Field and Sedimentological Studies of Nataco-Banda Sediments of Lokoja Formation, Southern Bida Basin, Nigeria: Implication for Depositional Environment

: The study area falls within the Lokoja Formation of the Southern Bida Basin. The objective of this study was to evaluate the field characteristics and sedimentological parameters of depositional environment of the Nataco-Banda Sediments of Lokoja Formation, Southern Bida Basin, Nigeria using appropriate techniques such as granulometric and pebble morphometric analysis. Field studies revealed a repeated alternating sequence of conglomeratic and pebbly to coarse-grained sandstone and overbank fine sediment that is indicative of a braided fluvial source while the presence of crossbedding is suggestive of tidal influence. The morphometric analysis shows a fluvial setting where pebbles plot in the river area than the beach area. Environmental discrimination formulae for Y1, Y2 and Y3 indicated the dominance of Beach and Shallow agitated marine in both Y1 and Y2, respectively, while the almost equal percentage values of Y3 in both fluvial and shallow marine settings are highly suggestive of a shallow marine environment that is largely influenced by fluvial deposition.

The study of sedimentary rocks and the processes by which they are formed, includes and is related to a large number of phenomena. Sedimentology includes the five fundamental processes defined by the term sediaentation -weathering, erosion, transportation, deposition and diagenesis. Sedimentology shares with geomorphology the study of the surface features of the earth (Wolf and Benedict, 1964). Adeleye (1971 and1974) studied the stratigraphy and sedimentation process of the Upper Cretaceous rocks in the basin. Omali and Imasuen (2011) did a sedimentological characterization of the Lokoja Sandstone while Alege and others (2020) undertook sedimentologic, lithofacies, palynofacies and sequence stratigraphic of the successions within the basin. The study area is located in the Nataco-Banda area of Lokoja, Kogi State (Fig. 1a), where good exposures are accessible through road-cuts, stream channels and mining sites. This study focuses on the field and sedimentological characteristics of the Cretaceous sedimentary rocks of Lokoja Formation in the Southern Bida sub-basin.

MATERIALS AND METHODS
Description of study area: The Southern Bida Basin is a NW-SE trending Upper Cretaceous rocks extending from Kotongora areas to some extent beyond Lokoja in the south (Ojo and Akande, 2009). The area of study is located on the exposures of Banda-Nataco area of Lokoja, Kogi State in the North-Central part of Nigeria within latitudes 07 0 46'0"N to 070 55'0N and longitudes 006 0 42'0'E to 006 0 48'0"E part of sheet Lokoja Sheet 247NW. The area is accessible by major roads and some minor roads. The area was mapped on a scale of 1:25,000 and the areal extent is about 12km2 (Fig.1a, b). The Bida Basin is a northwestern extension of the Anambra Basin . The stratigraphic succession of Bida Basin comprises a Northern Bida Basin and Southern Bida Basin (Fig. 2). The basin fill consists of a northwest trending belt of  (Obaje et al, 2011). The Southern Bida Basin is made up of three formations from oldest to youngest; the Lokoja, Patti and Agbaja Formations. The Lokoja formation is the oldest stratigraphic sequence in the Southern Bida Basin, and is made up of lithologies of conglomerates, coarse to fine-grained sandstones, siltstone, and poorly sorted claystones often cross-stratified and generally compositionally immature. This Formation rests unconformably on the Precambrian Basement Complex.
The Patti Formation consisting of sandstones, and shales usually with bioturbated ironstones, siltstones, and claystones interbeds lying conformably on the Lokoja Formation. The Agbaja Ironstone is the youngest rock sequence overlying the Patti Formation. It forms a cap for the Campanian-Maastrichtian sediments in the Southern subbasin while serving as a lateral equivalent of the Batati Formation on the northern subbasin. Sample Collection, Treatment and Analysis: A total of nineteen (19) composite samples from the Lokoja Formation outcropping in the Felele, Nataco and Banda areas in Lokoja Kogi State were collected and measured to investigate their field characteristics, textural and lithologic variations, as well as their pebble morphometry. Samples were carefully disaggregated with the use of a rubber-padded pestle and mortar. The weight of the selected empty sieves (4.0mm, 3.35mm, 2.0mm, 1.18mm, 600µm, 425µm, 150µm, 75µm, 63µm, and pan) was obtained using weighing balance and placed in sieves arranged in order of decreasing mesh sizes and shaken. The sediments retained in each of the sieves and the bottom pan were weighed and their weight recorded.
Other grain size parameters like mean, sorting, skewness and kurtosis were calculated using the method by Folk and Ward (1957). These were further used as independent functions and sometimes combined in multivariate analysis by Linear Discriminate Function (LDF) to deduce the depositional settings.
A total of two hundred (200) pebbles were collected, washed, dried and the broken pebbles removed. Measurement was done using the vernier calliper after Krumbeins (1941) method. 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) were undertaken with the visual estimation of pebbles roundness using the roundness chart of powers (1953).

