Sedimentary Geology 368 (2018) 114–131
Contents lists available at ScienceDirect
Sedimentary Geology
j ourna l homepage: www.e lsev ie r .com/ locate /sedgeoGeochemical compositions of Neoproterozoic to Lower Palaeozoic (?)
shales and siltstones in the Volta Basin (Ghana): Constraints on
provenance and tectonic settingChiri G. Amedjoe a,⁎, S.K.Y. Gawu a, B. Ali a, D.K. Aseidu b, P.M. Nude b
a Department of Geological Engineering, College of Engineering, Kwame Nkrumah University of Science and Technology, University Post Office, Kumasi, Ghana
b Department of Earth Sciences, University of Ghana, P.O. Box LG 58, Legon, Ghana⁎ Corresponding author.
E-mail addresses: chiri.amedjoe@gmail.com, gcamedjo
(C.G. Amedjoe).
https://doi.org/10.1016/j.sedgeo.2018.03.004
0037-0738/© 2018 Elsevier B.V. All rights reserved.a b s t r a c ta r t i c l e i n f oArticle history:
Received 10 July 2017
Received in revised form 4 March 2018
Accepted 5 March 2018
Available online 08 March 2018
Editor: Dr. B. JonesMany researchers have investigated the provenance and tectonic setting of theVoltaian sediments using the geo-
chemistry of sandstones in the basin. The shales and siltstones in the basin have not been usedmuch in the prov-
enance studies. In this paper, the geochemistry of shales and siltstones in the Kwahu Group and Oti Group of the
Voltaian Supergroup fromAgogo and environs in the southeastern section of the basin has constrained the prov-
enance and tectonic setting. Trace element ratios La/Sc, Th/Sc and Cr/Th and REEs sensitive to average source
compositions revealed sediments in the shales and siltstones may mainly be from felsic rocks, though contribu-
tions from old recycled sediments and some andesitic rock sediments were identified. The felsic rocks may be
granites and/or granodiorites. Some intermediate rocks of andesitic composition are also identified, while the
recycled sediments were probably derived from the basement metasedimentary rocks. The enrichment of light
REE (LaN/YbN c. 7.47), negative Eu anomalies (Eu/Eu* c. 0.59), andflat heavy REE chondrite-normalized patterns,
denote an upper-continental-crustal granitic source materials for the sediments. Trace-element ternary discrim-
inant diagrams reveal passive margin settings for sediments, though some continental island arc settings sedi-
ments were also depicted. Mixing calculations based on REE concentrations and modeled chondrite-
normalized REE patterns suggest that the Birimian basement complex may be the source of detritus in the
Voltaian Basin. REEs aremore associatedwith shales than siltstones. On this basis chondrite-normalized REE pat-
terns show that shale lithostratigraphic units may be distinguished from siltstone lithostratigraphic units. The
significant variability in shales elemental ratios can therefore be used to distinguish between shales of the Oti
Group from that of the Kwahu Group.
© 2018 Elsevier B.V. All rights reserved.Keywords:
Provenance
Kwahu
Oti
Mpraeso
Anyaboni
Afram and Volta Basin1. Introduction
Geochemical investigations are widely used to determine the rela-
tive abundance of the components of siliciclastic rocks such asmajor de-
trital phases, matrix, heavyminerals andmetamorphic minerals (Roser,
2000). These investigations highlight the provenance of the sediments,
source areaweathering, tectonic setting and paleoclimatic environment
(Roser and Korsch, 1988; von Eyntten et al., 2008; Gabo et al., 2009).
The source(s) of the Voltaian sediments have been investigated by
many researchers (Kalsbeek et al., 2008; Carney et al., 2008b, 2010;
Kalsbeek and Frei, 2010; Couëffé and Vecoli, 2011) using different ap-
proaches including whole-rock geochemistry, regional mapping, detri-
tal zircon dating and petrographic studies. The detrital zircon dating
approach, provided a wide range of ages (1000–2200 Ma.) such that
the determination of sediments provenance varied. While the agee.coe@knust.edu.ghrange 2000–2200 Ma indicates the Birimian Supergroup with inputs
from the West African Craton as the sediments provenance, another
age range (1000–1200 Ma) suggests the amalgamated West African
Craton and Amazonian Craton as sediments' provenance. Considering
recycled materials in the sandstones, Anani et al. (2013) have indicated
possible sediment contributions from the Pan-African orogenic rocks.
The textural and mineralogical alterations of sediments by sedimentary
processes have obscured the source characteristics of the sediments,
making petrographic interpretations difficult (Weltje and von
Eynatten, 2004). Geochemical data of mainly sandstones have been
used to deduce the provenance of Voltaian sediments (Anani, 1999,
2000; Kalsbeek et al., 2008; Kalsbeek and Frei, 2010) with little atten-
tion to the mudstones.
In other basins, geochemistry of shales and siltstones have com-
monly been used and found to be better sediments provenance indica-
tors (Ajayi et al., 2006; Abd El-Rahman et al., 2010; Adel et al., 2011).
However, in the Volta Basin the shales and siltstones have not been
thoroughly investigated for provenance studies. This study used geo-
chemical data, mainly trace elements and rare earth elements
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 115
Fig. 1. The Geology of the Volta Basin showing the distribution of its main stratigraphical units after Affaton (1990) and the location of study area in the southeastern section of the Volta
Basin.
116 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131components of the shales and siltstones in the Kwahu Group and Oti
Group of the Voltaian Supergroup from the southeastern part of
Ghana to identify the source of the sediments. This is to help resolve
controversies of the provenance of the Voltaian sediments.
2. Geological setting
2.1. Voltaian Supergroup
The Voltaian Supergroup in Ghana (Fig. 1) consists of a succession of
Neoproterozoic to Lower Palaeozoic (?) sandstones and mudstones
with intercalations of limestone deposited on the Birimian Supergroup
basement rocks (Kalsbeek et al., 2008).
The Voltaian Supergroup is stratigraphically classified as Kwahu-
Bombouaka Group, Oti Group and Obosum-Tamale Group separated
from each other by unconformities (Affaton, 1990; Deynoux et al.,
2006). The Kwahu Group at the base, is found mainly in the western/
south-western portion of the basin. This is followed by the Oti Group
in the northern sector with minor occurrences in the southwest. At
the top of the sequence is the centrally located Obosum Group. The
Kwahu Group comprises Mpraeso Formation, Abetifi Formation and
Anyaboni Formation in order of younging. TheOti Group has Afram For-
mation and Ejura Formation while the Obosum Group is made up of
Obosum Formation and Tamale Formation (Carney et al., 2010). The for-
mations consist of cyclic repetition of sandstones, siltstones, shales with
minor contents of limestones (Bozhko, 1969; Annan-Yorke, 1971;
Affaton et al., 1980; Affaton, 1990; Carney et al., 2010).
The formations in the Volta Basin are generally horizontalwith occa-
sional shallow dips (1–2°) to the east or south-east. However, the east-
ern margins of the basin, has been deformed by Pan-African orogeny
(~600 Ma) (Kalsbeek et al., 2008) such that steeper dips in the forma-
tions are displayed.Fig. 2. Geological map of the study area in the south western section of the Voltaian Supergro
collected from the shales and siltstones composing the Kwahu and Oti Groups.2.2. Study area
The study area is located in the southeastern portion of the
Volta Basin in Agogo and its environs (Fig. 1) and bounded by latitudes
6° 42′, 57.07″ and 7° 1′ 19.9″ N, and longitudes 1° 16′ 23.41″ and
0° 54′ 42.44″ W (Fig. 2). The study area geology comprise of medium
grained sandstones, alternating shaly and siltstones beds in the
Mpraeso Formation. The Anyaboni Formation which consists of sand-
stones and shales intercalated with minor siltstones unconformably
overlying the Mpraeso Formation. The Afram Formation is made of
mainly shales, however, minor siltstones occur.
In the Mpraeso Formation, the alternating shaly and siltstones beds
are the lower unit and the medium-grained, moderately sorted and
cross-bedded sandstones form the upper unit. Similar textural and
structural observations were made by earlier workers around
Koforidua, Nkawkaw, Mpraeso, Agogo, etc. (Mason, 1963; Saunders,
1970; Carney et al., 2008a, 2008b, 2010; Couëffé and Vecoli, 2011).
Within the Anyaboni Formation, the lower unit consists of multiple
coloured shales intercalated with lenticular siltstones. The upper unit
is predominantly, medium-grained, feldspathic, ferruginous sand-
stones. According to Saunders (1970), occasional lenticular conglomer-
atic intercalations and altered feldspars do occur in the ferruginous
sandstones. The Afram shales are flaggy and micaceous with sporadic
lenticular siltstones beds.
3. Materials and analytical methods
3.1. Materials
A total of 106 samples comprising shales, siltstones and sandstones
were collected from Mpraeso Formation, Anyaboni Formation and
Afram Formation in the Kwahu Group and Oti Group respectively;up unconformably overlying the Birimian Supergroup and showing locations of samples
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 117Fig. 2 shows the sampling points. Forty samples consisting of six shales
and two siltstones fromAframFormation, four shales, five siltstones and
six sandstones fromAnyaboni Formation and seven shales, six siltstones
and four siltstones from Mpraeso Formation were selected for
geochemical analysis at Analytical Laboratory System (ALS) Minerals
Division in Vancouver, Canada. Another fifteen samples, comprising
eleven shales and four siltstones, were selected from the three forma-
tions for XRD analysis at the Physics Department Laboratory, University
of Ghana, Legon in Accra.
