Diagenetic transformations and silcrete–calcrete intergrade duricrust formation in palaeo-estuary...

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms (2013)Copyright © 2013 John Wiley & Sons, Ltd.Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.3516

Diagenetic transformations and silcrete–calcreteintergrade duricrust formation in palaeo-estuarysedimentsS. Ringrose,1*,† L. Cassidy,2† S. Diskin,3† S. Coetzee,4 W. Matheson,1 A. W. Mackay5 and C. Harris61 Independent Consultants, PO Box HA 65, HAK, Maun, Botswana2 Ecosurv Environmental Consultants, P.O. Box 201306, Gaborone, Botswana3 HESAS, University of Glamorgan, Clyntaff, UK4 Department of Physics, University of Botswana, P/Bag 0022, Gaborone, Botswana5 ECRC, Department of Geography, University College London, Gower Street, London, UK6 Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa

Received 30 April 2013; Revised 25 November 2013; Accepted 25 November 2013

*Correspondence to: Susan Ringrose, Independent Consultants, PO Box HA 65, HAK, Maun, Botswana. E-mail: [email protected]†Former address: Okavango Research Institute, University of Botswana, Private Bag 285, Maun, Botswana.

ABSTRACT: The Boteti palaeo-estuary in northern Botswana is located where the endoreic Boteti river, an overflow from theregional Okavango river system, enters the Makgadikgadi pans. The present work considers diagenetic silica and calcium carbonatedominated transformations. The aims are to help identify precursor conditions for the origin of microcrystalline silcrete–calcreteintergrade deposits while developing insight into pene-contemporaneous silica and calcite matrix formation. General precursorconditions require the presence of cyclical endoreic freshwater inflow into a saline pan. The pan should be deep enough tosustain a permanent watertable under climatic conditions sufficient to cause carbonate fractionation within the groundwater.Freshwater inflow into a saline pan drives the geochemistry of the system (from freshwater to saline, from neutral to high pH).The geochemistry is controlled by the periodicity of inflow relative to salinity levels of phreatic groundwater in the receptorsaline pan. The source of most silica and localized CaCO3 is derived from the dissolution and precipitation of micro-fossils, while more general CaCO3 enrichment stems from saline pan based carbonate fractionation. Diagenetic change leadsto colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed by transformations into opal-ine silica over time. Irregular zones of siliceous sediment forming in otherwise calcareous deposits may relate to the irregularoccurrence of biogenic silica in the source sediments, inferring a source for local silica mobilization in intergrade deposits.The distribution of calcareous micro-fossils may have a similar converse effect. Diagenetic evidence from an intergradedeposit with a low SiO2/CaO ratio suggests that transformation occurred more into the pan, while an intergrade deposit witha high SiO2/CaO ratio more likely formed closer to a land margin and was frequently inundated by freshwater. Pene-contemporaneous silcrete–calcrete intergrade formation under the above conditions may take place where dissolved silicacrystallizes out in the vicinity of calcite crystals due to local decreases in pH. The continuing consolidation of bSiO2/CaOaggregates may be facilitated by the presence of increasing amounts of calcite. It appears that CaCO3 may act as a catalystleading to pene-contemporaneous bSiO2/CaO aggregate formation. However the processes involved require further work.Copyright © 2013 John Wiley & Sons, Ltd.

KEYWORDS: Makgadikgadi Pans; micro-fossils; bSiO2/CaO aggregates; freshwater to saline pan gradient; calcrete–silcrete intergrade precursors

Introduction

Intergrade duricrusts are defined as comprising CaCO3 <50%and SiO2 <85% as described in Summerfield (1983a);Nash and Shaw (1998); Shaw and Nash (1998); Nash et al.(2004) and Ringrose et al. (2005, 2009). Silcrete–calcrete inter-grade deposits are prevalent throughout Botswana and othersemi-arid regions within North and South America, and Australia(Nash et al., 2004 and references therein). Important sites havealso been investigated in southern Africa and reported in

Watts (1980); Summerfield (1983a, 1983b); Shaw and de Vries(1988); Nash et al. (1994a); Nash and Shaw (1998); Ringroseet al. (2002, 2005, 2008); Kampunzu et al. (2007). Shaw andNash (1998) developed a conceptual model for fluvial silcretesand intergrades exposed within river-marginal or valley settingsin the upper Boteti river. Morphological and petrological evi-dence suggested that surface silcretes developed by river-bornesilica accumulations in seasonal pools remaining after the annualOkavango flood, while sub-surface horizons of silcrete–calcreteintergrade deposits appeared to have formed under conditions

S. RINGROSE ET AL.

of varying pH associated with fluctuating groundwater levelsbeneath the channel floor. The sub-surface horizons arecharacterized by concentric pisoliths which show alternatingphases of silica and calcium carbonate deposition, withcarbonate precipitation often associated with silica dissolution.Further analyses led Nash et al. (2004) to describe threesilcrete–calcrete intergrade types in different Kalahari settingson the basis of silica–carbonate associations within duricrustcement (Nash and Shaw, 1998). The three different cementtypes comprise: (a) duricrusts where extensive secondarysilicification has occurred within a calcareous matrix, (b) va-rieties where secondary carbonate has been precipitatedwithin a siliceous matrix, and (c) materials where silicaand carbonate matrix cements appear to have beenprecipitated contemporaneously or in close succession’(Nash et al., 2004). Precursors of this latter type of pene-contemporaneous matrix and conditions for its developmentare further considered in this work.Pene-contemporaneous silica and carbonate formation is

taking place in the contemporary Okavango delta (McCarthyet al., 2012) where transpiration from plant growth results inthe progressive accumulation of dissolved salts under islands.As the solute-enriched groundwater becomes increasinglysaline, silica and then magnesium calcite are the first to pre-cipitate (McCarthy et al., 1993). Calcite precipitation (henceCaCO3 deposition) occurs later as the greater loss of solutesby transpiration beneath island riverine forest fringes ultimatelyleads to carbonate saturation in sub-surface waters (McCarthyand Ellery, 1994, 1995). Ringrose et al. (2008) describe similarOkavango precipitates in island, floodplain and dune sedi-ments, as silica-enriched diagenetic products termed clayenhanced amorphous silica (CEAS). CEAS is enriched with bothCaCO3 and silica reflecting the changing water table geochem-istry which ranges from smectite-dominated flood periods(along with silica precipitation) to sepiolite-dominated drierperiods (which promote CaCO3 precipitation). These conceptsare further reviewed in the present work, where a case study

Figure 1. Extent of Okavango river catchment from equatorial Angola to thStudy area shown by rectangle over southwestern Makgadikgadi Pans. Botetfigure is available in colour online at wileyonlinelibrary.com/journal/espl

Copyright © 2013 John Wiley & Sons, Ltd.

describes diagenetic changes at a location where the Botetiriver formerly entered the Makgadikgadi Pans.

TheMakgadikgadi Pans sub-basin (MSB) in northern Botswanais a large ( ~63 000 km2) depression which has been discussedby authors primarily concerned with extended mid-Kalaharipalaeo-lake systems and their links to Zambezi–Kwando–Okavango early drainage (Heine, 1982; Butzer, 1984;Lancaster, 1989; Thomas and Shaw, 1991, 2002; McFarlaneand Eckardt, 2006; Burrough and Thomas, 2009; Burroughet al., 2009; Moore et al., 2012, Podgorski et al. 2013).Others have considered duricrust related processes ongoingwithin the MSB boundaries in relation to regionalpalaeo-environmental interpretations (Cooke and Verstappen,1984; Ringrose et al., 2005, 2009;White and Eckardt, 2006; Riedelet al., 2009). The dynamics of the evolution of theMSB (Figure 1) inrelation to the history of Boteti river drainage into the basin, via theOkavango river and delta system, has received little recent cohe-sive attention. A number of authors have, however, consideredaspects of river drainage evolution, diatomite deposition andpalaeo-limnological change along the Boteti river (Cooke andVerstappen, 1984; Shaw et al., 1997; Shaw and Nash, 1998;Ringrose et al., 1999a; Riedel et al., 2009) as it extends both aboveand below the Gidikwe ridge 945 m strandline (Figure 1).

The main aim of this work is to identify precursor(environmental) conditions leading to silcrete–calcrete intergradeformationwhile attempting to identify geochemical changes con-ducive to pene-contemporaneous silcrete–calcrete formation.

Location and Methods

The study area forms part of the south-western limit of theMakgadikgadi pans in north central Botswana centred onNtwetwe pan at 21° 00" south and 25° 00" east. The location isprimarily where the endoreic Boteti river formerly enteredNtwetwe pan along a fault-bound depression (Figure 1 and 2(a)).The Boteti river formed a palaeo-estuary where it entered the

e palaeo-Boteti estuary relative to the Makgadikgadi Pan terminal area.i river indicated linking Okavango Delta with Makgadikgadi Pans. This

Earth Surf. Process. Landforms, (2013)

Figure 2. (a) Detail of Makgadikgadi Pans with Boteti river inflow from Okavango delta. Ntwetwe pan to west and Sua pan to east. (b) Ntwetwe pansample location. (c) Inter-pan channel sample location – both relative to the palaeo-Boteti estuary. This figure is available in colour online atwileyonlinelibrary.com/journal/espl

Figure 3. The palaeo-Boteti estuary with proximal and distal sample locations. Darker zones on estuary surface are grassed islands (see Figure 4).Inter-pan channel leaves the estuary from the northeast. Note arcuate islands and abandoned channels towards distal margin and incised (recent)course of Boteti river along western margin and across in to the inter pan channel. This figure is available in colour online at wileyonlinelibrary.com/journal/espl

AN INNOVATIVE PERSPECTIVE ON HOLOCENE DIAGENESIS

Copyright © 2013 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms, (2013)

