DEPOSITIONAL ENVIRONMENT OF THE GOMBE FORMATION IN THE GONGOLA SUB-BASIN OF THE NORTHERN BENUE TROUGH: USING GRAIN SIZE PARAMETERS

The depositional environment of the Gombe Formation was determined using grain size parameters in which sixteen sandstone samples and ninety nine pebbles were subjected to granulometric and pebbles morphometric analysis respectively. The granulometric analysis for the sixteen (16) samples of the Gombe Formation show an average graphic mean of 2.51φ (fine grained sandstone), mean standard deviation of 0.58φ (moderately well sorted sandstone), mean skewness value of 0.09φ (nearly symmetrical) and mean kurtosis value of 0.89φ (platykurtic). The Bivariate plot of standard deviation vs. skewness indicated dominance of fluvial environment. While the probability curves plots showed a dominance of three sand populations indicating influence of marine processes. Environmental discrimination formulae for Y1, Y2 and Y3 indicated dominance of Aeolian, shallow agitated marine environment and shallow marine environment respectively. The plots of Y2 vs.Y1 and Y3 vs. Y2 showed a dominance shallow marine environment. The morphometric analysis indicates both fluvial and beach environment with dominance of fluvial environment.


INTRODUCTION
The Benue Trough trends NE -SW for about 1000 km in length and 150 km width.It formed during early Cretaceous rifting and strike slip movement of central West African Basement (Benkhelil, 1989).Benue Trough is bounded by Niger-Delta Basin at the southern end and by Chad Basin to the North (Fig. 1) (Zaborski 1998).The trough is a sedimentary basin containing up to 6000 m of Cretaceous-Tertiary sediments associated with volcanic.The Benue Trough of Nigeria is part of the West and Central African Rift System (WCARS); formed during separation of the African and South American plates during the Early Cretaceous (Fitton, 1983;Genik, 1992).
In the study area the environment of deposition of Gombe Formation was interpreted as fluvial dominated delta by Carter el al. (1963) and Abubakar (2006).Shetima et al. (2012) regarded the environment of deposition of the formation as marginal marine based on textural and lithologic analysis.Hamidu (2012) regarded red sandstone facies, bedded facies and interbedded facies of Zaborski et al. (1997) as alluvial, beach, shoreface, and sublitoral environment respectively.
The study area is located mainly at Gombe and Environs in the Gongola sub-basin.The purpose of this study is to use pebble morphometric analysis and Sahu (1964) method in addition to granulometric analysis used by Shetima et al. (2012) to interpret the depositional environment of Campano-Maastrichtian Gombe Formation.

GEOLOGICAL SETTING
The Gombe Formation is the youngest Cretaceous lithostratigraphic unit in the N-S trending Gongola sub-basin of the Northern Benue Trough (Popoff et al. 1986 andNwajide 2013).It unconformably overlies Fika Shale and is overlain by Palaeocene Keri-Keri Formation.The lithologic units of the Gombe Formation are divided into three (Zaborski et al. 1997): basal interbedded unit, middle bedded facie and upper red sandstone facies.The basal unit comprise alternating thin beds of silty shalves, with some plant remains and fine to medium grained sandstone intercalated with flaggy ironstones.The middle part consists of regularly horizontally bedded, fine to medium grained quartz arenite, interbedded with Silts, Silty Clays and ironstones while, the upper part of the Formation consists of brick red coloured sandstone.The grain sizes range from pebble to medium grained sandstones with trough, tabular, and planar cross-bedding.The Gombe Formation is regarded as Maastrichtian in age (Carter et al. 1963;Kogbe 1976;Popoff et al. 1986).