3.2. Geochemical analysis
Major elements oxides and loss on Ignition (LOI) were detected by
inductively coupled plasma- atomic emission spectrometry (ICP-AES).
Pulverized samples (0.2 g) were decomposed in lithium metaborate/
lithium tetraborate and nitric acid digestion, then fused at 1000 °C. LOI
was calculated by weight difference after ignition at 1000 °C. The trace
elements including rare earth elements (REE) were detected by ICP-
mass spectrometry after decomposition and fusion of samples just as
for major elements. Corrections for possible isobaric interferences
were monitored and corrected on measured masses.
3.3. XRD analysis
The powdered samples were treated with trihexylamine acetate
to expand the smectites and then analysed using Empyrean powder
X-ray diffractometer (PANanalytical), operated at 45 kV, 40 mA. The
prepared samples were randomly X-rayed, within the range 2° to 120°
2θ, a step size of 0.1° and scan step time of 2 s.
4. Results
4.1. Mineralogy
The XRD patterns (Fig. 3) indicate quartz as the predominant
mineral, with feldspar, sillimanite, illite and/or muscovite occurring asFig. 3. Representative X-ray diffraction patternsminor minerals. Other minor minerals identified but not visible in
Fig. 3 include montmorillonite and hematite. High quartz contents
characterised the siltstones, with feldspar, sillimanite and illite as mi-
nors in some of the siltstones. The shales also have high quartz content
withminor amounts of illites, muscovites and kaolinite; themontmoril-
lonite occurred mainly in the shales. The presence of some iron base
suspected to be hematite and/or chlorite incorporated in the clay min-
erals were observed in the shale. It is possible that the hematite may
be coming from the cementing material in some of the rock samples.
4.2. Geochemistry
The summarized geochemical results for the shales and siltstones in
the Anyaboni Formation, Mpraeso Formation and Afram Formation of
the Kwahu Group and Oti Group, respectively, are presented in
Tables 1, 2 and 3.
4.2.1. Major elements
The SiO2 content in the siltstones (70–89 wt%) is higher than in the
shales (57–69 wt%) for all the formations (Table 1).While the siltstones
plot in SiO2 high concentration regions on the bivariate plot (Fig. 4), the
shales plot in high Al2O3 concentration regions. This plot, which relates
elemental composition tomineralogy, shows clear segregation between
quartz dominant siltstones, represented by high SiO2 content, from the
shales, also composed of significant clay/sericite minerals represented
by Al2O3.
4.2.2. Trace elements
All the samples are characterized by varying trace element concen-
trations as shown in Table 2. With the high field strength elements
(HFSE), Zr recorded high concentrations, Hf and Nb were moderate,
and Ta and U showed low to very low concentrations. In the light field
strength elements (LFSE), Rb, Sr and Ba have high concentrations, but
Cs and Pb were moderate. In the case of the transitional trace elements
(TTE), V and Cr recorded high concentrations, Ni and Coweremoderate,
while Sn was low.of shales and siltstones in the study area.
118 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131
Table 1
Major element concentration (wt%) in studied lithologies of Kwahu and Oti Groups.
Sample Litho SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O MnO TiO2 P2O5 LOI Total
Afram Formation - Oti group
CAFR 14 Shale 58.70 16.85 7.48 2.01 0.25 0.43 3.48 0.03 0.67 0.03 9.81 99.74
CAFR 15 Shale 61.10 16.15 7.26 2.30 0.25 1.06 3.74 0.05 0.69 0.04 7.80 100.44
CAFR 16 Shale 62.20 16.30 7.12 2.99 0.47 1.61 3.75 0.08 0.70 0.13 6.38 101.73
CAFR 17 Shale 63.10 16.15 6.31 2.80 0.48 1.93 3.27 0.11 0.80 0.16 6.15 101.26
CAFR 18 Shale 63.70 16.70 4.21 2.83 0.36 1.63 3.88 0.06 0.69 0.10 6.58 100.74
CAFR 19 Shale 62.10 16.35 6.57 2.59 0.39 1.74 3.15 0.08 0.80 0.15 7.68 101.60
CAFR 22 Siltstone 83.60 7.88 3.49 0.53 0.04 0.07 3.14 0.02 0.39 0.07 2.17 101.40
CAFR 23 Siltstone 75.60 13.15 4.35 0.37 0.01 0.06 2.24 0.01 0.69 0.02 4.94 101.44
Mpraeso Formation - Kwahu group
CAMP 25 Shale 62.90 16.25 7.46 1.24 0.16 0.11 4.68 0.02 0.86 0.07 6.61 100.36
CAMP 27 Shale 57.20 17.60 10.45 1.74 0.18 0.13 5.08 0.06 1.05 0.07 6.59 100.15
CAMP 29 Shale 60.30 17.00 10.05 1.31 0.14 0.09 4.78 0.02 0.87 0.09 6.63 101.28
CAMP 30 Shale 64.60 17.45 5.74 0.81 0.01 0.08 3.40 0.01 0.82 0.09 7.51 100.52
CAMP 33 Shale 58.10 18.30 8.95 1.64 0.16 0.10 5.20 0.03 0.87 0.06 7.20 100.61
CAMP 35 Shale 61.20 17.60 7.90 1.40 0.13 0.11 5.03 0.02 0.84 0.07 6.37 100.67
CAMP 40 Shale 62.60 15.60 9.08 1.45 0.20 0.11 5.17 0.03 0.84 0.03 5.58 100.77
CAMP 24 Siltstone 85.00 6.97 3.94 0.64 0.05 0.23 2.40 0.11 0.26 0.01 2.10 101.71
CAMP 26 Siltstone 79.20 8.40 5.36 0.94 0.05 0.25 2.63 0.03 0.49 0.02 2.75 100.12
CAMP 28 Siltstone 85.60 4.52 6.90 0.46 0.04 0.16 2.00 0.04 0.17 0.04 1.25 101.18
CAMP 32 Siltstone 69.00 17.50 3.31 0.65 0.01 0.07 3.37 0.03 0.92 0.03 6.44 101.33
CAMP 36 Siltstone 73.40 11.90 5.98 0.98 0.10 0.08 4.19 0.02 0.65 0.06 3.43 100.79
CAMP 38 Siltstone 78.30 4.94 9.30 0.50 0.14 0.05 1.41 0.36 0.16 0.01 4.82 99.99
Anyaboni Formation - Kwahu group
CANY 5 Shale 57.50 21.10 7.02 0.90 0.04 0.06 4.58 0.02 1.13 0.06 7.32 99.73
CANY 8 Shale 68.20 17.60 3.66 0.71 0.07 0.09 3.20 0.02 0.92 0.08 6.78 101.33
CANY 10 Shale 64.00 19.95 3.73 0.67 0.02 0.09 3.36 0.01 1.03 0.10 7.91 100.87
CANY 12 Shale 67.80 17.75 2.34 0.45 0.01 0.07 2.91 0.01 0.92 0.02 6.38 98.66
CANY 1 Siltstone 86.70 6.42 2.15 0.46 0.06 0.57 2.41 0.11 0.43 0.02 1.60 100.93
CANY 2 Siltstone 86.90 6.09 2.37 0.43 0.06 0.30 2.04 0.02 0.60 0.01 1.95 100.77
CANY 11 Siltstone 88.30 5.30 3.67 0.15 0.01 0.04 1.53 0.01 0.28 0.01 1.92 101.22
CANY 20 Siltstone 69.40 13.75 3.61 0.98 2.08 3.39 2.78 0.05 0.42 0.11 2.98 99.55
CANY 21 Siltstone 83.20 8.59 3.16 0.18 0.02 0.05 2.82 0.02 0.44 0.04 1.77 100.29
Major elemental ratios in studied lithologies of Kwahu and Oti groups
Sample Litho CIA K2O/Na2O Na2O/K2O SiO2/Al2O3 SiO2/TiO2 K2O/Al2O3 Al2O3/TiO2 Al2O3/K2O Fe2O3/Al2O3 Fe2O3/K2O
Afram Formation - Oti group
CAFR 14 Shale 77.45 8.09 0.12 3.48 87.61 0.21 25.15 4.84 2.15 2.15
CAFR 15 Shale 72.20 3.53 0.28 3.78 88.55 0.23 23.41 4.32 1.94 1.94
CAFR 16 Shale 68.58 2.33 0.43 3.82 88.86 0.23 23.29 4.35 1.90 1.90
