ELSEVIER Sedimentary Geology 128 (1999) 83–98

Sandstone petrology and provenance of the Neoproterozoic VoltaianGroup in the southeastern Voltaian Basin, Ghana

Chris Anani *

Department of Geological Science, Graduate School of Science and Technology, Niigata University, Ikarashi 2-8050,Niigata 950-2181, Japan

Received 15 November 1998; accepted 5 May 1999

Abstract

The Neoproterozoic strata in the eastern part of the Voltaian Basin are divided into the Lower, Middle and UpperVoltaian formations. The Kwahu Sandstone and Anyaboni Sandstone members are within the Lower and Middle Voltaianformations, respectively. Modal plots for the Kwahu Sandstone indicate a craton interior provenance. Quartz-type analysissuggests a mostly plutonic origin, and the tourmaline chemistry indicates a major granitic source with associatedmetasedimentary rocks. The sandstones of the Anyaboni Sandstone plot mostly in the transitional continental and partlyin the craton interior provenances on QFL and QmFLt diagrams. Quartz-type analysis suggests a source of medium-grademetamorphic rocks; tourmaline analysis indicates a metasedimentary origin. The sedimentological data and interpretationssuggest that the Kwahu Sandstone originated from the Birimian Supergroup located in the west and south of the VoltaianBasin. The Anyaboni Sandstone is thought to have been derived from the same supergroup, but from the southwest of theVoltaian Basin. The differences in sandstone composition between the Kwahu Sandstone and Anyaboni Sandstone seemto be due to tectonic movements, probably intracontinental rifts, and to a possible climatic effect in the source areas andwithin the sedimentary basin. 1999 Elsevier Science B.V. All rights reserved.

Keywords: Ghana; Voltaian Basin; modal analysis; provenance; sandstone; petrology

1. Introduction

The Neoproterozoic to Early Cambrian VoltaianBasin in Ghana is considered to reflect more thanone tectonic setting (Affaton et al., 1980; Wrightet al., 1985). Its central and western parts representtypical platform cover, but the eastern part is deeplyburied and resembles a continental passive margin.

The sandstone compositions, their provenancesand their relationships to the tectonics of the Volta-ian Basin have not previously been examined in

Ł Tel.: C81 25 260 2130; Fax: C81 25 260 2130; E-mail:chris@geows.sc.niigata-u.ac.jp

detail. Such studies are essential to the understand-ing of the paleogeography of the region, and shouldmake it possible to identify possible source areas.Comparison of the sedimentology and mineralogy ofmodern river sands with those of the sandstones ofthe Voltaian Group may help disclose the sources ofvaluable minerals such as gold, and thus might leadto more successful prospecting.

This paper describes the petrography of sand-stones in the Voltaian Group, including analyses ofquartz types and varietal studies of tourmalines, andcomments on the provenance and tectonic setting ofthe Voltaian Group sediments.

0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 6 3 - 9

84 C. Anani / Sedimentary Geology 128 (1999) 83–98

2. Geological setting

There are three main structural units in WestAfrica: (1) a Precambrian basement referred to asthe West African Craton; (2) Pan-African (around600 Ma) and Hercynian (350–300 Ma) orogenic foldbelts; (3) Mesozoic to Cenozoic sedimentary basins(Affaton et al., 1980).

The Voltaian Basin is situated on the southeast-ern margin of the West African Craton (Fig. 1). Itis bounded to the south, west and north mostly byrocks of the Paleoproterozoic Birimian Supergroup,which have yielded an Sm:Nd whole-rock isochronage of tholeiitic basalts of 2166 š 66 Ma (Taylor etal., 1988, quoted in Hirdes et al., 1992) and a U=Pbzircon dating age on belt granitoid plutons of ¾2175Ma (Hirdes et al., 1992), and partly by the Tark-waian Group (Fig. 1). Hirdes et al. (1992) reportedthat both the Birimian supracrustal rocks and the

Fig. 1. Generalized geological map of the West African Craton. Modified after Ako and Wellman (1985). Belts and basins in the Birimianare defined after Leube et al. (1990) as follows: A D Kibi–Winneba belt; B D Cape Coast Basin; C D Ashanti belt; D D Kumasi Basin;E D Sefwi belt; F D Sunyani basin; G D Bui belt; H D Maluwe Basin; I D Bole–Navrongo belt; J D Lawra belt.

Tarkwaian Group were deformed, metamorphosedto greenschist facies in the course of the Eburneantectono-thermal event (¾2102 Ma) to form part ofthe West African Craton.

The Birimian Supergroup consists of metasedi-mentary rocks which were deposited in basinal ar-eas separated by volcanic belts, that trend northeast–southwest in which mainly tholeiitic basalts occur.The metasedimentary rocks in the basinal areas are(1) volcaniclastic rocks, (2) turbidite-related wackes,(3) argillitic rocks, and (4) chemical sediments (Leubeet al., 1990). The rocks of the Tarkwaian Group rep-resent the erosional products of the Birimian, and aredominated by coarse-clastic sediments. They are lo-cated mostly within the volcanic belts. The wholecomplex of basins and volcanic belts have been ex-tensively intruded by granitoids (Eisenlohr, 1992).