S. RINGROSE ET AL.

pan with evidence of continued former flow through an inter-pan channel prior to a second entry into Ntwetwe pan itself(Figure 2 and 3). Although currently flowing annually(2011–2013) the Boteti river no longer enters the palaeo-estuary and terminates at Lake Xau (Figure 2), since itwas diverted by channelization works (SMEC, 1987). Thestudy area comprises bare pan-like surfaces or is sparselyvegetated with fringing pan grassland merging into mixedmopane trees and shrubs as soils thicken away from thepan edge. The area has an average rainfall of ~350 mmyr-1 which is anomalously low in terms of annual rainfallacross Botswana (Ringrose et al., 1999b). Annual precipita-tion is exceeded by evapo-transpiration by a factor of threeas temperatures range from 5–25°C in winter to 20–40°Cin summer (Water Surveys, 2010). Within this semi-arid re-gime, most of the Boteti river channel in the palaeo-estuaryis dry with pools and river channels being periodicallyfilled with rainwater which rapidly becomes saline (Figure 4(b)).Throughout most of Cenozoic time (McCarthy and Rubridge,2005) repeated filling and drying of the Makgadikgadi pansdepression led to a clay, silt and fine sand infill which is currentlysaturated by a near surface brine aquifer of the Na-CO3-SO4-Cltype (Shaw et al., 1990). The local bedrock geology comprisesArchean basement at depth covered by >250 m of Karoo(Lebung and Ecca Group) sandstones and shales, crossed by laterdyke swarms and covered by Kalahari Group sediments(Geological Survey of Botswana, 2000). Throughout this work

Figure 4. (a) Section through the upper friable silts of the proximal palaeo-Bbeds just visible towards lower end of tape.Tape extended 2.5 m. (b) Part of pradjacent lake infilled with rainwater which rapidly became saline. (c) Collecthe palaeo-estuary surface, north of IND 9A (see Figure 3). (d) Present day teTypha cappensis on channel margin. This figure is available in colour online


‘estuary’ is taken to mean the Boteti river palaeo-estuary whilethe phrase ‘Boteti estuary area sediments’ incorporates the Botetiriver palaeo-estuary along with inter-pan channel and Ntwetwepan deposits.

A sediment sampling strategy was devised to provide cover-age from proximal (southern) to distal (northern) parts of theestuary (Figure 3) and extended into the peripheral pan andinter-channel areas (Figure 2(b) and 2(c)). Samples reportedhere were obtained from fifteen pre-selected surface locations(hand-dug pits to 50 cm) and six auger holes. Surface locationswere used to sample upper units (friable silts) from the proximal(FP sample series) and distal (FD sample series) portions of thepalaeo-estuary (Figure 3). Deeper augured profiles (to c. 2 m)were drilled manually using a stainless steel posthole augerthrough a hole previously dug to 50 cm. Profile samples weretaken at 20 cm intervals through the proximal estuary (BPP1and BPP2), the distal estuary (BPD2 and BDP6), the inter-panchannel (ICP) and Ntwetwe pan (NPP) (Figure 2(b) and 2(c)and Figure 3). All samples were initially examined for sedi-mentary structures and analysed using an optical microscopeto ascertain visual characteristics. Twenty bulk samples fromthe proximal and distal estuary were subject to X-ray diffractionpowder analysis. The equipment used was a Phillips PW 3710based XRD unit, operated at 45 kVand 40 mA, employing Cu-Kalpha radiation and a graphite monochronometer. The sampleswere scanned from 3° to 70° 2θ and their diffractogramsrecorded. After initial scanning bulk samples were leached in

oteti estuary near BPP2. Note diatomaceous horizons. Lower silty clayoximal estuary near FP1 looking NE, showing former Boteti channel andtion of silcretised possible Typha cappensis root remnants washed ontorminus of the Boteti river at lake Xau (see Figure 2). Note prevalence ofat wileyonlinelibrary.com/journal/espl



a dilute 10% HCl solution. Leached residues were scanned forminor and accessory insoluble silicate phases in the bulk sam-ple. Mineral identification took place using Panalytical X’PertHiscore software based on the International Centre for Diffrac-tion Data’s Powder Diffraction File (PDF-2.2007) database. Ma-jor element compositions from the estuary area sediments weredetermined by Chemex, Johannesburg using an ICP-AES with adetection limit of 0.01 wt% and a relative precision of ± 1%(Kane, 1992). Loss on ignition (LOI) was undertaken to deter-mine the amount of inorganic carbon. One gram samples wereplaced in an oven at 1000° C for one hour, cooled thenreweighed. The percentage LOI was calculated from the differ-ence in weight. Inorganic CO2 was determined using a Leco-Gasometer with detection limits of 0.2 wt%. A Phillips6000 scanning electron microscope (SEM) and electron mi-croprobe was used to obtain micro-textural information, todetermine whether micro-organisms were present and toprovide EDAX based element data. The samples were car-bon coated and naturally occurring surfaces were examinedwithout preparatory polishing to preserve the micro-texturesintact.Carbon and oxygen isotope ratios from the proximal estuary

and inter-pan channel were determined at the University ofCape Town (UCT) on powdered silts and silty clays containingdisseminated calcium carbonate. The CO2 was extracted byreaction of 3–10 mg of powered sample and 5 mL of 100%H3PO4 at 25°C using the classical method of McRae (1950).Samples contained between 6.5 and 38.5 wt% calcite. TheCO2 extracted was analysed for both carbon and oxygen usinga Thermo Delta XP mass spectrometer in dual inlet mode, andthe data were corrected using the CO2–calcite fractionationfactor of 1.01025. Data are reported in the familiar δ notationwhere δ= (Rsample/Rstandard --1) * 1000 and R= 18O/16O or13C/12C. An in-house carbonate standard, Namaqualandmarble (NM), was run in duplicate with each batch of samples.The data obtained on the NM standard were used to convertthe raw data to the PDB and SMOW scales. The long-termduplication of NM suggests that the precision for both δ 18Oand δ 13C is better than 0.1 per mil. Three samples from estuarysediments (friable silts, cf. Figure 4(a)) were sent for conven-tional radiocarbon and accelerated mass spectrometry (AMS)assays to the Laboratory of Archaeometry, University of Athens,Greece and the Curt-Engelhorn Zentrum Archaeometrie,Mannheim, Germany. NET11 from the outermost reaches ofthe estuary (Figure 3) comprised the carbonized remains oforganic material from 20 cm below surface and was radio-carbon dated by conventional techniques. The two othersamples, BOT 09A and IND 9A were dated using AMStechniques. BOT 09A which occurred peripheral to a distalestuary channel (Figure 3) consisted of organic matter from75 cm below surface. Sample IND 9A was obtained from themargin of a channel in the proximal estuary and compriseda silcretized macrophyte root (probably Typha cappensis).The root fragments (Figure 4(c)) were exposed after havingbeen washed down onto the estuary surface by recentdrainage. The results were calibrated using Oxcal, (Ramsey,2010) based on the 14C datasets of Reimer et al. (2009–2012).

Results

Topography and sediment description

Elevation data obtained from the Shuttle Radar Topography Mis-sion (SRTM) show that the Boteti estuary is incised into a lowerpalaeo-Makgadikgadi strandline surface which is composed ofolder multi-phase silcretes and silcrete calcrete intergrades


occurring between 912–920 m asl (Cooke and Verstappen,1984; Thomas and Shaw, 1991; Ringrose et al., 2005, 2009).The more recent Boteti estuary lies below the 912 m strandlinesurface and extends for about 20 km north–south. The estuary is2 km wide at its southern proximal margin and 5 km wide atits distal margin, where it enters Ntwetwe Pan through a seriesof arcuate islands and remnant channels (Figure 3). The sur-face elevation drops from 908 m asl in the proximal area(where the Boteti river runs into the former estuary) to 906 masl distally at the pan margin, lowering to 904 m asl at theNtwetwe Pan surface. The latest dry Boteti river channel is in-cised into the surface of the Boteti estuary and runs northwardthrough the proximal to the distal areas before turning west-ward through the inter-pan channel (Figure 3).

The proximal and distal parts of the estuary comprise 2.0 mof semi-continuous lower light brown silty clays overlain bydark grey organic rich friable silts (Figure 5). Due to post-depositional deflation, parts of the upper friable silts areexposed beneath intermittent grass islands (Figure 3 and 4(a)).In the proximal estuary (BPP2) the friable silts are organicrich and interbedded with diatomite horizons over silty clay(Figure 5). In the distal estuary (BPD6) the organic rich friablesilts are capped by an intermittent burnt peat horizon. Thelower silty clays are more homogeneous and lack distinctiveorganic detritus. In the proximal reaches these sedimentsconsist of light brownish grey silty clay interspersed bysemi-indurated angular (blocky) silica rich fragments withintermittent Fe-oxide staining. In the distal reaches the siltyclay is less consolidated and lacks siliceous fragments. Bothproximal and distal silty clays overlie micro-fossil beds(Figure 5). The inter-pan channel deposits (IPD) consist ofdiatomites over a siliceous indurated layer overlying quartzrich silty clay. Ntwetwe pan sediments (NPP) consist ofdiatomaceous light brownish grey clayey silts which areinterrupted by a dark brown organic rich horizon (Figure 5).In summary the estuary area sediments consistently compriseupper and lower units defined by differing sediment charac-teristics. In the Boteti estuary these comprise upper friable siltand lower silty clay beds whereas the inter-pan channelsediments are characterized by lower silty clay and upper di-atomite beds. In the Ntwetwe pan profile the upper silty clayis separated from the lower silty clay by an organic rich hori-zon suggesting a hiatus between two depositional intervals.This general dichoctomy suggests that the palaeo-estuaryarea evolved as a result of two separate depositional events.

X-ray diffraction and major elements

Down-profile proximal sediment (BPP2) X-ray diffractogramtraces have characteristic peaks indicating the abundance ofquartz and calcite, with some plagioclase and muscovite(Figure 6). Clay minerals include smectites with illite andhalloysite (Table I). The mineral proportions change littledown-profile. Distal down-profile traces (from BDP6) show asimilar mineralogy but at this location amorphous silica,verified by SEM analysis was recognized in the lower beds(silty clays) as an enlarged ‘hump’ on the left side of thequartz peak (Figure 6). The distal sediment traces (BPD6)show that quartz and calcite are still dominant but haliteand muscovite are also present. The distal profile is charac-terized by Mg enriched calcite along with small quantitiesof smectite, illite and halloysite. No glauconite was foundin the samples (Table I).

