Two member subdivision of the Bima Sandstone, Upper Benue Trough, Nigeria: Based on sedimentological data

Accepted Manuscript Two Member Subdivision of the Bima Sandstone, Upper Benue Trough, Nigeri: Based on Sedimentological Data A. Tukur, N.K. Samaila, S.T. Grimes, I.I. Kariya, M.S. Chaanda PII: DOI: Reference:

S1464-343X(14)00358-6 http://dx.doi.org/10.1016/j.jafrearsci.2014.10.015 AES 2170

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

22 November 2013 28 October 2014 31 October 2014

Please cite this article as: Tukur, A., Samaila, N.K., Grimes, S.T., Kariya, I.I., Chaanda, M.S., Two Member Subdivision of the Bima Sandstone, Upper Benue Trough, Nigeri: Based on Sedimentological Data, African Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jafrearsci.2014.10.015

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TWO MEMBER SUBDIVISION OF THE BIMA SANDSTONE, UPPER BENUE TROUGH, NIGERI: BASED ON SEDIMENTOLOGICAL DATA A. Tukur1*; N. K. Samaila1; S.T. Grimes2; I. I. Kariya1 and M. S. Chaanda2 1 Geology Programme, Abubakar Tafawa Balewa University, Bauchi, PMB 0248, Bauchi, Nigeria 2 School of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, PL4 8AA, Devon, Plymouth, UK Corresponding author’s email: [email protected]. ABSTRACT The Early Cretaceous Bima Sandstone is a continental succession in the Upper Benue Trough, C

Nigeria formally divided into the Lower, Middle and Upper Bima Members. Sedimentological data presented here indicates a two member model (Lower and Upper Members) is more appropriate for the formation. The boundary separating the two proposed members is exposed at the Bollere River, Bima Hill, Wuyo II, and Kaltungo sections. The lithological differences between the two members are perhaps to a large extent a reflection of the sediments sources. The Lower Bima Sandstone Member was deposited in aggradational braided alluvial systems and contains well preserved overbank fines, avulsive and crevasse splay sandstones, and channel deposits. Pedogenic carbonates are also common features of these alluvial deposits in the Bima Hill. The Kaltungo Fault section exposes sediments of brief lacustrine setting within the Lower Bima Sandstone Member. The Upper Bima Sandstone Member was deposited in fully matured braided river with well-developed accommodation space in both shallow and deep fluvial channels. Sedimentation in this braided river was mostly on braid bars and with scarce channels. Preliminary δ13CTOC data along the Bollere River shows lack of any significant carbon isotope excursion suggesting climate, especially changes in aridity was not 1

a major contributor in differences between the two members of the Bima Sandstone. Keywords: Bima Sandstone; lithofacies; two-member model; aggradational deposits; braid bar deposits. 1.0 INTRODUCTION The Early Cretaceous Bima Sandstone is the basal unit of the sedimentary succession in the Upper Benue Trough (Fig. 1). It was named by Falconer (1911) and described by Carter et al. (1963), whose work was the basis for subsequent reviews e.g. Guiraud (1990), Zaborski et al. (1997), Zaborski (1998, 2003), Braide (1992a, 1992b). Cant and Walker (1978) and Miall (1978) regarded braided rivers as devoid of overbank deposits. However, works by Bentham et al. (1993); Bristow et al. (1999); Sabaou et al. (2005) and Hajek and Wolinsky, (2012) have questioned these early braided river’s models which were based purely on non-aggrading systems. The aim of this work is to critically look at the sedimentology of the Bima Sandstone in light of the more recent literature on fluvial deposits and also to provide preliminary stable carbon isotope data in order to test for possible excursion or otherwise. 2.0 GEOLOGICAL SETTING The Bima Sandstone uncomfortably overlies the crystalline Basement Complex throughout the Upper Benue Trough (Fig. 1). In most places it represents by far the greatest proportion of the lithostratigraphic succession present in the Upper Benue Trough (Zaborski, 1998). The type section of the formation is to the south in the Lamurde anticline (Zaborski et al., 1997). The Bima Sandstone was named by Falconer (1911). Carter et al. (1963) studied the formation and established the type section at the Bollori (correct name “Bollere”) section and its type locality at the Bima Hill. Carter et al (1963) divided the Bima Sandstone into three members principally based on physical sedimentary structures, mainly the paucity and/or abundance of 2

cross-beddings. The Lower Bima Sandstone Member, the base of which is not exposed in the type section (Carter et al. 1963), consists of coarse-grained, feldspathic sandstones with occasional pebble horizons alternating with purple and reddish clays and shales. Calcareous sandstone, well-bedded, medium grained sandstone and thin siltstone also occur within this sequence. The Middle Bima Sandstone Member consists of very coarse-grained, feldspathic sandstones, thin clays, shales, calcareous sandstone and impure limestone with numerous bivalves. The Upper Bima Sandstone Member, consists almost wholly of whitish, cream, white-grey to buff coloured, sandstone.

medium- to coarse- and very coarse- grained feldspathic

The sandstones are commonly cross-bedded and frequently contain scattered

pebbles and occasionally layers of rounded quartz pebbles. Allix (1983) and Guiraud (1990) also recognized three subdivisions from the Bollere section and the Zambuk Ridge respectively. Allix (1983) referred to these subdivisions as formations, whereas Guiraud (1990) called them members. However, the thicknesses of the subdivisions reported by all the workers differ (Table 1). Guiraud (1990) reviewed the sedimentology of the Bima Sandstone, mostly from the Zambuk Ridge and reported that the Bima 1 is a lithologically variable reaching up to 1,500 m thick and with a basin- wide angular unconformity at its upper boundary. The Bima 2, 50-200m thick was described as gravels to coarse- grained sandstones d o min at ed by large scale trough cross-bedding. Bima 3 is homogeneous, medium- to finegrained sandstone with oblique tabular cross-bedding. 3.0 METHODS The type section of the Bima Sandstone (i.e., the Bollere River section) and several other sections in the Upper Benue Trough (Fig. 1) were logged in the present study from exposures in

3

gullies along rivers, road cuts, and hills. Lithological profiles (Figs. 2) were constructed using CorelDraw X5 software, and described. Sedimentological attributes measured in the field include bed thickness, textures, structures, paleocurrent direction, colour, and their field relationships. Samples were collected from the type section for stable carbon isotope analysis. The type section is the longest section of the formation and exposes both the Lower and the Upper Bima Members (as proposed here, 5.0). Whole rock samples (mudstone and sandstone) were oven dried (30°C, 24 hours), crushed using a granite pestle and mortar, and de-carbonated following Domingo et al. (2009) using excess hydrochloric acid (10% v/v) until any visible sign of reaction had ceased. This was followed by repeated washing with deionised water until a neutral solution was obtained, then oven drying (30°C, 24 hours). Stable carbo n isotope analyses were conducted at the Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Keyworth- Nottingham, United Kingdom. Total organic carbon content was measured using a Carlo Erba 1500 elemental analyser with acetanilide used as the calibration standard. Replicate analyses indicated a precision of ±0.1% in wellmixed samples (1 Standard Deviation, SD). For δ13C analysis a Carlo Erba 1500 EA online to a VG Triple Trap was used. This setup also included a secondary cryogenic trap in the mass spectrometer for samples with very low carbon content. The mean standard deviation on replicate δ13C analyses of laboratory standard broccoli (BROC1) and soil (SOILB) was between 0.1 and 0.4‰. From here on the carbon isotope composition of the total organic carbon within the samples is referred to as δ13CTOC. 4.0 LITHOSTRATIGRAPHY 4.1 Nonconformity between Basement Complex Rocks and the Lower Bima Sandstone 4

Member The contact between the Pan African Basement and the Early Cretaceous Lower Bima Sandstone Member (as proposed in section 5.0) is exposed at a small outcrop about 4km to Bahai village on Kwaya Kusar – Mulla road. The contact is defined by poorly sorted massive sandstone that contains highly angular and unimbricated gravels (Fig. 3A) and abundant concretions. The size of the gravels ranges from 3cm to 10cm along their long axes. This bed is overlaid by kaolinitic tabular cross-bedded sandstone which seems similar to some beds of the Bima Sandstone west of Dadin Kowa town. The whole section is about 2.2m thick. The nonconformity surface at the Bima Hill section is overlaid by gray sandstone and purple mudstone. The exposed and weathered (at the contact) rock of the Basement is up to 5m thick and contains calcite 4.2 Bollere River Lithosection The lithological characteristics of the Bima Sandstone at the Bollere River section are defined by two m a j o r d i s t i n c t i v e depositional sequences, each subdivided in to subsequences. Sequence one is about 62m thick (Figs. 2 & 4A). S u b s e q u e n c e o n e of this sequence shows a coarsening and thickening upward t rend (Fig. 4A) and is typified by purple and greenish mudstone lithofacies (Fig. 3B), fine-grained, tabular cross-bedded sandstone lithofacies, massive sandstone lithofacies and a medium- to coarse- grained, trough crossbedded sandstone lithofacies (Fig. 4). The upper most lithofacies of this subsequence contains pebbles less than 15mm in diameter and mud intercalations. This depositional package is followed by the deposition of very coarse grained sandstone (subsequence two), which internally contains parallel laminations.

