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Geochronology of the Manyara Beds, northern Tanzania new tephrostratigraphy, magnetostratigraphy andAr/Ar ages

Geochronology of the Manyara Beds, northern Tanzania new tephrostratigraphy, magnetostratigraphy andAr/Ar ages

The 25 m-thick Manyara Beds section near Makuyuni, northern Tanzania, contain abundant Acheulean lithics and vertebrate fossils, including possibleHomo erectusremains. However, the age of the unit (and its most productive fossil and archaeological localities) has been provisional. To address this we measured 2 new stratigraphic sections and collected approximately 3 tephra and volcanic rock samples forAr/Ar 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 forAr/Ar 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 PleistoceneAr/Ar age of .633 .39 Ma. Two-step analysis of 1 feldspars from a nephelinite underlying the Manyara Beds produced a weighted meanAr/Ar age of 6. .3 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 .633 Ma tephra) we estimate that the base of the Manyara Beds was deposited between .98 and 1.3 Ma and the top between .27 and .44 Ma. Beds III-IV and the Masek Beds at Olduvai were probably also deposited in this 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.The upper member of the Manyara Beds is .626 .38 Ma (Ar/Ar analysis). The Manyara Beds probably range from 1.3-1.1 Ma to .42-.3 Ma. Rift processes climate change caused major facies and paleofaunal changes at .626 Ma. Analysis of 3-grain aliquots provided the most preciseAr/Ar pumice age..

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