Mineralogy and Micromorphology of a late Neogene Palaeosol Sequence at Langebaanweg, South Africa: Inference of Palaeoclimates

Palaeogeography, Palaeoclimatology, Palaeoecology 409 (2014) 205–216

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Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo

Mineralogy andmicromorphology of a late Neogene paleosol sequence atLangebaanweg, South Africa: Inference of paleoclimates

Peter N. Eze a,⁎, Michael E. Meadows b

a School of Geography and Environmental Studies, University of the Witwatersrand, Wits 2050, Johannesburg, South Africab Department of Environmental and Geographical Science, University of Cape Town, Rondenbosch 7701, Cape Town, South Africa

⁎ Corresponding author at: School of GeographyUniversity of the Witwatersrand, Wits 2050, Johannes11 7176590; fax: +27 11 7176529.

E-mail address: [email protected] (P.N. Eze).

http://dx.doi.org/10.1016/j.palaeo.2014.05.0080031-0182/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Article history:Received 10 March 2014Received in revised form 30 April 2014Accepted 8 May 2014Available online 17 May 2014

Keywords:Paleo-drainageMicromorphologyClay mineralogyClimate changePedogenic carbonates

Paleosol–sediment sequences at the West Coast Fossil Park in Langebaanweg, South Africa have a stratigraphyextending over ten units occupying a total depth of 18 m ranging in age from Mid-Miocene to Holocene. Six ofthese units qualify as paleosols. The mineralogical assemblages and micromorphology of the paleosols andpedofacies have been studied with the objective of interpreting and reconstructing the paleoenvironments ofthe area. Physico-chemical parameters,mineralogical properties andmicromorphology (described from thin sec-tions) of the paleosols and pedofacies were analyzed using routine laboratorymethods. The units are named fol-lowing the FAO and Soil Survey Staff classification systems. An alternating stack of paleosols and sedimentspreserve repeated phases of paleoenvironmental changes. Rhizoliths in the Pleistocene Ck layer lend credenceto a shallow standing paleowatertable during the Pliocene. Remarkable differences are observed in the mineral-ogy of the paleosols and sediments. Paleosols of the Middle Miocene have mixed clay mineralogy of halloysite,chlorite, muscovite mica, and kaolinite — an indication that the clay sources could have either formedpedogenically or from different parent materials which may have taken place during transportation. Allophaneand imogolite in the Early Pliocene paleosols are most likely to be stream-depositedwhile palygorskite and sepi-olite indicate dry paleoclimates. Vertic properties of the paleosols were observed from their open porphyric c/f-related distribution, blocky microstructure and striated b-fabrics. Properties including calcareousness, vertic,gleyic, illuviation suggest cyclic patterns of erosion and deposition, which correlates with paleoenvironmentaland climatic changes. The observed pedofeatures indicate the reddish Mid-Miocene paleosols and Pliocene ped-ogenic carbonates possibly formed under subtropical and Mediterranean climate conditions, while thepedosediments reflect a (semi) arid climate.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Likemodern soils, paleosols show characteristics that reflect the con-ditions under which they form, such as climate, organisms, topographyand parent material, all of which influence their development. In recentyears, paleosols have provided increasingly useful and reliable proxiesthroughwhich scientists successfully reconstructed paleoenvironmentaland paleoclimatic changes in Earth's history (Mack and James, 1994;Blum, 2005; Sheldon and Tabor, 2009). Many paleosol based proxies,for example clay mineralogy (e.g. Nedachi et al., 2005; Srivastava et al.,2009; Du et al., 2010; Presley et al., 2010; Rostási et al., 2011; Hong,2012; Watanabe et al., 2012), micromorphology (e.g. Todisco andBhiry, 2008; Khormali and Kehl, 2011; Kemp, 2013), and scanning elec-tronmicroscopy (e.g.Mahaney et al., 1988;Mahaney andVortisch, 1989;

and Environmental Studies,burg, South Africa. Tel.: +27

Retallack andKrinsley, 1993; Xie et al., 2013), have been successfully andwidely applied in the paleoenvironmental and paleoclimate reconstruc-tion through careful description and interpretation of various pedogenicproperties of soils and paleosols.

Clayminerals are products of chemical weathering. They are formedwhen parent material interacts with water at or near the surface of theearth (Velde, 1995). Consequently, they are of secondary origin in soilsand do not necessarily reflect the primary compositions of the geneticparent materials. Varying degrees of geothermal, hydrothermal andcontact metamorphic conditions promote the formation of differentclay minerals (Velde, 1995; Thiry, 2000). In soils, there are diverse pro-cesses capable of forming different types of clay minerals (Birkeland,1984) and the formation of clay minerals during pedogenesis is themost fundamental among these processes such that temperature, pre-cipitation and drainage exert the foremost controls on clay mineral for-mation (Blaise, 1989).

In general, five assumptions form the basis for using pedogenic claysas proxies for paleoclimate reconstruction, viz.: (1) clay mineral forma-tion is a function of climate; (2) once formed in a weathering environ-ment, clay minerals are stable and do not change subsequently as long

206 P.N. Eze, M.E. Meadows / Palaeogeography, Palaeoclimatology, Palaeoecology 409 (2014) 205–216

as the climate remains unchanged (pre-burial stability); (3) there isuniformity in clay mineral accumulation throughout a weathering pro-file; (4) once present and buried, clay minerals could be stable (post-burial stability); (5) uniform sensitivity of clay minerals towards envi-ronmental factors (Singer, 1984). Though the sensitivity of clay min-erals to environmental and climatic factors is variable, clay mineralspresent in paleosols in well-defined environments have been adjudgedsuitable for paleoclimate interpretation (Velde, 1995).

