Evidence for early Pleistocene Glaciation(s) in tropical Africa: Stratigraphy, Paleomagnetism, Paleosols and Paleoclimate of the Gorges Moraine System, Mount Kenya

Moraines marking an Early Pleistocene glaciation on Mt. Kenya are known from several valleys marking the lowermost extent of the Early Pleistocene Gorges Glaciation (2850 m a.s.l.). The lowermost paleosols at these sites formed either in till predating the Gorges Glaciation or in weathered phonolitic bedrock and related lavas, similar to the lithology forming the base of the Mt. Kenya volcanic series of Miocene/Pliocene age. Paleomagnetism and weathering characteristics have been used to refine the age of sediments assigned to the Gorges Glaciation. These deposits are normally magnetized but carry a persistent reversed overprint, suggesting that they were deposited during one of the normal subchrons within the Matuyama Reversed Chron. They are underlain either by a reversely magnetized weathered till (GOR 68), or weathered bedrock (GOR 64 and GOR 69), the latter exhibiting normal magnetization (Gauss?) with reversed overprint (Matuyama?). The sediments are overlain at all three sites by normally magnetized loesses and paleosols of presumable Brunhes age. The normal magnetization and reversed overprint recorded in sediments of the Gorges Glaciation most likely span a considerable portion of the Olduvai subchron (1.78-1.950 Ma.), which persisted for sufficient time to accommodate an extensive montane glaciation and prolonged period of weathering and soil formation. While the somewhat younger Jaramillo subchron cannot be ruled out, the extensive sediment and weathering record is more easily accommodated within the longer-lived Olduvai subchron. The characteristics of the lowermost buried paleosols and weathered bedrock substrate indicate wetter conditions prior to the onset of Pleistocene glaciation, a period that initially may have fostered a higher elevation forest cover of substantial spatial extent. These wetter conditions, punctuated with dry events, depict a progressive transition to alpine grassland at the start of the Quaternary. In light of new paleomagnetic and paleosol evidence, at least two tropical glaciations during the Matuyama Reversed Chron are documented from Mt. Kenya, suggesting this volcanic edifice had attained sufficient relief to form an ice mass and outlet glaciers, despite its equatorial latitude.