The estuary area sediments are rich in SiO2 with CaO andAl2O3 being the other abundant major element oxides (Table II).Within the friable silts (F locations on Figure 3) the SiO2 content


Figure 5. Boteti estuary sediment profiles, showing relationships between upper organic rich friable silts and lower clayey silts at proximal (BPP2)and distal (BPD1, BPD2) locations including inter-pan channel (IPD) and pan (NPP) profiles. Note elevation of profiles at ground surface. This figure isavailable in colour online at wileyonlinelibrary.com/journal/espl

S. RINGROSE ET AL.

is higher in the proximal area of the estuary (43.9 to 76.1 wt%)than in the distal area (32.3 to 64.9 wt%). This varies somewhatfrom the XRD results which imply a higher amorphous silicacontent in the distal reaches. The CaO content of the friablesilts in the proximal reaches of the estuary ranges from 0.74to 3.31 wt% (with the exception of an anomalous high valueat FP3 at 16.05 wt%) while the range in the distal reaches ishigher at between 1.25 to 23.4 wt%. CaO is generally higherin the distal area. The Al2O3 content in the proximal estuaryranges from 2.84–6.98 wt% while the distal range occursbetween 1.45 to 7.59 wt% (Table II). Major element resultsfor estuary silty clays were obtained from BPD1 (Table II) withdown-profile changes shown on Figure 7. Results show thatSiO2 values range between 49.7 and 75.7 wt% while CaOvalues span 0.65 to 15.35 wt%. The Al2O3 content ranges from2.68 to 4.62 wt%. Inter-pan channel (IPD) sediments showminor differences in major element ranges between upperdiatomites and lower silty clays (Table III, Figure 7). In thediatomites, SiO2 increases from 42.1–69.3 wt%, with minimaldown-profile change through the lower silty clay (65.3 to67.3 wt%). CaO values are high at the surface ranging between9.17 to 17.53 wt% and decrease through the silty clays tobetween 10.3 to13.7 wt%. The Al2O3 content in the diatomiteranges from 2.43 to 3.38 wt%, while Al2O3 values in the siltyclays are similar at around 2.68 wt%. In Ntwetwe pan theCaO content is high in the upper silty clays ranging between19.25–22.7 wt% while the SiO2 content is low, ranging from32.5–37.2 wt%. These values change in the lower silty clayto a lower SiO2 range (28.1 to 30.6 wt%) and a higher CaOrange (23.6 to 25.1 wt%.). Al2O3 values are low and changelittle throughout the profile (Table III). Profile data comprisingboth the upper and lower units of Boteti estuary sediment pro-files highlight the extent of co-variation between SiO2 and CaOand element variation (Figure 7), a characteristic which is also


typical of calcrete–silcrete intergrade deposits (Thomas andShaw, 1991; Ringrose et al., 2002; Nash et al., 2004).

Correlation analysis (SPSS) on the distribution of major ele-ments (Table III) in the estuary silts and silty clays shows rela-tively strong positive associations (at the 95% significancelevel) between SiO2 with Al2O3 and SiO2 with TiO2. (r

2 =0.63and 0.67, respectively). Al2O3 is also correlated with TiO2 atthe 99% confidence level (r2 = 0.93) while positive associationsbetween Al2O3 and both K2O and Fe2O3 occur at the 95%level (r2 = 0.60 and 0.61, respectively). Significant negative as-sociations at the 99% confidence level are found betweenAl2O3 and both CaO and SrO (r2 =�0.72 and �0.68). CaO isalso positively correlated at the 99% confidence level withMgO and Sr (r2 = 0.70 and 0.95). The results are similar to thosefound elsewhere in intergrade duricrusts derived from Kalaharisands (Nash et al., 2004; Ringrose et al., 2005, 2009) and CapeCoastal silcretes (Nash et al., 1994(b)). The associations suggestthat Boteti estuary samples form two geochemical groups; onebeing SiO2/Al2O3 dominated, while the second group com-prises CaCO3 dominated associations. A CaO vs SiO2 plot ofthe estuary friable silts also shows a high degree of scatterwhich can be resolved into two fields (Figure 8). The mainlydistal estuary samples which form field (A) are characterizedby both high SiO2 and high CaO values while field (B) showshigh SiO2 and low CaO values from mainly proximal estuarysamples (Figure 8(a)). Data from silty clay samples from the es-tuary, inter-pan channel and Ntwetwe pan show a tighter clus-tering (hence higher correlation) of SiO2 and CaO (Figure 8(b);r2 =�0.956). Assuming a similar estuarine origin for both thefriable silts and silty clays this kind of increased clusteringsuggests that ongoing transformations incorporate increasingCaO into the sediment. Further analysis of the friable siltsshows high SiO2/Ca ratios associated predictably with highAl2O3 values (field A) in the case of proximal samples, while


Table I. X-ray diffraction results showing mineralogical change with depth at proximal and distal locations of the Boteti estuary (see profile dataFigure 5)

Sediment type Proximal delta (BPP2) Distal delta (BPD6)

Friable organic-rich silt Quartz, calcite, smectite, illite, plagioclase feldspar Quartz, halite, illite/smectite, halloysite,Upper silty clay Quartz, calcite, smectite, muscovite, possible nontronite Quartz, calcite, halite, illite, smectite, halloysite,Mid silty clay Quartz, calcite, smectite, possible nontronite Quartz, calcite, illite, smectite, halloysiteLower silty clay Quartz, amorphous silica, calcite, smectite, halloysite Quartz, amorphous silica calcite (Mg), halite,

halloysite, smectiteDiatomaceous silty clay Quartz, amorphous silica, calcite, smectite, halloysite Quartz, amorphous silica calcite (Mg), halite,

halloysite, smectite, muscovite

Figure 6. X-ray diffraction traces of BPD2 (proximal delta) and BDP6 (distal delta) showing main mineral peaks and amorphous silica hump to theleft of the main quartz peak, which is mainly prevalent in BDP6.


field B is mainly from distal samples (Figure 8(c)). For the siltyclay samples (Figure 8(d)) the SiO2/Ca ratio is decreased bya factor of 10 due to the increased CaO content. Nonethelesshigher SiO2/Ca ratio values in the A field circumscribe sam-ples from the estuary and inter-channel areas while lower ra-tios (B field) are exclusively from Ntwetwe pan. These datasuggest that friable silts which are younger have undergoneminimal transformation and infer that more advanced


transformations in the older, lower silty clays are associatedwith a higher (later introduced) CaCO3 source.

Scanning electron microscope

Scanning electron microscope (SEM) data were obtained toverify the kinds of transformational changes taking place


TableII.

Majorelem

entsfrom

(upp

er)friable

siltan

d(lo

wer)silty

clay

bed

sat

proximal

(P)an

ddistal(D

)locatio

nsin

Botetiestuary(P

2O

5missing,

seeFigu

re3forsample

locatio

ns)

Proximal

friable

silt

SiO

2wt%

Al 2O

3wt%

Fe2O

3wt%

CaO

wt%

MgO

wt%

Na 2O

wt%

K2O

wt%

TiO

2wt%

MnO

wt%

SrO

wt%

BaO

wt%

LOIwt%

FP1-35*

59.4

6.98

2.09

1.66

1.13

8.5

1.28

0.39

0.03

0.02

0.08

18.6

FP2-20

69.3

6.13

1.88

1.75

1.13

4.54

1.4

0.33

0.03

0.02

0.11

12.9

IND9A

**76.1

6.13

2.06

3.31

1.46

4.0

1.03

0.33

0.02

0.02

0.1

9.68

FP3-15

46.1

2.84

1.86

16.05

2.04

4.55

1.26

0.16

0.06

0.07

0.14

25.3

FP4-10

63.9

4.34

1.88

0.84

1.36

5.95

1.03

0.23

0.02

0.01

0.07

20.1

FP5-15

43.9

4.39

2.36

0.74

0.71

22.3

1.22

0.28

0.02

0.01

0.06

24

FP6-30

62.6

4.97

1.99

1.12

1.3

10.75

0.96

0.28

0.02

0.01

0.08

17.4

Distalfriab

lesilt

FD8-20*

32.3

2.4

1.32

23.4

4.27

5.06

0.98

0.13

0.05

0.19

0.07

31.5

FD9-35

67.3

1.45

0.63

14.35

1.4

0.24

0.55

0.15

0.02

0.11

0.15

12.8

FD10

-15

28.3

2.77

1.33

13.2

3.61

15.85

1.01

0.12

0.07

0.11

0.09

32.3

FD11

-10

45.8

1.92

0.69

18.35

1.42

6.07

0.6

0.19

0.02

0.16

0.18

26.2

FD12

-15

61.0

7.59

2.35

1.25

0.95

4.04

1.3

0.43

0.02

0.01

0.09

18.5

FD13

-30

61.6

4.38

0.95

14.25

1.85

1.27

1.11

0.32

0.03

0.12

0.18

13.9

BPD

1-10

29.2

1.76

1.61

4.55

1.55

25.3

1.08

0.14

0.03

0.03

0.05

22.5

BPD

1-20

42

2.66

2.13

19.65

1.94

2.5

0.95

0.16

0.11

0.1

0.21

26.3

BPD

1-30

64.9

4.45

2.63

6.46

2.02

2.43

1.37

0.32

0.04

0.05

0.13

14.9

BOT0

9A**

63.6

5.49

1.36

1.53

0.97

9.1

0.84

0.24

0.02

0.02

0.06

16.9

Distallow

ersilty

clay

SiO

2wt%

Al 2O

3wt%

Fe2O

3wt%

CaO

wt%

MgO

wt%

Na 2O

wt%

K2O

wt%

TiO

2wt%

MnO

wt%

SrO

wt%

BaO

wt%

LOIwt%

BPD

1-50*

75.7

3.65

2.63

0.65

2.11

2.72

1.23

0.22

0.03

0.02

0.11

11.5

BPD

1-70

55.7

2.68

1.95

13.35

2.6

2.92

0.93

0.12

0.04

0.1

0.08

20.6

BPD

1-90

49.7

4.62

1.9

15.35

2.97

2.87

1.36

0.26

0.05

0.11

0.13

20.2

BPD

1-110

68.7

4.57

1.45

10.6

1.38

1.71

1.36

0.3

0.03

0.07

0.14

10.6

BPD

1-130

59.6

2.72

1.92

12.6

1.4

3.69

1.06

0.16

0.03

0.09

0.12

17.8

BPD

1-150

61.6

3.34

2.04

12.15

1.88

2.91

1.28

0.23

0.03

0.1

0.14

16.1

*Sam

pledep

thin

cm**AMSdatelocatio

n

S. RINGROSE ET AL.


Figure 7. Down-profile changes in major element content in (a) the distal palaeo-Boteti estuary at BDP1, (b) the inter-pan channel area (IPD), (c)Ntwetwe pan (NPP) and Boteti delta BDP1. Note: changes in vertical (cm) and horizontal (wt%) scales.


