Pebble Morphogenesis of the Cretaceous Conglomerates from the Abeokuta Formation near the Oluwa River, Eastern Dahomey Basin, Southwestern Nigeria

Pebbles taken from the bed of the Cretaceous Abeokuta Formation paraconglomerate near the Oluwa River were used to infer the depositional setting and the nature of the source area, through the integration of bivariate and ternary (sphe -ricity-form and Zingg diagrams) analyses. Deposition in a river environment is indicated by the high sphericity values, ranging from 0.59 to 0.88 (average 0.70). Also, bivariate plots of the maximum projection sphericity (ψ p ) vs. oblate-prolate index (OPI) and flatness index (FI) vs. maximum projection sphericity (ψ p ) point to the domination of fluvial processes. Dominantly elongated, compact elongated, compact and compact-bladed pebbles are typical for sedimentary regime with prevalence of fluvial over beach processes. Co-existence of various pebbles shapes (mainly disc, rod-, and sphere-shaped), despite of the similar, predominantly quartz composition, may occur due to the different clast fabrics. This heterogeneity also indicates various transport distances and water energies, pointing to the multiple source areas.


Introduction
The Abeokuta Formation is composed of conglomerates and sandstones, exposed along the Ore-Ode Aye Road in the southwestern part of Nigeria, and also forms the basal stratigraphic unit of the eastern Dahomey (Benin) Basin (Okosun, 1990). It unconformably overlies the Precambrian Basement Complex of Nigeria (Obaje, 2009). The formation is well exposed near the Oluwa River, which is located 6.4 kilometers to the north of Ode-Aye (see Figure 1).
Several authors have carried out sedimentological studies on the Abeokuta Formation (Nton, 2001;Okosun, 1990; Ikhane et al., 2013;Madukwe, 2016), but they did not apply the pebble morphometric approach, a method previously successfully applied in some other research areas in Nigeria (Nwajide and Hoque, 1982;Olugbenro and Nwajide, 1997 The aim of this study is to apply the pebble morphometric analyses to interpret the nature of the source area and the depositional environment of Abeokuta conglomerates exposed near the Oluwa River.

Geology of the area
The Abeokuta and Araromi Formations are two lithostratigraphic formations that make up the Cretaceous sequence of the eastern Dahomey Embayment (Figure 2; Okosun, 1990). The Abeokuta Formation is mostly composed of unconsolidated sands with grey shale, mudstone, silt, and clay intercalations (Okosun, 1990;Obaje, 2009). These deposits are dated from the late Albian to the Santonian (Okosun, 1990). Dark grey and black shales with sandstone, limestone, marl, siltstone, and glauconite shale interbeds make up the Araromi Formation (Obaje, 2009).
The Paleocene Imo Shale/Ewekoro Formation, the Eocene Oshosun and Akinbo formations, the Oligocene-Miocene Ilaro Formation, and the Miocene-Quaternary Benin Formation are all Paleogene-Neogene deposits of the eastern Dahomey (Benin) Basin (Figure 2; Ogbe, 1972;Olabode, 2006). The Imo Shale is made up of laminated and calcareous grey, dark grey, and black shale with occasional white to brown sands (Okosun and Alkali, 2012), while the Ewekoro Formation consists of limestone with thin bands of sand, shale, and marl (Okosun and Alkali, 2012).
Flaggy grey and black shales, as well as clay, make up the Akinbo Formation (Ogbe, 1972 separated by glauconitic rock bands and phosphatic beds (Enu and Adegoke, 1988). Phosphate-bearing, greenishgrey or beige clay and shale with interbeds of sandstone make up the Oshosun Formation, which unconformably overlies the Akinbo Formation (Okosun, 1998).
The Ilaro Formation is composed of large, yellowish, weakly cemented fine to coarse cross-bedded sandstones, clays, and shales with occasional thin bands of phosphate beds that overlie the Oshosun Formation (Madukwe et al., 2016). The Benin Formation consists of cross-bedded, poorly sorted sandstones with estuarine to continental characteristics (Kogbe, 1974;Nton et al., 2006).

Methods
The outcrop description and logging, sampling and data analyses are discussed below.