Characteristically,

the sandstones

of this

subsequence contain sub-rounded to rounded pebbles and cobbles less than 70mm in diameter

5

(Fig. 3C) that are imbricated in NE direction of transport, few of the grains are dispersed from this trend. Along the southern flank of the Lamurde anticline, t he sequence one contains thick purple mudstone and poorly sorted massive tabular sandstone. Towards the west, along this flank of the anticline, thin beds of fine- to medium- grained, well sorted, trough cross-bedded sandstone overlies the purple and greenish mudstone. Most of these sandstone beds have been diagenetinetically altered by quartz cementation. The thickness of sequence two of the Bima Sandstone at the Bollere River section is about 1050m (Figs. 2 & 4A) and is made up of two depositional s u b s e q u e n c e s with subtle differences. S u bs e q u e nc e o ne (692.5m thick) is made up of white, coarse, tabular crossbedded sandstone lithofacies (Fig. 4A). Contacts between beds of this unit are erosional and in some cases are lined up by pebbles (20mm in diameter or less) lag deposits (Fig. 5A). At the lower part of this subsequence, the beds are indurated and physically unsorted. Sets of crossbedding are homogeneous and of the lambda type (Miall, 1990). A bed of white silty mudstone has been observed in this subsequence (Fig. 5B). The second subsequence (357m thick) is an upward fining, and is composed of white, very fine-, fine- to coarse-grained sandstone with tabular cross-bedding; massive and deformed cross-beddings (Fig. 5C). Measurements o f the deformed cross-beddings indicate NE displacement of the foresets. At an outcrop after the Yolde Bridge on the Gombe-Yola road, the Bima Sandstone also contains soft-sediments deformation structures. Pebbles and cobbles that are common in the subsequence one are completely absent in subsequence two. Contacts between beds are erosional and sharp. 4.3 Bima Hill Lithosection The Bima Hill section (Fig. 1) is located on low level terrain at the south-eastern side of the

6

Bima Hill. Over 250m of sediments of the Bima Sandstone (Figs. 2 & 4B) are exposed by the Bima drag fold in five subsequences (SI-SV, Figs. 2 section 2 & 4B). Three lithofacies were identified, these are: mudstone, massive sandstone, and trough cross-bedded sandstone (Table 2). Mudstone lithofacies comprises of purple and greenish mudstone, at the lower part of the section (SI), this lithofacies is characterized by desiccation cracks, which were filled with pedogenic calcite (Fig. 3D); the desiccation cracks are discordant and concordant to the bedding planes and their sizes ranged from 0.5cm to 2cm. At S I V ( F ig s. 2 se ct io n 2 & 4 B) , the beds of this lithofacies occur and alternate with very coarse to medium grained trough cross-bedded sandstone lithofacies. Most beds of the mudstone lithofacies contain sand and pebble size grains and are termed sandy mudstone. Beds thicknesses are variable; the thickest bed measured is up to 5m; laterally they can be traced for a distance of over 1.5km. Many of the beds of this lithofacies have been weathered. The massive sandstone lithofacies (Fig. 4B, SI) represents a structure less sandstone. It is gray to white in colour and up to 2.6m thick. A bed of this lithofacies at the lower p a r t o f S I displays elliptical soft-sediment deformation structure (Fig 3D). The trough cross-bedded sandstone lithofacies o c cur s in S I I , S I I I a nd S I V su bs eq ue nc e s. It is subdivided into two sub-facies; pebbly trough cross-bedded sandstone and muddy trough cross–bedded sandstone. The former has pebbles 40mm or less in diameter whereas the later contains mud. The pebbles are imbricated in a NE direction and in some instances lined the bases of trough foresets. This sub-facies contains sub-rounded to rounded concretions with a thin ferruginised layer lined their outer wall separating them from the sediments of the host beds. The muddy trough cross-bedded sandstone sub-facies is very coarse- to coarse- grained with very high content of mud. It is gray-pink to gray-red in colour. Set thicknesses within both sub-facies

7

are on the order of 10cm-50cm. Some of the sandstone beds, especially towards the lower part of the section contain diagenetic calcite (Fig. 3E). 4.4 Bahai lithosection Sediments of the Bima Sandstone, about 29m thick, are exposed at an outcrop, three kilometres after Bahai Village along Kwaya Kusar – Mulla road (Fig. 6A). This lithosection comprises of four

different

lithofacies:

purple

to d a r k

mudstone lithofacies; massive

sandstone lithofacies; parallel laminated sandstone lithofacies; and trough cross-bedded sandstone lithofacies. The mudstone is purple to dark gray at the lower part of the section but becomes dark gray (Fig. 6B) towards the top, with abundant desiccation cracks which were filled with the deposits of the overlying beds. The massive sandstone lithofacies is gray–white and has some load structures. The size of the load structures ranged from 15cm to 40cm in diameter and 30cm in depth. The structures occur in rows w it h a distance of 10cm to 40cm between two neighbouring rows. Thin ferruginous layer separates the in-filled of these structures and the parent material. This lithofacies also contains slickensides surfaces with films of water marks. Contacts between beds of this lithosection are irregular and erosional (Fig. 6B). The sandstone lithofacies are fine to very coarse and are very poorly sorted. Paleocurrent measurement from imbricated pebbles in the massive sandstone lithofacies points to a SE sediments transport direction. 4.5 Wuyo I Lithosection The Wuyo I lithosection was logged along a stream channel located on northern side of Biu-Wuyo

road

at

Wuyo

village.

The section exposes a total thickness of 16.20m

sediments of the Bima Sandstone in two subsequences (Fig. 2 section 4). S ubsequence one begins with fine grained massive sandstone lithofacies, this is overlaid by a coarse- to very

8

coarse grained parallel laminated sandstone lithofacies and a purple to reddish mudstone lithofacies (Fig. 7A). Subsequence two has coarse- to very coarse grained trough crossbedded sandstone lithofacies, coarse to very coarse massive sandstone lithofacies, parallel laminated sandstone lithofacies and remnants of a sedimentary breccia lithofacies. The later lithofacies contains boulders of K-feldspar; granite; pegmatites; and mud and sand size grains. Out sized clasts of the boulders ranged from 35cm to 88cm in diameter. The sandstones are poorly sorted. Contacts between beds are erosional. A m o n g the sandstone beds, the contacts are characterized by imbricated pebbles and mud lags. The contact between the mudstone of subsequence o ne and the overlying sandstone of the subsequence t wo is characterized by load structures of various dimensions (Fig. 7A). 4. 6 Wuyo II Lithosection The Bima Sandstone at Wuyo II lithosection which is about 1.2km west of Wuyo I section is made up o f t wo sequences. Sequence one (Fig. 2 sect ion 5) comprises of coarsegrained trough cross-bedded sandstone lithofacies; coarse-grained tabular cross-bedded s a n d s t o n e lithofacies; ferruginous paleosol lithofacies; coarse grained massive sandstone lithofacies; dark gray mudstone lithofacies. Contacts between beds are typified by deposition of mud lags along erosional contact surfaces (Fig. 7B). The sizes of most of the mud lags are estimated to range between 1m to 1.5m in length, and 30cm to 40cm in height. The ferruginous paleosol lithofacies is composed of 13cm, dark brown ferruginised sandstone layer (Fig. 7C). Off the section and towards the east, where the sandstone beds overlying the ferruginised layer had wedged out, the bed of the paleosol has been eroded and the fragments formed part of the overlying mudstone lithofacies (Fig. 7D) which

itself is

presently undergoing pedogenesis. The mudstone lithofacies is dark gray in colour but

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where it is exposed along with the ferruginous paleosol it changes to dark red colour due to pedogenesis. The mudstone lithofacies is overlaid by ver y t hick deposit s of braid bar complex (Fig. 8A) on a conformable surface and this constitutes sequence two (Fig. 2 section 5). Architectural analysis of this bar complex is presented in Fig. 8B. The bar is oriented in SWNE direction. The lower contact of the bar is a six order bounding surface while the upper contact is a convex firth order bounding surface (Miall, 1989). Architectural elements are both DA and LA and contained either convex or concave first and second order or third order bounding surfaces. Texturally, the deposit fines upwards along the section. West wards this bar pinched out and the deposits of the Bima Sandstone is characterized by series of smaller braid bars with total thickness of 12.3m and contained different types and sizes of bed forms. 4.7 Kaltungo Fault Lithosection The Kaltungo Fault lithosection starts with 5-6m thick dark shale (Fig. 9A & B ) in sequence one (Fig. 2 s e c t i o n 6 ), this shale overlies the Basement of the Kaltungo inlier. The shale contains ho r izo n of nodular carbonate c lo se to its lower contact wit h the Basement Complex rocks. This lithofacies is overlaid by well sorted fine- to medium-grained parallel laminated sandstone followed by interbedded sandstone and greenish mudstone; trough crossbedded sandstone and laminated shale. This interbedded unit is capped by 30cm thick carbonate layer (Fig. 9C & D) followed by 80cm layer of dark gray shale with rounded nodular carbonate horizon, these nodules ranged in size from 36cm x 26cm to 62cm x 29cm. The shale is also capped by a 5cm thick carbonate layer. Fine to medium grained sandstone with mesh-like microfractures overlies the thin carbonate layer. Outer surfaces of the carbonates are fibrous and black but internally they are massive and white. The lower beds of

10

this sequence dip and p inc he d o ut west wards (Fig. 9 A). S equ e nc e t wo ( F ig . 2 sect io n

