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A revised stratigraphy of the Bida Basin, Nigeria
Accepted Manuscript A revised stratigraphy of the Bida Basin, Nigeria M.A.O. Rahaman, S.L. Fadiya, S.A. Adekola, S.J. Coker, R.B. Bale, O.A. Olawoki, I.J. Omada, N.G. Obaje, O.T. Akinsanpe, G.A. Ojo, W.G. Akande PII:
S1464-343X(18)30357-1
DOI:
https://doi.org/10.1016/j.jafrearsci.2018.11.016
Reference:
AES 3371
To appear in:
Journal of African Earth Sciences
Received Date: 4 March 2018 Revised Date:
12 November 2018
Accepted Date: 14 November 2018
Please cite this article as: Rahaman, M.A.O., Fadiya, S.L., Adekola, S.A., Coker, S.J., Bale, R.B., Olawoki, O.A., Omada, I.J., Obaje, N.G., Akinsanpe, O.T., Ojo, G.A., Akande, W.G., A revised stratigraphy of the Bida Basin, Nigeria, Journal of African Earth Sciences (2018), doi: https:// doi.org/10.1016/j.jafrearsci.2018.11.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A REVISED STRATIGRAPHY OF THE BIDA BASIN, NIGERIA
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New Geological Map of the Bida Basin.
Section showing the regional stratigraphic succession of the Bida Basin (Line A-B)
ACCEPTED MANUSCRIPT A Revised Stratigraphy of the Bida Basin, Nigeria Rahaman, M. A. O.1, *Fadiya, S. L.1, Adekola, S. A.1, Coker, S. J.2, Bale, R. B.3, Olawoki, O. A.4, Omada, I. J.5, Obaje, N. G.6, Akinsanpe, O. T.1, Ojo, G. A.7and Akande, W. G.8 5
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Department of Geology, Obafemi Awolowo University, Ile- Ife, Nigeria Skangix Development Limited, Nigeria 3 Department of Geology and Mineral Sciences, University of Ilorin, Nigeria 4 Geospectra Nigeria Limited, Lagos. 5 Department of Earth Sciences, Kogi State University, Ayangba, Nigeria 6 Department of Geology, Ibrahim Badamosi Babangida University, Lapai, Niger State. 7 Project Office, Dangote Cement Plc, Ikeja, Lagos, Nigeria 8 Department of Geology, Federal University of Technology, Minna. *Corresponding Author: FADIYA Suyi Lawrence Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria Telephone: +234-80333-20230 e-mail: [email protected]; [email protected]
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ABSTRACT
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The Bida Basin is a NW-SE trending inland basin, stretching from Shegwa (NW) to Dekina (SE). It is approximately 350 km long and varies in width from 75 - 150 km. It is roughly elliptical in ground plan and runs perpendicularly to the western margin of the NE-SW trending Benue Trough Complex.
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Unlike other sedimentary basins in Nigeria, it is characterized by the absence of volcanics, carbonates and rocks of Tertiary age. Dedicated fieldwork carried out over the past four years revealed that the basin is underlain by mainly continental sediments with minor occurrence of
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marginal/shallow marine to freshwater flood-plain deposits. The initial basin fill is characterized
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largely by several cycles of simple, but amalgamated and/or stacked fanglomerates (alluvial fan) systems developed from several single stream breaches onto the proximal setting of the basin. These fanglomerates directly overly the Basement Complex and display a fining upward sequence. This 30
unit is generally accepted as the Lokoja/Bida Sandstone. Our studies revealed that the Lokoja/Bida Sandstone is overlain directly by the oolitic/pisolitic ironstone of the Agbaja/Sakpe Ironstone which is in turn succeeded by a sequence of rocks generally referred to as the Patti/Enagi Formation, which is comprised of predominantly fine to medium-grained, grey and white sandstones, grey clays, carbonaceous silts and shales, and ubiquitous concretionary ironstone bands. This observation is
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opposed to the widely accepted view of previous workers that considered the Agbaja Ironstone as the youngest formation in the basin. This study further revealed that the sequence of rocks used to define the Patti Formation does not exist at the location of Mount Patti in Lokoja, rather, it is found in road cut sections, quarries and boreholes around the Ahoko Village. Following the rules for stratigraphic
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nomenclature, it is hereby proposed that the name ‘Patti Formation’ should be changed to ‘Ahoko Formation.’ A new geological map was made for the basin which clearly demonstrates the
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relationship of the formations.
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KEYWORDS: Bida Basin, Stratigraphy, Patti Formation, Ahoko Formation, Geological Map
INTRODUCTION
The Bida Basin is a NW-SE trending intracratonic basin that extends northwestwards from the confluence of the Niger and Benue Rivers at the Lokoja/Dekina axis to the basement complex that
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separates it from the basal continental early Cretaceous to late Jurassic Illo and Gundumi Formations (Kogbe, 1989) of the Sokoto Basin at Shegwa (NW) in the Kainji Dam area. The basin is roughly 50
elliptical in shape and runs perpendicular to the NE-SW trending Benue Trough Complex. The basin
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is approximately 350 km in its longest (NW-SE) axis and varies from 75 - 150 km in width (Zaborski, 1998). The Bida Basin is unique among the other Nigerian sedimentary basins in that it is
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characterized by an absence of volcanics, carbonate rocks and sediments of Tertiary age.
Tectonic Setting
The tectonic setting of the Bida Basin has been a subject of discussion over the years and till present, a satisfying explanation of the origin of the basin is yet to be accepted by geoscientists. Three main tectonic models have been proposed for the origin: (i) 60
The simple sag model: Adeleye (1976) suggested that the Bida Basin was formed as a simple sag structure in response to Santonian folding in the Benue Trough. Ojo and
ACCEPTED MANUSCRIPT Ajakaiye (1976, 1989), based on gravity studies, on the other hand, concluded that the contacts at the edges of the basin are probably not faulted and considered the basin as an area of local subsidence resulting from the outflow of lower crustal material associated with intermittent igneous activity associated with the ‘Younger Granites’
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magmatism. Whiteman (1982), on the basis of field relationship and the absence of
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fault scarps around the basin proposed that the basin is a post-Santonian shallow cratonic basin.
Rift model: Landsat imageries studied by Kogbe et al. (1981) indicated that the basin
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(ii)
is controlled by NW-SE trending faults in the surrounding basement and thereby proposed a rift origin for the basin. The rift school of thought was further strengthened
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by Ojo (1990) who identified in the area an elongated E-W negative aeromagnetic anomaly flanked to the south by a positive anomaly and concluded that the anomaly was caused by basic rocks at depths of between 4 km – 6 km. This, according to him
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suggests that, a deep-seated rift may exist in the crust under the basin. Agyingi (1993) believed the basin formed as a pre-Santonian rift. Likkason (1993) believed that the
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basin was initiated during Campano-Maastrichtian times and related its formation to a
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mantle plume to be located close to the confluence of the Benue and Niger rivers in the southeastern part of the basin.
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The strike slip model: Braide (1990; 1992) proposed a strike slip origin for the basin
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based on the lateral continuity of the linear northeastern boundary of the basin and the ‘Gboko transform fault’ of Whiteman (1982). Braide (op cit.) suggested that the basin was formed as a pull-apart basin with multiple phases of transtension and basin infilling with little internal deformation.
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While the interpretation of the proposed origin of the basin has been based largely on gravity and aeromagnetic data, there is the need for seismic data over the region to help determine a more plausible explanation.
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Stratigraphy
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The earliest description of the geology of the lithic fill in the Bida Basin was made by Falconer (1911) who labelled it the ‘Lokoja series’ to cover sediments extending from Idah in the southeastern
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end of the basin to Kontagora in the northwestern part. Russ (1930) described the sandstones around Bida and referred to them as Nupe sandstones. More recent stratigraphic and paleoenvironmental
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studies were carried out by Adeleye (1973; 1974), Braide (1990; 1992), Ladipo et al. (1994), Ojo and Akande (2003; 2008; 2009) and Obaje et al. (2011). and The results of more intensive and detailed field work across the basin now reveal additional information about the basin fill successions and history, which improves the understanding of the relationships and controls of sedimentation,
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including sea level cyclicy and tectonics.
Summary of Previous Works
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The Bida Basin has been broadly subdivided into the Northern and Southern Bida Basins (Fig. 1) based on the geographical setting. The northern sector is popularly referred to as the Bida area while
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the southern sector is named the Lokoja area. The stratigraphic succession of each region was established and correlated across the basin and to the 105
adjoining Anambra Basin. Russ (1930), Adeleye (1974), Whiteman (1982), Braide (1992), Idowu and Enu (1992), Akande et al. (2005), Obaje et al. (2013), Obaje et al. (2013) and Ojo and Akande (2009) among others, established the lithostratigraphic succession shown in Table 1. It was generally agreed by these workers that the Lokoja Sandstone overlies the Basement Complex nonconformably in the Southern Bida Basin. Its equivalent in the Northern Bida Basin – the Bida Sandstone, also lies
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unconformably on the basement rocks in that region. The Lokoja and Bida Sandstones range from
ACCEPTED MANUSCRIPT massive, immature (texturally and mineralogically), matrix to clast supported conglomerate, which fines upward to conglomeratic sandstone, to medium-grained sandstone, siltstone and surbordinate claystone. Russ (1930), Adeleye (1974), Whiteman (1982) and Akande et al. (2005) believed the Lokoja and Bida Sandstones are directly overlain by the Patti Formation, which is equivalent to the Enagi Formation in the Northern Basin and in turn overlain by the Agbaja Formation.
