ESTIMATING THE AGE OF FORMATION OF LAKES - AN EXAMPLE FROM LAKE TANGANYIKA, EAST-AFRICAN RIFT SYSTEM

Age estimates for ancient lakes are important for determining their hi stories and their rates of biotic and tectonic evolution. In the absen ce of dated core material from the lake's sedimentary basement, severa l techniques have been used to generate such age estimates. The most c ommon of these, herein called the reflection seismic-radiocarbon metho d (RSRM), combines estimates of short-term sediment-accumulation rates derived from radiocarbon-dated cores and depth-to-basement estimates derived from reflection-seismic data at or near the same locality to e stimate an age to basement. Age estimates from the RSRM suggest that t he structural basins of central Lake Tanganyika began to form between 9 and 12 Ma. Estimates for the northern and southern basins are younge r (7 to 8 Ma and 2 to 4 Ma, respectively). The diachroneity of estimat es for different segments of the lake is equivocal, and may be due to erosional loss of record in the northern and southern structural basin s or to progressive opening of the rift. The RSRM age estimates for La ke Tanganyika are considerably younger than most prior estimates and c larify the extensional history of the western branch of the East Afric an Rift system.