Open Journal of Geology, 2015, 5, 562-576
Published Online August 2015 in SciRes. http://www.scirp.org/journal/ojg
How to cite this paper: Ejeh, O.I., Akpoborie, I.A. and Etobro, A.A.I (2015) Heavy Minerals and Geochemical Characteristics
of Sandstones as Indices of Provenance and Source Area Tectonics of the Ogwashi-Asaba Formation, Niger Delta Basin.
Open Journal of Geology, 5, 562-576. http://dx.doi.org/10.4236/ojg.2015.58051
Heavy Minerals and Geochemical
Characteristics of Sandstones as Indices
of Provenance and Source Area Tectonics
of the Ogwashi-Asaba Formation,
Niger Delta Basin
O. Innocent Ejeh, I. Anthony Akpoborie, A. A. Israel Etobro
Department of Geology, Delta State University, Abraka, Nigeria
Received 8 July 2015; accepted 23 August 2015; published 26 August 2015
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
Heavy mineral petrographic and geochemical compositions (major and trace/rare earth elements)
of sandstones obtained from the Oligocene-Miocene Ogw ashi -Asaba Formation, Niger Delta were
studied to determine their provenance, source area weathering conditions and tectonic setting.
The heavy mineral suite (opaque minerals, zircon, tourmaline, and rutile) revealed that the sand-
stones are mineralogically mature and implied rapid disintegration and chemical decomposition
of sediments mostly of recycled orogen. The sandstones were geochemically classified as Fe-sand
and partly quartz arenitic. Chemical Index of Alteration and Chemical Index of Weathering values
of 89.92% and 91.87% respectively suggest that the source region was predominantly felsic and
was subjected to intense chemical weathering probably under tropical palaeoclimatic conditions
with abundant rainfall that enhanced sediment recycling. Major element concentration discrimi-
nant plots also indicated that the sediments were derived from mixed sources (granitic, gneissic
or recycled orogen) under passive margin setting. Chondrite normalized plot of the rare earth
element pattern is marked by light rare earth element enrichment and negative Eu anomalies, in-
terpreted to mean that provenance was mainly continental crustal rocks. Trace elemental ratios
that are provenance diagnostic (La/Sc, Th/Sc, Cr/ T h , La/Co , Th/Co , Th/Cr, Eu /Eu*, and Eu*) all
point to sediments derived from felsic source and upper continental crust. The mixed provenance
of the sandstones can be traced to the southwestern and southeastern Basement Complex (con-
sisting of granites, g neiss es, e tc.) and sediments derived from the adjacent sedimentary basins
(Anambra and Benue Trough).
O. I. Ejeh et al.
Provenance, Source Area Tectonics, Heavy Minerals, Geochemical Characteristics, Sandst ones,
Ogwash i-Asaba Formation, Niger Delta Basin
Provenance and other related studies associated with tectonic settings and palaeo-climatic conditio ns that deter-
mined weathering processes in the source areas of the Akata For mation, the Agbada Formation and Benin For-
mation that constitute the sedimentary fill of the Niger Delta Basin are lacking as empha sis has a lways be en on
the hydrocarbon potential of these for mations. The lignite-bearing Ogwashi-Asaba Formation which is the sur-
face equivalent of the Agbada Formation is no exception as only limited studies have specifically examined its
hydrogeology ; aspects of the lignite geochemistry ; combined petrological, geochemical and sedimento-
logical study ; and a textural and geochemical study for the determination of its d e positional setting .
The pr imary obje ctive of this investi gation is to utilize hea vy mineral and geochemical d ata to determine the
pro vena nce , tectonic setting of source area and palaeo-climatic conditions associated with the deposition of the
Og wa s h i -Asaba Formation. Similar research on provenance of siliciclastic sedimentary rocks has usually been
focused on sandstones and has in the mai n been ba sed on petrographic analysis o f quartz, feldspars, micas and
heavy minera ls. However, ancillary techniques such as cathodoluminescence microscopy  and the ge oche mi-
cal studies of major, trace and rare earth elements and isotopes in siliciclastic s have also been used. The use of
heavy mineral s (the compone nts of silicicla stics whose spe cific gravity is greater tha n that of bromoform (2.8))
is a universally accepted method for determining the provenance of siliciclastics   because they resist rela-
tively the physiochemical alteration associated with source area weathering, erosion and di agenes is  and thus
retain and bear the source rock characteristics from which they were derived.