METHODS
Sixteen ( 16) sandstones samples and ninety nine pebbles were collected from four outcrop sections of Gombe Formation for granulometric analysis and pebble morphometric study respectively.Samples obtained from Arawa stream, Pantami stream and Mallam Sidi road cut setion were sieved using Folk and Ward (1957) method.The graphical parameters of graphic mean, standard deviation, skewness, and kurtosis were calculated using Folk and Ward (1957) formula.The calculated graphical parameters were used for palaeoenvironment discrimination using the method proposal by Sahu (1964).The following Sahu (1964)  The bivariate plots of Friedman (1979), Sahu (1964) and Visher (1969) sand population were applied to interpret the depositional environments of Gombe Formation.
In pebbles morphometric analysis, the long axis (a), the intermediate axis (b) and the short axis (c) were measured with vernier calliper according to Wentworth (1922) method.The values obtained were used for computing the statistical parameters based on Zingg (1935), Wentworth (1922) and Dobkins and Folks (1970).Axial ration were calculated according to Zingg (1935).Maximum projection sphericity and coefficient of flatness were calculated according to the method of Dobkins and Folks (1970) and Stratten (1974) respectively.

RESULTS
The granulometric analysis results of percentile value for the sixteen sandstone samples of Gombe Formation are presented in Table 1.The graphic mean, Standard deviation, skewness, kurtosis of the analysed samples are presented in Table 2.
The graphic mean for the various samples ranges from 0.8 to 3.1 with an average of 2.5ϕ (fine grained sandstone) (Table 2) which indicate coarse to very fine grained sandstone.The standard deviation values range from 0.4 (Table 2) with average of 0.6ϕ (moderately well sorted sandstone), which indicate well to poorly sorted.However, moderately sorted to well sorted values predominate.The values for the skewness range from -0.3to (Table 2) which indicate negative skewed to positive skewed.However the positive skewed values predominate.The values of Kurtosis for the various sample range from 0.6to2.1(very platykurtic to very leptokurtic) with an average of 1.3 (leptokurtic).The Bivariate plot of standard deviation vs. skewness indicated dominance of fluvial environment (fig.3).While the probability curves plots showed a dominance of three sand populations and subordinate results of two sand populations (figs.4).The results obtained from environmental discrimination functions (Y1, Y2 and Y3) of Sahu (1964) are presented in Table 3.The values of maximum projection sphericity and coefficient of flatness obtained are shown in Table 4, 5 and 6.

Univariate grain size parameters
The graphic mean size for the various sample of Gombe Formation range from 0.8ϕ to 3.1ϕ, which indicate coarse to very fine grained.Freidman (1967) pointed out that grained size distributions are controlled by processes rather than environment.Therefore the fluctuations in mean values suggest change in energy of the depositional conditions.Sorting is important factor that is use for environmental analysis.It is very significant in differentiating between fluvial and wave deposits (Freidman 1967).Sorting for various samples of the Gombe Formation range from 0.37ϕ to 1.01ϕ, this indicates well to poorly sorting.However moderately to well sorted values predominate.The well to moderately sorted nature of Gombe Formation may suggest reworking by wave activities, which removed clay material from the sand (Mason and Folk 1958;Freidman 1967).
The values of skewness for various samples range from 0.31ϕ to 0.4ϕ which indicates negative skewed to positive skewed.However the positive skewed values predominate.The predominance of positive skewed in coarsening upward deposit of Gombe Formation suggest transitional environment.This interpretation is supported by the work of Freidman (1967 and1979), in which he suggested positive skewed for deeper part of the continental shelf/lagoon and beach where abundance of fine exceeds the energy available for dispersing them.The values for kurtosis from various sample range from 0.56ϕ to 2.05ϕ (very platykurtic to very leptokurtic) with an average of 1.3ϕ (leptokurtic).Kurtosis has little Geologic information that could be obtained (Boggs 2006), however, the fluctuation in values may suggest change in energy of depositional medium ( Abdel-Wahab 1988, Shetima et al., 2012).
For the discrimination between aeolian and littoral (intertidal zone) environments, the Y1equation is used.Sahu (1964) suggest that when Y1 is less than -2.7411 it is an Aeolian deposit whereas if Y1 is greater than -2.7411 a beach environment is suggested.A Aeolian environment was infer, since 62.5% of the values of Y1 are greater than -2.7411 (Table 3).This may suggest aeolian deposits associated with bar deposit.For the discrimination between beach (backshore) and shallow agitated marine environments (sub-tidal environment) the Y2 equation is used.If the value of Y2 is less than 65.3650 a beach deposition is suggested, whereas if it is greater than 65.3650, a shallow agitated marine environment is inferred.Shallow agitated marine environment is suggested since 93.25% values of Y2 calculated are greater than 65.3650 (Table 3).This agitated marine environment may be taken to be a deltaic realm, where Gombe Formation is inferred as fluvial-deltaic in nature.For discrimination between shallow marine and the fluvial environments, the discrimination equation of Y3 is used.If Y3 is less than -7.419 the sample is identified as a fluvial deposits whereas if Y3 is greater than -7.419 the sample is identified as a shallow marine deposit.The analyzed results showed 93.75% of the plotted Y3 values from the total number of samples has values more than -7.419, suggestive of shallow marine environment while 6.25 % has Y3 less than -7.419 inferring fluvial setting (Table 3).
The result obtained from Y2, Y3 and probability plot have supported the earlier interpretation of the positive skewed values as transitional environment rather than fluvial environment.

