NEW SEISMIC STRATIGRAPHY AND LATE TERTIARY HISTORY OF THE NORTH TANGANYIKA BASIN, EAST-AFRICAN RIFT SYSTEM, DEDUCED FROM MULTICHANNEL AND HIGH-RESOLUTION REFLECTION SEISMIC DATA AND PISTON CORE EVIDENCE

We present here the initial results of a high-resolution (sparker) ref lection seismic survey in Northern Lake Tanganyika, East African Rift system. We have combined these results with data from earlier multicha nnel reflection seismic and 5-kHz echosounding surveys. The combinatio n of the three complementary seismic investigation methods has allowed us to propose a new scenario for the late Miocene to Recent sedimenta ry evolution of the North Tanganyika Basin. Seismic sequences and regi onal tectonic information permit us to deduce the palaeotopography at the end of each stratigraphic sequence. The basin history comprises si x phases interpreted to be responses to variations in regional tectoni sm and/or climate. Using the reflection seismic-radiocarbon method (RS RM), the minimum ages for the start of each phase (above each sequence boundary) are estimated to be: similar to 7.4 Ma, similar to 1.1 Ma, similar to 393-363 ka, similar to 295-262 ka, similar to 193-169 ka, s imilar to 40-35 ka. Corresponding lowstand lake elevations below prese nt lake level for the last five phases are estimated to have been: sim ilar to 650-700 m, similar to 350 m, similar to 350 m, similar to 250 m and similar to 160 m, respectively. The latest phase from similar to 40-35 ka until the present can be subdivided into three subphases sep arated by two lowstand periods, dated at similar to 23 ka and similar to 18 ka. From the late Miocene until the mid Pleistocene, large-scale patterns of sedimentation within the basin were primarily controlled by tectonism. In contrast, from the mid Pleistocene to the present, se dimentation in Lake Tanganyika seems to have responded dramatically to climatic changes as suggested by repeated patterns of lake level fluc tuations. During this period, the basin infill history is characterize d by the recurrent association of three types of deposits: 'basin fill ' accumulations; lens-shaped 'deep lacustrine fans'; and 'sheet drape' deposits. The successive low-lake-level fluctuations decreased in int ensity with time as a consequence of rapid sedimentary filling under c onditions of declining tectonic subsidence. The climate signal has thu s been more pronounced in recent sedimentary phases as tectonic effect s have waned.