Mineralogy and stratigraphy of three deep lateritic profiles of the Jos plateau (Central Nigeria)

CATIENA ELSEVIER

Catena 21 (1994) 195-214

Mineralogy and stratigraphy of three deep lateritic profiles of the Jos plateau (Central Nigeria) R. Zeese a, U. Schwertmann b, G.F. T i e t z c a n d U.

Jux d

aGeographisehes Institut, Univ. zu K61n, Albertus-Magnus-Platz, 50923 KO'ln 41, Germany bLehrstuhl fiir Bodenkunde, TUM-Weihenstephan, 85350 Freising, German)' ~Geol. Pal. Inst. u. Museum, Bundesstr. 55, 20146 Hamburg 13, German)" dLehrstuhl /h'r Pala'ontologie, Geol.-Pal. Inst., Univ. zu K6ln. Albertus-Magnus-Platz, 50923 K(iln 41, Germany

Abstract

The Fluviovolcanic Series (= FVS) of the Jos plateau, Central Nigeria are an upper Eocene to middle Miocene time-equivalent to the Continental Terminal of the surrounding basins. It can be divided into a lower part with mafic to intermediate and even felsic volcanic rocks (according to the Zr/TiO2), fluvial or limnic sediments and an upper part of marie volcanic rocks with intercalated transported pisoids and quartz grains. Palynomorphs of a limnic clay reflect a subtropical to montane tropical vegetation rich in plant species. The lower part of the FVS often shows distinct tilting as a result of neotectonic movement along a reactivated conservative plate boundary in continuation of the transatlantic Romanche fracture zone. The various rocks have been completely converted into deep (often > 80 m) polygenetic soils which consist of repetitive sequences of saprolite, mottled horizon and ferricrete. The ferricrete represents a paleosurface and protects the profiles against erosion. The mineralogy of 3 profiles consists essentially of kaolinite with various amounts of goethite and hematite; gibbsite appears in one confined horizon, whereas anatase and residual (weathered) ilmenite and quartz occur as accessories. The presence or absence of ilmenite, quartz and anatase in the various sections of the profiles were taken as an indicator for different parent rocks. The crystallinity of kaolinite (Hinckley index), the crystal size and AI substitution of goethite and hematite varied sectionwise indicating different weathering enviromnents. For example, the goethite in laminar-massive ferricretes appearing at the base of all 3 profiles had lower AI substitution and somewhat larger crystals than that of the pisolitic-vermiform ferricretes and was associated with redoximorphic conditions. On the basis of mineralogical, paleontological, sedimentological and absolute K/Ar age of basalts, the profiles were placed into the transition from Paleogene to Neogene and stratigraphic correlation was suggested.

0341-8162:94/$07.00 ~ 1994Elsevier Science Publishers B.V. All rights reserved SSDI 0341-8162(93)E0039-3

196

R. Zeese et al./Catena 2l (1994) 195 214

1. Introduction

The Fluviovolcanic Series (FVS) of the Jos Plateau, thecentral uplands of Nigeria (Fig. 1), were first described by Falconer (1921) as sequences of sedimentary and volcanic rocks, which are often entirely decomposed. McLeod et al. (1971) described them as lateritized Older Basalts. The age of ferricrete Iormation on top of the FVS is controversial (McLeod et al., 1971; Boulang6 and Eschenbrenner, 1971; Hill and Rackham, 1976; Valeton and Beissner, 1986). Pedologically, the profiles, which are often more than 80 m thick, consist of polygenetic soils * (Valeton, 1991; Zeese, 1991 ) from different parent materials within one profile. Usually the fresh parent rock of these profiles cannot be found anymore, but some fresh basalts are preserved in different stratigraphic positions and have been K/At dated to between 0.5 and 11 Ma old (Grant et al., 1972: Rundle, 1975, 1976). Limnic clays below the soil profiles

Fig. 1. Geology of Nigeria.

* Soil is used here in its pedological definition (Fairbridge and Finkl, 1979, p. 434) as "'the upper portion of the lithosphere that has been altered into horizons that differ from ... the underlying unaltered material {pedosphere)" (see also Matthes, 1990, p. 271).

R. Zeese et al./Catena 21 (1994) 195 214

197

occasionally contain carbonized leaves and microfossils. These palynomorphs give the maximum age of the soils. As a result of younger valley incision, profiles often occur as buttes, whose ferricrete caps protect the underlying soft material against erosion. Additionally, due to the mining activities in the tin placer-bearing sediments at the base, numerous profiles have been opened. The objectives of this study were to investigate and explain the characteristics of selected profiles, to determine their stratigraphic position and to gain insight into the paleoenvironmental conditions of their formation. Some results were already published by Valeton (1991) and Becker (1989) and the palynomorphs of an underlying clay were determined by Takahashi and Jux (1989).

