PEBBLE MORPHOMETRIC ANALYSIS OF THE BASAL SECTION OF THE AWI FORMATION, CALABAR FLANK, SOUTHEAST NIGERIA

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1 PEBBLE MORPHOMETRIC ANALYSIS OF THE BASAL SECTION OF THE AWI FORMATION, CALABAR FLANK, SOUTHEAST NIGERIA ESSIEN, NSE UDO; ITAM, ASUKWO ESSIEN; OYAMA, ADAMS A, AND OBIM, V.N DEPARTMENT OF GEOLOGY UNIVERSITY OF CALABAR, CALABAR, NIGERIA Abstract:A pebble morphometric study was carried out on 400 pebbles samples retrieved from conglomeritic deposits of the basal section of Awi Formation, Calabar flank, exposed along Calabar- Oban road, Southeastern Nigeria. The three orthogonal dimensions of Long (L), Intermediate (I) and Short(S) axes were measured and the various pebbles parameters computed. The computed pebble morphometric parameters have ranges of values as follow; Flatness Ratio (FR = ), Flatness Index (FI=51% - 67%), Elongation Ratio (ER= ), Maximum Projection Sphericity Index (MPSI= ), and Oblate Prolate Index (OPI = ) these values strongly suggest fluvial deposits. Roundness Index determined based on Sames chart ranges between 43% and 52% (sub angular to subrounded) and inferred short to long intermediate distance of transport from the provenance. The diagnostic forms for pebbles of fluvial origin (Compact, Compact Bladed, Compact Platy and Compact Elongation) of 68% predominate over beach origin (Very Platy, Very Bladed, Bladed and Platy) of 23%. The bivariate plots of FI versus MPSI and MPSI against OPI, agreed with this result. The results from the study area favours continental setting of dominantly fluvial origin. Keywords: Pebble morphometric, Calabar flank, Awi Formation, provenance, fluvial origin. I. INTRODUCTION The recent excavations as a result of the construction and expansion of United Cement Company (Unicem) Southeastern Nigeria (figure 1), exposed good sections of conglomeratic sand/ironstone unit of the basal unit of the Awi Formation. The pebbly basal unit have been studied and subjected to pebble morphometric investigation. The study area has not received any geologic attention due to the fact that it was covered with thick layers of overburden. Pebble morphometric research studies have has shown that pebble form indices or parameters are good indicators of paleodepositional environment ( Sames, 1966; Stratten,1974; Nwajide and Hoque,1982; Els,1988; Inyang and Enang,,2002; Odumodu and Ephraim, 2007a and 2007b ; Odumodu, 2014 ; Itam et al., 2015 and Itam and Ugar, 2016) This study will therefore attempt to use the different unweathered quartz pebbles encountered in the investigated area to interpret the paleodepositional environment of the deposit.the results obtained will therefore be compared with results obtained in other part of the basin previously All Rights Reserved 208

2 Figure 1: Map of the study area showing location of the samples. Location of the Study Area The area of study is located along Calabar - Oban Road (figure 1), Cross River State, Southeastern Nigeria. The geographic position lies within latitude N05 06ʹ 42 ʹʹ to N05 10 ʹ 05ʹʹ and longitude E ʹ 50ʹʹ to E ʹ 57ʹʹ and is part of Calabar Flank basin. II. GEOLOGICAL SETTING The Calabar Flank is an epirogenic sedimentary basin in southeastern Nigeria (Murat, 1972).The basin according to Nyong (1995) is bounded by the Oban Massif in the north with the Calabar hinge line separates the basin from the Niger Delta basin in the south and the Ikpe platform and Cameroon Volcanic Line delineate it in the west and east respectively (figure 2). The origin of this basin is associated with the opening of the South Atlantic in the Cretaceous times when the South American plate drifted away from African plate. The major structural elements within the basin include the Ikang Trough (graben structure) and Ituk High (horst) which were mobile depression and stable mobile submarine ridge that influenced the distribution sedimentary of facies (Murat, 1972 and Nyong, 1995). The stratigraphic succession in the Calabar Flank is shown in Table 1. Sediment thickness is over 3500m with the onlap (or featheredge) of the outcropping units exposed along the fringes of the Oban Massif basement complex. The Formations are best exposed along Calabar Ikom road and a succession consists of five (5) Cretaceous and a Tertiary lithostratigraphic units. Awi Formation is the oldest basal unit and sits nonconformably on the basement complex of Oban Massif. The Formation is Aptian in Age (Adeleye and Fayose, 1978).This is overlain by Mfamosing Limestone of Middle- Upper- Albian age (Petters, 1982) deposited during the first marine transgression in the South Atlantic. This in turn is succeeded by Late Albian- Cenomanian to Turonian, Ekenkpon Shale. Subsidence on the faulted blocks of horst and graben allowed wide spread deposition of shales with minor marl and mudstone intercalation. The New Netim Marl which is Coniacian in age, succeeded the shale unit. The Santonian period was marked by a major unconformity in the Calabar Flank. Nkporo Shale of All Rights Reserved 209