The Buem Formation and the Togo Formationform the eastern boundary of the Voltaian Basin

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(Fig. 1). The Buem Formation is gently folded withN–S-trending axes, and displays an anchizonal meta-morphism (Castaing et al., 1993). It is composed ofcalcareous, argillaceous and=or sandy shale, arkoseand graywacke sandstone, ophiolitic units that in-clude alkaline and calc-alkaline basalts, and serpen-tinites with chromite and cross-cutting dolerites. TheTogo Formation is mainly composed of aluminousmetaquartzite, albitic micaschist, leucocratic gneissand scarce metabasic and ultrabasic rocks (Fig. 1;Affaton et al., 1980). To the east of the TogoFormation, the Benin–Nigerian Province (Caby,1989, quoted in Petters, 1991) comprises mus-covite-bearing quartzites, quartzitic sericite schists,gneisses, granites, granitic gneisses and leucocraticmigmatites, and basic rocks (Affaton et al., 1980).The Benin–Nigerian Province thus comprises blocksof the Birimian Supergroup subsequently reworkedduring the Pan-African and perhaps Kibaran oroge-nies, a metasedimentary cover mainly equivalent tothe Togo Formation (Affaton et al., 1980).

The sediments of the Voltaian Group cover anarea of 103,600 km2, almost one-third of the areaof Ghana (Kesse, 1985). They are generally flat-lying and unmetamorphosed, except at their east-ern margin. Ako and Wellman (1985) reported thatthe Compagnie General de Geophysique (CGG) hascarried out a total magnetic intensity aeromagneticsurvey over the whole of the Voltaian sedimentarybasin, and has mapped depths to magnetic basement.Magnetic basement has its greatest depth of 4–5 kmalong a N–S trough at 0º longitude between 7º and9ºN latitude. The western and northern thirds of thebasin are less than 3 km thick, and the southeasternthird generally over 3 km thick. The Voltaian Groupconsists mainly of sandstone, shale, mudstone andconglomerate.

2.1. The Voltaian Group

There are few detailed geologic descriptions ofsediments of the Voltaian Group sequences; subdi-vision of the group is difficult due to poor expo-sure and the lack of laterally persistent lithologicalmarker beds or fossils. The group has generallybeen divided into three formations, each separatedby an unconformity marked by a tillite (Fig. 2). TheLower Voltaian Formation consists of a massive to

cross-bedded arkosic sequence. The Middle Volta-ian Formation consists of a flyschoid sequence, andthe Upper Voltaian Formation consists of a molassesequence (Affaton et al., 1980).

The Lower Voltaian Formation unconformablyoverlies the Birimian Supergroup. A radiometric ageof 993 š 62 Ma from the lower part of the LowerVoltaian Formation gives the approximate period forthe beginning of sedimentation of the group. Thecalculated age was based on a three-point regressionline and no analytical data were published (Clauer,1976, quoted in Cahen et al., 1984). The depositionalenvironment for the Lower Voltaian Formation islikely to have been shallow marine or fluviatile. Itreflects a fairly stable tectonic setting throughout thedepositional area.

The Middle Voltaian Formation overlies theLower Voltaian Formation with a slight angular un-conformity. Conglomeratic beds interpreted as tillitesform the basal part of the Middle Voltaian Formation(Petters, 1991). Shales, siltstones and sandstones,glauconitic in part, constitute the principal litholo-gies of the Middle Voltaian Formation. K–Ar dat-ing of glauconite from a borehole core at Tibagonayielded a Vendian age of 600š20 Ma (Bozhko, 1969,1974, 1984, quoted in Bozhko, 1994); the presenceof abundant early Vendian microfossils confirms thisage.

The Upper Voltaian Formation is divided intoa lower and an upper unit. The lower unit con-sists mostly of dirty-yellow, fine-grained, thinly bed-ded, micaceous, feldspathic quartz sandstones withsubordinate argillite intercalations. The upper unitconsists of white to whitish-yellow, massive, fine- tomedium-grained, cross-bedded arkosic and quartzosesandstones. The Upper Voltaian Formation occurs asscattered outcrops in the central part of the Volta-ian Basin, with an average thickness of about 400m. The Upper Voltaian Formation constitutes themolasse deposits formed in the Cambro–Ordovicianperiod by the erosion of some horizons in the Pan-African mobile belt (Grant, 1969).

