THE LITHOSTRATIGRAPHY OF THE NEAR-SURFACE IN PART OF SEDIMENTARY KOLMANI FIELD IN NORTHERN BENUE TROUGH, NIGERIA, USING SOIL CORE AND SEISMIC REFRACTION DATA

Soil samples from 31 shallow boreholes were acquired at depths 0m, 1m, 2m, 3m, 4m, 5m, 7m, 10m, 15m, 20m, 25m, 30m, 35m, 40m, 45m, 50m, 55m


INTRODUCTION
The lithology of a rock unit is its physical characteristics seen and described in terms of colour, texture, grain size, and composition (US Geological Survey, 2010; Bates and Jackson, 1984;Allaby and Allaby, 1999;American Heritage Dictionary, 2005).Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas.In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts which are used to determine which rock naming system to use.In this system, sandstones and conglomerates, a word describing the grain size range is added to the rock name.Examples are "pebble conglomerate" and "fine quartz arenite".In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description.
In the case of sequences possibly including carbonates, calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate) using dilute hydrochloric acid and looking for effervescence (Geology.com, 2016).The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded based on the Munsell colour system (US Bureau of Reclamation, 1998).Stratigraphy is concerned with the rock layers (strata) and layering (stratification).It is primarily used in the study of sedimentary and layered volcanic rocks.Though stratigraphy has two related subfields: lithostratigraphy (lithologic stratigraphy) and biostratigraphy (biologic stratigraphy).Most of the studies that have been carried out on the Northern Benue Trough focused on the stratigraphy, sedimentology, palaeoenvironmental interpretation, palaeontological and structural studies (Dike, 1993;Adegoke et al., 1986;Carter et al., 1963;Adegoke et al., 1978).However, studies focusing on seismic velocity layering are very limited.In this regard, this paper integrates geological lithostratigraphy with seismic stratigraphy of the Northern Benue Trough.Near-surface refers to the uppermost tens of meters beneath Earth's surface.This portion of Earth both affects and is impacted by human activities such as building, excavating, tunnelling, and storing or accidentally releasing hazardous materials.Many of the planet's mineral and groundwater resources are located in the uppermost subsurface layers.The techniques of near-surface geophysics are being increasingly used to benefit society in activities such as nuclear-waste storage, carbon sequestration, precision agriculture, archaeology, crime-scene investigations, and cultural-resources management.Near-surface geophysicists conduct a huge range of scientific and engineering investigations in service of society (Doll et al., 2012;Everett, 2013).Moreover, near-surface geophysicists address basic scientific questions relevant to a spectrum of natural processes that include biogeochemistry, coastal processes, climate change, ecology, hydrology, tectonics, volcanology, and glaciology (Slater et al., 2006).
Near-surface geophysical data are now routinely used, for example, in basic geoscience investigations such as environmental remediation, natural-hazards risk assessment, water resource management, gas hydrates and permafrost studies, glacier and ice-sheet mass transport, watershed-scale and coastal hydrology, fault-zone characterization, and the reconstruction of Earth's tectonic, volcanic, and extra-terrestrial impact history.The aim of the paper is to delineate the near-surface lithology and velocity layering using core and seismic refraction data acquired in Kolmani Field in the Northern Benue Trough during reflection seismic survey.

Uphole Seismic Refraction Method
In Uphole seismic method (Figure 1), a deep hole is drilled, and an uphole tool, a detector cable is placed in the hole.An energy source (hammer) placed on earth surface at an offset of 3m away from the well.A seismograph on the surface measures the arrival times, T, of the generated sound energy to the detector down the hole.These arrival times are corrected for the slant travel and source paths are then plotted against the detector depths, X.The velocities of the layers are computed automatically from the reciprocals of the slopes of the straight-line segments.The generalized depth equations of the subsurface layers are:

(
) The thicknesses, for a three-layer case are: The time intercepts t1 and t2 are read from the plot and their values used to determine the thickness of the subsurface layers.

