World Applied Sciences Journal 11 (3): 245-255, 2010ISSN 1818-4952© IDOSI Publications, 2010

Corresponding Author: Jude E. Ogala, Department of Geology, Delta State University, P.M.B. 1, Abraka, Nigeria.E-mail: etunimogala@yahoo.com.

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Granulometric Analysis and Pebble Morphometric Studies as Indicators ofDepositional Environments of the Sandstone Facies Around Okanyan

and Evirons in the Benin Formation, Southwestern Nigeria

Jude E. Ogala, Edwin O. Adaikpoh, Omoleomo O. Omo-Irabor and Rosemary U. Onotu

Department of Geology, Delta State University, P.M.B. 1, Abraka, Nigeria

Abstract: Quartz pebbles (225 samples) and sandstones (5 samples) from quarry pits, outcrops and streams inOkanyan and environs were subjected to sieve analysis and pebble morphometric studies. The aim of this studyis to determine the textural characteristics and depositional environment of the sediments in the BeninFormation. The sedimentary structures typical of the study area include bedding planes, herringbone andplanar cross beds. Herringbone cross beds are bi-directional structures indicative of tidal origin and signifiesdeposition in a shallow marine (fluvial or deltaic) environment. The results of grain size analysis showed thatthe sandstone range from fine to coarse grained, poorly sorted, mostly mesokurtic and positively skewedsuggesting a river origin for the sediments. Scatter plots of simple sorting measure versus simple skewnessmeasure, skewness versus standard deviation and mean versus standard deviation clearly define thesandstones as products of fluvial regimes. The pebbles have (total mean) flatness ratio = 0.544 (marine),elongation ratio = 0.794 (torrent, brooks and rivulets), maximum projection sphericity index (M.P.S.I) = 0.713(fluvial) and oblate-prolate index = -0.955 (fluvial). Scatter plots of Roundness versus E.R. indicate variableenvironments for the pebbles ranging from littoral to transitional to fluvial while that of M.P.S.I versus O.P.index is more diagnostic of fluvial depositional environment.

Key words: Pebble morphometry Sandstone facies Fluvial Herringbone cross beds Benin formation

INTRODUCTION Previous geological studies in the Niger Delta Basin

The present study covers an area of 61km within hydrocarbon source potential. [5] classified sediments in2

longitudes 5° 32’ to 5° 41’E and latitudes 6° 33’ to 6° 41’N eastern Niger Delta using chemical parameters of(Figure 1). The Niger Delta Basin is located on the SiO /Al O ratio, Fe O /K O ratio and CaO content.continental margin of southern Nigeria. It is bounded in The present study utilizes textural parametersthe south by the Gulf of Guinea while the Benin and obtained from sieve analysis of sandstones andCalabar Flanks mark the northwestern and eastern morphometric analysis of pebbles to infer the depositionalboundaries of the delta respectively [1] recognized three environments of the sandstone facies in Okanyan areadiachronous units which show an overall upward and environs of the Benin Formation.transition from marine pro-delta shales (Akata Formation)through sand-shale paralic sequence (Agbada Formation) MATERIALS AND METHODS OF STUDYto continental sands and gravels (Benin Formation).

The Miocene-Recent Benin Formation is an extensive Two hundred and twenty five (225) quartzstratigraphic unit in the southern Nigeria sedimentary pebbles and five ( 5) sandstone samples frombasin. This formation previously called Coastal Plain outcrops, quarry pits and streams (Plate 1) were subjectedSands [2-3] and was named Benin Formation by [4]. to sieve analysis and pebble morphometry. 100 g ofThe formation consists of unconsolidated and friable each sandstone samples that were amenable tosands, with intercalation of gravely units and clay lenses. disaggregation were sieved using a sieve mesh of ½The total thickness of the formation is about 2,000 m. interval on a Pascal automatic shaker for 15 minutes [6].

have concentrated mainly on the sedimentology and

2 2 3 2 3 2

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Fig. 1: Map of study area showing sample location

Plate 1: Photographs showing (a) outcrop section with sedimentary structures and (b) Quarry pit with sand andpebble units.