Outcrop description and logging
The geographic coordinates of the studied outcrop are 06 0 38'49''N and longitude 004 0 45'16''E, near the Oluwa River, southwestern part of Nigeria (see Figures 1, 2  and 3). The outcrop section is underlain by the Precambrian Basement Complex rock, upon which deposition of basal conglomerate and sandstone occurred (see Figure 3). The Precambrian Basement Complex rock is highly weathered (see Figure 3), and unconformably overlain by 0.20 -1.00 meters of basal conglomerate. The basal conglomerate comprises weakly imbricated quartz pebbles held together by a clayey and/or sandy matrix. Weathered, coarse-grained, poorly sorted sandstone overlying the paraconglomerate is 0.80 -1.2 meters thick.

Sampling
An outcrop section exposed near the Oluwa River was carefully examined and described. One hundred and sixty-eight (168) unbroken pebble size clasts were randomly picked from the exposed section. The study involved measurement of the orthogonal axes of the pebbles which includes the long (D L ), the intermediate (D I ) and the short axes (D S ). The clast diameter size (φ) and the roundness (R) of the pebbles were also measured. Data from the measurement of these axes (D L , D I , and D S ) were used to compute the form indices, namely the coefficient of flatness (CF), the elongation ratio (ER), the maximum projection sphericity (ψ p ), and the oblateprolate index (OPI) of each pebble (see Tables 1 to 5).
Where: D L represents the long axis of selected pebbles, D S represents the short axis of selected pebbles.
Where: D L represents the long axis of selected pebbles, D I represents the intermediate axis of selected pebbles.  The maximum projection sphericity (ψp) of the pebbles describes particle behaviour and is defined mathematically as the ratio between the maximum projection area of a sphere with the same volume as the particle (Boggs, 1987). The maximum projection sphericity (ψ p ), according to Sneed and Folk (1958), was defined using Equation 3 and was calculated for each selected pebble. maximum projection sphericity Where: D L represents the long axis of selected pebbles, D I represents the intermediate axis of selected pebbles, D S represents the short axis of selected pebbles.

oblate-prolate Index (OPI) =
(4) Where: D L represents the long axis of selected pebbles, D I represents the intermediate axis of selected pebbles, D S represents the short axis of selected pebbles. The Equation 5 was applied in calculating the flatness index (FI) of each selected pebble (Illenberger, 1991) flatness index (FI) = Where: D L represents the long axis of selected pebbles, D I represents the intermediate axis of selected pebbles, D S represents the short axis of selected pebbles. The calculated values obtained from Equations 1, 2, 3, and 4 were used to construct bivariate and ternary plots. The pebble roundness (R) was estimated using the visual charts of Powers (1953). Pebble forms were thereafter plotted using Tri-plot version 1.3 (Graham and Midgley, 2000). The Zingg (1935) classification plot was used to categorize the pebbles into descriptive shape fields defined by Sneed and Folk (1958).

Results
The clast diameter size (φ) of the pebbles generally ranges from -3.5 to 5.5 and the average roundness (R) value for the pebbles is 39.96 (see Tables 1 to 5). The results of pebble morphometric analysis (see Tables 1 to 5) indicate that the oblate-prolate values of the studied pebbles range from -1.96 to 12.15 with a mean value of 0.98 (see Tables 1 to 5). Over 90% of the pebbles have oblateprolate index (OPI) values greater than -1.5, suggesting a fluvial environment (Dobkins and Folk, 1970). The pebbles' elongation ratio ranges from 0.69-0.85. The maxi-mum projection sphericity (ψ p ) of the pebbles obtained from the exposed outcrop varies from 0.59 to 0.88, with a mean value of 0.70. The percentage of pebbles with maximum projection sphericity (ψ p ) values greater than 0.65 is 92%, indicating a fluvial setting. The flatness index (FI) values range from 68.18 to 96.00, with a mean FI value of 77.50. All the measured pebbles have flatness index (FI) values greater than 45% (see Tables 1 to 5), suggesting deposition by fluvial-influenced processes.

s/n D L D I D S D S /D L D I /D L D S /D L D L -D I D L -D S OPI
The bivariate plots of flatness index (FI) vs. maximum projection sphericity (ψ p ) (see Figure 4) and maximum projection sphericity (ψ p ) vs. oblate-prolate index (OPI) (see Figure 5) revealed that most of the pebbles are located in the fluvial field.

