Textural characteristics and depositional processes 3 of sediments from a 47 km transect in the Niger 4 Delta , Southern Nigeria

6 120 sediment samples collected along a 47km transect from Ikot Abasi to Eket, Akwa Ibom State were subjected to granulometric analysis to determine their textural characteristics and depositional processes. Standard sedimentological methods involving sieve analysis were utilized to determine the various size distributions for each sample. This was further subjected to statistical treatment (mean size, sorting, kurtosis, skewness, bivariate and multivariate analyses). Results show that the samples were very fine to pebbly (3.23 to -1.53) Ø diameter in size and varies from very poorly sorted to very well sorted (2.069 to 0.294) Ø with about 86% of the samples being poorly sorted. The sediments are predominantly leptokurtic (91%), with only few (9 samples) being platykurtic (range 8.148 to -1.082) Ø and are coarse to very fine skewed in nature. The dominant modes of transportation reflected by these sediments are saltation and surface creep attributed mostly to current and channel action. Bivariate analysis revealed that most of the sediments are associated with fluvial processes of deposition with beach processes being subordinate. Multivariate analysis showed that shallow marine agitated environment mostly characterized the sediments of this study area. CM pattern for the sediments reveal deposition was mainly by graded suspension to bottom suspension and rolling. These features characterize sediments deposited by fluvial processes dominated by tractive current patterns in a shallow marine depositional environment. 7


INTRODUCTION
The area under investigation is a 47km transect in the southern part of Akwa Ibom State Nigeria 11 between Eket and Ikot Abasi (Fig 1). It is drained by two notable rivers, the Imo and Qua-Iboe rivers. 12 In between these two rivers are three major creeks (Essene, Jaja and Awa creeks) which are tidal and 13 constitute the main source of water that feed the swamps in most part of the area. To date, no 14 systematic study has been carried out on sediments from this area. Grain size analysis has been a 15 significant tool for relating textual characteristic of sediments to their depositional processes [1][2]. The 16 size distribution is a reflection of the fluidity factor of the depositing medium and the energy factor of 17 the environment of deposition. Several works have been carried out to distinguish depositional 18 environment based on their textural characteristics [3][4][5][6]. 19 This study was undertaken to describe the textural characteristics of about 120 samples collected 20 across the Ikot Abasi -Eket transect in order to understand the depositional processes (energy and 21 transport mechanism) and paleoenvironment of deposition. 22

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The study area lies between latitude 4 ̊ 30' -4 45' and longitude 7 35' -7 53'E covering about 47km 25 stretch within the southern part of Nigeria coastline, which was progressively being re-shaped as the delta grew [16]. Climatic variations and the 41 proximity and nature of sediment source areas also contributed to the factors that controlled the 42 growth of the Niger Delta. The various stages of growth of the delta marks the delimitation of the 43 seven (7) depobelts that characterizes the Niger Delta. 44 Stratigraphically, the Niger Delta is made up of three main formations. These include the Akata, 45 Agbada and Benin Formations. The Akata Formation is characterized predominantly of shales. The 46 shales are dark grey, silty and uncompacted and represent the pro-deltaic sediments of the basin. 47 They contain several lenses of abnormally high pressures siltstones/fine grain sandstones [8]. 48 The Agbada Formation consist of intercalations of sandstones and shales of the delta front deposit, 49 distributary channel and delta plain origin [8]. The alternation of sandstone-shale sequence has been 50 interpreted as a cyclic marine-fluvial sequence during deposition of the Agbada Formation [18]. The 51 sands are fairly clean, locally calcareous and glauconitic. The shales are dark grey, hard, silty in 52 places and locally glauconitic. The Benin Formation was first described by [19] and expanded upon by 53 [10] who erected a type section for this formation at Elele -1 well, 34km northwest of Port Harcourt. It 54 consists predominantly of massive, highly porous, freshwater bearing sandstones, with some shale 55 interbeds which are considered to be of braided stream origin [8]. The area under study is generally flat and low lying with alluvial plains, sand ridges and rolling sandy 65 plains as the principal features [21] and characterized by creeks and estuarine processes shaping it 66 consistently in different areas. Notable creeks within the area include the Jaja, Essene and Awo 67 creeks. Two major rivers that influence the area under investigation are the Imo and Qua Iboe rivers, 68 with large network of tributaries, streams, creeks, rivulets and tidal inlets [21]. 69