2. Geomorphology of the investigation area

The three profiles described here are situated in the western part of the Jos plateau near the upper reaches of the river Werram (Fig. 2, left) and contain all horizons viz. saprolite, mottled horizon and ferricrete. The terrain is dominated by flat to bevelled plains from 550 to 600 m a.s.1, in the west and 1300 to 1400 m a.s.1, in the east of the area shown in Fig. 2, which represents the central watershed. The planation surfaces, partly covered by extended Plio/ Pleistocene basalt sheets or small remnants of older basalts, are separated by scarps and ramps i.e. steeper parts of the flat terrain. The latters are commonly developed in more easily weatherable rocks like schists and, less frequently, in more resistant, but deeply weathered rocks, like the Panafrican Older Granites (for geology see McLeod et al., 1971). The ramps are interpreted as a response to neogene uplift (Zeese, 1989). Scarps either follow prominent tectonic lines (Fig. 2, right) or are controlled by outcrops of plutons of Jurassic age (the so-called Younger Granites). Scarp formation occurs to be concentrated along the supposed continuation of the Transatlantic Romanche fracture zone. Hills and mountains are composed of granites, except for the buttes, where ferricrete caps protect the soil profiles against erosion. In the granitic area closed depressions with internal drainage have been described by Thorp (1967). Numerous greater depressions are associated with rocky plains within the mountain areas, where sometimesferricrete buttes testify the former deeply weathered soil mantle, as in the Kagoro hills in the NW corner (Fig. 2).

3. Materials and methods

The whole area was mapped geomorphologically at a scale of ca. 1 : 30 000 using aerial photographs. Three profiles were selected for a detailed chemical, mineralogical and micromorphological study. In a first part of this study, bulk chemistry (x-ray fluorescence spectrometry), bulk mineralogy (XRD) and porosity were determined by Kemink (1989) and published by Valeton (1991). These results are included in this paper for completeness. The second part comprised a more detailed mineralogical analysis of goethite, hematite, kaolinite, anatase and ilmenite. For this purpose,

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etchplain, undiff, deeply weathered etchplain in parts with gully incision and ferricrete table hills dissected etchplain and partly stripped etchplain rocky plain and knolls

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R. Zeese et al./Catena 21 (1994) 195-214

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subsamples from sections where Fe oxides were concentrated were also analysed in additon to bulk samples. Chemical quantification of Fe oxides (Fed) was done by the dithionite-citrate-bicarbonate method (CBD). For X R D quantification of goethite and hematite, the intensity of the (021) X R D line of hematite and the (110) line of goethite were used (K/impf and Schwertmann, 1982). X R D line width was turned into crystal size (MCD) using the Scherer formula (Klug and Alexander, 1974). The crystallinity of kaolinite was characterized by the Hinckley index (HI) (Hinckley, 1967) and unit cell size of anatase and ilmenite were measured by XRD. Where needed, concentration procedures were used for goethite and hematite (boiling in 5 M NaOH) and ilmenite and anatase (boiling in 5 M NaOH + CBD). From selected samples thin sections were produced. Scanning electron micrographs and microprobe analyses were carried out with a Cam Scan CS24 instrument. To obtain the AI substitution of goethites, synthetic AI goethites were used as standards. To characterize the frequency distribution of mineral properties, kernel densities were computed, which avoid arbitrary classification* (Victor, 1978; Gordon, 1987). Detailed profile descriptions and colored copies of thin sections may be obtained from the senior author.

4. Results and discussion

4.1. Soil profiles (Figs. 3, 4, 5) 4.1.1. General characteristics The profiles display a common repetitive succession (from below) of saprolite, mottled zone and ferricrete. This succession varies in completeness, thickness, morphology and mineralogy. Fe concentration (mainly as hematite and goethite) generally increases upwards from saprolite to ferricrete. Within the saprolite there may be a bauxitic horizon, which seems to be restricted to a specific stratigraphic position. Two types of ferricretes can be distinguished: one type is laminated to massive and occurs predominantly in the deeper parts of the profiles. The other is pisolitic or vermiform and may represent older land surfaces. Goethite and hematite form the dominant phases and are mineralogically rather monotonous with kaolinite. However, considerable variation results from the proportion of these minerals, the crystallinity and Fe content of the kaolinite (Hinckley index, HI), the crystal size and AI substitution of goethite and hematite and the presence or absence of anatase, pseudorutile, residual ilmenite and quartz. The latter two may be used to distinguish between different basalts or other rocks as parent materials. Furthermore, ilmenite appears to be partly oxidized and associated with pseudorutile, and anatase seems to have some Fe in the structure (to be published elsewhere).

* The program may be obtained from Dr. W. Martin, Bayer. Geol. Landesamt, HeBstr. 128, D-80798 Mtinchen, Germany.

R. Zeese et al./Catena 21 (1994) 195 214

200

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F i g . 3. (Upper) Profile 1: The presence of accessory minerals, the Hinckley index of kaolinite and the amount, crystal size and AI substitution of goethite and hematite (dots in the profile indicate sample sites). (Lower) Profiles 1, 4 : P o r o s i t y , S i , A I , F e and Ti content and mineralogical composition.

