4.1 Lithologic description from outcrop observations.
The four (4) outcrops investigated were logged stratigraphically at 3 points each and described in terms of their lithology types, grain size, colour, fossil content, and sedimentary structures. Outcrops ER, IJ, and OB are road cut exposures while OK is a low-lying outcrop surrounded by dense vegetation (Figures 3 and 4).
Outcrop OB (Figure 4a)
Outcrop OB is covered by sparse vegetation and has an altitude of 53 m above sea level. It is a road cut section with a vertical extent of ca. 8.2 m, length of ca. 30 m and strike in the SW-NE direction. Heights of points OB1, OB2, and OB3 at the outcrop location are 8.2 m, 7.8 m, and 4.8 m (Figures 4a and 6). The basal unit at OB1 is characterized by brownish-yellow sandstone (Figure 5d) and with clasts that are generally rounded and medium- to coarse- grained (Figure 4a, 5a and 6). The clasts become finer upwards as well as the clay content up till about 3.7 m where angular to subangular clasts are observed (Figure 5b). At the top of the section, the color of the sandstone changes to brown grey. The uppermost unit of the outcrop at this point is lateritic (Figure 6). Clayey-sandstone is observed as the basal unit at point OB2, albeit the sandstones are light brown-whitish grey, and with rounded, fine- to medium- grained clasts. The middle/intermediate units are quite like those of OB1 (Figure 6) while the uppermost unit is also reddish-brown laterite (Figure 6). Clayey sandstones with sub-angular clasts were also identified at the base of OB3 (Figure 5c). Here, the clasts are medium- to coarse- grained, white to light brown in color (Figure 5e), and their shape changes from rounded in the middle of the section (Figure 6) to sub-rounded towards the top (Figure 6).
Outcrop IJ (Figure 4c and d)
The outcrop has an altitude of 85 m above sea level and length of ca. 60 m. Heights of the log points IJ1, IJ2, IJ3 are 7.8 m, 6.4 m, and 5.3 m respectively (Figure 4 c, d and 6). The surface of the outcrop is heavily scrapped due to sand quarrying in the area. At point IJ1, the outcrop grades from medium- to coarse-grained, reddish-brown sandstone with sub-angular clasts at the base to coarse to very coarse-grained, greyish-brown sandstone in the middle (Figures 4c and d). At the top of the outcrop at point IJ1, reddish-brown to grey, medium-grained sandstone with sub-rounded clasts are present (Figure 5i). The reddish color is attributed to increasing laterite content from base to top (Figure 5f). The stratigraphy at points IJ2 and IJ3 are quite like those at IJ1 in terms of color and texture. The base of the exposure at IJ2 is characterized by greyish-brown sandstone of medium- to very coarse texture and sub-angular clasts (Figure 6). The sandstone becomes a little clayey at a depth of 1.7 to 4.1 m, while the upper sections show brown, yellow to red-purple color (Figure 6). The sand clasts are very coarse-grained and with inclusion of well-rounded pebbles (Figure 6). The base of the section at IJ3 is made up of coarse-grained sandstone with angular clasts, while the top is characterized by brownish grey, coarse-grained sandstone, with sub-angular clasts and laterite (Figure 5g).
Outcrop ER (Figures 4b)
Outcrop ER has a length of ca. 30 m and a vertical extent of 1.93 m, 1.21 m, and 1.07 m at points ER1, ER2, and ER3, respectively (Figure 7). Additionally, a thin layer comprising cobbles with diameters of 64 mm to 210 mm is observed. This is a paraconformity that is inclined at about 20 to the south (Figure 4b). From base to top, point ER1 consists of grey to yellowish-brown, medium to very coarse-grained sandstones with sub-angular to well-rounded clasts (Figures 5h and 7). They become coarser upward until about 1.5m where the cobble-paraconformity is present. The cobbles are generally ferruginous in nature as indicated by their reddish-brown color. At the uppermost section, the sandstone becomes yellowish-brown and grey but with more sub-angular clasts (Figure 5h). The same stratigraphy is observed at the other two logging points: ER2 and ER3.
Outcrop OK (Figure 4e and f)
Outcrop OK is a low-lying outcrop that extends laterally for ca. 15 m. Heights at log points OK1, OK2 and OK3 are 1.4 m, 1.23 m, 1.55 m, respectively. The strata at the base of the outcrop non-conformably overlie the weathered basement rock. Tabular crossbedding was also observed within the outcrop (Figures 4e, 4f and 7). The basal unit across OK1-OK3 is a weathered basement rock composed of kaolinite (Figure 4e and 4f). Clasts within the weathered unit are weathered feldspathic minerals that are suggestive of a granitic protolith. Above the basement lies grey to red, medium to coarse-grained sandstone with sub-rounded clasts (Figures 4e and 4f). The overlying sandstone is slightly muddy and light brown in colour and with clasts of white pebbles. At about 0.84 m to 1.16 m in the upper part of the section, the sandstone becomes conglomeratic although the texture remains the same as that of the older underlying sandstones (Figure 4e and 4f). The uppermost sandstone unit is brownish to white and coarse-grained with quartz pebble clasts ranging from 15 to 20 mm in diameter. Similar observations were made at points OK2 and OK3, except that the size of the quartz granules at about 0.3 to 0.7 m at OK2 ranges from 2.0 mm to 3.5 mm (Figure 7).