CAFR 17 Shale 68.37 1.69 0.59 3.91 78.88 0.20 20.19 4.94 1.93 1.93
CAFR 18 Shale 69.11 2.38 0.42 3.81 92.32 0.23 24.20 4.30 1.09 1.09
CAFR 19 Shale 70.40 1.81 0.55 3.80 77.63 0.19 20.44 5.19 2.09 2.09
CAFR 22 Siltstone 69.02 44.86 0.02 10.61 214.36 0.40 20.21 2.51 1.11 1.11
CAFR 23 Siltstone 83.93 37.33 0.03 5.75 109.57 0.17 19.06 5.87 1.94 1.94
Mpraeso Formation - Kwahu group
CAMP 25 Shale 74.76 42.55 0.02 3.87 73.14 0.29 18.90 3.47 1.59 1.59
CAMP 27 Shale 74.61 39.08 0.03 3.25 54.48 0.29 16.76 3.46 2.06 2.06
CAMP 29 Shale 75.52 53.11 0.02 3.55 69.31 0.28 19.54 3.56 2.10 2.10
CAMP 30 Shale 82.25 42.50 0.02 3.70 78.78 0.19 21.28 5.13 1.69 1.69
CAMP 33 Shale 75.18 52.00 0.02 3.17 66.78 0.28 21.03 3.52 1.72 1.72
CAMP 35 Shale 75.18 45.73 0.02 3.48 72.86 0.29 20.95 3.50 1.57 1.57
CAMP 40 Shale 72.02 47.00 0.02 4.01 74.52 0.33 18.57 3.02 1.76 1.76
CAMP 24 Siltstone 69.47 10.43 0.10 12.20 326.92 0.34 26.81 2.90 1.64 1.64
CAMP 26 Siltstone 71.58 10.52 0.10 9.43 161.63 0.31 17.14 3.19 2.04 2.04
CAMP 28 Siltstone 64.65 12.50 0.08 18.94 503.53 0.44 26.59 2.26 3.45 3.45
CAMP 32 Siltstone 82.35 48.14 0.02 3.94 75.00 0.19 19.02 5.19 0.98 0.98
CAMP 36 Siltstone 71.23 52.38 0.02 6.17 112.92 0.35 18.31 2.84 1.43 1.43
CAMP 38 Siltstone 72.69 28.20 0.04 15.85 489.38 0.29 30.88 3.50 6.60 6.60
Anyaboni Formation - Kwahu group
CANY 5 Shale 81.03 76.33 0.01 2.73 50.88 0.22 18.67 4.61 1.53 1.53
CANY 8 Shale 83.55 35.56 0.03 3.88 74.13 0.18 19.13 5.50 1.14 1.14
CANY 10 Shale 85.16 37.33 0.03 3.21 62.14 0.17 19.37 5.94 1.11 1.11
CANY 12 Shale 84.90 41.57 0.02 3.82 73.70 0.16 19.29 6.10 0.80 0.80
CANY 1 Siltstone 64.17 4.23 0.24 13.50 201.63 0.38 14.93 2.66 0.89 0.89
CANY 2 Siltstone 68.78 6.80 0.15 14.27 144.83 0.33 10.15 2.99 1.16 1.16
CANY 11 Siltstone 76.02 38.25 0.03 16.66 315.36 0.29 18.93 3.46 2.40 2.40
CANY 20 Siltstone 53.23 0.82 1.22 5.05 165.24 0.20 32.74 4.95 1.30 1.30
CANY 21 Siltstone 73.71 56.40 0.02 9.69 189.09 0.33 19.52 3.05 1.12 1.12
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 119
Table 2
Trace element concentrations (ppm) in studied lithologies of Kwahu and Oti Groups.
Sample Litho LFSE HFSE TTE
Rb Sr Ba Cs Pb Zr Nb Hf Ta Th U Y Sc V Cr Co Ni Cu Ga
Afram Formation - Oti group
CAFR 14 Shale 137.00 87.90 363.00 8.04 12.00 157.00 11.80 4.20 0.80 9.62 2.44 39.70 16.00 125.00 80.00 14.00 32.00 29.00 23.70
CAFR 15 Shale 147.50 106.50 547.00 8.78 10.00 150.00 11.70 4.20 0.80 9.51 1.70 42.80 16.00 115.00 80.00 22.00 53.00 21.00 22.40
CAFR 16 Shale 139.00 68.90 289.00 8.62 12.00 157.00 11.50 4.20 0.70 9.48 1.48 22.20 16.00 104.00 70.00 17.00 41.00 15.00 21.10
CAFR 17 Shale 120.50 70.70 423.00 6.72 11.00 197.00 13.10 5.30 0.90 10.65 1.97 33.40 15.00 111.00 80.00 18.00 87.00 36.00 20.10
CAFR 18 Shale 147.50 71.60 279.00 8.46 3.00 166.00 12.60 4.60 0.90 9.90 1.84 22.30 16.00 115.00 80.00 17.00 41.00 29.00 23.70
CAFR 19 Shale 116.00 84.00 528.00 6.16 8.00 197.00 12.70 5.30 0.80 9.94 2.10 38.60 15.00 118.00 80.00 16.00 54.00 36.00 20.80
CAFR 22 Siltstone 111.00 81.60 729.00 2.05 34.00 581.00 9.00 14.40 0.70 12.15 1.71 32.90 4.00 16.00 40.00 4.00 10.00 8.00 10.50
CAFR 23 Siltstone 92.20 56.20 290.00 4.25 12.00 493.00 14.60 11.90 1.00 14.90 3.23 32.70 10.00 71.00 50.00 3.00 9.00 14.00 16.00
Mpraeso Formation - Kwahu group
CAMP 25 Shale 211.00 92.90 373.00 11.75 12.00 381.00 16.70 9.50 1.20 17.15 3.08 41.50 15.00 87.00 60.00 12.00 19.00 19.00 21.50
CAMP 27 Shale 234.00 110.00 424.00 14.25 17.00 398.00 20.60 9.90 1.40 20.10 3.61 47.80 18.00 113.00 80.00 55.00 24.00 54.00 26.10
CAMP 29 Shale 219.00 103.00 405.00 11.70 12.00 326.00 17.10 8.70 1.20 17.20 3.49 39.40 16.00 94.00 70.00 9.00 18.00 13.00 23.60
CAMP 30 Shale 158.50 68.10 368.00 8.65 16.00 255.00 17.50 6.90 1.30 17.95 2.64 37.30 16.00 93.00 60.00 9.00 15.00 31.00 23.70
CAMP 33 Shale 202.00 93.90 539.00 11.10 26.00 457.00 17.20 11.00 1.20 18.10 3.30 34.10 12.00 89.00 70.00 19.00 34.00 17.00 23.00
CAMP 35 Shale 198.50 95.30 609.00 10.75 24.00 645.00 17.30 15.90 1.20 18.65 3.71 41.60 12.00 86.00 60.00 19.00 34.00 21.00 21.10
CAMP 40 Shale 218.00 138.00 476.00 18.70 36.00 365.00 13.80 9.50 0.90 12.65 3.39 32.30 16.00 91.00 70.00 22.00 25.00 13.00 20.50
CAMP 24 Siltstone 72.50 42.50 693.00 1.09 43.00 199.00 5.20 5.20 0.30 4.55 1.16 16.20 6.00 25.00 30.00 14.00 17.00 15.00 8.50
CAMP 26 Siltstone 85.20 47.10 631.00 2.18 12.00 486.00 8.80 11.80 0.70 8.30 3.42 31.20 6.00 33.00 30.00 34.00 34.00 12.00 11.70
CAMP 28 Siltstone 61.90 40.80 588.00 1.53 4.00 145.00 3.60 3.80 0.20 2.86 1.02 10.30 3.00 20.00 40.00 6.00 8.00 8.00 6.80
CAMP 32 Siltstone 144.50 66.30 450.00 7.39 13.00 432.00 18.70 10.70 1.40 18.10 3.70 44.30 15.00 100.00 60.00 5.00 8.00 16.00 22.60
CAMP 36 Siltstone 151.00 91.60 458.00 6.84 22.00 598.00 12.10 14.30 0.80 10.60 2.43 37.10 10.00 58.00 60.00 24.00 43.00 12.00 15.00
CAMP 38 Siltstone 44.40 72.60 4510.00 1.28 8.00 147.00 4.10 4.10 0.30 4.29 0.95 13.20 6.00 19.00 50.00 9.00 12.00 8.00 5.50
Anyaboni Formation - Kwahu group
CANY 5 Shale 226.00 149.50 285.00 16.20 20.00 496.00 23.60 12.10 1.70 23.30 3.71 62.70 18.00 122.00 80.00 8.00 10.00 11.00 28.40
CANY 8 Shale 142.00 83.00 618.00 6.90 14.00 424.00 23.90 11.00 1.60 21.30 4.06 38.50 13.00 97.00 70.00 10.00 19.00 24.00 22.90
CANY 10 Shale 141.50 75.40 573.00 6.84 26.00 349.00 21.30 10.00 1.60 23.30 4.66 43.30 15.00 111.00 70.00 5.00 8.00 15.00 25.20
CANY 12 Shale 119.50 72.30 580.00 5.10 16.00 566.00 20.80 13.70 1.50 21.40 4.14 40.50 15.00 102.00 70.00 3.00 15.00 13.00 22.30
CANY 1 Siltstone 82.00 42.40 629.00 1.57 18.00 495.00 7.80 11.90 0.60 10.20 3.16 28.30 4.00 27.00 30.00 7.00 12.00 17.00 7.60
CANY 2 Siltstone 73.80 33.70 357.00 1.32 10.00 864.00 11.20 20.60 0.80 14.65 3.40 38.20 4.00 26.00 40.00 5.00 12.00 9.00 7.10
CANY 11 Siltstone 57.70 50.00 2730.00 1.93 13.00 344.00 7.00 8.60 0.50 6.30 1.63 19.30 4.00 24.00 30.00 3.00 6.00 11.00 7.40
CANY 20 Siltstone 105.50 388.00 777.00 3.01 13.00 161.00 9.50 4.10 0.60 8.07 1.60 20.70 7.00 50.00 40.00 8.00 18.00 11.00 17.20
CANY 21 Siltstone 88.60 58.90 417.00 1.80 21.00 357.00 10.00 9.10 0.80 12.10 1.81 30.80 4.00 26.00 20.00 5.00 9.00 5.00 9.40
Trace elements and elemental ratios in studied lithologies of Kwahu and Oti groups
Sample Litho Zn Li Sn Th/U Sc/Th Cr/Th Cr/Ni Zr/Th Rb/Sr Zr/Hf Co/Th Th/Sc Zr/10 Zr/Sc
