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GEOLOGIC HISTORY % % %

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% % Legend

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HOLOCENE %

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% Pl-na

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% Alasho ash

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Evaporite Series (0-9 ka): with interbedded clays. The Magadi Soda Company currently mines the % % % Sediments Metamorphics %%

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trona at Magadi in the region to the north. Samples of this formation from indicate significantly % %% %%

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% Alasho centers

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%% % G% eology of the Area South of Magadi,

more than trona compared to (Bell and Simonetti 1996). %% %

% Holocene Kurase Group

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% % Pl-pt %

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% % Magadi Trachyte Xl % % %

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PLEISTOCENE % % % % Trona Crystalline Limestone %

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% % For area to North see: Geology of the Magadi Area, KGS Report 42; Digital version by A. Guth %

% % 36.5 °E

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% %

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36.0 °E % %

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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

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a period of higher lake levels in both Natron and Magadi. Coarser pebble beds seen near Len% derut Alluvial fan Undiff. Pelitic host %%

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% Pkb %%

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% are thought to be fluvial deposits associated with the higher lake stand (Baker 1963). %

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% Pl-ogl

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% Xq

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% Shompole Volcanics % % Lacustrine Sediments Quartzite

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Natron Lake Beds: reworked volcanic material, fine to medium sand, clay, gravel, and rounded che% rt %

%

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% % Pl-shv

% pebbles. Magnesite and bentonite clay are found in small outcrops near the shore of (% 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

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% Plv Sh-cb %

Magadi region by Baker (1963), they are now known to be significantly younger (Baker 1971). % %%

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% Pl-nl Carbonatite breccia Xkg

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% Alangarua Pl-sb

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% Natron Lake Beds Kyanite Gneiss

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% !

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% Kabongo

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% Plv %%

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% 0

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Oloronga Beds (300-800 ka): yellow water-lain tuffs are the remnants of a larger and fresher lake 0 %%

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1 %%

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% Pleistocene: Gelasian 0

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0

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(Crossley 1979). About 45 m in outcrop, these sediments overlie the Magadi Trachytes, and contain % chert, %%

% % 0

% % Volcanics

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% 1

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% 1

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% Pl-ogl 0

% kunkar limestone, and layers of boxwork (Baker 1958, Potts et al. 1988). Some hominin %

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% 0 Pkb

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% 0 Kirikiti Basalt

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% 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 %%

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over 18m thick, mantle the plain between Alasho and Shompole (Baker 1963). %

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% Plv

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% Sh-c

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% Lenderut

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% 00 Sh-shv

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% 10

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% % 0

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% 0

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% K

0 0 %

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% 0

0 % %

% e 0

0

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Plateau/Magadi Trachyte (0.8-1.4 Ma): fine grained, peralkaline flood trachyte with a medium gree%n to n 0

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y 0 0 %

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% a % 0

1

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% - 1 % 2

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% T

% 8

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a %

1 %

% Pl-nv

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brown-grey matrix, and feldspar phenocrysts up to 0.5cm. This prominent trachyte is found between Lake z % 0

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a 0 %

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n % % % 2

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0 %

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1

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B 8 %

8 %

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Natron and Suswa, and is one of the several expansive "flood trachytes" that cover the rift floor. % o %

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% r % %

% d %

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% e %

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% %%%%%%%%%%%%%%%% ! % %

% % City Road-major Lake- ephemeral %

% % faults-large %

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% 1 %

Gelai (1-1.5 Ma): shield volcano that lacks a summit crater, reaching an elevation of 2942m. Slopes are %

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% % Town Road-minor

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% Springs

% 0 % % faults-small

% % composed of olivine basalts, and peralkaline trachytes. Peridotite xenoliths may also be present in %

% 0 % %

% %

8 0

% %

% %

% % 0

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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

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Pl-na F %

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% o

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r

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Shompole (Shombole) (1.96-2 Ma): highly weathered stratovolcano composed of nephelinites, % % r % % % e

1

% % %

% % %

a

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% % % t

% carbonatites and phonolites (Dawson 2008). % % o

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%% % WATER RESOURCES %

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7 %

% E

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% The Magadi region receives on the order of 475 mm of precipitation per year, making this a semi-arid climate.

