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Geology of the Area South of Magadi, Kenya

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Geology of the Area South of Magadi, Kenya %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %% % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % %% %% % % % % % % %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% % GEOLOGIC HISTORY % % % % % % % % % % % % % % % % % % % Legend % % % % % % % % % % % % % % % % % % % %% % %% %% % HOLOCENE % %% %% %% %% %% % % Pl-na %% %% %% % % Alasho ash %% %% %% % Evaporite Series (0-9 ka): trona with interbedded clays. The Magadi Soda Company currently mines the % % % Sediments Metamorphics %% %% %% %% %% %% %% %% %% %% %% %% % trona at Lake Magadi in the region to the north. Samples of this formation from Natron indicate significantly % %% %% %% %% %% %% %% % % Pl-nv %% % % Alasho centers %% %% %% % G% eology of the Area South of Magadi, Kenya more halite than trona compared to Lake Magadi (Bell and Simonetti 1996). %% % % Holocene Kurase Group %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% %% % % % % % Pl-pt % % % % Magadi Trachyte Xl % % % % PLEISTOCENE % % % % Trona Crystalline Limestone % % % % For area to North see: Geology of the Magadi Area, KGS Report 42; Digital version by A. Guth % % % 36.5 °E % % % % 36.0 °E % % % % % High Magadi Beds (9-23.7 ka): yellow-brown silts over laminated clays with fish remains. Deposited % during % % Pl-gv % -2.0 ° -2.0 ° % % Gelai Xp % %% % a period of higher lake levels in both lakes Natron and Magadi. Coarser pebble beds seen near Len% derut Alluvial fan Undiff. Pelitic host %% %% %% % % Pkb %% %% % are thought to be fluvial deposits associated with the higher lake stand (Baker 1963). % %% %% %% % % Pl-ogl % % Xq %% % % Shompole Volcanics % % Lacustrine Sediments Quartzite %% %% %% % Natron Lake Beds: reworked volcanic material, fine to medium sand, clay, gravel, and rounded che% rt % % % % % Pl-shv % pebbles. Magnesite and bentonite clay are found in small outcrops near the shore of Lake Natron (% Guest Shompole % 0 % % Pleistocene Karigau Group % 0 % % 8 & Pickering, 1966). % % % % % % % Sh-c % Central vent deposits Xfg % Pl-ml' % Pl-ml % Pebble Beds Feldspathic Gneiss % 0 % % 0 Pl-sb Chert Series/Green Beds (40-96 ka): lacustrine chert and associated sediments up to 30 meters th% ick Pl-pt % 2 % % Pl-pt 0 1 % Sh-p3 % 0 % 8 above Oloronga beds. Cherts are typically surrounded by a green matrix of erionite tuffs and pyroc% lastic Type III phonolite % 8 Pl-ml Xmg % % High Magadi Beds Muscovite-biotite gneisses 0 % Pl-ml' % 0 % 1 silts, and may rise diapirically through the High Magadi Beds in the Magadi region (Behr & R hrich% t 2000, 8 % Mbakasi 0 0 % 0 % ! 0 % 0 % Sh-p2 % Behr 2002). % % Pl-c Type II phonolite Xhg % Magadi Green Beds Hornblend gneiss % 0 %% 1000 %% 0 %% 800 2 % 1 %% Sh-p1 Lengorale Trachyte (630-640 ka): quartz trachytes and trachytic tuffs at the southern end of the Ngu%%ruman %% 800 Pl-ogl Type I phonolite Xbg %% Oloronga Lake Beds Biotite/Banded Gneiss %% Pl-ml % Escarpment that rest directly on the Kirikiti basalts. Originally associated with the Lengitoto trachytes % % in the %% %% %% %% % % Plv Sh-cb % Magadi region by