6)

w ho l l y

co mpr is e d

of

co ar se

gr a i ned

t abu lar

cr o ss- be d d e d

sa nd st o ne w it h a bu nd a nt pet r if ie d wo o d fr ag me nt s. 4.8 Jauro Bello, Hawa, Gamajigo Lithosections In a quarry 1km to Jauro Bello village along Biu-Guyuk road, 2.79m thick sediments of the Lower Bima Member are exposed, and comprises of purple and greenish mudstones (Fig. 9E); buffed to pinkish trough cross-bedded sandstone, with sets ranging in thickness from 2.5cm to 12cm; well sorted medium to coarse grained, small scale trough cross-bedded sandstone and fine grained trough cross-bedded rippled sandstone. The sandstones are highly indurated and contain biotite flakes. At Hawa, 1km to Murke on Yola-Mubi road high way, the Bima Sandstone is made up of very coarse-grained trough cross- bedded sandstone and purple to greenish mudstone lithofacies. Some beds of the sandstone lithofacies had their primary sedimentary structures completely deformed making the sand paper mottled (Fig. 9 F ). In the Gamajigo area, the Bima Sandstone comprises of well sorted fine- to mediumgrained sandstone with large scale tabular cross-beddings and trough cross-beddings, this lithofacies is overlaid by purple to gray mudstone and at the upper part of the section is a sedimentary breccias lithofacies. Below the section there

seems

to

be

a

weathered

mudstone that may probably be similar to the shale of the Kaltungo Fault. 4.9 Tula, Lakwaime Lithosections The area covered by Ture, Awak, Tula, and Balanga to the north and Lakwaime, Pobauli, Dogon Dutsi to the south has excellent exposures of the Bima Sandstone and therefore is a good laboratory for the study of the unit. The section at Tula is 22.5m thick (Fig. 2 section 8) and explicitly exposes vertically stacked bar deposits (Fig. 1 0 A). These braid bars are made

11

up of tabular cross-bedded sandstone deposited by bedforms of different sizes. Sets sizes vary among and within bars, as a whole they ranged from 12 cm to 1.4 m thick. A foreset within the tabular cross-bedded lithofacies has been deformed to overturned soft-sediment structure (Fig. 10B). First- and second- order bounding surfaces are slightly inclined. All fourth order bounding surfaces along this section are erosional and sharp. Texturally, this lithofacies is medium to coarse grained and light brown in colour. The lower part of this lithosection c o mp r i s e s o f greenish-gray mudstone with patches of purple red colouration and parallel laminated sandstone lithofacies. The first bed of this lithofacies is interbedded with the mudstone and thin layers of ferruginised sandstone. The Bima Sandstone is well exposed east of Lakwaime village. The section is 12.91m thick, and comprises of two lithofacies: planar cross-bedded sandstone and rippled sandstone. The tabular cross-bedded sandstone is fine- to medium-grained sandstone, this soft, white, light brown and light pink sand formed the entire section, and the rippled sandstone only occurs as capping of the section. The cross-beddings are both large-scale (Fig. 10C) and small-scale with a horizon that contains water escape soft-sediments deformation structures. The rippled sandstone is defined by asymmetrical ripples that are internally planar cross–bedded, fine to medium texture. The ripples have wavelengths ranging from 11cm to 48cm, a width of 20cm and vertical depths of 3cm to 4cm. Another important characteristic of this lithofacies is the presence of ladder ripples. This second order structures have widths that hardly exceed 10cm but with well-developed stoss and lee sides. . 4.10 The Dogon Dutsi I Lithosection The Dogon Dutsi I lithostratigraphic section is located about 250m north east of Dogon Dutsi village which is about 5km to Bambam on the Gombe – Yola road. The section is 37.4m thick;

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and comprises of tabular cross-bedded sandstone lithofacies (Figs. 2 & 10D), rippled sandstone, massive sandstone, and convolute laminated sandstone lithofacies (Fig. 10E). The tabular cross-bedded lithofacies contains white and light brown, very fine-, fine- and coarsegrained sandstones; some of the beds have fresh K- feldspar grains. Tabular crossbeddings ranged from small-scale to very large-scale. A bed of this lithofacies has reactivation surface with third order bounding surface. First and second order bounding surfaces are erosional and inclined while fourth order bounding surfaces are sharp. Some sets of the tabular cross-beddings have been deformed (Fig. 10F). The uppermost part of the section is dominated by current ripples and massive sandstones. 4.11 Dogon Dutsi II and Pobauli Lithosection The Dogon Dutsi II lithostratigraphic sect io n is located less than a kilometre north of Dogon Dutsi village at a road cut along the Gombe-Yola road. The section is 15.35m thick (Fig. 2). Four different lithofacies make up this lithosection: a fine grained, white, massive sandstone lithofacies; fine- to medium-to coarse-grained sandstone, with white and light brown coarse-grained planar cross-bedded sandstone lithofacies; rippled, fine-grained sandstone/siltstone lithofacies; and greenish-gray mudstone lithofacies (Fig. 10G). Some beds of the planar cross-bedded lithofacies contain fresh K-feldspar. Sets within the planar crossbedded lithofacies are grouped, and are texturally homogeneous. Characteristically, third order bounding surfaces of all the lithofacies are sharp. The mudstone bed thins and wedges out northwards and thickened southwards. East of Pobauli village on the Gombe –Yola road, an exposure of the Bima Sandstone is made up of 8.5m thick sediments comprising of white, small-scale planar cross–bedded sandstone capped by sandy and pebbly greenish-gray mudstone lithofacies, the latter is overlaid by white and light brown, small- and large-scale

13

homogeneous planar cross-bedded sandstone lithofacies. 4.12 Gire (MAUTECH) and Sangere Lithosection The Bima Sandstone is also well exposed at Gire and Sangere near Yola, here the lithology comprised of well sorted coarse-grained sandstone with both large-scale trough and tabular cross-beddings (Fig.10H), the trough cross-bedded sandstone is the dominant lithofacies and constitutes part of braid bars macroforms deposits. The bars range in thickness from 1m to 7m. From the same area, Braide (1992a) also reported large-scale trough cross-bedding in the Upper Bima Sandstone Member. The maximum trough score measured at an outcrop at MAUTECH, Yola is around 1.4m wavelength and 6.3m in width. This lithosection has vertically stacked braid bars deposits with bedforms of the lower bars exhibiting large-scale trough cross- bedding. The braid bars have convex upper bounding surfaces. The upper bars of the section have planar cross-beddings which are capped by current ripples as in Wuyo II, Dogon Dutsi I and Tula lithosections. 4.13 Result of Stable Carbon Isotope Analysis

The results of the carbon isotope analysis are presented in Table 1. Sample numbers BL2 to BR6 are from the Lower Bima Sandstone Member (as presented in section 5.0) and BR7 to BL15 are from the Upper Bima Sandstone. The entire sample set shows very low total organic carbon content with δ13C values exhibiting a very narrow range of values from -23.9‰ to -27.7‰ (excluding BL12), with an average value of -26.1 ±0.9‰.

5 .0 DISCUSSION The works of Carter, et al. (1963); Allix (1983); Guiraud, (1990); Zaborski, (1998, 2003) among others formed the basis of this research. These workers, as earlier observed,

14

subdivided the Bima Sandstone into three members in error. Its sedimentological descriptions were conflicting; e.g. Zaborski (1998) observed that on the regional scale the subdivisions of Allix (1983) and Guiraud (1990) of the Bima Group do not correspond. The angular unconformity at the upper boundary of the Lower Bima Member (sensu Carter et al., 1963; Guiraud, 1990) as contained in Guiraud (1990) was not observed in all the sections and outcrops visited and was not reported by any other author working on the Bima Sandstone and also, neither did any of these authors demonstrated a section exposing the upper and lower boundary of the Lower and Middle Bima Members respectively and the Upper and Lower boundary of the Middle and Upper Bima Members respectively. The impure limestone with numerous indeterminate bivalves documented by Carter et al. (1963) from the type section of the formation was not reported by any other author and was not observed in this work. Another problem associated with the three-member model of the Bima Sandstone, is the inconsistent and variables thicknesses (Table 1) measured from the Bollere River section by Carter et al (1963) and Allix (1983), and those estimated by Guiraud (1990). Pictorial models of the three members were presented by Guiraud (1990) in his Figs 2, 3, 4 & 7. All these figures are at variance with regards to the three member model. For example the cross section in his Fig. 7 constructed from Fig. 2 along several localities would have exposed rocks of the basement, volcanic and the conglo merates of the Lower Bima Member but the figure shows contrary; his Figs. 3 & 4 are hypothetical because they are not tied to any location. According to Guiraud (1990), pedogenesis occurred in all the sediments of the Lower and Middle Bima Members (Fig. 4A & B of Guiraud, 1990), this we believed is not realistic and this trend has not been observed by other previous workers and us. Another misinterpretation of the Bima Sandstone was presented by

15

Braid (1992a, 1992b); he introduced deltaic depositional model in to purely continental braided river deposits of the Upper Bima Sandstone from sections anomalously presented; the sections were from far different places but to surprise all the section are almost of equal height (100m), how can depositional agents, uplift, and erosion operate at the same rate in far different locations. Scaling up or down of sedimentary units is a normal practice in sedimentology, this is normally motivated by improved knowledge in sedimentology and attendant focus attention on sedimentary basin for economic exploitation. Due to the deficiencies and discrepancies in the three-member model highlighted above, a two-member model is here proposed. This model is purely based on lithofacies associations that can lead to better paleoenvironmental interpretations. The boundary separating the two members proposed here is well exposed in many exposures of the Bima Sandstone in both Gongola and Yola Arms of the Upper Benue Trough. The Lower and Upper Bima Members of Carter et al (1963) and Guiraud (1990) have been retained as the names of the two members proposed here. It is believed here that there are no such lithological characteristics that warrant a member status to the Middle Bima Sandstone Member and should therefore be expunged from the nomenclature of the Bima Sandstone. Each of the two members can be identified by its lithofacies association which is defined based on lithofacies. The lit ho facies association of the Lower Bima Sandstone Member (Fig. 2, Table 3) is defined by the following lithofacies: purple and greenish mudstone; trough cross-bedded sandstone; tabular cross-bedded sandstone; parallel laminated sandstone; massive sandstone; shale; sedimentary breccias, ferrugineous,

and carbonate (calcrete)

paleosols. Two or more of these lithofacies may make up a lithosection. 16

The Upper Bima Sandstone Member has simple lithofacies association (Fig. 2, table 4) dominantly of tabular/planar cross-bedded sandstone; other minor facies are trough crossbedded sandstone; massive sandstone; parallel laminated sandstone; greenish mudstone and rippled sandstone. The lithological differences between the two members are perhaps a reflection of the sediments source. Ubiquitous mudstone both as individual beds and as minor component in sandstone in the Lower Bima Member coupled with early basinal tectonics may have had significant control on channels geometries and flow patterns. On the other hand the Upper Bima Member was deposited in more stable paleochannels with reworked sediments forming braid bars and some minor channel deposits. 5.1 Characterization of the Boundary between Lower and Upper Bima Members The boundary between the Lower and the Upper Bima Members is exposed at the Bollere River section; Wuyo II section; Kaltungo Fault section and the Bima Hill section. At Bollere River section (Fig. 2 section 1, Fig 11), the Lower Bima Sandstone Member is characterized by mudstone, trough pebbly

cross-bedded

sandstone,

tabular

cross-bedded

sandstone

and

and g r a v e l l y sandstone lithofacies. The later lithofacies topped the Lower