Depositional environment
The roundness (R) of pebbles is a function of transport and tends to increase downstream from rivers to beaches (Sneed and Folk, 1958;Lutig 1962 (Humbert, 1968;Pettijohn, 1975). This means that the roundness (R) value suggests deposition under a fluvial-influenced process (Okoro et al. 2012;Anyiam et al., 2017). The plot of roundness (R) vs. elongation ratio (ER) for the Abeokuta Formation pebbles (see Figure 6) shows that the bulk of the pebbles occur in the fluvial field (Sames, 1966 ; Figure 6). Most of the measured pebbles are plotted in the fluvial field, indicating the dominance of fluvial processes, and the occurrence of some proportion of beach pebbles suggests that the river reached the marginal marine environment during its flow (Figure 6; Madi and Ndlazi, 2020). The integration of the pebble morphometric data and bivariate plots reflects a fluvial depositional environment for pebbles of the Cretaceous Abeokuta Formation.

Character of the source area
The majority of the measured pebbles plotted on the bivariate plot of Zingg (1935), fall on the disc, spheroid, and rod fields (see Figure 7). Spheres and rollers are transported more readily than blades and discs having the same mass, thus owing to preferential transport of spheres and rollers, downstream changes in pebble form may occur in rivers (Boggs, 1987). Homogeneous rocks (massive limestones, dolomites, quartzites, or marbles) form sphere and disc pebble shapes (Barudžija et al.,  2020). The rod-shaped pebbles were formed from highly cleaved or schistose rocks (Tucker, 2003). The distribution of pebbles shapes of mainly disc-, rod-, and sphereshaped may reflect different clast fabrics and indicating multiple sources (e.g., Sremac et al., 2018; Barudžija et al., 2020).
The compact (C), compact platy (CP), compact bladed (CB), and compact elongated (CE) pebbles are diagnostic of fluvial environments; platy, bladed, and elongated pebbles are diagnostic of a transitional environment; while very platy, very bladed, and very elongated are diagnostic of marine (beach) environment (Ocheli et al., 2018). The compact, compact platy, compact bladed, and compact elongated pebbles constitute 2%, 0% and   42.26%, respectively. The percentages of points that correspond to platy, bladed, and elongated fields are 0%, 40.48%, and 13.10% respectively, while none of the pebbles fall in very platy, very bladed, and very elongated fields. The plots on the ternary diagram of Sneed and Folk (1958) indicate that the pebbles of the Abeokuta Formation fall on the fluviatile-transitional depositional environment field (see Figure 8).
The present study has shown that the high proportion of fluvial plots indicates the dominance of fluvial processes, and the occurrence of some proportion of beach pebbles suggests that the river reached the marginal marine environment during its flow. This study has shown the reliability of using the pebble morphometric approach in deciphering a depositional environment and source area. The outcome of this study is consistent with the results of previous authors (

Conclusions
The Cretaceous Abeokuta Formation exposed near the Oluwa River consists of paraconglomerate and coarse-grained poorly sorted sandstone. The morphometric analysis of the pebbles from the paraconglomerate strata allows the following conclusions to be made on the source area, transport process, and depositional settings.
1. The mean pebble form indices and average sphericity of 0.70 indicate the deposition under a fluvial setting. 2. The maximum projection sphericity (ψ p ) vs. oblateprolate index (OPI), flatness index (FI) vs. maximum projection sphericity (ψ p ), and coefficient of flatness (CF) vs. roundness (R) are bivariate graphs also indicative of a fluvial environment. 3. Bivariate and ternary plots of pebbles obtained from the bed of the Cretaceous Abeokuta Formation support the deposition in a river system. 4. Most of the pebbles yielded elongated, compact elongated, compact, and compact-bladed forms, indicative of the dominance of fluvial processes over beach actions. 5. The scattered distribution of pebble shapes of mainly disc-, rod-, and sphere-shaped pebbles reflects different clast fabrics, indicating multiple sources.