Facies Model of a Semiarid Freshwater Wetland, Olduvai Gorge, Tanzania

Abstract

The sedimentology and stratigraphy of a freshwater wet- land in early Pleistocene (; 1.75 Ma) volcaniclastic deposits, Olduvai Gorge, Tanzania, are characterized using texture, mineralogy, micro- fossils, and stratigraphic description to create a depositional facies model. The freshwater wetland was located on the margin of a semi- arid, closed basin containing a playa lake. The lake-margin deposits are dominated by two distinct lithologies: waxy claystones and earthy sediments. Waxy sediments are olive green, dense claystones that con- tain authigenic minerals such as trona, and represent a fluctuating saline, alkaline lake (Hay 1976). Earthy sediments are beige, friable siltstones that contain siliceous microfossils, bone fragments, and minor pebbles; they represent the freshwater wetland. Microfossils from the earthy sediments and diatomites indicate freshwater marsh biota with some rare salt-tolerant species. Associated lithofacies are: diatomite, carbonate, and sandstone. Interpretation of vertical and lateral lithofacies variations in the 2 km 2 wetland, based on ; 50 excavations, identified seven distinct sub- environments: spring deposits, perennial marsh, pool, ephemeral wet- land, fringing wetland, wetland channel, and ephemeral stream. The stratigraphic sequence, estimated to represent an ; 40-50 kyr long interval, indicates that persistent (; 10 3 yr) wet periods prevailed dur- ing the deposition of thick waxy clays when the lake expanded and flooded the wetlands, and equally long intervening dry periods caused the lake to contract, allowing the flourishing of wetlands and accretion of earthy sediment. The lithofacies associations in the Olduvai paleo- wetland provides an important first step in the development of a de- positional environmental model for freshwater wetlands that can be tested elsewhere, particularly in arid and semiarid settings.