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A total of 120 sediment samples were collected across southern Nigeria along a 47km transect from 71 Ikot-Abasi to Eket in Akwa Ibom state. This area falls within the Nigeria's Niger Delta sedimentary 72 basin. Sediment samples were collected in polyethene bags for laboratory analysis. During this 73 process, biases was avoided by collecting samples that were fresh and devoid of contamination from 74 nearby engineering structures. In the laboratory, the dried samples were subjected to coning and 75 quartering. The samples were then subjected to sieve analysis using a Ro-tap sieve shaker for a 76 period of 15mins for each sample. The measurement of the weight retained was carried out and 77 tabulated accordingly. Cumulative curves were plotted and statistical parameters such as mean, 78 median, standard deviation, skewness, and kurtosis were all computed following the technique 79 proposed by [3]. Log -plots were prepared in order to characterize the various transport and 80 depositional mechanisms and compared with the findings of [1]. Bivariate and linear discriminate 81 function (multivariate) analyses were also carried out to properly characterize the sediments and  The result of textural analysis of the samples was used to determine the statistical parameters and is 89 presented in Table 1. The result for the various locations have been grouped in order to facilitate data 90 handling. Their range and average for each unit was calculated (Table 1). From the results, the 91 sediments range from fine to coarse grain, with medium grain class being predominant. They are 92 poorly to moderately sorted, leptokurtic and very fine skewed sands. Distinction between 93 environments by grainsize distribution is informed by the specific properties of such environments, 94 and is illustrated by the unique properties of one or more subpopulations within their size distribution 95 [1]. This majorly characterizes the index of energy conditions during deposition. In general, the mean 98 grain size in the study area range from (-1.533 to 3.233 ɸ) with an average value of (2.12 ɸ). From the 99 average value for mean grain size, fine to medium grain sands predominate the study area. This 100 indicates that the energy condition of the depositing agent was moderate, however the presence of a 101 wide range of phi (ɸ) grain size values signifies fluctuating energy levels of the depositing medium.  turbulence sediments tend to be negatively skewed and characteristically becomes positively skewed 117 in calmer environmental conditions. As observed in this study, strongly fine skewed to fine skewed 118 sediments implies introduction of excess fine materials, mostly from the suspension fraction, 119 associated with less turbulence during deposition. 120 121

Kurtosis (K G ) 122
The graphic kurtosis quantitatively measures how the sediments depart from normality. It clearly 123 describes the sorting at the tails of the curve and relate them to the central portion. In this study, the 124 kurtosis values range from -1.082 to 8.148 ɸ, with an average of 3.21 ɸ, suggesting very leptokurtic to 125 platykurtic character. It has been pointed out that where the kurtosis values are very high (as in this 126 case), sediments may have achieved sorting from elsewhere in a higher energy environment before 127 being redeposited in a different environment of completely different environmental [5,24]. 128 129

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Bivariate analysis varies dual parameters and aid in the interpretation of energy conditions, 131 transportation medium and mode of deposition. The bivariate plot between mean and sorting (Fig 3)  132 show samples being almost restricted to the right wing of the inverted -V trend suggesting that the 133 variation in size classes was not too large [3]. It further shows that the sediments are characterized by 134 predominantly poorly sorted, medium to fine grain sands. Plots of skewness against mean (Fig 4), 135 skewness against sorting (Fig 5) and Kurtosis against Skewness (Fig. 6) also point to the same 136 depositional character and suggests fluvial process predominating the study area. The CM plot of [26] was utilized to characterize the transportation mechanism of the sediments within 148 the study area. He suggested that two patterns for which bottom sediments are transported include 149 tractive currents and turbidity currents. 150 The sediments from this study showed marked resemblance to deposits of tractive current, whereby 151 the sediment load is transported by rolling and suspension (Fig 7). These where further subjected to bivariate scatter plots ( Fig. 7 and Fig. 8) to further improve the 178 success rate of deposition mechanism characterization. From the plots it was shown that the 179 sediments were characteristically from shallow marine (Intertidal) and deltaic (Delta front) 180 environment. 181

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Based on the sediments characteristics and the respective graphic and discrimination analysis carried 183 out on them, the analyzed sediments may have been under the influence of an environment 184 characterized by both shallow water agitation (high energy) and poorly to moderately sorted low 185 energy conditions. The sediments admixture is likely to be as a result of re-sedimentation of previous 186 sedimentary rocks with sediments from first cycle of deposition. The sediments may have been 187 subjected to intertidal influence in a deltaic setting. Thus, the sediments along the studied transect 188 show energy setting associated with a mix of high and relaxed energy at different times during 189 depositional process, with moderate sorting, predominantly positively skewed and leptokurtic nature.

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It is hereby emphasized that despite the huge success in the characteristic of sediments in 191 discriminating depositional environments and processes, uncertainties that still persist should be 192 reduced by the integration of other available sedimentary features like sedimentary structures as well 193 as available fossils or trace fossils. 194 195