Koobi Fora Formation Research Articles

Petrology and Geochemistry of Rhizoliths from Plio-Pleistocene Fluvial and Marginal Lacustrine Deposits, East Lake Turkana, Kenya

ABSTRACT Recent studies of rhizoliths from the Koobi Fora Formation (Plio-Pleistocene) of East Lake Turkana, Kenya, indicate that their shapes vary with the depositional environment of their host sediment. Vertical rhizoliths are associated with channel-bar and overbank deposits of fluvial origin. These rhizoliths represent the tap roots of phreatophytes that lived in well-drained, upland settings. Horizontal rhizoliths predominate in facies ascribed to beaches, lagoons, and floodplains associated with ancient Lake Turkana. They are produced by water-emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments. The rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems. The vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements. These attributes reflect precipitation of the calcite in well-oxygenated, well-drained vadose conditions. In contrast, the cements of the horizontal root systems lack textures indicative of vadose conditions, contain abundant clay and plant debris, and display Mn concentrations as high as 4.5 cation percent. These features confirm that the horizontal rhizoliths developed in chemically reduced, water-saturated conditions. Bacterial decay of the plant debris maintained reduced Eh conditions while also supply ng abundant Mn2+ to the pore solutions. Groundwater that had moved through volcanic strata may have also contributed Mn2+. The Mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of Mn2+ as well as by inclusion of manganese oxides.

40Ar/39Ar age spectra from the KBS Tuff, Koobi Fora Formation.

40Ar/39Ar age spectra on anorthoclase phenocrysts from three pumice clasts in the KBS Tuff yield nearly ideal flat patterns, providing good evidence that the samples have remained undisturbed since crystallization. The ages are concordant at 1.88±0.02 Myr, and confirm that the KBS Tuff, a key marker bed in the Koobi Fora Formation, northern Kenya, is now very well dated. These results resolve the conflict between earlier 40Ar/39Ar and conventional K-Ar dating measurements on the KBS Tuff.

Preliminary correlations between the Koobi Fora and Shungura Formations, East Africa

Major, minor and trace element analyses of glass from pumice and tuffs from the Koobi Fora and Shungura Formations show that the Chari, Karari and Tuff L are correlative ; that the KBS Tuff and Tuff H2 are correlative; and that an unnamed tuff in the Koobi Fora Formation is correlative with Tuff H4 of the Shungura Formation.

Environmental Significance of Fossil Root Casts from Koobi Fora Formation (Pliocene-Pleistocene), East Turkana, Kenya: ABSTRACT

Roots and fossil root casts at East Turkana, Kenya, are valuable as paleo-environmental indicators. Roots are preserved within the Koobi Fora Formation as calcified concentric and nodular casts in lithologies independently interpreted as fluviodeltaic or lacustrine-coastal plain. Five common root cast types are recognized: (1) horizontal, planar root mats; (2) vertical or vertically branching root structures; (3) horizontal or horizontally branching root structures; (4) diagonally branching root structures; and (5) root clusters or balls. Root mats are usually in blocky or laminated muds. Typically, the mats extend laterally over tens of square meters, although an individual mat will be only about 5 cm thick. They appear to have grown in quiet-water lagoons. Vertical ro t structures are best developed in coarse-grained sands interpreted as fluvial channel, and secondarily in fine-grained silts believed to be interdistributary overbank deposits. They are uncommon in blocky or laminated lagoonal muds. Comparison of root morphologies among living East Turkana plants supports interpretation of the fossil associations. In arid environments water availability is a dominant factor in root morphology. Plants growing along intermittent streams or flood plains must frequently rely on perched water bodies or underground seepage, and therefore send out long vertical tap roots to take advantage of this limiting resource. Plants living within shallow reed swamps have no such requirement, as their roots are nearly continually immersed in ground water; during floods th plant stalk itself is subaqueous. The roots of these plants tend to extend laterally in thin mats over much of the lagoon. End_of_Article - Last_Page 434------------

A statistical approach to temporal biostratigraphy

Various statistics (similarity coefficients) can be constructed for faunal assemblages which vary more or less regularly as these assemblages change. Methods are described for placing assemblages on an evolutionary scale, the units of which are equal amounts of evolutionary transformation in the species makeup of assemblages. The greatest single factor involved in the scaling appears to be time; the effects of zoogeography are also discussed. Characteristics of different similarity coefficients differ, and some of these characteristics are discussed. Applying our method to faunal assemblages from East African hominid sites, reasonable correlations can be established between Olduvai Gorge, Tanzania, the Shungura Formation, Ethiopia, and the Koobi Fora Formation, Kenya.