3. Stratigraphy of the study area

The study area, bounded by 6º150N and 6º300Nlatitude and 0º000W and 0º150W longitude, lies in

86 C. Anani / Sedimentary Geology 128 (1999) 83–98

Fig. 2. Sequence of the Voltaian Group (redrawn after Cahen et al., 1984 and Petters, 1991). The horizons of both the Kwahu SandstoneMember and the Anyaboni Sandstone Member are indicated. Radiometric age, 993š 62 Ma, and Rb–Sr isochron age, 660 š 65 Ma,from Cahen et al., 1984.

the southeastern part of the Voltaian Basin (Fig. 1).The Kwahu and Anyaboni sandstones crop out overmost of the study area, with a small area of BuemFormation in the southeast (Fig. 3). There is no com-pletely exposed section through the Voltaian Groupdue to deep weathering and low topographic relief.

Correlations within the group and the formations ofthe area within it are therefore difficult.

Two subdivisions were identified in the presentwork, the Kwahu Sandstone Member of the LowerVoltaian Formation and the Anyaboni SandstoneMember of the Middle Voltaian Formation (Fig. 3

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Fig. 3. Geological map of the Voltaian Group and the Buem Formation in the southeastern part of the Voltaian Basin with sampling localities. See insert map in Fig. 1 forlocation.

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Table 1Stratigraphic and lithologic divisions of the Voltaian Group in the southeastern Voltaian Basin

and Table 1). The former corresponds to the upperpart of the Kwahu Sandstone of Saunders (1970),and the latter is the correlative of the Anyaboni For-mation of Saunders (1970). A normal fault with aNW–SE trend, the Bukudo Fault, separates the twosandstone members (Fig. 3).

3.1. The Kwahu Sandstone Member

Saunders (1970) divided the strata of the KwahuPlateau into three units, a lower thin-bedded sand-stone, a middle shale, and an upper cross-stratified

orthoquartzite (quartzose sandstone of this paper),which he called the Kwahu Sandstone. The lowestunit unconformably overlies the Proterozoic Tarkwa-ian Group and the underlying Birimian Supergroup.The Kwahu Sandstone Member is 500 m thick in thestudy area and is exposed at the Osonson, Sekesua andOtrokper localities (Fig. 3). It consists of a lower hori-zon of mostly coarse- to medium-grained quartzosesandstones (Table 1). The beds are thick to massive(>20 m), and generally horizontal. The quartz grainsare subrounded to rounded and often poorly sorted.The sandstone characteristically contains clay clasts

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(or clay galls), and sporadic interbeds of gray shaleoccur. The shales are fine-grained, quite smooth-tex-tured and exhibit remarkably thin laminae. The thick-to massive bedded quartz sandstones are underlainby granule conglomerates, which often form rela-tively thin beds. These features are typically observedaround the Osonson locality.

The upper horizon consists of 10 to 15 m ofmedium- to fine-grained quartzose sandstones. Thebeds are generally whitish in color, but often lo-cally stained by reddish Fe2O3. Grains are also sub-rounded to rounded.

3.2. The Anyaboni Sandstone Member

Saunders (1970) showed that the Bukudo Faultmarks the contact between the upper part of theKwahu Sandstone and the Anyaboni Formation (theAnyaboni Sandstone Member of this paper). Thefault is well exposed in the bed of the Bukudo streamat a point 5 km west of Sekesua (Fig. 3).

The Anyaboni Sandstone Member is exposed atthe Anyaboni, Apimso, Bisa, Asesewa and Anyilesulocalities. Its type area is in and around the Anyabonilocality. The member is about 400 m thick and hashere been divided into two units, a lower unit domi-nated by beds ranging from 1 to 5 m in thickness, andan upper unit of thick sandstone beds, about 10 mon average. Both units are composed of feldspathicsandstone, although feldspar is less frequent in thelower unit which is characterised by relatively highmica contents (especially muscovite). Mica is rare orabsent in the upper unit. The boundary between thetwo units is gradational and, in some places, poorlydefined. The feldspar in both units imparts a pinkcolor. The sandstones are mostly chocolate in color,and a reddish brown color (probably hematite) is alsocommonly present. Gray or white sandstones occurat a few localities. Both units contain cross-beddingand=or parallel laminations, with other sedimentarystructures such as poorly developed graded-beddingat some localities.

3.3. The Buem Formation

One locality in the study area, at Pawnpawn,is situated in the lower part of the Middle BuemMember of the Buem Formation. This consists of

coarse-grained quartz sandstones in thick to massive(>20 m) beds. The dip is generally to the south-southeast (Fig. 3). The quartz grains are coarse- tomedium-grained, subrounded to rounded, and oftenpoorly sorted. The sandstones characteristically con-tain clay clasts.

4. Petrographic sampling and modalcharacteristics of the Voltaian Group and theBuem Formation sandstones

4.1. Sampling

A suite of samples was collected from the Volta-ian Group and the Buem Formation of the studyarea. 109 samples were collected from 9 localities(Fig. 3). These were made up of 11 samples fromOsonson, 14 from Sekesua, 5 from Otrokper, 45from Anyaboni, 4 from Apimso, 10 from Pawnpawn,8 from Bisa, 7 from Asesewa, and 5 from Anyilesu.Most of the sampled rocks are medium-grained witha few fine- and coarse-grained sandstones. Thin sec-tions were prepared of all of them for petrographicanalysis. Only 96 were point-counted. The remaining13 thin sections were not good to produce favourablecounting statistics due to hematite invading micro-pores in the rock fragments, sufficient enough toprevent other mineral grain identification.

