Crevasse-splay and associated depositional environments of the hominin-bearing lower Okote Member, Koobi Fora Formation (Plio-Pleistocene), Kenya

摘要

Abstract The Okote Member from the northeast Turkana Basin of Kenya represents an exceptionally good archive of Early Pleistocene archaeological and fossil sites. Field study of the lower Okote Member and underlying deposits was conducted in detail at two sets of outcrops (500 to 1000 m long) separated by some 22 km of modern landscape. The examined sections (10 to 15 m thick) preserve two facies: interbedded sandstones, tuffs and mudstones (crevasse splay) and ribbon-like bodies of sandstone/tuff (crevasse channel). Mudstones are overprinted with small carbonate nodules and sparse slickensides that are interpreted as evidence of weak soil development after the cessation of suspension deposition in a floodplain. The immaturity of the ancient soils implies that sediment accumulation was fairly rapid, yet the nodules and slickensides indicate seasonal contrasts in moisture. Episodically rapid sedimentation is suggested by the types of depositional environments and their structures, and indications of soft-sediment deformation. Crevassing may have been influenced by rivers that flooded in response to monsoonal rainfall variations and the voluminous influx of fluvially reworked volcaniclastics to the basin. A well-developed palaeosol directly underlying the lower Okote Member suggests that the deposits accumulated following a significant depositional hiatus. This basal contact, in addition to the apparent pervasiveness and penecontemporaneousness of the sedimentary facies, resembles a crevasse-splay system deposited by a sub-delta lobe or river avulsion. A paucity of lacustrine/deltaic indicators might indicate that the studied deposits correspond better with an avulsion origin. Sedimentation rates calculated from prior radiometric dating of tuffs tend to overestimate the timeframe of accumulation for individual strata of the lower Okote Member. Many appear to have formed on the order of 10 0 to 10 3 years, as suggested by facies, palaeosols and observations of modern crevasse splays. This cautions against using sedimentation rates alone to infer how much time a stratum preserving fossil/archaeological information represents.