Lake Turkana - Geology

Geology

Lake Turkana is an East African Rift feature. A rift is a weak place in the Earth's crust due to the separation of two tectonic plates, often accompanied by a graben, or trough, in which lake water can collect. The rift began when East Africa, impelled by currents in the mantle, began separating from the rest of Africa, moving to the northeast. Currently, the graben is 320 km wide in the north of the lake, 170 km in the south. This rift is one of two, and is called the Great or Eastern Rift. There is another to the west, the Western Rift.

Lake Turkana is a unique feature of the East African landscape. Besides being a permanent desert sea, it is the only sea that retains the waters originating from two separate catchment areas of the Nile. The Lake Turkana drainage basin draws its waters mainly from Kenya Highlands and Ethiopian Highlands.

The basement rocks of the region have been dated by two analytical determinations to 522 and 510 million years ago (mya). No rift was in the offing at that time. A rift is signalled by volcanic activity through the weakened crust. The oldest volcanic activity of the region occurred in the Nabwal Hills northeast of Turkana and is dated to 34.8 mya in the late Eocene.

The visible tectonic features of the region result from extensive extrusions of basalt over the Turkana-Omo basin in the window 4.18-3.99 mya. These are called the Gombe Group Basalts. They are subdivided into the Mursi Basalts and the Gombi Basalts.

The two latter basalts are identified as the outcrops forming the rocky mountains and badlands around the lake. In the Omo portion of the basin, of the Mursi Basalts, the Mursi Formation is on the west side of the Omo, the Nkalabong on the Omo, and the Usno and Shungura east of the Omo. Probably the best known of the formations are the Koobi Fora on the east side of Turkana and the Nachukui on the west.

Short-term fluctuations in lake level combined with periodic volcanic ash spewings over the region have resulted in a fortuitous layering of the ground cover over the basal rocks. These horizons can be dated more precisely by chemical analysis of the tuff. As this region is believed to have been an evolutionary nest of hominins, the dates are important for generating a diachronic array of fossils, both hominoid and nonhominoid. Many thousands have been excavated.

Terraces representing ancient shores are visible in the Turkana Basin. The highest is 75 m above the surface of the lake (only approximate, as the lake level fluctuates), which was current about 9500 years ago, at the end of the Pleistocene. It is generally theorized that Turkana was part of the upper Nile system at that time, connecting to Lake Baringo at the southern end and the White Nile in the north, and that volcanic land adjustments severed the connection. Such a hypothesis explains the Nile species in the lake, such as the crocodiles and the Nile perch.