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Present Study
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Recent and more detailed field studies by the authors of this paper revealed that the lithostratigraphic sequence of the basin is at variance with those proposed by the earlier studies (Adeleye, 1971; Adeleye and Dessauvagie, 1972; Nwajide et al., 19998; Ojo and Akande, 2003). While some
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formation names remain valid, others are not because the formations were not exposed at the locations after which they were named.
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METHODOLOGY
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A section by section description of major outcrops around the basin during yearly field trips between 2013 – 2016 was made based on field observation. Our focus is on the lithology, texture, sedimentary
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structures, fossil assemblage and other information relevant to the interpretation of each outcrop. A North-South traverse made on the southern part of the basin involved the study of several outcrops
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including those around Lokoja, Agbaja, Koton-karfi, Gegu, Ahoko and Abaji. The North west-South east traverse cutting across the southern and northern part of the basin provide opportunity to study outcrops around Gerinya, Gulu, Kpada, Doko, Bida, Jima, Sakpe, Batati and Enagi. Outcrops around Wushishi and Share at the Southwestern fringe of the basin were also studied. The results from the studied sections were used for assigning the sections into lithostratigraphic units based on the initial description of the character of the units. A new geological map was made for the basin and a revision 135
of the lithostratigraphic succession shown in a cross section across the map. A description of the existing stratigraphy of the basin are made as follows:
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2.1
Lokoja Area
2.1.1
Lokoja Sandstone
Falconer (1911) described the ‘Lokoja series’ from various locations around the basin as a shallow
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water accumulation of sandstones, grits, conglomerates and ironstones. He observed that, the lowermost rocks of the Lokoja series are almost invariably light-coloured sandstones and conglomerates. He described the lowest rock exposed at the base of Mt. Patti at Lokoja as soft, white,
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felspathic sandstones with pebbles of quartz, kaolinized feldspar and pegmatite which occasionally accumulate into conglomeratic bands. According to Falconer (op cit.), the greater part of the hill is
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composed of white sandstone and grits indistinctly bedded and stained regularly, and in places to a considerable depth, with iron oxides. Falconer (1911) described the upper part of the hill as white sandstone and ferruginous sandstones with concretionary knots of earthly limonite and intercalated bands and irregular lenticular patches of oolitic ironstone. The uppermost 46 m (150 ft) are made up of oolitic, pisolitic and earthly ironstones enclosing irregular mass of hard lustrous concretionary
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limonite, while the summit is capped by a pebbly ironstone composed of small rounded and elliptical pebbles of compact ironstone cemented by earthly limonite containing a few grains of quartz.
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According to Jones (1958), Lokoja sandstones are composed of pebbly and clayey grits and sandstones, coarse false-bedded sandstones and a few thin oolitic ironstone. Jones (1958) described the rarely exposed basal conglomerate as consisting of well-rounded quartz pebbles in a matrix of
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white clay.
2.1.2
Patti Formation
The Patti Formation was recognised by Tattam (1943), Jones (1958) and Ladipo et al. (1994) as a 160
distinct formation (approx. 100 m thick) overlying the Lokoja Basal Sandstone. The formation is said to be comprised mainly of yellowish siltstones, shales and black carbonaceous shales with minor interbeds of claystones, sandstones, impure coal seams and concretionary bands of ironstone as can
ACCEPTED MANUSCRIPT be seenon the outcrops between Koton-karfi (N080 08’ 34.0’’, E0060 46’52.3’’) and Abaji (N080 27’ 52.3’’, E0060 56’ 24.6’’). Jones (1958) described the formation as consisting of fine-medium grained, 165
grey and white sandstones, grey clays, carbonaceous silts and shales and oolitic ironstone. Thin coals have been recorded and massive, white, gritty clays are common. Recorded fossils are plant remains
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in the carbonaceous beds. Fossil seeds and other plant fragments are preserved in detail where these rocks have been ferruginized near the surface. Pollen and spores from angiosperms, pteridophytes, gymnosperms and palmae as well as marine indicator dinoflagellates (Dynogymnium acuminatum, Senegalinium bicavatum and Paleocystodinium australinium) were recovered from the formation
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(Jan du Chene et al., 1978; Ojo and Akande, 2003). It is significant to note that the Patti Formation as
rocks/lithologies on Mount Patti.
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2.1.3
Agbaja Ironstone
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described by these authors was not found by Falconer (1911) in his description of the
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Adeleye (1974), Ladipo et al. (1994), Ojo and Akande (2008) present the Agbaja Ironstone as the youngest rock sequence in the Lokoja area, that overlies the Patti Formation and caps the mesas around Lokoja. It is composed of oolitic ironstone to pisolitic, concretional and massive ironstone
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facies. According to Adeleye (1971; 1975), the Agbaja Ironstone is characterised by oolitic and pisolitic textures, often massive, but locally flat bedded and exhibiting cyclic sedimentation patterns.
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The oolites become coarser upwards and become pisolitic toward the top. The formation contains laterite and interbeds of clay and sandstone. Fossil remains within the Agbaja Ironstone include oysters and gastropod shells, as well as plant remains (Adeleye 1971; 1975).
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Bida Area
2.2.1
Bida Sandstone
Much like its lateral equivalent (Lokoja Sandstone), the Bida Sandstone lies unconformably on the crystalline basement complex and consist of a basal conglomerate with a succession of cross-bedded,
ACCEPTED MANUSCRIPT white to grey sandstones intercalated by kaolinitic clays believed to have been derived from nearby 190
deeply weathered basement rocks. The conglomerate contains evidence of mud and debris flows. While the basal beds are massive and flat, the upper parts shows clear cross-stratification. The deposits making up the Bida Sandstone are cyclic in nature and similar to those seen in the
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contiguous Lokoja Sandstone. Two members of the Bida Sandstone were recognised by Adeleye (1971), the Doko and Jima members. The Doko Member is the basal unit and about 183 m thick 195
(Adeleye, 1974). It is coarse grained, arkosic to feldspathic, with sandy siltstones and
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intraformational breccias. The Jima Member consists of cross-bedded quartzose sandstone,siltstone,
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claystone and breccia.
Sakpe Ironstone
Adeleye and Dessauvagie (1972) described the Sakpe Ironstone, which occurs over an extensive area west of Bida and forms a sharp contact with underlying Bida Sandstone. The formation consists of
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oolitic – pisolitic ironstone with oolites at the base, pisolites in the middle and a return to oolites at the top. Two laterally equivalent members – the Wuya Ironstone Member and the Baro Ironstone Member were recognised. The Wuya Ironstone Member consists of both oolitic and pisolitic ironstones, with lesser amounts of locally developed claystone. The Baro Member is oolitic ironstone
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and ferruginous sandstone, which are locally pyritic and concretionary. It is overlain by the Enagi
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Siltstone.
Enagi Siltstone
This Enagi Siltstone as described by Adeleye (1971) is overwhelmingly dominated by siltstones with interbeds of mudstones and fine-grained sandstone. The Enagi siltstone is believed to conformably overlie the Bida Sandstone Formation except in the areas of the occurrence of the Sakpe Ironstone Formation.
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2.2.4
Batati Ironstone
This Batati Ironstone was described from the central Bida Basin by Adeleye (1971; 1973), who recognized two sub-facies. The basal Kutigi Member is finely oolitic (0.2 - 1 mm), whereas the overlying Edozhigi Member is coarsely oolitic (0.2 - 1.5 mm). According to Adeleye (2016), when
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both are present they are separated by an erosional surface. The overlying beds overstep the finegrained subfacies eastwards and southwards. Occasional vertical burrows are observed and the sorting of oolites very variable. It generally appears massive. A correlation between the formations in
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the Southern Bida Basin (Lokoja area) with those from the Northern Bida Basin (Bida area) is
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Revised Stratigraphy
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summarized in Table 1 as modified from Akande et al. (2005)
The purpose of this section is not to redescribe the lithology, sedimentology, sedimentary structures or the palaeontology of these units, but to put the formations in the correct stratigraphic order. To
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achieve this, it is important to do a section by section description of each of the outcrops and make a comparison with the original description of the formation. 230
The Lokoja-Agbaja Road Section.
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2.4.1
Along the Lokoja-Agbaja road section, the basal conglomeratic sandstone of the Lokoja Sandstone is
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exposed in river channels below Agbaja Hill where it rests unconformably on porphyritic granite. The conglomerate is succeeded by coarse-grained sandstone. Both units appear massive, flat bedded 235
and poorly cross-stratified. At higher elevation on the hill, lensoid clay units were encountered within the coarse sandstones. The clay interbeds become thicker upward and culminate in a clay unit that is thick enough to act as a seal (impermeable layer) to prevent further downward percolation of water from the overlying sand. The clay units here are discontinuous lenticular beds. The clay units were interpreted as flood basin, abandoned ponds and lacustrine facies (Adeleye, 1971; 1972; Braide,
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1992). Above the clay layer, the Lokoja Sandstone consists of medium to coarse-grained, friable
ACCEPTED MANUSCRIPT cross-stratified sandstone unit. These observations fit perfectly into the original description by Jones (1958) of the Lokoja sandstones in which he described the formation as being characterized by pebbly and clayey grits and sandstones, coarse false-bedded sandstones and a few thin oolitic ironstones layers. An erosional surface is visible between this white friable sandstone unit and the succeeding Agbaja Ironstone. This implies that the Patti Formation is absent in the Lokoja – Agbaja
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Road section. A reinterpreted litholog of Ojo and Akande (2008) for the Agbaja section is as shown
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in Figure 2.
