Methods of dating

Journal ArticleThe fossil record of primates begins in the latest Cretaceous period, so primate palaeontologists are interested in techniques of dating applicable over the past 70 million years. Fossil bones themselves are rarely datable with any precision, and these are mainly of late Pleistocene or Holocene age. In general, it is the geological materials with which they are found that are dated. For this reason, dating usually begins with an attempt to order past events, and to relate fossils to rock layers that can themselves be dated. Once this is done, the ages of fossils can be estimated by determining the ages of rocks that lie lower and higher in a stratigraphic section

Dating

Journal ArticleThere are basically three lines of investigation that are involved in geochronology ? the dating of rocks and other earth materials. First, there are the physical and chemical dating methods which give us numerical estimates of age. Second, there is the reconstruction of the order of events in sections of sedimentary rocks ? those laid down by water and wind. What is done is to find rocks which can be dated within a sequence of strata in order to establish the ages of fossils found within it, since the fossils themselves are not directly datable. The third line of investigation is called geologic correlation, in which one establishes that two or more events happened at the same time, or very nearly so

Geological development of the Omo-Turkana Basin during the pliocene and pleistocene epochs

Journal ArticlePliocene and Pleistocene deposits of the Omo-Turkana Basin extend from 6?N latitude to about 2?N latitude (-450 km), and laterally from about 35.5?E to 36.5?E. Sediments are brought to the basin by the Omo River which rises in Ethiopia, and by the Turkwel and Kerio Rivers which rise in Kenya and Uganda. Pliocene deposition began -4.3 Ma ago, and is recorded in the Mursi, Nkalabong, Usno, and Shungura Formations in southern Ethiopia, and by the Koobi Fora, Nachukui, Lothagam, and Kanapoi Formations in neighboring Kenya. The composite stratigraphic thickness of exposed Pliocene strata is on the order of 400 m. The stratigraphy of the basin is well integrated through tephrostratigraphic correlations, and deposits are well-dated through 40Ar/39Ar and K/Ar analyses of lavas and reworked ash layers, In addition the paleontological record in the basin is tied firmly to marine records in the Gulf of Aden. Paleoclimatic changes are recorded not only through isotopic compositions of paleosols but also in the distribution of strata of particular ages. Pleistocene strata have a composite thickness of about 400 m, but there is a gap in the record from about 0.7 Ma to 0.2 Ma. Seismic work in the greater basin suggests that a more complete record exists beneath Lake Turkana, where combined Pliocene and Pleistocene strata appear to be -4000 m thick. Pleistocene history is recorded in the Shungura, Koobi Fora, Nachukui, Kibish, and Galana Boi Formations. Of particular interest is that the locus of deposition in the basin appears to have controlled to some extent by the same climatic factors that are responsible for development of sapropels in the eastern Mediterranean Sea. For example, deposition of the Kibish Formation near the northern end of the basin is episodic, and new dates suggest that each episode of deposition can be correlated with particular sapropels in the Mediterranean sequence. As the sapropels are related to Nile discharge, this is understandable because the Omo River shares a drainage divide with the Blue Nile. Tectonic and volcanic events have also played a role in the development of the basin during the Pliocene and Pleistocene

Observations on the stratigraphy and rediometric age of the omo beds, lower omo basin, southern Ethiopia

Journal ArticleDuring September and October, 1966, the author measured a stratigraphic section for a portion of the Omo Beds, as exposed north of Kalam, Ethiopia. Additional field observations were also made at the time in other parts of the lower Omo Basin. Samples were collected for analysis and for radiometric age determination, and the first potassium-argon measurements were made on feldspars from tuffs in this succession in late 1966 and early 1967. As part of the work of the University of Chicago contingent of the Omo Research Expedition, further field work was possible in the main exposures during the summer of 1967, with additional units being added to the stratigraphic section recorded previously. Materials suitable for isotopic age determination were recollected and further potassium-argon measurements were made during the fall of 1967. These results substantiated and extended the earlier findings

A reappraisal of the geomagnetic polarity time scale to 4 MA using data from the turkana basin, East Africa

Journal ArticleRecalibration of the Pliocene and early Pleistocene geomagnetic time scale using the K-Ar dated fluvial sequence of the Turkana Basin in East Africa agrees with calibrations based on astronomical calculations. Ages estimated here are: Olduvai Subchron, 1.78-1.96 Ma; Reunion Subchrons, 2.11-2.15 Ma and 2.19-2.27 Ma; Matuyama-Gauss boundary, 2.60 Ma; Kaena Subchron 3.02-3.09 Ma; Mammoth Subchron, 3.21-3.29 Ma; Gauss-Gilbert boundary, 3.57 Ma