Bima Sandstone Member, though the unexposed surface overlying this lithofacies may be mudstone lithofacies that has been weathered. The Upper Bima Sandstone Member at the Bollere River section is made up of tabular cross-bedded sandstone lithofacies with some few massive sandstone beds and a bed of white silty mudstone lithofacies towards the upper part. The lower boundary of this member is defined by the appearance of tabular cross-bedded sandstone that lies on a basaltic plug. At the Bima Hill, the type locality of the Bima Sandstone, the boundary between the Lower and the Upper Bima Sandstone Members is

17

exposed at the northern tip of the hill on the western side of Gwani-Shinga road about 300m after Wade-Shinga junction. Here purple and greenish mudstone marks the upper part of the Lower Bima Sandstone Member (Fig. 12A) and the lower boundary of the Upper Bima Sandstone Member is marked by the appearance of tabular cross-bedded sandstone of braid bar origin. The boundary between the two members at the Wuyo11 section which is about 28km east of Bima Hill is marked by dark gray mudstone (Fig. 12B). At the upper part of the Lower Bima Sandstone Member (Fig. 2, section 5 sequence one), this mudstone is overlaid by thick braid bar deposits of the Upper Bima Sandstone Member (Fig. 2, section 5 sequence two,). The Lower Bima Sandstone Member at the Kaltungo Fault section is composed of dark fissile shale, greenish gray shale (Fig. 9A), parallel laminated sandstone, trough cross-bedded sandstone and carbonate lithofacies. The Upper Bima Sandstone Member is wholly composed of tabular cross-bedded sandstone lithofacies. The two members of the Bima Sandstone exhibit different appearances topographically. The Lower Bima Sandstone Member occurs in low label terrains e.g. the core of the Lamurde Anticline, the south eastern side of the Bima Hill and the area flanking the north western side of the Guburunde Horst. The Upper Bima Sandstone Member occurs as hills, as observed at the Bima Hill, Tula Hill and hills around Ture.

The Lower and Upper Bima Sandstone Members are separated by sharp but conformable boundary at the Bima Hill; Wuyo II and Kaltungo Fault lithosection (Fig. 12A; B & C), A disconformity surface occurs very close to the boundary at the Wuyo II lithosection (Fig. 3C & D). This surface is defined by ferruginised sandstone. About 30m thick sediments have been eroded at the boundary in the Bollere lithosction but the two members can easily be 18

spotted out by their distinctive textural and structural properties. δ13CTOC chemostratigraphy (Fig. 11) along this lithosection shows a carbon isotope values ranging from -23.9‰ to -27.7‰ (excluding BL12), with an average value of -26.1 ±0.9‰. The lack of any significant carbon isotope excursion along the Bollere River lithosection suggests that climate, especially changes in aridity, was not a major contributor in the lithological and lithostratigraphic differences between the two members of the Bima Sandstone. Despite the small nature of the magnitude of the Carbon Isotope Excursion (CIE) -3.8‰ seen in the Bollere River section, there was an abrupt perturbation preceding the conglomerate of the Lower Bima Sandstone Member and gradual/transient recovery which continued to oscillate within the sandy body of the Upper Bima Sandstone Member.

Based on the two members proposed here in, a new stratigraphic successio n is presented in figure 13 for the Bima Sandstone and the Upper Benue Trough at large.

5.2 Lower Bima Sandstone Member The Lower Bima Sandstone Member (Fig. 2) is exposed in both Gongola and Yola arms of the Upper Benue Trough. In both sub basins it occurs at low level topography except a few hills e.g. the Bahai lithosection that may not be more than 30m high. The cropping out of the Lower Bima Sandstone Member on low level terrains is attributed to its mud and shale beds that made the unit to be highly ductile and could not be seriously fractured and uplifted during the Santonian tectonic episode. The Lower Bima Sandstone Member represents a very important depositional unit in the sedimentary succession of the Upper Benue Trough; however its sedimentological a t t r i b u t e s have been misunderstood and this has led to wrong interpretations of its depositional processes and paleoenvironment by previous authors (e. g.

19

Carter et al., 1963; Allix, 1983; Guiraud, 1990; Braide, 1992a, 1992b; Zaborski, 1998, 2003; and Samaila, 2012). Principally, these authors gave little recognition to the presence of mudstones in the Lower Bima Member (sensu Carter et al. 1963 and Guiraud, 1990). They also underestimated its volume, lateral extent, and vertical stacking in association with other lithofacies. Guiraud (1990) described the deposits of the Lower Bima Member as alluvial, spatially restricted to regions along fractures bordering the Early Cretaceous basins. It is obvious from our study that the Lower Bima Sandstone Member is laterally wide and its lithofacies have been observed in lithosections far away from the three Early Cretaceous basins (Guiraud, 1990). At Shani, braid alluvial deposits of the Lower Bima Sandstone Member are exposed and represent channel belt deposits. Laterally, these channel belt deposit is up to 1.5km wide, pointing to the fact that the alluvial landscape in which the Lower Bima Sandstone Member was deposited may be large enough to form laterally extensive floodplain deposits during aggradations regime. The present

day semi- arid

fan, the

Okavango alluvial fan in Botswana, has a surface area in excess of 20,000km2 (Mccarthy and Ellery, 1995). The present data interprets the purple and greenish mudstones in the Lower Bima Sandstone Member as floodplain fines. Associated with this mudstone deposits are avulsive channels deposits, crevasse splay deposits and channel belt deposits. In the Kaltungo Fault lithosection, the Lower Bima Sandstone Member is characterized by lacustrine deposits. 5.2.1 Floodplain Mudstone Deposits Most studies of lithofacies developed within modern braided rivers have concerned themselves with channel deposits, while associated overbank or vertical accretion deposits have been largely neglected (Bentham et al., 1993). These earlier studies of braided rivers by Douglas

20

(1962); William and Rust (1969); Miall (1977); and Cant and Walker (1978) have completely rejected the presence of floodplain fine deposits in the braided river’s landscape. Bristow et al (1999) and Bentham et al. (1993) regarded these models as purely based on non aggradational rivers with less over bank fines preservation potential. Like the Escanilla Formation of the Spanish Pyrenees, described by Bentham et al. (1993), the Early Cretaceous Bima Sandstone. Member has also defied these earlier braided river models. The Lower Bima Sandstone Member represents a classical ancient braided alluvial deposit with well-preserved floodplain fine deposits (Figs. 2; 3B, 3D, 3F; 6A & B; 7A; 9E; 12A; 12B,). The floodplain fine deposits ar e expressed by the purple and greenish mudstone lithofacies; and dark gray mudstone lithofacies with variable occurrences in the lithosections comprising the unit. In the Bima Hill and Bahai lithosections, this floodplain deposits are characterized by numerous desiccation cracks that are filled with pedogenic calcite (Fig. 3D) and ferruginous sands of the overlying beds (Fig. 6B) respectively. The floodplain mudstone containing the pedogenic calcite and desiccation cracks in the two lithosections could be interpreted as calcisol and vertisol respectively (Kraus, 1999; Paik and Il Lee, 1998) developed in arid to semi-arid climatic conditions with scarce vegetation. Mack (1991) suggested that the presence of calcic soil horizons in the Lower Cretaceous paleosols, New Mexico, points to a precipitation that was probably less than 60cm/year, whereas the absence of pedogenic carbonate in the Upper Cretaceous climate

experiencing

more

paleosols

is consistent

with a

than 60cm/year precipitation. The time available for paleosol

development was the time between successive avulsion deposits (Kraus, 2002). In the rest of the lithosections, this unconfined flow (North and Davidson, 2012) deposits lacks pedogenic features. This difference could possibly be due to local variations in sediments supply and/or

21

local subsidence rate. In the Bima Hill lithosection, the calcisol was formed close to the nonconformity surface (Fig. 2), at the time when the basin had started receiving sediments; subsidence rate probably became slower giving adequate room for the formation of the pedogenic carbonates. The sandy purple mudstone lithofacies at the southern flank of the Bollere River and the Bima Hill lithosections (Table 3) is interpreted as a product of advection flow (Pizzuto et al, 2008). This interpretation is based on the presence of various grain sizes in the sandy mudstone lithofacies. Elsewhere this lithofacies results from turbulent eddies diffusion. 5.2.2 Crevasse Splay Deposits Crevasse splay deposit in the Lower Bima Sandstone Member was only observed at the Jauro Bello ( Fig. 9E). Here, the splay sandstone beds overlie the overbank floodplain fine deposits and are overlaid by tabular cross-bedded sandstone of the Upper Bima Sandstone Member. This section represents terminal distal fan. Crevassing is a common in d i s t a l fan a r e a s

phenomenon

because of its associated low gradients, low depositional energy,

death and creation of channels. The crevasse splay interpretation is based on the sharp base of the crevasse sandstone beds and occurrence of ripple-size bedforms. This indicates active bedforms formation as contained in Bristow et al. (1999); relatively thin beds; and presence of biotite grain flakes which are very light and remained in suspension until they reach the distal portion and were deposited. 5.2.3 Avulsive Channel Deposits Channel migration commonly occurs through avulsion, which refers to the process by which a new channel belt is established through the diversion of discharge and sediments from an existing channel to a floodplain (Buehler, et al., 2011).