down-profile and along the proximal estuary to saline pangradient. Proximal friable silt (BPP2) samples reveal that themajor SiO2 source is diatom frustules which occur in a partiallyconsolidated amorphous silica matrix along with neoformedspongy aggregates (Figure 9(A) and 9(B)). The aggregatesbecome progressively consolidated down profile (Table IV).The diatoms are identified as mainly periphyton species whichprefer shallow, fresh water conditions (Mackay et al., 2012).Minor quantities of Al2O3 Na2O, MgO3 and K2O suggest thepresence of smectite or illite (Table V). The SiO2 content is veryhigh in the proximal friable silts which are composed almostentirely of amorphous silica aggregates and matrix material,visually forming from degraded diatom frustules (hereafterbSiO2 aggregates). The proximal friable silt beds register a localSiO2:CaO ratio >200:1. This contrasts with the (lower) siltyclay part of the proximal profile which also consists of biogenicsilica rich aggregates but with increasing quantities down-profile of disseminated calcite and CaCO3 micro-fossil platelets(Table IV and cf. Figure 8(b)). The micro-fossil platelets are alsobiogenic in origin but cannot be identified because of theirdegradation and because they are frequently covered by calcite


rhombs. The bSiO2 aggregates increasingly incorporate calcar-eous platelets with depth thereby becoming bSiO2/CaO aggre-gates. The combined aggregates in turn become increasinglyconsolidated down-profile, where in the proximal profile theytransform into a smooth variably calcareous (amorphous) silicapaste interspersed with incipient opaline fragments. Thesetransformations cease at the base of the profile (Table IV) whichcomprises large micro-fossil remnants including bivalves anddiatoms in an open silica matrix (c. Figure 5). The trace elementcomposition of the silty clay shows low quantities of SO3 andFe2O3 suggesting slightly anoxic porewaters (Table V) hencethe likely prevalence of saturated conditions during deposition(Berner et al., 1985; Nash et al., 2004). At one location theFe2O3 and SO3 content was observed as being related to a hon-eycomb microstructure surrounded by amorphous iron sulphatein a bSiO2/CaO aggregate (Figure 9(C)). In the microstructure,Fe2O3 reaches 48.3 wt% and SO3 attains 35.8 wt%.

The distal estuary upper friable silts (BDP6) are composed ofbSiO2 aggregates in addition to numerous unaltered andpartially degraded diatoms (Table IV, Figure 9(D)). These forma diverse shallow water periphyton assemblage with mostly


TableIII.

Major

elem

entsfrom

inter-ch

annel

area

(IPD)an

dNtw

etwePan

(NPP)Botetiestuaryarea

sedim

ents(P

2O

5missing,

seeFigu

re2forsample

locatio

ns)

Inter-pan

chan

nel

SiO

2wt%

Al 2O

3wt%

Fe2O

3wt%

CaO

wt%

MgO

wt%

Na 2O

wt%

K2O

wt%

TiO

2wt%

MnO

wt%

SrO

wt%

BaO

wt%

LOIwt%

IPD-5

D42.1

2.43

1.51

17.55

0.94

6.87

1.24

0.12

0.04

0.13

0.19

28.5

IPD-30D

69.3

3.38

1.28

9.17

0.92

3.14

1.19

0.22

0.03

0.06

0.18

12.9

IPD-50LSC

67.3

2.93

1.5

10.3

0.94

2.85

1.16

0.24

0.03

0.09

0.15

13.9

IPD-70LSC

66.8

2.89

1.46

12.4

0.99

1.9

1.05

0.23

0.04

0.1

0.13

13.2

IPD-100LSC

65.7

2.85

1.55

13.1

1.05

1.4

0.96

0.24

0.04

0.1

0.12

12.9

IPD-120LSC

65.3

2.68

1.19

13.7

0.97

1.3

0.85

0.24

0.03

0.1

0.12

13.3

Ntw

etwePa

nSiO

2wt%

Al 2O

3wt%

Fe2O

3wt%

CaO

wt%

MgO

wt%

Na 2O

wt%

K2O

wt%

TiO

2wt%

MnO

wt%

SrO

wt%

BaO

wt%

LOIwt%

NPP-0

PC

27.4

1.95

1.09

23.8

3.98

6.53

0.97

0.11

0.04

0.17

0.19

34.6

NPP-10USC

32.5

2.19

1.69

22.7

5.05

4.44

0.92

0.11

0.05

0.15

0.18

31.0

NPP-20USC

34.5

2.46

1.34

20.7

6.25

3.81

0.98

0.12

0.05

0.13

0.18

29.5

NPP-30USC

33.1

2.29

1.36

21.6

7.14

3.54

0.94

0.11

0.05

0.14

0.16

30.0

NPP-40USC

37.2

2.4

1.5

19.25

7.28

3.64

1.00

0.11

0.05

0.12

0.16

28.3

NPP-50OH

32.3

2.47

1.46

23.4

6.29

2.77

0.98

0.12

0.05

0.14

0.20

29.5

NPP-60LSC

30.6

2.24

1.52

23.6

6.86

2.93

0.85

0.11

0.05

0.14

0.19

30.8

NPP-70LSC

28.1

2.0

1.28

25.1

6.78

2.92

0.78

0.09

0.05

0.15

0.19

32.0

*Sam

pledep

thin

cm.D=diatomite

;LSC=lower

silty

clay;USC

=upper

silty

clay;

PC=pan

crust;OH

Organ

ichorizo

n.

S. RINGROSE ET AL.


freshwater phyla including Amphora, Rhopolodia, Synedra andAulacoseira whose habitat preferences are indicated onTable VI (cf. MacKay et al., 2012). The friable silt aggregatesbecome gradually more calcareous with depth as low quanti-ties of fine calcareous platelets become apparent (Table IV).The distal silty clays (BDP6) are formed mainly of bSiO2/CaOaggregates which become increasingly calcareous andconsolidated with depth in the presence of trace elements(Tables IV and V). The CaO may be derived from CaCO3 richpatterned micro-fossil platelets or Ca2+in the groundwater andforms secondary calcite rhombs on the platelets (Figure 9(E)and 9(F)), in a dispersed amorphous silica matrix (Table IV).Hence the proximal BPP2 and distal BDP6 estuary profilesare similar being characterised by (a) diatomaceous friable silts(comprising mainly bSiO2 aggregates) overlying mixed bSiO2/CaO aggregates mainly in the silty clays, while (b) the proximalsilty clay profile shows more evidence of aggregate change to avariably calcareous silica rich paste and incipient opalinefragments than the distal profile which is generally more calcar-eous due to higher proportions of shelly micro-fossils anddisseminated calcite.

The presence of opaline fragments in the proximal silty claysled to further analysis of former Boteti river bed samples (BRS,Figure 3) to appreciate possible relationships between amor-phous silica transformation in near saline pan environmentssubject to freshwater flow. The results indicate that the river‘clay’ comprises relatively abundant opaline particles, alongwith diatom frustules, calcite crystals and CaCO3 micro-fossilplatelets (Figure 10(A) and 10(B)). The SEM images showopaline particles with a bright, angular crystallinity comparedto dull, sub-rounded, clastic quartz particles (Figure 10(C)).The river bed setting implies that the opaline particles mayhave neoformed as a result of crystallization under riverinefreshwater flow conditions, following saturation by silicic acid(bH4SiO4) during an earlier, drier period.

Further comparative insight into aggregate modification andopaline formation was sought through the SEM analyses of in-ter-pan channel (IPD) sediments. The IPD samples were foundto contain a relatively high quartz grain composition, despitewhich the overall matrix Si:Ca ratios (Table VII) and major ele-ment content (Table III) remain comparable to those occurringin the Boteti estuary sediments (Table IV). However, in contrastto the estuary sediments, the IPD diatomite horizon comprisesbSiO2/Ca aggregates which are more highly calcareous to-wards the surface with the CaO content decreasing down-profile (cf. Figure 7). The high bSiO2 content again relates tobeds of tightly packed diatom frustules which include freshwa-ter Synedra spp. and Rhopalodia rupestris in addition to themore brackish tolerant Cyclotella, Nitzchia and Stenopterobisspp (Figure 10(D)). Below the diatomite bed, dispersed CaCO3

occurs in the sediment matrix leading to the renewed develop-ment of bSiO2/Ca aggregates with superimposed salt (NaCl)crystals. Throughout the lower silty clay beds, the calcareousnature of the sediment increases (local SiO2/CaO 0.4:1) and(rare) neoformed silica and calcareous particles occur together(Table VII and Figure 10(E)). In the inter-pan channel silty clays,neoformation appears to be taking place in a milieu of cal-careous particles and bSiO2/Ca aggregates suggesting pene-contemporaneous deposition. The dissolved silica appears tohave been derived mainly from diatom frustules. Despiteapparent weathering effects (Figure 10(F)), most quartz clastsare intact (cf Figure 10(C)), suggesting that the overall role ofquartz grain dissolution in amorphous silica formation isnegligible even under potentially extremely saline conditions.

The SEM data from Ntwetwe pan (NPP) silty clays showthat these samples contain more calcareous fossil platelets(Figure 11(A)) and disseminated calcite than estuary deposits.


