GEOCHRONOLOGY OF THE MANYARA BEDS, NORTHERN TANZANIA: STRATIGRAPHY AND NEW MAGNETOSTRATIGRAPHY AND 40Ar/39Ar AGES

The ~30m-thick Manyara Beds fluvio-lacustrine section near Makuyuni, northern Tanzania, contains abundant Acheulian lithics and vertebrate fossils, including possible Homo erectus remains. However, the age of the unit (and its most productive fossil and archaeological localities) has been provisional since the 1930’s. To address this we measured 20 new stratigraphic sections and collected approximately 30 tephra and volcanic rock samples for ⁴⁰Ar/³⁹Ar geochronology and 12 mudstone/siltstone samples for a trial paleomagnetic analysis. The Manyara Beds consist of a greenish-gray, lacustrine lower member and reddish-brown, fluvial upper member. Most sampled tephra deposits from the lower member proved too fine-grained and/or lithologically heterogeneous for ⁴⁰Ar/³⁹Ar analysis. However, crystals from a pumice lapilli-rich unit (the ‘Hollywood Tuff’) at the base of the upper member, analyzed in three batches, yielded a middle Pleistocene ⁴⁰Ar/³⁹Ar age of 0.626 ± 0.038 Ma. Additionally, nineteen laser fusion analyses of feldspars from a nephelinite underlying the Manyara Beds produced a weighted mean ⁴⁰Ar/³⁹Ar age of 5.95 ± 0.03 Ma, linking it to the nearby stratovolcano Essimingor. Paleomagnetic analysis shows that the Manyara Beds samples carry a stable ancient remanence, and characteristic directions obtained by AF demagnetization define a preliminary magnetostratigraphy for the unit. Four magnetozones over 21 m of section include two intervals of normal polarity tentatively correlated with the Jaramillo Subchron and Brunhes Chron. The Matuyama-Brunhes boundary is located within the upper half of the lower member, at least 6 m below the lower-upper member contact. Using undecompacted sedimentation rates of 24-36 m/Ma (calculated using geomagnetic polarity time scale correlations and the stratigraphic position of the ~0.626 Ma tephra) we estimate that the base of the Manyara Beds was deposited between 1.3 and 1.1 Ma and the top between 0.42-0.3 Ma. Beds III-IV and the Masek Beds at Olduvai were probably deposited coevally during this early-middle Pleistocene interval, when the Mid-Pleistocene Revolution climatic event and active rifting and volcanism in the southernmost Gregory Rift jointly contributed to landscape and ecosystem instability.