Spatial and temporal geochemical variability in

Sedimentology (2018)

doi: 10.1111/sed.12443

Spatial and temporal geochemical variability in lacustrine sedimentation in the East African Rift System: Evidence from the Kenya Rift and regional analyses R I C H A R D B E R N H A R T O W E N * , R O B I N W . R E N A U T † and T I M K . L O W E N S T E I N ‡ *Department of Geography, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China (E-mail: [email protected]) †Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada ‡Department of Geological Sciences and Environmental Studies, State University of New York, Binghamton, NY 13902, USA Associate Editor – Alexander Brasier ABSTRACT

Many previous studies on lacustrine basins in the East African Rift System have directed their attention to climatic controls on contemporary sedimentation or climate change as part of palaeoenvironmental reconstruction. In contrast, this research focuses on the impact of tectonism and volcanism on rift deposition and develops models that help to explain their roles and relative importance. The study focuses on the spatial and temporal variability in bulk sediment geochemistry from a diverse range of modern and ancient rift sediments through an analysis of 519 samples and 50 major and trace elements. The basins examined variously include, or have contained, wetlands and/or shallow to deep, fresh to hypersaline lakes. Substantial spatial variability is documented for Holocene to modern deposits in lakes Turkana, Baringo, Bogoria, Magadi and Malawi. Mio-Pleistocene sediments in the Central Kenya Rift and Quaternary deposits of the southern Kenya Rift illustrate temporal variability. Tectonic and volcanic controls on geochemical variability are explained in terms of: (i) primary controlling factors (faulting, subsidence, uplift, volcanism, magma evolution and antecedent lithologies and landscapes); (ii) secondary controls (bedrock types, rift shoulder and axis elevations, accommodation space, meteoric and hydrothermal fluids and mantle CO2); and (iii) response factors (catchment area size, orographic rains, rain shadows, vegetation densities, erosion and weathering rates, and spring/runoff ratios). The models developed have, in turn, important implications for palaeoenvironmental interpretation in other depositional basins. Keywords East Africa, geochemistry, lakes, rifts, sediments, tectonic, volcanics.

INTRODUCTION The 3500 km long East African Rift System (EARS) extends from the Afar triple junction in Ethiopia to Lake Malawi and beyond, with young rifts extending to the Okavango Delta (Fig. 1A). The eastern and western branches of the rift pass around the sides of the Archean

Tanzanian Craton, propagating locally into its margins. Thermal, plume-related, processes have been linked to igneous activity and faulting (Macdonald et al., 1994; Smith, 1994; Huerta et al., 2009) with widespread volcanism developing in the eastern branch. This compares with restricted volcanism in the western branch, with this contrast having significant implications for

© 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists

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sedimentation. The EARS was probably initiated in the Early Miocene, although earlier Palaeogene igneous activity took place in northern Kenya and Ethiopia (Ebinger et al., 1993; Hendrie et al., 1994) with rifting migrating southward from the Afar, at about 35 Ma, to the southern Kenya Rift by about 5 to 8 Ma (Crossley & Knight, 1981), although Le Gall et al. (2008) have noted that several basins in northern Kenya and Ethiopia formed at about the same time (ca 35 Ma) as others in the Central Kenya Rift (Mugisha et al., 1997; Hautot et al., 2000). The modern and ancient soda lakes of the Kenya Rift formed partly in response to weathering of alkali volcanics erupted during rifting (Baker, 1987; Schagerl & Renaut, 2016). However, faulting is also important in creating basins in which closed and open lakes have developed. In some cases, such as lakes Naivasha and Baringo (Fig. 1A and C), topographically closed lakes are fresh because faulting has allowed subsurface drainage. Fault-controlled hot and cold spring waters have also maintained lakes where otherwise no permanent water exists, as happens today at Nasikie Engida (Fig. 1D). Several researchers have attempted to explain sediment accumulation in rift lakes in terms of climate-driven processes (Johnson, 1993; Bergner et al., 2009; Trauth et al., 2010) with some studies focusing on models that document contrasts in sedimentation between high and low lake-level stages (Scholz & Rosendahl, 1990; Scholz et al., 1998). Other models have combined both tectonic and climate controls in explaining facies variability in subsiding basins (Carroll & Bohacs, 1999; Bohacs et al., 2000; Hinderer & Einsele, 2002). Many geochemical studies of specific elements and isotopes have also been interpreted in terms of climate-related factors such as evaporation–precipitation balances, lake-level change, variations in water chemistry, wind-driven mixing and redox depths (Talbot & Lærdal, 2000; Russell et al., 2003; Cohen et al., 2006; Brown et al., 2007; Brucker et al., 2011; Foerster et al., 2012; Wilson et al., 2014; McManus et al., 2015; Deocampo et al., 2017). Despite the common use of climate controls to explain sediment geochemistry, some studies have focused on tectonic influences. Several geochemical studies have attempted to identify signatures that discriminate between a broad variety of tectonic settings, including passive or active continental margins and oceanic or continental island arcs (Bhatia & Crook, 1986;