Geologic Lithostratigraphic Descriptions
The nature and structure of a given soil depends on weathering, decomposition, erosion processes (US Brureau of Reclamation, 1998).Soils possess a number of physical characteristics which can be used as aids to size identification in the field.A handful of soil rubbed through the fingers can yield sand (and coarser) particles which are visible to the naked eye.Silt particles become dusty when dry and are easily brushed off hands and boots.Clay particles are greasy and sticky when wet and hard when dry, and have to be scraped or washed off hands and boots.Clays include silty clays and clay-silt.Clay grains are usually the product of chemical weathering or rocks and soils.Clay minerals are produced mainly from the chemical weathering and decomposition of feldspars, such as orthoclase and plagioclase, and some micas.They are small in size and very flaky in shape.The key to some of the properties of clay soils, e.g.plasticity, compressibility, swelling/shrinkage potential, lies in the structure of clay minerals.The range of particle sizes encountered in soil is very large: from boulders with a controlling dimension of over 200mm down to clay particles less than 0.002mm (2mm) (Figure 2).Some clays contain particles less than 1 mm in size which behave as colloids.

The Benue Trough
The Benue Trough (Figure 3) is regarded as rift basin in central West Africa that extends NNE-SSW for over 1000km in length and 150km in width.The Benue Trough is bounded to the North and South by Chad and Niger Delta basins, respectively (Nwajide, 2013;Matheis, 1976;Abubakar, 2014;Abubakar et al., 2019).Cretaceous sediments with some Tertiary sediments filled in the Benue Trough (Figure 3) and attained a total thickness of about 6000 m (Carter et al., 1963;Abubakar, 2014;Abubakar et al., 2019;Zaborski et al., 1997).The sediments were associated with some Tertiary volcanism (Zaborski et al., 1997).The Benue Trough was formed during the breakup of Gondwanaland in the early Cretaceous that led to the separation of Africa and South American plate (Matheis, 1976).
King and Stoneley proposed a rift system model associated with tensional movement and host and graben model, respectively, for the evolution of the Benue Trough (King, 1950;Stoneley, 1966).Grant proposed a Rift Rift Failed (RRF) triple junction model that led to plate dilation and the opening of the Gulf of Guinea (Grant, 1971).
Aulacogen model was proposed whereas a wrench faulting was considered the prevailing tectonic process during the evolution of the Benue Trough (Olade, 1975;Benkhelil, 1982;Matheis, 1976).The Benue Trough is subdivided into Northern, Central and Southern portions (Nwajide, 2013).The study area is situated in the Northern Benue Trough.
The Northern Benue Trough is the North-eastern end of the Benue Trough and it is divided into the N-S trending Gongola Sub-Basin and the E-W trending Yola Sub-Basin (Nwajide, 2013;Zaborski, 1988).In the Gongola Sub-basin, the oldest lithostratigraphic unit (Figure 4) is the Aptian-Albian thick continental Bima Sandstone that unconformably overlies the Precambrian crystalline basement rocks.Kolmani is in the Benue Trough of Nigeria within which the study area is located is the most important of all the Cretaceous sedimentary basins of Nigeria.It is an elongated rift approximately 1000km long and 50-150km wide tending NE-SW and overlying the pre-Cambrian shield of the West African Mobile Belt (Benkhelil, 1989;Guiraud, 1990;Sidi et al., 2015).The Benue Trough is an elongated partly fault-bounded depression occupied by up to 6,000m of marine and fluvio-deltaic sediments that have been compressionally folded into a non-orogenic shield environment (Carter et al., 1963;Wright, 1985).The northern limit of the trough is the southern boundary of the Chad Basin, which is in turn separated from the trough by an anticlinal feature called while the southern limit of the trough is the northern boundary of the Niger-Delta (Zaborski et al., 1997).Marine and fluvio-deltaic sediments infill the entire Benue Trough ranging from Late Aptian to Paleocene in age (Adegoke et al., 1986;Allix, 1983).The Benue Trough runs through the NE-SW central parts of Nigeria from the Niger Delta in the SW right up to the Bornu Basin in the NE of Nigeria.It is subdivided arbitrarily into lower, middle and upper portions.
The Northern Benue Trough comprises of the area extending from the Bashar-Mutum Biyu line as far north as the 'Dumbulwa-Bage High' which separates it from the Borno Basin (Zaborski, 1998).It includes an trending Yola Arm and an N-S trending Gongola Arm which are separated by an area structurally dominated by four major NE-SW trending sinistral strike-slip faults, the Gombe fault, Bima-Teli fault, Kaltungo fault and Burashika fault.A group researchers referred to the median zone as Zambuk Ridge (Carter et al., 1963).The Cretaceous succession in the Upper Benue Trough comprises of early Cretaceous continental clastics, the Bima Group, Yolde Formation, Pindiga Formation and lateral equivalents in the Yola Arm which are the Dukul Formation, Jessu Formation, Sekule Formation, Numanha Shale and Lamja sandstone, sandy members of the middle part of the Pindiga Formation in the Gongola Basin, Gombe Sandstone, Kerri-Kerri Formation and Neogene to Quanternary Basalts (Zaborski et al., 1997).