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Vernier caliper was used in pebble morphometric RESULTS AND DISCUSSIONSstudies in the measurement of the long (L), intermediate (I)and short (S) axis of pebbles. Morphometric parameters The results of sieve analysis and pebbleobtained include flatness ratio (FR) and elongation ratio morphometry are presented in Tables 1a-e. The result(ER) after [8], maximum projection sphericity index show that the sandstones have graphic mean ranging(M.P.S.I) after [9]. Roundness of pebbles was estimated from 0.200 - 0.525 with an average mean size of 0.345 visually according to the chart of Sames (1966). These (medium grained) (Table 1a). The mean grain size in aparameters are evaluated as single functions and as deposit is largely a function of the energy of thedependent variables on scatter plots of M.P.S.I Vs O.P. processes controlling transport and deposition that isindex [10] and Roundness Vs E.R [11] to discriminate particles are segregated according to their hydrodynamicdepositional environment of pebbles. behavior which depends on size, specific gravity and

Table 1a: Results of grain size analysesGraphic Standard Skewness Kurtosis Simple Sorting Simple Skewness

Sample number Mean ( ) Deviation ( ) (sk) (K ) Measure (S ) Measure (S ) InterpretationG SO SK

L2 P1 0.303 1.327 0.788 1.165 2.275 1.750 Medium grained, poorly sorted,positively skewed and leptokurtic.

L2 P2 0.477 1.597 0.345 0.900 2.52 1.760 Fine grained, poorly sorted,positively skewed and mesokurtic.

L2 P3 0.217 1.542 0.355 0.984 2.52 1.640 Medium grained, poorly,positively skewed and mesokurtic.

L3 Stream Sediment 0.200 1.006 0.041 1.119 1.72 0.280 Fine grained, moderately medium sorted,positively skewed and mesokurtic.

L4 0.527 1.356 0.370 1.232 2.330 1.580 Coarse grained, poorly sorted,positively skewed and leptokurtic.

Average 0.345 1.366 0.380 1.080 2.273 1.402 Medium grain, poorly sorted,positively skewed and Mesokurtic

Table 1b: Pebble Morphometric data for Pebble Set (Location 2 Pit 1)S/N S(cm) I (cm) L(cm) S/L I/L MPSI OP INDEX ROUNDNESS %1 1.310 3.320 3.920 0.334 0.847 0.509 -8.087 802 3.000 3.500 4.850 0.619 0.722 0.809 3.711 803 1.670 2.520 3.810 0.438 0.661 0.539 2.347 804 2.600 3.300 4.74 0.549 0.696 0.756 3.149 405 1.930 2.820 4.460 0.433 0.632 0.667 3.423 606 2.620 3.370 4.540 0.577 0.742 0.766 1.896 807 1.150 3.080 4.900 0.235 0.629 0.444 -0.624 508 2.000 3.420 3.800 0.526 0.900 0.675 -5.492 609 2.000 2.990 3.140 0.637 0.952 0.752 -5.784 9010 2.600 3.720 4.930 0.527 0.755 0.717 0.366 5011 2.000 2.500 2.900 0.690 0.862 0.820 -0.805 9012 1.900 3.310 3.600 0.528 0.919 0.672 -6.239 5013 2.110 3.930 4.550 0.464 0.864 0.629 -5.300 6014 2.840 3.200 5.100 0.557 0.627 0.791 6.177 8015 2.100 3.230 4.000 0.525 0.808 0.699 -1.805 6016 1.990 3.900 5.230 0.380 0.746 0.579 -2.355 8017 3.550 3.920 4.340 0.818 0.903 0.905 0.387 2518 1.200 3.100 4.910 0.244 0.631 0.456 -0.497 4019 1.850 3.240 5.000 0.370 0.648 0.596 1.587 6020 2.650 3.790 4.950 0.533 0.766 0.721 0.081 9021 3.450 4.250 6.740 0.512 0.631 0.746 5.016 4022 2.910 3.600 5.500 0.529 0.655 0.753 4.416 8023 2.100 2.510 3.210 0.654 0.782 0.818 1.997 8024 1.350 2.100 2.260 0.597 0.929 0.727 -5.430 8025 1.050 1.100 1.250 0.840 0.880 0.929 2.976 8026 2.300 4.400 4.550 0.505 0.967 0.642 -8.581 80AVERAGE 2.163 3.235 4.276 0.524 0.775 0.697 -0.520 67