Bivariate grain size parameters
The bivariate plots of standard deviation vs. skewness is based on Friedman (1979) which distinguishes Inland Dune Sand from Beach Sand, the plots indicates that 68.75% of the samples plotted within the river field and 31.25% of the samples plotted within the Beach Sand field (Fig. 3).The dominance of samples within Inland Dune field was due to river pumps of tremendous load of fines into the nearshore environment and the available energy was unable to remove them (Friedman, 1967) Plotting of the three discriminate functions (Y1, Y2 and Y3) as bivariate scatter plots was used to improve the success rate and refinement of the discrimination of the depositional environment.A bivariate plot of Y1 and Y2 (Fig. 5) show most samples to be littoral or shallow agitated marine environment.A bivariate plot of Y2 and Y3 (Fig. 6) show that most samples are of fluvial/deltaic environment.

Probability plots
According to Visher (1969)  Fourteen samples obtained from coarsening upward units have three sand populations.Eleven of the samples have saltation population from 2.0 to 3.5and little suspension population (Fig. 4A and B) similar to Visher (1969) figure 9A.This type of curves were interpreted as marine sand from wave zone, while sample 12 and 14 have saltation population between 2.7 to 3.9 and 10 percent surface creep (Fig. 4C) similar to Visher (1969) figure 13A which was interpreted as offshore marine sand in the area of tidal delta at depth of 3.05 to 12.2 m.

Pebbles morphometric
Maximum projection sphericity and coefficient of flatness are environmentally diagnostic (Stratten 1974).Pebbles from Gombe Formation were measured and the value of maximum projection sphericity and coefficient of flatness were calculated to determine its depositional environment.The pebbles have mean maximum projection sphericity of 0.646, 0.627 and 0.667 table 4, 5 and 6 respectively and mean coefficient of flatness of 52.013, 42.714 and 45.796 table 4, 5 and 6 respectively.
Sphericity is a measure of the approach of a pebble to sphere.It can be measured in different ways (Krumbein, 1941, Sneed andFolk 1958).In this study the maximum projection sphericity of Sneed and Folk has been used.The maximum projection sphericity values for range from 0.4 to 0.9 with mean value of 0.65.The average value of maximum projection sphericity is exactly at a line of separation between beach and fluviatile of Stratten (1974), indicating beach environment.

Figure 3 :Figure 4 :
Figure 3: Bivariate plot of skewness vs. standard deviation (afterFriedman, 1979) different sand population are of environmental value.The sand population curves are characteristic of either fluvial, beach or wave zone.Two sand populations are characteristic of fluvial setting, while three sand populations are characteristic of shallow marine setting.Probability plots of samples from bedded facies of Gombe Formation (Figs. 4A, B and C) show three sand populations which indicate deposit of shallow marine environment.Samples of red sandstone facie show two sand populations which suggest fluvial environment (Fig.4D).

Table 1 :
Percentile value from cumulative curve plots of sandstone samples analyzed.

Table 2
Grain size distribution and quantitative parameters for the sandstone samples analyzed

Table 3 :
The discriminate function and environment of deposition

ENVIRONMENT OF THE GOMBE FORMATION IN THE GONGOLA SUB-BASINTable 4 :
Summary of pebble morphometric analysis results at 10

Table 5 :
Summary of Pebble Morphometric Analysis Results at 10

Table 6 :
Summary of Pebble Morphometric Analysis Results 10