Sabarei And Allia Bay Areas

The large area bounded on the west by Lake Turkana, the north by the Ethiopian border and the south by the 3°N line of latitude is described under the names of the Sabarei and Allia Bay areas in reports by Key and Watkins (1988) and Wilkinson (1988). The more northerly Sabarei area extends much further eastwards than Allia Bay, the ground to the south of the more easterly part of Sabarei not yet having been described in detail, but the general geology is indicated on the 1:1,000,000 Geological Map of Kenya (1987). The greater part of the area, which includes two volcanic islands in Lake Turkana (see No. 085-00-03 - Central Island), is occupied by volcanic and some intrusive rocks, only along the western margin and in the southeast are there extensive areas of upper Miocene to Recent sedimentary rocks and minor Precambrian gneisses and marble. The volcanic rocks range from Oligocene to Recent in age and details of stratigraphy are given by Key and Watkins (1988), but correlation of the sequences west and east of the Lake Stephanie Rift and its lacustrine sediments has not yet been clearly made. Three distinct periods of Cenozoic volcanism can be distinguished in the Sabarei area: an early Oligocene to middle Miocene period of activity related to the rifting, which was succeeded after 6 million years by eruption of extensive plateau lavas with isolated central volcanoes. The final Quaternary volcanism was more localised with multi-centred trachytic and basaltic volcanoes. The earliest group comprises a wide range of lavas and pyroclastic rocks including basalt, ankaramite, hawaiite, trachyandesite, trachyte, phonolite and peralkaline rhyolite, many of them ignimbrites; there are also some tholeiites in the sequence. The plateau lavas are hawaiite, alkali basalt and quartz tholeiite. The third period of activity in the Sabarei area occurred in the extreme east where the Dukana volcanic rocks consist of basaltic breccias, ashes and tuffs. Fitch et al. (1975) described carbonate-rich rocks from within the Gum Dura pantelleritic ignimbrites as being ash-flow and air-fall deposits, which they refer to as carbonatites. However, Key and Watkins (1988) consider these rocks to be of pedogenic origin and/or the result of groundwater mineralisation in the vicinity of hot springs.The volcanic rocks of the Allia Bay area consist of alkali and sub-alkali basalts, basanite, trachyte, phonolite, tephrite and ignimbrite. The most alkaline rocks are the Jarigole phonolites, which occupy a large area immediately east of Lake Turkana, and include flows of phonolite, nephelinite and tephrite, phonolitic pyroclastics, swarms of dykes and several intrusive centres with which abundant pyroclastic rocks, including ash-flow tuffs and ignimbrites, are associated. The phonolites frequently contain glomerophorphyritic aggregates of sanidine and nepheline and microphenocrysts of nepheline, sanidine and analcime, in a groundmass including aegirine-augite and aenigmatite. Nephelinites comprise aegirine or ‘enstatite-augite’, nepheline, partly replaced by sodalite, and an opaque phase in a groundmass of the same minerals. Tephrites are distinguished by the presence of augite and plagioclase (An40-55) nepheline and aenigmatite; olivine may be present. The Hafari rhyolites which extend over a considerable area in the northeast of the Sabarei region include several ignimbrite units some of which are aegirine-bearing and a rock analysis proved to be pantelleritic (Key and Watkins, 1988); these peralkaline rocks may have emanated from the Jibisa ring complex (No. 085-00-04). The major intrusive centres in the Allia Bay area are Moiti-Longondoti, Kubi Algi, Sibilot and Shin, and in the Sabarei area the Jibisa ring complex, which is described separately (No. 085-00-04). The Allia Bay intrusive centres, all of which are associated with the Jarigole phonolites, have one or more of the following features: nepheline syenite, trachyte, an associated dyke swarm and abundant pyroclastic rocks. The largest centre, Moiti-Longondoti, is located on the eastern shore of Lake Turkana and comprises two breached craters the northern one of which is cut by a 2 km diameter nepheline syenite plug; there are numerous dykes of microfoyaite. The foyaite of the plug consists of about 90% orthoclase, a little oligoclase, interstitial colourless pyroxene plus aegirine-augite, aenigmatite and interstitial nepheline. Another nepheline syenite plug occurs at Shin while others are found over the Jarigole phonolite outcrop and there are numerous fine-grained nepheline syenite dykes.