SEDIMENTOLOGICAL AND PALEOENVIRONMENTAL IMPLICATIONS OF THE NGORORA FORMATION, TUGEN HILLS (CENTRAL KENYA RIFT)

The influence of tectonics versus climate on Miocene lacustrine environments are not well understood in the Central Kenya Rift of the East African Rift System. The Ngorora Formation of the Tugen Hills within this portion of the rift preserves highly fossiliferous sediments of various depositional environments for the middle to late Miocene (9-12 Ma). The exceptional preserved fish fossils and their paleoenvironments remain largely uninvestigated. The objective of this study is the sedimentological reconstruction of Ngorora Formation paleoenvironments in the Kapkiamu Graben and bordering Kerio Valley in the Central Kenya Rift, as well as the investigation of the cause(s) for the fish mortality levels preserved in the unit.

In order to study the paleoenvironment of the Ngorora Formation, 15 sedimentary profiles were logged at 11 different localities. In addition >120 sediment samples were collected for further XRD and palynological analysis, and >200 articulated fish fossils were excavated for further study.

Three different lakes at three different locations could be reconstructed. Furthermore it was possible to correlate the lake development with the classified member of the Ngorora Formation. Stratigraphic analysis revealed that several fluctuations between lacustrine and fluvial depositional environments occurred within one lake system. Generally terrestrial stages remain rare. In addition, the Ngorora Formation experienced multiple episodes of explosive volcanism that punctuated many intervals of sedimentary deposition.

The sedimentological data indicate a close relationship between this volcanism and fish mortality. Moreover the pollen data in correlation with the rifting stages reveals that the main control of lake evolution was tectonic rather than climatic. Notable is that no complete lacustrine or fluviatile succession could be observed. This makes assumptions about average sedimentation rates between the different lacustrine episodes difficult.