Indeed, geochemical data from sandstones sequences deposited in various environments provide invaluable
clues on ancient source area weathering conditions, va riati ons i n prove nance co mpositio ns a nd tect onic setti ngs
-. The characteristic compositions of source rocks are also typically well recorded in the sedimentary fills
which furnish val uable informatio n abo ut the type of source ro cks and their tectonic setting - .
2. Geology and Stratigraphic Setting of the Niger Delta Basin
Firs Adediran et al.  described the coastal sedimentary basins (Dahomey, Anambra, Niger Delta and Calabar
Flank) of Nigeria as open-type , d evel op ed a long t he p assi ve c onti nenta l margi n o f the G ulf o f Gui nea a nd filled
with co ntine ntal d epos its. T he dr ifting a part o f the So uth Ame rican c onti nent fr om the Afric an con tinent duri ng
the late Jurassic or early Cretaceous has been closely related to the spatial distribution of these basins . T he
Niger Delta basin occupies the southern end of the Anambra basin that is adjacent to the Benue Tro ugh, formed
as a result of a failed arm of a triple junction such that rifting ceased in the late Cretaceous . The Niger De lta
thus evolved in the late Cretaceous through rift induced tectonics that ended with the opening of the Gulf of
The coastal sedimentary basins of Nigeria have been through three depositional cycles   : first, a mid-
Cretaceous marine incursion that ended by mild folding during Santonian; a second cycle that led to the devel-
opment of the proto-Niger De l ta d uri ng t he Camp a nia n a nd co nc lud ed i n a maj or t ransgression in the Paleocene.
The third sedimentary cycle lasted from Eocene to Recent, leading to the unremitting development of the main
Niger Delta. The major thrust of this paper deals with the third cycle, especially from Oligocene to Miocene.
The spatial and temporal distribution of the Niger Delta facies can be grouped into the outcropping and sub-
surface Tertiary sections Tabl e 1  -. The Akata Formation, Agbada Formation and the youngest Be-
nin Formation make up the subsurface section while the surface and outcropping facies equivalents are respec-
tively, the Imo Formation, Ameki Formation, Ogwashi-Asaba Formation and Benin Formation . The Imo
Formation (Paleocene to early Eocene) is known to exhibit blue-grey shales with sand lenses, marls and fossili-
ferous limestones. The Ameki Formation (Eocene to early Oligocene), sometimes referred to as the Ameki Group
 is characterized by calcareous clays and silts with thin shelly limestone, rich in foraminifera, also mainly
sands with minor silt and clay intercalations. Reyment  suggested the name Ogwashi-Asaba Formation (also
O. I. Ejeh et al.
Table 1. Comparisonof the facies of present-day with the Teriary units of the Niger Delt a ( after Maron  ).
Present Niger Delta Outcrops of tertiary strain Subsurface formation
NEDECO Report, 1954 R eymen t , 1965 S hort and S täuble, 1967
Contine ntal (fluviatile) depo-
sits, mainly sands Benin Format ion Benin Formation
Afam C la y Memb er
Mixed contine ntal brac kish
water and marine deposits,
sands and clays
Ameki Formation Agbada Formation
Marine deposits, mainly clays Imo shales Akata Formation
the age, Oligoce ne -Miocene) to replace the “lignite series” of Parkinson  and is made up of lignite seams,
clay, shales/carbonaceous mudstones and cross-bedded sandstones. The Benin Formation (Oligocene to Recent)
is also marked by cross-bedded, coarse-pebbly continental sands, with cla y lenses.