Point-counting was carried out to identify individ-ual grains or crystals larger than 0.0625 mm usingthe Gazzi–Dickinson method (Dickinson, 1970; In-gersoll et al., 1984). The constituent minerals of thesandstones were classified into seven groups: mono-crystalline quartz, polycrystalline quartz, feldspar,aphanitic lithic fragments (volcanics, meta-volcanics,sediments, meta-sediments and cherts), heavy min-erals, monocrystalline phyllosilicates (sericites, chlo-rites, kaolinites, etc), and miscellaneous and uniden-tified framework grains.

For each thin section, 500 points were counted,using the maximum grid spacing to give full cover-age of the slide. In a very few cases the thin sectionwas of poor quality and the grid spacing was reducedin order to obtain at least 500 counts. All the thinsections were stained with sodium cobaltinitrite todistinguish the potassium feldspar from quartz andchert. Full data are reported in Anani (1997).

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Table 2Mean modal composition of sandstones from the Voltaian Group and Buem Formation from the southeastern margin of the VoltaianBasin

Member name Number of samples Quartz Feldspar Aphanitic lithic fragment D M Mc

Qm Qp total K P total Lv Ls ch

Anyaboni 62 71.8 2.5 74.4 20 0 20 0.7 0.03 0.5 0.3 1.1 3.5Sandstone (SD) 5.90 1.90 6.20 5.3 0 5.3 0.5 0.1 0.5 0.5 0.9 2.2Kwahu 25 96.3 1.4 97.6 0.4 0 0.4 0.1 0.1 0.7 0.1 1 0.2Sandstone (SD) 2.5 1.1 2.6 1.5 0 1.5 0.3 0.2 0.6 0.1 1.1 0.4Middle 9 92.8 2.5 95.3 0.3 0 0.3 0.2 1.3 0.7 0.2 1.5 0.6Buem (SD) 5.2 0.7 4.7 0.7 0 0.7 0.2 2.3 0.4 0.3 1.7 0.7

Qm D monocrystalline quartz, Qp D polycrystalline quartz, K D potassic feldspar, Lv D volcanic and metavolcanic lithics, Ls Dsedimentary and metasedimentary lithics excluding chert, ch D chert, D D dense minerals, M D monocrystalline phyllosilicates, Mc Dmiscellaneous and unidentified framework grains, (SD) D standard deviation.

Compositional fields are shown as triangularQFL (quartz–feldspar–lithic fragments) and QmFLt(monocrystalline quartz–felspar–total lithic frag-ments) diagrams to differentiate maturity and sourcerocks (Dickinson and Suczek, 1979; Dickinson etal., 1983; Dickinson, 1985.). The three apices whichrepresent the recalculated proportions of key cate-gories of grain types are those used by Dickinson(1970). The mean compositions of each membersampled here are shown in Table 2. The QFL andQmFLt compositions are plotted in Figs. 4 and 5,respectively.

4.2. The Kwahu Sandstone Member

The Osonson, Sekesua and Otrokper localities aresituated in the upper part of the Kwahu SandstoneMember. The absence of diagnostic fossils and litho-logical marker beds makes correlation between thelocalities difficult, but the same horizons seem tooutcrop at each locality. The beds are mostly hori-zontal; a few dip gently eastwards.

The petrographic characteristics of all three local-ities in the Kwahu Sandstone Member are broadlysimilar. All three are quartz-rich sandstones withvery few feldspars or rock fragments (Table 2).

The framework quartz occurs mainly as mono-crystalline quartz, some of which has strained (undu-latory) extinction. Very few show polycrystallinity.The grains are subrounded to rounded and are mod-erately to poorly sorted. A few have overgrowths,and some of those show multiple overgrowth rim-ming on detrital quartz grains. Sphene, tourmaline,

zircon, apatite and rutile occur as accessory minerals.A few clay pellets are present in the form of aggre-gates of minute phyllosilicates. The samples fromthe Kwahu Sandstone Member mostly fall in the Qpole area within the craton interior field of the QFLdiagram (Fig. 4). The QmFLt plots of the KwahuSandstone Member show a similar result due to theextremely low content of lithic fragments (Fig. 5).Both plots emphasise the highly quartzose nature ofthe Kwahu Sandstone Member.

4.3. The Anyaboni Sandstone Member

Anyaboni, Apimso, Bisa, Asesewa, and Anyilesu(Fig. 3) localities are situated within the AnyaboniSandstone Member. The stratigraphic relations arepoorly known due to the absence of diagnostic fossilsand lithological marker beds. The horizons sampledmay not cover the whole of the Anyaboni SandstoneMember sequence. The beds are mostly horizontal.