Afram Formation - Oti Group
CAFR 14 Shale 94.00 10.00 3.00 3.94 1.66 8.32 2.50 16.32 1.56 37.38 1.46 0.60 15.70 9.81
CAFR 15 Shale 132.00 30.00 3.00 5.59 1.68 8.41 1.51 15.77 1.38 35.71 2.31 0.59 15.00 9.38
CAFR 16 Shale 113.00 40.00 2.00 6.41 1.69 7.38 1.71 16.56 2.02 37.38 1.79 0.59 15.70 9.81
CAFR 17 Shale 103.00 40.00 3.00 5.41 1.41 7.51 0.92 18.50 1.70 37.17 1.69 0.71 19.70 13.13
CAFR 18 Shale 117.00 40.00 3.00 5.38 1.62 8.08 1.95 16.77 2.06 36.09 1.72 0.62 16.60 10.38
CAFR 19 Shale 110.00 30.00 3.00 4.73 1.51 8.05 1.48 19.82 1.38 37.17 1.61 0.66 19.70 13.13
CAFR 22 Siltstone 30.00 10.00 2.00 7.11 0.33 3.29 4.00 47.82 1.36 40.35 0.33 3.04 58.10 145.25
CAFR 23 Siltstone 27.00 20.00 3.00 4.61 0.67 3.36 5.56 33.09 1.64 41.43 0.20 1.49 49.30 49.30
Mpraeso Formation - Kwahu group
CAMP 25 Shale 73.00 30.00 4.00 5.57 0.87 3.50 3.16 22.22 2.27 40.11 0.70 1.14 38.10 25.40
CAMP 27 Shale 71.00 30.00 5.00 5.57 0.90 3.98 3.33 19.80 2.13 40.20 2.74 1.12 39.80 22.11
CAMP 29 Shale 68.00 30.00 4.00 4.93 0.93 4.07 3.89 18.95 2.13 37.47 0.52 1.08 32.60 20.38
CAMP 30 Shale 78.00 40.00 4.00 6.80 0.89 3.34 4.00 14.21 2.33 36.96 0.50 1.12 25.50 15.94
CAMP 33 Shale 67.00 30.00 3.00 5.48 0.66 3.87 2.06 25.25 2.15 41.55 1.05 1.51 45.70 38.08
CAMP 35 Shale 63.00 30.00 3.00 5.03 0.64 3.22 1.76 34.58 2.08 40.57 1.02 1.55 64.50 53.75
CAMP 40 Shale 88.00 20.00 3.00 3.73 1.26 5.53 2.80 28.85 1.58 38.42 1.74 0.79 36.50 22.81
CAMP 24 Siltstone 83.00 30.00 1.00 3.92 1.32 6.59 1.76 43.74 1.71 38.27 3.08 0.76 19.90 33.17
CAMP 26 Siltstone 81.00 50.00 2.00 2.43 0.72 3.61 0.88 58.55 1.81 41.19 4.10 1.38 48.60 81.00
CAMP 28 Siltstone 38.00 40.00 1.00 2.80 1.05 13.99 5.00 50.70 1.52 38.16 2.10 0.95 14.50 48.33
CAMP 32 Siltstone 41.00 40.00 4.00 4.89 0.83 3.31 7.50 23.87 2.18 40.37 0.28 1.21 43.20 28.80
CAMP 36 Siltstone 67.00 10.00 2.00 4.36 0.94 5.66 1.40 56.42 1.65 41.82 2.26 1.06 59.80 59.80
CAMP 38 Siltstone 370.00 40.00 1.00 4.52 1.40 11.66 4.17 34.27 0.61 35.85 2.10 0.72 14.70 24.50
Anyaboni Formation - Kwahu group
CANY 5 Shale 44.00 10.00 5.00 6.28 0.77 3.43 8.00 21.29 1.51 40.99 0.34 1.29 49.60 27.56
CANY 8 Shale 70.00 70.00 4.00 5.25 0.61 3.29 3.68 19.91 1.71 38.55 0.47 1.64 42.40 32.62
CANY 10 Shale 41.00 50.00 4.00 5.00 0.64 3.00 8.75 14.98 1.88 34.90 0.21 1.55 34.90 23.27
CANY 12 Shale 23.00 50.00 4.00 5.17 0.70 3.27 4.67 26.45 1.65 41.31 0.14 1.43 56.60 37.73
CANY 1 Siltstone 25.00 10.00 2.00 3.23 0.39 2.94 2.50 48.53 1.93 41.60 0.69 2.55 49.50 123.75
CANY 2 Siltstone 21.00 10.00 2.00 4.31 0.27 2.73 3.33 58.98 2.19 41.94 0.34 3.66 86.40 216.00
CANY 11 Siltstone 44.00 30.00 1.00 3.87 0.63 4.76 5.00 54.60 1.15 40.00 0.48 1.58 34.40 86.00
CANY 20 Siltstone 43.00 20.00 2.00 5.04 0.87 4.96 2.22 19.95 0.27 39.27 0.99 1.15 16.10 23.00
CANY 21 Siltstone 17.00 5.00 2.00 6.69 0.33 1.65 2.22 29.50 1.50 39.23 0.41 3.03 35.70 89.25
120 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131
Table 3
Rare earth elements (ppm) and elemental ratios in studied lithologies of Kwahu and Oti Groups.
Sample Litho La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La/Yb Eu/Eu* La/Sc La/Th ∑REE
Afram Formation - Oti group
CAFR 14 Shale 97.80 246.00 31.00 122.50 22.70 4.03 14.40 1.92 10.05 1.71 4.84 0.60 4.12 0.57 23.74 0.68 6.11 10.17 617.94
CAFR 15 Shale 98.40 194.50 24.70 94.20 17.10 3.29 12.35 1.65 9.10 1.62 4.46 0.60 3.87 0.58 25.43 0.69 6.15 10.35 525.22
CAFR 16 Shale 31.80 56.40 7.21 28.70 5.72 1.13 4.40 0.67 4.22 0.88 2.82 0.38 2.68 0.36 11.87 0.69 1.99 3.35 185.57
CAFR 17 Shale 30.70 55.30 7.41 29.50 6.28 1.33 5.51 0.88 5.38 1.08 3.35 0.44 3.09 0.42 9.94 0.69 2.05 2.88 199.07
CAFR 18 Shale 28.20 51.70 6.50 25.40 4.95 1.01 4.05 0.63 4.07 0.83 2.53 0.36 2.61 0.37 10.80 0.6 1.76 2.85 171.51
CAFR 19 Shale 70.70 104.00 15.30 59.10 11.60 2.31 8.94 1.28 7.62 1.43 4.11 0.57 3.68 0.52 19.21 0.69 4.71 7.11 344.76
CAFR 22 Siltstone 69.90 142.50 17.30 67.20 12.65 2.12 8.84 1.20 6.31 1.22 3.98 0.53 3.50 0.59 19.97 0.61 17.48 5.75 374.74
CAFR 23 Siltstone 41.30 83.80 9.22 33.20 5.94 3.91 5.01 0.87 5.76 1.17 3.91 0.56 3.93 0.61 19.97 0.59 4.13 2.77 241.89
Mpraeso Formation - Kwahu group
CAMP 25 Shale 50.10 105.00 12.30 48.10 9.81 1.66 8.14 1.21 8.17 1.63 5.16 0.63 4.98 0.72 10.06 0.57 3.34 2.92 313.66
CAMP 27 Shale 72.00 150.00 16.80 65.30 11.35 1.88 9.24 1.43 8.77 1.79 5.70 0.78 5.55 0.88 12.97 0.56 4.00 3.58 417.02
CAMP 29 Shale 53.00 109.00 13.10 51.30 9.74 1.60 7.48 1.21 7.53 1.49 5.12 0.68 4.98 0.75 10.64 0.57 3.31 3.08 321.81
CAMP 30 Shale 46.70 98.80 11.10 42.00 7.62 1.29 6.23 1.08 6.64 1.37 4.37 0.66 4.26 0.62 10.96 0.57 2.92 2.60 286.19
CAMP 33 Shale 51.90 106.50 11.80 45.40 8.71 1.64 6.68 1.07 6.32 1.29 4.02 0.55 4.20 0.64 12.36 0.66 4.33 2.87 296.77
CAMP 35 Shale 56.30 127.00 14.85 59.30 11.70 2.17 8.94 1.30 7.47 1.58 4.77 0.66 4.80 0.73 11.73 0.65 4.69 3.02 354.66
CAMP 40 Shale 40.70 79.90 8.61 32.10 6.10 1.07 5.08 0.90 5.75 1.16 3.93 0.62 3.91 0.59 10.41 0.59 2.54 3.22 238.75
CAMP 24 Siltstone 20.10 49.10 5.97 25.10 4.92 0.87 3.29 0.54 3.35 0.65 1.96 0.27 2.03 0.27 9.90 0.66 3.35 4.42 140.62
CAMP 26 Siltstone 20.90 49.50 6.14 25.60 5.60 0.94 5.05 0.88 5.59 1.13 3.48 0.55 3.97 0.63 5.26 0.54 3.48 2.52 167.16
CAMP 28 Siltstone 12.90 30.00 3.56 15.00 3.15 0.48 2.19 1.98 0.37 1.11 0.17 1.21 0.18 10.66 0.56 4.30 4.51 87.60
CAMP 32 Siltstone 48.50 97.60 10.50 38.50 6.83 1.14 6.16 1.07 7.20 1.50 4.98 0.71 5.24 0.77 9.26 0.54 3.23 2.68 290.00
CAMP 36 Siltstone 41.90 90.00 10.30 39.10 7.75 1.33 6.32 1.07 6.70 1.42 4.20 0.59 4.31 0.59 9.72 0.58 4.19 3.95 262.68
CAMP 38 Siltstone 10.40 23.80 2.74 11.10 2.29 0.39 2.35 0.44 2.43 0.54 1.42 0.22 1.43 0.22 7.27 0.51 1.73 2.42 78.97
Anyaboni Formation - Kwahu group
CANY 5 Shale 78.20 164.50 18.65 73.80 13.80 2.41 11.00 1.88 11.30 2.34 7.15 0.99 7.42 1.13 10.54 0.6 4.34 3.36 475.27
CANY 8 Shale 50.20 108.00 12.15 46.50 8.98 1.42 7.14 1.15 7.03 1.49 4.64 0.69 4.79 0.69 10.48 0.54 3.86 2.36 306.37
CANY 10 Shale 52.50 115.50 12.75 47.80 8.76 1.56 7.52 1.17 6.90 1.43 4.20 0.63 4.41 0.69 11.90 0.59 3.50 2.25 324.12
CANY 12 Shale 52.10 112.00 12.55 47.90 9.51 1.58 7.79 1.27 7.79 1.60 4.80 0.72 5.13 0.75 10.16 0.56 3.47 2.43 320.99
CANY 1 Siltstone 39.60 81.20 9.67 37.00 6.96 1.16 5.44 0.83 5.20 0.99 3.26 0.47 3.48 0.48 11.38 0.58 9.90 3.88 227.36
