Great Rift Valley and Kenyan Cenozoic Stratigraphy

Rift Valleys Form in regions of extensional stress Form in regions of extensional stress As the lithosphere and crust pull apart, the area between sinks down. As the lithosphere and crust pull apart, the area between sinks down. This is called the graben This is called the graben The graben is flanked by steep boundaries The graben is flanked by steep boundaries They start when plumes form beneath continents They start when plumes form beneath continents The lighter crust cracks, and subduction zones continue to pull the crust apart The lighter crust cracks, and subduction zones continue to pull the crust apart

Rift Valley Formation

Great Rift Valley The largest rift valley in world The largest rift valley in world 6400 km long and km wide 6400 km long and km wide Stretches from southern Turkey to Mozambique Stretches from southern Turkey to Mozambique Visible from the Moon Visible from the Moon Began 30 Mya in Gulf of Aden Began 30 Mya in Gulf of Aden East Africa Rift Valley began 14 Mya East Africa Rift Valley began 14 Mya

East Africa Rift Valley Valley is filled with interbeded volcanic rock and sediments Valley is filled with interbeded volcanic rock and sediments Ethiopia’s Afar Triangle is triple junction of the African, Arabian, and Somalian plates Ethiopia’s Afar Triangle is triple junction of the African, Arabian, and Somalian plates Early hominid fossils have been found here Early hominid fossils have been found here Place of human origins Place of human origins

Kenyan Stratigraphy - Marine AgeFormationComponents M Pleistocene Mombasa Island Crags, calcareous sand Kilindi Sands Pliocene Marafa Beds White sand, red sand, clay, pebbles L Miocene Baratum Beds marls, sand, clay, conglomerates, limestone Fundi Isa Beds limestone

Kenyan Stratigraphy - Continental AgeFormationComponents M Pleistocene Kanjera Ashes, clay, limestone Rawi Clay, gravel, volcanic SS Kanam Clay, gravel, ash L Miocene Kiaherra Series tuffs, limestone, grit, pisolithic earth Hiwegei Red clay, boulders, limestone Kathawant Point Dull, flaggy mudstones Kathawanage mudstone, sandstone

Mpesida Beds Miocene age Miocene age Volcanoclastic horizons interbedded with trachytic flows Volcanoclastic horizons interbedded with trachytic flows Examination of three sites helped reveal Miocene conditions, time when humans were evolving Examination of three sites helped reveal Miocene conditions, time when humans were evolving Chimps and humans split 6-5 Mya, Miocene starts ~ 5 Mya Chimps and humans split 6-5 Mya, Miocene starts ~ 5 Mya

Mpesida Beds Site 1: Site 1: Pyroclastic rock is massive, unsorted, entirely ash Pyroclastic rock is massive, unsorted, entirely ash contains silicified wood contains silicified wood Fossil trees are up to 50 m tall Fossil trees are up to 50 m tall Indicative of a wet forest Indicative of a wet forest From Kingston et al. 2002

Mpesida Beds Site 2: Site 2: Pyroclastic layer that is grey to buff colored, coarse sandstone Pyroclastic layer that is grey to buff colored, coarse sandstone Vertebrate fossils confined to this layer Vertebrate fossils confined to this layer Shows new families establishing themselves in Sub- Saharan Africa Shows new families establishing themselves in Sub- Saharan Africa Savannah and forest environments Savannah and forest environments Site 3: Site 3: Lacustrine facies of clays, silts, fine sandstone interbedded with diatomite Lacustrine facies of clays, silts, fine sandstone interbedded with diatomite A hiatus from volcanic activity A hiatus from volcanic activity

Fossil Mammals of Great Rift Valley 1) 3) 2) Creodont 2 Chalicotherium 3 Deinotherium 1

Fossil Mammals of Great Rift Valley 3) 2) 1) Hippopotamus 1 Leporid 3 Equus 2

Fossil Mammals of Great Rift Valley 2) 3) 1) Orrorin turgenensis 1 Australopithecus afarensis 2 Homo rudolfensis 3