RESULTS AND DISCUSSION
Field characteristics: Four locations (A, B, C and D) were mapped in the study area (Fig. 1a) and the outcrops have similar facies characteristics (Fig. 3). The exposure in Location A is composed of seven members. The sandstone in each bed ranges from brown-grey, dark brown, pink, and grey to light brown. It is a matrix-supported conglomeratic sandstone with the alignment of cobbles. The textures are generally coarse to medium grained with cross-beddings interbedded with pebbles. Dark brown, lateritic clay is also present on the overburden.
Location B comprises four beds with matrix-supported conglomeratic sandstone. It ranges in colour from milky to white, brown to grey. The beds are composed of sandy claystone, with an overburden of ironstone. Texture ranges from coarse to medium grain size.
Location C consists of brown to grey, grey to light brown coarse-grained sandy claystone with the alignment of pebbles. The beds consist majorly of matrix-supported conglomeratic sandstone.
Location D beds range from grey to white, pinkish, brownish in colour. The beds are colour-beaded pebbles and capping of brownish ironstone which is brownish and composed mainly of boulders that are angular to subangular.  (Fig. 3). It overlies the Basement Complex non-conformably and occurs as grain supported conglomerate subfacies with colour ranging from brown to grey to light brown. This subfacies is observed in all locations of the study area making sharp contact with the overlying graded matrix supported conglomerate subfacies with the alignment of cobbles. In the studied section from Nataco to Banda, the conglomerates are massive with supporting matrix that grades into poorly sorted muddy sediments towards the upper part of the section.
b. Sandstone facies: The sandstone facies occurs in all four locations of the study area. They are made up of grey, coarse-grained, poorly to moderately sorted, near-symmetrical to positively skewed, platykurtic to mesokurtic, massive sandstone. This facies overlies the basal conglomerate facies (Figure 3) in the upper part of the sections. They occur as massive crossbedded pebbly sandstone subfacies that show repeated upward cycle which begins with conglomerates ( Fig.  2) in which clasts are pebble to cobble-sized and oriented parallel to the bounding surfaces where they make erosive contact with the lower bed.
The result of the pebbles morphometric analysis (Table 2) of the Banda-Nataco exposures of the Lokoja Formation reveals that the sandstone has low sphericity that ranges from angular to subangular.
The pebbles morphometric analysis shows a fluvial setting of the Banda-Nataco exposure of the Lokoja Formation. Furthermore, scatter plots of the calculated form indices such as MPSI and OPI suggest that pebbles are concentrated more in river areas than the beach area, which indicates a fluvial condition for the pebbles.
The roundness is an indication of the extent of abrasion (ability to wear or rub off) determined by the distance of transportation of pebbles rather than the depositional environment. The measured pebbles' roundness results show that 83% of the pebbles vary from angular to subangular, while the remaining percentage plot within rounded to subrounded. This result is indicative of textural immaturity, suggestive of close provenance. The scatter plots of the calculated pebbles ( Figure 5) show the majority of the pebbles plot in the fluvial environment of deposition. The Y1 equation according to Sahu (1964) was used in the discrimination between aeolian and littoral (intertidal zone) environments. The equation suggests that when Y1 is less than -2.7411, it is an Aeolian deposit, and if Y1 is greater than -2.7411, a beach environment. From the result of the linear discriminant function analysis of the Banda-Nataco exposures of the Lokoja Formation (Table 4), all (100%) values of Y1 are greater than -2.7411 (Table 4) indicating a beach environment. According to Sahu (1964), the value of Y2 greater than 63.3650 is indicative of a shallow agitated marine environment is inferred. Thus, a shallow agitated marine environment is suggested since Y2 is 89.47% of values of Y2 (Table 4).
The Y3 equation was used for the discrimination between shallow marine (subtidal) and fluvial deltaic environments. The results of the analysis revealed that 52.63% of the plotted Y3 values are suggestive of a shallow marine environment while 47.37% % has Y3 values less than -7.419 indicative of a fluvial deltaic setting (Fig. 6). By implication, the almost equal percentage values of Y3 in both fluvial and shallow marine setting is suggestive of a shallow marine environment that is largely influenced by fluvial deposition.

Conclusion:
The field associations and sedimentological studies of the Nataco-Banda outcrops of the Lokoja Formation have been studied. Results of the granulometric analysis show a mix of poorly to moderately sorted, positively skewed mesokurtic to platykurtic coarse-grained conglomerate and sandstone facies suggestive of strong to moderate energy of deposition. Pebble morphometric analysis indicates the dominance of the fluvial environment while the linear discriminate analysis suggests a shallow agitated marine environment that is strongly influenced by fluvial deposition (fluvial-deltaic).