3. Material and Methods
Eighteen sandstone samples were obtained from active quarry sites located in the Asaba Capital Territory and
Ibusa, Figure 1. T hese representative samples were carefully obtained from different levels of each quarry from
the base to the to p, Figure 2. A subset of twelve (12) sa mples out of the sampled masses was reduced to 10 g of
sediments each by coning and quartering and prepared for heavy mineral separation. The samples were thereaf-
ter washed separately to remove clay-sized fractions and boiled with HCl acid (1:1) for 10 minutes to remove
iro n coati ngs and ceme nting material. They were then washed and dried a second time and thus made read y for
heavy mineral separation (gravity method). Separation of the heavy from light minerals was done using bromo-
form (CHBr3) as the separating liquid. The extracts of bromoform co ntai ni ng the heavy minerals were rinsed
with acetone and thereafter mounted on glass slides. A quantitative estimation [point-counting method of the
opaque and non-opaque fractions (ultra-stable and meta-stable groups)] of the heavy minerals present in the
sandstones was done by studying them under the binocular petrological microscope at the Petrology Laboratory,
Department of Geology, University of Ibadan, Nigeria.
A second subset of six (6) selected samples was air dried, homo ge niz ed by coni n g and q uar tering and reduced
to 10 g. Aliquots of 10 g of each dried sample were powdered in an agate mortar. Further preparation and ana-
lytical procedures of samples are as outlined in Jarvis and Jarvis  and Pearce et al. .
Major element oxides and trace/rare earth elements (Sc, Be, V, Sr, Y, Zr , and Ba) concentrations were deter-
mined by Fusion Inductively Coupled Plasma (FUS-ICP) with a detection limit of 0.001% - 0.1%. Other trace
and rare earth elements were determined by Fusion Mass Spectrometry (FUS-MS) with a dete ction limit of 0.04
- 30 ppm. These two analytical packages gave results that comprised 10 major elements (oxides and loss on ig-
nition), in addition to 18 trace and 14 rare earth elements respectively. These analyses were done at the Activa-
tion Laborato ries Limited, O nt ar io , Canada.
4.1. Heavy Mineral Petrography
The heavy mineral assemblage consists of opaque minerals (e.g. haematite), zircon, rutile, t ourmal ine, garne t ,
sillimanite and apatite, in decreasing order of abundance displayed in Table 2 and Figure 3. Thus the opaques
have the hi ghest prop ortion o f 42% of the total heavies while the no n-opaques, zircon and rutile account for 26%
in equal proportion, followed by tourmaline which is 12%. Garnet, sillimanite and apatite consist of 8%, 7%,
and 5% respectively.
Zircon sho wed prismatic, s ub -hedral grains that have pro minent crystal faces. It’s colourless, e xhibiti ng hig h
relief and b irefringence. Modal composition varies fro m 9.09 % to 17.78% of the total he avies. Elongated, pris-
matic, pleochroic, and sub-rounded grains were indicated by tourmaline with colour variations from brown,
gree ni sh br own to d ar k gre en. Tourmaline modal composition varies from 8.89% to 13.95% of the total heavies.
Rutile is redd ish brown, weakly pleochroic, and with very high relief. Rutile percentage proportion varies from
11.11% to 18.60% of the total heavies. Garnet was identified as colourless to light purple, sub -a ngular a nd hig h
O. I. Ejeh et al.
Figure 1. Sample locations on a geological map of Asaba and surrounding areas (modified
after Akpoborie et al. ).
Figure 2. A representative lithological section of the Ogwashi-Asaba Formation observed at
Ibuse quarry site.
relief. Apatite is usually colourless with moderate relie f. The modal composition of apatite r anges from 3.03% to
7.14% while the modal percentage composition of garnet varies from 4.76% to 11.62% of total heavies.
Estimates of the Z.T.R. mineralogical maturity indices  derived as the ratio of the combined modal com-
position of zircon (Z), tour mal ine (T ) , and r utile (R) to that of the transp arent, non-micaceous and detrital heavy
O. I. Ejeh et al.
Figure 3. Relative proportions of heavy minerals in the sanstones of the study area.
Table 2. Relative proportions of heavy ninerals (in %) in the sanstones of the Ogwashi-Asaba Formation .