Hollings et al., 2007). Research has also been directed towards characterization of sediment provenance (Schoenborn, 2010; Heinrichs et al., 2012) with relatively immobile trace elements such as La, Ce, Y, Th, Zr, Hf, Nb, Ti and Sc being especially suited for such investigations since they are transported with minimal quantitative change (McLennan et al., 1983). In contrast, studies of bulk major and trace elements in EARS sediments are scarce, except for a few investigations of individual basins that have focused on chemical index of alteration (CIA) data and rare earth element (REE) compositions (Huntsman-Mapila et al., 2005; Owen et al., 2011, 2012, 2014). Huntsman-Mapila et al. (2009) carried out a comparative study of the bulk geochemistry of two basins representative of early (Okavango Delta) and later rifting (Lake Tanganyika). In recent decades, research into deposits of the Kenya Rift and other parts of the EARS have focused on sedimentology, microfossils and aspects of geochemistry (for example, carbon and oxygen isotopes) to reconstruct past climates (Cohen et al., 2007; Stone et al., 2011). The bulk geochemistry of sediments in the EARS reflects their provenance and weathering, as well as their depositional, chemical and diagenetic histories, which result from a complex interplay between geological and climatological controls (WoldeGabriel et al., 2000; Owen et al., 2012). Weathering, for example, leads to loss of labile elements (Nesbitt & Young, 1984), but the weathering intensity might reflect either climate (Ivory et al., 2017) or relative uplift and subsidence rates, which control exposure time to weathering. This study attempts to answer questions about how much major and trace element spatial variability there is in Late Quaternary lacustrine deposits from contrasting tectonic settings (Fig. 1A). It also examines the temporal variability in: (i) the Central Kenya Rift, which has a discontinuous 15 Ma record of lacustrine deposition; and (ii) the South Kenya Rift with sediments that accumulated over the last 2 Ma. New analyses from the Kenya and Malawi rifts are supplemented by published analyses from Lake Tanganyika and the Okavango Delta. The objectives of this research are to explore the influence of tectonism and volcanism in rift sedimentation and to provide a geochemical framework for studies elsewhere to enable more comprehensive palaeoenvironmental reconstructions. To achieve these objectives, this study

© 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology

Geochemical variability in East African rifts

L. Tanganyika

20 km

Volcanics F Sandstones Siliciclastic & carbonate Okavango sediments

E

K2

LK1

N1

C2

Hills

Indian Ocean

Tanzanian Craton

Ta MPU10 ng a MPU3 nyik a

GB2 GB72

C

L. Naivasha

L.

F

D

GB16

KYS1

Tuge n

L. Victoria

Galana Boi Formation Holocene littoral sand Plio-Pleistocene sediments Road

Kerio V alley

Western rift

C

N

ment

L. Albert

Elgeyo Escarp

L. Turkana

B Koobi La Fora ke Tu rk an 2 km a

B Eastern rift

A

T1

L. Baringo

C1

K1 L1

L. Malawi

G

L. Bogoria

82 & 83

81

3

Maun

KIM1

50 km

BL (multiple shoreline sites)

5 km

Okavango Delta 500 km

Section/ sample site

Holocene sed.; Loboi silts Kapthurin F. Chemeron; Kaperyon F.

Munya wa Gicheru

Suswa

MG1 & 2

Age (Ma)

H

D

Tugen Hills/Lakes Baringo/Bogoria Elegeyo Escarpment

0

Eremit

Olorgesailie TP

E

Chemeron F.

M527 M511

Lukeino F. Mpesida Beds

6

SEL/GB/EL2/SEM 5 km OLOGB3

Olorgesailie F. High Magadi Beds Surficial Green Beds Oloronga Beds sediments

Fluvial and lacustrine sediments

N

M164 M225

10

Koora

GB2

M151

8

Ngorora F. 12

14

Tambach F.