Micromorphology is the identification, description and interpretationof the components, features and fabrics of soils and sediments at amicro-scopic level (Bullock et al., 1985; Stoops et al., 2010). It provides evidenceon the operational pedogenic processes, hydrology and geology of soilsand sediments. Althoughmicromorphology has been applied extensive-ly to the understanding of past and present environmental processes, sci-entists encourage a combination of field and bulk analytical data as itprovides a stronger platform for soil genesis interpretation. One majorinterpretative problem associated with micromorphology is theacceptance by scientists of the distinctiveness of some pedofeatures–pedogenic process–macroenvironments relationships. It is true thatsuch relationships are often not fully understood; indeed, a growingbody of evidence suggests that different local and regional blends of ped-ogenetic processes and/or environments may produce similar horizonsand pedofeatures (Stoops et al., 2010; Badía et al., 2009). Nevertheless,holistic interpretations of pedogenic features, includingmicromorpholo-gy, may provide important insights into key processes related to envi-ronmental controls such as clay illuviation, calcite redistribution andthe formation of cryogenic microstructures (Kemp, 1998).

TheWest Coast Fossil Park at Langebaanweg (LBW), South Africa, isa regionally and globally important Late Neogene geoarchaeological siterenowned for its well preserved buried fossils extending to circa 5.1Ma(Olson, 1984; Roberts et al., 2011). Typical of coastal sedimentary envi-ronments (Panagiotaras et al., 2012; Parkinson et al., 2012; Roy et al.,2012), the geomorphology, soils and sediments of theWest Coast Fossilpark vicinity have been subjected to several cycles of fluvial, eolian andmarine processes over time, including sediment transport, coastal ero-sion and biogeochemical processes, which have shaped their character-istics. Global sea level changes in response to tectonics and climatechange are believed to have had a significant impact on the geomor-phology of this locality and the fossil animal remains deposited duringa marine transgression have been very intensively studied giving riseto detailed paleoenvironmental reconstructions (Hendey, 1982;Roberts et al., 2011). Quarry excavation at this site has revealed alithostratigraphic section with pedogenically modified horizons(Roberts, 2006; Roberts et al., 2011). The exposed surfaces, dated onthe basis of relative stratigraphy and OSL, consists of Early Pliocenepedogenically modified buried soils at the bottom of the sectionthrough to Holocene eolian sediments at the top of the succession.

Hitherto, the West Coast Fossil Park has been extensively studiedmainly for its rich paleo-biodiversity (e.g. Hendey, 1982; Manegold,2010; Scott et al., 2011), and complex stratigraphy e.g. (Roberts, 2006;Roberts et al., 2011). At the wake of global consciousness for climatechange, Chase and Meadows (2007) opined that paleoenvironmentalevidence for southern Africa remains largely incomplete as paleoclimat-ic proxy records are scarce and often discontinuous. Thestudy ofpaleosol-based proxies as tools for inferences of paleoclimates, particu-larly clay mineralogy and micromorphology is largely lacking in south-ern Africa even though there is presence of good paleosol exposureswith an important fossil-bearing site at Langebaanweg which certainlyneeds such detailed paleoenvironmental studies. In fact, it has beendemonstrated that paleosols archive reliable and finer scale paleo-cli-matic and environmental imprints (May et al., 2008; Sheldon andTabor, 2009; Von Suchodoletz et al., 2009).

The overarching aim of this study, therefore, is to improve the un-derstanding of paleoenvironments and paleoclimate dynamics of theexposed sequence at Langebaanweg using paleosol-based proxies. Thespecific objective is to provide a detailed description and genetic

interpretation of the: i) micromorphology of the paleosols, ii) selectedphysico-chemical properties, and iii) mineral assemblages.

2. Geographical and geological setting

TheWest Coast Fossil Park is located approximately 120 km north ofCape Town. The exposed paleosol–sediment sequences are situated atlatitude 32°57.784″ S and longitude 18°06.367″ E approximately 30 mabove sea level (Fig. 1). This site is also famous for a number of Mid-to Late Quaternary paleontological sites, for example, Elandsfontyn(where the partial cranium of early archaic Homo sapiens, “Saldanhaman” was discovered by Singer and Wymer (1968)), Langebaan (thesite of last interglacial fossil human footprints, Roberts et al., 2009),and theMiddle StoneAge (~250–25 kya) sites of Hoedjiespunt, SeaHar-vest, Yzerfontein and Duinefontyn (Hendey, 1982; Roberts and Berger,1997; Halkett et al., 2003). The climate of LBW is generally semi-aridto arid and falls within the winter rainfall zone of South Africa(Hopley et al., 2006).

The geology of LBW comprises Varswater formation (Fm) of theSandveld group overlain by the Springfontyn Formation and calcareouseolian deposit of the Langebaan formation (Fm) and Varswater forma-tion (Roberts et al., 2011) (Fig. 2). The Varswater formation is furthersubdivided into four members, viz: the Langeenheid Clayey Sand(LCSM), the Konings Vlei Gravel (KGM), the Langeberg Quartz Sand(LQSM) and the Muishond Fontein Pelletal Phosphorite Members(MPPM) (Roberts et al., 2011). A lithostratigraphic summary of the fos-sil park shows the spatio-temporal relationships with approximatethickness, lithology, and depositional environment of the paleosol–sed-iment sequences (Fig. 2).