3 Campanian to Early Maastrichtian (Edet and Nyong, 1994) capped marine transgression and Cretaceous sedimentation in Calabar Flank.The Tertiary continental sands and gravel of the Benin Formation completes the sedimentation in the basin (Table 1). Figure 2: Structural map of the Calabar Flank and adjoining areas. (Modified after Nyong and Ramanathan, All Rights Reserved 210

4 Table 1: Lithostratigraphic correlation between Calabar Flank. Abakaliki Trough,Anambra Basin and the Middle Benue Trough (Petters et al;2010) III. Materials and Methods A total of four hundred (400) unweathered quartz pebbles were collected with fifty (50) pebbles each from eight (8) different locations (L1-L8). Pebble morphometric measurement using Vernier caliper after Krumbeins (1941) method was used. The Long (L), Intermediate (I) and Short (S) axes of the pebbles were measured. Pebble indices such as Flatness Ratio (FR) Flatness Index (FI), Elongation Ratio (ER), Maximum Projection Sphericity Index (MPSI) and Oblate Prolate Index (OPI) with the visual estimation of pebble roundness using Roundness Chart of Sames (1966).The results obtained were compared with those recorded in other localities previously studied. IV. RESULTS AND DISCUSSION Field description of the study area The lithologic description of study area consists of conglomeratic sandstone, pebbly sandstone and ironstone (figure 3).The conglomeratic sandstone is reddish brown, clast supported to matrix supported, imbricated conglomerate, with coarse medium, poorly sorted. The lithologic cross section of outcrop sample in location 5 (L5) is about 2.0m thick and made up of pebbly sandstone ( figure 3a).This unit is characterized by reddish brown, dominantly angular to subrounded clast supported at the base to matrix support at the top. The top of this unit is texturally characterized by mudstone units. The sandstone is coarse to medium, with poorly sorted grains. The overall sequence represent fining upward succeession (graded bed). At locations 6-8 (L6-8) the thickness of the outcrop units in local quarry ranges from 3.8m to 4.0m (figure 3b) and consists of pebbly sandstone and a thin bed of reddish brown ironstone. The pebbly sandstone exhibits similar characteristics features as previously All Rights Reserved 211

5 Fluvial - Beach Deposits Fluvial - Channel Deposit International Journal of Recent Trends in Engineering & Research (IJRTER) Thickness Lithology Description Depositional Environment 1.9m Reddish brown, matrix suppoted, poorly sorted, coarse - medium grain pebbly sandstone Thickness Lithology Description 3a Depositional Environment 0.96m Reddish brown coarse - medium grain 1.4m Reddish brown, matrix suppoted, poorly sorted, coarse - medium grain pebbly sandstone 0.4m Reddish brown iron stone Reddish brown,. matrix suppoted, poorly sorted, coarse - medium grain pebbly sandstone Figure 3: 3b Lithostratigraphic cross sections of the mapped area in: (a) location 5, L5 (b) location 6 8, L6 L8 3b Figure 3: Lithologic cross sections of outcrop samples from the study area of: (a) Location 5, L5 (b) Location 8, All Rights Reserved 212

