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Facies variations in peralkaline ash-flow tuffs from the Kenya Rift Valley

Published online by Cambridge University Press:  01 May 2009

P. T. Leat
P. T. Leat
Affiliation:
Department of Environmental Sciences, University of Lancaster, Bailrigg, Lancaster, LA1 4YQ, U.K.
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Abstract

Two late Quaternary ash-flow tuffs erupted as single flow units from the Menengai caldera volcano, Kenya, show marked facies variations. Dense welding of the tuffs close to source may have been promoted by their peralkaline trachyte compositions. Coarse, near-vent breccias are partly welded and were emplaced as lithic-rich parts of the body of pyroclastic flows. Non-welded tuff forms the bulk of the outflow sheets which include valley-fill deposits up to 520 m above the present caldera rim, indicating ash-flow generation by collapse of moderately high eruption columns. The flows overran varied topography which controlled development of ash-flow tuff facies. On hilly terrain, the flows became turbulent and separated into a lithic-rich, rapidly deposited part and a fine-grained lithic-and crystal-poor part which was subsequently deposited farther from source. Caldera walls belonging to an earlier caldera probably formed a major barrier to the spread of the second ash-flow from its site of collapse. There are many similarities in the facies variations in these peralkaline tuffs and subalkaline tuffs. Differences in welding characteristics and the shape of ash particles reflect the lower viscosity of peralkaline magma.

Type
Articles
Information
Geological Magazine , Volume 122 , Issue 2 , March 1985 , pp. 139 - 150
Copyright
Copyright © Cambridge University Press 1985