3. Materials and methods

3.1. Field sampling

The quarrywas excavated to fresh exposures by scraping off the longexposed surfaces so as to avoid any altered samples. Representative un-disturbed and/or core samples were taken from each horizon of thepaleosol sequences (Fig. 3) for thin section preparations. Additionally,bulk sampling from each horizon was done for laboratory analyses. Itwas not possible to sample the LBW excavation along the same planedue to a rocky layer; so in order to facilitate a complete study of the ex-posed surfaces to a depth of 30 m, samples were taken in two subsec-tions: the so-called “high wall” and “low wall” (Fig. 3) which areseparated by an interbedded thick layer of unweathered pelletal phos-phorite rock. In the field, color was described using Munsell soil colorchart (Munsell Colour Co., 2000). Morphological properties were de-scribed and horizon designation was done following guidelines for soilprofile description (FAO, 2006).

3.2. Laboratory analysis

The collected paleosol and pedosediment samples were gentlyground to break up clods and subsequently passed through a 2 mmsieve in accordance with standards described in van Reeuwijk (2002).Dry and moist colors were determined using a Munsell Colour Co.(2000) color chart (measurements taken in triplicates). The soil rednessrating (Hurst, 1977) was calculated using the formula RR = H⋅C/Vwhere C = chroma (intensity), V = value (lightness) and H = hue(shade) (12.5 for hue 7.5R; 10 for hue 10R; 7.5 for 2.5YR; 5 for 5YR,2.5 for 7.5YR; and 0 for 10YR of the Munsell color nomenclature) afterTorrent et al. (1980) and Barrón and Torrent (1986). Particle size distri-bution was measured using the hydrometer method of Bouyoucos(1962). For soil pH(H2O) and electrical conductivity, a soil/solutionratio of 1: 2.5 was used and the values were read from digital pH andMilwaukee SM302 EC meters respectively. Calcium carbonate contentof samples was determined by the gravimetric method as described

Study site

Fig. 1. Geographical location of the West Coast Fossil Park, Langebaanweg, showing its seasonal rainfall pattern (adapted from Roberts et al., 2009).

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by the U.S. Salinity Laboratory Staff (1954). Bulk and clay mineral sam-ples (b20 μm) were treated with 10% H2O2 to remove organic matter.Mineral suspensions were washed repeatedly with deionized water toremove excess ion and help defloculate the clay particles. Thiswas centri-fuged to concentrate themineral solution into paste. Air-dried samples ofthe pastes mounted on slides to obtain oriented amounts were analyzedusing XRD Phillips PW 3830/40 Generator with a PW 3710 mpdcontrol X-ray diffraction system. Minerals were identified following theposition of the (001) series of basal reflection of samples in air-dry stateusing Xpert data collector/identification software at the Department ofGeological Sciences, University of Cape Town.Micromorphological analy-ses were carried out on the thin section slides viewed with a polarizingpetrographic microscope (Nikon) and images captured with an OlympusALTRA 20 camera. Scanning electronmicroscopywas conductedwith theOxford X-Max silicon drift detector and a high resolution Carl ZeissΣigmaAdvanced Analytical Microscope. The energy dispersive spectrum wasanalyzed with Oxford INCA software. Reported values are the average ofmeasurements taken in triplicates.

4. Results

4.1. Field observations and pedostratigraphy

The LBW section shows interesting and complex paleosol–sedimentsequences. General stratigraphy of a higher spatial resolution of the sec-tion shows that it qualifies as an example of a buried soil with a mantle,i.e. recently transported material lying above a buried horizon (Figs. 2and 3). In accordance with the FAO and Soil Survey Staff nomenclaturesystems, the horizons were named and described. The total height ofthe exposed paleosol–sediment sequences is approximately 18 m. Atthe bottom of the sequences is a Mid-Miocene hydromorphic buriedsoil profile (3ABgb–3Bgb1–3Bgb2) also developed from the LCSM atthe low wall of the pedocomplex (Fig. 3). Above these paleosols is aLate Miocene high marine energy deposit of unweathered/unalteredphosphocretes which developed from the Varswater formation. Overly-ing these rocks is the Early to Late Pliocene 260 cm section

which constitutes the ABb–2Bkmb1–2Bkmb2 horizons. The Quaternary“A–Bk–C” horizons of overlying Langebaan formation constitute themantle of the paleosol–sediment sequences and have a characteristicochric epipedon (Fig. 3). Rhizoliths are abundantly present in the “C”horizon of the Pleistocene sediment layer. In summary, the studied sed-iments consist of three stacks of pedogenically modified layers (MiddleMiocene paleosols, Early to Late Pliocene and the uppermost Holocenepaleosol profile) interbedded by pedosediments which have basicallyundergone diagenesis.

4.2. Macromorphology and physico-chemical properties

Paleosols were identified by the presence of at least one of: (i) undis-turbed features from soil floral and faunal activity, such as passage offeatures and channels with root residues and excrements; (ii) cutans;(iii) structure; or (iv) one or more types of undisturbed pedofeatures.On the other hand, pedosediments were recognized by: (i) an absenceof in situ biogenic features, but the commonpresence of fragmentary fau-nal or floral remains; (ii) massive structure, and occasionally a structuredominated by packing of rounded peds/aggregates; or (iii) preservedsedimentary features (Fedoroff et al., 2010; Mücher et al., 2010).Throughout the section, significant differences in macromorphologicalproperties particularly color and texture characteristics between thepaleosols and pedosediments are observed (Table 1). The different faciesstudied comprise eolian sediments, calcrete paleosols, phoscrete andpedogenically modified estuarine sandy deposits at the low wall. TheMiddle Miocene paleosols (3ABb–3Bgb1–3Bgb2) on the low wallhave colors ranging from yellowish red (5YR 4/6) to brownish yellow(10YR 6/6) with strong evidence of dark yellow mottles, while thePliocene buried paleosols (2Bkmb1 and 2Bkmb2) show colors rangingfrom brown (7.5 YR 5/4) eluvial soil to pale yellow (2.5Y 8/2) calcrete.The Quaternary section is composed of moderately cemented Pleisto-cene Bk and Ck horizons and a Holocene A horizon- and the color variesfrom light olive brown (2.5 Y 5/4) to dark reddish brown (5YR 3/2).Fragments greater than 2 mm in diameter are present in some of thehorizons, especially within the interbedded pedosediment layers

Fig. 2. Annotated summary stratigraphic column for LBW (after Roberts et al., 2011).