22

Avulsive channel deposit is an

important component of the Lower Bima Sandstone Member floodplain deposits. In the Bima Hill (Fig. 4B subsequence I and IV and Fig. 3F) and Bahai lithosection (Fig. 6A), the avulsive channels may constitutes over 40% of the floodplain deposits and are vertically stacked with the overbank fines and intermittently associated with channelized deposits, this indicates high sedimentation rate and high rate of avulsion frequency. Most of the avulsive channel deposits es pe c ia l l y at the Bahai and Bima Hill lithosections are structureless and display lower erosive contacts. The occurrence of large flood events, a consequence of irregular precipitation regime characteristics of dry land is sufficient to gradually scour new anabranches in to floodplain. This can occur incrementally over a great number of flood events by the slow expansion of the selected floodplain-surface channels until they become large enough to capture bankfull flows (North et al. 2007). Martin and Turner (1998) and North and Davidson (2012) interpreted floodplain massive sandstone deposits with erosive base as product of hyper-concentrated flow. High sediment concentration results in dampening of turbulence and preclusion of traction carpets, consequently inhibiting dune bedforms development, this results in deposition occurring directly from suspension. Deposits of massive-type sand formed in this way are prone to the development of secondary water escape structures because of their loose primary packing (Lowe, 1982). The water film marks in the massive sandstone at the Bahai section indicates that movement has occurred before the sediments lost its water contents. In the Gamajigo the trough cross-bedded sandstone could be a product of local avulsion process controlled mainly by local sedimentation rate (Heller and Paola, 1996). Some of the beds of the massive sandstone lithofacies have pebbles size component and Lowe (1982) interpreted that coarse particles within high-density flow were probably maintained in

23

suspension by the combined effect of turbulence, buoyant support and dispersive pressure. In subsequence IV of the Bima Hill and the lower part of the Bollere lithosections (Figs. 2, 4A & B) the avulsive channel deposits, internally contained trough and tabular cross-beds reflecting segregated bed load dominated flow conditions which generated bedforms in relatively shallow

channels.

In subsequences I and IV of the Bima Hill lithosection, the avulsive

channels deposits are thick relative to the overbank fines (Figs. 4B, 3F). Repeated avulsion of channels across a common stratigraphic horizon

has t h e c a p a c i t y

to r e w o r k e d

previous deposits, thereby preserving only s m a l l proportions of overbank fine sediments (Cain and Mountney, 2009). Overbank fines and avulsive channel deposits occur in both proximal and distal reaches of the braided alluvial basin of the Lower Bima Sandstone Member, the lower part of the Lower Bima Sandstone Member at the Bollere River lithofacies exposes sediments of distal alluvial paleoenvironment. Approximation to a distal alluvial paleoenvironment is governed by the prevalence of coarsening and thickening sequences of trough cross-bedded sandstone lithofacies (Heyward, 1978), the complete absence of ripple marks, and localization

of large scale planar cross-bedding, Vanderneut and Caijaghan

(1991) used these parameters to identify flood deposits in lower fan (distal fan) setting of Wigeriver Formation, Waterberg Group. Another indicator is the thick mud beds relative to channelized sheet sand deposits. Distal alluvial fan channels are normally shallow due to decrease in gradient of depositional slope (Nilsen, 1993). Sheet flood (unconfined flow, sensu North and Davidson, 2012) and fairly well-sorted sand in alluvial setting is also characteristic of distal plain (Dike, 2000). Most of the avulsive channel sandstone are coarse grained, the condition necessary to deposit coarse-grained sediment onto floodplain are most easily achieved in settings where fluvial discharge is highly variable (Alexander and Fielding,

24

2006). Some of the avulsive channel sandstone deposits in the Bima Hill lithosection also contain carbonates (Fig. 3E). The Carbonates occur as sheets along bedding planes. Presence of carbonate sheets along stratification planes suggests that the carbonates are inorganically formed from meteoric waters travelling preferentially along such planes (Netterberg, 1969). Similar carbonate in the Olduvai Gorge, Tanzania has been referred to as “tufa‟ by Ashley et al. (2010) and they interpreted it to be formed from freshwater sources in semi-arid paleoenvironment. At the Bima Hill, the upper bed in f igur e 3D have been deformed into elliptical pillows and fluidization channels similar to those reported by Gilbert, et al (2011) from Tecopa Basin, California. 5.2.4 Channelized Alluvial Deposits Overlying the distal floodplain deposit at the Bollere River lithosection is a small-scale cycle of prograding proximal alluvial deposit which is typified by horizontal bedding and pebbles imbrications (Fig. 3C). This lithofacies is interpreted to be a product of lateral accretion on longitudinal bars that formed in shallow channels of a proximal braided river system. It also indicates change in flow direction and flow stage and strength. The deposits perhaps resulted from a high energy non-turbulent bed load traction currents with flow velocities probably ranging from 2-3cm-1 to 20-30cm-1 based on Hjulstron (1939) diagram. The Gamajigo lithosection and the sequence two of Wuyo I are also progradational alluvial fan sequences. Both sequences are capped by sedimentary breccias lithofacies and this shows an outward migration of proximal facies over distal facies (Nilsen, 1993). Leeder (1982) described progradational sequences in alluvial fan setting as a product of increasing basin margin faulting. The finer fraction of the sedimentary breccias lithofacies has been washed away leaving scattered coarser fraction composed of quartz, granite and silicified wood 25

fragments. The composition of this lithofacies depicts a setting resulting from dry climatic condition in which weathering process do not run to break down source rock completely. In the Bima Hill (subsequences II, III and V), Bahai, Wuyo I, Wuyo II and Hawa lithosections, the Lower Bima Sandstone Member has well preserved channelized deposits. These channel deposits are characterized by prominent erosional bases and pebbles and/or mud lag deposits. The channel deposits at the Bima Hill; Wuyo I; Wuyo II and Hawa are cross– bedded, deposited by high energy bed load traction carpet which had the strength to rolled down pebbles and mud lumps in a spectrum of segregated flows. Some of the channelized deposits at Hawa have been deformed into mottled sandstone soft-sediment structure (Fig. 6D). The softsediment structures in the Lower Bima Sandstone Member were perhaps as a result of seismic shock generated probably when the basin was pundergoing through tectonic evolution. Also in the Wuyo I lithosection the basal part of channelized deposits contain load structures (Fig. 7A). This is interpreted to result from gravitational displacement due to reverse density between the underlying mud and the sandstone. The mudstone may have contained some large desiccation cracks which accelerated the vertical displacement of some portions of the sand. 5.2.5 Lacustrine Deposits The Lower Bima Sandstone Member at the Kaltungo Fault lithosection is interpreted to represent a shallow lake deposits formed at the eastern margin of the outcropping Basement rocks of the Kaltungo Inlier. The control of the Basement rocks on the evolution of the lake can be seen in the dipping of the lake sediments‟ beds westwards (Fig. 9A). The lake margin appeared to have suffered significant erosion during its depositional history; eroding completely the first phase of the lake margin sediments before the sedimentation of the second 26

phase. This was probably triggered by local uplift of the sediments source or subsidence of the western margin of the lake. The lake was probably formed in an arid to semi-arid climatic condition as demonstrated by the presence of lacustrine carbonate horizons. The shale lithofacies ( F ig. 9 A) occurs at the centre of the lake and was deposited by suspension fallout. No marine fossil was recovered from this lithofacies and this favoured the continental interpretation of the lake. The clastic component of the lake margin comprises of parallel laminated sandstone and cross-bedded sandstone. The first two lithofacies were formed by bed load currents in migrating longitudinal sinuous-crested and straight-crested dune bedforms respectively.

The later was formed by hyperconcentrated high density currents. The

carbonates lithofacies ( F ig , 9 C & D) is inferred to have precipitated out of solution during flow fluctuations. The source material for the Lower Bima Sandstone Member contained a lot of carbonate which dissolved and the depositional fluid was supersaturated with respect to carbonate and therefore when the depositional fluid was ponded the carbonate precipitated in form of nodules and layers. 5.3 Upper Bima Sandstone Member The Upper Bima Sandstone Member (Sequence one, Fig 2) overlies the Lower Bima Sandstone Member (Figs. 2, 4A, 12A; B; & C). Characteristically this member is found on the hilly terrains. The textural homogeneity of the Upper Bima Sandstone Member as described by Carter et al. (1963) and Guiraud (1990) is quite obvious in all the lithosections and outcrops (Bollere River; Bima Hill; Ture town near Kaltungo; Tula; Balanga; Sangere and Gire near Yola town) of the Member. Characteristically, the sediments of the Upper Bima Sandstone Member were deposited as microforms or macroforms with little presence of channelized and overbank fines; this is contrary to the Lower Bima Sandstone Member.