Timing of volcanism and evolution of the northern Kenya Rift

Journal ArticleThe northern Kenya Rift is bounded on the west by uplands of Turkana which comprise horst-like blocks that include metamorphic basement rocks, locally overlain unconformably by the Cretaceous Lubur Sandstone, in turn overlain by predominantly volcanic sequences in which relatively thin sedimentary packages occur. Amphibolite facies crystalline rocks of the basement yield Early Palaeozoic K-Ar cooling ages reflecting the Pan-African Orogeny. Volcanism in Turkana was initiated through voluminous eruptions of transitional tholeiitic basalts commencing about 36 Ma ago in the Late Eocene, with some evidence for concomitant rhyolitic volcanism. Volcanism became dominantly rhyolitic in the interval from about 27 to 23 Ma ago, but remained bimodal as basaltic lavas are also known from this period. From about 19 to 15 Ma or younger, basaltic volcanism again dominated, often alkaline in nature, with thin but significant sedimentary sequences interleaved that have yielded important vertebrate faunal assemblages. Parallels exist between the volcanic history recorded in Turkana and that found in the Nabwal Hills east of Lake Turkana. In the southern Turkana region, oil exploration by seismic methods and deep drill holes has shown the existence of northerly-trending half-graben with up to 7 km of fill, and that these developed from at least Oligocene and possibly Late Eocene times. This suggests that the widespread basaltic volcanism at about 36 Ma ago (Late Eocene) heralds an earlier initiation of the Kenya Rift in northern Kenya than most workers have previously suggested

Gombe group basalts and initiation of pliocene deposition in the Turkana depression, northern Kenya and southern Ethiopia

Journal ArticleA little before 4 Ma ago, deposition of Pliocene and Pleistocene strata described as the Omo Group began in the Turkana and Omo basins of northern Kenya and southern Ethiopia. Soon after, basaltic magma erupted as thin lava flows, and intruded as dykes into the oldest Pliocene strata of the basin. These flows and intrusions are similar petrographically and geochemically, and mark a basaltic magmatic event spanning latitudes from 2?45' N to 6?45' N at a longitude of about 36? E. By 3.94 Ma, this basaltic magmatic activity had ceased. Previous researchers used these lavas as an important seismic marker in their study of the southern part of the Turkana Basin. Subsequent volcanic eruptions formed North, Central and South islands in Lake Turkana, and the Korath Range in southern Ethiopia. Thus there was a hiatus in basaltic magmatic activity of nearly 4 Ma in the area presently occupied by Lake Turkana and the lower Omo Valley, although volcanism continued on the eastern margin of the basin. Here we review the field occurrences of these basalts, their distinctive petrography, composition, age and significance to Pliocene deposition in the basin

Correlation of tephra layers from the Western Rift Valley (Uganda) to the Turkana Basin (Ethiopia/Kenya) and the Gulf of Aden

Journal ArticleFour volcanic ashes have been mapped in Albertine Group strata (Warwire, Kaiso Village and Nyabusosi Formations) Western Rift Valley, Uganda. Three of these tuffs have geochemical fingerprints similar to tuffs in the Turkana Basin (Ethiopia, Kenya), in Afar (Ethiopia) and the Gulf of Aden, permitting correlation between sequences. Radioisotopic ages can be inferred for the faunas from the Albert Basin, including two of Gautier's Molluscan Associations which are the basis for fine scale biostratigraphic correlations between scattered outcrops of sediments within the Albert and Edward Basins in both the Uganda and Zaire sectors of the Western Rift Valley. The age of the Warwire Tuff, the Kyampanga Tuff and the Kagusa Tuff can be estimated at 3.6, 3.4 and 1.5 Ma

Pleistocene rhyolite of the mineral mountains, Utah geothermal and archeological significance

Journal Articl

The evolution of neogene terrestrial ecosystems in Europe

Journal ArticleThe Pliocene-Pleistocene chronology of hominid and other vertebrate evolution in East Africa is largely constrained by isotopic dating and regional intercorrelation of volcanic ash layers. Some eruptions were of sufficient magnitude or duration that their widespread tephra dispersal defines a series of dated marker horizons throughout the fossil-bearing sedimentary deposits of Tanzania, Uganda, Kenya, and Ethiopia (Brown, 1982; Feibel et al, 1989; Haileab&Brown, 1992,1994; Pickforderal, 1991; WoldeGabrieler al, 1994). Although many of the larger eruptive events have been dated directly the ages of many tephra are only constrained by indirect stratigraphic interpolation between dated levels. The geochcmica! compositions of volcanic glasses from each eruption are unique, providing a definitive means to establish broad tephrostratigraphic correlations linking the regional climatic, tectonic, and biologic histories of this distinctive archive of Earth history