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sandwiching the paleosols (Table 1). Where present, and depending onthe amount, carbonates were largely responsible for the cementation ofboth paleosols and pedosediments. Only paleosols and pedosedimentson the “high wall” reacted to dilute hydrochloric acid, in some casesvigorously.

Where applicable, both the wet and dry redness rating for thepedocomplex follow the same pattern and are highest for the Pliocenepaleosol (Table 2). Particle size distribution across the sequence is vari-able. Both the soils and pedosediments record alkaline soil pH values.Although pH fluctuates down the sequences, the Pliocene paleosolshave higher values than the other samples. Electrical conductivityvalues of the soils and sediments are low,with the exception of theMid-dle Miocene paleosol in the “low wall” section in which conductivity issomewhat higher. Calcium carbonate equivalent values also vary downthe sequences of the “high wall” where it is present.

4.3. Mineralogy

The X-ray diffractograms of the untreated fine silt and clay sized frac-tion (b20 μm) of the samples vary markedly across the paleosol–sedi-ment sequence (Fig. 4a–i). The Mid-Miocene paleosol at the low wallof the section had scanned peaks corresponding to known peaks ofgreenalite, muscovite, kaolinite and sepiolite at the 3ABb horizon(Fig. 4a). The underlying B horizons (“3Bgb1 and 3Bgb2”) had mottling,an evidence of hydromorphism in the profile. Both horizons had scannedpeaks primarily corresponding to the theoretical peaks of chlorite andkaolinite (Fig. 4b and c). Additionally, 3Bgb2 had peaks of noneexpanding 1:1 clay minerals — halloysite and mica. The two B-horizonsof the paleosol (Bkmb1 and Bkmb2) had low broad peaks correspondingto short range amorphous clay minerals allophane and imogolite. Calcitepeaks were also present in the paleosols (Fig. 4d and e). SEM

Fig. 3. Pedostratigraphic section of the LBW section (depth in “cm”) showing high and low wall respectively (the horizontal thin white straight line demarcates the “low wall” from the“high wall”).

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micrographs also confirm the presence of needle-fiber shaped andwhitefluffy calcites in the carbonate paleosol (Fig. 5a and b). The AB horizon ofthe buried Early Pliocene paleosol has peaks matching that of mica,dickite, oxides, sepiolite, palygorskite and calcite (Fig. 4f). Evidence oflayering and neo-formation of clays in this horizon was seen from theSEM (Fig. 5b) and the compositional elements from the energy disper-sive spectrum (EDS) (Fig. 6c). The gritty Pleistocene eolian parent mate-rial of themantle (C-horizon) reveals peaks corresponding to calcite only(Fig. 4g). The overlying horizon (Bk) has peaks matching smectite,quartz, oxides and sepiolite (Fig. 4h). A scanning electron microscopy(SEM) study of the b20 μm “Bk” sample also reveals the fibrous textureof sepiolite as it appears to coat quartz grains (Fig. 5a). The Holocene sur-face horizon (epipedon of the mantle) has scanned peaks that show pri-mary correspondence with theoretical peaks for quartz, oxides,palygorskite and calcite (Fig. 4i).

4.4. Micromorphology

Only thin sections for the pedogenically modified horizons (3Bgb2,3Bgb1, 3ABb, 2Bkmb2, 2Bkmb1, and ABb) were prepared as thepedosediments (A, Ck, 2C and 2C2 horizons) have single grain structure

which made it very difficult for sampling. Photomicrographs capturedunder both plane polarized (PPL) light (Fig. 6a–f) show varying degreesof pedogenesis. A summary of themicromorphological properties of thepaleosols (Table 3) is presented under three subheadings for ease of de-scription, viz. groundmass, voids and microstructure and pedofeatureswhich show major difference in composition of the paleosols andpedosediments.

4.4.1. Mineral componentsQuartz is the dominant mineral in the coarse mineral fraction

(N100 μm) in all samples. Feldspars (mainly plagioclase) andphosphoriteare also common in theMid-Miocene paleosols in the “lowwall” subsec-tion (Fig. 6a–c). These paleosols have an intergrainmicroaggregate struc-ture, complex packing voids with enaulic to chitonic distribution. Theoverlying Pliocene 2Bkmb1 and 2Bkmb2 paleosols are dominated byfine to medium (250–1000 μm) sand particles which are poorly to mod-erately sorted with finemonic distribution (Fig. 6d and e). The ABb hori-zon of the same parent material and epoch comprises a loosely arrangedsingle grain structure, simple packing voidswith enaulic to chitonic relat-ed distribution (Fig. 6f). The Quaternary mantle sediments are well tomoderately sorted fine to medium sand particles (N63–250 μm). Most

Table 1Macromorphological properties of the LBW paleosol–sediment sequences.