The petrographic characteristics of the five locali-ties in the Anyaboni Sandstone Member are similarto each other and are therefore described togetherhere. The study showed some pore-filling quartz aswell as significant amounts of hematite infilling porespaces.

Opaque oxide pigments, probably mainly hema-tite, are present locally in a few of the sandstones.They probably result from the oxidation of ferromag-nesian minerals; the same process occurs on a largerscale throughout the outcrops. In some samples, thehematite in the micropores prevents the identificationof other mineral grains.

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Fig. 4. QFL plots of detrital modes for the Kwahu and AnyaboniSandstone suites of the Voltaian Group and the Buem Formation.See Table 2 for data and explanation.

A petrographic study of 62 thin sections from thefive Anyaboni Sandstone localities (Table 2) showedthe sandstones to be mostly arkose to subarkose, ac-cording to the classification of Pettijohn et al. (1972).

Fig. 5. QmFLt plots of detrital modes for the Kwahu andAnyaboni Sandstone suites of the Voltaian Group and the BuemFormation. See Table 2 for data and explanation, and legend ofFig. 4.

The main constituents are quartz and feldspar withvery few rock fragments. The heavy minerals tourma-line, zircon, sphene, apatite, epidote and rutile occur

92 C. Anani / Sedimentary Geology 128 (1999) 83–98

as accessories. Tourmaline and zircon are the mostcommon. The micas (0.2%–3%) are mostly mus-covite with traces of biotite. The secondary miner-als sericite, illite and, more rarely, kaolinite are alsopresent, generally forming 0.8%–5.4% of the matrix.Grain-to-grain boundaries are mostly straight witha few crenulate contacts between strongly elongategrains. Detrital-grain sizes range mostly between 0.12mm and 0.65 mm. Most of the grains are subangularto subrounded, and are moderately to well sorted, us-ing the definition of Pettijohn et al. (1972).

The QFL and QmFLt plots in Figs. 4 and 5, re-spectively, were compared with those of Dickinsonet al. (1983) to identify possible source areas for thedetrital grains. The QFL plots of the Anyaboni Sand-stone Member fall in both the transitional continentalfield and the craton interior field (Fig. 4). The QmFLtplots for the corresponding samples also fall withinthe transitional continental field and the craton inte-rior field (Fig. 5). In the QmFLt diagram, all lithicfragments are plotted together; thus, the emphasis isdirected toward the grain size of the source rocks,since finer-grained rocks yield more lithic fragmentsin the sand-size range.

4.4. The Middle Buem Member

A small part of the lower horizon of the Mid-dle Buem Member was sampled at the Pawnpawnlocality (Fig. 3).

The sandstones at the Pawnpawn locality arequartz-rich, medium- to coarse-grained. Most of thecoarse grains and some of the medium grains aresubrounded to rounded: most of the medium grainsare subangular to subrounded. The sandstones aremoderately to poorly sorted. A few of the quartzgrains have overgrowths which can be recognised bya dust ring. Clay clasts can be seen to be formedof aggregates of minute phyllosilicate crystals. Thetextural characteristics are similar to those of thesandstones of the Kwahu Sandstone. The only ob-servable difference between the two members is thehigher polycrystalline quartz content of the MiddleBuem Member.

Samples of the Middle Buem Member mostly plotin the craton interior field in the QFL and QmFLtdiagrams (Figs. 4 and 5). The QmFLt plot showshigher lithic fragment content in the Middle Buem

Member of the Buem Formation compared to thoseof the Anyaboni and Kwahu sandstones.

5. Quartz types of selected sandstones

According to Basu et al. (1975) comparison ofundulosity of the monocrystalline quartz with theamount of polycrystalline quartz can be used todifferentiate recent and ancient sands of plutonicand low- and high-rank metamorphic origins. Themethod was used for quartz-types from some ofthe medium-grained sandstones of the Kwahu andAnyaboni sandstones.

5.1. Procedure

Fourteen sandstone samples, five from the KwahuSandstone and nine from the Anyaboni Sandstone,were examined in thin section. Each thin section wasstained with sodium cobaltinitrite to differentiatequartz from potassium feldspar. Point-counts weremostly in the range 330–500. Four counting bankswere reserved for data collected simultaneously, fol-lowing the procedure described by Basu et al. (1975).These were: (1) monocrystalline quartz, subdividedinto two banks of (a) undulatory quartz (extinctionangle >5º) and (b) non-undulatory quartz (extinctionangle �5º); and (2) polycrystalline quartz, subdi-vided into two banks, namely (a) 2–3 crystals pergrain and (b) >3 crystals per grain. In a very fewcases, mostly in the Kwahu Sandstone, grain bound-ary identification was difficult, but in most cases dustrings made boundary identification easy.

