アフリカ大地溝帯の地球科学的特異性と Olduvai 峡谷の洪積統

抄録

I. Geological and geophysical characteristics of the African Rift Valley are summarized in this article. The African Rift Valley forms a part of a world-wide fracture zone, extending from the Red Sea and the Gulf of Aden to the Indian Ocean to link up with the midoceanic ridges, whose medial part are often occupied by the rift structure. Various independent evidences suggest the tensional origin of the rift valleys.
The pattern of alkaline igneous activity shows a remarkable correspondence to the rift pattern, and these are probably ascribed to the same major process. The occurrence of the carbonatites is the most striking petrological feature found in the region of the Rift System, and the active volcano Oldoinyo Lengai near the Kenya-Tanzania border, one of the outstanding examples of this activity, yielded lavas of high alkali carbonatite in its last eruption. Emphasis was given especially on petrological and geochemical properties of carbonatite and other volcanic rocks in the African Rift Valley.
II. The Olduvai Gorge, located at the transitional area between the eastern Rift Valley and the central African peneplane, became famous for the discoveries of the oldest human bones named zinjanthropus boisei by Leakey. At the Gorge the Pleistocene sedimentaries, divided into the Beds I-V in ascending, consist of the alternation of lake deposits and eolian ones, which suggest the repetitions of wet and dry climates during the Pleistocene. The Balbal depression at the eastern extremity of the Gorge and several faults cutting the Beds I-IV are oriented in NE-SW direction and reveal a movement of the Rift Valley at that time. Recently, K-Ar age of the Bed I is determined and the problems on the correlation to the international standard and the Pleistocene chronology of east Africa are debated. After the re-examination of the stratigraphy and the paleontology of the Beds I-IV, and the Pluvial-Glacial correlation, we come to the conclusion given in Table 5.