Horizon Depth(m)

Facies Colora

(moist)N2 mmFragments

Structureb Root Boundaryc Consistency(moist)

Fieldtexture

Cementation React HCl Otherfeatures

A 0− Eolian 5YR 3/2 Absent 1gr Common gw Loose Sand None Moderate Snail shells(dark reddish brown)

Bkm 0.15− Calcrete 5Y 7/1 Absent 3sbk Common aw Firm Loamy sand Carbonate Strong –

(light gray)Ck 0.33− Eolian 2.5 Y 5/4 Very few 3sbk Few cw Very firm Sand Carbonate Moderate Rhyzolith

(light olive brown)ABb 1.20− Fluvial 7.5YR 5/4 Few 1gr None cs Friable Sand None Occasional Burrows

(brown)2Bkmb1 1.38− Calcrete 5YR 3/4 Common 3abk None gw Very firm Sand Carbonate Strong –

(dark reddish brown)2Bkmb2 3.48− Calcrete 2.5Y 8/2 Absent 3abk Few gw Firm Clay Carbonate Strong Burrows

(pale yellow)2C 3.97− Phoscrete 2.5Y 8/1 Abundant 1gr Common cw Friable Sand None None –

(pale yellow)2C2 15.97− Cal-phosphocrete nd Common blk None Cw Very firm Blocky None None

(not determined)3ABb 16.07 Estuarine 5YR 4/6 Common 2gr.s Few cs Loose Gr. sand None Occasional Stone line

(yellowish red)3Bgb1 16.41 Estuarine 10YR 6/3 Occasional 2sbk Few cs Firm Loamy sand None None Mottles

(pale brown)3Bgb2 17.00+ Estuarine 10YR 6/6 Absent 2sbk Few – Firm Loamy sand None None Few mottles

(brownish yellow)

a nd— not determined.b 1 — weak; 2 — medium; 3 — strong; gr— granular; sbk — subangular blocky; abk — angular blocky; blk— blocky; Gr. S — gravelly sand; l. sand — loamy sand.c a — abrupt; c — clear; s — smooth; g — gradual; w — wavy.

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of the grains in these horizons are loosely arranged and although there isweak tomoderatemicroaggregate structure in the Pleistocene Bkhorizonwhose micromorphology is apparently similar with that of 2Bkmb2(Fig. 6d).

4.4.2. Marine shell, rhizogenic structure and terrestrial humuscharacterization

There is no evidence of marine shell and rhizogenic structure in theMiddle Miocene paleosols of the LBW pedocomplex. Marine shells werenot observed in the thin section of the Pliocene paleosols, butmacroscop-ic rhizogenic structures are common. The Pleistocene pedosedimentforming on coastal eolian sediments houses many marine shells withrhizoliths. In general, the paleosol–sediment sequences had common tovery few organic matter traces. Organic matter pigmentation is seen inthe ABb horizon 3ABb samples and proved useful in the designation ofsuch horizons. Traces of soil organic matter, indicated by darker patchesare presentmostly in the A and Bk horizons of the Quaternarymantle, al-beit in very low amounts.

Table 2Selected physico-chemical properties of Langebaanweg paleosol–sediment sequences.

Horizon Color (dry) RRa Sand Silt

Dry Moist (b2 mm) (g kg−1)

A 5YR 5/2 2.0 3.3 908 22Bkm 5Y 8/1 – – 638 137Ck 2.5Y 7/3 – – 907 22ABb 7.5YR 7/6 2.1 2.0 899 112Bkmb1 5YR 5/8 8.0 6.7 729 412Bkmb2 2.5 Y 8/1 – – 473 1842C 2.5Y 8/1 – – 613 1503ABb 5YR 5/6 6.0 7.4 712 633Bgb1 10YR 8/2 0 0 264 1123Bgb2 10YR 6/8 0 0 562 113

a Redness rating.b Sa— sand; ClLo — clay loam; LoSa — loamy sand; SaClLo — sandy clay loam; Cl — clay.c nd— not determined.

4.4.3. Pedological processes

4.4.3.1. Mineral component alteration. Evidence of mineral alteration ismore pronounced in the paleosols — “AB” and “B” horizons (Fig. 6a–f)whereas the mineral components of the pedosediments are relativelyunaltered, subrounded to subangular as seen with the aid of handlens. Additionally, most of the mineral components of the interbeddedpedosediments did not show evidence of mechanical cracking fissures,which indicates theywere not in situ. For the paleosols, there is evidenceof advanced chemical weathering of the mineral components asreddish-brown Fe oxide and/or Fe hydroxide staining commonly occursalong cleavage lines and fractures in grains (Fig. 6a and b).

4.4.3.2. Redoximorphic features (redox concentrations). Evidence for ac-tive redoximorphism in the sequences is present only in theMiddleMio-cene paleosols (Figs. 3, 6a and b). They have abundant reddish, brown-reddish, dark brown to black nodules rich in Fe or Fe–Mn oxide noduleswith sharp boundaries. Simple, composite or nucleic nodules are alsopresent. The vertic property of the Pliocene paleosol (2Bkmb2) as seen

Clay Textureb pH(H2O)

EC mS(cm−1)

CaCO3c

(g kg−1)(IUSS)

70 Sa 8.3 0.08 183225 ClLo 9.2 0.11 43771 Sa 7.7 0.06 21890 LoSa 9.5 0.36 238

230 SaClLo 9.2 0.46 475343 SaClLo 9.3 0.62 626228 SaClLo 8.6 0.17 nd225 SaClLo 7.9 15.07 nd624 Cl 8.0 11.15 nd325 SaClLo 8.3 2.53 nd

Fig. 4. Diffractograms of the LBW paleosol–sediment sequence horizons: a) 3Bgb2, b) 3Bgb1, c) 3ABb, d) Bkmb2, e) Bkmb1, f) ABb, g) Ck, h) Bk, i) A.