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0 t

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0 Xq s %

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Singaraini Basalt (2.31-2.33 Ma): olivine basalts with occasional, small, feldspar phenocrysts. Five flows

% % %

t

e

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% % 0 The Ewaso Ngiro river drains into Lake Natron, which is its terminal basin. This river and associated swamps

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e %

% 1 e %

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e %

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% % % are exposed in a fault scarp at the Singaraini trigonometrical station, all have normal magnetic polarity, X

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% % provide adequate water for the Maasai and their cattle herds, however, the water supply is not sufficient during

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% e %

and outcrops are bouldery. See Baker and Mitchell (1976) for a discussion regarding previous correlations o

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e the dry season to keep the trona surface submerged. The only open water around Lake Natron during the dry l % %

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o

% r 0 % %

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% Ole Seni % g % g

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and dating of this formation. % % ! %

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% y

e 8 %

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% % seaso% n is supplied by brackish or saline springs.

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d

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Lenderut (2.5-2.7 Ma): highly eroded remains of a volcanic center composed of andesites, tephrites and % z

%

%

% %

N

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% 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. seeps have been reported from ground fissures in the area (Baker 1963). 0 %

l %

l % 4

p %

A % of the map along the . See Crossley (1979) for a discussion of issues regarding 1 % 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 i o 0 14 Xq 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 Pl-sb 200 y 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 fashion. The formation of these faults is thought to have coincided in time with the Lake Lolasko 16 deposition of the Oloronga lake beds (LeGall et al. 2008), although additional minor faulting has affected the younger lake beds as well (Baker 1958). The large Nguruman escarpment defines the western rift Engare Naibor ! boundary in this area, and the seismic data indicate 3.5 km of rift fill lies in the rift next to this fault (Simiyu Xq 0 140 0 & Keller 2001). 0 0 0 2 0 Pl-sb Xmg 8 0 Xbg 0 The metamorphosed sediments that comprise the basement series exposed in this area were affected by Xmg 8 1 0 2000

0

orogenic folding that produced a NNW-SSE foliation. Three folds located in this region (the Ropet 8 00 Colored DEM with hillshade for the mapped region. Pl-gv Xmg 22 syncline, Losirua anticline and Kileu syncline) plunge gently to the NNE (Baker 1963). 2200 Elevation range from 579 - 2260m (cyan - red). Reference map showing the source maps and their geographic coverage 0 40 Xq PALEONTOLOGY 1 No sites are known from this specific mapped area, however hominin sites are known to exist on the 1000 Xmg 0 0 0 Pl-gv 0 2 western shore of Lake Natron, where the Peninj Group Sediments (1.1-1.2 Ma) contain a variety of stone 0 2 1 0 tools and animal remains (Domı´nguez-Rodrigo et al. 2002, Deino et al. 2006). 0 6 1 Xbg

ECONOMIC DEPOSITS 0 Xfg 0 0 Xq 0 8 1 6 Xq 0 1 0 While the Magadi Soda Company has occupied the shores of the lake Magadi since 1911, there is no 2 0 2 0 active mining of the trona at Lake Natron. Potentially economic quantities of kyanite are associated with 0 1 Xmg Xmg the gneisses in the Losirua area, and the same area also contains a source of graphite (Baker 1963). 0 Xq

0

2 Xmg

1 Xfg 0 Xmg 1 0 8 6 0 1 Xbg 0 1200 Xmg -2.5 ° -2.5 °

36.0 °E 36.5 °E 0 5 10 20 30 Kilometers Cross Section Legend

Lacustrine Sediments Diagrammatic cross section from A to A'. Vertical exaggeration = 2X ² A A' Magadi Trachyte Geological Map of the Pl-shv Shompole Singaraini Southern Kenya Rift contour interval 200m Plv Lenderut Pl-shv Plv Location: South of Magadi, Kenya Metamorphic 36.0 E - 36.5 E, 2.0 S - 2.5 S A. Guth, J. Wood (2013)

Above: Location of presented geologic map (grey-square) Coordinate System: Geographic WGS84 Michigan Technological University in relation to the major rift bounding faults and .

DIGITAL MAP AND CHART SERIES DMCH016 DOI: 10.1130/2014.DMCH016.S5 Published by The Geological Society of America, Inc., 3300 Penrose Place • P.O. Box 9140, Boulder, Colorado 80301-9140 © 2014 The Geological Society of America. All rights reserved.