L. Magadi

ES

Munya wa Gicheru Beds

B01

Nasikie Engida NE

High Magadi Beds/Evaporite series

Volcanic rocks No sediment record

Precambrian

Muruyur Beds 16

lawi

G05

OLT

Gneiss/ marble

L. Ma

D01; D99; D98; D91; OLO; B99 MT TR

4

Surficial sediments; Holocene Bogoria; Loboi silts

Faults

SuswaMagadi Olorgesailie PostGreen Beds Olorgesailie F. Kapthurin F. Oloronga Beds Olorgesailie F.

}

2

Lukeino F. Ngorora F. Tambach F. Kimwarer F.

Core 2A

G

KOI

Metamorphic basement

M586

50 km

M601

Superficial Sedimentary rocks Metamorphic rocks Granitic rocks

MM78 M61

Fig. 1. (A) Sediment sample locations. Locations are shown by italicized letters ‘B’ to ‘G’. (B) to (G) Map locations of sampled sediments including (E) locations of sampled sediments in Lake Malawi. (H) Stratigraphy of key sediment sequences reported in the text. © 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology

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specifically aimed to: (i) characterize the spatial variability of major and trace elements in lacustrine sediments of the EARS; (ii) relate rift processes to geochemical sedimentation; and (iii) develop models that may consider climate, but which focus on tectonic and volcanic influences on sedimentation.

METHODS Geochemical analyses (original data in Table S1) were carried out on fine-grained (clay to silt) lake sediments collected over a period of 40 years from the EARS. Samples were collected from diatomaceous silts in the Holocene Galana Boi Formation (n = 41; Fig. 1B), Quaternary diatomaceous and non-diatomaceous silts and limestones from the Lake Bogoria and Lake Baringo basins (n = 30; Fig. 1C), limestones, zeolitic silts, muds and trona from the Magadi Basin (n = 94; Fig. 1D) and diatomaceous silts, muds and laminated diatomite-mud deposits from Lake Malawi (n = 37; Fig. 1E). These original analyses are supplemented by data reported for southern Lake Tanganyika (n = 30; Huntsman-Mapila et al., 2009; Fig. 1F) and the Okavango Delta (n = 26; Huntsman-Mapila et al., 2005; Fig. 1G). Older deposits were collected from the Elgeyo Escarpment and the Tugen Hills (Fig. 1C). These included Miocene fluvial sandstones from the Kamego Formation (n = 3) and lake deposits, including: zeolitic silts and clays from the Tambach Formation (n = 22); clays, zeolitic clays and diatomaceous silts from the Ngorora Formation (n = 7); diatomites from the Lukeino Formation (n = 1); and diatomaceous silts from Kaparaina Basalt Formation interbeds (n = 2). Other samples include diatomites, diatomaceous silts and silts from the Plio-Pleistocene Chemeron Formation (n = 30), the Pleistocene Kapthurin Formation (n = 4), Loboi Silts (n = 2) and the Kampi ya Samaki Beds (n = 2). Pleistocene diatomaceous silts, diatomites, clays, silts and tufas were sampled from the Munya wa Gicheru Beds (n = 38; Fig. 1D), the Olorgesailie Formation and postOlorgesailie Formation sediments (n = 205; Fig. 1D) in the southern Kenya Rift. Chemical analyses of bulk samples were performed by Activation Laboratories Limited, Ancaster, Ontario, Canada (Code 4E-exploration). Major and trace elements were determined using fusion inductively coupled plasma/optical emission spectrometry (instrumental neutron activation analysis and total digestion

inductively coupled plasma techniques). Details of which methods were used for each element are given in Table S1. Detection limits for majors were 0
01% (0
005% for TiO2). Trace element detection limits (mg kg 1) were as follows: Ag (0
5), As (2), Ba (3), Be (1), Bi (2), Br (1), Cd (0
5), Co (1), Cr (1), Cs (0
5), Cu (1), Hf (0
5), Hg (1), Mo (2), Ni (1), Pb (5), Rb (20), Sb (0
2), Sc (0
1), Se (3), Sr (2), Ta (1), Th (0
5), U (0
5), V (5), W (3), Y (1), Zn (1), Zr (2), La (0
2), Ce (3), Nd (5), Sm (0
1), Eu (0
1), Tb (0
5), Yb (0
1), Lu (0
05) with Au and Ir limits of 5 lg kg 1 and a S limit of 0
001%. Loss on ignition (LOI) data includes H2O+, CO2 and other volatiles and was determined from weight loss after heating samples to 1000°C. Calculation of the CIA follows the method of Nesbitt & Young (1984) with CIA = [Al2O3/ (Al2O3 + K2O + Na2O + CaO*) 9 100], using molar proportions and with CaO*, excluding calcium in carbonates and phosphates. The REE data were normalized using CI chondrite compositions (Sun & McDonough, 1989): Eu# records Eu anomalies relative to Sm and Tb.

