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Geologic Map of Oldonyo Lengai (Oldoinyo Lengai) Volcano and Surroundings, Arusha Region, United Republic of Tanzania

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Geologic Map of Oldonyo Lengai (Oldoinyo Lengai) Volcano and Surroundings, Arusha Region, United Republic of Tanzania Geologic Map of Oldonyo Lengai (Oldoinyo Lengai) Volcano and Surroundings, Arusha Region, United Republic of Tanzania By David R. Sherrod, Masota M. Magigita, and Shimba Kwelwa Pamphlet to accompany Open-File Report 2013-1306 2013 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2013 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. Suggested citation: Sherrod, D.R., Magitita, M.M., and Kwelwa, S., 2013, Geologic map of Oldonyo Lengai (Oldoinyo Lengai) and surroundings, Arusha Region, United Republic of Tanzania: U.S. Geological Survey Open-File Report 2013-1306, pamphlet 65 p., 1 sheet, scale 1:50,000, with GIS database, http://dx.doi.org/10.3133/ofr20131306. ISSN 2331-1258 Contents Introduction . 1 Brief Geologic History of the Southernmost Lake Natron Basin . 2 Setting the Stage . 2 Enter the Youngest Volcano . 3 Debris-Avalanche Deposits . 3 Stratigraphic and Structural Features of the Natron Escarpment . 3 Natron Escarpment and Area West of It Expose Oldest Rocks in Map Area . 3 Mosonik Volcano . 3 Escarpment Volcanic and Sedimentary Rocks . 3 Volcanic Rocks . 4 Embalulu Sekenge . 6 Sedimentary Rocks in the Area of Mto Wa Mbagai . 6 Escarpment Structure and the Timing of Range-Front Faulting . 6 Gelai Volcano Forms Eastside Basin Boundary . 7 Natron-Engaruka Volcanic Field . 11 Tuff Rings . 11 Tephra Cones . 11 Widespread Extensive Tuff . 11 Lava Flows . 12 Ildonyo Loolmurwak . 12 Olmoiton (Armykon Hill) . 12 Oremit (“One Mile North of Lalarasi”) . 12 Five Kilometers North of Lalarasi . 12 Phonolite Lava Dome 10 km Northeast of Summit . 14 Ancient Lake Natron Beds on the Basin Floor . 16 Remnant Erosion Surfaces and Gelai Alluvial Fan Overlie the Ancient Lake Natron Beds . 17 Oldonyo Lengai . 19 Summit Craters and Summit Structure . 26 Summit Carbonatite Lava Flows . 27 Lava Flows and Vents on the Lower Flanks . 28 Phonolite Lava on the East Flank . 28 Embalulu Oltatwa (Oltatwa Crater) . 28 Nasira Cones . 29 Downwind Fallout Tephra . 30 Volume of Oldonyo Lengai’s Cone . 32 Age of Oldonyo Lengai . 32 New Radiometric Ages . 32 Previously Published Ages from Oldonyo Lengai . 33 Evidence from Dating at Olduvai Gorge . 33 Geochemistry and Stratigraphic Relations at Oldonyo Lengai . 34 Origin of Oldonyo Lengai Magma . 35 Debris-Avalanche Deposits . 37 Earliest Known Avalanche Events Probably Those Exposed Northeast of Oldonyo Lengai . 37 Widespread Deposits North of the Volcano . 37 Distribution . 37 Age . 38 Engare Sero Fan . 40 Leshuta Canyon . 40 Deposits South and Southeast of the Volcano . 40 Young Debris-Avalanche Deposits at Foot of Eastern Chasm and the Small Lake That Formed above Them . 40 Late Pleistocene and Holocene Faults in the Southern Basin . 42 Holocene Faults Recently Active . 42 Enarok Kohoke Fault . 42 Sag Troughs in Alluvium near Sidan Indare Stream . 43 Liquefaction Features along Gaping Cracks, Holocene or Late Pleistocene in Age . 44 Volcano Hazards . 45 Tephra Fallout . 45 Pyroclastic Flows . ..
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