Olorgesailie Formation Research Articles

Quaternary diatoms and palaeoenvironments of the Koora Plain, southern Kenya rift

The Koora Basin (south Kenya Rift) preserves a continental, tropical, one-million-year record of environmental change driven by global climate, regional tectonism and volcanism. Diatom-based reconstructions from Olorgesailie Drilling Project (ODP) cores indicate lakes that expanded and contracted with conductivities ranging between ∼200 and > 25,000 μS.cm−1 and pH of 7.9–11.2. Benthic and planktonic diatoms document mostly shallow fresh water between 1 Ma and 870 ka with deeper freshwater lakes from 870 to 470 ka. After the Mid-Brunhes Event at about 430 ka, diatoms record many transgression-regression cycles with both freshwater and saline-alkaline lakes present. Palaeosols also indicate episodes of desiccation and lower water tables. Carbonates and zeolites are present in younger sediments, especially after 400 ka. Many high-lake-level stages correlate with low values in ocean benthic δ18O stack data. Most, but not all, low lake levels occurred during higher δ18O MIS intervals, suggesting tectonic and/or volcanic events, in addition to climatic forcing, influenced the drainage, outlet heights and accommodation space.The 870–470 ka period of deeper freshwater lakes at Koora correlates well with the neighbouring Lake Magadi pollen record that suggests generally wetter conditions at this time. Wet-dry cycles after 470 ka at Koora developed when the Magadi record indicates a change towards drier conditions, but with many wetter intervals. High lake level periods at Koora also correlate with phases of diatom-inferred flooding at Magadi. Outcrops north of Koora also document several large lakes during deposition of parts of the Olorgesailie Formation prior to ∼500 ka. The Koora environmental history helps to fill an environmental gap (500–320 ka) encompassing critical changes in hominin lithic technology caused by a hiatus at Olorgesailie. During the first part of this interval (470–390 ka), Koora was occupied by a shallow alkaline lake, suggesting relatively dry conditions. The second part (390–320 ka) was characterised by fluctuating deeper lakes that imply greater variability and wetter conditions. Subsequently, both the Olorgesailie and Koora records indicate variable environments.

Frontal bone virtual reconstruction and geometric morphometric analysis of the mid-Pleistocene hominin KNM-OG 45500 (Olorgesailie, Kenya).

KNM-OG 45500 is a hominin fossil composed of parts of a frontal bone, left temporal bone, and cranial vault pieces. Since its discovery along the Olorgesailie Formation (Kenya) in 2003, it has been associated with the Homo erectus hypodigm. The specimen, derived from a geological context dated to ca. 900 Ka BP, has been described as a very small individual of probable female sex. However, despite its status as an important hominin specimen, it has not been used in a quantitative comparative framework because of its fragmentary condition. Here, we undertake a virtual reconstruction of the better-preserved fragment, the frontal bone. We additionally apply geometric morphometric analyses, using a geographically diverse fossil and modern human sample, in order to investigate the morphological affinities of KNM-OG 45500. Our results show that the frontal shape of KNM-OG 45500 exhibits similarities with Early Pleistocene fossils from Eurasia and Africa that are assigned to H. erectus sensu lato (s.l.). Its size, on the other hand, is notably smaller than most other Homo erectus fossils and modern humans and similar to the specimens from Dmanisi (Georgia) and to Homo naledi. Taken together, our analyses of the frontal bone suggest a taxonomic attribution of KNM-OG 45500 to H. erectus s.l. and extend even further the range of size variability associated with this taxon around 900 Ka BP.

An integrated study of the Homo-bearing Aalat stratigraphic section (Eritrea): An expanded continental record at the Early–Middle Pleistocene transition