Sample code Zircon Rutile Tourmaline Sillimanite Garnet Apatite Opaque mineral s
IBS L4A 13.95 11. 62 13. 95 6.97 6.97 4.65 41.86
IBS_L2D 12.50 12.50 1 0.42 8.33 6.25 4.17 45.83
IBS_L5A 9.30 13.95 11.63 6.97 11.62 6.97 39.53
IBS_L4B 11. 11 11.11 1 3.89 5.56 8.33 5.56 44.44
IBS_L1B 9.09 12.12 9.09 6.06 6.06 3.03 54.54
IBS_L2A 14.63 12.19 12. 19 4.87 9.76 4.88 41.46
IBS_L 3B 9.30 18.60 1 1.63 6.98 6.97 4.65 41.86
IBS_L2C 16.27 13.95 11. 62 6.97 9.30 4.65 37.21
IBS_L4D 14.28 14.28 11. 90 7.14 7.14 4.77 40.47
IBS_L 3E 17.78 11. 11 8.89 6.67 8.89 4.44 42.22
IBS_L3A 14.28 14.28 9.52 7.14 4.76 7.14 42.85
IBS_L1A 15.90 13.63 13. 63 9.09 6.82 4.54 36.36
Averag e 13.20 13.28 11. 53 6.89 7.74 4.95 42.39
minerals and expressed as percent for each sample (Table 3), range from 58% to 68% with an average of
4.2. Major Element Geochemistry
The compositions (wt.% of oxides) of major elements of the representative sandstone samples are presented in
Table 4. The clearly quartz-rich sandstones have SiO2 content that range between 71.42 wt% and 90.84 wt%.
They are also depleted of TiO2, MnO, P2O5, the alkalis (Na2O and K2O) and alkali earth elements (MgO and
CaO). This parallel depletion can be attributed to the weathering conditions in the source area. In contrast the
Fe2O3 and Al 2O3 show high and slight enrichment re spectively. Herro n’s  log -log plot of (SiO2/Al2O3) versus
O. I. Ejeh et al.
Table 3. Percentages of Zir con, Tourmaline, Rutile and Z.T.R. index of sandstones of the Ogwashi-Asaba Formation .
Sample code Zircon Tourmaline Rutile ZTR inde x
IBS L4A 24.0 2 4.0 2 0.0 68
IBS_ L2D 23.0 1 9.2 2 3.1 65
IBS_L5A 15.4 19.2 2 3. 1 58
IBS_ L4B 20 25.0 2 0.0 65
IBS _L1B 20.0 20.0 2 6.7 67
IBS_L2A 25.0 20.8 2 0. 8 67
IBS_L 3B 16.0 2 0.0 3 2. 0 68
IBS_L2C 25.9 18.5 2 2. 2 67
IBS_L4D 24.0 20.0 2 4. 0 68
IBS_L 3E 30.8 1 5.4 1 9. 2 65
IBS_L3A 25.0 16.7 2 5. 0 67
IBS_L1A 25.0 21.4 2 1. 4 68
Averag e 66. 99
Table 4. Major element compositions (wt.%) for sandstones from the Ogwashi-Asaba Formation .
Rock type Ferruginized sandstones Non-ferruginized sands tone s
Sample # IBS_L 2E IBS_L 3E IBS _L2B IBS_L1C IBS_L2A IBS_L3B
SiO2 71.42 76.57 7 8. 51 87. 48 90.84 92.76
TiO2 0.294 0.033 0.041 0.119 0.177 0.098
Al2O3 1.59 1.13 1.46 1.13 1.85 1.38
Fe2O3* 21.42 19. 12 16. 79 9.48 4.84 3.63
MnO 0.025 0.012 0.014 0.015 0.01 0.011
MgO 0.03 0.02 0.02 0.01 0.01 0.02
CaO 0.02 0.02 0.09 0.02 0.02 0.44
Na2O 0.05 0.04 0.05 0.03 0.04 0.05