6 Pebble morphometric parameters The average results of the various computed pebbles morphometric parameters and roundness values are presented in Tables 2 and 3 Table 2: Average pebble morphometric parameters from the various study locations. LOCATION L(cm) I(cm) S(cm) FR FI ER L-I/L-S MPSI OP R (%) L1(n=50) L2(n=50) L3 (n=50) L4(n=50) L5 (n=50) L6 (n=50) L7 (n=50) L8 (n=50) OVERALL MEAN Legend: L=Long axis, I =Intermediate axis, S = Short axis, FR =Flatness Ratio, FI= Flatness Index, ER =Elongation Ratio, MPSI =Maximum Projection Sphericity Index, OPI = Oblate Prolate Index, R =Roundness, n = number of samples per location. Table 3: Characteristic features and paleoenvironmental significant of the various computed parameters. Pebble Characteristics Defined limits from Interpretation of morphometric parameters previous studies depositional environment/ Flatness Index (FI) Elongation Ratio (ER) Maximum Projection Sphericity Index ( MPSI) Oblate Prolate Index (OPI) FI consists of over 84% above fluvial limit and 16 % below this limit Over 78% has values between % of the MPSI sampled occurred above the imaginary limit (0.66) Characteristics by 69% of the OPI value plotted above -1.5 limit line and 33% below Roundness( R) 15% has values below 0.35,while 35% has values above 0.45 Form Geometry 21%C, 7.25%E, 24%CB, 15%CE,3.5%P,.75%VB, 0.5%VP,17.25%B, 8.75%CP, VE1% Lutig, 1962 Beach (<45%) Fluvial (> 45%) Hubert,1968 Fluvial( ) Dobkins and Folk, 1970 Beach (< 0.66) Fluvial (> 0.66) Sneed and Folk,1958 Beach ( < -1.5) Fluvial ( > -1.5) Sames,1966 Fluvial(<.35%) Littoral(>0.45%) Sneed and Folk, 1958; Dobkins. and Folk, 1970 and Gale, Fluvial(C,E,CB, CE) Beach (P,VP,VB,B) processses Predominantly fluvial Fluvial processes Predominantly fluvial Dominantly fluvial with few beach influences. Mainly littoral/beach action with little fluvial Dominantly fluvial with few beach influences. The interpretation of these results are carried out in conjunction with other works (Sneed and Folk, 1958 ; Lutig 1962; Sames, 1966; Hubert, 1968 ; Dobkins. and Folk,1970 ; Gale, 1990; Odumodu and Ephraim 2007a and b ; Ogala,et al,.2010; Udo and Mode, 2013; Odumodu,2014; Itam and David,2015; Itam et al., 2015 and Ideozu and Ikoro,2015).The result obtained show that the mean Flatness Ratio (FR) has value of The FI ranges from 50.94% to 67.40% with average mean of 58.46% All Rights Reserved 213

7 FLATNESS INDEX International Journal of Recent Trends in Engineering & Research (IJRTER) fluvial depositional environment (Lutig, 1962). The mean ER for pebbles evaluated is 0.79 which is approximately within the range for fluvial setting (Hubert, 1968 and Itam and Inyang, 2015). The mean value of MPSI in the study area is 0.75 and ranges from All the sampled pebbles have values above the 0.66 limit (Dobkins and Folk, 1970), that separates fluvial from beach pebbles (Tables 2 and 3). These values suggest fluvial depositional environment. The OPI ranges from to 2.48 with a total mean value of These values are greater than -1.5; lower empirical limits (Dobkins and Folk, 1970), which distinguishes beach dominated pebbles from river. This analysis shows the dominance of fluvial influence over beach. From the analysed data the mean roundness values is 47 % (43%-52% implying angular to subroundness).this infer short/intermediate travel distance of the pebble from the provenance. This sediments are likely to be sourced from the weathering of the basement rocks of the adjoining Oban Massif Nigeria and intrabasinal activities (Itam and David, 2015). The results obtained from the pebble morphometric analysis were further analysed using bivariate plots. This was carried to further discriminate the paleoenvironment of deposition. Morphometric plots of FI against MPSI (Stratten, 1974) and MPSI versus OPI (figure 4a and b) of all the pebbles sampled point to predominantly fluvial influence. Also, the bivariate plot of Roundness (R) against Elongation ratio (ER) in figure 4c using the upper limits of 35% and 45% Roundness of pebbles shaped in river and littoral environment of Sames ( 1966 ) and the fluvial limit of Elongation ratio of 0.60 to 0.90 for pebbles influence by fluvial processes (Hubert,1968 ), this infer fluvial beach environment BEACH FIELD FLUVIAL BEACH MPSI All Rights Reserved 214

