VARIATION IN TAPHONOMIC CHARACTER OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA: PALEOENVIRONMENTAL AND STRATIGRAPHIC IMPLICATIONS OF SHELL BEDS IN LAKES

Large regions of the littoral lake bottom along structural platforms in Lake Tanganyika, Africa, are carpeted with sub-modern shell accumulations (shell beds). Diverse organisms such as endemic mollusks, sponges, crabs, and cichlid fish inhabit these shell beds. The viviparous gastropod Neothauma tanganyicense (Neothauma) is the lead contributor to these accumulations but the snail itself is rarely found living on or adjacent to the shell accumulations (poor live-dead agreement), which makes it difficult to develop a model for the process(es) of shell accumulation and consequent community structuring of those organisms that occupy the shell beds. This study examines the shell beds located in the low-gradient platform of Kungwe Bay, north of the Mahale Mountains, which is fed by the Lagosa and Lokoma River deltas. Bulk shell samples were collected along transects perpendicular to the shoreline at 9, 12, 15, and 20 m.

The taphonomic features examined at these depths are encrustation, abrasion, fragmentation, oxidation patina and manganese oxidation. A qualitative 3-pt scoring of the degree of each taphonomic attribute was applied to each shell. Highly encrusted shells have lower fragmentation and abrasion scores, suggesting that encrustation forms a protective layer increasing shell preservation. Shells in shallow water are more abraded, likely attributable to exposure to wave action. Fragmentation increases from north to south across the bay at water depths of 9, 12, and 15 m. Shells coated with black manganese oxide and reddish-orange oxidation patinas show signs of exposure and burial. Manganese oxidation increases as water depth increases. By examining the taphonomic character of Neothauma we can begin to understand the sedimentological and environmental processes contributing to the formation of the shell beds, which has implications for stratal development in platform settings. Future work involving age- dating of individual shells will allow us to infer the degree of time-averaging of these shell accumulations to determine if the process of formation of the shell accumulations are lake-wide or more local, non-synchronous events.