The Early to Middle Pleistocene continental transition in East Africa is widely documented from lacustrine and deep-sea records, although significant insights are also provided by fluvio-lacustrine successions of the central and southern African Rift Valley, such as the at Olduvai Gorge succession (Tanzania), the Bouri Formation (Ethiopia) and the Olorgesailie Formation (Kenya). The Early to Middle Pleistocene Dandiero Basin fill (Eritrean Danakil) represents the only continental succession in the northernmost sector of the African Rift Valley that provided abundant fossil vertebrates, including human remains. The present study integrates already available data with new sedimentological, pedological, magnetostratigraphic, paleontological and paleoanthropological investigations of the 300m thick Aalat section (North Dandiero Basin). This sedimentary succession records repeated shifts from fluvial to lacustrine depositional settings, which occurred under the tight interaction between local tectonics and Pleistocene climate changes. Accumulation was associated with axial sedimentation in a NS-trending extensional basin, with an overall tectono-sedimentary setting comparable with that of the coeval Bouri Formation (Ethiopia). Because of the high rates of sedimentation, a poor to moderate degree of soil development characterizes the whole succession. Sporadic soil horizons testify to carbonate dissolution, leaching and accumulation in calcic and petrocalcic horizons (indicating an overall dry climate). The alternate with local to extensive iron-oxide/hydroxide segregation, promoted by water infiltration under varying drainage conditions and/or seasonal contrast, that record more humid conditions. Magnetostratigraphic dating and correlation indicates that this section is among the world's thickest record embracing the Early-Middle Pleistocene transition, spanning from the Jaramillo to the base of Brunhes chron. The terrestrial vertebrate fauna includes a typical Early to Middle Pleistocene East African mammalian assemblage for this age and is dominated by taxa characterized by strong water dependence. The ichthyofauna, with its abundant Clariidae, is also consistent with the shallow water, fluvio-lacustrine paleobiotopes. The cranial, dental and postcranial human remains from the lower part of the Aalat succession add valuable evidence about the patterns of variation and evolutionary dynamics in African Homo erectus/ergaster near the end of the Early Pleistocene.

Correlation of volcanic ash layers between the Early Pleistocene Acheulean sites of Isinya, Kariandusi, and Olorgesailie, Kenya

Olorgesailie, Kariandusi, and Isinya are archeological sites with Acheulean artifacts in or on the flanks of the southern Rift Valley of Kenya. 40Ar/39Ar ages on feldspar from tuffs in the Olorgesailie Formation of 992 ± 39 ka (Member 1), 974 ± 7 ka (Member 5), and 747 ± 6 ka (Member 9), 662 ± 4 ka (Member 10), and 601 ± 3 ka (Member 11) bracket most of the Acheulean sites there. A 40Ar/39 age on a tuff in the Kariandusi sequence is 977 ± 10 ka, and both sites are associated with thick diatomite deposits. No age control has been available for the site of Isinya. Electron microprobe analyses of glass from volcanic ash layers from these sites allow tephrostratigraphic correlations between them. Here we report that samples from Kariandusi below the archeological levels correlate with samples in Member 4 and Member 2 at Olorgesailie (992 ka–974 ka). Samples from a volcanic ash at Isinya above the archeological site correlate with an ash layer in Olorgesailie Member 4, showing that these artifacts are >974 ka, providing the first age control for the artifacts at Isinya. The correlation from Olorgesailie to Kariandusi shows that the diatomites at those sites were deposited at the same time ∼120 km apart but now differ in elevation by ∼700 m. Chemical similarity with obsidians from localities in the Naivasha region suggest that some of the ash layers may originate from that area.

Quaternary geochemical stratigraphy of the Kedong–Olorgesailie section of the southern Kenya Rift valley