CANY 2 Siltstone 38.30 83.10 9.61 37.00 7.11 1.12 5.65 0.95 5.74 1.30 4.22 0.65 4.59 0.71 8.34 0.54 9.58 2.61 241.84
CANY 11 Siltstone 17.30 40.30 4.60 18.60 4.03 0.67 3.01 0.53 3.40 0.70 2.09 0.34 2.13 0.33 8.12 0.59 4.33 2.75 120.99
CANY 20 Siltstone 42.90 75.60 8.70 32.70 5.13 1.13 4.31 0.58 3.54 0.69 2.14 0.30 1.88 0.27 22.82 0.73 6.13 5.32 206.71
CANY 21 Siltstone 47.00 88.70 9.66 36.80 6.72 1.26 6.00 0.96 5.60 1.14 3.45 0.48 3.47 0.50 13.54 0.61 11.75 3.88 245.78
Fig. 4. Bivariate plot of SiO2 against Al2O3 showing separation between shales and siltstones.
Fig. 5. Spider-diagrams of trace element compositions in the shales and siltstones in the Afram Formation, Anyaboni Frmation andMpraeso Formation normalized against UCC values after
Taylor and McLennan (1985).
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 121
122 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131The abundance of the trace elements were normalized to average
upper continental crust (UCC) values, and displayed in spider diagrams
(Fig. 5). These diagrams show that while K and Th abundance are close
to that of the UCC in some rock samples, La, Nb, Ce, aremoderately high,
but Sr, Nb, Ta and P appear to be depleted. The patterns appear to show
enrichment of Cs, Rb, La, Ce and Nd in all the shales, and some siltstone.
However, the Ba results show some level of inconsistency;whereas they
show depletion in the shales, the two siltstone samples fall on opposite
sides of the UCC (Fig. 5). The elements Zr, Hf, Nb, U and Th show varying
concentrations, which may be due to the carbonate content in the
samples.4.2.3. Rare earth elements
The rare earth elements (REE) recorded variable concentrations as
shown in Table 3. The total REE content is higher in the shales
(171.51–617.94 ppm) than in the siltstones (78.97–290.00 ppm), and
the range of variation is also wider in the shales than in the siltstone.
The chondrite-normalized REE patterns of the shales and siltstones
(Fig. 6) appear similar to the distribution patterns of UCC as presented
by Taylor and McLennan (1985). However, the shales appear clearly
separated from the siltstones, with the shales showingmore abundance
REE than the siltstones. These patterns signify that the REEs concentra-
tions are a result of higher clays and zircon content in the shales than in
the siltstones. The patterns showdistinctly enriched LREE (La, Ce, Pr, Nd,
Sm) with variable negative Eu anomalies averaging 0.62 in the shales
and 0. 59 in the siltstones, and almost flat HREEs (Gd, Tb, Dy, Ho, Er,
Tm, Yb). Negative Eu anomalies range between 0.51 and 0.72
(Table 3) in all the shales and siltstones samples. The Afram shales of
the Oti Group have moderate Eu anomalies (0.64–0.68), while the
shales of the Anyaboni Formation and Mpraeso Formation of the
Kwahu Group, showed relatively strong to moderate Eu anomalies
(0.52–0.63).5. Discussions
5.1. Geochemistry
The chemical compositions of clastic rocks are principally controlled
by source area rocks, erosion, sorting, redox environment and diagene-
sis (Johnsson, 1993). However, these compositions could be altered by
the extent ofmetamorphicmineral reactions (Johnsson, 1993).With lit-
tle modification by secondary processes, clastic sediments whole-rock
geochemistry is useful in fingerprinting provenance and depositional
setting of sedimentary rocks.5.1.1. Major elements
The generally higher content of K2O, Fe2O3 and TiO2 in the shales
than in the siltstones when compared with Al2O3, show that K2O,
Fe2O3 and TiO2 are associated with clay-size phases (Madhavaraju and
Lee, 2010). Positive correlations between Al2O3 and K2O, TiO2 and
Fe2O3 indicate that K, Ti, Fe and Al are associated entirely with detrital
phases as Al concentration in sediments is considered a reasonable
good measure of detrital flux (Peinerud, 2000; Nagarajan, et al., 2007).
However, the SiO2 correlating negatively with Al2O3 in all the siltstones
(Mpraeso siltstones r = −0.88 and Anyaboni siltstones r = −0.99)
signifies detrital quartz influence on sediments. It further suggests the
presence of biogenic siliceous components of aluminosilicates that are
not associated with detrital quartz (Robertson, 2007). The elements K
and Rb are likely to have been obtained from phyllosilicates in the
source rocks due to the positive correlation between Al2O3 and K2O.Fig. 6. Chondrite-normalized REE patterns of shales and siltstones in Afram Formation, Anyabo
similar to upper continental crust.5.1.2. Trace elements
The shales and some siltstones are enriched in Cs and Rb, but have
varied Ba concentrations because theymay have been preferentially ex-
changed and adsorbed onto clays in these rocks due to their large ionic
sizes (Nesbitt et al., 1980; Wronkiewicz and Condie, 1987). The high Rb
concentrations found in the shales and siltstones may be due to its
lower mobility during weathering processes than K. The Rb may
therefore have been preferentially retained in weathered illite (Taylor
and Eggleton, 2001; Li et al., 2005). The high abundance of Ba in some
shales and siltstones may be an indication of detrital flux (Temraz,
2005; Aly, 2015). The high Ba content in siltstones may also be an indi-
cation of original syn-sedimentary barite crystals in composite sedi-
ments (Aly, 2015). The strong positive correlation between Al2O3 and
the trace elements Cr, Sc and V in the Anyaboni shales (r=0.79, 0.85,
0.98), siltstones (r = 0.28, 0.93, 0.95), Mpraeso siltstones (r = 0.63,
0.97, 0.99) suggests bonding of the Cr, Sc and V with clay minerals as
noted by Fedo et al. (1996). It is possible that weathering of some sili-
cate minerals released these TTEs that were later concentrated in the
sediments (Fedo et al., 1996; Bauluz et al., 2000).
5.1.3. Rare earth elements
Weathering of silicateminerals, probably the phyllosilicates,mayhave
introduced someREEs into sediments composing the shales and siltstones
(Liaghati, 2004; Sanz-Montero et al., 2009). Similarly, weathering may
have produced K-bearing clay minerals such as illite (Dudek, 2012) that
primarily controlled the abundance and distribution of LFSE in the
sediments (Feng and Kerrich, 1990; McLennan et al., 1993).