Baker (1963), they are now known to be significantly younger (Baker 1971). % %% % % Pl-nl Carbonatite breccia Xkg % % Alangarua Pl-sb % % Natron Lake Beds Kyanite Gneiss % % ! %% % % Kabongo % % Plv %% % % 0 %% % % 0 %% %% Oloronga Beds (300-800 ka): yellow water-lain tuffs are the remnants of a larger and fresher lake 0 %% %% 1 %% % % Pleistocene: Gelasian 0 %% %% 0 % (Crossley 1979). About 45 m in outcrop, these sediments overlie the Magadi Trachytes, and contain % chert, %% % % 0 % % Volcanics % % 1 % % 1 %% % % Pl-ogl 0 % kunkar limestone, and layers of carbonate boxwork (Baker 1958, Potts et al. 1988). Some hominin % % % 0 Pkb %% % % 0 Kirikiti Basalt %% %% %% % artifacts are also associated with these sediments (Shipman et al. 1983). % % % Pleistocene % % % % 1 0 % 0 Pl-sb % 4 0 % 0 % A 0 Singaraini 10 2 % 8 00 % 0 % 1 % Ol Doinyo Alasho: small trachytic cone composed of ashes, tuffs and glass scoria. Pumice tuffs, reac% hing Plv A' Pl-lgt %% % % Plv Lengorale %% %% %% % over 18m thick, mantle the plain between Alasho and Shompole (Baker 1963). % % % Plv % % Sh-c %% % % Lenderut % % 00 Sh-shv %% % % 10 %% %% % % 0 % % 0 % % 0 % % K 0 0 % % % 0 0 % % % e 0 0 % % Plateau/Magadi Trachyte (0.8-1.4 Ma): fine grained, peralkaline flood trachyte with a medium gree%n to n 0 % % 1 y 0 0 % % % a % 0 1 % % - 1 % 2 % % T % 8 % % 0 a % 1 % % Pl-nv n % %% % brown-grey matrix, and feldspar phenocrysts up to 0.5cm. This prominent trachyte is found between Lake z % 0 %% % a 0 % %% % n % % % 2 % 0 % % % i 0 % a 0 % 1 % B 8 % 8 % % % % Natron and Suswa, and is one of the several expansive "flood trachytes" that cover the rift floor. % o % % % r % % % d % % % e % % % r % % % %%%%%%%%%%%%%%%% ! % % % % City Road-major Lake- ephemeral % % % faults-large % % % 0 Pl-sb % %% % 0 % % % % % % 0 %% % % % % 1 % Gelai (1-1.5 Ma): shield volcano that lacks a summit crater, reaching an elevation of 2942m. Slopes are % % % % % % % ! % % % % % % % Town Road-minor % % Springs % 0 % % faults-small % % composed of alkali olivine basalts, and peralkaline trachytes. Peridotite xenoliths may also be present in % % 0 % % % % 8 0 % % % % % % 0 % % % % ! % some basalts and tuff cones. Numerous small scoria and tuff cones are aligned with faults that cut the % % 2 % % % 0 % % % % % 1 0 Village Road-track Swamp % 2 rivers % % % % 1 volcano’s slopes (Dawson 2008). A series of seismic events from July-Sept 2007 were focused on t% he % 0 % % % % 0 % % % % 0 2 % % % Pl-pt 0 1 % % % % southern flank of this volcano, and may have been related to magmatic dyking (Delvaux et al. 2008)% . % 0 % % % rail % % % 1 200m-contour % % % Pl-na F % %% % %% % % % % % o % % % % % % r % % % 0 % % % % % % 0 a % % % % % % 2 % Shompole (Shombole) (1.96-2 Ma): highly weathered stratovolcano composed of nephelinites, % % r % % % e 1 % % % % % % a % % % % % % % % % % % % t % carbonatites and phonolites (Dawson 2008). % % o ; % %% % WATER RESOURCES % % % % 7 % % E % % % % % 2 % % % % a % % % % % + % s % % The Magadi region receives on the order of 475 mm of precipitation per year, making this a semi-arid climate. % % % 6 % % % 0 t % % % 1 % % % 0 Xq s % % % Singaraini Basalt (2.31-2.33 Ma): olivine basalts with occasional, small, feldspar phenocrysts. Five