5.2. Results

The Kwahu Sandstone samples are mostly mono-crystalline quartz with 75 to 80% of the quartz grainsshowing non-undulatory extinction. Polycrystallinegrains make up between 3 and 9% of the total quartz.In contrast, the Anyaboni Sandstone samples showundulatory extinction in 25–36% of total quartz pop-ulation and polycrystallinity in 5–12% of the quartzgrains.

When plotted on a diamond diagram of Basu etal. (1975) (Fig. 6), the Kwahu Sandstone samplesfall in the plutonic field and around the plutonic–

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Fig. 6. Four-variable plot of nature of quartz population in the selected sandstones from the Voltaian Group. Diagram is after Basu et al.(1975).

metamorphic field boundary. The Anyaboni Sand-stone samples plot in the middle and upper rankmetamorphic field.

6. Tourmaline as a petrogenetic indicator

Within the study area, zircon, tourmaline andopaque minerals are the most common heavy miner-als in the Voltaian Group, although this may not betrue for the Voltaian Basin as a whole. Tourmalineanalysis was used following the procedure describedby Henry and Guidotti (1985), to try to identify pos-sible source rocks and hence source areas. Tourma-

line is mechanically and chemically stable and eachtourmaline grain can potentially provide informationon its origin.

Henry and Guidotti (1985) showed that tourma-line compositions plotted on Al–Fe(tot)–Mg andCa–Fe(tot)–Mg ternary diagrams can be used toidentify tourmalines from different rock types.

6.1. Procedure

Sandstone samples were crushed in a jaw-crusherand each sample was panned to produce a heavy-minerals concentrate. Carbonates were removed withdilute (10%) HCl, and the residues were dried and

94 C. Anani / Sedimentary Geology 128 (1999) 83–98

sieved using a 44 µm sieve and a 250 µm sieve. Theheavy minerals were concentrated using bromoform(tribromoethane), and a tourmaline concentrate wasprepared using a magnetic separator. The tourmalinegrains were then hand-picked and mounted on glassslides using petropoxy 154. Thin sections were madeof 98 grains from six sandstone samples for opticaland chemical analysis.

A JSM-5310E Scanning Electron Microscope(SEM) equipped with an Energy-Dispersive Spectro-scope (EDS) was used for the analyses. The proce-dure followed for the normalisation of cations is thatof Henry and Guidotti (1985) and Henry and Dutrow(1996). This provides a good first approximation:24.5 oxygens were applied in the normalisation ofthe cations since the EDS analyses each grain to100%, thus boron cannot be calculated. Microprobeanalytical data are reported in Anani (1997).

6.2. Microscopic identification and chemicalanalyses

Under the optical microscope some of the tour-maline grains occur as short slender prisms withterminations at one, or rarely, both ends. Mostly theyappear as curved near-triangular sections, and as egg-shaped or long ellipsoidal grains. Of the tourmalines45 show zoning and 53 appear to be homogeneous.Both the cores and the rims of the zoned grains werechemically analysed, but they showed little varia-tion. Only the results from the cores are shown here(Fig. 7b and Fig. 8b). Analyses from both the un-zoned and zoned grains give an indication as to thenature of the potential source rocks (Figs. 7 and 8).The homogeneous grains show more compositionalvariation and may be more detrital in origin.

6.3. The Kwahu Sandstone Member

The unzoned tourmalines from the Kwahu Sand-stone (samples 126, 129 and 115) fall in distinct fields(Fig. 7a). In the Al–Fe(tot)–Mg diagram (Fig. 7a)sample 115 falls in the field defined for metapelitesand metapsammites not coexisting with an Al-saturat-ing phase. Sample 126 tourmalines fall in the follow-ing respective fields: Li-poor granitoids, metapelitesand metapsammites coexisting with an Al-saturat-ing phase, with a few tourmalines straddling in and

around the field for metapelites and metapsammitesnot coexisting with an Al-saturating phase. Tourma-lines from sample 129 distinctly fall into the fields ofLi-poor granitoids, metapelites and metapsammitescoexisting with an Al-saturating phase.

The core of zoned tourmalines from the KwahuSandstone (samples 126, 129, 115) fall in the fol-lowing fields (Fig. 7b). Sample 126 tourmalines fallrelatively well in the field defined for metapelitesand metapsammites coexisting with an Al-saturat-ing phase. A few of them fall well within the fielddefined for Li-poor granitoids. Tourmalines fromsample number 129 fall within 3 fields, the Li-poorgranitoids, metapelites and metapsammites coexist-ing with an Al-saturating phase, and metapelites andmetapsammites not coexisting with an Al-saturat-ing phase. Sample number 115 tourmaline straddlesaround the Li-poor granitoid fields. This distinctpositioning of the tourmalines is similarly shownby their corresponding Ca–Fe(tot)–Mg diagrams(Fig. 8).

The chemical compositions of the tourmalinespoint to source rocks that are mostly granites, withsome metasediments.