4.1.2. The saprolites The saprolites are formed from different parent materials. The lower section of profile 1, which rests on weathered granite, consists of kaolinitic sand. The concentration of quartz grains suggests fluvial transport and the deposition of material probably derived from weathered granite as seen from the Zr/TiO2 ratio which is well in the felsic range (Fig. 6). The other saprolites all result from the weathering of volcanic rocks. From their Zr/ TiO2 ratio mafic rocks in profile 1, profile 4 and in the upper section of profile 2 can be separated from an intermediate volcanic rock at the lower section of profile 2. The latter contains dispersed quartz grains with deep resorption caverns as seen in thin sections. The uppermost saprolite of profile 2 also contains dispersed quartz grains, but its Zr/TiO2 ratio is still in the mafic field. Within the mafic rocks those with ilmenite (profile 1, upper sections of profile 2) can be separated from mafic rocks without ilmenite (profile 4). Unweathered basalt on top of profile 4 contains ilmenite suggesting a higher stratigraphic position of the ilmenitic sequence. Its K/Ar age of 27 Ma sets a minimum age to profile 4 and eventually a maximum age to profile 1. Porosity exceeds 40 vol.% in the saprolites. All saprolites from mafic volcanic rocks contain reasonably well crystalline kaolinite (HI > 0.8) and are low in Fed. Ilmenitic basalts are free of anatase which regularly occurs in ilmenite free basalts.

R. Zeese et al./Catena 21 (1994) 195 214

201

major elements

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Fig. 4. (Upper) Profile 2: The presence of accessory minerals, the Hinckley index of kaolinite and the amount, crystal size and AI substitution of goethite and hematite (dots in the profile indicate sample sites). (Lower) Profiles 2, 4: Porosity, Si, A1, Fe and Ti content and mineralogical composition.

202

R. Zeese et al./Catena 21 (1994) 195 214 m a j o r elements

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Fig. 5. (Upper) Profile 4: The presence of accessory minerals, the Hinckley index of kaolinite and the amount, crystal size and AI substitution of goethite and hematite (dots in the profile indicate sample sites). (Lower) Profile 4: Porosity, Si, A1, Fe and Ti content and mineralogical composition.

203

R. Zeese et al./Catena 21 (1994) 195 214

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Fig. 6. Zr/TiO2 ratios and parent rock type after Hallberg (1984). 4.1.3. T h e b a u x i t i c z o n e

The saprolite of the 13 m thick second section of profile 2 is divided into two parts by a bauxitic zone, where gibbsite rather than kaolinite dominates. Gibbsite crystals of relatively small size (< 1 #m) occupy the former space of feldspars and are associated with a well crystalline (50 60 nm) hematite. Hematite is often concentrated as a halo around the former augite crystals. The growth of larger gibbsite crystals (2-10 #m and larger) is oriented from this hematite framework into the leached center of the former augites. Larger crystals also occupy other open pores. Probably as a later formation hematite aggregates grew at the surface of the large gibbsite crystals (Fig. 7e). A small reduction of porosity seems to result from partial pore space filling by gibbsite growth. 4.1.4. T h e m o t t l e d z o n e

Faint red to purple mottling or relic structures traced by these colors may occur already in the saprolites. Hematite of medium crystal size (30-40 nm) and A1 substitution (ca. 5 tool%) causes its striking purple coloration (Munsell hue of up to 5RP, i.e. in the red-purple range). The multicolored saprolites (="argiles bariol~es"; Leprun, 1979) grade into a distinct mottled zone in the upper parts of the profiles 1 and 2. In profile 1 a gradual increase in Fed going upwards signifies the pedogenic homogeneity of this section. At the top of the mottled zone, pure goethitic parts alternate with parts rich in hematite (up to 50% of the Fe oxides), with both oxides having relatively high A1 substitution. In the upper 2-3 m of the mottled zone, relic textures of the former basalt occur side by side with channels. The filling of these channels contains quartz grains of allochthonous origin (crotovina?). Clay illuviation is visible as clay coatings along the channel walls. Some loose concretions at the surface of the profile are free of ilmenite and contain primary quartz, suggesting profile truncation. Presently, biotic activity modifies the fabric of the truncated saprolite.