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Table 1c: Pebble Morphometric data for Pebble Set (Location 2 Pit 3)S/N S (cm) I (cm) L (cm) S/L I/L MPSI OP INDEX ROUNDNESS %1 1.600 1.800 2.710 0.590 0.664 0.807 5.421 502 0.850 1.650 1.650 0.515 1.000 0.643 -9.709 903 1.310 2.060 2.700 0.485 0.763 0.676 -0.816 604 1.450 1.840 2.050 0.707 0.898 0.823 -2.122 905 1.390 2.210 3.550 0.392 0.623 0.627 3.071 606 1.130 1.840 1.940 0.582 0.984 0.710 -6.470 807 1.350 1.550 2.130 0.634 0.869 0.773 -2.224 808 1.360 1.900 2.820 0.482 0.674 0.701 2.700 809 1.350 2.150 2.390 0.565 0.900 0.708 -4.765 8010 1.000 1.740 1.800 0.555 0.967 0.683 -7.658 8011 1.240 1.600 1.610 0.770 0.994 0.842 -6.143 9012 1.110 1.400 1.810 0.613 0.773 0.786 1.398 8013 0.950 1.350 1.900 0.500 0.711 0.706 1.579 6014 1.020 1.110 1.340 0.761 0.828 0.888 2.875 9015 1.100 1.360 1.420 0.775 0.958 0.856 -4.032 9016 1.100 1.700 1.900 0.579 0.895 0.721 -4.318 8017 0.850 1.210 1.310 0.649 0.924 0.770 -4.355 9018 0.850 1.000 1.100 0.773 0.909 0.869 -1.294 8019 1.320 1.520 1.620 0.815 0.938 0.891 -2.045 8020 1.900 1.150 2.860 0.664 0.752 0.837 3.608 8021 1.400 1.950 3.000 0.467 0.650 0.695 3.346 6022 1.400 1.800 2.750 0.509 0.655 0.734 4.002 6023 2.100 2.240 2.780 0.755 0.806 0.708 3.896 6024 1.400 2.450 2.630 0.532 0.932 0.673 -6.648 8025 1.230 1.990 2.400 0.513 0.829 0.682 -2.916 6026 1.220 2.000 2.440 0.500 0.820 0.673 -2.787 6027 1.160 1.300 1.760 0.659 0.739 0.838 4.047 9028 0.970 1.220 1.910 0.508 0.639 0.739 4.607 8029 0.840 1.560 2.300 0.365 0.678 0.582 0.188 6030 1.500 1.730 2.300 0.652 0.752 0.867 3.259 5031 1.190 2.650 3.270 0.364 0.810 0.547 -5.547 6032 2.000 2.860 3.810 0.525 0.751 0.716 0.474 5033 1.400 2.000 3.110 0.450 0.643 0.680 3.314 80AVERAGE 1.274 1.794 2.275 0.588 0.815 0.740 -0.790 73

Table 1d: Pebble Morphometric data for Pebble Set (Location 3-Stream Sediments)S/N S(cm) I(cm) L(cm) S/L I/L MPSI OP INDEX ROUNDNESS %1 2.260 2.310 2.630 0.859 0.878 0.944 4.248 80.002 1.460 2.250 2.650 0.547 0.849 0.706 -3.047 60.003 1.000 1.750 2.630 0.380 0.665 0.601 1.857 90.004 1.680 2.020 2.600 0.646 0.777 0.813 2.019 60.005 1.610 2.820 2.950 0.546 0.956 0.678 -7.381 40.006 1.350 1.890 2.450 0.551 0.771 0.733 0.165 30.007 2.060 2.110 3.520 0.585 0.599 0.830 7.962 50.008 2.020 2.560 3.940 0.513 0.650 0.740 4.264 30.009 1.010 1.720 2.130 0.474 0.808 0.653 -2.825 25.0010 1.950 2.120 2.920 0.668 0.726 0.850 4.861 40.0011 1.000 1.390 1.900 0.526 0.732 0.723 1.267 25.0012 1.310 2.000 2.520 0.520 0.794 0.698 -1.351 50.0013 1.030 1.830 2.050 0.502 0.893 0.656 -5.664 60.0014 1.190 1.690 1.680 0.708 0.994 0.796 -6.774 60.0015 1.390 1.610 2.420 0.574 0.665 0.792 4.990 40.0016 0.990 1.550 1.970 0.503 0.787 0.685 -1.420 40.0017 0.830 1.510 1.600 0.519 0.944 0.658 -7.382 40.0018 1.320 1.410 1.620 0.815 0.870 0.914 2.454 40.0019 2.000 2.130 2.750 0.727 0.775 0.881 4.493 40.0020 1.370 1.860 2.110 0.649 0.882 0.782 -2.499 90.0021 1.200 1.420 1.500 0.800 0.947 0.878 -2.917 90.00