This study uses a variety of geochemical parameters to reconstruct a Quaternary environmental history of the Kedong–Olorgesailie sector of the southern Kenya Rift Valley. Lacustrine, wetland and fluvial deposition took place between about 1.9 and 0.72Ma at Munyu wa Gicheru in the northern part of the study area. Sedimentation continued about 40km further south to produce the lake, wetland, fluvial and pedogenic deposits of the Olorgesailie Formation between about 1.2 and 0.5Ma. Faulting subsequently led to basin breakup at Olorgesailie and the development of Middle to Late Pleistocene cut-and-fill fluvial deposits, as well as pond, wetland and terrestrial sediments that were locally altered by pedogenesis. Tufas and calcretes also accumulated during this latter time interval.Sedimentation in these different depositional sequences reflects several long-term trends, including: 1) an increase in carbonate deposition; 2) a decrease in biogenic silica; and 3) decreased chemical weathering indices (Chemical Index of Alteration, Vogt ratios), which imply increasing Quaternary aridity and/or erosion of catchment soils. The biophile elements Br and S also increase upwards, suggesting pulses of erosion of organic soils. Plants permineralized by silica became widespread after about 0.5Ma, providing evidence that groundwater bodies were becoming silica-enriched, possibly as a result of deeper water circulation along active faults. Several fault-aligned fossil hot spring deposits with siliceous vent, pedogenic and swamp facies have been identified at Emerit in the central part of the Kedong–Olorgesailie rift. Tectonism also provided pathways for meteoric water that led to the formation of carbonate springs in the Olorgesailie area. REE patterns in clastic sediments are stable throughout the Quaternary, reflecting the dominant trachytic source rocks, but these patterns become variable in later Pleistocene calcretes and tufas documenting changing hydrochemical conditions.The geochemical evolution of the area partly reflects climate change through time. However, it also documents the influence of Quaternary rifting processes. Rift faulting affected the geochemistry of the basin sediments by 1) causing changes in the depth of fracture systems that controlled groundwater movements and sourcing of fluids, 2) changing accommodation space in which sediments could accumulate, and 3) altering relief and thus rainfall and evaporation patterns across the rift system, as well as erosion and weathering rates.

Facies, geochemistry and diatoms of late Pleistocene Olorgesailie tufas, southern Kenya Rift

This research reconstructs paleoenvironments of late Pleistocene tufas at Olorgesailie in the southern Kenya Rift and examines the relationship between climate, tectonism and tufa precipitation. The deposits crop out at three main sites, forming at 290kyr and at least twice between 220 and 12kyr ago. Ten carbonate facies comprise the bulk of the tufas, which accumulated at spring vents, in swamps and shallow lakes, along fissures, and along fluvial channels.The tufas contain diatoms dominated by shallow-water, swamp and fluvial taxa associated with fresh to mildly saline conditions. δ13C isotopes are relatively enriched (0.25 to 1.96‰) compared with other East African tufas, but uniform within each set of tufa deposits, suggesting similar sources of CO2 derived from similar plants and soil organic matter during each interval of deposition. δ18O ranges from −7.17‰ to +5.16‰ suggesting variable evaporation and rainfall/groundwater sources or changing water temperatures. Variable REE patterns suggest spatial and/or temporal contrasts in groundwater chemistry.The former springs may have developed in response to climate changes, but there are no clear correlations with known East African wet phases. The deposition of tufa probably also relates to basin breakup and faulting, which likely influenced groundwater movements. Increased permineralized silica in plants in tufas and non-carbonate sediments reflects the introduction of deeper groundwaters than those during earlier Olorgesailie Formation times. Whether climatically or tectonically controlled, the tufa spring sites would have provided a supply of potable water, at least intermittently, during the Pleistocene, and would have provided attractive places for wildlife and possibly for Middle Stone Age human inhabitants of the region, especially during relatively dry times.

Geochemical trends through time and lateral variability of diatom floras in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

The Olorgesailie Formation (1.2–0.49 Ma) consists of fluvial and lacustrine rift sediments that have yielded abundant Acheulean artifacts and a fossil hominin ( Homo cf. erectus). In testing prior understandings of the paleoenvironmental context, we define nine new geochemical zones. A Chemical Index of Alteration suggests increased catchment weathering during deposition of Members 1, 2, 7, 11, and 13. Biophile elements (Br, S) peak in M8–9 and lower M13 possibly reflecting increased input from soil erosion. REE data show that the Magadi Trachytes supplied most siliciclastic grains. Sixteen diatom stages indicate conductivities of 200–16,000 μS cm − 1 and pH of 7.5–9.5 for five deep-water lakes, ten shallow lakes and sixteen wetlands. These results are compared with diatom data from other sections in the basin and show aquatic spatial variability over km-scale distances. Similar floras are traceable over several kilometers for M2, M3 and M9, indicating broadly homogeneous lacustrine conditions during these times, but diatoms in other members imply variable conditions, some related to local tectonic controls. This lateral and temporal variability emphasizes the importance of carrying out stratigraphic sampling at multiple sites within a basin in efforts to define the environmental context relevant to human evolution.