204

R. Zeese et al./Catena 21 (1994) 195--214

~:i:a

Fig. 7. (a) Conchoidal goethite cortices around former quartz grains in a laminar ferricrete (profile 2, sample 12). (bl Partially dissolved quartz grain embedded in goethite cortices (profile 2, sample 12). (c) Surface of the quartz grain of (b) showing healing (or dissolution?) features (profile 2, sample 12). (d) Location of

R. Zeese et al./Catena 21 (1994) 195 214

205

In profile 2 the mottled zone below the second ferricrete preserves the residual, onion-shaped structure of the basalt with parallel zones of Fe depletion and concentration. A detailed microprobe study of a transect through such a zone (sample 24) is documented in Fig. 7f. The crust consists of two goethite zones (points 2 - 5 and 14) embedded in a kaolinitic matrix (1, 18) and embracing a gibbsitic zone in the center (12). The A1 substitution of the goethite as shown by spot analysis is up to 15---20 m o l % , whereas the bulk sample gave a value of 13 m o l % based on X R D line shift. This higher value, as compared to those in the laminar ferricretes (see below), possibly reflects an environment richer in A1, i.e. the immediate neighbourhood of weathering primary silicates or of kaolinite as against quartz in the laminar ferricrete. 4.1.5. T h e J e r r i c r e t e s

Massive to laminated ferricretes (type A) with high Fed (> 40%) are developed in sediment layers with abundant sandy to gravelly quartz grains. The wide range of the Zr/TiO2 ratios points to different source rocks which supplied weathered material. In Profile 1 the felsic character coincides with the underlying weathered granite. In the other profiles connections are less evident. Well crystalline, low-A1 goethite (AI substitution ca. 5 m o l % ) is dominant in ferricrete type A, which in profile 2 contains some hematite-rich zones as well. The goethite has crystallized around quartz grains, filling a substantial part of the pores (porosity of 10-5%). It has formed cortices with a conchoidal surface and a palisadic interior (Fig. 7a). After cortex formation, the quartz grains were partially (Fig. 7b) or completely dissolved indicating a leaching environment of low Si activity. The surface of the quartz grains (Fig. 7c) may either be the result of a structurally controlled dissolution or, more likely, a subsequent healing process fed by solutions richer in Si which permeated into the material from the overlying altered basalt. Microprobe analysis at various points of the goethite cortex (Fig. 7d) showed A1 substitution usually below 10 m o l % , in agreement with the X R D line shift of the bulk sample. In contrast to the underlying saprolite the kaolinite in the ferricrete is poorly crystalline (HI < 0.3). It is suggested that this kaolinite, formed in the presence of excess Fe, incorporated some Fe into the structure creating some disorder. Pisolitic to vermiform ferricretes (type B) seem to be more a group than a clearly definable genetic type. Their consistency varies from friable to extremely hard. Thick brown coatings around pisoids and pores as well as partly laminated material can be observed. The ferricretes may contain anatase or quartz. The iron oxides may be dominantly goethite or hematite or a mixture of both. The second ferricrete from profile 2 may illustrate the situation. It is free of ilmenite but contains quartz and anatase suggesting a less mafic parent rock (Fig. 6) than the over- and underlying mafic saprolites. Different relic textures in the pisoids demonstrate their formation

points for microprobeanalysisin a goethite cortex of a laminar ferricrete(profile2, sample 14) (see text). (e) Hematite crystal aggergateson large euhedral gibbsite crystals (profile 2, sample 22). (f) Location of points for microprobe analysis of a transect through a goethite cortex in the mottled zone of section 2 of sample 2 (profile 24) (see text).

206

R. Zeese et al./Catena 21 (1994) 195 214

from different parent materials. It is therefore proposed that this section consists of allochthonous weathered material redistributed by water and cemented in situ. Wind born sedimentation can be excluded since quartz sand and gravel are mixed with the gravelly pisoids. The upper ferricrete of profile 2 also can be identified as a recemented sediment by its mineralogy. Eventually even more ferricretes of type B originate from transported material (see Ollier, 1991). Ferricrete type B significantly differs from type A by a higher AI substitution of goethite and hematite (10-20 mol% A1). On the other hand Fe d is high ( 4 0 - > 50%) and crystallinity of kaolinite is low (HI < 0.5) as in ferricrete type A.