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Continued table 1d: Pebble Morphometric data for Pebble Set (Location 3-Stream Sediments)S/N S(cm) I(cm) L(cm) S/L I/L MPSI OP INDEX ROUNDNESS %22 1.070 1.620 2.120 0.505 0.764 0.693 -0.471 40.0023 0.890 1.700 1.720 0.517 0.988 0.647 -9.205 90.0024 1.050 2.500 2.790 0.376 0.896 0.541 -8.865 25.0025 1.150 2.110 2.690 0.428 0.784 0.615 -2.883 40.0026 1.060 1.600 2.500 0.4245 0.640 0.655 2.948 50.0027 1.400 1.970 2.320 0.603 0.849 0.754 -1.983 50.0028 1.460 1.810 1.820 0.802 0.995 0.865 -5.888 95.0029 1.200 2.000 2.300 0.522 0.870 0.679 -4.354 95.0030 0.880 1.850 3.290 0.267 0.562 0.503 3.652 40.0031 1.790 2.450 3.000 0.597 0.817 0.758 -0.761 30.0032 1.250 2.000 2.290 0.546 0.873 0.699 -4.050 25.0033 1.250 1.870 3.010 0.415 0.621 0.652 3.560 20.0034 1.200 1.3500 2.150 0.558 0.628 0.792 6.131 40.0035 1.200 1.800 1.850 0.649 0.973 0.756 -6.420 90.0036 1.090 2.360 2.500 0.436 0.944 0.586 -9.191 40.0037 1.160 1.730 2.290 0.507 0.755 0.698 -0.087 40.0038 0.870 1.130 1.270 0.685 0.890 0.808 -2.190 30.0039 0.830 1.280 1.300 0.638 .985 0.745 -7.170 50.0040 0.610 1.150 1.250 0.488 0.92 0.637 -7.044 60.0041 0.720 0.880 1.580 0.456 0.557 0.803 6.885 40.0042 0.730 1.000 1.310 0.664 0.763 0.741 0.519 30.0043 0.950 1.130 1.500 0.633 0.753 0.811 2.729 30.0044 0.820 1.120 1.510 0.543 0.742 0.733 1.201 30.0045 0.500 0.930 1.450 0.345 0.641 0.570 1.373 30.0046 0.790 1.050 1.460 0.541 0.719 0.802 2.069 50.0047 0.420 0.960 1.220 .590 0.787 0.762 0.339 30.0048 0.700 1.150 1.500 0.467 0.767 0.657 -1.338 25.0049 0.780 1.070 1.280 0.609 0.836 0.763 -1.314 30.0050 0.720 0.790 1.550 0.465 0.510 0.751 8.939 25.0051 0.600 1.000 1.310 .458 0.763 0.650 -1.384 30.0052 0.720 1.040 1.250 0.576 0.832 0.736 -1.802 40.0053 0.690 0.900 1.430 0.483 0.629 0.718 4.477 40.0054 0.840 1.810 1.980 0.424 0.914 0.582 -8.275 40.0055 1.250 2.080 2.290 0.546 0.908 0.690 -5.459 60.0056 0.840 1.520 1.620 0.519 0.938 0.659 -7.164 60.0057 1.420 2.190 3.210 0.442 0.682 0.659 1.580 40.0058 1.780 2.000 3.200 0.556 0.625 0.791 6.206 90.0059 1.950 2.550 3.000 0.650 0.850 0.792 -1.099 90.0060 1.290 2.200 3.050 0.423 0.721 0.628 -0.403 25.0061 1.110 1.870 2.500 0.444 0.748 0.641 -1.053 25.0062 1.650 1.720 2.650 0.623 0.649 0.842 6.902 95.0063 0.810 1.190 2.000 0.405 0.595 0.651 4.461 30.0064 1.420 2.200 2.730 0.520 0.806 0.695 -1.835 50.0065 1.100 1.950 2.000 0.550 0.975 0.677 -8.081 90.0066 1.110 2.000 2.200 0.505 0.909 0.654 -6.268 90.0067 1.220 1.920 2.430 0.502 0.790 0.683 -1.564 40.0068 1.320 2.190 3.920 0.401 0.666 0.623 1.456 40.0069 1.900 2.750 4.320 0.440 0.637 0.672 3.381 80.0070 1.080 2.280 3.280 0.329 0.695 0.538 -1.382 50.0071 1.120 1.300 1.760 0.636 0.739 0.818 3.439 50.0072 2.050 2.620 4.730 0.433 0.554 0.697 6.635 95.0073 1.240 2.230 3.270 0.379 0.682 0.595 0.325 95.0074 1.120 2.270 3.690 0.304 0.615 0.531 1.728 90.0075 1.720 2.130 2.650 0.649 0.804 0.806 0.911 40.0076 1.130 2.040 2.500 0.452 0.816 0.630 -3.633 40.0077 0.650 1.250 1.290 0.504 0.969 0.640 -8.681 40.0078 0.860 1.230 1.340 0.642 0.918 0.766 -4.219 40.0079 1.220 1.530 2.120 0.575 0.722 0.771 +2.705 40.0080 1.210 1.390 1.820 0.665 0.764 0.833 3.081 30.00

