Lacustrine Shell Accumulations in Continental Rifts: Actualistic Examples of Depositional Style and Taphonomy from the Littoral of Lake Tanganyika, East Africa

Shell-rich deposits are an important yet understudied component of the littoral zone in many tropical rift lake deposystems. Lake Tanganyika, the world's second largest rift lake, houses a remarkable suite of shallow-water shelly deposits that can be used to guide interpretations of ancient strata. Grab samples, vibracores and direct observation from SCUBA were used to investigate styles of deposition and preservation potential of shelly deposits near Kigoma, Tanzania. Both monotypic and polytypic accumulations of mollusks exist in the study area. Polytypic assemblages, composed dominantly of the endemic gastropod Neothauma tanganyicense and thumbnail clams of the genus Coelatura, occur along faulted headlands and gently sloping inter-deltaic platforms. Wave action, gravity flows, and activities of shell-brooding fish contribute to the development of para-autochthonous, internally complex beds of gravel-rich molluscan hash along headlands. Large bioclasts in this facies are commonly fragmented and encrusted, but otherwise well preserved. Downdip, these deposits grade laterally into stromatolite boundstones that incorporate both bioclasts and lithoclasts. In contrast, densely packed beds of mud-rich molluscan hash are routinely encountered 15 – 25 m below the lake surface along low gradient platform margins. These deposits thicken away from the lakeshore and appear to develop in response to sediment starvation, periodic winnowing and fish nesting. Monotypic lenses of N. tanganyicense occur along the beaches of platform margins, as well as adjacent to faults in deeper water (> 45 m), where shell preservation is enhanced by heavy reduction coatings. Preservation in the beach environment is low due to sand abrasion. However, gastropod-rich sandstones from paleo-highstand deposits show that shoreline shelly deposits are preserved in the rock record. Our results: 1) document the diversity of shell-rich deposits in Lake Tanganyika's littoral, and 2) highlight an example of biogenic feedback, as shell beds create a unique substrate utilized by a community of specialized benthic organisms.