It is important to note that the Jones (1958) interpretation of the Patti Formation from a borehole was based on the observation of a clayey layer which he referred to as argilaceous. Our present
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interpretation is that the clay units are lensoid in shape and not continuous units. This can be seen in the Lokoja - Agbaja road section, at the Gada Biu quarry near Abaji along the Abuja road and at the Kpada (Pategi) hill section. It is important to mention that the carbonaceous shale and marine fossils
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characteristic of the Patti Formation were not found anywhere in the sections of the Lokoja and Bida Sandstones that we studied nor on Patti Hill which is made up entirely of the Lokoja Sandstone. It therefore follows that the Agbaja Ironstone is capping the Lokoja Sandstone and not the Patti
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Formation. This conclusion agrees with the comment of Jones (1958) reported that Falconer (1911) did not record the presence of carbonaceous shale at the top of the succession in his description of the
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2.4.2
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type section at Mount Patti.
Abaji Roadcut Section
The Abaji roadcut section shows the characteristic rhythmic layering of the Lokoja Sandstone. A close examination of the outcrop reveals a conglomeratic sandstone at the base that is succeeded by a poorly sorted coarse sandstone and medium grained sandstone. Figure 3 is a photograph of the 265
conglomeratic unit within the poorly sorted sandstone. This textural relationship is cyclic and observed repeatedly in the section until it is succeeded by a strongly cross-stratified, coarse to
ACCEPTED MANUSCRIPT medium-grained sandstone unit before giving way to the claystone and kaolinitic clay at the top (Fig. 4). This is a typical Lokoja Sandstone sequence and not Patti Formation as interpreted by some workers (Jones, 1958). 270
Gada-Biyu Quarry, Abuja Road.
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2.4.3
In the Gada-Biyu quarry, large lenticular clay interbeds (Fig.5) are quarried for foundry purposes. The associated coarse, poorly sorted sandstones are typical of the Lokoja Sandstone. The materials
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were believed to be derived from the nearby crystalline basement rocks judging from the textural characteristic and the similarity in mineralogical composition of the sediments and the basement
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rocks.
BIDA SANDSTONE
Adeleye (1971) recognized two members for the Bida Sandstone: Doko and Jima. Both are mappable on Jima ridge.
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Jima Sandstone Section
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The Jima Member mapped at Jima ridge consists of conglomeratic, arkosic sandstone at the base. Clasts in the conglomeratic sandstone are composed of angular to subrounded quartz and feldspar, and range between 0.8-10 cm. The arkosic sandstone is succeeded by a 2 m thick conglomerate
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containing weathered granitic (porphyritic) rock fragments derived from nearby basement complex. Bedding structures become apparent in the middle section where the sandstone is light grey, arkosic and displays planar cross beds. From the middle section upward, a rhythmic repetition of conglomeratic, matrix-supported, pebbly, arkosic sandstone filled with weathered rock fragments at 290
the base is observed. This is succeeded by planar, cross bedded, medium-grained, feldspathic sandstone overlain by siltstone and claystone as shown in Figure 6. The cyclic conglomerate,
ACCEPTED MANUSCRIPT sandstone, siltstone and claystone interval with repeating fining upward cyclic layering is overlain by the oolitic - pisolitic ironstone of the Sakpe Ironstone.
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It is worthy of note that, a striking similarity exist between the Agbaja Hill and the Jima Ridge
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sections. The basal massively bedded unit seen at the Agbaja Hill section, which was succeeded by the cross bedded sandstone and the siltstone, exhibits the same patterns observed in the Jima section. Strongly cross bedded medium-grained sand units were also observed below the Sakpe Ironstone cap.
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Most importantly, the succession of pisolitic to oolitic and back to pisolitic textures (Fig. 7) of the Sakpe Ironstone on the Jima Hill is the same pattern observed in the Agbaja Hill section, a similarity
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that is interpreted to establish equivalency between the sections.
Kpada Section
At Kpada, a location 40 km west of Pategi, Kwara State, the Bida Sandstone is exposed. The section is about 150 m thick and consists of clast-supported, conglomeratic sandstone at the base and
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overlain by massive, arkosic sandstone with gravel-boulder sized fragments of weathered basement rocks. This unit fines upward into siltstone before the beginning of the next depositional cycle (Fig.
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8). This series of stacked fining-upward cycles is characteristic of the Lokoja Sandstone. The discontinuous, lenticular clay beds are conspicuous in the section at Kpada. Moreover, the friable, cross-bedded sandstones are clearly evident below the capping oolitic ironstone of the Sakpe
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Ironstone. This similarity is considered strong evidence to support the contention that the same sequence described earlier for Bida Sandstone exposed at Jima is equivalent to the Lokoja Sandstone exposed variously around Lokoja, where it is capped by the Agbaja Ironstone.
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2.4.7
Patti Formation: Kotonkarfi - Ahoko – Acheni and Gegu – Gerinya Sections
The evidence presented supports that the ‘Patti Formation’ is likely absent in the Patti Hill and the Lokoja – Agbaja and Abaji sections. However, continuous parallel beds characteristic of marine
ACCEPTED MANUSCRIPT deposition were noticed from Koton-karfi (N80 6’ 25.4’’ and E60 47’ 32.2’’) northwards to Ahoko (N80 18’ 3.8’’ and E60 52’ 31.2’’) as well as those exposed along the Gegu – Gerinya Road. These 320
deposits match the description of the Patti Formation as given by Jones (1958) in which he described the formation as consisting of fine-medium grained, grey and white sandstones, grey clays,
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carbonaceous silts and shales and oolitic ironstone. Thin coals have been recorded and massive, white, gritty clays are common. Recorded fossils are plant remains in the carbonaceous beds. Fossil seeds and other plant fragments are preserved in detail where these rocks have been ferruginized near the surface. Pollen and spores from angiosperms, pteridophytes, gymnosperms and palmae as well as
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marine indicator dinoflagellates (Dynogymnium acuminatum, Senegalinium bicavatum and
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Paleocystodinium australinium) were recovered from the formation (Jan du Chene et al., 1978; Ojo and Akande, 2003). These as well as some agglutinated foraminifera species were recovered from the samples collected at the Ahoko section which typified the description by Jones (1958). The Patti 330
Formation is clearly exposed along a road cut some hundred meters east of Gerinya village (N80 14’
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46’’ and E60 46’ 6.9’’).
Change of name for the Patti Formation
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Based on the presented evidence, it is proposed that the Patti Formation should be changed to ‘Ahoko Formation.’ The principal reason for the name change is that a well exposed and complete
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section of the formation, which typifies the original description of the Patti Formation is located at Ahoko village. This proposition is further supported by the absence of the Patti Formation on Patti Hill. A typical litholog of the proposed Ahoko Formation as seen at Ahoko village and the Gegu – Gerinya Road is shown in Figures 9 and 10, respectively. The marine nature of the sediments is 340
evident
from
the
recovery
of
shallow
marine
benthic
foraminifera
(Ammobaculites,
Haplophragmoides and Ammodiscus), echinoids, brachiopods and pelecypod shells. The carbonaceous units within the Ahoko Formation are interpreted as swamp or salt marsh deposits. Given that this sequence of rocks does not occur on Mount Patti, it is suggested that the formation be
ACCEPTED MANUSCRIPT renamed ‘Ahoko Formation’ after the village where a complete section is exposed. Discontinuous 345
concretionary ironstone interbeds/bands characteristic of the Patti/Ahoko Formation and its equivalent in the Anambra Basin (Mamu Formation) are clearly visible in this outcrop (Fig. 11). The unique characteristic of the Ahoko Formation observed at the Ahoko village, but not seen in the
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2.4.8
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Agbaja section, include the Skolithus/Ophiomorpha borings (Fig. 12) and brachiopod shells (Fig. 13).
The Enagi Siltstone
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Good exposures of the Enagi Siltstone are visible at the Patti Shabakolo section. There is strong evidence that the section at Shabakolo is equivalent to the Ahoko (old Patti) Formation in the Lokoja
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area as the characteristic concretionary iron bands are clearly evident. At Batati, the Enagi Siltstone occurs as a white silty unit with abundant clay balls. 355
2.4.9
Batati Ironstone?
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At Batati, the Enagi Siltstone is capped by a lateritic layer which has been designated the Batati Ironstone. However, near the top of the siltstone is a ferruginised clay layer with a high iron content deduced from its heavy weight and jagged edges from breakage. Obviously, this unit cannot be the one referred to as the Batati Ironstone as there are no oolitic structures and it lies within the Enagi
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Siltstone. Without a better exposed location, the Batati Ironstone type or reference section cannot be
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verified at any location examined in the course of our fieldwork.
A redrawn map of the Bida Basin showing the relationship between the different formations is shown 365
in Figure 14. The proposed stratigraphic relationships for the different lithostratigraphic units of the Bida Basin is presented in Figure 15. In the northwestern part of the basin, the Enagi Siltstone can be seen lying directly on top of the Bida Sandstone. In the middle part of the basin, the Bida Sandstone is overlain by the Sakpe Ironstone. In thesoutheastern part of the basin, the Agbaja Ironstone can be
ACCEPTED MANUSCRIPT seen overlying the Lokoja Sandstone around Lokoja and Agbaja. The Ahoko Formation lies directly 370
on top of the Agbaja Ironstone at a location in Koton-karfi. Lokoja Sandstone at Gerinya and Ahoko.
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CONCLUSION:
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The widely adopted stratigraphic succession placing the Ahoko (formerly Patti) Formation on top of the Lokoja Sandstone has been revised as the Agbaja Ironstone and its northern equivalent (Sakpe 375
Ironstone) can be seen to lie on top of the Lokoja sandstone and the Bida sandstone respectively. The
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Sakpe Irostone in the Northern Bida Basin and the Agbaja Ironstone in the Southern Bida Basin are lateral equivalents just as the Enagi Siltstone and the Ahoko Formation in the northern and southern
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parts of the basin are lateral equivalents. Undoubtedly, the Bida Sandstone in the northern part of the basin and the Lokoja sandstone in the southern parts are lateral equivalents. Since the old ‘Patti’ 380
Formation type section cannot be seen on the Patti Hill, it is appropriate that the name of the carbonaceous shale unit be changed to reflect a type or reference locality where the unit is observed
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as originally described. The old Patti Formation is therefore renamed Ahoko Formation after Ahoko village, where outcrops that match the original description was found.