27

Formation of microforms and macroforms in this member is a manifestation of segregated turbulent flow conditions. Greater percent of the sedimentation in the Upper Bima Sandstone Member had occurred on these bedforms while the small percentage observed only at the lower part of the member at the Bollere River lithosection is a channelized deposit. Analysis of the sedimentation pattern in a large braided river by Bristow (1987) indicates that channel fill is a relatively minor component of braided river deposits (15%) and that the dominant component is accretion onto bars (53%) (Bristow, 1993b). 5.3.1 Braid Bar Deposits Braid bar deposits of the Upper Bima Sandstone Member are well exposed at the Wuyo II, Tula, Lakwaime, Dogon Dutsi I, Dogon Dutsi II, Sangere and Gire, lithosections. These bedforms are vertically stacked. Stacking of bedforms is a characteristic feature of many ancient braided alluvial rivers. A typical example is the Cretaceous Atane Formation, Central West Greenland (Jensen and Pedersen, 2010).

The Upper Bima Member was probably

deposited within deep and large braided stream systems (Guiraud, 1990), however, in the Bollere River, the Upper Bima Sandstone was probably deposited in relatively shallow fluvial channels. The braid bars in the Upper Bima Sandstone Member can be comparable to those of the present day Brahmaputra

River in t e r m s of thickness a n d length. Sedimentation

of

the Upper Bima Sandstone Member at Wuyo 11 was accompanied by a relative increase in accommodation space leading to the formation and growth of mid-channel bars (Fig. 2, section 5, sequence two). A compound mid–channel bar (Fig. 8B) measured along the Wuyo II section is about 7-8m thick, indicating paleochannel depth of 14-16m. Miall and Jones (2003) expressed that bankfull depth of channel is undoubtedly greater than the depth indicated by bar height. Bars typically ranged from half to slightly less than bankfull depth,

28

therefore in channel of 12m deep, bars would typically be about 7m high (Bristow, 2001). A large mid-channel sand braid bar of 1.5km long and 12m high has been reported by Best et al (2003) in the Jamuna River of Bangladesh. The compound mid-channel braid bar deposit at Wuyo II grew by vertical accretion through stacking of different types of sandy dunes bedforms, this reflects paleoflow hydrodynamics. The dunes were of variable scales reflecting disequilibrium in flow strength (Bristow, 1993a). The architectural analysis of this bar is shown in figure 8B. The dominant hierarchies

features of sandy bed load rivers are

of repetitive bedforms, ripples and dunes often with more than one scale of

coexisting dunes. Dunes of different sizes and morphologies are superimposed at low depositional angle upon one another due to lag effect as they attempt to equilibrate to changing

discharge

prior

to

emergence (Reading, 1996). The bar started growing by

downstream accretion of 2D dunes with first-order bounding surfaces enclosing at its lower part DA elements. Deposition of the DA elements has occurred at the upper flow regime and was interrupted at two stratigraphic levels by low energy LA elements. The latter has slightly inclined and convex-up upper bounding surface and a third order lower bounding surface. At upstream and downstream ends it changes to DA element. Texturally, the DA and LA elements are coarse grained and grades into one another, this character has also been reported by Miall and Jones 2003) from Hawkesbury Sandstone. The upper part of the bar records periods of low energy discharge, this is reflected by thick upper flow regime plane beds and current ripples. The braided river deposits at the Tula lithosection (Fig. 10A) comprises of four vertically stacked braid bars which are fully preserved indicating depositional system dominated by aggradation and syndepositional generation of accommodation (Skelly, et al, 2003). These

29

bars contained 2D straight crested dunes with tabular cross-bedding, indicating low current velocity (Bristow, 1993b), 2D dune also characterizes the Upper Bima Sandstone Member at Lakwaime, Pobauli, Kaltungo Fault and Dogon Dutsi I and II areas. Planar cross-bedding was hitherto used by Carter et al. (1963) and Guiraud (1990) in identifying the Upper Bima Member and this has been cited by subsequent authors.

Set sizes at Tula generally thin

upward reflecting waning flood condition. At Sangere and Gire areas, the Upper Bima Sandstone Member contained large proportion of large-scale trough cross-bedding formed by 3D sinuous-crested dunes which alternate with 2D dunes. This alternation of 2D and 3D based cross-beddings has also been observed at the southern flank of the Lamurde Anticline close to Bollere village. Rapid changes in sedimentary structures can in part be attributed to flow acceleration and deceleration over bedforms (Bristow, 1993a). Braide (1992a, 1992b) described both the 2D and 3D deposits of the Upper Bima Sandstone Member at Gire as crevasse splay sand within deltaic and proximal/distal alluvial. This interpretation is in error. neglected

the

use

The

author

appears

to

have

of appropriate sedimentological terminologies to describe the 2D and

3D deposits of the Upper Bima Sandstone Member at Gire and elsewhere. The eight lithosections presented by Braide (1992a; 1992b) where from far different locations but are of the same height. The Upper Bima Sandstone Member shows uniform pattern of braided river origin in all its exposures. In the Bollere River lithosection, the greater percent of the macroforms are in centimetre scale with very few scaling to metre(s), the later hardly exceed 4m to 5m. Therefore greater percentage of these bars was deposited in shallow constructional braided channel; this is consistent with Guiraud (1990) interpretation.

30

5.3.2 Ripples and Parallel Laminations Asymmetrical current ripples and parallel laminations capped braid bar sedimentations at the Wuyo II (Figs. 2 & 8A); Dogon Dutsi I; Lakwaime and Tula lithsections (Fig. 2). This waning flood (Collinson and Thompson, 1992; Jensen and Pedersen, 2010) deposits terminates sedimentation of the Upper Bima Sandstone Member in these areas. The ripples contained planar cross-bedding which points that they were formed by migrating straight-crested 2D dune in a very shallow water paleoenvironment. In the Lakwaime lithosection, the current ripples are ferruginised and contained ladder ripples whose paleocurrent are oriented in NE direction and reflects retreating of depositional fluid. 5.4 Soft-sediments Deformation Structures Soft-sediment deformation structures are important component of the Upper Bima Sandstone (Fig.10B, E, F) and therefore deserve a separate paragraph, a lt ho u g h so me soft–sediment d e f o r m a t i o n st r u ct u r e s have been reported here from the Lower Bima Sandstone Member (Figs. 3D, 9F). Soft-sediment deformation is typically driven by stresses that are normally incapable of deforming sedimentary deposits (Owen, et al., 2011). Soft–sediment deformations in the Bima Sandstone have been documented by Carter et al. (1963) and Samaila et al. (2006). The later authors in particular, dwelled on the convolute laminations, cusps, droplets, sand volcanoes and deformed cross- beddings all within the Upper Bima Sandstone Member. T he present study in addition has identified water-escape structures (Fig. 10F). Soft-sediment structures such as convolute lamination, deformed cross- bedding, load structures and water escape structures are common in sands and sandstone (Owen and Moretti, 2011). The interpretation that these soft-sediment structures in the Upper Bima Member were formed by fluidization process by Samaila et al. (2006) is a valid contribution and is upheld as

31

the possible mechanism responsible for the formation of water-escape structures. The control (driving force/mechanism) of this process being episodic syndepositional Mesozoic volcanism of the Jurassic to Albian times, however, may be questioned based on the fact that most of the structures occur at the upper most part of the Upper Bima Sandstone Member which is presumed to have been deposited when the basin was relatively tectonically stable. For example, at the Dogon Dutsi I, the sediments were extruded by a volcanic eruption which formed a peak and displaced the sediments on either side; however, field evidences disclosed that the volcanism had no effect on the water-escape structures (Fig. 10F) and the chaotic deformed cross-beddings respectively. The deformed cross-beds at the Bollere River lithosection are tabular cross-beds and were displaced eastwards (5C), similar trend and morphology is exhibited by those at the Tula lithosection indicating regional control on the deforming mechanism, which was probably internally generated. Owen, et al. (2011) argued that for endogenic triggers, soft-sediment deformation structures should be consistently present in most occurrences of a given facies type. The submission here is that the deformed cross-beddings in the Upper Bima Sandstone Member was generated by lateral shear stress displacement of fluid supersaturated beds triggered and supported by unequal loading of overlying beds. 5.5 Conclusion The Early Cretaceous continental Bima Sandstone was studied based on field data and stable carbon isotope. The result of the field work shows that the Bima Sandstone contained ten distinctive l it ho fa c ie s, viz: mudstone; tabular cross bedded sandstone; trough cross bedded sandstone; massive sandstone; shale; parallel laminated sandstone; ripple laminated sandstone; sedimentary breccias; carbonates (paleosol); and ferruginised sandstone (paleosol). The

32

mudstone ranged in colour from purple; dark and to greenish. The sandstone ranged from very fine to very coarse and conglomeratic and mostly white or brownish. The carbonate lithofacies and the ferruginised layers in the Kaltungo fault and the Bima Hill and Wuyo II represents soil horizons respectively. Critical analysis of these lithofacies indicated that the Bima Sandstone has two major lithofacies associations, and this formed the basis of reclassifying the Bima Sandstone in to two members against the known three members. The Lower Bima Sandstone Member comprises of purple; dark and greenish mudstones; tabular cross bedded sandstone; trough cross bedded sandstone, parallel laminated sandstone; massive sandstone; sedimentary breccias; carbonate paleosol; and ferruginised paleosol lithofacies. While the Upper Bima Sandstone member is predominantly tabular cross bedded with minor components of rippled sandstone; parallel laminated sandstone; greenish mudstone; and massive sandstone lithofacies. Sedimentary processes in the Lower Bima Sandstone Member include suspension fall out; turbulent traction carpet; hyperconcentrated gravity flow and pedogenesis. In the Upper Bima Sandstone Member, turbulent traction carpet was the dominant sedimentary process with suspension fall out and hyperconcentrated gravity flow contributing in the deposition of minor components of greenish mudstone and massive sandstone respectively. The stable isotope chemostratigraphy shows very insignificant δ 13CTOC excursion. 6.0 Acknowledgements The study was funded by Abubakar Tafawa Balewa University, Bauchi and Tertiary Education Training Fund (TetFund), as part of the first author’s Ph D thesis. Our appreciation goes to the Plymouth University for granting the first author a short term research visit under the ERASMUS Programme and the financial assistance enjoyed from the National Centre for