4.1.6. Interpretation As stated in the introduction, it is generally assumed that the minerals, particularly those formed by weathering, store important information about the environment in which the profiles developed over a long timespan during the Tertiary. This is a field which is not yet well developed and only rather vague suggestions can be made. One has to keep in mind, however, that certain mineralogical features may only be associated with a certain geological period if similar conditions have not occurred repeatedly during pedogenesis. For example, Fe oxides, once formed under aerobic conditions, can be redissolved and reprecipitated if exposed later to a high ground water level. If this takes place repeatedly, a profile may contain several generations of the same mineral with identical properties. The following mineralogical data may be useful: (A) llmenite and quartz must be related to the parent rock. In particular, the presence or absence of ilmenite in the various sections of the same profile is most likely associated with basalts of different flows and ages. (B) Anatase and pseudorutile are of pedogenic origin. Whereas anatase occurs only in ilmenite-free sections of the profiles and vice versa, pseudorutile is always associated with ilmenite. Thus, the primary Ti source to form anatase must come from other, more easily weatherable Ti-containing minerals such as augite, olivine, biotite and titanomagnetite rather thanfrom ilmenite, whose (early?) weathering product appears to be pseudorutile. As seen from its slightly larger unit cell, the anatase seems to have some structural Fe m. Whether or not the presence of anatase would only help to identify the type of parent rock or would also supply additional information on the weathering conditions remains to be understood later. (C) Goethite and hematite, the only secondary Fe Ill oxides vary widely in relative proportion, crystal size and Al-for-Fe substitution. Kernel frequency densitograms for the A1 substitution of all goethites showed a strong maximum at c a 6 mol% AI and two weaker peaks at ca. 12 and 17 mol% (Fig. 8a). Subdividing the goethites into those from laminar to massive (L) ferricretes and those from pisolitic and vermiform ferricretes (M) coincides with the separation. The separation within the higher substitution range cannot be explained yet. Anand and Gilkes (1987a,b) also found higher (20 34%) substitutions in pisolitic ferricretes of Western Australia. The frequency densitogram for the AI hematites (n = 45) (Fig. 8) also shows two maxima which, however, can not be attributed to the type of ferricrete. The majority of the hematites with A1 substitution around 6 mol% were recovered

207

R. Zeese et al./Catena 21 (1994) 195-214

a

123

5

10

15

20

25

AI substitution of hematites (mol%)

AI substitution of goethites (tool%) 7I

6

d

5

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o1 0

lo

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6'o

70

0

10

20

30

40

50

60

7'0

80

90

MCDa of hematites (nm)

Fig. 8. Frequency distribution of crystal size and AI substitution of goethite and hematite.

from those samples containing gibbsite. Again, higher (up to 15 mol%) substitutions were reported for West Australian ferricretes (Anand and Gilkes 1987a,b). The kernel densitogram of the crystal size of goethite perpendicular to the (111) face (MCDlll), shows a maximum at ca. 20 nm (Fig. 8) (Anand and Gilkes, 1987b; Schwertmann, 1988a). The shoulder at the larger crystal size range is caused by goethites from laminar ferricretes. The MCDa densitograms of hematites have two maxima and the larger crystals (> 40 nm) are partly associated either with ferricretes or with gibbsitic zones. In conclusion, these frequency considerations seem to justify the assumption of a different environment for goethite formation with respect to structural incorporation of A1 and crystal size, and of hematite with respect to crystal size (Schwertmann, 1988b). Fitzpatrick and Schwertmann (1982) have associated low AI goethites with redoximorphic conditions. Goethites with larger crystal size and less A1 substitution occur in all laminar to massive ferricretes. They were formed under stagnant water conditions from Fe 2+ in a porous, quartz-rich substrate from solutions low in A1 and crystallization inhibitors. In contrast, higher rates of substitution and smaller crystal sizes of goethites occur where the goethite was formed in immediate contact with an AI source, such as weathering primary silicates or kaolinite. Furthermore, the presence of gibbsite appears to favour hematite over goethite formation due to more AI in the system.

208

R. Zeese et a/./Catena 21 (1994) 195 214 25

20 ~ L

samples(24)

_'~ ,5

Hsamples(23)

~o 5

0 0.0

, O.2

04

O.6

O.S

1.0

1,2

Hinkley Index

~.4

1.6

1.8

Fig. 9. Frequencydistribution of the Hinckleyindex of kaolinites. (D) The significance of purple hematite with colors as purple-red as 5RP, often occurring in these profiles, is not known yet. The purple color is typical for larger (ca. 1 #m) idiomorphic hematite crystals. Purple colors with much smaller crystals as in these profiles (Fig. 8) were explained by oriented aggregates of smaller platy crystals providing for the same optical effect (Torrent and Schwertmann, 1987). This phenomenon seems to be restricted to older formations, possibly with long periods of slow oriented growth of hematite crystals from a mineral such as biotite. (E) In contrast to well-crystalline kaolinite, the poorly-crystalline form occurs strictly sectionwise. This is evident from a kernel densitogram (Fig. 9) computed after separating complete sections with "low" and "high". Again, their association with certain weathering conditions is still obscure. Mestdagh et al. (19801) and Muller and Bocquier (1987) have linked structural Fe of kaolinites with low crystallinity and this is supported by the observation that, in our profiles, kaolinites in ferricretes have low His. Another possibility is to consider primary (book) kaolinite vs. secondary kaolinites, Ambrosi et al. (1986) suggested that the oxidation and hydrolysis of incoming Fe z+ produces protons which dissolve the primary kaolinite and lead to a neoformation of Fe containing poorly crystalline secondary kaolinite. The clear separation between sections of low and high HI kaolinites may be either due to different conditions of formation or, as suggested by the above authors, to the transformation of high-HI kaolinite into low-HI kaolinites in only some sections. (F) Two groups of gibbsite crystals can be observed: Relatively small crystals replaced antecedent mineral aggregates, probably kaolinite. Larger crystals grew into open pores. The latter may be explained as precipitates from migrating solutions. According to B~rdossy and Aleva (1990) and others, a seasonally fluctuating groundwater table, high rainfall and a short drier season favours desilification and gibbsite formation. A seasonal pore water deficit may also explain hematite formation. Under these conditions Fe z+ may have been oxidized rapidly to form ferrihydrite at the surface of gibbsite crystals and at the rim of the decomposing pyroxene. During the dry season ferrihydrite was transformed to hematite. The predominance of hematite over goethite in the bauxite is in accordance with the results of laboratory experiments according to which A1 in the system strongly favours hematite over goethite formation from ferrihydrite (Schwertmann, 1988a).