Acknowledgements
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The authors are grateful to the Department of Geology, Obafemi Awolowo University Professional
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Masters Program on whose platform the field trip exercises that led to this work was carried out. This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors. 390
References: Adeleye, D.R., 1971. Stratigraphy and sedimentation of the Upper Cretaceous strata around Bida, Nigeria. Unpubl.Ph.D. Thesis, University of Ibadan, 297 pp.
ACCEPTED MANUSCRIPT Adeleye, D.R., 1973. Origin of ironstones: An example from the middle Niger Valley. Journal of 395
Sedimentary Petrology, 43, p. 709-727 Adeleye, D.R., 1974. Sedimentology of the fluvial Bida Sandstone (Cretaceous), Nigeria. Sedimentary Geology, 12, p. 1-24.
Petroleum Geologists Bulletin, 59, 2302-2313. 400
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Adeleye, D.R., 1975. Nigeria Cretaceous Stratigraphy and Paleogeography. American Association of
Adeleye, D.R., 1976. The Geology of the Middle Niger Basin.In: Kogbe, C. A. (ed.), Geology of
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Nigeria. Elizabethan Publishing Company, Lagos, pp. 283-287.
Adeleye, D.R., 2016. Bida Basin and the Economic Backbone of Nigeria; 3rd Inaugural Lecture,
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Ajayi Crowther University, Oyo. 44pp.
Adeleye, D. R. and Dessauvagie, T.F.J., 1972. Stratigraphy of the Niger Embayment near Bida, 405
Nigeria. In: Desssauvagie, T.F.J., Whiteman, A.J. (eds.), African Geology. University of Ibadan Press, pp. 181-186.
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Agyingi, C.M., 1993. Palynological evidence for a late Cretaceous age for the Patti Formation, eastern Bida Basin, Nigeria. Journal of African Earth Sciences. 17 (4):513-523. Akande, S.O., Ojo, O.J., Erdtmann, B., Hetenyi, M., 2005. Paleoenvironments, Organic Petrology and Rock-eval Studies on Source Rock Facies of the Lower Maastrichtian Patti Formation,
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Southern Bida Basin, Nigeria. Journal of African Earth Sciences. 41, 394-406.
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Braide, S.P., 1990. Petroleum geology of the Southern Bida Basin, Nigeria. American Association of Petroleum Geologists Bulletin, 74 (5): 717.
Braide, S.P., 1992. Syntectonic fluvial sedimentation in the central Bida Basin. Journal of Mining 415
and Geology, 28: 55-64.
Falconer, J. D., 1911. The Geology and Geography of Nothern Nigeria. London: Macmillan (101pp.) Idowu, J.O., Enu, E.I., 1992. Petroleum Geochemistry of some Late Cretaceous Shales from the Lokoja Sandstone of the Middle Niger Basin, Nigeria. Journal of African Earth Sciences, 14, pp. 433-455.
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Jan du Chene, J., Adegoke, O. S., Adediran, S. A. and Petters, S. W., 1978. Palynology and Foraminifera of the Lokoja Sandstone (Maastrichtian), Bida Basin, Nigeria. Revista Espanola de Micropalontologia, 10 (3), 379-393.
Geological Survey of Nigeria(1955), 20-43. 425
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Jones, H.A., 1958. The oolitic ironstones of Agbaja Plateau, kabba Province. Record of the
Kogbe, C.A., 1989. Cretaceous and Tertiary of the Iullemmeden Basin in Nigeria. In: Kogbe, C. A. (ed.), Geology of Nigeria, 2nd edition. Pg 377-421.
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Kogbe, C.A., Ajakaiye, D.E., Matheis, G., 1981: Confirmation of a rift structure along the mid-Niger valley, Nigeria. Journal of African Earth Sciences, 1, 127-131
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Ladipo, K.O., Akande, S.O., Mucke, A., 1994. Genesis of Ironstones from the Middle Niger Sedimentary Basin, Evidence from Ore Microscopic and Geochemical Studies. Journal of Mining and Geology, Vol. 20, pp. 161-168.
Likkason, O.K., 1993. Application of trend surface analysis to gravity data over the Middle Niger
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Basin, Nigeria. Journal of Mining and Geology, 29 (2), 11-19. Nwajide, C. S., Oni, A. O. and Oluyide, P. O., 1998. The Geology of Ilorin Area: Sedimentary Rocks 435
of Bida Basin. Geological Survey of Nigeria Bulletin, 42: 60-65.
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Obaje, N.G., Musa, M.K., Odoma, A.N. and Hamza, H., 2011. The Bida Basin in north-centreal Nigeria: sedimentology and petroleum geology. Journal of Petroleum and Gas Exploration
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Research. Vol. 1 (1 ), pp 001-013.
Obaje, N.G., Balogun, D.O., Idris-Nda, A., Goro, I.A., Ibrahim, S.I., Musa, M.K., Dantata, S.H, 440
Yusuf, I, Mamud-Dadi, N., Kolo, I.A., 2013. Preliminary Integrated Hydrocarbon
Prospectivity Evaluation of the Bida Basin in North Central Nigeria. Petroleum Technology Development Journal: An International Journal; July 2013 v.3 No.2, p. 36 –
65. Ojo, S. B., 1990. Origin of a major aeromagnetic anomaly in the Middle Niger Basin, Nigeria. 445
Tectonophysics, 185: 153-162.
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Ojo, O.J., Akande, S.O., 2003. Facies Relationship and Depositional Environment of the Upper Cretaceous Lokoja Formation in the Bida Basin, Nigeria. Journal of mining and Geology, 39: 39-48. Ojo, O.J., Akande, S.O., 2008. Microfloral assemblage, age and paleoenvironment of the Upper
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Cretaceous Patti Formation, southeastern Bida Basin, Nigeria: Journal of Mining and Geology, 44: 71-78.
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Ojo, O.J., Akande, S.O., 2009. Sedimentology and depositional environments of the Maastrichtian Patti Formation, southeastern Bida Basin, Nigeria. Cretaceous Research, 30: 1415 – 1425. Ojo, S.B., Ajakaiye, D.E., 1976. Preliminary Interpretation of gravity measurements in the middle
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Niger Basin area, Nigeria,in: Kogbe, C.A. (ed.), Geology of Nigeria. Elizabethan Publishing Company, Lagos, pp. 295-307.
Ojo, S.B., Ajakaiye, D.E., 1989. Preliminary Interpretation of gravity measurements in the middle
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Niger Basin area, Nigeria,in: Kogbe, C. A. (ed.), Geology of Nigeria. 2nd edition. Elizabethan Publishing Company, Lagos, pp. 347-358. Russ, W., 1930. The Minna-Birnin Gwari belt. Geol. Surv. Nigeria Rept. 21pp.
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Tattam, C.M., 1943. A review of Nigeria stratigraphy. Rep. Geol.Surv. Nigeria, pp. 27-46. Whiteman, A.J., 1982. Nigeria: Its Petroleum Geology, Resources and Potential. Graham and
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Trotman Publication, London. 394pp.
Zaborski, P.M., 1998. A review of the Cretaceous Systems in Nigeria. African Geoscience Review, 5, 385-483.
ACCEPTED MANUSCRIPT Table 1: The lithostratigraphic units of Bida Basin (based on Russ, 1930; Adeleye, 1974; Whiteman, 1982; Idowu and Enu, 1992).
Southern Bida Basin (Lokoja Area)
Nothern Bida Basin (Bida Area)
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Agbaja Formation Maastrichtian Patti Formation (75 m) ?
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Lokoja Formation (100 – 300 m) Basal Sandstone
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Campanian
Batati Formation (Upper Ironstone) Edozighi Member Kutugi Member Enagi Formation (70 m) Sakpe Formation (Lower Ironstone) Wuya Member Baro Member Bima Formation (80 m) Jima Member Doko Member
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Base of lithic fill (unexposed)
Crystalline Basement Complex
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Precambrian
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Figure 1: Approximate location map for stops during the field mapping exercise. (Modified from Obaje et al., 2013).
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Figure 10: Lithologic Log of ‘Ahoko Formation’ Exposed at Gegu – Gerinya Road (N 08014’46’’, E 006046’06.9’’)
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Figure 11: Concretional ironstone bands of the Patti/Ahoko Formation. Similar concretionary band occur in the Mamu Formation.
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Figure 12: Scolithus/Ophiomorpha borings within the Ahoko Formation.
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Figure 13: Brachiopods within the ‘Ahoko Formation’ indicating marine deposition.
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Figure 14: New Geological Map of the Bida Basin.
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Figure 15: Section showing the regional stratigraphic succession of the Bida Basin from this work (Section A-B in Figure 14)
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Figure 2: Lithologic log of the section exposed on the Agbaja Plateau N 07056’55.6’’ E 006039’40.7’’ (Modified from Ojo and Akande, 2008)
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Figure 3: Conglomeratic Sandstone exposed in the Abaji Road Cut section – Lokoja Sandstone (named Patti Formation by Ojo and Akande, 2006) .
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Figure 4: Lithologic log of Lokoja Sandstone exposed at Abaji, Federal Capital Territory (N 08027’51.8’’, E006056’24.7’’).
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Lokoja Sandstone
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Figure 5: Lithologic log of the Lokoja Sandstone exposed at Gada-biyu Quarry, Federal Capital Territory, Abuja (N 08036’42.1’’, E 006054’52.0’’).