6.4. The Anyaboni Sandstone Member

The plots of tourmalines from the Anyaboni Sand-stone (samples 151, 165 and 185) allow some con-straints to be placed more confidently on the possiblesource rocks. In Fig. 7a sample 151 falls in bothfields defined for metapelites and metapsammites.Sample 165 tourmalines fall in the field definedfor Li-poor granitoids. Tourmalines from sample 185distinctly fall into the following fields: Li-poor grani-toids, metapelites and metapsammites not coexistingwith an Al-saturating phase and a third field forcalc-silicate rocks and metapelites.

Fig. 7b shows the core of zoned tourmalines fromthe Anyaboni Sandstone (samples 151, 165 and 185).Sample 151 tourmalines fall well within the fielddefined for metapelites and metapsammites coexist-ing with an Al-saturating phase. Tourmalines fromsample 165 fall well within the Li-poor granitoidfield, while some straddle around metapelites andmetapsammites not coexisting with an Al-saturatingphase. Sample 185 tourmalines straddle around thefield for metapelites and metapsammites not coexist-

C. Anani / Sedimentary Geology 128 (1999) 83–98 95

Fig. 7. (a) Unzoned tourmalines. (b) Zoned tourmalines. Al–Fe(tot)–Mg diagram (in molecular proportions) for tourmalines fromVoltaian Group sandstones. This diagram is divided into regions that define the compositional ranges of tourmalines from differentparent-rock types (after Henry and Guidotti, 1985); 1 D Li-rich granitoid pegmatites and aplites; 2 D Li-poor granitoids and theirassociated pegmatites and aplites; 3 D Fe3C-rich quartz–tourmaline rocks (hydrothermally altered granites); 4 D metapelites andmetapsammites coexisting with an Al-saturating phase; 5 D metapelites and metapsammites not coexisting with an Al-saturating phase;6 D Fe3C-rich quartz–tourmaline rocks, calc-silicate rocks, and metapelites; 7 D low-Ca meta-ultramafics and Cr, V-rich metasediments;8 D metacarbonates and meta-pyroxenites.

ing with an Al-saturating phase. This distinct posi-tioning of the tourmalines is similarly shown by theircorresponding Ca–Fe(tot)–Mg diagrams (Fig. 8).

The chemical composition of the tourmalines heresuggests source rocks that are mostly metasedimentswith some granites.

7. Discussion

The possible effects of tectonic movements whichoccurred during the deposition of the Voltaian Groupin the southeastern Voltaian Basin on the mineralcompositions of the Kwahu and Anyaboni sand-stones are considered here. The characteristics ofthe quartz populations of the sandstones and thechemical compositions of their tourmalines probablyreflect these crustal movements and can therefore beused to decipher them. The rock constitution and

tectonic setting of the surrounding terranes, the Bir-imian Supergroup, the Buem and Togo formationsare also discussed to deduce possible source rocks.

7.1. Provenance characteristics deduced fromclastic sediments

The sandstones of the Kwahu Sandstone arequartz arenites which consist almost exclusively ofquartz with a minor component of aphanitic lithicfragments. QFL and QmFLt plots indicate that al-most all these sediments were formed within thecraton interior (Figs. 4 and 5). Quartz-type analy-ses (Fig. 6) and tourmaline compositions (Figs. 7and 8) indicate that the sandstones were probablyderived largely from granites with some input frommetasediments. The predominance of quartz in thesandstones may indicate severe chemical weatheringin Neoproterozoic times, and may also be partly as-

96 C. Anani / Sedimentary Geology 128 (1999) 83–98

Fig. 8. (a) Unzoned tourmalines. (b) Zoned tourmalines. Ca–Fe–Mg diagram (in molecular proportions) for tourmalines from VoltaianGroup sandstones. This diagram is divided into regions that define the compositional ranges of tourmalines from different parent-rocktypes after (Henry and Guidotti, 1985); 1 D Li-rich granitoid pegmatites and aplites; 2 D Li-poor granitoids and their associatedpegmatites and aplites; 3 D Ca-rich metapelites, metapsammites, and calc-silicate rocks; 4 D Ca-poor metapelites, metapsammites andquartz–tourmaline rocks; 5 D metacarbonates; 6 D meta-ultramafics. See also legend of Fig. 7.

cribed to the present-day tropical weathering, and toa possibly long transport distance.

The sandstones of Anyaboni Sandstone are mostlysubarkoses. Their QFL and QmFLt plots indicate thatthe sediments were derived from both transitionalcontinental and craton interior sources (Figs. 4 and 5).Dickinson and Suczek (1979) and Dickinson et al.(1983) defined a transitional continental source asone lying between a craton interior and an upliftedbasement. In a craton interior province, granites andhigh-grade metamorphic rocks crop out in an areaof low topographic relief subjected to deep weather-ing. An uplifted basement province is characterisedby high topographical relief which may be associatedwith wrench tectonism. Quartz-types indicate that theAnyaboni Sandstone was derived from metamorphicsource rocks (Fig. 6). The tourmaline compositions(Figs. 7 and 8) suggest derivation from meta-sedi-ments with a small contribution from granitic rocks.