209

R. Zeese et al./Catena 21 (1994) 195 214

The formation of distinct gibbsitic horizons cannot be explained by differences in the parent rock. It seems to postdate the first occurrence of ilmenitic basalts and may be a result of changing environmental conditions. Therefore age and stratigraphic position of the weathering profiles have to be clarified the more so as correlations of AI rich and Fe rich duricrusts with landscape evolution are controversial (Boulang6 and Eschenbrenner, 1971; Hill and Rackham, 1976).

4.2. Age and stratigraphic position of the profiles The stratigraphic position of the profiles can be estimated by the absolute age of several fresh basalts, and by the relative age of palynomorphs of a lacustrine clay, comprising part of the FVS.

4.2.1. K/Ar age of the basalts West of the city of Jos, a fresh basalt (profile 0 in Fig. 10) covers sandy sediments and a laminated to massive ferricrete of type A, similar to those at the base of profiles 1, 2 and 4. The K/Ar age of this basalt is 34.7 + 0.2 Ma. The K/At age of fresh ilmenitic basalt on top of profile 4 is 27 ± 0.2 Ma. Another basalt, which covers a truncated profile close to profile 4, has a K/Ar age of 8.43 to 8.63 +0.1 Ma. Therefore the timespan of the FVS embraces the whole Oligocene and the Lower and Middle Miocene. Soil profiles formed in this period have the following main characteristics distinguishing them from those formed on younger rocks (< 10 Ma): strong bleaching in the basal part, high iron content in massive, laminated or pisolitic ferricretes, gibbsite-hematite associations occasionally below the ferricretes, corroded primary =rofile 0

Profile 4

NW

SE

34,7Ma

Profile1

s

N

8,4 Ha

2 7M

40 -

Profile 2

ProfileT

7

"~20-

FUpperOl~qocene ItoLower~iocene]

w

(JOS PLATEAU, CENTRALNIGERIA) BASALT, UNWEATHERED

CLAY WITH FOSSIL LEAVES AND PALYNOMORPHS

BASALT, WEATHERED

z

# -~

WEATHERED VOLCANIC MATERIAL WITH FEW QUARTZ GRAINS GRANITE

] FERRICRETE(VERMIFORM, NODULAR, PISOLITIC) }~ IL

FERRICRETE(MASSIVE, LAMINATED) ILMENITE

GI

GIBBSITE'.

WEATHERED GRANITE

27M(~ K/Ar DATA FROM AMDEL (AUSTRALIA)

SAND OR SANDSTONE

....

SUPPOSED CORRESPONDENCE

Fig. 10. Stratigraphic position of the profiles (slightly generalized; profiles 0 and T were not described in the text).

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R. Zeese et al./Catena 21 (1994) 195 214

quartz grains and purple colors. These features, which can be easily observed in field, facilitate distinguishing the older profiles from the younger ones.