Shales are generally known to have higher total REE content than
siltstones even though both silts and clay sediments host the bulk of
REEs (Cullers et al., 1987, 1988). The higher total REE content in the
shales (Table 3) suggests that the clay minerals in the shales, especially
those rich in alumina and Fe3+ are hosting the REEs (Cullers et al., 1987,
1988; Condie, 1991) and are not affected by post-depositional diage-
netic activities and redistribution (Pettijohn, 1975; Condie, 1991). Com-
parably, quartz dilution (SiO2 N70%) may have lowered composite total
REE content in the siltstones (Taylor and McLennan, 1985).
The enriched REE in chondrite-normalized plot (Fig. 6) suggest felsic
sources for sediments in the shales and siltstones (Taylor and
McLennan, 1985; Wronkiewicz and Condie, 1989). Also the negative
Eu anomalies (Fig. 6) and high LREE/HREE values (between 5 and 11,
average 6.7) further suggest felsic rocks for composite sediments
(Cullers and Graf, 1984; Taylor and McLennan, 1985). However, the
lower LREE/HREE values (4.2 and 4.5) with little Eu anomalies at some
locations indicate the presence of some mafic derived sediments in
some of the shales and siltstones (Cullers and Graf, 1983, 1984;
Cullers, 1994). The negative Eu anomalies (0.60 to 0.68) suggest felsic
source as argued by some researchers (Cullers, 1994; Cullers et al.,
1988; and Cullers and Podkovyrov, 2000), who place the range for felsic
sources as 0.40 to 0.94. The depletion of Eu in the shales and siltstones
may be due to the absence of plagioclases (Bhatia, 1985); in fact, Eu
anomaly is used to evaluate plagioclase in sediments (McLennan et al.,
1990). Sediments re-working or intracrustal differentiation of upper
continental crust at shallow depth could also account for the absence
of plagioclase (McLennan, 1989). Perhaps, source rocks fractionations
were limited and therefore sediments/seawater inherited this Eu anom-
aly from their source igneous rocks in the upper crust (Taylor and
McLennan, 1985; McLennan and Taylor, 1991; Awwiller, 1994).
Zircon presence or the co-occurrence of zircon and other heavymin-
erals in the shales and siltstones is shown by the Zr and Th correlations,
(Afram shales r = 0.84; Anyaboni siltstones r = 0.78; Mpraeso
shales r = 0.69). The increase abundance of Zr in shales suggest
that zircon minerals were selectively concentrated by grain sizeni Formation and Mpraeso Formation (after Taylor and McLennan, 1985). The patterns are
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 123
124 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131fractionation of the sediments. Thus the high Zr/Hf values (34.90–
41.94) signify the control of zircon abundance in all the shales and
siltstones. According to Bauluz et al. (2000), felsic rocks are enriched
in HFSE than mafic rocks, implying that the enrichment of HFSE in
the shales and siltstones, denotes mainly felsic source for composite
sediments.
5.2. Provenance and compositional variation
The REE, Th and Sc in sedimentary rocks are considered reliable indi-
cators of provenance and tectonic setting (Taylor and McLennan, 1985;
McLennan, 1989). This is because their distributions are not seriously
affected by secondary processes –diagenesis and metamorphism
(McLennan and Taylor, 1991; Condie, 1993). This means the degree of
sediment reworking and compositional variation could be evaluated
with the Th/Sc against Zr/Sc diagram (McLennan et al., 1993). The bivar-
iate plot of Th/Sc versus Zr/Sc (Fig. 7) shows felsic provenance, and
recycled materials for many of the shales and siltstones (Fedo et al.,
1997; Taylor and McLennan, 1985). The enrichment of Zr and Th
(HFSE) in the sediments is more indicative of felsic provenance than
mafic source areas (Bauluz et al., 2000); however, minor andesitic prop-
erties were revealed in some of the shales. A similar range of Th/Sc
values were recorded in both the shales and siltstones – i.e. Afram
0.59–3.04 and Anyaboni 1.15–3.66 and Mpraeso 0.76–1.55 in the Oti
Group and Kwahu Group respectively. This is an indication that, sorting
may not have had any serious effect on concentrating Th and Sc
(Mongelli et al., 2006) as it did on Zr that resulted in a wide range of
Zr/Sc values (9.38 to 216.00). Therefore, the elements Th and Sc are
not derived from easily depleted minerals during weathering. On the
strength of this, Th and Sc were considered to be potential source finger
printers (Nyakairu and Koeberl, 2001), and were used to infer the felsic
sources of the sediments. The positive linear correlation trend between
Th/Sc and Zr/Sc (R2 = 0.81), indicates chemical differentiation of igne-
ous rocks for the elements (Meinhold et al., 2007), and the recorded
narrow low range of values for Th/Sc (0.59–3.66), is characteristic of
sediments from felsic source materials.
Many heavy minerals in sedimentary rocks are dominated by trace
elements (Zr in zircon, REE in monazite and allanite) (Johnsson and
Basu, 1993), hence the Zr abundance in sediments signifies zircon en-
richment (Garzanti et al., 2011). Though Zr is heavily enriched in zircon,
it is also easily recycled with the zircon in sediments (Johnsson and
Basu, 1993). However, Sc is present in sediments in labile phases
(McLennan, 1993). It implies that, Zr/Sc values are capable of being
used to analyse heavy minerals concentrations in sediments during
sorting. The Zr/Sc ratio is also one of the useful proxies in evaluating
the presence or absence of recycling in sediments (Cole et al., 2009).
The Zr/Sc values recorded in the shales and siltstones, spanned over a
wide range (9.38–216.00) to signify significant zircon enrichment
consistent with reworking during transport (McLennan et al., 1993).
The observed wide range of Zr/Sc values, demonstrates that the entire
sediments did not originate from the source regions but consists of ap-
preciable recycled materials. Comparatively, the Zr/Sc values in the
shales are lower (Afram 9.81–13.13; Anyaboni 23.27–37.73; Mpraeso
15.94–53.75) than the Zr/Sc values in the siltstones (Afram 49.30–
145.25; Anyaboni 86.00–216.00; Mpraeso 24.50–81.00). The above
range of Zr/Sc values demonstrates that heavy minerals were concen-
trated in the sediments mainly by recycling. The Th/Sc versus Zr/Sc
plot further suggests that typical upper continental crust is the source
for sediments rather than lower continental crust. The trend of Rb/Sr
values (0.27 to 2.27) in the shales and siltstones affirm the upper
continental crustal origin of the sediments.
The chondrite-normalized REE diagram (Fig. 6) shows LREE enrich-
ment and an almost flat HREE patterns in the shales and siltstones that
denote felsic sources for the sediments (Wang et al., 2016). The negative
Eu anomalies in this diagram signify source rocks differentiation that is
similar to granite (Rudnick, 1992). The observed LREE enrichment andHREE depletion may be due to low heavy minerals (zircon) concentra-
tions in the sediments (Nyakairu and Koeberl, 2001). It can also be ar-
gued that, the shales and siltstones had had blend of various source
sediments through a number of sedimentary cycles. It is on basis of
enriched LREE and depleted HREEs that detrital (siliciclastic) origin for
the trace elements was inferred instead of seawater fractionation (Ali
et al., 2014). According to Taylor and McLennan (1985), Eu anomalies
better fingerprint source-rock characteristics, such that the range of
negative Eu anomalies 0.51 to 0.66 (Table 3) in the shales and the silt-
stones, affirm the inferred felsic source(s) for the sediments (Cullers
and Graf, 1984). It further suggests that the sediments were eroded
from magmatic arcs (Taylor and McLennan, 1985). The negative Eu
anomalies in the range 0.67 to 0.73 could still be considered as indicat-
ing felsic source(s) as noted by Cullers, 1994, Cullers et al., 1988, and
Cullers and Podkovyrov, 2000.
The REE ratios differentiate felsic source components from that of
mafic and avoid quartz dilution effects (Taylor and McLennan, 1985;
Bhatia and Crook, 1986; Girty et al., 1996). The established La/Sc, Th/
Sc, Cr/Th, Eu/Eu*, La/Th, Th/Co, Th/Cr, Zr/Y, and Nb/Y values which differ
significantly in mafic and felsic source rocks (Taylor and McLennan,
1985; Cullers et al., 1987, 1988; Cullers, 1994, 2000; Cullers and
Podkovyrov, 2000, 2002; Armstrong-Altrin et al., 2004), have been com-
pared with the same elemental ratio values obtained in this study
(Table 4), to provide further information on the sediments felsic prove-
nance in the shales and siltstones.
The Afram shales have strong to moderate Eu anomalies than the
Anyaboni shales and Mpraeso shales (Table 4). The siltstones in
the Anyaboni Formation and Mpraeso Formation have low Th/Sc
ratio values than Afram Formation siltstones. With the exception of
the Afram shales having average Th/Sc value of 0.63 (n= 6), the rest
of lithologies recorded higher average Th/Sc values of 2.26 for the
Afram siltstones (n = 2), 1.48 for the Anyaboni shales (n = 4) and
2.39 for siltstones is, (n=5), 1.19 for the Mpraeso shales (n=7) and
1.01 for Mpraeso siltstones (n = 6) than the value of 0.9 for UCC
(Taylor and McLennan, 1985). These average values suggest relatively
felsic source-rock composition for the sediments. According to
Cullers (1994), clastic rocks with Cr/Th values in the range 2.5 to 17.5
and Eu/Eu* values between 0.48 and 0.78, originate mostly from felsic
sources rather than from mafic sources. In Table 4, Cr/Th and Eu/Eu*
values for the studied shales and siltstones also fall in felsic provenance
range.