RESULTS

Spatio-temporal variability Major elements are plotted against Al2O3 in Fig. 2, with shaded or outlined areas representing specific basins. Silica is negatively correlated with Al2O3 in most basins, but also shows positive relationships (Fig. 2A). There is a clear separation of fields for Late Quaternary samples from Lake Malawi, southern Lake Tanganyika and the Okavango, with Turkana sediments overlapping with the Malawi analyses. There is a positive correlation between TiO2 and Al2O3 (r = 0
57), with Lake Turkana samples plotting separately from deposits at Malawi, Magadi, Tanganyika and Okavango (Fig. 2B). Al2O3 is positively correlated with Fe2O3 (0
8) and K2O (0
59) with poor correlations for MgO (0
14) and Na2O (0
04). CaO is negatively correlated with SiO2 ( 0
83) and Al2O3 ( 0
59) and positively related to LOI (0
82). Plots for older rift sediments, such as those forming the Miocene Tambach and Ngorora Formations (Central Kenya Rift), also show separation of major elements (Fig. 2A to D). Contrasts between major and trace elements can also be documented using principal component analyses (PCAs) with samples from

© 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology

Geochemical variability in East African rifts A

100

5

Late Quaternary Late Holocene sediments (L. Malawi)

80

Holocene sediments (L. Tanganyika)

SiO2 (%)

Holocene Galana Boi F. (L. Turkana) 60

Late Pleistocene/Holocene (Okavango) Holocene trona (Magadi)

40

Holocene High Magadi Beds (Magadi) Late Quaternary silts/clays (Magadi)

20

Late Quaternary cherts (Magadi) Holocene lake limestone (Bogoria)

0 3

B

Early to Middle Pleistocene Post-Olorgesailie Formation (silts; Olorgesailie) Post-Olorgesailie Formation (tufa; Olorgesailie)

TiO2 (%)

2

Olorgesailie Formation (Olorgesailie) Munya wa Gicheru Beds (Kedong) 1

Kapthurin Formation lake Beds (Baringo) Plio-Pleistocene

0

Chemeron Formation (Baringo) C

Miocene Ngorora Formation (Kabarsero)

Fe2O3 (%)

10

Ngorora Formation (Kapkiamu) Tambach Formation (Elgeyo)

5

Sedimentary basins 0

8

Okavango sediments F Trona (Na2O = 13·06%–39·28%)

D

L. Malawi sediments L. Tanganyika sediments

Na2O (%)

6

L. Turkana sediments L. Magadi cherts

4

Carbonate seds in the Pleistocene Olorgesailie (tufas, calcretes) and Holocene Bogoria (lake lmst.) basins

2

L. Magadi trona 0 0

5

10

15

20

Al2O3

L. Magadi Pleistocene silts and clays

Fig. 2. Variation diagrams for major elements versus Al2O3, showing separation of sedimentary basins. Symbols show samples for individual sedimentary units. Only representative samples are presented for Olorgesailie Formation and Munya wa Gicheru sediments to reduce crowding. © 2017 The Authors. Sedimentology © 2017 International Association of Sedimentologists, Sedimentology

R. B. Owen et al.

different sedimentary basins plotting in contrasting parts of the diagram (Figs 3 and 4). CaO and LOI plot with positive values on axis 1 and negative values on axis 2 (Fig. 3), with MnO, P2O5, MgO, Fe2O3, TiO2, Al2O3 and K2O showing negative values for both axes. In contrast, total SiO2 shows negative axis 1 and positive axis 2 values. The trace elements Au, Ag, As, Bi, Cd, Hg, Ir, Mo, Se and W were commonly, but not always, below detection limits. The transition metals Sc, Ni, Co, Cr, Cu and V display negative values along axis 1 (Fig. 4), but with Y showing a positive value along with: (i) the lanthanides La, Ce, Sm, Eu, Tb; (ii) the alkaline earth metals Sr and Ba; and (iii) the actinide Th. Uranium plots close to zero on axis 1, but has a similar negative value to most of the trace elements on axis 2. Other trace elements are positive along axis 2 with clustering of the alkali metals Cs and Rb

4

and the transition metals Ta, Hf and Zn. The inset in Fig. 3 presents a broader PCA field that includes chemical sediments (limestone, tufa, chert) that plot in contrasting parts of the PCA. Most EARS sediments have chondrite-normalized Lan/Ybn ratios