Ultrafine Clay Minerals of the Pleistocene Olorgesailie Formation, Southern Kenya Rift: Diagenesis and Paleoenvironments of Early Hominins

AbstractThe Pleistocene Olorgesailie Formation in the southern Kenya Rift has yielded a remarkable assemblage of Acheulean artifacts and vertebrate fossils, including hominin specimen KNM-OG 45500. Members 1 and 7 both contain clay-rich deposits that have been pedogenically modified into paleosols (UM1p and UM7p, respectively). This study provides the first detailed mineralogical and geochemical analyses of the clays of this important Pleistocene basin. The smectitic clays, which show abundant evidence for pedogenesis, were apparently originally deposited under lacustrine conditions. They have an average structural formula of (Ca0.01Na0.32K0.26)(Si3.76Al0.24)(Al0.86Ti0.04Fe0.68Mg0.42)O10(OH)2. The high layer charge clays indicate diagenetic alteration of detrital clay derived from the volcanic drainage basin, probably involving alkaline waters of variable salinity. Despite overall lower salinity compared to other Plio–Pleistocene basins of the region (e.g. Olduvai Gorge), the basin still shows evidence for authigenic clay mineral precipitation. Clay chemistry and bulk geochemical indicators of pedogenesis imply that UM1p clays more closely reflect depositional paleo-waters, whereas UM7p clays have been more pedogenically altered due to subaerial exposure. UM1p smectites show some Mg enrichment near the western Lava Hump locality, consistent with discharge of Mg-bearing paleo-waters from a volcanic aquifer into a siliceous and alkaline (though not highly saline) paleo-lake. UM7p smectites were deposited in a more saline paleo-lake, but have lost substantial amounts of Mg due to post-depositional weathering. Locally abundant artifacts and vertebrate fossils found in these deposits accumulated at times following deposition of the lacustrine clay, probably concurrent with pedogenesis. The limnological conditions associated with initial clay deposition, therefore, preceded hominin occupation of the exposed surfaces.

Comment on “Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift” by R.B. Owen, R. Potts, A.K. Behrensmeyer and P. Ditchfield [Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 17–37

Owen et al. [Owen, R.B., Potts, R., Behrensmeyer, A.K., Ditchfield, P., 2008, Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley. Palaeogeography, Palaeoclimatology, Palaeoecology, 269, 17–37], Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift, Palaeogeography Palaeoclimatology Palaeoecology, 269, 17–37) argued that diatom assemblage variations in the Olorgesailie Formation indicate considerable environmental instability with both wetter and drier periods, contradicting the proposed period of lake stability and wet climatic conditions between ca. 1.1 and 0.9 million years ago as proposed by Trauth et al. [Trauth, M.H., Maslin, M.A., Deino, A., Strecker, M.R., 2005. Late Cenozoic moisture history of East Africa. Science 309, 2051–2053., Trauth, M.H., Maslin, M.A., Deino, A., Bergner, A.G.N., Dühnforth, M., Strecker, M.R., 2007. High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution. Journal of Human Evolution 53, 475–486]. Contrary to the interpretation of our work by Owen et al. [Owen, R.B., Potts, R., Behrensmeyer, A.K., Ditchfield, P., 2008, we never said that the proposed periods of large lakes were characterized by stable conditions.

Reply to the comment on “Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley” by R.B. Owen, R. Potts, A.K. Behrensmeyer and P. Ditchfield [Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 17–37

We welcome the reply by Trauth and Maslin (Comment on “Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley” by R.B. Owen, R. Potts, A.K. Behrensmeyer and P. Ditchfield [Palaeogeography, Palaeoclimatology, Palaeoecology 269 (2008) 17–37], Palaeogeography, Palaeoclimatology, Palaeoecology, 2009-this issue) to Owen et al. (2008, Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley, Palaeogeography, Palaeoclimatology, Palaeoecology, 269, 17–37) and accept the clarifications made concerning their earlier work. However, we also note some of the differences between their broad characterization of the Olorgesailie record and our more detailed approach.