In the La/Th vs. Hf discrimination diagram (Fig. 8), for differentiating
various arc components and source rocks by Floyd and Leveridge
(1987), three Afram shales depicted slightly high La/Th values (10.17,
10.35 and 7.11) while the rest of the shales and siltstones have uni-
formly low La/Th values (2.25–5.75). The Hf concentrations in some
shales and siltstoneswere low (3.80–5.30) ppm,while in others, slightly
Hf high concentrations (8.70–20.60) ppm were recorded. Perhaps the
low Hf concentrations may be attributed to low zircon contents in sed-
iments. The Afram shales andMpraeso siltstoneswith La/Th values (b5)
and Hf contents (2–6) ppm are felsic in composition. This is because
Floyd and Leveridge (1987) established that, arcs of felsic compositions
have both low or uniform La/Th values (b5) and Hf concentrations be-
tween 3 and 7 ppm. The obtained La/Th values in the shales and silt-
stones are similar to those in granites and granodiorites (McLennan,
2001). In shales and siltstones with Hf contents above 10 ppm, much
higher than what felsic rocks are known to contain, may be originating
from metasedimentary sources (Taylor and McLennan, 1985) and sug-
gest passive margin tectonic setting. However shales and siltstones
with low La/Th values and increasing Hf concentrations (the Anyaboni
shales and siltstones, Mpraeso shales) indicate sediments derived
from felsic source rocks with some input from recycled old sediment.
Probably the progressive unroofing of arc rocks and/or incorporation
of sedimentary basement rocks increased theHf content through the re-
lease of zircon (Floyd et al., 1989). Some Afram shales and an Anyaboni
siltstone consist of low Hf concentrations and slightly high La/Th values
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 125
Fig. 7. Th/Sc v. Zr/Sc discrimination diagram (after McLennan et al., 1993) illustrating compositional variations and sediment recycling of sediments in the shales and siltstones.to suggest a mixture of felsic and mafic source region (Fig. 8) that most
likely involve oceanic crust obducted onto continental margin (Floyd
et al., 1989). The general depletion of Nb and Ta, Cs enrichment
(Fig. 5) and anomalous REE patterns in especially the shales affirm
mafic source materials input (Floyd et al., 1989).
Constraining felsic or mafic provenance of the sediments, a mixing
model using Th/Sc vs Cr/Th ratios (Condie and Wronkiewicz, 1990)
was generated (Fig. 9). Some siltstones in the Afram Formation and
Anyaboni Formation characterized as average Archean crustal granite
(3.75–10) with Th/Sc values from 2.5 to 4.5 and Cr/Th values b3.5.Table 4
Range of elemental ratios values in studied shales and siltstones compared to similar ratios valu
1988; Cullers and Podkovyrov, 2000).
Elemental ratio Range of values in shales and siltstones in Formations in Kwahu a
Afram Anyaboni
Shale Siltstone Shale Siltst
Eu/Eu* 0.64–0.68 0.57–0.58 0.53–0.58 0.52–
La/Sc 1.76–6.11 4.13–17.48 3.47–4.34 4.33–
Th/Sc 0.59–0.71 1.49–3.04 1.29–1.64 1.15–
Cr/Th 8.05–8.41 3.29–3.36 3.00–3.43 1.65–
La/Th 2.85–10.35 2.77–5.75 2.25–3.36 2.61–
Th/Co 0.56–0.69 3.04–4.97 2.13–7.13 1.01–
Th/Cr 0.12–0.14 0.30–0.30 0.29–0.33 0.20–
Zr/Y 3.50–7.44 15.08–17.49 7.91–13.98 7.78–
Nb/Y 0.27–0.57 0.27–0.45 0.38–0.62 0.28–
(La/Yb)N 6.02–15.14 6.37–12.10 6.39–7.22 4.92–Probably these siltstones received sediments from mainly granitic
rocks (felsic sources). Sediment sources with mixed materials (felsic
and mafic), the Th/Sc values range between 0.5 and 2.0 with corre-
sponding Cr/Th values between 3.0 and 8.0 (Cullers, 1994; Lahtinen,
2000). The values for both Th/Sc and Cr/Th in some of the studied shales
and siltstones are within the above stated ranges, hence characterizing
them as mixed felsic and mafic materials. However, two Mpraeso silt-
stones and some Afram shales recorded Cr/Th values N8 and low Th/Sc
values (b1.5) to suggest the presence of localized strong mafic source
sediments in them. In general, sediments composing the studied shaleses in fractions derived from felsic andmafic rocks (after Cullers, 1994, 2000; Cullers et al.,
nd Oti Range of values in sediments in
felsic
Mpraeso
and mafic rocks
one Shale Siltstone Felsic Mafic
0.72 0.55–0.63 0.51–0.62 0.40–0.94 0.71–0.95
11.75 2.54–4.69 1.73–4.30 2.50–16.30 0.43–0.86
3.66 0.79–1.55 0.72–1.38 0.84–20.50 0.05–0.22
4.96 3.34–5.53 3.31–13.99 4.00–15.00 25.0–500
5.32 2.60–3.58 2.42–4.42 16.30–25.0 0.43–0.86
2.93 0.37–1.99 0.24–0.48 0.67–19.40 0.04–1.40
0.61 0.18–0.31 0.09–0.30 0.13–2.70 0.018–0.046
22.62 8.27–5.50 9.75–16.12 – –
0.46 0.40–0.50 0.28–0.42 – –
13.83 6.10–7.86 3.19–6.00 – –
126 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131
Fig. 8. Source rock La/Th-Hf discrimination diagram to constrain provenance compositions of the clastic sediments in the shales and siltstones.and siltstones were sourced from mainly felsic materials with inputs
from mixed felsic and mafic areas alongside old recycled materials.
The abundance of least fractionated trace elements (REE Th, and Sc)
in sediments are related to that of their provenance (McLennan et al.,
1980). Implying that the REE, Th, and Sc in the sediments composingFig. 9. Th/Sc versus Cr/Th diagram (after Totten et al., 2000) to furththe studied shales and siltstones must be related to their source rocks
by these trace elements. To investigate this relationship, amixing calcu-
lations was performed to model REE data of the studied shales, silt-
stones and selected end members REE data of Birimian basement
complex, obtained from Block et al. (2016) and UCC data from Condieer constrain provenance characteristics of shales and siltstones.
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 127(1993) to determine if the Birimian basement complex contributedma-
terials into the Volta Basin. The mixing calculations followed thematrix
procedure put forward by Kasanzu et al. (2008):
2 E 3u
66 E 
 77 2 32 3 2 36 u 7 0:90 0:87 1:04 x 0:666 La6 77 ¼ 411:00 45:23 6:0054 y5 ¼ 412:0754YbGd 5 2:20 3:62 1:40 z 2:07 shale matrix
Yb
2 E 3u
66 E 
 7 2 32 3 2 36 u 77 0:90 0:87 1:04 x 0:566 La6 77 ¼ 4 11:00 45:23 6:00 54 y5 ¼ 410:9354YbGd 5 2:20 3:62 1:40 z 1:82 siltstone matrix
Yb
where x = TTG (tonalite-trondhjemite-granodiorite), y = granite and
z = basalt. The mixing calculations results (Table 5), modeled with
chondrite-normalized REE patterns (Fig. 10) show reasonable well
matching between sediments composing the shales and siltstones and
sediments derived from Birimian basement complex. Thus it is inferred
that the Birimian basement complex is the source terrain that mainly
supplied sediments into the Volta Basin.Table 5
Results from mixing calculations of Birimian basement complex end members compared with
Elements/elemental Kwahu and Oti groups
ratios
Afram Formation Anyaboni Formation
Ave. Shale Ave. siltstone Ave. shale Ave. siltsto
(n = 6) (n = 2) (n = 4) (n= 5)
ppm N ppm N ppm N ppm N
La 59.6 251.48 55.60 234.60 58.25 245.78 37.02 15
Ce 117.98 192.78 113.15 184.89 125.00 204.25 73.78 12
Nd 59.90 128.27 50.20 107.49 54.00 115.63 32,42 69
Sm 11.39 74.46 9.30 60.75 10.26 67.07 5.99 39
Eu 2.18 37.64 1.59 27.33 1.74 30.04 1.07 18
Gd 8.78 42.70 8.03 39.05 9.54 46.44 5.78 28
Tb 1.17 31.32 1.04 27.67 1.34 36.56 0.77 20
Yb 3.34 19.66 2.95 21.85 4.00 31.99 2.11 18
Lu 0.47 18.50 0.45 23.62 0.58 32,09 0.3 18
Eu/Eu* 0.66 0.56 0.53 0.5
La N/YbN 12.07 10.93 7.73 9.2
La N/SmN 3.48 4.03 3.67 3.9
Gd N/YbN 2.07 1.82 1.47 1.6
Note: TTG, Granite and Basalt values were from Condie (1993) as sourced from Asiedu et
normalization standards were from Sun and McDonough (1989) standard values. GR =
Elements/elemental Mixing results
ratios
Afram Formation Anyaboni F
Shales Siltstones Shales
88%TTG:12%GR:0% 90%TTG:10%GR0% 78%TTG22%
BA BA BA
ppm N ppm N ppm
La 28.58 120.59 28.05 118.73 30.92
Ce 53.21 86.94 51.54 84.21 60.66
Nd 23.99 51.36 23.17 49.62 27.61
Sm 4.99 32.63 4.89 31.97 5.44
Eu 1.26 21.81 1.27 21.93 1.23
Gd 4.58 22.29 4.48 21.81 5.02
Tb 0.68 18.24 0.67 17.79 0.76
Yb 1.73 10.20 1.69 9.92 1.95
Lu 0.25 9.72 0.24 9.34 0.29
Eu/Eu* 0.81 0.83
La N/YbN 11.82 11.83
La N/SmN 3.70 3.70
Gd N/YbN 2.18 2.195.3. Tectonic setting
The trace elements La, Th, Zr and Sc are quantitatively transferred
into clastic sediments during primary weathering and transportation,
due to their relatively low mobility and residence time in ocean water,
therefore are used to fingerprint tectonic settings (Bhatia and Crook,
1986). The low mobility of La, Zr, Sc and Th informed the choice of
Bhatia and Crook (1986) ternary diagrams which is based on Th, Zr,
and Sc to determine the depositional tectonic setting in the study area.