The Bima Formation
The continental Bima Group comprises the oldest sediments in the Northern Benue Trough which directly and unconformably overlie the crystalline Basement rocks.Several workers described the Bima Group; however, provided the most detailed account and described its three parts as (Guiraud, 1990): Lower Bima Sandstone (B1) is a highly variable unit with an overall thickness of 0-1500m.Individual lithofacies distribution was controlled by synsedimentary tectonic activity which created a number of sub-basins with associated volcanism.Lithofacies association within a sub-basin consist of conglomeratic alluvial fan or debris flow deposits, adjacent to active basin-forming faults, grading laterally into fining-upward fluvial sequences into lacustrine deposits with interbedded clays, fine-grained sandstones and calcareous sandstones.In the sector of the Gombe Inlier, the Bima Group is greatly attenuated.It is represented by pebble conglomerates and poorly sorted arkosic sandstone similar to the lower Bima Formation elsewhere.Radiometric ages from intercalated lavas suggest an age from Albian to close to Jurassic-Cretacous boundary (Guiraud, 1990).
The Middle Bima Sandstone (B2) which is widely distributed, fairly uniform unit consisting of fining upward cycles each 5-10m thick.It has trough and tabular cross bedding and clays and palaeosols may occur at the top of individual cycles.Overall thickness ranges from 100-500m.In a study they reported fossil wood, silicified Metadopocarpoxylon libanoticum (Edwards) to which a Late Aptain age was assigned (Guiraud, 1990).The Upper Bima Sandstone (B3); a fairly homogeneous, relatively mature, fine to coarse grained sandstone characterized by tabular crossbedding with sets of a few tens of centimeters to a few meters thick.Convolute bedding and overturned cross bedding are common.Its thickness ranges from 500-1500m.

The Yolde Formation
The name Yolde Formation was proposed for the transition beds recognized earlier between the Bima Group and the Pindiga Formation (Carter et al., 1963;Falconer, 1911).A type section was designated in the Yolde stream in the western part of the Yola Arm.The Yolde Formation gives rise to a subdued topography often with sparse vegetation cover and is poorly exposed in most part of the Gongola basin.The Yolde Formation is indeed a transitional sequence between the continental Bima Group and the marine deposits of the lower part of the Pindiga Formation.The lower sandstone-mudstone portion of the Yolde Formation is interpreted to be of fluvial origin, the upper portion with more thinly and regularly bedded bioturbated sandstones and low diversity bivalve faunas at the top is of shallow marine origin.Adegoke suggested a Late Albian to Late Cenomanian age for the Yolde Formation on the basis of its palynofossils (Adegoke et al., 1986;Adegoke et al., 1978).

The Pindiga Formation
The name Pindiga Formation was proposed for the calcareous beds and clay shales previously described (Carter et al., 1963;Suleiman et al., 2015).The Pindiga Formation developed in the Gongola Basin is equivalent to Dukul Formation, Jessu Formation, Sekkuliye Formation, Numanha Shale and Lamja Sandstone of the Yola Basin.In the Gongola-Gombe sub basin, the Gongila Formation and Fika Shale are the lateral equivalents of the Pindiga Formation.Lithologically, the Pindiga Formation is made up of dark/black carbonaceous shales and limestones intercalated with pale coloured limestone, shale and minor sandstone.Earlier, regarded the Pindiga Formation as consisting of five members, from bottom to top, the members are the Kanawa member, Dunbulwa member, Deba-Fulani member, Gulani member and lastly the Fika member (Zaborski et al., 1999).