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Figure 6: Lithologic Log of Bida Sandstone exposed on Jima Ridge (N 08059’11.6’’ E 50 55’0.7’’), Jima Village, Niger State.
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Pisolitic
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Pisolitic
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Oolitic
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Figure 7: Alternating beds of pisolitic and oolitic ironstone, Jima Hill. This pattern is also observed on Agbaja Hill
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Figure 8: Lithologic log of Bida Sandstone exposed at Kpada.
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Figure 9: Lithologic log of ‘Ahoko Formation’ exposed at Ahoko (N80 18’ 3.8’’ and E60 52’ 31.2’’), along the Lokoja-Abuja Expressway.
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RESEARCH HIGHLIGHTS Detailed Fieldwork involving lithologic description Nigerian Bida Basin sequences.
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Erection of a revised lithostratigraphic column for the Bida Basin.
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Re-naming of the Patti to ‘Ahoko Formation’ with the type section in Ahoko Village.
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New Geological Map drawn to show the formations’ relationship in the Bida Basin
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S1464-343X(18)30357-1
DOI:
https://doi.org/10.1016/j.jafrearsci.2018.11.016
Reference:
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To appear in:
Journal of African Earth Sciences
Received Date: 4 March 2018 Revised Date:
12 November 2018
Accepted Date: 14 November 2018
Please cite this article as: Rahaman, M.A.O., Fadiya, S.L., Adekola, S.A., Coker, S.J., Bale, R.B., Olawoki, O.A., Omada, I.J., Obaje, N.G., Akinsanpe, O.T., Ojo, G.A., Akande, W.G., A revised stratigraphy of the Bida Basin, Nigeria, Journal of African Earth Sciences (2018), doi: https:// doi.org/10.1016/j.jafrearsci.2018.11.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A REVISED STRATIGRAPHY OF THE BIDA BASIN, NIGERIA
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New Geological Map of the Bida Basin.
Section showing the regional stratigraphic succession of the Bida Basin (Line A-B)
ACCEPTED MANUSCRIPT A Revised Stratigraphy of the Bida Basin, Nigeria Rahaman, M. A. O.1, *Fadiya, S. L.1, Adekola, S. A.1, Coker, S. J.2, Bale, R. B.3, Olawoki, O. A.4, Omada, I. J.5, Obaje, N. G.6, Akinsanpe, O. T.1, Ojo, G. A.7and Akande, W. G.8 5
1
Department of Geology, Obafemi Awolowo University, Ile- Ife, Nigeria Skangix Development Limited, Nigeria 3 Department of Geology and Mineral Sciences, University of Ilorin, Nigeria 4 Geospectra Nigeria Limited, Lagos. 5 Department of Earth Sciences, Kogi State University, Ayangba, Nigeria 6 Department of Geology, Ibrahim Badamosi Babangida University, Lapai, Niger State. 7 Project Office, Dangote Cement Plc, Ikeja, Lagos, Nigeria 8 Department of Geology, Federal University of Technology, Minna. *Corresponding Author: FADIYA Suyi Lawrence Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria Telephone: +234-80333-20230 e-mail: [email protected]; [email protected]
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ABSTRACT
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The Bida Basin is a NW-SE trending inland basin, stretching from Shegwa (NW) to Dekina (SE). It is approximately 350 km long and varies in width from 75 - 150 km. It is roughly elliptical in ground plan and runs perpendicularly to the western margin of the NE-SW trending Benue Trough Complex.
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Unlike other sedimentary basins in Nigeria, it is characterized by the absence of volcanics, carbonates and rocks of Tertiary age. Dedicated fieldwork carried out over the past four years revealed that the basin is underlain by mainly continental sediments with minor occurrence of
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marginal/shallow marine to freshwater flood-plain deposits. The initial basin fill is characterized
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largely by several cycles of simple, but amalgamated and/or stacked fanglomerates (alluvial fan) systems developed from several single stream breaches onto the proximal setting of the basin. These fanglomerates directly overly the Basement Complex and display a fining upward sequence. This 30
unit is generally accepted as the Lokoja/Bida Sandstone. Our studies revealed that the Lokoja/Bida Sandstone is overlain directly by the oolitic/pisolitic ironstone of the Agbaja/Sakpe Ironstone which is in turn succeeded by a sequence of rocks generally referred to as the Patti/Enagi Formation, which is comprised of predominantly fine to medium-grained, grey and white sandstones, grey clays, carbonaceous silts and shales, and ubiquitous concretionary ironstone bands. This observation is
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opposed to the widely accepted view of previous workers that considered the Agbaja Ironstone as the youngest formation in the basin. This study further revealed that the sequence of rocks used to define the Patti Formation does not exist at the location of Mount Patti in Lokoja, rather, it is found in road cut sections, quarries and boreholes around the Ahoko Village. Following the rules for stratigraphic
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nomenclature, it is hereby proposed that the name ‘Patti Formation’ should be changed to ‘Ahoko Formation.’ A new geological map was made for the basin which clearly demonstrates the
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relationship of the formations.
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KEYWORDS: Bida Basin, Stratigraphy, Patti Formation, Ahoko Formation, Geological Map
INTRODUCTION
The Bida Basin is a NW-SE trending intracratonic basin that extends northwestwards from the confluence of the Niger and Benue Rivers at the Lokoja/Dekina axis to the basement complex that
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separates it from the basal continental early Cretaceous to late Jurassic Illo and Gundumi Formations (Kogbe, 1989) of the Sokoto Basin at Shegwa (NW) in the Kainji Dam area. The basin is roughly 50
elliptical in shape and runs perpendicular to the NE-SW trending Benue Trough Complex. The basin
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is approximately 350 km in its longest (NW-SE) axis and varies from 75 - 150 km in width (Zaborski, 1998). The Bida Basin is unique among the other Nigerian sedimentary basins in that it is
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characterized by an absence of volcanics, carbonate rocks and sediments of Tertiary age.
Tectonic Setting
The tectonic setting of the Bida Basin has been a subject of discussion over the years and till present, a satisfying explanation of the origin of the basin is yet to be accepted by geoscientists. Three main tectonic models have been proposed for the origin: (i) 60
The simple sag model: Adeleye (1976) suggested that the Bida Basin was formed as a simple sag structure in response to Santonian folding in the Benue Trough. Ojo and
ACCEPTED MANUSCRIPT Ajakaiye (1976, 1989), based on gravity studies, on the other hand, concluded that the contacts at the edges of the basin are probably not faulted and considered the basin as an area of local subsidence resulting from the outflow of lower crustal material associated with intermittent igneous activity associated with the ‘Younger Granites’
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magmatism. Whiteman (1982), on the basis of field relationship and the absence of
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fault scarps around the basin proposed that the basin is a post-Santonian shallow cratonic basin.
Rift model: Landsat imageries studied by Kogbe et al. (1981) indicated that the basin
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(ii)
is controlled by NW-SE trending faults in the surrounding basement and thereby proposed a rift origin for the basin. The rift school of thought was further strengthened
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by Ojo (1990) who identified in the area an elongated E-W negative aeromagnetic anomaly flanked to the south by a positive anomaly and concluded that the anomaly was caused by basic rocks at depths of between 4 km – 6 km. This, according to him
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suggests that, a deep-seated rift may exist in the crust under the basin. Agyingi (1993) believed the basin formed as a pre-Santonian rift. Likkason (1993) believed that the
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basin was initiated during Campano-Maastrichtian times and related its formation to a
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mantle plume to be located close to the confluence of the Benue and Niger rivers in the southeastern part of the basin.
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The strike slip model: Braide (1990; 1992) proposed a strike slip origin for the basin
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based on the lateral continuity of the linear northeastern boundary of the basin and the ‘Gboko transform fault’ of Whiteman (1982). Braide (op cit.) suggested that the basin was formed as a pull-apart basin with multiple phases of transtension and basin infilling with little internal deformation.
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While the interpretation of the proposed origin of the basin has been based largely on gravity and aeromagnetic data, there is the need for seismic data over the region to help determine a more plausible explanation.
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Stratigraphy
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The earliest description of the geology of the lithic fill in the Bida Basin was made by Falconer (1911) who labelled it the ‘Lokoja series’ to cover sediments extending from Idah in the southeastern
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end of the basin to Kontagora in the northwestern part. Russ (1930) described the sandstones around Bida and referred to them as Nupe sandstones. More recent stratigraphic and paleoenvironmental
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studies were carried out by Adeleye (1973; 1974), Braide (1990; 1992), Ladipo et al. (1994), Ojo and Akande (2003; 2008; 2009) and Obaje et al. (2011). and The results of more intensive and detailed field work across the basin now reveal additional information about the basin fill successions and history, which improves the understanding of the relationships and controls of sedimentation,
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including sea level cyclicy and tectonics.
Summary of Previous Works
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The Bida Basin has been broadly subdivided into the Northern and Southern Bida Basins (Fig. 1) based on the geographical setting. The northern sector is popularly referred to as the Bida area while
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the southern sector is named the Lokoja area. The stratigraphic succession of each region was established and correlated across the basin and to the 105
adjoining Anambra Basin. Russ (1930), Adeleye (1974), Whiteman (1982), Braide (1992), Idowu and Enu (1992), Akande et al. (2005), Obaje et al. (2013), Obaje et al. (2013) and Ojo and Akande (2009) among others, established the lithostratigraphic succession shown in Table 1. It was generally agreed by these workers that the Lokoja Sandstone overlies the Basement Complex nonconformably in the Southern Bida Basin. Its equivalent in the Northern Bida Basin – the Bida Sandstone, also lies
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unconformably on the basement rocks in that region. The Lokoja and Bida Sandstones range from
ACCEPTED MANUSCRIPT massive, immature (texturally and mineralogically), matrix to clast supported conglomerate, which fines upward to conglomeratic sandstone, to medium-grained sandstone, siltstone and surbordinate claystone. Russ (1930), Adeleye (1974), Whiteman (1982) and Akande et al. (2005) believed the Lokoja and Bida Sandstones are directly overlain by the Patti Formation, which is equivalent to the Enagi Formation in the Northern Basin and in turn overlain by the Agbaja Formation.