8 MPSI International Journal of Recent Trends in Engineering & Research (IJRTER) Fluvial Beach OPSI All Rights Reserved 215

9 ROUNDNESS International Journal of Recent Trends in Engineering & Research (IJRTER) Sames % R>45% Hubert, ER limit Sames % R<35% ELONGATION RATIO c Figure 4: Bivariate plots of; (a) FI against MPSI (b) MPSI against OPI (c) R against ER According to (Sneed and Folk, 1958; Dobkins. and Folk,1970 and Gale, 1990 ), Compact (C), Elongation ( E), Compact Bladed (CB) and Compact Elongate (CE) are most indicative of fluvial action whereas Platy( P), Very Platy ( VP),Very Bladed (VB) and Bladed ( B ) are diagnostic of beach setting (C), Elongation ( E), Compact Bladed (CB) and Compact Elongate (CE) are most indicative of fluvial action whereas Platy( P), Very Platy ( VP),Very Bladed (VB) and Bladed ( B ) are diagnostic of beach setting.the result of the analysed pebbles values of 21%C, 7.25%E, 24%CB, 15%CE, 3.5%P, 1.75%VB, 0.5%VP, 17.25%B, 8.75%CP,and 1%VE (figure 5).These values point to predominantly fluvial than beach processes in the shaping of the analysed pebbles in the study area. Comparing this result obtained with those recorded by previous authors (Itam and Inyang, 2015 and Itam et al; 2015) in the distant location of the Awi Formation, the result is in agreement with predominantly fluvial over beach All Rights Reserved 216

10 B 17% VP VB VE 1% 2% 1% E 7% C 21% CP 9% C CP CB CE P 3% CE 15% CB 24% P B E VP VB VE Figure 5: Pie chart showing form names from the study area Legend: C= Compact, CP =Compact Platy, CB = Compact Bladed, CE = Compact Elongate, P=Platy, E= Elongation, VP =Very Platy, VB =Very Bladed, VE= Very Elongate V. CONCLUSION The pebble morphometric parameters have proven to be good indicators for distinguishing paleodepositional environments in situations where fossils are lacking especially in a continental setting such as Awi Formation. The various parameters obtained in the pebble morphometric analysis together, with the different bivariate scatter plots have shown that the depositional environment of the basal of the section of the Awi Formation is predominantly influence by fluvial processes with beach / littoral influence.the results obtained corroborate with the previous work, carried out on elsewhere within the Awi Formation REFERENCES 1. Adeleye, D.R and Fayose, E.A. (1978).Stratigraphy of type section of Awi Formation, Odukpani area, Southeastern Nigeria, Nigeria. Journal of Mining Geology.15: Dobkins, J.E. and Folk, R.L. (1970). Shape development of Tahiti- Nui. Journal of Sedimentary Petrology, 40, pp Edet, J.J and Nyong,.E.E. (1994). Palynostratigraphy of Nkporo Shale 4. exposures (Late Campanian Early Maastrichtian) on the Calabar Flank, 5. Southeastern Nigeria. Paleobotany and Palynology, pp. 80, Gale, J. (1990). The Shape of Beach Gravels. Journal of Sedimentary Petrology,pp Hubert, F.L. (1968). Selection and wear of pebbles on Gravel Beach, University of Groningen, Geological Institute Publication. Number, pp Ideozu, R.U and Ikoro, D.O (2015).Sedimentology of the conglomeratic beds within Odoro Ikpe- Arochukwu Axis: Afikpo Basin, Southeastern Nigeria. International Research Journal of Geology and Mining.Vol 5. (3), pp Inyang, D.O and Enang, E.I (2002).Use of statistical parameters in sedimentological study of conglomerates in Northeastern Niger Delta, Nigeria. Nigerian Journal of Mining and Geology.40 (1), pp Itam, A.E and Inyang D.O (2015). Granulometry and Pebble Morphometry of Awi Sandstone, Calabar Flank Nigeria. International Journal of Engineering and Applied Sciences. Vol 6(4), pp. All Rights Reserved 217

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