Fig. 5. SEM of the paleosol at LBW: a) Lowmagnification of the Bkm sample showing the coating of the tangential fibrous sepiolite (arrows) on quartz (Qtz) grains, “C”: charging; b) layeringand neoformed clay in the 2ABb horizon; c) energy dispersive spectrum of the rectangular area highlighted in Fig. 5b showing elemental composition; d) tangential filaments needle fibercalcite of the 2Bkmb1 horizon showing elemental composition (%); e) high resolution SEM of Bkmb1 showing layering and white fluffy surface coating of calcite mineral; f) high resolutionSEM of 2Bkmb2 clays showing abundant white fluffy calcitic fiber; g) a close up higher resolution Σigma SEM of Bkmb2.

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a b

c d

e f

3Bgb2 3Bgb1

2Bkmb1 ABb

2Bkmb23ABb

Fig. 6. Thin section photomicrographs of the paleosol sequence: a)well sortedmineralmatrix of quartz and feldsparswith clay coatings (3Bgb2, XPL); b) impregnative redox pedofeaturesof iron and manganese oxide hypocoatings and granular matrix with signs of reduction (3Bgb1, XPL); c) moderately sorted with rounded to sub-rounded particles, with little hierarchybetween pedofeatures (iron oxide coatings and simultaneous faunal activity) (3ABb, XPL); d) biphase pedogenesis, depletion redox pedofeatures of iron oxide depletion hypocoating, ironoxide quasicoating and calcium carbonate illuvial pedogenic facies (2Bkmb2, XPL); e) impregnative redox pedofeatures of iron oxide hypocoating and granular matrix with no sign of re-duction (2Bkmb1, XPL); f) well sorted granular microstructure with subrounded aggregates and encircling fine grained iron oxide coatings (ABb, XPL).

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from the cracks on the field shows that hydromorphism could have pos-sibly been operational at a time in which lessivage (argilliturbation),high pH and subsequent dessication of the horizon could have lead to aremarkably cracked horizon.

4.4.3.3. Translocation of fine particles. Based on the features observedfrom the thin sections there is clear evidence of varying degrees oftranslocation of fine materials (mostly silt and clays) across adjacentlayers. Texture and drainage conditions are likely the principal factorsthat influenced the observed illuvial features. The higher moisture con-tent of the Mid-Miocene paleosol at the base of the sequences points toa finer particle size distribution which promotes higher water and clay

retention in the hygroscopic capillaries. Since chemical weathering isnot advanced in these horizons as seen from earlier geochemical study(Eze andMeadows, 2014). It could be possible that these finer particlescame from sedimentary differentiation of deposited material duringLate Miocene high energy marine activities in the area. Translocationof fine materials is also reflected in the redistribution of the chemicalcomponents leading to the formation of iron and Mn oxides/hydroxidenodules within the soil fabrics (Fig. 6a–c). Evidence of prolonged trans-location of organic to organo-iron coatings is generally rare in the min-eral components of the horizons, probably due to erosion. Illuvialfeatures of the Early Pliocene paleosols indicate a high paleowatertableand subsequent paleodrainage.

Table 3Summary of the micromorphological description of the LBW paleosols.

Horizon Groundmass Voids and microstructure Pedofeatures

Bk C/F distribution: gefuric to chitonic; C/F limit: silt/sand; coarsecomponent: 80% of the t.t.s.a., silt andfine to coarse sand, smooth andspheroidal fracture, shape rounded to subrounded variability withinrandom fabric associated with shape, elongated grains are randomlydistributed; fine component: fabric: 10% t.t.s.a., calcium carbonatecreates quasi- and hypo-coatings on walls of peds, plant residues

5–10% of the t.t.s.a.; few simple to frequent compoundpacking voids; microstructure: single to bridged grainstructure

Textural sandy pedofeatures associatedwith coastal/eolian transportation; calciumcarbonate coatings around peds

ABb C/F distribution: enaulic; C/F limit: silt/sand; coarse component: 80%of the t.t.s.a., silt and fine to coarse sand, smooth and spheroidalfracture, shape subrounded to subangular high variability withinrandom fabric associated with shape, elongated grains are randomlydistributed; fine component: fabric: crystallitic b-fabric, 15% t.t.s.a.,iron oxide creates quasi- and hypo-coatings onwalls of peds; organicmatter pigment.

5% of the t.t.s.a.; few simple packing voids, withcommon channel voids; microstructure: single tobridged grain structure

Depletion and crystalline pedofeaturesassociated with iron oxide eluviation andtranslocation within the soil fabric

2Bkmb1 C/F distribution: porphyric; C/F limit: silt/sand; coarse component:70% of the t.t.s.a., silt and fine to coarse sand, smooth and spheroidalfracture, shape subrounded to subangular high variability withinrandom fabric associated with shape, elongated grains are randomlydistributed;fine component: fabric: striated b-fabric; 25% t.t.s.a., ironoxide creates coatings on walls of peds, plant residues

5% of the t.t.s.a.; channel, dominant voids arechambers; microstructure: massive

Impregnative pedofeatures associatedwithilluviation and impure clay with hypocoatings around peds

