Great Rift Valley and Kenyan Cenozoic Stratigraphy http://earth.imagico.de/comment.php?site=eafrica Rift Valleys   Form in regions of extensional stress As the lithosphere and crust pull apart, the area between sinks down.     This is called the graben The graben is flanked by steep boundaries They start when plumes form beneath continents 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     6400 km long and 48-64 km wide Stretches from southern Turkey to Mozambique Visible from the Moon Began 30 Mya in Gulf of Aden  East Africa Rift Valley began 14 Mya http://neuropolitics.org/defaultjun06.asp East Africa Rift Valley    Valley is filled with interbeded volcanic rock and sediments Ethiopia’s Afar Triangle is triple junction of the African, Arabian, and Somalian plates Early hominid fossils have been found here http://www.answers.com/topic/great-rift-valley  Place of human origins Kenyan Stratigraphy - Marine Age Formation M Pleistocene Mombasa Island Kilindi Sands Components Pliocene Marafa Beds L Miocene Baratum Beds White sand, red sand, clay, pebbles marls, sand, clay, conglomerates, limestone limestone Fundi Isa Beds Crags, calcareous sand Kenyan Stratigraphy - Continental Age Formation M Pleistocene Kanjera Rawi Kanam Kiaherra Series L Miocene Hiwegei Components Ashes, clay, limestone Clay, gravel, volcanic SS Clay, gravel, ash tuffs, limestone, grit, pisolithic earth Red clay, boulders, limestone Kathawant Point Dull, flaggy mudstones Kathawanage mudstone, sandstone Mpesida Beds  Miocene age   Volcanoclastic horizons interbedded with trachytic flows Examination of three sites helped reveal Miocene conditions, time when humans were evolving  Chimps and humans split 6-5 Mya, Miocene starts ~ 5 Mya Mpesida Beds From Kingston et al. 2002  Site 1:    Pyroclastic rock is massive, unsorted, entirely ash contains silicified wood Fossil trees are up to 50 m tall  Indicative of a wet forest Mpesida Beds  Site 2:  Pyroclastic layer that is grey to buff colored, coarse sandstone     Vertebrate fossils confined to this layer Shows new families establishing themselves in SubSaharan Africa Savannah and forest environments Site 3:  Lacustrine facies of clays, silts, fine sandstone interbedded with diatomite  A hiatus from volcanic activity Fossil Mammals of Great Rift Valley Creodont2 Deinotherium1 Chalicotherium3 1) http://www.answers.com/topic/deinotherium 2) http://www.ucmp.berkeley.edu/mammal/eutheria/creodonta.html 3) http://paleo.amnh.org/bjburger/fossilmammal/u3.html Fossil Mammals of Great Rift Valley Equus2 Hippopotamus1 Leporid3 1) http://www.nature-wildlife.com/girskel11.html 2) http://www.crockerfarm.org/ac/rm02/pratt99/pratt99mammals.htm 3) http://www.nature-wildlife.com/girskel08.html Fossil Mammals of Great Rift Valley Australopithecus afarensis2 Orrorin turgenensis1 1) http://www.oursci.org/magazine/200108/010814.htm 2) http://www.achievement.org/autodoc/photocredit/achievers/joh1-021 3) http://en.wikipedia.org/wiki/Homo_rudolfensis Homo rudolfensis3