In these diagrams (Figs. 11 and 12), the shales and the siltstones in
the Afram Formation, Anyaboni Formation and Mpraeso Formation
plotted mainly in the Passive Margin, with a few in the Continental
Island Arc fields. It is possible that the shales and siltstones that plotted
in Continental Island Arc field, formed on either ‘well-developed
continental crust or on thin continental margin’ with a provenance of
a “dissected magmatic arc-recycled orogen” (Bhatia, 1983; Bhatia and
Crook, 1986).
It was realized that Th/Sc values greater than three (N3) and La/Sc
values greater than six (La/Sc N 6) in some of the shales and siltstones
are not characteristic of passive-margin setting (Bhatia and Crook,
1986). These values rather suggest controls by certain localized source
factors that enriched sedimentswith Th (Descourvieres et al., 2011), ac-
counting for the plotting of some shales and siltstones outside defined
tectonic settings boundaries (Figs. 11 and 12). Probably, hydraulicaverage shales and siltstones in Kwahu and Oti groups.
Birimian basement complex
Mpraeso Formation Mixing end members
ne Ave. shale Ave. siltstone TTG GR BA (n = 5)
(n = 7) (n= 6) (n = 19)
ppm N ppm N ppm N ppm N ppm N
6.20 52.96 223.45 25.78 108.79 26 70 48 130 11 30
0.56 110.89 181.19 56.67 92.59 45 47 115 120 27 28
.42 49.07 105.08 25.73 55.10 20 28 54 76 15 21
.15 9.29 60.72 5.09 33.27 4.5 19 8.7 38 3.8 16
.41 1.62 27.86 0.86 14.78 1.3 15 1.0 11.0 1.3 15
.14 8.80 42.82 4.23 20.57 4.1 13 8.2 27 4.2 14
.59 1.17 31.32 0.72 19.21 0.6 11 1.3 22 0.7 12
.29 2.10 27.46 2.00 17.83 1.5 6.1 3.5 14 2.7 11
.03 0.33 27.73 0.26 17.45 0.2 6.0 0.6 15 0.5 12
6 0.54 0.57 0.93 0.36 0.99
1 8.11 6.23 11.62 9.19 2.73
7 3.73 3.12 3.60 3.4 1.80
3 1.55 1.24 2.21 1.89 1.27
al., 2017; N represents chondrite normalized values; Eu/Eu* = EuN/√(SmN*GdN); REE
Granite TTG = Tonalite-Trondjemite-Granodiorite and BA = Basalt.
ormation Mpraeso Formation
Siltstones Shales Siltstones
GR0% 88%TTG12%GR0% 70%TTG30%GR0% 87%TTG0%GR13%
BA BA BA
N ppm N ppm N ppm N
130.47 28.63 120.81 32.55 137.34 24.10 101.70
99.12 53.38 87.22 65.84 107.58 42.72 69.81
59.12 24.07 51,54 30.12 64.50 19.37 41,47
35.55 5.00 32.70 5.75 37.58 4.41 28.83
21.26 1.26 21.79 1.21 20.27 1.30 22.41
24.42 4.59 22.34 5.32 25.89 4.11 20.01
20.23 0.68 18.28 0.81 21.61 0.61 16.38
11.46 1.74 10.23 2.10 12.32 1.65 9.72
11.40 0.25 9.76 0.32 12.56 0.24 9.37
0.72 0.81 0.67 0.93
11.39 11.81 11.14 10.47
3.67 3.69 3.65 3.53
2.13 2.18 2.10 2.06
128 C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131
Fig. 10.Results of themixing calculations for theREEs plottedwith average shales and siltstones inAfram Formation, Anyaboni Formation andMpraeso Formation inKwahuGroup andOti
Group.sortingmay have had significant influence on the chemical composition
of the clastic sediments composing the shales and siltstones and there-
fore controlled some trace elements (Th, U, Zr, Hf and Nb) distribution
(Garcia et al., 2004; Armstrong-Altrin, 2009). The clear distinction be-
tween the siltstones enriched with Zr from recycled older sediments
and other shales and siltstones compose of source Zr is shown in
Fig. 7. The high Zr/Sc values (14.21–89.25) in the siltstones, signify
heavy mineral accumulation through sediments recycling and sorting,
resulting in Zr enrichment (McLennan et al., 1993), with far less Th/ScFig. 11. Ternary Th-Sc-Zr/10 discrimination plot after Bhatia and Crook (1986). A) Oceanicvalues (Bhatia and Crook, 1986). The low Th/Sc values (0.59–3.04) sug-
gest the depletion of plagioclase and unstable mineral in the Passive
Margin setting.
Some previous researchers in the Volta Basin have also interpreted
the tectonic settings of the Kwahu-Bombouaka Group as a continental
succession and the Oti Group interpreted as passive margin deposits
(Affaton et al., 1980; Trompette, 1994). Though sediment geochemistry
does not provide age constrains of Continental Island Arc materials, it is
likely that, older Continental Island Arc basement relics from exposedisland arc; B) Active continental margin; C) Continental island arc; D) Passive margin.
C.G. Amedjoe et al. / Sedimentary Geology 368 (2018) 114–131 129
Fig. 12. Ternary Th–Co–Zr/10 discrimination plot (after Bhatia and Crook, 1986). ACMActive continentalmargin, CIA Continental island arc, OIA Oceanic island arc and PMPassivemargin.rocks at the Passive Margin environment may have imparted its signa-
ture on the whole-rock chemistry of the sediments.
6. Conclusion
Comprehensive composition and provenance analysis of the shales
and siltstones in the Kwahu Group and Oti Group of the Voltaian
Supergroup from Agogo and its environs have been undertaken using
trace element geochemistry and mineralogy. The conclusions drawn
concerning the provenance and tectonic setting of the shales and silt-
stones are:
a) Trace element geochemistry in the siltstones were controlled by
quartz dilution resulting in low composition. Relative abundance
of quartz and K-feldspar concentrations in siltstones were due to se-
lective depletion of plagioclase and micas principally, biotite. The
shales are clay dominated and have higher trace element concentra-
tions resulting from incorporated aluminosilicates and sulphide
minerals than the siltstones.
b) The provenance of sediments composing the shales and siltstones
are from mainly felsic areas and recycled materials with minor
mafic inputs as revealed from chondrite- normalized REE patterns
and geochemical ratios (Th/Sc, Zr/Sc, La/Sc, Th/Co and Cr/Th).
Though the chondrite-normalized REE patterns and Eu anomalies
demonstrate that the sediments are Upper Continental Crustal
materials, Th/Sc and Cr/Th values indicate the presence of some
Archean cratonic basement granitic materials in the siltstones.
c) Mixing calculations based on REE concentrations show that the
shales are modeled as: Afram - 88%TTG, 12%GR, 0%BA, Anyaboni -
78%TTG, 22%GR, 0%BA while Mpraeso - 70%TTG, 30%GR, 0%BA, and
the siltstones are modeled as: Afram - 90%TTG, 10%GR, 0%BA,
Anyaboni - 88%TTG, 12%GR, 0%BA and Mpraeso - 87%TTG, 0%GR
and 13%BA.
d) The shales and siltstones were deposited mainly in Passive Margintectonic setting that overlaps into Continental Island Arc setting.
The detritus supplied mainly by the Birimian basement complex
underlying the Voltaian Basin.
Acknowledgement
This research was supported by research funds awarded by the
KNUST Research Fund (KReF) (OGR/33/KReF/3) and the College of Engi-
neering (COE) (VC/OGR/15) in KNUST. The authors would like to ac-
knowledge the encouragement and assistance of Prof. M. Adom-
Asamoah (Provost of COE, KNUST).We also thank reviewers for improv-
ing the quality of the manuscript.
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