33

Petroleum Research and Development (NCPRD), ATBU, Bauchi. We also thank Usman Abubakar, Muhammad Ahmad Isma’il and Jibrilla Muhammad Tashar Alaja for helping during the field work. The effort of Ian King and Helen Hughes both of SoGGS Plymouth University in coordinating the samples for the carbon isotope study is well acknowledged. Lastly we thank Prof Melanie Leng of the NERC Isotope Geosciences Laboratory, UK. and Sulaiman Ibrahim Musa of Land Surveying Programme, ATBU for his assistance in digitization of the map. 7.0 REFERENCE Alexander, J. and Fielding, C. R. (2006), Coarse-grained floodplain deposits in the seasonal tropics: Towards a better facies model. Journal of Sedimentary Research, 76, 539-556. Allix , P. (1983). EnvironmentMesozoiques de la Partie Nord-orientale du Fosse de la Benoue (Nigeria): Stratigraphie, Sedimentologie, E‟volutiongeodynamique. In: Zaborski, P. M. (1998), A review of the Cretaceous system in Nigeria. Africa. 5 (4), 385-483. Ashley, G. M., Dominguez - Rodrigo, M., Bunon, H. T., Mabulla, A, Z. P., and Baquedano, E. (2010), Sedimentary geology and human origins: A fresh look at Olduvai gorge, Tanzania. Journal of Sedimentary Research, 80, 703 – 709. Avbovbo, A. A., Ayoola, E. O., and Osahon, G. A. (1986), Depositional and structural styles in Chad Basin of NE. Nigeria. AAPG.70 (12): 1787 – 1798. Bentham, P. A., Talling, P. J., and Burbank, D. W. (1993), Braided stream and flood-plain deposition in a rapidly aggrading basin: the Escanilla Formation, Spannish Pyrenees, In: Best, J. L. and Bristow (ed.) Braided Rivers. 177 - 194. Best, J. L., Ashworth, P. J., Bristow, C. S. and Roden, J. (2003).Three-dimensional sedimentary architecture of a large, mid-channel sand braid bar, Jamuna River, Bangladesh. Journal of Sedimentary Research, 73 (4), 516-530. Braide, S. P. (1992a), Tectonic origin of preconsolidation deformation Bima sandstone, Journal of Nigerian Association of Petroleum Geologists, 7 (1) 39 – 45. Braide, S. P. (1992b), Morphology and depositional history of an Aptian-Albian crevasse splay in in the AlbianBima Sandstone, Yola basin, Benue trough, Journal of Nigerian Association of Petroleum Geologists, 7 (1) 39 – 45. 34

Bristow, C. S. (1987), Sedimentology of large braided rivers ancient and modern. Ph D thesis, University of Leeds, UK. Bristow, C. S. (1993a), Sedimentary structures exposed in bar tops in the Brahmaputra River, Bangladesh. In: Best, J. L. and Bristow (ed.) Braided Rivers. 277-289 Bristow, C. S. (1993b), Sedimentology of the Rough Rock: a Carboniferous braided river sheet sandstone in northern England, In: Best, J. L. and Bristow (ed.) Braided Rivers. 191304. Bristow, C. S., Skelly, R. L., and Ethridge, F. G. (1999), Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara R iver , Nebraska: effect of base-level r is e . Sedimentology, 46, 1029 – 1047. Buehler, H. A., Weissman, G. S., Scuder, L. A. and Hartley, A. J. (2011), Spatial and temporal evolution of an avulsion on the Taquari River distributive fluvial system from satellite images analysis. Journal of Sedimentary Research, 81, 630-640. Cain, S. A. and Mountney, N. P. (2009), Spatial and temporal evolution of a terminal fluvial fan system: the Permian Organ Rock Formation, South-east Utah, USA. Sedimentology, 56, 1774-1800. Cant, D. J. and Walker, R. G. (1978), Fluvial processes and facies sequences in the sandybraided South Saakatchewan River, Canada. Sedimentology, 25, 625-648. Carter, J. D., Barber, W. D. F. and Tait, E. A. (1963), The geology of parts of Adamawa, Bauchiand Bornu provinces in north-eastern Nigeria. Geological Survey of Nigeria Bulletin, 30. Collinson, J. D. And Thomson, D. B. (1992), Sedimentary Structures, Allen and Unwin, London, 194. Dike, E. F. C. (2000). Sedimentology: the power behind the heart of petroleum industry, th 15 inaugural series, Abubakar Tafawa Balewa University, Bauchi. Dike, E. F. C.(2002), Sedimentation and tectonic evolution of the Upper Benue Trough Bornu Basin of NE Nigeria, Nigerian mining and geosciences society 38 annual international conference, Port Harcourt, Nigeria.

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Mack, G . H. (1991). Paleosols as indicators of climatic change at early-late Cretaceous boundary, southwestern New Mexico, Journal of sedimentary petrology, 62 (3) 483-494. Martin, C. A. L., and Turner, B. R. (1998), Origins of massive-type sandstones in braided river sytems. Earth Science Reviews 44, 15-38. Mccarthy, T. S. and Ellery, W. N. (1995). Sedimentation on the distal reaches of the Okavango fan, Boswana, and its bearing calcrete and silcrite (Ganister) formation. Journal of Sedimentary Research, A65 (1), 77-90. Miall,A. D. (1977), A review of the braided rivers depositional environment. Earth Science review, 5, 597-664. Miall, A. D. (1978), Lithofacies types and vertical profile models in braided rivers: a summary. In: Bristow, C. S., Skelly, R. L., and Ethridge, F. G. (1999), Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara River, Nebraska: effect of base-level rise. Sedimentology, 46, 1029 – 1047. Miall, A. D. (1989), Architectural elementsn and bounding surfaces in channelized clastic deposits: Notes on comparisons between fluvial and turbidite systems. In: Taira, A. and Masuda, F. (ed), Sedimentary facies in the active plate margin, 3-15. Miall, A. D. (1990). Principles of Sedimentary Basin Analysis, second edition. Springerverlag 668.

Miall, A. D. And Jones, B. G. (2003), Fluvial architecture of the Hawkesbury Sandstone (Triassic), near Sydney, Australia. Journal of Sedimentary Research, 73 (4), 531-545. Netterberg, F. (1969), The interpretation of basic calcrete types. South African Archeological bulletin, 24, 117 122. In: Khadkikar, A. S., Merh, S. S., Malik, J. A., and Chamyal, L. S. (1998). Calcrete in semi arid alluvial system: formative pathways and sinks. Sedimentary Geology, 116, 251-260. Nilsen, T. H. (1993), Alluvial fan deposits, In: Scholle, P. A. and Spearing, D. R. (ed), Sandstone Depositional Environment, AAPG. 31, 50-86. North, C. P., Nanson, G. C. and Fagan, S. D. (2007), Recognition of the sedimentary arcthitecture of drylandanabranching (anostomosing) rivers. Journal of Sedimentary Research, 77 (11), 925-938. North, C. P. And Davidson, S. K. (2012), Unconfined alluvial flow processes: Recognition and interpretation of their deposits; and the significance for paleogeographic reconstruction. Earth Sciences Review, 111, 199 – 223. 37

Owen, G. and Mretti, M. (2011), Identifying triggers for liquefaction induced soft-sediment deformation in sands. Sedimentary Geology, 235, 141-147. Owen, G., Moretti, M., and Alfaro, R. (2011), Recognising triggers for soft-sediment deformation: current understanding and future directions. Sedimentary Geology, 235, 133-140. Paik, I. S. And Il Lee, Y. (1998), Desiccation cracks in verticpaleosols of the Cretaceous Hasandong Formation, Korea: Genesis and paleoenvironmental implication. Sedimentary Geology, 119, 161 – 179. Pizzuto, J. E., Moody, J. A. and Meade, R. H. (2008), anatomy and dynamic of a floodplain, Powder, Montana, U.S.A. Journal of Sedimentary Research, 78, 16-28. Reading, H. G. (1996), Sedimentary Environment: Processes, Facies and Stratigraphy, third edition. Blackwell Science, London. 688. Sabaou, S. Lawtton, D. E., Turner, P., and Pilling, D. (2005), Floodplain deposits and soil classification: The prediction of channel sand distribution within the Triassic ArgiloGresseuxinferieur, Berkine basin Algeria. Journal of Petroleum Geology, 28 (3) 223-239. Samaila, N. K. (2012), Reservoir Potentials of the Upper Bima Sandstone, Upper Benue Trough, N.E. Nigeria, 169pp., LAP LAMBERT Academic Publishing GmbH & Co. KG HeinrichBöcking-Str. 6-8 66121, Saarbrücken, Germany (ISBN 978-3-8465-8916-8).

Samaila, N. K., Abubakar, M. B., Dike, E. F. C., and Obaje, N. G. (2006), Descriptionof softsediment deformation structures in the Cretaceous Bima sandstone from the Yola Arm, Upper Benue Trough Northeast Nigeria. Journal of African Earth Sciences, 44, 66-74.

