depths, which correlate with substrate saturation levels. Soft sediment deformation around the ancient footprints, including push-up mounds and slide marks, suggest a similar saturated substrate. Springs and ephemeral rivers feed the modern lake-margin by small ephemeral channels that exhibit asymmetrical ripples marks during high flow events. Similar ripple patterns are preserved in the ash near several animal trackways. Macrofossils of broad-leafed plants (1 to 15cm in length), sedges, and thorns (acacia?) are pressed into and preserved in the ash, and in some instances are pressed into human footprints. Strong winds introduce winged insects, whole leaves, sedge stems, and other plant debris to the lakeshore where they become trapped in the damp sediment. Vertical tubes (0.5 to 1cm across) preserved in the ash indicate either insect burrows or plant molds. Only salt-tolerant insects and plants colonize the modern mudflat but leave their traces as vertical burrows and tubes. Features are well preserved on the margin of Lake Natron because the lake’s chemistry excludes most plants and animals that could bioturbate the sediment. These features can be an appropriate proxy for shoreline deposits of other saline, alkaline rift lakes in the geologic record.