Wetland sedimentation and associated diatoms in the Pleistocene Olorgesailie Basin, southern Kenya Rift Valley

The Olorgesailie Basin in the southern Kenya Rift Valley contains a variety of lacustrine, wetland and terrestrial facies that were laid down during the last 1.2 million years. This study integrates diatom, ichnofossil, and rhizolith (silica and carbonate) evidence in order to identify and characterize shallow wetland deposition at the top (Member 14; ca. 493 ka) of the Olorgesailie Formation. A key feature of these deposits is the presence of facies with aquatic indicators (diatoms) that are overprinted by ichnofossils and rhizoliths that suggest drier conditions. Diatom floras include several fresh water Aulacoseira spp. and saline indicators such as Thalassiosira rudolfi, T. faurii, Cyclotella meneghiniana and Anomoeoneis sphaerophora. The main indicators of shallow fresh to mildly saline waters include Epithemia argus, Rhopalodia gibberula, Encyonema muelleri, and Synedra spp. Three main suites of ichnofossils indicate varying water table depths during their formation. These consist of 1) horizontal tunnels that suggest saturated substrates; 2) vertical burrows formed in substrates with relatively lower water tables; and 3) termite, hymenopteran, and beetle ichnofossils that indicate dry substrates. Member 14 of the Olorgesailie Formation also contains carbonate and siliceous rhizoliths, which occur both together and separately. Four possible models are suggested to explain the silica and carbonate rhizolith formation: 1) a complex of spring-fed marshes and dry areas; 2) ephemeral wetlands and floodplains; 3) wetlands with abundant organic decay, and 4) initial carbonate rhizoliths that were replaced by silica. Preservation of cellular structures and the reworking and incorporation of rhizoliths in overlying younger strata suggest that they formed early during diagenesis.

Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

The Olorgesailie Formation is comprised of lacustrine, volcaniclastic and alluvial sediments that formed in the southern Kenya Rift between about 1.2 million and 490,000 years ago. Diatoms are common in much of the sequence and preserve a record of environmental change within the basin. A high-resolution diatom stratigraphy has been developed for these deposits. The data document the presence of freshwater and saline lakes as well as wetlands. Transfer functions indicate that these water bodies ranged in conductivity between about 200–20,000 μS cm − 1 , with pH varying between about 7.5 and 10.3. Pedogenesis affected multiple horizons within the succession, reflecting periods of emergence that encompass much of the time represented by the Olorgesailie Formation. A variety of other sedimentological indicators (carbonates, rhizoliths, mudcracks, erosional channels) also record periods when the sampled portions of the basin were dominated by terrestrial conditions. Stone tools are common at several levels, indicating the use of the area by hominins. Lakes and wetlands were potentially usable as sources of potable water by hominins for part of the basin history, but at other times were undrinkable. Other water sources (springs, rivers) would have been necessary during these periods. Paleoenvironments in the basin were complex and changed frequently with time. Such shifts in resources and habitat distribution during Olorgesailie Formation time seem likely to have influenced the behavior and evolution of local plant and animal populations, including Homo.

High-resolution records of location and stratigraphic provenance from the rare earth element composition of fossil bones

Bone apatite acts as a natural, timed sampling device, scavenging trace elements from local pore waters over timescales of ca. 1–50 ka. The rare earth element (REE) and U/Th composition of fossil bones reflects associated pore water compositions during the period of recrystallisation. The REE composition of fossil bones is controlled by partitioning of REE between pore waters and particle surfaces, and the REE composition of fossil bones reflects the REE composition of pore waters which vary spatially and temporally. Light REE are preferentially sorped onto particle surfaces, thus the high La/Yb values seen in many bones from coastal marine and aeolian environments are best explained by release of REE from light REE-enriched particles to local pore waters and subsequent immobilisation in recrystallising bones. The REE compositions of bones recovered from pedogenically altered diatomite sediments of the Olorgesailie Formation of southern Kenya vary over spatial scales of less than 10 m. Location accounts for 48% of the observed variation in bone chemistry and bones recovered from eight discrete excavations within the same time-equivalent stratigraphic layer can be assigned to their excavation location with >70% accuracy based on a discriminant analysis of REE, U, and Th composition. Despite this within-layer variation, bones recovered from different stratigraphic horizons within the Olorgesailie Formation can also be distinguished on the basis of their trace element composition. Bones recovered from four stratigraphic horizons spanning ca. 0.5 million years were assigned to their correct stratigraphic layer with >90% accuracy. Where sedimentological conditions are favourable, the trace element composition of fossil bone may be used to test stratigraphic provenance and burial location in excavated bone with a temporal resolution of <10 ka and a spatial resolution of <10 m. The trace element composition of fossil bone may also be used to investigate the accumulation history of vertebrate assemblages and to reconstruct pore water variability across land surfaces.