K2O 0.03 0.04 0.03 0.03 0.03 0.03
P2O5 0.38 0.28 0.25 0.18 0.1 0.03
LOI 3.52 3.02 2.84 1.62 1.32 1.69
Total 98. 78 100.3 100.1 100.1 99.24 100.1
MgO + Fe2O3* 21.45 19. 14 16. 81 9.49 4.85 3.65
Fe2O3*/K2O 714 478 559.7 316 161.3 121
CaO* + Na2O 0.0012 0 .0010 0 .0016 0.0009 0.0010 0 .0016
Al2O3/Ti O2 5.408 34.242 35. 609 9.496 10.452 14.082
K2O/Na2O 0.6 1 .0 0 .6 1.0 0.75 0.6
SiO2/ Al2O3 44.92 67. 76 53. 77 7 7.42 49.10 67. 22
SiO2 1.1886 1 .2743 1 .3065 1.4558 1.5117 1 .5437
Al2O3 0.0156 0 .0111 0 .0143 0.0111 0.0181 0 .0135
CaO* 0 .0004 0 .0004 0 .0008 0.0004 0.0004 0 .0008
Na2O 0.0008 0 .0006 0 .0008 0.0005 0.0006 0 .0008
K2O 0.0003 0 .0004 0 .0003 0.0003 0.0003 0 .0003
CIA 91.228 88.800 8 8.272 90.244 93.299 8 7.663
CIW 92.857 91.735 89. 937 92.500 94.764 8 9.404
Note: Fe2O3* refers to total Fe, C aO* repr esents Ca in silicate miner als only (i.e. , ex cluding calcit e, dolom ite an d apatite) [46 ]. An approxi mate cor-
rection for carbonate cont ent was done by assum ing reasonable Ca/Na ratios in silicate materials .
O. I. Ejeh et al.
(Fe2O3/K2O) fo r the sample s is shown in Figure 4, i n which s andsto nes of the st udy area fall mostl y wit hin the
Fe sand field.
4.3. Trace Element Geochemistry
Concentrations of trace elements and elemental ratios of the sandstones are shown in Table 5. The sandstones
are enriched in Zr, with minor enrichment in Y, Nb, Th, Hf, and U. Ta is depleted. These elements are essential
components of heavy minerals such as zircon. They are less common in mafic rocks, but enriched in felsic rocks.
Ratios of trace ele ments such as La/Sc, Th/Sc, Co/T h, Cr/Th, La/Co, Th/Co, Th/Cr, E u/Eu*, and Eu* in silicic-
lastic rocks such as sandstones have been used to infer the average provenance composition   [ 40 ]-.
4.4. Rare Earth Element Geochemistry
The rare-earth element concentrations of the sandstones are presented in Table 6. Figure 5 indicates their cho n-
drite normalized rare earth element (REE) pattern, marked by light rare earth element (LREE) enrichment and
Figure 4. Log (Fe2O3/K2O) versus lo g (SiO2/Al 2O3) (after Herron, [3 8] ) for sandstones of the Ogwashi-Asaba Formation.
Figure 5. Chondrite normalized REE patterns for sandstones of the Ogwashi-Asaba Formation.
00.5 11.5 22.5
Quar tz ar enit e
Fe Sa nd
Fe Sh al e
Su b-lithic -arenite
O. I. Ejeh et al.
Table 5. Trace element compositions (ppm) and their ratios for sandstone samples obtained from the Ogwashi-Asaba