flows % % % t e % % % 0 The Ewaso Ngiro river drains into Lake Natron, which is its terminal basin. This river and associated swamps % % % % e % % 1 e % % % % e % % % % : % % % are exposed in a fault scarp at the Singaraini trigonometrical station, all have normal magnetic polarity, X % % h % G %% % % % provide adequate water for the Maasai and their cattle herds, however, the water supply is not sufficient during % % % s % % % % % % e % %% % % % % % e % and outcrops are bouldery. See Baker and Mitchell (1976) for a discussion regarding previous correlations o % % % %% % % % e the dry season to keep the trona surface submerged. The only open water around Lake Natron during the dry l % % % % o % r 0 % % % % % % % Ole Seni % g % g % % % 0 % % and dating of this formation. % % ! % % % y e 8 % % % % seaso% n is supplied by brackish or saline springs. % % % d % % % o % % % % % % f % % % a %% % % % i %% %% %% % % % t %% %% %% %% %% %% % % % h n %% % % e a % % % % % Lenderut (2.5-2.7 Ma): highly eroded remains of a volcanic center composed of andesites, tephrites and % z % % % % N %% % % GE% OTHERMAL PHENOMENA n Xq % % % % % % a % a % % %% % % % % % % m basanites (Baker 1963). T % Th% e Kenya Rift has a number of geothermally active sites, and both Lakes Magadi and Natron are surrounded , % a %% o % n % d % Pl-nl g b%% y thermal springs. There have been some studies on the potential of geothermal power generation at Lake n % a % o % i % - l % 0 PLIOCENE B %% o 0 % Magadi (e.g. McKnitt et al. 1989, Clarke et al. 1990), but this section of the rift has yet to be tapped for power i % 8 s L %% s % Kirikiti Basalts (2.5-3.1 Ma): olivine basalts with rare plagioclase phenocrysts found in the western section f % 0 % e o % generation. Carbon dioxide seeps have been reported from ground fissures in the area (Baker 1963). 0 % l % l % 4 p % A % of the map along the Nguruman escarpment. See Crossley (1979) for a 1 % discussion of issues regarding a % r %% e % m % Xq 0 a %% previous age dates for these basalts. c 0 % , i 4 % 1 % K % g %% o G % l 0 %% 0 % o %% S 6 e 0 % 0 % 6 R BASEMENT SYSTEM: 1 1 % G % % % e : % % p e %% Precambrian metamorphic rocks exposed here are part of the Mozambique Belt, which represents the % o e %% r s Xmg % % t % t % 0 %% 3 closure of the Mozambique ocean during the Pan-African Orogeny (Nyamai et al. 2003). The Kurase s % 0 % 9 e % % 8 % ; Xq % 1 % D W % Group has been interpreted as a former shallow shelf environment, while the metamorphosed arkose, 0 % % i o 0 %% 0 g 0 t 4 % % 0 i 0 1 Xkg % t %8 a greywackes, and basic lavas of the Kasigau group were deposited within a subsiding basin (Warden & a 0 %1% l e 1 0 % r 0 Xbg v a 2 Horkel 1984). These sediments have been subjected to several stages of deformation (descriptions in e 1 Xp r 0 Xq 0 r 0 0 s o 4 Xq 0 1 i o Warden & Horkel 1984), with all but the most recent associated sediments reaching upper F 1 n amphibolite/granulite grade (Nyamai et al. 2003). Xbg b 00 y Pl-sb 2 1 Xkg Xkg A . Pl-nl G 0 0 u Xkg 0 t 2 0 h 0 0 2 STRUCTURE Xbg 0 Xq 0 14 0 This area is cut by numerous "grid faults" that can be seen running roughly parallel to each other in a 00 2 00 Xq northeast-southwest
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