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Continued table 1d: Pebble Morphometric data for Pebble Set (Location 3-Stream Sediments)S/N S(cm) I(cm) L(cm) S/L I/L MPSI OP INDEX ROUNDNESS %81 1.240 1.720 2.000 0.620 0.860 0.765 -2.122 50.0082 0.850 1.350 1.400 0.607 0.964 0.726 -6.740 60.0083 0.910 1.110 1.200 0.758 0.925 0.853 -2.502 80.0084 0.720 1.110 1.290 0.558 0.860 0.713 -3.301 50.0085 1.000 1.170 1.730 0.578 0.676 0.791 4.622 50.0086 1.330 1.730 2.150 0.617 0.805 0.781 0.198 60.0087 0.900 1.330 1.530 0.588 0.869 0.736 -3.104 40.0088 0.880 1.090 1.300 0.677 0.838 0.818 0.000 40.0089 0.860 1.130 1.510 0.570 0.748 0.757 1.484 40.0090 0.700 1.510 1.760 0.398 0.858 0.569 -6.637 90.0091 0.890 1.100 1.400 0.636 0.786 0.801 1.387 90.0092 0.590 1.000 1.220 0.484 0.820 0.658 -3.116 20.0093 1.180 1.730 1.900 0.621 0.911 0.751 -4.249 25.0094 0.900 1.090 1.100 0.818 0.991 0.877 -5.501 25.0095 0.960 1.320 1.500 0.640 0.880 0.774 -2.604 30.0096 1.410 2.100 2.12 0.665 0.991 0.764 -7.095 25.0097 0.940 1.430 1.680 0.560 0.851 0.716 -2.896 30.0098 1.200 1.660 2.690 0.446 0.617 0.686 4.289 30.0099 1.040 2.000 2.260 0.460 0.885 0.621 -6.237 90.00100 1.070 1.380 2.000 0.535 0.690 0.746 3.115 40.00101 1.100 1.600 1.900 0.579 0.842 0.736 -2.159 50.00102 0.870 1.430 2.300 0.378 0.623 0.613 2.868 50.00103 0.850 1.300 1.820 0.467 0.714 0.673 0.778 40.00104 0.860 1.620 1.800 0.477 0.900 0.633 -6.468 40.00105 1.320 1.850 1.960 0.673 0.944 0.783 -4.876 80.00106 1.000 1.300 1.600 0.625 0.813 0.783 0.000 80.00107 1.130 1.450 1.850 0.611 0.784 0.781 0.909 50.00108 1.580 2.440 2.650 0.596 0.921 0.728 -5.096 90.00109 0.900 1.700 2.020 0.446 0.842 0.618 -4.805 50.00110 0.700 1.230 1.720 0.407 0.715 0.614 -0.482 30.00111 0.890 1.300 1.860 0.478 0.699 0.689 1.618 30.00112 1.680 2.300 2.920 0.575 0.788 0.749 0.000 30.00113 0.960 1.390 1.840 0.522 0.755 0.712 0.218 40.00114 0.860 1.010 1.830 0.470 0.552 0.737 7.348 40.00115 1.570 2.190 3.010 0.522 0.728 0.720 0.689 60.00116 1.110 1.900 2.090 0.531 0.909 0.677 -5.765 50.00117 0.960 1.120 1.520 0.632 0.737 0.815 3.391 50.00118 1.080 1.650 1.900 0.568 0.868 0.719 -3.435 40.00119 0.890 1.450 1.650 0.540 0.879 0.692 -4.386 40.00120 0.790 1.430 1.780 0.444 0.803 0.626 -0.341 40.00121 1.110 2.000 2.120 0.524 0.943 0.662 -7.275 60.00122 1.340 1.630 2.060 0.650 0.791 0.812 1.496 60.00123 1.110 1.730 1.970 0.563 0.878 0.712 -3.924 60.00124 0.820 1.150 1.360 0.603 0.846 0.755 -1.843 40.00125 1.150 1.640 1.730 0.665 0.948 0.755 -5.185 30.00126 1.130 1.500 1.770 0.638 0.847 0.783 -1.225 40.00127 0.790 1.630 1.820 0.434 0.896 0.595 -7.270 40.00128 0.680 1.620 1.900 0.358 0.853 0.532 -7.556 40.00129 0.900 1.560 1.700 0.529 0.918 0.673 -6.144 40.00130 1.370 1.690 1.900 0.721 0.889 0.836 -1.439 40.00131 0.850 1.320 1.330 0.639 0.992 0.744 -7.499 30.00132 0.740 1.000 1.720 0.430 0.581 0.683 5.458 30.00133 0.720 1.050 1.200 0.6000 0.875 0.744 -3.125 40.00134 1.470 1.570 2.200 0.668 0.714 0.850 5.434 30.00AVERAGE 1.112 1.644 2.09 0.547 0.788 0.719 -1.143 49.08