4.2.2. Relative dating by palynomorphs in lacustrine clays At Major Porter tin mine (profile T in Fig. 10, pos. T in Fig. 2, right) a carbonaceous clay bed which was exposed near the groundwater table below the soil contains abundant fossil leaves and palynomorphs. A total of 147 species of spores and pollen were identified by Takahashi and Jux (1989). 65 of them could be used for relative dating and indicate a Middle Tertiary age (Late Oligocene Early Miocene). 4.2.3. Corresponding sediments in the neighbouring basins From both relative palynological and absolute K/Ar ages, the older profiles can be assigned into the transition from Paleogene to Neogene. The conclusions based on locally obtained data can be supported by overregional comparison. In West Africa, Lower Eocene and older sediments ("Continental intercalaire") are separated from the younger Paleogene continental sediments ("Continental Terminal") by a widespread erosion disconformity (Boudouresque et al., 1982). Gwandu and Kerri-Kerri sediments can be regarded as Nigerian equivalents to the "Continental Terminal" (Kogbe, 1981). They consist of multicolored sandy to clayey fluvial and lacustrine sediments. Beside fossil soils with mottling, pisoid layers and often purple colors in the profiles, massive to laminated ferricretes also occur. All these features are characteristics of the older soil profiles. Gwandu and Kerri Kerri sediments as members of the Continental Terminal are, therefore, considered as time equivalents to the FVS. 4.3. Paleoenvironment and environmental changes 4.3.1. Mid Tertiary vegetation cover As can be deduced from the geologic section, the sedimentary structures, the mineralogical and geochemical composition, the fossiliferous clay was deposited in one of the many sloughs embanked by basalt flows on an undulating surface. In these swampy depressions the stagnant ponds were partly covered by salivinaceans (Hydrosporis cf levis) with marginal clusters of typhaceans (Tetradomonoporites typhinus n. sp.), whereas exposed mudflats may have been invaded by chenopodiaceans ( Chenopodipollis dispersus n. sp.) and other herbs (perhaps Euphorbiacites africanus n. sp.). Boggy grounds which lined thelower reaches of the sloughs seem to have been grasslands (e.g. Graminidites gracilis, G. minor n. sp.) as well as thickets of shrubs and trees composed of cyrillaceans (Cyrillaceaepollenites exactus), salicaceans (eventually Tricolpites cf. microreticulatus) and cupressaceans (Cupressacites cuspidataeformis). The adjacent hardwood forest which grew on the poorly drained soils may have consisted of some betulaceans (Triporopollenites parvus, Carpinuspollis sp.), moraceans ( Triporopollenites subrotundus n. sp.), many legumes ( e.g. Margocolporites vanwijhei, Striatopollis variabilis, S. catacumbus, S. nigericus n. sp., Fagraepollis reticulatus n. gen. et n. sp.), several olaceans (i.e. Retitrescolpites typicus) and caprifoliaceans (Rhoipites cf. finitus) with an underwood of manifold polypodiaceans

R. Zeese et al./Catena 21 (1994) 195 214

21 l

(e.g. Laevigatosporites haardti, L. josensis n. sp., L. oviformis n. sp., Verrucatosporites alienus) and schizaeaceans ( Leiotriletes maxoides, Triplanosporites sinuosus). At some distance, beeches (Faguspollenites globosus n. sp.), elms (Ulmipollenites semiundolosus n. sp.) and maples (Striatocolporites sp. b) may have been associated with this flatwood. On the slopes, above the sloughs, conifers (Araucariacites australis, Psophasphaera pseudotsugoides) and hardwoods grew. The latter are recorded by the pollen of rather abundant fagaceans (e.g. Quercoidites henriei, Q. microhenrici, Cupuli[eroidaepollenites cf liblarensis, C.josensis n. sp.), magnoliaceans ( Magnolipollis graciliexinus, M. micropunctatus) and sapotaceans (e.g. Tetracolporopollenites laevigatus, T. kirch-

heimeri, T. africanus). No species of the assemblage became abundantly fossilized, obviously emphasizing the genetic diversity of this dispersed microflora. The clay beds with leaf impressions were deposited in seasonal rhythms in a shallow lake depression fringed by gentle slopes. Therefore, the fossil microflora can not only be taken as a reliable biostratigraphic marker but also as a good indicator of former plant communities in either subtropical or montane tropical environments, which, astonishing as it may seem, contrast little from other synchronous assemblages of most of the Northern Hemisphere.

4.3.2. Crustal movement and dislocation of soil profiles The layers of profile 4, as well as the intermediate and felsic part of profile 2, are tilted. Tilting of ferricretes and of sediments, such as channel fills, can be observed at many other places where the lower sequence of the FVS is exposed (Fig. 10). N 160°E and N 35 60°E are very prominent directions of fracturing in profile 4. Therefore reactivation of horizontal and vertical tectonic movements near the conservative plate boundary of the Benue trough (Benkhelil et al., 1989) can be presumed. Weathered ilmenitic basalts with associated ferricretes are either oriented horizontally or, at least, much less tilted than the quartz-containing weathered volcanic rocks. Therefore we suppose that synchronous crustal movements influenced the formation of the FVS and its soils. Dislocations render correlations of stratigraphic members of the FVS more difficult.

4.4. The importance of the deep soil profiles for the understanding of the Tertiary paleoenvironments in Central Nigeria Soil profiles store informations on paleoenvironments if they are not erased by subsequent processes. The record in our profiles starts with a sequence of sedimentary and intermediate to mafic volcanic material. The oldest deposits contain abundant tin placers. They are the weathered residues of Jurassic granites, containing cassiterite and columbite, which were redistributed as the result of initial uplift and were subsequently covered by volcanic material. Both sediments and volcanics are strongly weathered. The most important features of the oldest sections

