| Description |
The KBS and the upper Burgi Members of the Koobi Fora Formation are exposed on the eastern shore of Lake Turkana in northern Kenya ~11 km southeast of Koobi Fora in paleontologic collecting areas 106, 107 and 109. The upper Burgi Member is only exposed in very low lying outcrops in the southern portion of the study area therefore a stratigraphic section for this unit was not established. All other strata in the areas listed belong to the KBS Member and total ~145 m of section. During KBS Member time a large lake was present in this region, and most of the KBS Member in this area was deposited in nearshore lacustrine settings as lake level fluctuated. The most prominent mappable beds in the study area are mollusc-packed sandstones, also called arenaceous bioclastic carbonate beds (ABC), a term that is also applied to algal beds. These beds formed in the nearshore environment of ancient Lake Turkana. They are poorly- to well-cemented and form easily mapped pavements that provide both stratigraphic and structural control. Four tuffs known from previous studies have been identified within the KBS Member in the study area: the KBS Tuff, the Brown Tuff, the Orange Tuff, and the Asa Tuff. In addition to these named tuffs, another tuff, K09-572, lies about 5 m above the Asa Tuff. The Asa Tuff has not been described elsewhere and is defined here. The KBS Tuff is well dated at 1.869 ± 0.021 Ma, but direct ages have not been measured on any of the other tuffs mentioned. Although this is the case, the Orange Tuff has been correlated with Tuff J of the Shungura Formation, and also with the Kayle Tuff-1 at Konso in Ethiopia where it lies 2.5 m below the Kayle Tuff-2, which has been dated at 1.735 ± 0.03 Ma. In the Nachukui Formation, west of Lake Turkana, the Asa Tuff lies below the Morutot Tuff, hence its age is between 1.735 and 1.61 Ma. Tuff J is known to lie above the top of the Olduvai Event, for which the current age estimate is 1.778 Ma. Other workers relate fluctuations in the level of Lake Turkana to Milankovitch cycles which, from studies of sapropels in the Mediterranean Sea, are known to drive the intensity of monsoonal rains over the Ethiopian highlands. These highlands are the headwaters of the Omo River, which feeds Lake Turkana. Thus any change in precipitation affects the level of Lake Turkana. It has been proposed that ABCs only form during dry periods of time in near shore environments. Milankovitch cycles have been proposed to control the formation of ABCs, because in paleontological collection Areas 102 and 103, which lie north of the study area, available chronological control provided a reasonable fit to the number and approximate age of these units. Additional age control provided by the Orange Tuff suggests that at least in Areas 106, 107, and 109, the number of ABCs present within a local section is location dependent, and cannot obviously be correlated with Milankovitch cycles. |