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Table 1e: Pebble Morphometric data for Pebble Set (Location 4)S/N S(cm) I (cm) L(cm) S/L I/L MPSI OP INDEX ROUNDNESS %1 1.450 3.100 3.460 0.419 0.896 0.581 -0.766 802 1.580 2.000 2.300 0.687 0.870 0.816 -0.121 603 2.000 2.400 3.200 0.625 0.750 0.805 2.667 804 1.090 1.850 2.770 0.394 0.668 0.614 1.209 905 1.480 2.260 3.310 0.447 0.683 0.664 1.650 906 1.810 2.350 3.000 0.603 0.783 0.775 0.766 907 1.800 2.460 3.210 0.561 0.766 0.743 0.569 608 2.360 2.810 3.800 0.621 0.739 0.805 3.019 409 1.760 2.360 3.460 0.59 0.682 0.724 2.889 5010 1.260 1.610 1.710 0.737 0.942 0.832 -3.769 9011 1.230 1.460 2.420 0.508 0.603 0.754 6.038 8012 1.170 1.850 2.070 0.565 0.894 0.710 -4.523 6013 1.640 2.150 2.830 0.580 0.760 0.762 1.232 2014 0.900 1.740 1.760 0.511 0.989 0.642 -9.330 6015 1.400 1.750 1.800 0.778 0.972 0.854 -4.820 6016 0.900 1.670 1.710 0.526 0.997 0.657 -8.567 6017 1.270 2.300 2.930 0.433 0.785 0.621 -2.720 9018 1.200 2.350 2.400 0.500 0.979 0.674 -9.167 9019 1.200 2.000 2.680 0.448 0.746 0.645 -0.905 8020 1.320 1.720 2.890 0.457 0.595 0.705 5.366 9021 1.060 1.780 2.430 0.436 0.733 0.638 -0.586 9022 1.070 1.650 2.200 0.486 0.750 0.681 -0.273 9023 1.250 1.720 2.100 0.595 0.819 0.756 -0.890 9024 1.150 1.950 2.860 0.402 0.682 0.619 -0.800 9025 1.350 2.010 2.550 0.529 0.788 0.708 -0.945 8026 1.400 2.080 2.830 0.495 0.735 0.693 0.494 8027 1.020 2.080 2.400 0.425 0.867 0.593 -6.309 8028 1.490 2.640 3.220 0.465 0.820 0.639 -3.558 6029 1.350 2.070 3.000 0.450 0.690 0.665 1.414 8030 1.330 2.370 2.960 0.449 0.801 0.632 -3.074 9031 1.050 2.040 2.600 0.404 0.785 0.592 -3.433 8032 1.330 2.340 2.530 0.526 0.925 0.669 -6.496 60Average 1.364 2.091 2.668 0.518 0.797 0.696 -1.367 74.69