212

R. Zeese et al./Catena 21 (1994) 195 214

of the deep profiles are distinct tilting and Fe oxide removal (bleaching). It is suggested that the Fe z+ formed in a lowland groundwater environment with ample supply of biomass leading to 02 deficiency and to reduction and mobilization of iron. Ferricretes, intercalated between fluvial sediments and the intermediate to mafic volcanic suite, point to such extensive Fe mobilisation and migration before subsequent tectonics dislocated the oldest part of the profiles and erosion increased. In contrast, the deep kaolinitic and gibbsitic saprolites, in which hematite dominates over goethite, must have formed well above the permanent ground water table. Only thus the soluble products of weathering (Na, K, Mg, Ca and Si) could be perfectly removed. For all these parts (middle section of profile 2 and 4), a climatic seasonality must be assumed with a pore water deficit in the dry season facilitating the transformation of ferrihydrite to hematite. The formation of the gibbsitic-hematitic zone in profile 2, which also occurs in other profiles formed on ilmenite-containing basalts, indicates the total desilification of at least 2 m of basalt at least 4 to 8 m below the horizon of biotic activity and demands long lasting flow of non-saturated water under a climate with high rainfall and short dry seasons. Similar conditions must be assumed for the corrosion of primary quartz grains and also for the formation of closed depressions in felsic rocks ( = silica karst). The perfect drainage can be considered to result from continuous uplift and gentle tilting at the margins along the upraising Jos plateau. It is postulated that after the period of higher precipitation the climate became drier again, while the uplift continued or even accelerated. The soil profiles, which developed on basalts since that time, are less differentiated and gibbsite formation is restricted to better drained channels penetrating the uppermost part of the profiles. Consequently, the different soil profiles reflect environmental changes in Central Nigeria throughout the last 40 Ma, of which the first 30 million years are documented by soil formation on rocks of the FVS. This can be seen as the time-equivalent of the "'Continental Terminal" in the surrounding basins. The environmental changes have resulted from complex interrelations between endogenically and exogenically controlled processes. Further interdisciplinary work will help to understand better these interrelations.

Acknowledgements Our work has been generously sponsored by the Deutsche Forschungsgemeinschaft. One of us (U.S.) acknowledges the skilled analytical work of Ms. U. Maul. We are also thankful for the support we received from colleagues and companies, vicarious for many of them A M D E L laboratory, S.A., from where we got our basalt data and Julius Berger Nig., whose infrastructure we could use, may be mentioned. Thanks also to Dr. R. Bourman and Ms. S. Glasauer who kindly revised the text. Dr. K. Auerswald suggested and helped in the use of kernel density computation.

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213

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and surrounding clayey matrices in a laterite from Cameroon. In: L.G. Schultz, H. van Olphen and F.A. Mumpton (Editors), Proc. Int. Clay Conf,, Denver, CO, 1985. Clay Min. Soc., Bloomington, IN, pp. 186-194. Ollier, C,D., 1991. Laterite profiles, ferricrete and landscape evolution. Z. Geomorph. N.F., 35:165 173. Rundle, C.C., 1975. K - A r dating of basalts from the Jos Plateau, Nigeria. Rep. 75/1, IGS, London. Rundle, C.C., 1976. Further K - A t age determination on basalts from the Jos Plateau, Nigeria. Rep. IGS 76/6, IGS, London. Schwertmann, U., 1988a. Occurrence and formation of iron in various pedoenvironments. In: J.W. Stucki, B.A. Goodman and U. Schwertmann (Editors), Iron in Soils and Clay Minerals. Reidel, Dordrecht, pp. 267 302. Schwertrnann, U., 1988b. Some properties of soil and synthetic iron oxides. In: J.W. Stucki, B.A. Goodman and U. Schwertmann (Editors), Iron in Soils and Clay Minerals. Reidel, Dordrecht, pp. 203 244. Takahashi, K. and Jux, U., 1989. Palynology of Middle Tertiary lacustrine deposits from the Jos Plateau, Nigeria. Bull. Fac. Lib. Arts, Nagasaki Univ., Nat. Sci., 29/2:181 367. Thorp, M.B., 1967. Closed basins in younger granite massifs, Northern Nigeria. Z. Geomorph., I 1:459 480. Torrent, J. and Schwertmann, U., 1987. Influence of hematite on the color of red beds. J. Sediment. Petrol., 57:121 125. Valeton, I., 1991. Bauxites and associated terrestrial sediments in Nigeria and their position in the bauxite belts ofAfrica. In: J. Lang (Editor), Sedimentary and Diagenetic Dynamics of Continental Phanerozoic Sediments in Africa. J. Afr. Earth Sci., 12(1/2): 297 310. Valeton, I. and Beissner, H., 1986. Geochemistry and mineralogy of the Lower Tertiary in situ laterites on the Jos Plateau, Nigeria. J. Afr. Earth Sci., 5:535 550. Zeese, R., 1989. Einwirkungen junger Tektonik auf die Reliefentwicklung in der Umgebung des JosPlateaus Nigeria. Z. Geomorph. Suppl., 74:83 93. Zeese, R , 1991. Paleosols of different age in Central and Northeast Nigeria. In: J. Lang (Editor), Sedimentary and diagenetic dynamics of continental Phanerozoic sediments in Africa. J. Afr. Earth Sci., 12(1"2): 311 318.