Rock type Ferruginous sandstones Non-ferruginous sandstones
Sample # IBS_L 2E IBS_L 3E IBS _L2B IBS_L1C IBS_L2A IBS_L3B
Sc 4 3 5 3 2 1
Ni 19 18 19 17 18 17
Ga 3 2 8 2 2 3
Nb 10 5 5 7 7 6
Ba 31 11 12 44 48 19
Ta 0.5 <0.1 < 0 .1 0 .1 0 .2 0 .2
Co 5 2 3 2 1 1
Cu 9 9.5 8.9 9.9 9 .6 9 .8
Sr 12 5 9 16 17 22
V 46 28 37 18 12 20
Zn 40 <30 <30 <30 <30 <30
Th 3.9 1 .3 2.3 2 .2 2 .1 2 .3
U 3.7 2 .8 3.4 1.8 1 .3 0 .9
Cr 40 20 40 20 17 30
Rb 1.8 2 1.8 1 .6 1 .5 1 .6
Zr 200 53 54 119 154 84
Hf 4.4 1.2 1.3 2 .6 3 .4 2 .0
Zr/1 0 20 5.3 5.4 1.19 1.54 8.4
Th/U 1.05 0.46 0.68 1.22 1.62 2.56
Rb/Sr 0.15 0.40 0.20 0.10 0.09 0.07
Cr/Th 10.26 15.38 1 7.39 9.09 8.10 13.04
Cr/ Zr 0.20 0.38 0.74 0.17 0.12 0.36
Th/Sc 0.98 0.43 0.42 0.73 1.05 2.30
Th/ Zr 0.020 0.025 0.043 0.018 0.014 0.027
La/Th 5.26 4.38 2.65 14.32 19. 57 3.70
Co/T h 1.28 1.54 1.30 0.91 0.48 0.43
La/Sc 5.13 1.90 1.22 10.50 20.55 8.50
La/Co 4.10 2.85 2.03 15.75 41. 10 8.50
Th/Co 0.78 0.65 0.77 1.10 2.10 2.30
Th/Cr 0.09 0.07 0.06 0.11 0.11 0.08
Ba/ La 1.51 1.93 1.97 1.40 1.17 2.24
O. I. Ejeh et al.
Table 6. Rare earth element co ncentration s (in ppm) for sandstones obtained from the Ogwashi-Asaba Formation.
Rock type Ferruginous sandstones Non-ferruginous sandstones
Sample code IBS_L 2E IBS_L 3E IBS _L2B IBS_L1C IBS_L2A IBS_L3B
La 20.5 5.7 6.1 31.5 41.1 8 .5
Ce 43. 5 10.3 11.2 63.1 8 2.4 1 5.7
Pr 5.39 1.21 1.28 7.56 9.97 1.67
Nd 21.4 4.5 4.6 29.5 3 6.6 5.9
Sm 4.5 1 .0 1.0 5 .7 6 .4 1 .1
Eu 0.85 0.21 0.20 0.99 1.17 0.21
Gd 2.7 0 .9 0.9 2 .8 2 .9 0 .9
Tb 0.4 0 .2 0.1 0 .4 0 .3 0 .1
Dy 1.8 0.9 0.7 1 .5 1 .3 0 .7
Ho 0.3 0.2 0.1 0 .2 0 .2 0 .1
Er 0.9 0 .5 0.4 0.5 0 .5 0 .4
Tm 0.14 0.08 0.06 0.08 0.07 0.06
Yb 1.0 0.5 0.4 0 .5 0 .5 0 .4
Lu 0.15 0.08 0.06 0.07 0.08 0.06
Y 7.0 4 .0 4.0 5.0 5 .0 3 .0
Eu/Eu* 0.69 0.66 0.63 0.67 0.72 0.63
Eu* 1.23 0.32 0.32 1.48 1.63 0.33
ΣLREE 95.29 22.7 2 4.18 137 .36 176.47 3 2.87
ΣHREE 7.39 3.36 2.72 6.05 5.85 2.72
Σ REE 103 .53 26.28 2 7.10 144.4 183.49 35.8
LRE E/H REE 12.89 6.76 8.89 22.70 30.17 1 2.08
negative Eu anomalie s , point ing to the fact that the pro venance wa s mainly continental c rust al rocks . T he pattern
of REE shows appreciable heavy rare earth element (HREE) depletion (a fairly flat HREE pattern).
5. Discussion and Interpretations
5.1. Source -Area Weathering
The Z.T.R. index calculated for the sandstones is a high (66.99%) indicating that they are mineralogically ma-
tured  . Such level of mineralogical maturity is suggestive of either rapid chemical weathering and erosion in
high relief source area or a reworking and recycling of pre-existin g ol der sand sto nes. The d epletion of the al kali
and a lka l i e ar th e le ment a nd e nr ic h me nt o f Al 2O3 is a consequence of the alteration of igneous and metamorphic
rocks during chemical decomposition (weathering). Thus following from Nesbitt and Young  and Fedo et al.