The Gombe Sandstone
Overlies the Pindiga Formation and represents the youngest Cretaceous sediments in the Gongola arm of the Upper Benue Trough, The Gombe Sandstone is a sequence of estuarine and deltaic sandstones, shales, siltstones and ironstones which overlie the Pindiga, Gongila and Fika shale Formations in the Gongola arm of the Upper Benue Trough.Falconer named Gombe Sandstone near Gombe town as Gombe grits and clays and assigned an Eocene age to it (Falconer, 1911).Ahmed described the Gombe Sandstone at Kware stream as consisting of well bedded fine to medium grained sandstone, sandy and silty micaceous shale and occasional mudstones (Ahmed, 2007).Akande described the Gombe as consisting of three (3) major lithofacies; lower bed characterised by mudstones and ironstones, middle bed composed of well bedded sandstones and siltstones, while the upper bed is composed of crossbedded sandstones and siltstones (Akande et al., 1998).
The Maastrichtian Gombe Sandstone is lithologically similar to the Bima sandstone, attesting to the reestablishment of the Albian palaeoenvironmental conditions (Obaje and Abaa, 1996).Gombe State consists of fourteen ( 14

The Kerri-Kerri Formation
represents the record of Early Tertiary sedimentation in North-eastern Nigeria and overlies the Cretaceous Gombe Sandstone unconformably in the Gongola Basin of the Upper Benue Trough.The Formation is essentially flat laying to gentle dipping of about 5 0 (Carter et al., 1963).The Kerri-Kerri Formation consist predominantly of grits and ferruginous sandstones, siltstones and claystones often kaolinite with well-developed crossbedding.The maximum thickness of the Kerri-Kerri Formation is about 300m, although it varies from 300m to over 320m (Dike, 1993).This sequence is deposited in a wide range of environments inducing fluviatile, deltaic and marginal lacustrine (Dike, 1993).The type section of this Formation is exposed at Kadi about 100km north of Kaforati.The Formation is considered to be Palaeocene in age.

Climate, Geomorphology and Soil of Study Area
According to a study, Gombe State which is popularly referred to as the 'Jewel in the Savannah' was carved out from the former Bauchi State by the Federal Military Government of Nigeria on 1st October, 1996 (Ikusemoran et al., 2016).The State shares boundaries with Yobe State to the north, Adamawa and Taraba States to the south, Borno State to the east, and Bauchi State to the west.In Gombe State Official Website, the topography of Gombe State is made up of mainly mountainous, undulating and hilly to the south-east and flat opens plains in the weatern part of the State (Gombe State Tourism Board, 2014).Gombe State Tourism Board also described the hills of Gombe State as having beautiful dome shaped rock formations with fascinating scenery which are ideal for climbing and camping (Gombe State Official Website, 2015).The Famous Tangale dome shaped rock/hill formation, Tula Hills, the Bima hills as well as Filiya-Dadiya hills and the Ndinijam Kargo Hill are some of the hills in these mountainous regions.
Most parts of the State are located within the valley of River Gongola and on some highlands and mountain ranges.The river basin is a generally lowland area with altitudes ranging from 184 to 351m heights above sea level.They concluded that more than 33% of the land of Gombe State lies within the Gongola valley.This River Gongola valley formed an east-west narrow land area in Dukku and Nafada LGA at the extreme northern part of the State, and except the intrusion of Kulani-Degri-Likkar hills in the southern part of the state, the entire western border of the state comprised the river basin.The highest peak of the highlands in Gombe State is Kilang hill in Shangom LGA with altitude of 1158m above the sea level.Only 5.8% of the land areas of Gombe State are of highlands, with Kaltungo, Balanga and Shongom LGAs having more than 10% of their land area as highlands (Ikusemoran et al., 2016).
River Gongola is the largest and the most important river in the State.The River enters the State in the north-west through Bauchi State and flows eastward through Dukku, and Nafada LGAs.At Nafada, the Gongola bends in a loop southward forming natural boundary between the state and Borno and Adamawa states in the eastern parts before it joins River Benue at Numan, outside the state (Ikusemoran et al., 2016).According to Abubakar, Gombe state has two distinct climates, the dry season (November-March) and the rainy season (April-October) with an average rainfall of 850mm (Abubakar, 2013).The mean annual temperature is about 32˚C, while the vegetation of the state is that of savanna woodland comprising scattered shrubs and trees.

Determination of Orthometric Heights
The data for this work was acquired in the course of acquiring 3D seismic reflection data.Field equipment used for the study include a map of the study area, sampling bags, Global Position System (GPS), Camera, measuring tape, masking tape, marker pens, field notebook, pen, pencil, shovels, and drilling rig.