shapes [12]. The standard deviation range from 1.006 - The grain size distribution curve typically1.597 , averaging 1.366 which indicates poorly sorted shows two straight line segments (traction andsand. The skewness values are consistently positive for saltation load) with the exception of the riverall the samples (0.041 - 0.788) inferring river/fluvial sands. sediments which has a third segment indicating theThe positive value of skewness indicates a tailing off suspension load with a break commonly occurringthe curve to the right of the mean. This corresponds to at a cumulative frequency between 85 and 90%excess fine to coarse grained particles. and falling within the range of 2.5 and 3.5 [13].

River sands are generally poorly sorted and Thus, the segment representing the fine fractionpositively skewed while beach sands are well sorted and usually constitutes 10% or more of the totalnegatively skewed, reflecting in the latter case high distribution. Figure 5 is a probability plot showingenergy conditions. Kurtosis values range from 0.900 - two major breaks at a cumulative frequency of 90%1.232 (mesokurtic to leptokurtic). Skewness and standard with 2.6 break with the exception of stream sedimentdeviation are effective in environmental diagnosis. having a third break at a cumulative frequency of 99%Scatter plots of simple skewness measures versus with 2.7 break. The traction, saltation and suspensionsimple sorting measure (Figure 2), skewnes versus population are in the ratio of 7:5:1, the curves shows astandard deviation (Figure 3) and mean versus standard gentle slope and indicates equivalence of fluvialdeviation (Figure 4) clearly define the sands of Okanyan hydrodynamic processes which infer a fluvial systemand environs to be fluvially deposited. (Figure 5).

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Fig. 2: Plot of Simple Skewness Measure versus Simple Sorting Measure

Fig. 3: Plot Skewness versus Standard Deviation

Fig. 4: Plot of Mean versus Standard Deviation

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Fig. 5: The grain size distribution curve illustrating the truncation of the curve into (a) traction population, (b) saltationpopulation and (c) suspension population

Table 2: Summary (total average) of pebble morphometric parameters

Morphometric Total average Total average Total average Total average Total average

Parameters Pebble Sets L P Pebble Sets L P Pebble Sets L Pebble Sets L of the Pebble Sets Interpretation2 1 2 3 4 3