Origin of Reddened and Melted Zones in Pleistocene Sediments of the Olorgesailie Basin, Southern Kenya Rift

Hardened red zones and rare melt rocks are puzzling and rare features in the Middle Pleistocene sediments of the Olorgesailie Formation of the southern Kenya Rift. We propose that combustion metamorphism and melting produced these zones. This is evidenced by field observations, chemical composition, X-ray diffraction and melting experiments. Plant-rich beds and disbursed plant matter served as fuel. Probably lightning generated surface fires that ignited carbonaceous beds exposed in gullies. The burning then proceeded underground. Melt rocks and red thermally metamorphosed zones within beds distinguish combustion metamorphism from reddening of paleosols by surface vegetation fires. Diagenetic oxidation has removed much of the carbonaceous material from the paleosols and marsh deposits, except where they are included in dense metasediments. Extraordinarily high temperatures of about 1200°C, which are essential to producing the melt rocks, required plant matter buildup, burial of the plant fuel under at most a few metres of sediment, and then rapid burning under dry conditions. This sequence of events points to one or more extreme droughts between 601,000 and 493,000 years ago, the period represented by upper Member 12 and Member 13 of the Olorgesailie Formation. Combustion metamorphism was not recognized in early studies at Olorgesailie, partly because it is not a widely taught nor published subject. Other Pleistocene East African basins and similar sites may contain as yet unrecognized products of combustion metamorphism that may prove of paleoclimatological importance.

Paleolandscape variation and Early Pleistocene hominid activities: Members 1 and 7, Olorgesailie Formation, Kenya

Grass woven baskets is a famous feature of Gumugou cemetery (3800 BP), an early Bronze Age necropolis in Tarim basin, northwestern China. They were suggested to be used for storage of liquid foods, for example gruel, ephedra soup or milk, however this notion has not been supported by analytical facts. We applied comprehensive multi-technique analytical approach, including FT-IR, proteomics, GC–MS and starch grains analyses to characterize minute amount of dried organic film preserved on the inner surface of one of the Gumugou grass woven baskets. The analysis revealed the presence of Bovidae milk proteins and traces of lactic acid bacteria. Together with the relative abundance of detected caseins and whey proteins it suggested that the organic deposit in the woven basket was strained sour milk. No ephedra biomarkers or starch grains were found. The protein composition of Gumugou sample differed from solid dairy products recently discovered at the nearby Xiaohe Cemetery. We speculate that the alterations in local dairy technologies might be the result of two processes: climatic changes in the region of Tarim basin and admix of early European settlers with lactose-intolerant Eastern ethnic group. European ancestry of Gumugou inhabitants suggested that the milking practice in the Tarim region was promoted by the eastward migration of European population which was predominantly lactose–persistent. Xiaohe site (3600 BP) was inhabited by a West-East admixed community for whom solid dairy products became a dietary requirement that offered its inhabitants an advantage in better adaptation to the worsening climate.

Paleolandscape variation and Early Pleistocene hominid activities: Members 1 and 7, Olorgesailie Formation, Kenya