Up-Hole Drilling, Sample Collection and Description
The ellipsoid heights of the selected Uphole points were determined using Leica Viva GS10 GPS.The mud rotary method was used to drill the boreholes.After drilling the required depth, the drill stem was pulled.The sample-catching tool was lowered with a depth-measurement string to bring soil/rock sample to the surface.The samples were placed in containers and labelled (Figure 4), and analysed.The general description included colour, grain size, texture and mineral assemblage (Wentworth, 1922)., 1m, 2m, 3m, 4m, 5m, 7m, 10m, 15m, 20m, 25m, 30m, 35m, 40m, 45m, 50m, 55m, and 60m.
A hammer was used as the energy source and placed 3m away from the hole to obtain the first breaks (Figure 1).The recording parameters are in Table 1.NZXP Strata Visor Refraction Seismograph equipment was used to record the arriving energy from hammer source.The processing of the data was carried out using UDISYS Version 1.0.0.0 processing software.
For each shot a single-trace record was produced (Figure 5).The first breaks pick were digitized and interpreted using UDISYS Version 1.0.0.0 software, and produce T-X graphs (Figure 6) from which velocities and depths were computed.

RESULTS AND DISCUSSION
Results are presented in Tables 3 and 4 and Figures 7 -13.

Lithostratigraphy
The Lithology has dominantly Laterite, Clay, and Coal with intercalations of cobblestones, shale, sandstone, and silt (Table 3).The lithologies of the wells could not be correlated due to the highly variable elevation and the intensive intercalations of the soils and rocks as shown in Figure 7.

CONCLUSION
The rock layers in the Kolmani Field have three distinct layers specified as follows, namely, top weathered and sub-weathered layers made up of intercalation of laterite cobble, pepple, clay, and silts.The consolidated layer is made up of intercalation of sandstone, gravel ash clay and muddy coal shale.The wells' lithologic strata are not correlatable, and the elevation is highly variable which ranged from 317m and 524m with average of 389.16m.Top weathered layer of laterite intercalated with cobblestones with compressional wave velocity ranging from 342 ms -1 to 517 ms -1 with an average of 405.03 ms -1 (b) sub-consolidated Clay intercalated with silt and laterite of compressional wave velocity ranging from 440 ms -1 to 1854 ms -1 of average of 826 ms -1 (c) underlying consolidated layer is the consolidated clay intercalated with shale and coal layer having compressional wave velocity ranging from 1518 ms -1 to 4201 ms -1 with an average of 2162.65 ms -1 .The dominant lithologic sequences encountered are laterite, clay, silt, sand, gravel, coal and shale.The results of this work can used in seismic processing, planning and assessing risk for engineering structures, and for groundwater exploration.The laterite, clay, silt, sand, gravel, coal and shale components can be utilized in agriculture, construction, process industries, and environmental remediation.

Figure 3 :
Figure 3: Map of Nigeria showing the Study Area

Figure 4 :
Figure 4: Examples of Rock/Soil Samples4.3Up-Hole Seismic Data Acquisition and ProcessingBefore starting data acquisition operations, the cables, geophone, energy source, and the NZXP Seismograph were tested for efficiency and proper timing.The refraction recording spread consisted of a single SM4, 10Hz geophone positioned as shown in Figure1.Each hole was drilled to a depth 65 meters in each case, out of which 60 meters was the maximum depth of recording in most cases.The receiver depths were: 0m, 1m, 2m, 3m, 4m, 5m, 7m, 10m, 15m, 20m, 25m, 30m, 35m, 40m, 45m, 50m, 55m, and 60m.A hammer was used as the energy source and placed 3m away from the hole to obtain the first breaks (Figure1).The recording parameters are in Table1.NZXP Strata Visor Refraction Seismograph equipment was used to record the arriving energy from hammer source.The processing of the data was carried out using UDISYS Version 1.0.0.0 processing software.For each shot a single-trace record was produced (Figure5).The first breaks pick were digitized and interpreted using UDISYS Version 1.0.0.0 software, and produce T-X graphs (Figure6) from which velocities and depths were computed.

Figure 5 :
Figure 5: Typical Seismic Monitor Record from the Survey with a single geophone.

Table 3 :
Core Samples descriptions from well-bore cuttings