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References

Bailey, D. K. 1980. Volcanism, earth degassing and replenished lithosphere mantle. Philosophical Transactions of the Royal Society of London A297, 309–22.Google Scholar
Boyd, F. R. 1961. Welded tuffs and flows in the rhyolite plateau of Yellowstone Park, Wyoming. Bulletin of the Geological Society of America 72, 387–426.CrossRefGoogle Scholar
Bristow, C. M. 1962. Kenya ignimbrites. Nature 196, 364–5.CrossRefGoogle Scholar
Chapin, C. E. & Lowell, G. R. 1979. Primary and secondary flow structures in ash-flow tuffs in the Gribbles Run palaeovalley, Central Colorado. Special Paper of the Geological Society of America no. 180, 137–54.CrossRefGoogle Scholar
Curtis, G. H. 1968. The stratigraphy of the ejecta from the 1912 eruption of Mt Katmai and Novarupta, Alaska. Memoir of the Geological Society of America 116, 153–210.CrossRefGoogle Scholar
Druitt, T. H. & Sparks, R. S. J. 1982. A proximal ignimbrite breccia facies on Santorini, Greece. Journal of Volcanology and Geothermal Research 13, 147–71.CrossRefGoogle Scholar
Fisher, R. V. 1977. Erosion by volcanic base surge density currents: U-shaped channels. Bulletin of the Geological Society of America 88, 1287–97.2.0.CO;2>CrossRefGoogle Scholar
Hay, R. L, Hildreth, W. & Lambe, R. H. 1979. Globule ignimbrite of Mount Suswa, Kenya. Special Paper of the Geological Society of America no. 180, 167–75.CrossRefGoogle Scholar
Jennings, D. J. 1971. Geology of the Molo area. Report of the Geological Survey of Kenya no. 86.Google Scholar
Johnson, R. W. 1968. Volcanic globule rock from Mount Suswa, Kenya. Bulletin of the Geological Society of America 79, 642–52.CrossRefGoogle Scholar
Jones, W. B. & Lippard, S. J. 1979. New age determinations and the geology of the Kenya rift-Kavirondo rift junction, W. Kenya. Journal of the Geological Society of London 136, 693–704.CrossRefGoogle Scholar
Leat, P. T. 1984. Geological evolution of the trachytic caldera volcano Menengai, Kenya rift valley. Journal of the Geological Society of London(in press).CrossRefGoogle Scholar
Leat, P. T., Macdonald, R. & Smith, R. L. 1984. Geochemical evolution of the Menengai caldera volcano, Kenya. Journal of Geophysical Research(in press).CrossRefGoogle Scholar
Mccall, G. J. H. 1962. Froth-flow lavas resembling ignimbrites in the East African rift valleys. Nature 194, 343–4.CrossRefGoogle Scholar
Mccall, G. J. H. 1964. Froth flows in Kenya. Geologische Rundschau 54, 1148–95.CrossRefGoogle Scholar
Mccall, G. J. H. 1967. Geology of the Nakuru-Thompson's Falls–Lake Hannington area. Report of the Geological Survey of Kenya no. 78.Google Scholar
Miller, T. P. & Smith, R. L. 1977. Spectacular mobility of ash-flows around Aniakchak and Fisher calderas, Alaska. Geology 5, 173–6.2.0.CO;2>CrossRefGoogle Scholar
Ross, C. S. & Smith, R. L. 1961. Ash-flow tuffs: their origin, geologic relations, and identification. Professional Paper of the U.S. Geological Survey no. 366.CrossRefGoogle Scholar
Schmincke, H. U. 1974. Volcanological aspects of peralkaline silicic welded ash-flow tuffs. Bulletin Volcanologique 38, 594–636.CrossRefGoogle Scholar
Schmincke, H. U. & Swanson, D. A. 1967. Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands. Journal of Geology 75, 641–64.CrossRefGoogle Scholar
Self, S. & Sparks, R. S. J. 1978. Characteristics of widespread pyroclastic deposits formed by the interaction of silicic magma and water. Bulletin Volcanologique 41, 196–212.CrossRefGoogle Scholar
Smith, R. L. 1960 a. Zones and zonal variations in welded ash-flows. Professional Paper of the U.S. Geological Survey no. 354–F, 149–59.CrossRefGoogle Scholar
Smith, R. L. 1960 b. Ash flows. Bulletin of the Geological Society of America 71, 795–842.CrossRefGoogle Scholar
Sparks, R. S. J., Self, S. & Walker, G. P. L. 1973. Products of ignimbrite eruptions. Geology 1, 115–8.2.0.CO;2>CrossRefGoogle Scholar
Sparks, R. S. J. & Wilson, L. 1976. A model for the formation of ignimbrite by gravitational column collapse. Journal of the Geological Society of London 132, 441–51.CrossRefGoogle Scholar
Sparks, R. S. J., Wilson, L. & Hulme, G. 1978. Theoretical modelling of the generation, movement and emplacement of pyroclastic flows by column collapse. Journal of Geophysical Research 83, 1727–39.CrossRefGoogle Scholar
Waitt, R. B. 1981. Devastating pyroclastic density flow and attendant air fall of May 18 – Stratigraphy and sedimentology of deposits. In The 1980 Eruptions of Mount St Helens, Washington (ed. Lipman, P. W. and Mullineaux, D. R.), pp. 439–58. Professional Paper of the U.S. Geological Survey no. 1250.Google Scholar
Walker, G. P. L., Heming, R. F. & Wilson, C. J. N. 1980. Low-aspect ratio ignimbrites. Nature 283, 286–7.CrossRefGoogle Scholar
Walker, G. P. L. & Mcbroome, L. A. 1983. Mount St Helens 1980 and Mount Pelee 1902–flow or surge? Geology 11, 571–4.2.0.CO;2>CrossRefGoogle Scholar
Walker, G. P. L., Self, S. & Froggatt, P. C. 1981. The ground layer of the Taupo ignimbrite: a striking example of sedimentation from a pyroclastic flow. Journal of Volcanology and Geothermal Research 10, 1–11.CrossRefGoogle Scholar
Walker, G. P. L., Wilson, C. J. N. & Froggatt, P. C. 1980. Fines-depleted ignimbrite in New Zealand - the product of a turbulent pyroclastic flow. Geology 8, 245–9.2.0.CO;2>CrossRefGoogle Scholar
Walker, G. P. L., Wilson, C. J. N. & Froggatt, P. C. 1981. An ignimbrite veneer deposit: the trail marker of apyroclastic flow. Journal of Volcanology and Geothermal Research 9, 409–21.CrossRefGoogle Scholar
Williams, L. A. J. 1978. Character of Quaternary volcanism in the Gregory Rift Valley. In Geological Background to Fossil Man(ed. Bishop, W. W.), pp. 55–69. Edinburgh: Scottish Academic Press.Google Scholar
Williams, L. A. J., Macdonald, R. & Chapman, G. R. 1984. Quaternary caldera volcanoes of the Kenya rift valley. Journal of Geophysical Research(in press).CrossRefGoogle Scholar
Wilson, C. J. N. 1980. The role of fluidization in the emplacement of pyroclastic flows: an experimental approach. Journal of Volcanology and Geothermal Research, 8 231–49.Google Scholar
Wilson, C. J. N. & Walker, G. P. L. 1982. Ignimbrite depositional facies: the anatomy of a pyroclastic flow. Journal of the Geological Society of London 139, 581–92.CrossRefGoogle Scholar
Wolff, J. A. & Wright, J. V. 1981. Rheomorphism of welded tuffs. Journal of Volcanology and Geothermal Research 10, 13–34.CrossRefGoogle Scholar
Wright, J. V. 1980. Stratigraphy and geology of the welded air-fall tuffs of Pantelleria, Italy. Geologische Rundschau 69, 263–91.CrossRefGoogle Scholar
Wright, J. V. 1981. The Rio Caliente ignimbrite: analysis of a compound intraplinian ignimbrite from a major Late Quaternary Mexican eruption. Bulletin Volcanologique 44, 189–212.CrossRefGoogle Scholar
Wright, J. V., Self, S. & Fisher, R. V. 1981. Towards a facies model for ignimbrite-forming eruptions. In Tephra Studies(ed. Self, S. and Sparks, R. S. J.), pp. 433–9. D. Reidel Publishing Company.CrossRefGoogle Scholar
Wright, J. V. & Walker, G. P. L. 1977. The ignimbrite source problem: significance of a co-ignimbrite lag-fall deposit. Geology 5, 729–32.2.0.CO;2>CrossRefGoogle Scholar
Wright, J. V. & Walker, G. P. L. 1981. Eruption, transport and deposition of ignimbrite: a case study from Mexico. Journal of Volcanology and Geothermal Research 9, 111–31.CrossRefGoogle Scholar