2Bkmb2 C/F distribution: porphyric; C/F limit: silt/sand; coarse component: 70%of the t.t.s.a., silt and fine to coarse sand, smooth and spheroidalfracture, shape subrounded to subangular high variability withinrandom fabric associated with shape, elongated grains are randomlydistributed; fine component: fabric: striated b-fabric; 30% t.t.s.a., CaCO3

creates quasi- and hypo-coatings on walls of peds, plant residues

5% of the t.t.s.a.; channel, dominant voids arechambers; microstructure: massive

Textural pedofeatures associated withcalcium carbonate coatings around peds

3ABb C/F distribution: enaulic; C/F limit: silt/sand; coarse component: 80% ofthe t.t.s.a., silt and fine to coarse sand, smooth and spheroidal fracture,shape subrounded to subangular high variability within random fabricassociatedwith shape; fine component: fabric: crystallitic b-fabric, 15%t.t.s.a., iron oxide creates quasi- and hypo-coatings on walls of peds,organic matter pigment

5% of the t.t.s.a.; compound packing voids, withcommon chambers voids; microstructure: vughy porestructure

Depletion and crystalline pedofeaturesassociated with iron oxide eluviation andtranslocation within the soil fabric

3Bgb1 C/F distribution: enaulic; C/F limit: silt/sand; coarse component: 80%ofthe t.t.s.a., silt and fine to coarse sand, smooth and spheroidal fracture,shape subrounded to subangular high variability within random fabricassociatedwith shape;fine component: fabric: crystallitic b-fabric, 15%t.t.s.a., iron oxide creates quasi- and hypo-coatings on walls of peds

5% of the t.t.s.a.; com packing voids, with commonchannel voids; microstructure: complex: a mixture ofvughy and crumb micro structure

Textural pedofeatures associated withfragments of clay and silt embeddedwithinthe soil matrix

3Bgb2 C/F distribution: enaulic; C/F limit: silt/sand; coarse component: 80%of the t.t.s.a., silt and fine to coarse sand, smooth and spheroidalfracture, shape subrounded to subangular high variability withinrandom fabric associated with shape; fine component: fabric:crystallitic b-fabric, 15% t.t.s.a., iron coatings on walls of peds

5% of the t.t.s.a.; few simple packing voids, withcommon channel voids; microstructure: complex: amixture compact and bridged grain structure

Textural pedofeatures associated withfragments of clay and silt embedded in thesoil matrix

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5. Discussion

The paleosol–sediment sequences at the West Coast Fossil Park maybe classified as a “pedocomplex” — a recent term that has been cited inliterature to describe alternating sequences of paleosols and sedimentsin deltas and alluvial plains (e.g. Srivastava and Parkash, 2002; Feng andWang, 2005; Fedoroff et al., 2010). Paleosols and sediments aresuperimposed upon each other and provide evidence of the inter-penetration of pedofeatures of the overlying into the underlyingpaleosols. Roberts et al. (2011) demonstrated that the depositionalevents of LBWreflect a glacio-eustatic sea level history (Fig. 7). Apparent-ly, changes in climate and tectonics (i.e. topography) affected the erosion-al and depositional processes which can be determined from the degreeof alteration of the sediments or pedogenesis. A breakdown of sequencesof events that lead to the development of the pedocomplex (Fig. 7) sup-ports that 3ABb, 3Bgb1 and 3Bgb2 section of the pedocomplex formedduringMid-Miocene sea level rise and deposition (Fig. 7b). Marine trans-gression and deposition of the Early Pliocene are responsible for the de-position of the Varswater formation on which ABb, 2Bkmb1 and2Bkmb2 paleosols formed (Fig. 7c).

Like modern day soil profiles, the pedogenically modified hori-zons may have developed during periods of minimum erosionaland depositional activities, i.e. periods of relative geomorphic stabil-ity. The Mid-Miocene paleosols are well developed with at leastthree distinct horizons. This indicates that there was a prolonged

period of minimal deposition and erosion before the overlying LateMiocene higher energy marine phosphates were deposited. Similar-ly, the Early Pliocene paleosols of the pedocomplex have undergoneintense weathering and display evidence of advanced pedogenesis.The 2Bkmb1 and 2Bkmb2 horizons have illuvial materials from the“ABb” horizon and the vertic properties of the horizons possiblyattest to hydro- and faunal pedoturbation during the globally record-ed Early Pliocene highstand (Haq et al., 1987; Zachos et al., 2001).Invariably, the little altered mineral components of sediments inter-bedded with the paleosols suggest either or both a very slow rate ofweathering and a short exposure time for pedogenic processes.

Tubular rhizoliths or root-caste are prolific in the Plio–Pleistocenesediments. Rhizoliths are faithful indicators of paleodrainage (Liutkus,2009) and are formed from low nutrient but cation-rich sand wherethere is transpiration-driven water flux mass flow to roots in excess ofwhat is required by the plant (Cramer and Hawkins, 2009). Workingon a Paleogene sedimentary environment in the Bighorn basin, USA,Kraus and Hasiotis (2006) reported that calcareous rhizocretions –

either calcareous, tubular concretions or micro-accumulations ofcarbonate within gray rhizotubules – are common in moderately well-drained red paleosols. The paleoenvironmental implication of theserhizoliths is standing shallowwater table and not necessarily a paleosol.

The redness rating of a soil is strongly correlated with hematite con-tent in soils (Torrent et al., 1983). Early Pliocene 2Bkmb1has the highestredness rating in the pedocomplex and may be an indication of

Fig. 7. Sequence of major Neogene erosional and depositional events in the LBW environs: a, Fluvial incision during Oligocene lowstands; b, Early–Middle Miocene sea level (base level)rise and deposition of the fluvial Elandsfontyn Formation; c, Major Early Pliocene transgression and deposition of the Varswater Formation (Roberts et al., 2011) (for interpretation of thereferences to color in this figure legend, the reader is referred to the web version of the article).

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formation of soil under a warm subtropical climate with/without pres-ence a well-drained geomorphic condition.