Flatness Ratio (F.R) 0.524 0.588 0.518 0.547 0.544 Marine

Elongation Ratio (E.R) 0.775 0.815 0.797 0.788 0.794 Torrent, brooks or rivulets

M.P.S.I 0.697 0.740 0.696 0.719 0.713 Fluvial

O.P. Index -0.520 -0.790 -1.367 -1.143 -0.955 Fluvial

Roundness (%) 67.120 73.330 74.890 49.080 66.110

A summary (total average) of the computed pebble plots of roundness vs. E.R. (Figure 6b), shows 68% of allmorphometric parameters is presented in Table 2. Total pebbles plot within the littoral fields, while 17.3% andmean F.R., E.R., M.P.S.I., O.P. Index and roundness for 14.7% plot within transitional and fluviatile fieldspebbles of Okanyan and environs in the Benin Formation respectively with roundness having the greatest influenceare 0.544, 0.794, 0.713, -0.955 and 66.11 respectively in determining depositional environment of pebbles on(Table 2). The scatter plots of M.P.S.I vs. O.P. Index this plot. That is the lower the roundness the higher(Fig. 6a) is more diagnostic of fluvial depositional the probability that the depositional environment willenvironment, showing 73.4% of pebbles from Okanyan be fluvial.and environs within the river environment and 26.6% Morphometric scatter plot and indices are notwithin the beach environment. The standard scatter in such unanimous agreement with the fluvialplot of M.P.S.I vs. O.P. Index [4] is based on the 0.66 demarcation based on sieve analysis. Mean values of F.R.sphericity (M.P.S.I.) line best separate beach and river obtained for pebbles of Okayan and its environs rangepebbles, while values < 0.66 are typical of beaches, higher from non-diagnostic of a particular environment tovalues(> 0.66) suggests fluvial origin. An O.P. Index marine. The total mean F.R. of pebbles of Okayan andvalue > -1.5 generally signifies fluvial conditions. Scatter its environs is 0.544 (marine), E.R. is 0.794 (fluviatile).

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Fig. 6: Environmental discrimination of pebbles (a) M.P.S.I versus O.P. Index (b) Roundness versus Elongation ratio

Although significant for various environments, the F.R. necessarily reflect the distance grains have travelled fromand E.R. are functions of lithology, tectonic stress their source.operating within the basin and thickness of bed formsin which the pebbles are produced. But this is unlikely CONCLUSIONthe cause of the non-diagnostic and marineinterpretations from values of F.R. obtained, since all The present study based on grain size distributionpebbles were selected from unconsolidated sand although and pebble morphometry have shown that the sands andof varying grain size. The E.R. as a single function pebbles of Okanyan and environs of the Benin Formationconsidered has mean values for all pebble sets within the in the Niger Delta Basin are products of fluvialfluviatile range. As a dependant function of roundness deposition. The sandstone range from fine to coarseon the scatter plot of E.R. vs. roundness points range grained, poorly sorted, mostly mesokurtic and positivelyfrom fluviatile to littoral, reflecting the importance and skewed suggesting a river origin for the sediments. Theinference of roundness on this plot. sedimentary structures typical of the study area include

Roundness is not particularly indicative of bedding planes, herringbone and planar cross beds.depositional environment, rather the extent of Herringbone cross beds are bi-directional structuresabrasion that the grains or pebbles have undergone; it indicative of tidal origin and signifies deposition in areflects overall transport history and does not shallow marine (fluvial or deltaic) environment.

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1. Short, K.C. and A.J. Stauble, 1967. Outline of the Pub., New York.Geology of the Niger Delta. AAPG Bulletin, 9. Sneed, E.D. and R.L. Folk, 1958. Pebbles in the lower51: 761-779. Colorado River, Texas: a study in particle

2. Simpson, A., 1954. The Nigerian coal field: The morphogenesis. J. Geol., 66: 114-150.geology of parts of Onitsha, Owerri and Benue 10. Dobkins, J.E. and R.L. Folk, 1970. Shape developmentprovinces. Geol. Surv. Nigeria Bull., 24: 85. on Tahiti-Nui. J. Sedimentary Petrol., 40: 1167-1203.

3. Tattam, C.M., 1944. A review of Nigeria Stratigraphy. 11. Sames, C.W., 1966. Morphometric data of someRept. Geol. Surv. Nigeria, pp: 27-47. recent pebble associations and their application to

4. Reyment, R.A., 1965. Aspects of geology of Nigeria. ancient deposits. J. Sedimentary Petrol., 36: 126-142.University of Ibadan Press, pp: 145. 12. Olugbemiro, R. and C.S. Nwajide, 1997. Grain size

5. Braide, S.P., 1993. Geochemical classification of distribution and particle morphogenesis assome Eastern Niger Delta sediments. NAPE Bull., signatures of depositional environments of8(01): 74-79. Cretaceous (non-ferriginous) facies in the Bida

6. Friedman, G.M., 1979. Differences in size Basin, Nigeria. J. Mining and Geol., 33(2): 89-101.distributions of populations of particles among 13. Friedman, G.M. and J.E. Sanders, 1978. Principles ofsands of various origins. Sedimentology, 20: 3-32. sedimentology. John Wiley and Sons, New York.

7. Luttig, G., 1962. The shape of pebbles in thecontinental, fluviatile and marine facies. Int. Assoc.Scientific Hydrology Pub., 59: 235-258.

Sedimentology, Second Edition Chapman and Hall