Figure 8. Relationship between SiO2,CaO and Al2O3 in palaeo-Boteti estuary area (a) friable silts where high CaO field A samples are from the distalestuary and high SiO2 field B samples are from the proximal estuary; (b) lower silty clays where high CaO field A samples are from Ntwetwe pan andhigh SiO2 field B samples are from the estuary and inter-pan channel; (c) NB: SiO2/CaO ratio (10X) and Al2O3 content higher in (c) friable silts than (d)silty clays. (c) friable silts A field shows considerably high SiO2/CaO ratio in proximal estuary compared to B field which shows lower values in thedistal estuary. (d) Values more tightly clustered in the silty clays where field A samples are from estuary and inter-pan channel and field B samples arefrom Ntwetwe pan. This figure is available in colour online at wileyonlinelibrary.com/journal/espl


The platelets adhere onto loosely formed bSiO2/Ca aggre-gates which become increasingly consolidated and enlargedwith depth as the CaCO3 content increases (Figure 11(B)).Identified diatoms in the NPP profile include the freshwatergenera Brachysira and Gomphonea, along with the salinepreferring species, Campylodiscus clypeus. The NPPsediments are also relatively Mg rich (Tables I and VII)suggesting that despite the diatom evidence for freshwaterconditions, Ntwetwe pan periodically dried out and thatMg-enriched carbonate precipitation took place probablyunder closed basin, in a highly evaporative setting (Eugsterand Jones, 1979; Ringrose et al., 2005).

Isotopes and age estimates

The δ13C and δ18O (vs SMOW) values of the carbonate mate-rial from the distal Boteti estuary (BDP1) and the inter-panchannel (IPD) range from �0.43 to +2.62 ‰, and 30.79 to33.61, respectively (Table VIII). No significant correlation existsbetween the estuary area δ13C and δ18O values. The range inδ13C values is much smaller (0.65 to 1.24 ‰) if sampleBDP1-25 is ignored. There is no systematic difference betweenthe δ13C and δ18O values for the friable silt and silty clay sam-ples, although the situation might be different if more sampleshad been analysed.No age estimates are available at present for the estuary

lower silty clays so determining when the Boteti river firstflowed into the Makgadikgadi pans awaits further work.Calibrated results of radiocarbon and AMS dating from estuaryupper friable silts are shown on Table IX. The midpoint 14Cresult (94.0% probability) from sample NET 11 is calculatedat 844 ± 48 cal years BP. The midpoint for the AMS result(93.7% probability) from sample BOT09-06A is calculated at1778 ± 36 cal years BP. The midpoints for the final AMS result(93.9% probability) from sample IND 9A is calculated at4481 ± 42 cal years BP. The IND 9A result is from a


mineralised macrophyte root (Figure 4(c) – probablyTyphacappensis) which was recently washed onto the proximalestuary palaeo-surface. Because of the high carbonate contentof the interstitial porewaters all these dates may be overesti-mates (Shotton, 1972).

Discussion

Environmental conditions

Whereas evaporative pan conditions in the Makgadikgadibasin are assumed to have been prevalent at least since the lateQuaternary (Ringrose et al., 2005; Burrough et al., 2009) theδ13C values suggest that during Boteti river flooding, the estuarycomprised a shallow fresh water marsh which provided ahabitat for diatoms and calcareous micro-fossils. Carbon iso-tope results suggest that the marsh supported mainly C4 plantspossibly macrophytes. Cerling (1991) shows that pure C3 andC4 plants produce δ13C values of �12‰ and +2‰, respec-tively (cf. Quade et al., 1995). Thus the plant contributions tothe calcareous sediment appear to have been almost entirelyproduced from C4 plants (cf. McCarthy et al., 2012). Theδ18O value of the carbonate depends on the δ18O value ofthe porewater from which the calcite precipitated and the tem-perature of formation. The expected δ18O value of ambientrainwater in the area is unknown but present day data forPretoria, Windhoek and Harare are available from the IAEAdata base (Rozanski et al., 1993) for the years 1961–1987.The weighted mean δ18O values for these locations are�3.7‰, �5.0‰ and �6.1‰, respectively. Hence the expectedweighted mean δ18O value of rainfall in the Boteti estuary areaat the present day is probably between �4‰ and �6‰, rela-tive to SMOW (cf. Ringrose et al., 2009). The measured carbon-ate δ18O values range from 30.79 to 33.61 ‰ (relative toSMOW). The differences between these d18O values and likelyrainfall are too large for them to be in equilibrium at any


Figure 9. SEM photographs from sample locations on Figure 3, showing (A) whole, fragmented and partially disintegrated diatom frustules in a var-iably open and dense siliceous matrix. Proximal estuary upper friable silts (BPP2). (B) Sponge-like aggregates formed from siliceous matrix material.Proximal estuary upper friable silts (BPP2). (C) Honeycomb microstructures with high iron sulphate content. Proximal delta lower silty clays (BPP2).(D) Diverse assemblage of diatom frustules in distal estuary upper friable silts (BPD6). Diatom phyla include Surirella (slightly alkaline waters) andfreshwater phyla Amphora, Rhopolodia, Synedra and Aulacoseira. (E) SiO2/Ca aggregates formed in and among CaCO3 rich micro-fossil plates. Arrowshows patterning on the plates confirming biogenic origin. Distal delta, lower silty clay (BPD6). (F) Detail of micro-fossil plates showing calcite pre-cipitation on upper side in a mixed matrix containing diatom frustules Distal estuary, lower silty clay (BPD6).

S. RINGROSE ET AL.

reasonable temperature. An example of such a temperature cal-culation is given in Table VIII where a water δ18O value of �4‰ is used. In all except one case, the temperature is negative.Using a water δ18O value of zero gives higher temperatures(5.1 to 16.3°C) but these are still lower than ambient averagetemperatures. However, if a water δ18O value of +2 ‰ is as-sumed, the range of temperatures is 13.2 to 25.4°C. The meanδ18O value is 32.52 ‰, and if this formed in equilibrium withwater having a δ18O of +2 ‰, the temperature of formationwould have been 17.7°C. This is close to the mean annual tem-perature of 18.8°C (Gaborone, Botswana in Cooke, 1987).Thus it seems likely that there was significant groundwaterevaporation during calcite formation in the estuary. To providea basis for comparison, the C and O isotope composition of theBoteti estuary samples is plotted alongside Moshaweng(Kampunzu et al., 2007) and Makgadikgadi (Ringrose et al.,2009) calcrete samples (Figure 12). Interestingly whereas theδ13C results are similar, the Boteti samples have significantlyhigher δ18O values than the older calcretes. Whereas it ispossible that the cause of the higher δ18O values in the Botetiestuary calcites is related to climate-induced differences inthe O-isotope composition of rainfall, it seems more likely thatthe high δ18O values of the Boteti samples are due to theirformation from water that had undergone greater degrees of


evaporation. Hence the Ca2+ forming within the estuary areasediments might have been introduced following heavy inflowsof fresh river water and later precipitated during times of highevapotranspiration possibly under warm/dry conditions, similarto those prevailing at present.

The overall environmental conditions of river inflow leadingto marsh formation, hence biogenic deposition and subsequentevaporative drying appear to have induced major sedimenttransformations. While these diagenetic changes took placepossibly during the late Quaternary or Holocene, the overallpalaeo-environmental setting which entails significant riverflooding interspersed with extensive evaporative drying, appearsimilar to those prevalent during the present day in northernBotswana. Present day environmental conditions include thepresence of the Makgadikadi pans which lie in a depressionwith a relatively high, saline phreatic groundwater table(Eckardt et al., 2008). Presently the related Boteti palaeo-estuary aquifers include (some) fresh surface water overlyingintermediate waters with an EC of 75 000 μS/cm with deeperwaters are characterised by an EC of 140 000 μS/cm, whilepH values range from 8–10 (Water Surveys, 2010). The aquifersare recharged by periodic river inflow and rainfall. Flow downthe Boteti river into the pan is slow due to low topography,while flood peaks presently are highly variable being derived


Table IV. Detail of SEM data from Boteti estuary profiles (see Figure 3 for sample locations)

Sample and depth DescriptionSiO2:CaO aggregates

and matrix

BPP2-10 FS Whole and fragmented heavily degraded diatom frustules in open amorphous silicamatrix interspersed with neoformed (hardened) bSiO2 spongy aggregates and fewquartz grains (Figures. 9A and 9B).

370:1

BPP2-20 FS Clear diatomaceous aggregations: disintegrating diatom frustules form nucleationpatches with few spongy fragments and quartz particles.

200:1

BPP2-75 FS Well formed bSiO2 aggregates – more consolidated with depth. Low quantities ofangular opaline fragments.

Zero CaO

BPP2-120 FS Partly consolidated biogenic aggregates in open amorphous silica matrix;disseminated CaO with Fe2O3 and SO3 suggesting anoxic porewaters.

16:1

BPP2-140 FS Similar bSiO2 aggregate configuration to 120 cm with small amounts of CaO asinfrequent calcareous platelets with calcite precipitates. CaO (19.8 wt%)contributes to the bSiO2 aggregates.

3:1

BPP2-160 LSC bSiO2/Ca aggregates consolidated into a smooth (amorphous) silica pasteinterspersed with CaCO3 platelets with calcite precipitates.

3.5:1

BPP2-200 LSC Small, dispersed bSiO2/CaO aggregates in open amorphous silica matrix mixedwith numerous large (to 200 μm) calcareous shells, diatom fragments and algaltubes. Matrix Si:Ca ratio reflects increased CaO content and shell fragments.

1.4:1

BDP6-10 FS Whole/degraded diatom frustules floating in a disaggregated matrix composed ofbSiO2 (49%) and salt (NaCl) with no CaO.

Zero CaO

BDP6-50 FS Partly formed bSiO2/Ca aggregates along with disintegrating diatoms and possibleclay forming elements (Table V) Fine calcareous platelets source of CaO.

1.2:1

BDP6-75-120 LSC Well-defined bSiO2/Ca aggregates (c. 100–400 μm long) with CaCO3 richpatterned micro-fossil platelets (Figure 9(E) and 9(F)) in amorphous silica matrixwith possible clay minerals and halite (Table V).CaCO3 platelets increasinglyabundant and larger (to 80 μm long) with depth.

0.8:1 (75 cm) 1.7:1(100 cm) 0.1:1 (120 cm)

*Sample depth (e.g.75) in cm; FS= friable silt; LSC= lower silty clay; D=diatomite; USC=upper silty clay.