Paleolandscape research tests for variation in the spatial distribution of hominid artefacts and establishes the association of hominid activities with paleoenvironmental features over distances of 100s to 1000s of meters. This approach requires (1) precise definition of narrow stratigraphic intervals based on sedimentary criteria that can be documented over a broad area, and (2) excavation of these intervals in order to establish taphonomic and paleoenvironmental contexts. In this report, excavations of three target intervals within the early Pleistocene deposits (992 to 780ka) of the Olorgesailie basin are described. Assessment of time-averaging and paleolandscape structure shows that each target interval represents a relatively brief period (≤1000yrs) and exhibits a unique distribution of environmental features (e.g., topographic gradients, channels, soil development). Stone artefacts and fossilized animal bones are distributed nonrandomly in each interval, and include clusters that were five to 293 times more densely concentrated than the laterally equivalent background scatter. A paleosol in upper Member 1 preserves a relatively continuous distribution of artefacts and fossils, in contrast with the more patchy distribution in two intervals of lower Member 7. We infer that the difference between the two members reflects a real variation in hominid land use—either a response to local environmental differences or perhaps a change through time in hominid interaction with the environment. By expanding the comparative analysis to diverse basins, it should be possible to test for broader evolutionary change in hominid activities. Examples drawn from East African Pliocene and early Pleistocene sites suggest that evolutionary change in land use entailed (1) wider ranging of hominids and longer distances of stone transport, (2) expansion of tool-assisted behaviors to a wider diversity of environmental settings, and (3) more strongly focused placement of particular artefact forms (e.g., bifaces) in different areas of the landscape in response to specific environmental features, such as lava outcrops, stream channels, and lake margins. \\

Age-probability spectra for examination of single-crystal [formula omitted] dating results: Examples from Olorgesailie, southern Kenya Rift

Age-probability spectra are employed in the analysis of single-crystal, 40 Ar 39 Ar age data obtained from volcanic strata of the Olorgesailie Formation, southern Kenya Rift. These spectra provide a powerful means of graphically representing a suite of age data. We believe this representation is an improvement over the box histograms that are often used because age-probability spectra avoid the problem of the choice of an appropriate bin width, specifically incorporate variable analytical uncertainties, and carry more visual detail.

Single‐crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift

Single‐crystal laser fusion 40Ar/39 Ar analyses and several conventional bulk fusion 40K‐40 Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains an ash flow dated at ∼1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single‐crystal 40Ar/39 Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. We have presented a computer‐modeling procedure based on an inverse‐isochron analysis that promotes a more objective approach to trimming 40Ar/39 Ar isotope data sets of this type.

Increased age estimate for the Lower Palaeolithic hominid site at Olorgesailie, Kenya

The Acheulean is the major stone-tool tradition of Africa, and of much of Asia and Europe, which lasted in Africa from ∼1.5 to 0.15 Myr BP. It is associated with Homo erectus at some sites, and is characterized especially by hand-axes. Cultural development within the Acheulean has been very hard to assess because so few sites can be dated. Here we report potassium–argon dates and geochemical correlation of tephra layers at Olorgesailie—a key Acheulean site in the Kenya rift valley. The results suggest that the upper part of the Olorgesailie Formation lies between 0.6 and 0.74 Myr BP, and that the maximum age is ∼0.9 Myr. Most of the artefact-bearing levels at this site are thus >0.70 Myr old, significantly older than formerly thought.

Red Beds of Middle Pleistocene Olorgesailie Formation, Kenya: ABSTRACT

Middle Pleistocene lake sediments of the Olorgesailie Formation from the East African Rift Valley of Kenya contain red beds, with colors ranging from moderate orange pink to dark reddish brown (Munsell color chart). Two distinct mechanisms of hematite formation, distinguished by thin section and scanning electron microscope study, have produced the red sediments. In the first mechanism, hematite was formed in situ by dehydration of limonitic minerals. The limonitic minerals formed in soils of the source area and were transported to the depositional site mainly by adhering to clay particles. The elevated pH of the depositional environment accelerated the rate of hematite formation, producing ultrafine red coating on the clays. Red sediment formed by this mechanism occurs both as an undisturbed bed and as a reworked intraclast conglomerate. In the second mechanism, hematite is precipitated from ground water, possibly at elevated temperatures produced by the interaction of basaltic magma with water-saturated sediment. The hematite occurs in three forms: as cement within diatomaceous clay, as replaced or stained plant material, and within siliceous sinters. The sinters are red, have a pumice-like texture, and include diatoms that show effects of dissolution. These characteristics indicate formation of the sinters by boiling of near-surface water-saturated sediment. Red beds formed by this mechanism are redeposited and each of the three hematite forms dominates in specific localities. End_of_Article - Last_Page 934------------