7.2. Geological history of the Voltaian Basin

The Voltaian Basin is located on the eastern mar-gin of the West African Craton. Sedimentation of

the Voltaian Group commenced about 1000 Ma ago,unconformably on a cratonic basement composed ofBirimian Supergroup and Tarkwaian Group rocks.The QFL, QmFLt, quartz-type data, and the tourma-line data for the Anyaboni and Kwahu sandstonescoupled with the proximity of Birimian Supergroupoutcrops, suggest that the Birimian Supergroup wasa source rock for the Voltaian Group. The QFL andQmFLt plots for the Anyaboni Sandstone suggestthat it was derived from a transitional continental aswell as craton interior. The former terrane may havedeveloped between rift basins in the Birimian Super-group. The presence of one such rift zone, betweenthe Cape Coast basin and the Kibi–Winneba belt(Hastings, 1982, 1983; Wright et al., 1985, quoted inLeube et al., 1990), close to the study area (Fig. 1)possibly shows up in the QFL and QmFLt plots as atransitional continental field.

The Birimian Supergroup was deposited in a se-ries of sedimentary basins and volcanic belts inwhich granitic rocks are widespread (Fig. 1). TheKwahu Sandstone appears from its quartz and tour-maline mineralogy to have been derived from ametasedimentary facies of the Birimian Supergroup

C. Anani / Sedimentary Geology 128 (1999) 83–98 97

that lay immediately to the southwest of the studyarea (Fig. 1). This was probably eroded and exposedlarge plutons to the south, southwest and west. Thesewere gradually unroofed and concurrently erodedwith the metasediments. The high quartz contentof the Voltaian Group sandstones may indicate thatparts of the source region had low relief and un-derwent deep weathering or that transport distanceswere long.

According to Cahen et al. (1984) and Petters(1991), the Lower and Middle Voltaian Formationsare separated by an unconformity. The presence ofthis unconformity is reflected in the differences inthe QFL and QmFLt results for the Kwahu Sand-stone (pre-unconformity) and Anyaboni Sandstone(post-unconformity).

The presence of subarkosic debris in theAnyaboni Sandstone might have been related to in-termittent uplift within the Cape Coast Basin andthe Kibi–Winneba belt (Fig. 1). There seems to bean upward decrease in maximum clast size whichmay imply that the topographical relief in the sourcearea was decreasing with time. The source rock forthe Anyaboni Sandstone Member was most probablymetasediments in the southwestern part of the studyarea (Fig. 1).

To the east, the Buem and Togo formations, lat-eral equivalents of the Middle Voltaian Formation(Grant, 1967), were deposited at greater topograph-ical depths than the Voltaian Basin. Sediment couldnot, therefore, have been supplied to the VoltaianBasin at that time.

This paper has examined the sedimentology ofonly part of the Voltaian Group sequence in a part ofthe Voltaian Basin. Much future research is neededbefore an understanding of the complex sedimento-logical history of the basin is understood.

8. Conclusions

(1) The strata in the southeastern part of theVoltaian Basin contain two sandstone members, theKwahu Sandstone in the Lower Voltaian Formationand the Anyaboni Sandstone in the Middle VoltaianFormation.

(2) The sandstones of the Kwahu SandstoneMember are quartz-arenites derived from a cra-

tonic interior terrane. Their source rocks weremostly granitic rocks with a minor contribution frommetasedimentary rocks.

(3) The sandstones of the Anyaboni SandstoneMember are mostly subarkoses, and were derivedfrom craton interior and transitional terranes.

(4) The sediments of the Kwahu Sandstone Mem-ber were possibly supplied from the Birimian Super-group. Their high granitic content suggests supplyfrom the south and west of the study area, with somecontribution from metasediments to the southwest ofthe study area.

(5) The Anyaboni Sandstone Member sedimentshave a high metasediment-source content that wasprobably derived from Birimian metasediments thatlay southwest of the study area.

(6) The difference in quartz and tourmaline min-eralogy between the Kwahu Sandstone Member andAnyaboni Sandstone Member might be due to tec-tonic movements that are represented by the un-conformity between the Lower and Middle Voltaianformations.

Acknowledgements

This research was for the most part undertaken asa masters thesis of the Faculty of Science, ShinshuUniversity, Japan. It benefitted enormously from thesupervision and encouragement of Associate Pro-fessor Fujio Kumon, and I am deeply indebted tohim. I am also grateful to Dr. K. Hoyanagi, Prof.M. Akiyama and Dr. T. Otsuka who gave useful ad-vice and reviewed the manuscript, to Dr K. Makinoand Dr Y. Miake who advised on the tourmalineanalyses and to the Director and staff of the Geo-logical Survey Department of Ghana for providingthe necessary logistics for field work. This work wassponsored by the Japanese Government–Ministry ofEducation, Sciences and Culture (Monbusho).

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