Table V. EDAX composition of selected major elements as wt% from scanning electron microscope data (see Figures 2 and 3 for sample locations)

Sample depth and type SiO2 wt% CaO wt% Al2O3 wt% K2O wt% SO3 wt% Fe2O3 wt% Na2O wt% Cl2O wt%

BPP2-20 FS 89.1 0.4 3.4 0.9 0.46 1.4 2.2 1.1BPP2-140 LSC 57.5 19.8 3.5 1.4 0.95 2.8 3.88 0.88BDP6-30 FS 51.4 28.2 4.0 2.2 0.32 3.0 3.4 1.6BDP6-75 LSC 23.7 29.7 1.5 1.2 0.52 4.0 9.8 23.8BRS-20 Boteti river clay 62.0 6.3 2.8 1.9 1.82 3.9 9.4 7.3IPD-15 D 42.3 22.2 3.0 2.46 2.5 2.03 11.58 10.76IPD-70 LSC 59.8 20.75 3.72 3.42 0.65 3.33 3.21 0.55NPP-30 USC 46.49 27.94 2.26 1.56 0.0 2.33 5.35 2.62NPP-65 LSC 44.26 30.9 2.6 1.4 0.86 2.3 4.9 1.4

*Sample depth in cm. FS = friable silt; LSC= lower silty clay; D=diatomite;USC=upper silty clay.

Table VI. Diatom composition in the friable silt of the Boteti distal delta (BPD6)

Main genus/species Habitat preference/lifestyle

Genus Surirella, possibly S. engleri Habitat not well known – has been found living on the bottomsediments of freshwater lakes – often in occurs in slightly alkaline waters

Genera Aulacoseira and Synedra plus mixed species Mainly freshwater flora – also in Okavango deltaGenus Aulacoseira either A. granulata (more likely) or A. ambigua Indicative of freshwater conditions – genus requires

high amounts of silicaSpecies Achnanthes rupestoides Indicative of freshwater conditionsGenus Cocconeis, possibly C. placentula var. pseudolineata C. placentula var. pseudolineata prefers slightly alkaline

freshwater, and grows attached to other plantsSpecies Rhopalodia rupestris R. rupestris can fix own nitrogen as contains endosymbiotic bacteria.

Indicative of freshwater conditionsGenus Synedra possibly S. acus Indicative of freshwater conditionsGenus Campylodiscus probably C. clypeus. Indicative of alkaline, brackish quite saline watersGenus Gomphonema Indicative of freshwater conditions



Figure 10. SEM photographs from sample locations on Figure 3, showing (A) Angular neoformed silica rich (opaline) particles in a matrix of NaClcoated silica flakes, Boteti river bed, (BRS). (B) Neoformed silica rich (opaline) particle with NaCl crystals adhering to curvi-linear depressions alongparticle side, Boteti river bed, (BRS). (C) Sub-angular quartz grains juxtaposed with biogenic silica rich aggregates and halite crystals (IPD). (D) Diatomassemblage in IPD upper sediments including Synedra spp., Rhopalodia rupestris, Cyclotella, Nitzchia and Stenopterobis spp. (E) Angular neoformedparticles of varying composition in lower IPD sediment. Particle on left contains 93% SiO2 whereas particle on right contains 49% SiO2 and 20%CaO. (F) Dendritic weathering pattern (etchmarks) on sub-angular quartz grains formerly under salt coating (IPD).

S. RINGROSE ET AL.

via the Okavango river and delta, from equatorial rainfall in theAngolan Highlands (Figure 1). The Okavango river is character-ized (inter alia) by multi-decadal hydrological variability whichcould well be reflected in former flow regimes. The long-termvariability of the entire Okavango river system is manifestedthough 20–30 year and longer cycles of above-average orbelow-average flood conditions (Wolski et al., 2012). This hasbeen observed within the period of instrumental record (sincethe 1930s) and in the proxy climate record of the last 800 years(Tyson, 1999; Tyson et al., 2002). The decadal or longer termvariability within the river system may also have been region-ally characterized by infrequent more substantial wet/floodevents (Nash et al., 2006, Chase et al., 2009) which in theBoteti estuary context, may have contributed to periods ofinfrequent higher than average flow during the Holocene (cf.Holmgren et al., 2003). These higher than normal floods mayhave led to the deposition of the lower silty clays and upperbeds in the estuary area. The inference here is that probablythroughout the Holocene (a) in general the natural decadalvariability of the Boteti hydrological system probably inducedperiods of cyclical flooding interspersed with dry periods whichled to high water table and high evapotranspiration rates whichdrove the observed diagenetic changes and also that (b) inter-mittently specific higher than average flood events occurred


which swept sediment into the estuary, inter-pan channel andadjacent pan at least on two observable occasions, leading tothe deposition of the lower silty clays some time before deposi-tion of the more recent upper silt beds (cf. Figure 5).

Amorphous silica aggregate formation

In the Boteti estuary sediments, most aggregates are formedfrom biogenic sources. Silica is also known to form a majorcomponent of Okavango waters (McCarthy, 2005) from whichthe Boteti river is derived, while SiO2 could also be infusedfrom the underlying Karoo sandstones (Water Surveys, 2010).SEM data shows that the amorphous silica is nonetheless pre-dominantly biogenically derived and forms bSiO2 aggregateswhich co-precipitated with a range of metal cations in associa-tion with CaCO3 and calcareous shell fragments (Tables IV andVII). After sediment build up in a freshwater marsh, the laterdegradation and dissolution of diatom frustules feasibly tookplace under saline groundwater conditions (Fritz et al., 1991).Theoretical considerations discussed in Loucaides (2010)indicate that the dissolution rates of silica are significantlyenhanced by alkali salts (e.g. NaCl, KCl, MgCl and CaCl2) atpH 7.5–8.4 such as those occurring in the Makgadikgadi pans


Table VII. Detail of SEM data from Boteti estuary area profiles (sample locations Figure 3)

Profile DescriptionSiO2:CaO aggregates

and matrix

IPD-20 D Well developed bSiO2 and bSiO2/Ca aggregates to 1.3 mm and sub-rounded quartzgrains in amorphous silica (and halite) (Figure 10©). Few calcareous platelets withcalcite precipitates. Locally matrix SiO2 content is 41.8 wt% and CaO is 32.5 wt%.

1.3:1

IPD-24 D Quartz grains (and zircon) in dense SiO2/CaO matrix with SiO2 content averaging49.3 wt%, whereas the CaO is 28.5 wt%. No platelets suggesting dispersedCaCO3 in the matrix.

1.7:1

IPD-35 D Numerous diatom frustules packed into dense bSiO2 aggregates which are encrustedwith salt (Figure 10(D)).

15:1

IPD-40 D Densely packed diatom frustules and quartz grains in diatomaceous matrix. DispersedCaCO3 in matrix leads to the renewed development of bSiO2/Ca aggregates.

4.4:1

IPD-70 LSC Sub-rounded quartz grains in a dense matrix of bSiO2 and CaOwith superimposed salt(NaCl) crystals (cf. Figure 10(C)). Rare angular neoformed particles vary incomposition from 93.2 wt% silica and 3.2 wt% CaO to 48.9 wt% silica and20.4% CaO (Figure 10(E)). Locally relatively high CaO content (up to 60%).

0.4:1.

IPD Silty clay 90–120 cm Quartz grains in dense bSiO2/CaO matrix with degraded diatoms. Low CaO content. 1.6:1 90 cm 1.2:1.120 cmNPP Silty clay 10 cm Loosely formed bSiO2/CaO aggregates (to 100 μm) of amorphous silica and minor clay

type elements (Table V) comprising 41.1 wt% SiO2 and 39.7 wt% CaO. Largecalcareous micro-fossil platelets (to 120 μm). Mg content relatively high at9.39 wt%.

1:1

NPP Silty clay 30 cm Closely packed (and sponge-like) bSiO2/CaO aggregates with disintegrating diatomsincluding the freshwater genera, e.g. Gomphonema and large platey shellfragments (Figure 11(B)).

2:1

NPP Silty clay 50 cm bSiO2/CaO aggregates consolidating with influx of large calcareous platelets. Someplatelets cemented over leading as calcareous fragments. MgO drops to 6.7 wt%.

0.5:1

NPP Silty clay 70 cm bSiO2/Ca aggregates cemented together forming macro-aggregates (to 800 μm) withcalcitic micro-fossil fragments on their upper surface (Figure 11(C)).

1.6:1

*Sample depth in cm. FS = friable silt LSC; = lower silty clay; D=diatomite; USC=upper silty clay.


at present (Eckardt et al., 2008). Krauskopf (1956) andLoucaides (2010) show that the effective dissolution of bSiO2

in saline brines leads to an efflux of dissolved silica into thewater column or sediment pore-waters. The rate of dissolutionis controlled by pH (Owen, 1975; Iler, 1979). According toLoucaides (2010) increasing pH leads to the deprotonation ofsilanol groups (>Si-OH0 ⇔ SiO- +H) which facilitates thebreaking of siloxane bonds which are the rate limiting step ofthe dissolution process. Dissolution rates double as pH in-creases from 6.3 to 8.1 under saline conditions. In additionabove pH 7, dissolution rates increase exponentially especiallyin brines containing Na. The increased solubility is caused bythe formation of sodium bisilicate, following the reaction:

Naþ þOH– þ SiO2 aqð Þ⇔NaHSiO3 aqð Þ(Park et al., 2006)

Since the Makgadikgadi pan environments are Na rich, thiskind of exponential increase appears plausible, such thatdiatom degeneration most likely took place rapidly once thesediment pile was submerged in extremely saline groundwa-ters. In the Boteti estuary sediments this required a geochemicalchange in the phreatic groundwater from predominantly freshto highly saline/alkaline. This requires a decline in freshwaterinflow and a period when the entire pan area was subject toa strong evaporative gradient inducing salinization from theunderlying groundwater. The salinization effect leads to thesupersaturation of pore spaces with silicic acid (bH4SiO4).Once established highly evaporative conditions also lead tothe concentration of Ca2+ and CO3 ions in the pore watersleading to the precipitation of CaCO3, as discussed below.The clay minerals (smectite, with illite and halloysite) are of

interest as these appear to be found in association with thealready formed bSiO2/CaO aggregates (Tables I and V). A rela-tively high Al2O3 content is evident particularly in aggregates


with a high SiO2/CaO ratio (Figure 8(c) and 8(d)). Clay mineralneoformation in Makgadikgadi pan strandlines has beenpreviously reported in Ringrose et al. (2005) while possiblyneoformed smectites have also been inferred from islandenvironments in the Okavango region (Ringrose et al., 2008).In terms of a possible clay origin, Owen (1975) indicates thatcolloidal particles in alkaline solutions are negatively chargedand so (may) co-precipitate with hydroxides of Fe, Al, Mn orMg creating clay-like substances at high salinities. Smectiteneoformation is also known to take place in highly saline envi-ronments such as the Bolivian salt pans (Badaut and Risacher,1983). However, for amorphous silica to precipitate, a decreasein both salinity and pH is required. The association of clay typeminerals with bSiO2/CaO aggregate formation infers the priordevelopment of clay mineral nuclei at high salinities and thelater precipitation of amorphous silica around the clay nucleus.This appears probable as clay-like substances are also knownto accelerate the aggregation of amorphous silica, especiallyin the presence of opaline sinter (Krauskopf, 1956). Theneoformation of smectite under mixed freshwater and salineconditions has been described as reverse weathering as clayforming elements combine under these conditions, rather thandisperse. The concept of reverse weathering which involves thedisintegration of marine diatoms in the presence of degradedclay minerals, has previously been described in the Amazonestuary where relatively fresh and saline waters mix(Mackenzie and Kump, 1995; Michalopoulos and Aller,1995). The relationship between the precipitation of clay-likesubstances and amorphous silica as exemplified in Boteti estu-ary sediments was previously tentatively described as reverseweathering by Frings et al. (2012), and appears justified in thelight of new SEM evidence (Tables IV, V and VII).