4.2 Sediment characterization and grain size parameters
The grain size ternary plot for the sediments from outcrops IJ, OB, OK, and ER is presented in Figure 8. Sediment samples from ER are classified as sandy gravel, those from OB plotted as slightly gravelly, gravelly sand to sandy gravel, while sediments from outcrops OK and IJ are sandy gravel and gravelly sand. Importantly, the calculated graphical Mean (Mz) ranges from -0.42 to 0.43 for IJ, 0.08 to 1.17 for OB, -0.86 to -0.27 for ER, and 0.22 to 0.84 for OK (Figure 9). Hence, the average grain size is 0.05 for IJ and 0.66 for OB, signifying the predominance of coarse-grained sediments. The average grain sizes of sediments from ER and OK are -0.66 and -0.17 reflecting the predominance of very coarse-grained sediments. Similarly, the standard deviation for outcrops IJ, OB, ER, OK (Figure 10a) ranges from 1.07 to 1.37, 1.27 to 1.52, 0.69 to 1.21, and 0.65 to 1.22 while graphic skewness (Sk) ranges from 0.60 to 0.79 for ER, 0.08 to 0.36 for OB, 0.13 to 0.65 for IJ, and -0.02 to 0.60 for OK (Figure 10b). Most of the samples from OK fall predominantly within the very fine skewed class, with only 4 samples having a nearly symmetrical skewness. The samples from ER are very finely skewed, while those from IJ have fine to very-fine skewness. Samples from OB predominantly fall within the finely skewed class, with 5 samples being nearly symmetrical (Figure 10b). Kurtosis for IJ, OB, ER and OK sediments are from 0.73 to 1.03, 0.73 to 0.99, 0.83 to 2.30, and 0.79 to 1.16, respectively. These values indicate predominantly platykurtic-mesokurtic, predominantly platykurtic–mesokurtic, mesokurtic–very leptokurtic), and predominantly platykurtic distribution, respectively (Figure 10c). Based on the kurtosis, sediments from outcrops OK, IJ, and OB are characterized as being poorly sorted those of outcrop ER are moderately sorted.
In addition to the calculation of the statistical parameters, bivariate plots are shown in Figures 9 to 11b. The plot of skewness versus mean size shows that the Ise Formation samples are symmetrical to very fine skewed, albeit the samples are predominantly very fine skewed (Figure 9). Grain size ranges from medium to very coarse, with the coarse to very coarse sediments dominating the distribution. The environment of deposition corresponding to various classes of correlation of skewness against mean size are plotted in reference to various authors 35–37. Accordingly, the samples plotted in the field of river processes and are classified as massive sandstones (Figure 9). The plots of standard deviation against mean size, and skewness against standard deviation reveal samples from the study area are moderately well sorted to poorly sorted, with the poorly sorted samples dominating the distribution. In terms of depositional setting, these bivariate plots (Figure 10a) show the influence of a riverine environment 35. The sediments are also observed to be dominantly poorly sorted and show a fine to very skewed distribution (Figure 10b). Furthermore, the plots of kurtosis versus skewness, and kurtosis versus mean size show the samples are platykurtic to very leptokurtic but are mainly platykurtic and are symmetrical to very fine skewed. The very finely skewed class dominate the distribution (Figures 10c and 11b). A predominant riverine depositional setting is associated with virtually all the sediments from OK, OB, IJ and a few sediments from ER, while a beach setting is attributed to majority of the sediments from outcrop ER (Figure 10c).
4.3 Provenance of sediments based of grain size analysis
The statistical parameters presented above have different interpretations in relation to provenance or sediment source area. Sorting reflects the distance the sediments have travelled from the source 34. The mean of sediments can indicate the depositional setting 32,34, while skewness and kurtosis can point to the difference in kinetic energy of the transporting medium 31,34. Based on the average mean grain sizes of 0.05 φ for IJ, 0.66 φ for OB (coarse grained), -0.66 φ for ER, and -0.17 for OK (very coarse grained), and the predominately fine and very fine skewness, the analyzed sediments have been transported and deposited under a fluvial depositional setting, with fluctuating energy 33,34. Poorly sorted sediments at outcrops OK, IJ and OB, indicate deposition of the sediments near their source areas 32,34. The dominance of moderately sorted sands, as well as very leptokurtic kurtosis of sediments at outcrop ER, suggests that the ER sediments achieved their sorting in a different high energy environment 31.