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Present Study
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Recent and more detailed field studies by the authors of this paper revealed that the lithostratigraphic sequence of the basin is at variance with those proposed by the earlier studies (Adeleye, 1971; Adeleye and Dessauvagie, 1972; Nwajide et al., 19998; Ojo and Akande, 2003). While some
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formation names remain valid, others are not because the formations were not exposed at the locations after which they were named.
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METHODOLOGY
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A section by section description of major outcrops around the basin during yearly field trips between 2013 – 2016 was made based on field observation. Our focus is on the lithology, texture, sedimentary
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structures, fossil assemblage and other information relevant to the interpretation of each outcrop. A North-South traverse made on the southern part of the basin involved the study of several outcrops
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including those around Lokoja, Agbaja, Koton-karfi, Gegu, Ahoko and Abaji. The North west-South east traverse cutting across the southern and northern part of the basin provide opportunity to study outcrops around Gerinya, Gulu, Kpada, Doko, Bida, Jima, Sakpe, Batati and Enagi. Outcrops around Wushishi and Share at the Southwestern fringe of the basin were also studied. The results from the studied sections were used for assigning the sections into lithostratigraphic units based on the initial description of the character of the units. A new geological map was made for the basin and a revision 135
of the lithostratigraphic succession shown in a cross section across the map. A description of the existing stratigraphy of the basin are made as follows:
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2.1
Lokoja Area
2.1.1
Lokoja Sandstone
Falconer (1911) described the ‘Lokoja series’ from various locations around the basin as a shallow
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water accumulation of sandstones, grits, conglomerates and ironstones. He observed that, the lowermost rocks of the Lokoja series are almost invariably light-coloured sandstones and conglomerates. He described the lowest rock exposed at the base of Mt. Patti at Lokoja as soft, white,
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felspathic sandstones with pebbles of quartz, kaolinized feldspar and pegmatite which occasionally accumulate into conglomeratic bands. According to Falconer (op cit.), the greater part of the hill is
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composed of white sandstone and grits indistinctly bedded and stained regularly, and in places to a considerable depth, with iron oxides. Falconer (1911) described the upper part of the hill as white sandstone and ferruginous sandstones with concretionary knots of earthly limonite and intercalated bands and irregular lenticular patches of oolitic ironstone. The uppermost 46 m (150 ft) are made up of oolitic, pisolitic and earthly ironstones enclosing irregular mass of hard lustrous concretionary
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limonite, while the summit is capped by a pebbly ironstone composed of small rounded and elliptical pebbles of compact ironstone cemented by earthly limonite containing a few grains of quartz.
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According to Jones (1958), Lokoja sandstones are composed of pebbly and clayey grits and sandstones, coarse false-bedded sandstones and a few thin oolitic ironstone. Jones (1958) described the rarely exposed basal conglomerate as consisting of well-rounded quartz pebbles in a matrix of
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white clay.
2.1.2
Patti Formation
The Patti Formation was recognised by Tattam (1943), Jones (1958) and Ladipo et al. (1994) as a 160
distinct formation (approx. 100 m thick) overlying the Lokoja Basal Sandstone. The formation is said to be comprised mainly of yellowish siltstones, shales and black carbonaceous shales with minor interbeds of claystones, sandstones, impure coal seams and concretionary bands of ironstone as can
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grey and white sandstones, grey clays, carbonaceous silts and shales and oolitic ironstone. Thin coals have been recorded and massive, white, gritty clays are common. Recorded fossils are plant remains
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in the carbonaceous beds. Fossil seeds and other plant fragments are preserved in detail where these rocks have been ferruginized near the surface. Pollen and spores from angiosperms, pteridophytes, gymnosperms and palmae as well as marine indicator dinoflagellates (Dynogymnium acuminatum, Senegalinium bicavatum and Paleocystodinium australinium) were recovered from the formation
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(Jan du Chene et al., 1978; Ojo and Akande, 2003). It is significant to note that the Patti Formation as
rocks/lithologies on Mount Patti.
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2.1.3
Agbaja Ironstone
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described by these authors was not found by Falconer (1911) in his description of the
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Adeleye (1974), Ladipo et al. (1994), Ojo and Akande (2008) present the Agbaja Ironstone as the youngest rock sequence in the Lokoja area, that overlies the Patti Formation and caps the mesas around Lokoja. It is composed of oolitic ironstone to pisolitic, concretional and massive ironstone
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facies. According to Adeleye (1971; 1975), the Agbaja Ironstone is characterised by oolitic and pisolitic textures, often massive, but locally flat bedded and exhibiting cyclic sedimentation patterns.
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The oolites become coarser upwards and become pisolitic toward the top. The formation contains laterite and interbeds of clay and sandstone. Fossil remains within the Agbaja Ironstone include oysters and gastropod shells, as well as plant remains (Adeleye 1971; 1975).
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2.2
Bida Area
2.2.1
Bida Sandstone
Much like its lateral equivalent (Lokoja Sandstone), the Bida Sandstone lies unconformably on the crystalline basement complex and consist of a basal conglomerate with a succession of cross-bedded,
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deeply weathered basement rocks. The conglomerate contains evidence of mud and debris flows. While the basal beds are massive and flat, the upper parts shows clear cross-stratification. The deposits making up the Bida Sandstone are cyclic in nature and similar to those seen in the
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contiguous Lokoja Sandstone. Two members of the Bida Sandstone were recognised by Adeleye (1971), the Doko and Jima members. The Doko Member is the basal unit and about 183 m thick 195
(Adeleye, 1974). It is coarse grained, arkosic to feldspathic, with sandy siltstones and
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intraformational breccias. The Jima Member consists of cross-bedded quartzose sandstone,siltstone,
2.2.2 200
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claystone and breccia.
Sakpe Ironstone
Adeleye and Dessauvagie (1972) described the Sakpe Ironstone, which occurs over an extensive area west of Bida and forms a sharp contact with underlying Bida Sandstone. The formation consists of
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oolitic – pisolitic ironstone with oolites at the base, pisolites in the middle and a return to oolites at the top. Two laterally equivalent members – the Wuya Ironstone Member and the Baro Ironstone Member were recognised. The Wuya Ironstone Member consists of both oolitic and pisolitic ironstones, with lesser amounts of locally developed claystone. The Baro Member is oolitic ironstone
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and ferruginous sandstone, which are locally pyritic and concretionary. It is overlain by the Enagi
2.2.3 210
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Siltstone.
Enagi Siltstone
This Enagi Siltstone as described by Adeleye (1971) is overwhelmingly dominated by siltstones with interbeds of mudstones and fine-grained sandstone. The Enagi siltstone is believed to conformably overlie the Bida Sandstone Formation except in the areas of the occurrence of the Sakpe Ironstone Formation.
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2.2.4
Batati Ironstone
This Batati Ironstone was described from the central Bida Basin by Adeleye (1971; 1973), who recognized two sub-facies. The basal Kutigi Member is finely oolitic (0.2 - 1 mm), whereas the overlying Edozhigi Member is coarsely oolitic (0.2 - 1.5 mm). According to Adeleye (2016), when
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both are present they are separated by an erosional surface. The overlying beds overstep the finegrained subfacies eastwards and southwards. Occasional vertical burrows are observed and the sorting of oolites very variable. It generally appears massive. A correlation between the formations in
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the Southern Bida Basin (Lokoja area) with those from the Northern Bida Basin (Bida area) is
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2.3
Revised Stratigraphy
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summarized in Table 1 as modified from Akande et al. (2005)
The purpose of this section is not to redescribe the lithology, sedimentology, sedimentary structures or the palaeontology of these units, but to put the formations in the correct stratigraphic order. To
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achieve this, it is important to do a section by section description of each of the outcrops and make a comparison with the original description of the formation. 230
The Lokoja-Agbaja Road Section.
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2.4.1
Along the Lokoja-Agbaja road section, the basal conglomeratic sandstone of the Lokoja Sandstone is
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exposed in river channels below Agbaja Hill where it rests unconformably on porphyritic granite. The conglomerate is succeeded by coarse-grained sandstone. Both units appear massive, flat bedded 235
and poorly cross-stratified. At higher elevation on the hill, lensoid clay units were encountered within the coarse sandstones. The clay interbeds become thicker upward and culminate in a clay unit that is thick enough to act as a seal (impermeable layer) to prevent further downward percolation of water from the overlying sand. The clay units here are discontinuous lenticular beds. The clay units were interpreted as flood basin, abandoned ponds and lacustrine facies (Adeleye, 1971; 1972; Braide,
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1992). Above the clay layer, the Lokoja Sandstone consists of medium to coarse-grained, friable
ACCEPTED MANUSCRIPT cross-stratified sandstone unit. These observations fit perfectly into the original description by Jones (1958) of the Lokoja sandstones in which he described the formation as being characterized by pebbly and clayey grits and sandstones, coarse false-bedded sandstones and a few thin oolitic ironstones layers. An erosional surface is visible between this white friable sandstone unit and the succeeding Agbaja Ironstone. This implies that the Patti Formation is absent in the Lokoja – Agbaja
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Road section. A reinterpreted litholog of Ojo and Akande (2008) for the Agbaja section is as shown
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in Figure 2.