; and in order to determine the source area weathering history of the Ogwashi-Asab a sand sto nes, the Che m-
ical Index of Alteration (CIA) and Chemical Index of Weathering (CIW) for all samples were calculated and
values are presented in Table 4 and Figure 6. The CIA values range from 87.7% to 93.3% (average 89.92%)
and CIW from 89.4% to 94.7% (average 91.87%) suggestive of the fact that the source region was subjected to
intense chemical weathering likely under tropical palaeo-climatic conditions with abundant rainfall. The CIA
and CI W values a lso signify a dominant felsic source and sediment recycling processes . The concentrations
of trace elements and REE in the sandstones are indicative of possible sediment weathering, rec ycli ng and sort-
ing during transportation and subsequent heavy mineral enrichment. Thus, the high concentrations of Zr and
LREE in the sandstones are indices to heavy mineral enrichment in zircon and monazites respectively. The steep
O. I. Ejeh et al.
Figure 6. Chemical Index of Alteration (CIA) ternary plot of molecular
proportions of Al2O3-(CaO* + Na2O)-K2O for sandstones from the Og-
was hi -Asaba Formation  [4 5] [ 46] . The solid arrow shows the actual
weathering trend for the samples.
chond ri te-normalized REE trend Figure 6, especially the unusually high La concentration is a reflection of
possible monazite enric hment.
5.2. Proven a n ce
The high percentage of opaque minerals (42%) is attributable to haematite resulting from the ferruginous sand
(average 19.11 % Fe2O3). The presence of zircon, tourmaline, rutile and the suite of opaque heavy minerals (e.g.
haematite) is an indication o f sediments derived mostly from reworked sources and partly from felsic or acidic
igneous rock s .
Major element compositions of sandstones have also been used to determine sedimentary provenance by the
applicatio n of discrimi nant functio n analysis  which disti nguishe s betwee n four fields of sed imenta ry prov-
enance, namely: mafic igneous (P1), intermediate igneous (P2), felsic igneous (P3) and quartzose sedimentary or
recycled (P4). The sandstones of the Ogwashi-Asaba Formation plot in the P3 and P4 fields, Figure 7, indicat-
ing mixed source rock (granitic-gneissic or sedimentary) derivation. Furthermore, evidence of passive margin
environment of deposition  is provided in Figure 8 which shows major element tectonic setting discriminant
function diagram for sandstones of the Ogwashi-Asaba Formation. Finally, the major element tectonic setting
discriminant diagram in which Log (K2O/Na2O)] is plotted against SiO2  shows that most of the Ogwa-
shi-Asaba sandstones plot in the passive margin depositional area, Figure 9. P assive marg in pro venance setti ng
consists of recycled sediments that may have been sourced from tectonic uplift of the adjacent sedimentary ba-
Trace elements such as the REE and the high field strength elements (Co, Sc, Hf, Ta, Nb, Ti and Y) are also
very useful for provenance studies because of their relatively low mobility during surface sedimentary processes.
The negat ive Eu a no malie s s ho wn i n Figure 6 and Ta ble 6 sugges t pr ove nance fro m mat eria ls t hat have unde r-
gone intra-crustal geochemical differentiation involving plagioclase or orthoclase feldspar acting as a residual
phase. The Eu/Eu* values obtained averaged 0.67, which falls within characteristic range of Post Archean Se-
When elemental ratios from the sandstones of the Ogwashi-Asaba Formation are compared with those de-
scribed by Condie , Ar mst ro ng-Altrin et al. , Osae et al. [4 4] and those from other areas (Table 7) , the
ratios La/Sc, Th/Sc, Cr/Th, and Eu/Eu* fall within the range of felsic sources and upper continental crust (UCC).
Furthermore, the La/Sc, Th/Sc, La/Co, Th/Co, Th/Cr, and Eu/Eu* ratios correspond with similar fractions ob-
tained from felsic and mafic rocks described by Cullers and Podkovyrov,  and UCC as described by Taylor
O. I. Ejeh et al.
Figure 7. Discrim i na nt f unc t ion di a g r am for provenance of the sandstones
obtained from the Ogwashi-Asaba Formatio n.
Figure 8. Major element tectonic setting discriminant function diagram
for sandstones obtained from the Ogwashi-Asaba Format ion.
Figure 9. Major elemen t t ectonic sett ing d iscri minan t diag ram [Log ( K2O/
Na2O)] versus SiO2 for sandstones of the Ogwashi-Asaba Formation.