Palaeomagnetic stratigraphy of the Koobi Fora Formation, east of Lake Turkana (Lake Rudolf), Kenya

Palaeomagnetic stratigraphy of the Koobi Fora Formation, east of Lake Turkana (Lake Rudolf), Kenya

40Ar/39Ar dating of the KBS Tuff in Koobi Fora Formation, East Rudolf, Kenya

40Ar/39Ar and conventional K–Ar age determinations on volcanic sanidine–anorthoclase crystal concentrates from pumice lumps contained within the KBS Tuff in the Koobi Fora Formation at East Rudolf indicate an apparent age near 2.42 Myr. Recomputation of data obtained in 1969 from this tuff suggests a more accurate apparent iisochron age of 2.42±0.01 Myr for the feldspar, coincident with total fusion conventional K–Ar and 40Ar/39Ar dates of ∼ 2.4 Myr from the same sample. Age spectra and total fusion dates obtained from other samples of the KBS Tuff confirm this conclusion. Thus, apparent ages of < 2.4 Myr derived from some of our samples and conventional total fusion K–Ar apparent ages in the range 1.6–1.8 Myr obtained from the KBS Tuff by other workers are regarded as discrepant, and may have been obtained from samples affected by argon loss. This latter conclusion is compatible with our sector interpretation of the age spectrum analyses of KBS pumice feldspars. Argument for there being more than one tuff complex or an admixture of older pumice in the KBS Tuff is not supported by our work.

Fission-track dating of pumice from the KBS Tuff, East Rudolf, Kenya

Fission-track dating of zircon separated from two pumice samples from the KBS Tuff in the Koobi Fora Formation, in Area 131, East Rudolf, Kenya, gives an age of 2.44±0.08 Myr for the eruption of the pumice. This result is compatible with the previously published K–Ar and 40Ar/39Ar age spectrum estimate of 2.61±0.26 Myr for the KBS Tuff in Area 105, but differs from the more recently published K–Ar date of 1.82±0.04 Myr for the KBS Tuff in Area 131. This study does not support the suggestion that pumice cobbles of different ages occur in the KBS Tuff.

New hominid fossils from the Koobi Fora formation in northern Kenya.

A WELL preserved partial innominate KNM-ER 3228, and a relatively complete cranium KNM-ER 3733 provide further support for the presence of Homo during the Plio–Pleistocene in eastern Africa1–5. These hominid specimens and others (Table 1) were collected during the 1974 and 1975 field programme in the Koobi Fora Formation, lying to the east of Lake Turkana (formerly Lake Rudolf) in northern Kenya. (This area was previously known as East Rudolf and has been studied since 1968; ref. 6.) The material reported here has been referred to either the Upper or Lower Member of the Koobi Fora Formation. Recent work on the dating of tuffs by Curtis et al.7 has raised important questions that will require time to resolve and may necessitate a complete re-examination of the reported chronology of the Koobi Fora succession and all other East African Plio–Pleistocene localities. Alternative dating methods such as fission track will be given close attention in the future and may offer, together with studies of certain taxa, useful indications about the reliability of isotopic age determinations.

Australopithecus, Homo erectus and the single species hypothesis.

AN enormous wealth of early hominid remains has been discovered over the past few years by expeditions within eastern Africa. Evidence has been presented for the existence over a considerable period of time of at least two contemporaneous hominid species1. Some of this evidence is compelling, but some less so for a variety of reasons such as the lack of association, fragmentary specimens, geological uncertainties, equivocal anatomical differences and suchlike. Many of these new specimens are of great antiquity and have led to suggestions that an early form of the genus Homo was contemporary with at least one species of Australopithecus. The evidence presented here deals not with the earlier stages of human evolution, but with the unequivocal occurrence of H. erectus from the Koobi Fora Formation, east of Lake Turkana (formerly Lake Rudolf).

Age of KBS Tuff in Koobi Fora Formation, East Rudolf, Kenya

Conventional K–Ar dates on pumice from the KBS Tuff horizon in the Koobi Fora Formation at East Rudolf, Kenya, distinguish two tuff units. Samples from area 10 and area 105 give an age of 1.60±0.05 Myr whereas those from area 131 give 1.82±0.04 Myr. Both ages differ from the previously published age of 2.61±0.26 Myr obtained by 40Ar/39Ar technique.

Age of KBS Tuff in Koobi Fora Formation, East Lake, Turkana, Kenya

Age of KBS Tuff in Koobi Fora Formation, East Lake, Turkana, Kenya