It is important to note that the Jones (1958) interpretation of the Patti Formation from a borehole was based on the observation of a clayey layer which he referred to as argilaceous. Our present
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interpretation is that the clay units are lensoid in shape and not continuous units. This can be seen in the Lokoja - Agbaja road section, at the Gada Biu quarry near Abaji along the Abuja road and at the Kpada (Pategi) hill section. It is important to mention that the carbonaceous shale and marine fossils
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characteristic of the Patti Formation were not found anywhere in the sections of the Lokoja and Bida Sandstones that we studied nor on Patti Hill which is made up entirely of the Lokoja Sandstone. It therefore follows that the Agbaja Ironstone is capping the Lokoja Sandstone and not the Patti
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Formation. This conclusion agrees with the comment of Jones (1958) reported that Falconer (1911) did not record the presence of carbonaceous shale at the top of the succession in his description of the
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2.4.2
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type section at Mount Patti.
Abaji Roadcut Section
The Abaji roadcut section shows the characteristic rhythmic layering of the Lokoja Sandstone. A close examination of the outcrop reveals a conglomeratic sandstone at the base that is succeeded by a poorly sorted coarse sandstone and medium grained sandstone. Figure 3 is a photograph of the 265
conglomeratic unit within the poorly sorted sandstone. This textural relationship is cyclic and observed repeatedly in the section until it is succeeded by a strongly cross-stratified, coarse to
ACCEPTED MANUSCRIPT medium-grained sandstone unit before giving way to the claystone and kaolinitic clay at the top (Fig. 4). This is a typical Lokoja Sandstone sequence and not Patti Formation as interpreted by some workers (Jones, 1958). 270
Gada-Biyu Quarry, Abuja Road.
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2.4.3
In the Gada-Biyu quarry, large lenticular clay interbeds (Fig.5) are quarried for foundry purposes. The associated coarse, poorly sorted sandstones are typical of the Lokoja Sandstone. The materials
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were believed to be derived from the nearby crystalline basement rocks judging from the textural characteristic and the similarity in mineralogical composition of the sediments and the basement
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rocks.
BIDA SANDSTONE
Adeleye (1971) recognized two members for the Bida Sandstone: Doko and Jima. Both are mappable on Jima ridge.
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Jima Sandstone Section
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The Jima Member mapped at Jima ridge consists of conglomeratic, arkosic sandstone at the base. Clasts in the conglomeratic sandstone are composed of angular to subrounded quartz and feldspar, and range between 0.8-10 cm. The arkosic sandstone is succeeded by a 2 m thick conglomerate
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containing weathered granitic (porphyritic) rock fragments derived from nearby basement complex. Bedding structures become apparent in the middle section where the sandstone is light grey, arkosic and displays planar cross beds. From the middle section upward, a rhythmic repetition of conglomeratic, matrix-supported, pebbly, arkosic sandstone filled with weathered rock fragments at 290
the base is observed. This is succeeded by planar, cross bedded, medium-grained, feldspathic sandstone overlain by siltstone and claystone as shown in Figure 6. The cyclic conglomerate,
ACCEPTED MANUSCRIPT sandstone, siltstone and claystone interval with repeating fining upward cyclic layering is overlain by the oolitic - pisolitic ironstone of the Sakpe Ironstone.
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It is worthy of note that, a striking similarity exist between the Agbaja Hill and the Jima Ridge
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sections. The basal massively bedded unit seen at the Agbaja Hill section, which was succeeded by the cross bedded sandstone and the siltstone, exhibits the same patterns observed in the Jima section. Strongly cross bedded medium-grained sand units were also observed below the Sakpe Ironstone cap.
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Most importantly, the succession of pisolitic to oolitic and back to pisolitic textures (Fig. 7) of the Sakpe Ironstone on the Jima Hill is the same pattern observed in the Agbaja Hill section, a similarity
2.4.6
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that is interpreted to establish equivalency between the sections.
Kpada Section
At Kpada, a location 40 km west of Pategi, Kwara State, the Bida Sandstone is exposed. The section is about 150 m thick and consists of clast-supported, conglomeratic sandstone at the base and
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overlain by massive, arkosic sandstone with gravel-boulder sized fragments of weathered basement rocks. This unit fines upward into siltstone before the beginning of the next depositional cycle (Fig.
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8). This series of stacked fining-upward cycles is characteristic of the Lokoja Sandstone. The discontinuous, lenticular clay beds are conspicuous in the section at Kpada. Moreover, the friable, cross-bedded sandstones are clearly evident below the capping oolitic ironstone of the Sakpe
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Ironstone. This similarity is considered strong evidence to support the contention that the same sequence described earlier for Bida Sandstone exposed at Jima is equivalent to the Lokoja Sandstone exposed variously around Lokoja, where it is capped by the Agbaja Ironstone.
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2.4.7
Patti Formation: Kotonkarfi - Ahoko – Acheni and Gegu – Gerinya Sections
The evidence presented supports that the ‘Patti Formation’ is likely absent in the Patti Hill and the Lokoja – Agbaja and Abaji sections. However, continuous parallel beds characteristic of marine
ACCEPTED MANUSCRIPT deposition were noticed from Koton-karfi (N80 6’ 25.4’’ and E60 47’ 32.2’’) northwards to Ahoko (N80 18’ 3.8’’ and E60 52’ 31.2’’) as well as those exposed along the Gegu – Gerinya Road. These 320
deposits match the description of the Patti Formation as given by Jones (1958) in which he described the formation as consisting of fine-medium grained, grey and white sandstones, grey clays,
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carbonaceous silts and shales and oolitic ironstone. Thin coals have been recorded and massive, white, gritty clays are common. Recorded fossils are plant remains in the carbonaceous beds. Fossil seeds and other plant fragments are preserved in detail where these rocks have been ferruginized near the surface. Pollen and spores from angiosperms, pteridophytes, gymnosperms and palmae as well as
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marine indicator dinoflagellates (Dynogymnium acuminatum, Senegalinium bicavatum and
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Paleocystodinium australinium) were recovered from the formation (Jan du Chene et al., 1978; Ojo and Akande, 2003). These as well as some agglutinated foraminifera species were recovered from the samples collected at the Ahoko section which typified the description by Jones (1958). The Patti 330
Formation is clearly exposed along a road cut some hundred meters east of Gerinya village (N80 14’
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46’’ and E60 46’ 6.9’’).
Change of name for the Patti Formation
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Based on the presented evidence, it is proposed that the Patti Formation should be changed to ‘Ahoko Formation.’ The principal reason for the name change is that a well exposed and complete
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section of the formation, which typifies the original description of the Patti Formation is located at Ahoko village. This proposition is further supported by the absence of the Patti Formation on Patti Hill. A typical litholog of the proposed Ahoko Formation as seen at Ahoko village and the Gegu – Gerinya Road is shown in Figures 9 and 10, respectively. The marine nature of the sediments is 340
evident
from
the
recovery
of
shallow
marine
benthic
foraminifera
(Ammobaculites,
Haplophragmoides and Ammodiscus), echinoids, brachiopods and pelecypod shells. The carbonaceous units within the Ahoko Formation are interpreted as swamp or salt marsh deposits. Given that this sequence of rocks does not occur on Mount Patti, it is suggested that the formation be
ACCEPTED MANUSCRIPT renamed ‘Ahoko Formation’ after the village where a complete section is exposed. Discontinuous 345
concretionary ironstone interbeds/bands characteristic of the Patti/Ahoko Formation and its equivalent in the Anambra Basin (Mamu Formation) are clearly visible in this outcrop (Fig. 11). The unique characteristic of the Ahoko Formation observed at the Ahoko village, but not seen in the
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2.4.8
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Agbaja section, include the Skolithus/Ophiomorpha borings (Fig. 12) and brachiopod shells (Fig. 13).
The Enagi Siltstone
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Good exposures of the Enagi Siltstone are visible at the Patti Shabakolo section. There is strong evidence that the section at Shabakolo is equivalent to the Ahoko (old Patti) Formation in the Lokoja
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area as the characteristic concretionary iron bands are clearly evident. At Batati, the Enagi Siltstone occurs as a white silty unit with abundant clay balls. 355
2.4.9
Batati Ironstone?
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At Batati, the Enagi Siltstone is capped by a lateritic layer which has been designated the Batati Ironstone. However, near the top of the siltstone is a ferruginised clay layer with a high iron content deduced from its heavy weight and jagged edges from breakage. Obviously, this unit cannot be the one referred to as the Batati Ironstone as there are no oolitic structures and it lies within the Enagi
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Siltstone. Without a better exposed location, the Batati Ironstone type or reference section cannot be
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verified at any location examined in the course of our fieldwork.
A redrawn map of the Bida Basin showing the relationship between the different formations is shown 365
in Figure 14. The proposed stratigraphic relationships for the different lithostratigraphic units of the Bida Basin is presented in Figure 15. In the northwestern part of the basin, the Enagi Siltstone can be seen lying directly on top of the Bida Sandstone. In the middle part of the basin, the Bida Sandstone is overlain by the Sakpe Ironstone. In thesoutheastern part of the basin, the Agbaja Ironstone can be
ACCEPTED MANUSCRIPT seen overlying the Lokoja Sandstone around Lokoja and Agbaja. The Ahoko Formation lies directly 370
on top of the Agbaja Ironstone at a location in Koton-karfi. Lokoja Sandstone at Gerinya and Ahoko.
2.5
CONCLUSION:
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The widely adopted stratigraphic succession placing the Ahoko (formerly Patti) Formation on top of the Lokoja Sandstone has been revised as the Agbaja Ironstone and its northern equivalent (Sakpe 375
Ironstone) can be seen to lie on top of the Lokoja sandstone and the Bida sandstone respectively. The
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Sakpe Irostone in the Northern Bida Basin and the Agbaja Ironstone in the Southern Bida Basin are lateral equivalents just as the Enagi Siltstone and the Ahoko Formation in the northern and southern
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parts of the basin are lateral equivalents. Undoubtedly, the Bida Sandstone in the northern part of the basin and the Lokoja sandstone in the southern parts are lateral equivalents. Since the old ‘Patti’ 380
Formation type section cannot be seen on the Patti Hill, it is appropriate that the name of the carbonaceous shale unit be changed to reflect a type or reference locality where the unit is observed
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as originally described. The old Patti Formation is therefore renamed Ahoko Formation after Ahoko village, where outcrops that match the original description was found.