After the dissolution of diatom frustules when estuarinesediment pore waters were supersaturated with bH4SiO4 undersaline conditions (described above), the previously saline phre-atic groundwater is required to revert to freshwater. Reversals


Figure 11. SEM photographs from sample locations on Figure 3,showing (A) perforated patterns on shelly micro-fossil from NtwetwePan (NPP); (B) development of amorphous silica (bSiO2) and CaO ag-gregates with minor clay below Ntwetwe Pan surface (NPP); (C) mi-cro-fossil shell with calcite crystallised on upper surface, embeddedin an amorphous silica and clay aggregate (NPP).

S. RINGROSE ET AL.

from phreatic saline to fresh groundwater take place semi-annually in the region under the variable flood regime de-scribed above (Wolski et al., 2012). This presently occurs under

Table VIII. Carbon and oxygen isotope results from Boteti estuary and inte

Sample depth (cm) and sediment type Calcite % δ13C PDB ‰ δ1

BDP1-25 friable silt 38.5 �0.43BDP1-70 friable silt 26.9 0.65BDP1-110 silty clay 6.5 1.01BDP1-130 silty clay 27.5 0.99BDP1-150 silty clay 13.6 1.23IPD-10 diatomite 29.4 0.77IPD-50 silty clay 14.0 1.24IPD-100 silty clay 18.2 1.22

The calcite percentage was calculated from the yield of CO2 during carbonaequilibrium with water having δ18O values of �4, 0 and +2 ‰. The calcite


Okavango delta islands for instance, as renewed groundwaterfrom seasonal flooding forms a freshwater lense overlyingdenser saline groundwater (McCarthy, 2005; Wolski andSavenije, 2006). By analogy, renewed freshwater flow into theBoteti estuary would be expected to replenish the groundwaterby initially forming a freshwater lense over the phreatic salinegroundwater surface. When this occurs the consequent changein pH (to c. pH 5–6) and salinity reductions would naturallyinduce the precipitation of amorphous silica. The resultingdilution of previously bH4SiO4 supersaturated saline ground-water initiates the polymerization of both monomeric andbisilicate silica species (if present) to particles of colloidaldimensions (Owen, 1975). As polymerization proceeds, thecolloidal gels later form silica rich aggregates associated withclay nuclei, similar to those found in the present study. Thesilica rich matrices seen for instance in Figures. 9(A) and 10(D), may represent the colloidal stage of bSiO2 formation. ThebSiO2 rich spongy and dense aggregates (Figures 9(B) and 11(B)) and opaline silica particles (Figure 10(A), 10(B) and 10(E))are indicative of later stages in the process of silica transfor-mation consequent upon repeated dilution effects (cf. Street–Perrott and Barker, 2008). The increase in silica enrichedopaline fragments in the inflow (proximal) estuarine locationcould well be explained by repeated inflow at this location.

Role of calcium carbonate

Whereas silica is the dominant major element, calcium carbon-ate plays a secondary role in the formation of SiO2/Ca aggre-gates and is particularly prevalent in Boteti estuary lower siltyclays and pan environments (Figures 7 and 8). CaCO3 in theestuarine sediments occurs in two phases, as micro-fossils andas CaCO3 precipitates. Evidence from Ntwetwe pan deposits(NPP) suggests that one controlling factor behind the morestrongly calcareous bSiO2/CaO aggregates is simply the rela-tive abundance of shelly micro-fossils. While the calcareousmicro-fossil habitats are unknown, their distribution suggests apreference for deeper, stiller waters relative to diatom specieswhich preferred shallower, possibly slow flowing water inwhich to photosynthesize (Table VI). If correct, this habitatpreference could help explain the overall increase of CaO(Figure 8) along the proximal to saline pan gradient whereasthe general increase in disseminated CaCO3 with depth isfurther considered below.

Calcium and bicarbonate would over the years beintroduced via freshwater inflow as high concentrations ofCa2+ and HCO-

3 ions are prevalent in the Okavango regionsurface and groundwater (Mackay et al., 2011; McCarthyet al., 2012). Other possible sources include the direct

r-pan channel (see Figures 2 and 3 for sample locations)

8O SMOW ‰ δ18O PDB ‰ toC �4‰ toC 0‰ toC +2‰

30.79 �0.12 0.3 16.3 25.433.59 2.60 �9.2 5.2 13.232.32 1.37 �5.0 10.1 18.632.94 1.97 �7.1 7.6 15.933.61 2.62 �9.2 5.1 13.232.31 1.36 �5.0 10.1 18.633.40 2.42 �8.6 5.9 14.031.23 0.31 �1.3 14.5 23.4

te reaction at 25°C. Temperatures of formation are calculated assuming–water fractionation equation used was that of O’Neil et al. (1969).


TableIX.

Detailsofradiocarbon

andAMSdates

from

friable

silts,Botetiestuary(see

Figu

re3forsample

locatio

ns)

Labo

ratory

Code

Sample/M

aterial

δ13C(‰

)Calibrateddate(calBP)

Calibrateddate(BC/AD)

Probab

ilitie

s

DEM

-2182

NET

11Dep

th:20cm

carbonised

organ

icmatter

�21.24

926–

963calBP751–

744calBP

1024–

1187AD

1200–

1206AD

94.0%

1.4%

DEM

-2485

(MAMS-14182)

BOT09-06ADep

th:75cm

carbonisedorgan

icmatter

�11.70

1860–

1850calBP1826–

1714calBP

90–

101AD

124–

236AD

2.7%

93.7%

DEM

-2171

(MAMS-14213

IND

9ADep

th:surface

mineralized

reed

rootpossibly

Typhacappen

sis

�24.70

4567–

4559calBP4530–

4418calBP

2618–

2610BC2581–

2469BC

1.5%

93.9%

igure 12. Plot of δ13C vs δ18O for Boteti estuary carbonates in com-arison to global and previous Botswana calcretes. Fields from theangetic Plains are taken from Srivastava (2001); the Thar Desert,W India Andrews et al. (1998); Loess fields of China, (Rowe andaher (2000); the Trevel Graben, central Spain, Alonso-Zarzaa andrenas (2004); and Broken Hill Australia, Schmid et al. (2006). Dataoints for the Makgadikgadi pan strandlines and Moshaweng (Kalahari)om Ringrose et al. (2009). Boteti samples show similar δ13C values toose found in the Makgadikgadi pan strandlines. Boteti δ18O valuesre higher indicating a greater influx of freshwater inflow and highlyvaporative conditions.



FpGNMApfrthae

weathering release of Ca2+ from silicate minerals and viacarbonate rich wind-blown dust (Eckardt et al., 2008). In theBoteti estuarine sediments, overall CaO concentration increasesdistally and towards the pan (Figure 8). The high concentrationof (both Ca2+ and Mg2+) carbonates in the Makgadkigadki panenvironment has been previously described in Eckardt et al.(2008) as a direct result of evaporative fractionation. A Ca2+

and Mg2+ increase, in combination with CO3, (at high pH levels)leads to a quantitative deposition of CaCO3 (c. Eugster and Jones,1979) in a saline pan environment. Hence it is reasonable toexpect that a relatively high incidence of carbonate fractionationtook place where saline/alkaline conditions were most prevalent,which in the Boteti estuary area occurred more towards the distalestuary and in Ntwetwe pan.

In terms of CaO involvement in otherwise bSiO2 aggregateformation, Kellermeier et al. (2010) indicate that CaCO3

crystallises through a multi-step pathway at high pH, fromsolutions containing different amounts of sodium silicate. Theauthors found experimentally that growing amorphous CaCO3

particles provoked the spontaneous polymerization of silica intheir vicinity, probably resulting from a local decrease of pHnearby the surface. Barker et al. (1994) also suggest an associa-tion whereby the precipitation of amorphous silica is invokedby the presence of calcareous fragments. However from SEMresults described here, it appears that in semi-arid aquatic envi-ronments initial biogenic silica precipitation can take placewith or without CaCO3. The increasing presence of CaO withdepth (Figure 8) where greater transformational changes takeplace (Tables IV and VII) suggest that increases in disseminatedCaCO3 may have a catalytic effect on bSiO2/CaO aggregateformation, and their further consolidation into opalinefragments. Normally CaCO3 precipitation takes place at highpH levels (8–9) which in pan groundwater environments alsocorresponds to high salinity levels which would induce diatomfrustule disintegration and silica saturation inferring inverseprecipitation/dissolution processes (Shaw and Nash, 1998).The observed evidence of pene-contemporaneous SiO2 andCaCO3 precipitation in Boteti estuary sediments needs further


S. RINGROSE ET AL.

work to ascertain the conditions supporting the local pH/salin-ity changes required. However SEM data suggest that followingan initial phase of bSiO2 aggregate formation, increased Ca2+

dissemination through the sediment via groundwater fluctua-tions may have aided in the generation of bSiO2 rich spongyand dense aggregates and opaline silica particles especially inthe silty clays.

