Geochronology of the Manyara Beds, northern Tanzania: New tephrostratigraphy, magnetostratigraphy and 40Ar/ 39Ar ages

Abstract

The ∼25 m-thick Manyara Beds section near Makuyuni, northern Tanzania, contain abundant Acheulean 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. 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. Most sampled tephra deposits (from the greenish-gray, lacustrine lower member) proved too fine-grained or lithologically heterogeneous for 40Ar/ 39Ar analysis, but crystals from a pumice-rich unit (the ‘Hollywood Tuff;’ at the base of the reddish-brown, fluvial upper member), analyzed in three batches, yielded a middle Pleistocene 40Ar/ 39Ar age of 0.633 ± 0.039 Ma. Two-step analysis of 10 feldspars from a nephelinite underlying the Manyara Beds produced a weighted mean 40Ar/ 39Ar age of 6.00 ± 0.03 Ma, linking it to the nearby stratovolcano Essimingor. Paleomagnetic analysis showed that the Manyara Beds 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 24–46 m/Ma (calculated using geomagnetic polarity time scale correlations and the stratigraphic position of the ∼0.633 Ma tephra) we estimate that the base of the Manyara Beds was deposited between 0.98 and 1.3 Ma and the top between 0.27 and 0.44 Ma. Beds III–IV and the Masek Beds at Olduvai were probably also deposited in the same early–middle Pleistocene interval, during which the Mid-Pleistocene Revolution climatic event and active rifting and volcanism in the southernmost Gregory Rift jointly contributed to landscape and ecosystem instability.