Acknowledgements
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The authors are grateful to the Department of Geology, Obafemi Awolowo University Professional
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Masters Program on whose platform the field trip exercises that led to this work was carried out. This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors. 390
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ACCEPTED MANUSCRIPT Adeleye, D.R., 1973. Origin of ironstones: An example from the middle Niger Valley. Journal of 395
Sedimentary Petrology, 43, p. 709-727 Adeleye, D.R., 1974. Sedimentology of the fluvial Bida Sandstone (Cretaceous), Nigeria. Sedimentary Geology, 12, p. 1-24.
Petroleum Geologists Bulletin, 59, 2302-2313. 400
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Adeleye, D.R., 1975. Nigeria Cretaceous Stratigraphy and Paleogeography. American Association of
Adeleye, D.R., 1976. The Geology of the Middle Niger Basin.In: Kogbe, C. A. (ed.), Geology of
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Nigeria. Elizabethan Publishing Company, Lagos, pp. 283-287.
Adeleye, D.R., 2016. Bida Basin and the Economic Backbone of Nigeria; 3rd Inaugural Lecture,
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Ajayi Crowther University, Oyo. 44pp.
Adeleye, D. R. and Dessauvagie, T.F.J., 1972. Stratigraphy of the Niger Embayment near Bida, 405
Nigeria. In: Desssauvagie, T.F.J., Whiteman, A.J. (eds.), African Geology. University of Ibadan Press, pp. 181-186.
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Agyingi, C.M., 1993. Palynological evidence for a late Cretaceous age for the Patti Formation, eastern Bida Basin, Nigeria. Journal of African Earth Sciences. 17 (4):513-523. Akande, S.O., Ojo, O.J., Erdtmann, B., Hetenyi, M., 2005. Paleoenvironments, Organic Petrology and Rock-eval Studies on Source Rock Facies of the Lower Maastrichtian Patti Formation,
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Southern Bida Basin, Nigeria. Journal of African Earth Sciences. 41, 394-406.
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Braide, S.P., 1990. Petroleum geology of the Southern Bida Basin, Nigeria. American Association of Petroleum Geologists Bulletin, 74 (5): 717.
Braide, S.P., 1992. Syntectonic fluvial sedimentation in the central Bida Basin. Journal of Mining 415
and Geology, 28: 55-64.
Falconer, J. D., 1911. The Geology and Geography of Nothern Nigeria. London: Macmillan (101pp.) Idowu, J.O., Enu, E.I., 1992. Petroleum Geochemistry of some Late Cretaceous Shales from the Lokoja Sandstone of the Middle Niger Basin, Nigeria. Journal of African Earth Sciences, 14, pp. 433-455.
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Jan du Chene, J., Adegoke, O. S., Adediran, S. A. and Petters, S. W., 1978. Palynology and Foraminifera of the Lokoja Sandstone (Maastrichtian), Bida Basin, Nigeria. Revista Espanola de Micropalontologia, 10 (3), 379-393.
Geological Survey of Nigeria(1955), 20-43. 425
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Jones, H.A., 1958. The oolitic ironstones of Agbaja Plateau, kabba Province. Record of the
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Kogbe, C.A., Ajakaiye, D.E., Matheis, G., 1981: Confirmation of a rift structure along the mid-Niger valley, Nigeria. Journal of African Earth Sciences, 1, 127-131
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Ladipo, K.O., Akande, S.O., Mucke, A., 1994. Genesis of Ironstones from the Middle Niger Sedimentary Basin, Evidence from Ore Microscopic and Geochemical Studies. Journal of Mining and Geology, Vol. 20, pp. 161-168.
Likkason, O.K., 1993. Application of trend surface analysis to gravity data over the Middle Niger
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Basin, Nigeria. Journal of Mining and Geology, 29 (2), 11-19. Nwajide, C. S., Oni, A. O. and Oluyide, P. O., 1998. The Geology of Ilorin Area: Sedimentary Rocks 435
of Bida Basin. Geological Survey of Nigeria Bulletin, 42: 60-65.
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Obaje, N.G., Musa, M.K., Odoma, A.N. and Hamza, H., 2011. The Bida Basin in north-centreal Nigeria: sedimentology and petroleum geology. Journal of Petroleum and Gas Exploration
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65. Ojo, S. B., 1990. Origin of a major aeromagnetic anomaly in the Middle Niger Basin, Nigeria. 445
Tectonophysics, 185: 153-162.
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Ojo, O.J., Akande, S.O., 2003. Facies Relationship and Depositional Environment of the Upper Cretaceous Lokoja Formation in the Bida Basin, Nigeria. Journal of mining and Geology, 39: 39-48. Ojo, O.J., Akande, S.O., 2008. Microfloral assemblage, age and paleoenvironment of the Upper
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Cretaceous Patti Formation, southeastern Bida Basin, Nigeria: Journal of Mining and Geology, 44: 71-78.
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Ojo, O.J., Akande, S.O., 2009. Sedimentology and depositional environments of the Maastrichtian Patti Formation, southeastern Bida Basin, Nigeria. Cretaceous Research, 30: 1415 – 1425. Ojo, S.B., Ajakaiye, D.E., 1976. Preliminary Interpretation of gravity measurements in the middle
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Niger Basin area, Nigeria,in: Kogbe, C.A. (ed.), Geology of Nigeria. Elizabethan Publishing Company, Lagos, pp. 295-307.
Ojo, S.B., Ajakaiye, D.E., 1989. Preliminary Interpretation of gravity measurements in the middle
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Niger Basin area, Nigeria,in: Kogbe, C. A. (ed.), Geology of Nigeria. 2nd edition. Elizabethan Publishing Company, Lagos, pp. 347-358. Russ, W., 1930. The Minna-Birnin Gwari belt. Geol. Surv. Nigeria Rept. 21pp.
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Tattam, C.M., 1943. A review of Nigeria stratigraphy. Rep. Geol.Surv. Nigeria, pp. 27-46. Whiteman, A.J., 1982. Nigeria: Its Petroleum Geology, Resources and Potential. Graham and
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Trotman Publication, London. 394pp.
Zaborski, P.M., 1998. A review of the Cretaceous Systems in Nigeria. African Geoscience Review, 5, 385-483.
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Southern Bida Basin (Lokoja Area)
Nothern Bida Basin (Bida Area)
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Agbaja Formation Maastrichtian Patti Formation (75 m) ?
?
Lokoja Formation (100 – 300 m) Basal Sandstone
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Campanian
Batati Formation (Upper Ironstone) Edozighi Member Kutugi Member Enagi Formation (70 m) Sakpe Formation (Lower Ironstone) Wuya Member Baro Member Bima Formation (80 m) Jima Member Doko Member
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Age
Base of lithic fill (unexposed)
Crystalline Basement Complex
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Precambrian
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Figure 1: Approximate location map for stops during the field mapping exercise. (Modified from Obaje et al., 2013).
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Figure 10: Lithologic Log of ‘Ahoko Formation’ Exposed at Gegu – Gerinya Road (N 08014’46’’, E 006046’06.9’’)
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Figure 11: Concretional ironstone bands of the Patti/Ahoko Formation. Similar concretionary band occur in the Mamu Formation.
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Figure 12: Scolithus/Ophiomorpha borings within the Ahoko Formation.
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Figure 13: Brachiopods within the ‘Ahoko Formation’ indicating marine deposition.
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Figure 14: New Geological Map of the Bida Basin.
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Figure 15: Section showing the regional stratigraphic succession of the Bida Basin from this work (Section A-B in Figure 14)
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Figure 2: Lithologic log of the section exposed on the Agbaja Plateau N 07056’55.6’’ E 006039’40.7’’ (Modified from Ojo and Akande, 2008)
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Figure 3: Conglomeratic Sandstone exposed in the Abaji Road Cut section – Lokoja Sandstone (named Patti Formation by Ojo and Akande, 2006) .
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Figure 4: Lithologic log of Lokoja Sandstone exposed at Abaji, Federal Capital Territory (N 08027’51.8’’, E006056’24.7’’).
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Lokoja Sandstone
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Figure 5: Lithologic log of the Lokoja Sandstone exposed at Gada-biyu Quarry, Federal Capital Territory, Abuja (N 08036’42.1’’, E 006054’52.0’’).
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Figure 6: Lithologic Log of Bida Sandstone exposed on Jima Ridge (N 08059’11.6’’ E 50 55’0.7’’), Jima Village, Niger State.
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Pisolitic
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Pisolitic
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Oolitic
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Figure 7: Alternating beds of pisolitic and oolitic ironstone, Jima Hill. This pattern is also observed on Agbaja Hill
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Figure 8: Lithologic log of Bida Sandstone exposed at Kpada.
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Figure 9: Lithologic log of ‘Ahoko Formation’ exposed at Ahoko (N80 18’ 3.8’’ and E60 52’ 31.2’’), along the Lokoja-Abuja Expressway.
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RESEARCH HIGHLIGHTS Detailed Fieldwork involving lithologic description Nigerian Bida Basin sequences.
•
Erection of a revised lithostratigraphic column for the Bida Basin.
•
Re-naming of the Patti to ‘Ahoko Formation’ with the type section in Ahoko Village.
•
New Geological Map drawn to show the formations’ relationship in the Bida Basin
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•