The geology of the Nasaken area, South Turkana, Kenya and the petrochemistry of the Pliocene volcanic rocks

The geology of a previously unmapped 170 square miles (440 sq. kms.) area of South Turkana, lying within the Kenya rift, is described. A contoured and coloured geological map is presented. The stratigraphical succession established which totals about 6000 feet (18OO m.) in thickness and consists largely of volcanic rocks, ranges in age from about 25 to 4 million years b.p. New potassium/argon age determinations on some of the lavas are presented and reinforce the established stratigraphy . Particular attention is paid to the Pliocene succession which consists of a thick, widespread basalt formation overlain by a number of trachyte volcanoes. The morphology of the multicentred trachyte volcanoes suggests that they are a type not described before. The potrography of the lavas of the Nasaken area is described in detail. Chemical analyses of rocks and minerals are presented and the petrogenesis of the Pliocene lavas is discussed withparticular reference to the relationship between the basalts and the voluminous trachytes and the origin of the chemical variation within the trachytes. The trace-element geochemistry of the Pliocene lavas is compared with the geochemistry of lavas from other areas within the East African rift system.