Plio-Pleistocene stratigraphy and paleogeography for hominin remains from areas 130 and 133, Koobi Fora, Kenya.

摘要

The Koobi Fora Formation archives a record of hominin evolution for a three-million-year period (ca. 4-1 Ma) within the northeast Turkana Basin, Kenya. Assessing probable controls on the fossil or archaeological patterns in the formation requires an understanding of the spatial-temporal contexts of the hominin remains and detailed knowledge of the paleoenvironmental settings in which evolution occurred. Toward these ends, research was conducted in the basin to interrelate paleoanthropological sites from collection Areas 130 and 133, refine the paleogeography of the north-central Karari Escarpment, and ultimately provide a framework for establishing associations between hominin adaptations and habitats features in the Koobi Fora region. Results of sedimentological and stratigraphic analyses demonstrate that deposits of the upper Burgi/lower KBS members (∼1.9 Ma) accumulated within a low-lying floodbasin setting, covering an area of about 8 km2 for the northern Karari. This setting supported vertisols and was locally incised in northern Area 133 by a channel that attained a minimal depth of 3 m and lateral extent of ≥100 m. Beginning in the middle KBS Member and continuing to the upper KBS Member, the landscape experienced repeated dissection, aggradation, and prolonged (103-105 years) calcic soil formation. Channel systems during this interval extended laterally for ≥2 km and evolved from a deeply incised (up to 8 m of erosive relief) sandy complex to a shallower braided network carrying abundant basaltic gravel. Deposits of the upper KBS/lower Okote members (∼1.6 Ma) accumulated within a meandering channel and associated proximal floodplains. This landscape mainly aggraded through crevasse-splay sedimentation and supported immature alluvial soils. A similar floodplain style in Area 133 persisted to or resumed in the upper Okote/lower Chari members (∼1.4 Ma). The oldest and youngest hominin remains are potentially represented by surface occurrences of stone tools in Area 133. Most hominin remains from Area 130 derive from both braided- and meandering-river environments of the upper KBS to lower Okote members. The paleogeography represented by the reconstructed depositional systems was influenced by monsoon-driven seasonality in an otherwise semi-arid climate, in addition to instances of extra-basin volcanism that caused rates of sediment supply to episodically increase. Additional controls may have stemmed from lake-level oscillations and/or increased regional aridity occurring at 1.8-1.7 Ma.