Diagenesis and silcrete–calcrete intergradeformation

An overall explanation for the deposition of precursor silcrete–calcrete intergrade deposits lies in the juxtaposition of contrast-ing environmental variables. These have been described in thepresent work from isotope data which indicated mainly C4plants being present in marsh-like conditions and δ18O datawhich infer highly evaporative conditions. To form intergradedeposits under these conditions cyclically based endoreicfreshwater inflow is required into a saline depression or panwhich is deep enough to sustain a permanent water table.The pan climate should incorporate long dry periods withtemperatures reaching around 45°C, being sufficient to causecarbonate fractionation within the groundwater (cf Eckardtet al., 2008). Such conditions are locally prevalent in semi-aridareas worldwide including the Andean plateau, interiorAustralia and southwest USA. Interestingly cyclical Boteti riverflow presently in northern Botswana has developed a freshwa-ter marsh in the Lake Xau depression (Figure 4(d)) providing aplausible current environment for intergrade precursordevelopment. The geochemistry of the system (from freshwaterto saline, from neutral to high pH) may change seasonally,decadally or over hundreds of years (Tyson et al., 2002). Inthe present work age estimates place the deposition of theupper friable silts (for instance) as taking place around 4000cal years ago and terminating around 1000 cal years ago, inthe late Holocene (Table IX). This is consistent with Khoiseanpottery and hand-tools which occur in the study area (Republicof Botswana, 2010) suggesting that under conditions of re-peated but highly intermittent inflow, diagenetic transforma-tions may take place over approximately 3000 years.In the Boteti palaeo-estuary freshwater inflow led to a

mostly freshwater pan peripheral marsh environment (similarto Lake Xau, Figure 4(d)) sustaining diatoms and calcareousmicro-fossils which became source material for silica andcalcium carbonate diagenesis. These particular biogenicsources may not always account for source materials formingsilcrete–calcrete intergrade deposits. SEM data reported here(Figure 10(C)) suggests that the dissolution of quartz grainswas not taking place, despite apparent weathering effects(Figure 10(F)) so a predominantly local biogenic source forsilica diagenesis is reasonable for silcrete–calcrete intergradeformation. The presence of abundant diatoms (in this case)which readily dissolve under saline high pH conditions(Loucaides, 2010) contributes mostly to the development ofsilica saturated groundwater. Calcium carbonate sourcesinclude calcareous micro-fossils which likely provide for thelocalized enrichment Ca2+ in intergrade deposits. The distri-bution of CaCO3 in the Boteti estuary sediments (enrichmentdownprofile and distally, Figures 7 and 8) suggests the contri-bution of a more general enrichment which is believed tostem from carbonate fractionation (Eckardt et al., 2008). Weare suggesting here that mostly the pre-existing biogenic silicaand disseminated calcite (including micro-fossils) providespecific loci for amorphous silica and CaCO3 dissolutionand precipitation.


In terms of their distribution throughout the sediment pile,the irregular occurrence of diatom frustules in the Botetipalaeo-estuary may also help explain the general occurrenceof irregular zones of siliceous sediment forming in otherwisecalcareous deposits, inferring a source for the local mobiliza-tion of silica. The distribution of calcareous micro-fossils mayhave a similar converse effect. Secondary emplacement andmovement of siliceous or carbonate enriched groundwaterthrough the sediment profile is seen to have taken place verti-cally and laterally away from the original source area due topedogenic and non-pedogenic processes (Nash et al., 2004).These processes mostly lead to silcrete–calcrete intergradedeposits being characterized by distinct zones or void fills ofeither siliceous or calcareous sediment but may also contributeto pene-contemporaneous intergrade formation. However thespatial distribution of more silica rich intergrade units relativeto more calcareous enriched units in older deposits may beinterpreted in terms of duricrust palaeo-environments. In theBoteti estuary more bSiO2 aggregates and opaline silica weredeposited close to the entry point of the river whereas moreCaCO3 is evident in the distal estuary and Ntwetwe pan(Tables II and III). Hence a higher proportion of precipitatedamorphous silica may be interpreted as occurring towards theproximal end of a pan peripheral deposit where freshwaterinflow takes place most frequently. This is because a higherfrequency of inundation leads to an increased incidence ofsalinity reduction, hence silica precipitation. Conversely ahigher proportion of CaCO3 in an intergrade deposit suggestsa drier, evaporative pan environment with a high phreaticgroundwater table and lower inundation frequency. The pres-ence of more calcareous intergrade deposits infers that longeror more intense evaporative fractionation (relative to freshwaterinflow) took place within the groundwater table. So the diage-netic evidence from an intergrade deposit with for instance alow SiO2/CaO ratio would suggest that transformation occurredtowards the distal/pan edge of a given pan peripheral depositwhile an intergrade with a high SiO2/CaO ratio more likelyformed as a result of a frequently recurrent freshwater system.

Specifics of the diagenetic processes described in theBoteti palaeo-estuary case study result in theoretical andobservational (SEM) evidence which is in part re-iteratedhere to provide insight to the possible dynamics of pene-contemporaneous silcrete–calcrete intergrade formation. Animportant consideration is that while silica is the dominantelement in intergrade deposits, CaCO3 is also generally pres-ent. The predominance of precipitated silica may involve claymineral nuclei and increasing amounts of collateral CaCO3.The clay nucleus issue requires further work but the presenceof clay minerals in silcrete–calcrete intergrade deposits suchas those reported in for instance Kampunzu et al. (2007) andRingrose et al. (2005) may have environmental significance.Ringrose et al. (2008) suggested that the kind of clay mineralpresent in Okavango area sediments suggested neoformationunder alternating flooding and dry regimes, similar to thosedescribed in the Boteti palaeo-estuary. Clay minerals in thestudy area (smectite, with illite and halloysite) are found inassociation with the already formed bSiO2/CaO aggregates(Tables I and V). Owen (1975) indicates that silica richcolloidal particles in alkaline solutions are negatively chargedand so (may) co-precipitate with hydroxides of Fe, Al, Mn orMg creating clay-like substances at high salinities. Clay-likesubstances accelerate the aggregation of amorphous silica, es-pecially in the presence of opaline sinter (Krauskopf, 1956).Hence the presence of clay minerals of a particular type mayaid in the identification of intergrade precursor conditions.The possibility of clay forming in intergrade deposits may bedescribed as reverse weathering (Frings et al., 2012).



Disseminated calcite in the sediment pile precipitates underhigh pH (>8–9) conditions while silica (in the form of bH4SiO4)remains dissolved in the highly saline groundwater. This suggeststhat pene-contemporaneous precipitation is unlikely as a timelapse of at least decades would be required to facilitate thenecessary change in groundwater geochemistry (cf. Shaw andNash, 1998). However, evidence developed in the present work(Tables IV and VII) shows that dissolved silica may begin tocrystallize out in the vicinity of calcite crystals due to localdecreases in pH. Such conditions would lead to a form ofpene-contemporaneous bSiO2/CaO aggregate formation. Thismay be similar to results developed by Barker et al. (1994) andKellermeier et al. (2010) who describe instances where theprecipitation of amorphous silica is invoked by the presence of cal-careous fragments. Further evidence in the Boteti palaeo-estuarysediments suggests that increasing degrees of bSiO2/CaO consoli-dation may be facilitated by the presence of increasing amountsof calcite. If further validated it appears that the presence of CaCO3

may act as a form of catalyst leading to pene-contemporaneousbSiO2/CaO aggregate formation. However the processes involvedare uncertain and require further work.

Conclusions

Precursor conditions under which silcrete–calcrete intergradedeposits are formed are here taken from extant environmentalvariables while inferred processes leading to possible pene-contemporaneous silcrete–calcrete deposits are both consid-ered from work on the Boteti palaeo estuary in northernBotswana. General conclusions from this work are:

1. The formation of silcrete–calcrete intergrade depositsrequires cyclical endoreic freshwater inflow into a salinedepression or pan which is deep enough to sustain a perma-nent water table under climatic conditions sufficient tocause carbonate fractionation within the groundwater.

2. The geochemistry of the system (from freshwater to saline,from neutral to high pH) relates to the periodicity of inflowrelative to salinity levels in the phreatic groundwater in thereceptor saline pan. Inflow may change seasonally, deca-dally or over hundreds of years while diagenetic transfor-mations may take place over approximately 3000 years.

3. The source of most silica and localized CaCO3 may bederived from the dissolution and precipitation of micro-fossils, while more general CaCO3 enrichment stems fromsaline pan based carbonate fractionation. Diageneticchange leads to colloidal then more consolidated bSiO2/CaO aggregate formation (amorphous silica) followed bytransformations into opaline silica.

4. Irregular zones of siliceous sediment forming in otherwisecalcareous deposits may relate to the irregular occurrenceof biogenic silica in the source sediments, inferring a sourcefor local mobilization of silica. The distribution of calcareousmicro-fossils may have a similar converse effect.

5. The spatial distribution of more silica rich intergrade unitsrelative to more calcareous enriched units in older depositsmay be interpreted in terms of pan peripheral palaeo-environments. Diagenetic evidence from an intergradedeposit with a low SiO2/CaO ratio would suggest that trans-formation occurred more into the pan while an intergradedeposit with a high SiO2/CaO ratio more likely formed closerto a land margin and was frequently inundated by freshwater.

6. Pene-contemporaneous silcrete–calcrete intergrade for-mation under the above conditions takes place wheredissolved silica may begin to crystallize out in the vicinityof calcite crystals causing local decreases in pH.


Increasing degrees of bSiO2/CaO consolidation may be fa-cilitated by the presence of increasing amounts of calcite.It appears that the presence of CaCO3 may act as a form ofcatalyst leading to pene-contemporaneous bSiO2/CaO ag-gregate formation. However, the processes involved re-quire further work.

Acknowledgements—This work was made possible through a grantfrom the University of Botswana, Research and Publications Commit-tee. Water Surveys (Pty) Ltd and Debswana Botswana provided usefuladditional data. Laboratory work was provided at the Okavango Re-search Institute by Mr Masole and Mr Thorego. Thanks are extendedto two unknown reviewers who assisted with an early draft of this work.Patrick Frings provided valuable comments on an early draft whileSallie Burrough advised on age estimates. Yannis Manastis provided in-sight into the radiocarbon and AMS dates while Judith Sealy providedfunding for C and O isotope analyses.

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