Skelly, R. L., Bristow, C. S. And Ethridge, F. G. (2003) Architecture of channel belt deposits in aggading shallow sandbed braided river: the lower Niobrara River, northeast Nebraska. Sedimentary Geology, 158, 249-270. In: Jensen, M. A. And Pedersen, G. K. (2010), Architecture of vertically stacked fluvial deposits, Atane formation, Cretaceous Nuussuaq, Central west Greenland. Sedimentology, 57, 1280 – 1314. Vanderneut, M., Ericsson, P. G. andCaijaghan, C. C. (1991), Distal alluvial fan in early Proterozoic red beds of Wilgeriver Formation, Waterberg Group, South Africa. Journal o f African Earth Sciences, 12 (4) 537-547. Williams, P. F. and Rust, B. R. (1969), The sedimentology of a braided river. Journal of Sedimentary

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Petrology, 39, 649-679. In: Bristow, C. S., Skelly, R. L., and Ethridge, F. G. (1999), Crevasse splays from the rapidly aggrading, sand-bed, braided Niobrara River, Nebraska: effect of base-level rise. Sedimentology, 46, 1029 – 1047. Zaborski, P. M (1998), A review of the Cretaceous system in Nigeria. Africa Geosciences Review 5, (4) 385 – 483. Zaborski, P. M (2003), Guide to the Cretaceous system in the upper part of the Upper Benue Trough, NE, Nigeria. African Geosciences Review, 10 (1 &2) 13-32. Zaborski, P. M.; Ugodulunwa, F.; Idornigie, A.; Nnabo, P. and Ibe, K. (1997).Stratigraphy and structure of the Cretaceous Gongola basin NE. Nigeria. Bulletin centre r e c h e r c e s , Elf exploration production, 21 (1), 153-185.

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Fig. 1: Geologic map of the Upper Benue Trough with location of the studied sections. (Modified after Benkhelil, 1986).

40

Fig. 2: Studied lithological sections of the Bima Sandstone (see the lithofacies associations of the two members proposed in 5.0. 1: Bollere river; 2: Bima Hill; 3: Bahai; 4: Wuyo I; 5: Wuy II; 6: Kaltungo fault; 7: Jauro Bello; 8: Tula; 9: Hawa; 10: Dogon dutsi I; 11: Dogon dutsi II; 12: Pobauli; 13: Gire (MAUTECH). See tables 3 and 4 for the lithofacies.

41

B

A

C

D

E

F

Fig. 3: (A) Nonconformity surface (hammer head) between the Pan African Basement Complex and the Upper Cretaceous Lower Bima Sandstone Member, 4km to Bahai village on Kwaya kusar – Mulla road; (B) Purple and greenish mudstone lithofacies (see pencil) overlaid by fine grained tabular cross bedded sandstone; (C) very coarse grained parallel laminated sandstone containing pebbles and cobbles; (D) Purple mudstone and massive, coarse to very coarse, unsorted sandstone. The mudstone contains desiccation cracks filled with diagenetic calcite (red arrow), the lower sandstone bed is highly indurated while the upper sandstone bed displayed elliptical pillows soft - sediment deformation structure (white 42

arrow) and fluidization channels (green arrow), B and C are from the Bollere River lithosection; (E) Calcite crystals in sandstone (red arrow); (F) Vertically stacked beds of floodplain deposits: black arrows indicate floodplain purple mudstone and red arrow shows avulsive channel deposits, Lower Bima Sandstone Member, Bima Hill lithosection. D, E and F are from Bima Hill lithosection

Fig. 4: Lithological sections of the Bollere River (A) and Bima Hill (B) showing sequences and subsequences of the two members proposed in section 5.0.

43

A

B

C

Fig. 5: (A) Pebbles lag deposits at bedding planes, (B) Silty mudstone, (C) deformed cross-beddings. A & B are from subsequence one of sequence two while (C) is from subsequence two. Bollere River lithosection (Fig. 2 section 1 & Fig. 4A).

44

A

B

Fig. 6: (A) An out crop of the Lower Bima Sandstone Member at the Bahai lithosection, the mudstone and the sandstone beds are interbedded (man for scale); (B) close view of the dark gray mudstone, yellowish patches are desiccation cracks filled with the material of the overlying massive sandstone bed, the sandstone has lower erosional contact.

45

A

B

1m

D

C

Fig. 7: (A) Purple mudstone with pillow load structures (see pen) in the Lower Bima Sandstone Member at the Wuyo I lithosection, purple colouration in the load structure indicates that it has rolled down and moulded across the mudstone for a considerable distance before sinking down in the mudstone bed. (B) Mud lag deposit at the lower contact of tabular cross bedded sandstone. (C & D) Ferruginous paleosol (Disconformity surface), hand and marker for scale respectively) near the boundary between the Lower and the Upper Bima Sandstone Members, Wuyo II lithosection.

46

A

B

Fig. 8: (A) A compound braid bar deposit at the Wuyo II lithosection, (B) Architectural analysis of the braid bar. DA: downstream accretion elements; LA: lateral accretion elements.

47

A

C

E

B

D

F

Fig. 9: (A) An out crop lithosection of the lacustrine unit of the Lower Bima Sandstone Member, the shale and the overlying sandstone are rapidly dipping west wards, arrow shows erosional contact between two phases of lake margin sedimentation; (B) Is a close view of the shale (men for scale); (C) A sequence of dark and greenish shale, very fine sandstone, and carbonate and red arrow shows a carbonate nodule; (D) Close view of the carbonate horizon. A-D are from Kaltungo Fault lithosection. (E) Purple and greenish mudstone overlaid by well sorted medium to coarse grained, trough cross bedded sandstone in a 48

quarry near Jauro Bello, Jauro Bello lithosection. (F) Mottled soft – sediment deformation structure, Hawa lithosecion.

49

A

B

C

D

E

F

G

H

Fig. 10: (A) Vertically stacked braid bars deposits at (Tula section). (B) Defor med tabular cr ossbeds (Tula section). (C & D) Large scale tabular c r o s s -beds at Lakwaime and Dogon Dutsi I respectively. (E) Convolute laminations (upper beds), Dogon Dutsi I. (F) Deformed tabular cross beds, Dogon Dutsi I. (G) Greenish mudstone lithofacies, Dogon dutsi II. (H) Trough and tabular cross-beds at MAUTECH, Yola. Note the complete change in the orientation (900) of the two bedforms. 50

13

δ CTOC (VPDB ‰ ) -30.0 M

-28.0

-26.0

-24.0

-22.0

-20.0

-18.0

Yolde Formatio n

10 62

8 62

6 62

462

262

30m N E

6 2 57

.

47

. . . . . . . . .

37

2 7

20 80m N E

..

.

.. .

..

10 . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

. .

. .

M

Vf

F

C

Vc

Fig. 11: Lithostratigraphic section and δ13 C chemostratigraphy of the Bima Sandstone, Bollere River lithosection, the green arrow indicates the position of the boundary between the Lower and the Upper Bima Sandstone Members.

51

A

Upper Bima Sandstone Member

Lower Bima Sandstone Member

B

Upper Bima Sandstone Member

Lower Bima Sandstone Member

C

Upper Bima Sandstone Member member

Lower Bima Sandstone member

Fig. 12: The boundary between the Lower and the Upper Bima Sandstone Members; (A) Bima Hill, about 300m after Wade-Shinga junction, white arrow shows the greenish and purple mudstone characteristics of 52

the Lower Bima Sandstone Member, the solid black line indicates the possible position of the boundary; (B) Wuyo II lithosection, the boundary is indicated by the red arrow, the braid bar deposits of the Upper Bima Sandstone Member thickened east wards; (C) Kaltungo Fault lithsection, the red arrow shows the greenish shale marking the upper most Lower Bima Sandstone Member.

Fig.13: Lithostratigraphic succession of the Upper Benue Trough, with a new stratigraphic subdivision of the Bima Sandstone (Modified after Samaila et al., 2006)

53

T a b le 1 : T h i c k n es s o f t h e me mb er s of t h e B i ma S a n d s t o n e a s p r es en t ed b y t h e p r ev i ou s a u t h or s a n d t h e p r es en t w or k. *r e a d i n gs n ot t i ed t o a n y s ect i o n of t h e f or ma t i o n, N n ot s een ,

1,2

m ea s u r ed a t t h e B o l l er e R i v er a n d B i ma H i l l

r es p ect i v el y .

Member Author Carter

Allix

Guiraud

19631

1983

1990*

Upper Bima

1,737m (5,700ft) 1,200m

600m

Middle Bima

823m (2,700ft)

700m

50-200m

Lower Bima

396m (1,300ft)

365m

1,500m

Total

2956m

2265m

2300m

54

Zaborski

Present work

2003* 500m

500m

1050m

N

˃1,500m

62m1, ˃250m2

2500m

1112m

Table 2: δ13CTOC (‰ VPDB) and Carbon (wt%) . Note: BL stands for samples collected in our first field work while BR is for samples collected during second field work.

Sample δ13C Number

%C

BL15

-26.7

0.04

BL14

-25.7

0.04

BL13

-25.5

0.04

BL12

-18.7

0.02

BL11

-26.0

0.06

BL10

-27.4

0.05

BL9

-25.7

0.05

BL8

-26.2

0.07

BL7

-25.5

0.04

BR8

-26.2

0.03

BR7

-26.6

0.04

BR6

-26.1

0.04

BR5

-25.5

0.04

BR3

-27.7

0.05

BL6

-25.8

0.05

BL5

-23.9

0.11

BL4

-26.5

0.05

BL3

-27.1

0.06

BL2

-26.1

0.09

55

Table 3: Lithofacies association of the Lower Bima Sandstone Member, Upper Benue Trough.

56

Table 4: Lithofacies association of the Upper Bima Sandstone Member, Upper Benue Trough.

57

• • • • •

We studied the Bima Sandstone in light of current literature on fluvial deposits. Lithofacies making the formation were clearly identified. Problems in three member model of the Bima Sandstone also identified. Two member models proposed for the Bima Sandstone, Benue Trough. Lower aggradational braided alluvial and upper braid bar deposits.

58