| Description |
Major and trace element analyses and mineralogical data are presented for selected Plio-Pleistocene tuffs from the Shungura Formation which are useful in documenting hominid and vertebrate evolution in East Africa. The mainly fluvially deposited, hydrated tuffs are indicated as being comendites and pantellerites in all respects except for low Na20 and K20 values for which leaching is shown to be responsible. Mineral assemblages uncommon among conmedites and pantellerties are portrayed by the occurrence of chevkinite, manganiferous ilmenite, orthopyroxene, and plagioclase. Separating the tuffs into genetically related groups is difficult, due to similar trace element abundances and the evolutionary nature of volcanic chemistries. Similar chemistries may indicate that they all originate from the same general source area. Correlations on chemical bases are demonstrated between tuffs from the Shungura Formation and tuffs from both the Usno and Koobi Fora formations which are situated nearby in the Lake Turkana (formerly Lake Rudolf) area. The proposed source area of these tuffs is near Soddu on the western margin of the Ethiopian Rift. This was concluded from the following onsiderations: 1) the Plio-Pleistocene peralkaline rocks of the Ethiopian Rift are chemically and mineralogically very simila to the Shungura tuffs, and 2) tuffs common to both the Shungura and Koobi Fora Formations must have a source that is accessible to both the drainages responsible for transporting these tuffs to their present location. |