Mafic IgneousQuartzose Sedimentary
Discriminant Function I
Discriminant Function II
O. I. Ejeh et al.
Table 7. Range of elemental ratio of sandstones from Ogwashi-Asaba compared to the ratio in average Proterozoic
sandstones, upper continental crust and sandstone derived from felsic rocks and mafic rocks.
ratio Range of Ogwashi-
Range of sediments
from felsic sourcesb Range of sediments
from mafic sourcesb
Average Prot ero zo ic
sandst onesc Upp er continental
crust (1.6 - 0.8 Ga)c
Range of Buem
La/Sc 1 .22 - 20.55 2.50 - 16.3 0 .4 3 - 0.86 4.21 1 .91 1.70 - 12.1
Th/Sc 0.42 - 2.30 0.84 - 20.5 0 .05 - 0.22 1.75 0 .71 0.53 - 1.82
Cr/Th 8 .10 - 17.39 4.00 - 15.0 2 5 .0 - 500 5.71 4 .46 5.74 - 21.1
Eu/Eu* 0.63 - 0.72 0.40 - 0.94 0.71 - 0.95 0.67 0 .5 9 0.60 - 1.09
aThis study; b; c ; d[4 4 ] .
Table 8. Range of elemental ratios of Ogwashi-Asaba sandstone compared with ratios in similar fractions derived from felsic
and mafic rocks [4 3] and upper continental crust  .
Elemental ratio Range of Ogwashi-Asaba
sandstone Range of sediments
from felsic sources Range of sediments from
mafic sources Upper continental
Eu* 0.32 - 1.63 0.40 - 0.94 0.71 - 0.95 0.63
La/Sc 1.22 - 20.55 2.50 - 16.3 0.43 - 0.86 2.21
Th/Sc 0.42 - 2.30 0.84 - 20.5 0.05 - 0.22 0.79
La/Co 2.03 - 41.10 1.80 - 13.8 0.14 - 0.38 1.76
Th/Co 0.65 - 2.30 0.67 - 19.4 0.04 - 1.40 0.63
Th/Cr 0.06 - 0.11 0.067 - 4.0 0.002 - 0.045 0.13
and McLennan  (Table 8).
The RE E result s, Tabl e 6 also show Eu* values that are attributable to felsic sources with some input from a
mafic so urce and a n UCC. This is a lso suppo rted by the Cr/ Zr ratio s  which as s hown in Ta ble 5 are all b e-
low unity and thus indicative of felsi c sources.
5.3. Tectonic Settin g
Roser a nd Korsch  identified three tectonic settings namely, a passive continental margin (PM), active con-
tinental margin (ACM) and oceanic island arc (ARC) on a K2O/Na2O-SiO2 discrimination diagra m, Figure 9 on
whic h sand sto ne sa mple s fro m the Og was hi-Asaba For mation plot mostly in the PM field with some overlap in-
to the ACM field. PM sediments are usually quartz rich sediments sourced from stable continental areas or in-
tra-plate regions and deposited in intra cratonic basins or passive continental margins.
Heavy mineral data from sandstone layers of the Ogwashi-Asaba For mation show that they are mineralo gically
mature and derived from both reworked sedimentary and felsic rock sources. Geochemical data also provide in-
dications of the possible palaeo-climate and ancient weathering conditions of the source area, provenance and
tectonic setting of deposition. The sandstones are SiO2 and Fe2O3 rich. CIA and CIW values indicate that the
source region was subjected to intense tropical weathering in warm tropical climate. Provenance discrimination
diagrams also support the interpretation that the source area was dominated by felsic igneous and recycled se-
diments. Concentrations of major and trace elements suggest a passive margin setting. It may thus be concluded
that t he sand stones o f the Og washi-Asaba Formation were likely derived from the SW and SE Basement Com-
plex rocks (granites, gneisses, etc.) and sedimentary fills of the adjacent basin (Anambra basin and the Benue
We wish to acknowledge the cooperation of the Lander Brothers, operators of the quarry at Ibusa who granted
O. I. Ejeh et al.
us permission to collect samples at the site as well as the undergraduates from the Department of Geology, Delta
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