The Middle Miocene paleosols have mixed clay mineralogy ofhalloysite, chlorite, muscovitemica, and kaolinite. This can be attributedto clay sources coming from different parent materials during transpor-tation. Chlorite is known for its high water retention properties thatpromote hydromorphism in soils from ground water (Lee et al., 2003).Most kaolinite in soils originate fromweathering of soil parentmaterialsin near surface environments and are likely to have been subjected toone or more sedimentary cycles subsequent to its original formation(Allen and Hajek, 1989). In this sequence, redistribution of calcitethroughout the overlying Plio–Pleistocene paleosols and sediments isevident, possibly due to the high solubility of carbonates in soil environ-ments. Allophane and imogolite peaks are noted here in the Early Plio-cene paleosols. These amorphous and short-range order minerals areof non-tephric origin since there is no history of Late Cenozoic volcanicactivity in the region (Roberts et al., 2011). The origin of these minerals

can therefore be interpreted asfluvial, i.e. deposited by a stream (Parfitt,2009). On the mechanism of the formation of imogolite and ofallophane with imogolite-like structures in B horizons, Farmer (1982)explained it only by their deposition from hydroxyaluminum silicate(proto-imogolite) soils, which are known to have the necessary chemi-cal and colloidal stability to act as the agent of transport of Al and, inpart, Fe. The presence of sepiolite in the Bk horizon indicates aridity.This fibrous clay mineral is climatically sensitive and highly unstablein the soil environments and is weathered to smectite at mean annualprecipitation in excess of 300 mm (Paquet and Millot, 1973).

From themicromorphology observations, it is apparent that both theMid-Miocene and Early Pliocene paleosols are affected by in situreworking and mass transportation. The evidence for this is clearlyshown in their massive microstructure with variable abundance ofclosed polyconcave vughs, often grading to vesicles, and a granular mi-crostructure with rounded to subrounded aggregates that are not of ex-cremental origin, and the presence of fragmented pedofeatures. Thin

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section observations indicate that the major cause of disruption or de-formation of the parent sediments is paleo water saturation. Fine parti-cle size distribution and humid paleoenvironments are responsible forthe high water retention properties and low permeability of thepaleosols and consequently for the reduction and mobilization ofFe and Mn, resulting in the formation of Fe and Mn oxide pedofeatures.In thin sections, the Bkmb1 and Bkmb2 have open porphyric c/f-relateddistribution, a blockymicrostructure and grano-striated b-fabrics. Thesefeatures are typical of vertic properties in paleosols (Kovda andMermut,2010). The vertic property results from shrink-swell processes,pedoturbation and lateral shearing due to the fluctuating water tablein the clayey components. Similar processes may account for themicritic/microsparitic calcite coatings and hypocoatings in the voids ofthe Bkmb2 horizon.

A schematic presentation of the reconstructed paleoenvironmentsfrom this study (Fig. 3) derived from a combination of the abovediscussed proxies support earlier environmental and climate changes asreported for LBW (Hendey, 1982; Roberts et al., 2011). The Late Cenozoicpedogenesis and sedimentation cycle of LBW depositional environmentgenerally reflect the global trend as reported by Haq et al. (1987) andglacio-eustacy trend of Zachos et al. (2001). The Quaternary sequencesdepict (semi) aridity in the environment. According to deMenocal(1995) marine records of African climate variability document a shifttoward more arid conditions after 2.8 million years ago (Ma), evidentlyresulting from remote forcing by cold North Atlantic sea-surface temper-atures associated with the onset of Northern Hemisphere glacial cycles.

6. Conclusions

The mineralogical assemblages of paleosols and pedosedimentsfrom the West Coast Fossil Park of South Africa and pedofeature evi-dence from their micromorphology indicates repeated changes of cli-mate during the Late Cenozoic. Warming and increased humidity inthe periods of soil formation prompted changes in clay mineralogy,with the formation of kaolinite and halloysite during the Mid-Mioceneand suggest conditions consistent with a subtropical climate. Themore advanced weathered nature of this paleosol is further shown inits higher clay content. While the overlying Plio–Pleistocene paleosolsand sediments contain, none is evident in the underlying paleosolswhichmay suggest inflow and sedimentation of marine sediments con-taining shells only after Mid-Miocene in the locality. There presence ofnon-tephric short-range order minerals – allophane and imogolite – inthe Pliocene paleosols may originate from paleo stream beds as this lo-cality was subsequently submerged under high paleowater table duringthe global Late Pliocene highstand. The 2Bkmb1 horizon is comparative-ly sandier than 2Bkmb2, and hence the minimal amount of iron oxidesappear to be sufficient formore intense reddening; the finer the particlesizes, the larger the surface area and themore iron oxide pigment need-ed to achieve same degree of coloration. In the Plio–Pliocene section,surface erosion and sedimentary fossilization are followed secondarycarbonitization and subsequent induration at varying intensities. BothMid-Miocene and Early Pliocene paleosols are affected by in situreworking and mass transportation. Structure and orientation of quartzof the Holocene surficial sediments indicate strong eolian processes. Amodel of the climatic cycling of pedogenesis and sedimentations inthe LBW environment shows five cycles ranging from arid to warmhumid paleoclimate. This study supports earlier works, which suggestthat LBW has been under recurrent sea level oscillations. In times ofgeomorphic stability, erosion and sedimentation are limited to thedepth contour of the coastal fan and pedogenesis progressed.

Acknowledgements

The authors hereby gratefully acknowledge the financial assistanceof the Palaeontological Scientific Trust (PAST) and its Scatterlings ofAfrica Programme which awarded a PhD bursary to P.N.E.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.palaeo.2014.05.008.These data include Google map of the most important areas describedin this article.

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