Sedimentological Characterization of the Pre-Santonian Siliciclastic Deposits of Lower Benue Trough, Southeastern Nigeria

Field observations revealed that the sandstones of the Awi Formation are angular to subangular, feldspathic, and generally fines upward and Amaseri are calcareous, indurated and laterally extensive. Agbani Sandstone consists of thick vertical sequences of ferruginized and indurated sandstones. Granulometric study shows medium to coarse-grained (0.78-1.30Φ), poorly to moderately sorted (0.87-1.19Φ), coarse to strongly fine skewed (-0.17-0.57Φ) and very platykurtic to leptokurtic (0.66-1.31Φ). This indicates mostly high energy condition and this is characteristic of fluvial environments. Petrographic studies for Awi samples indicate that they are texturally immature to submature, subarkosic arenites from a continental block provenance, while Amaseri Sandstones are texturally submature sublitharenites from a recycled orogen provenance. Agbani Sandstones are texturally immature to mature subarkosic arenites, sublitharenites and quartz arenites. Heavy mineral studies revealed zircon, tourmaline, rutile and hornblende to be common to all formations, sillimanite, kyanite, garnet, and staurolite are mostly common to Awi and Amaseri Sandstones while Agbani Sandstone has a dominance apatite with spinel. This suite of minerals suggests an acid igneous rock to typical dynamothermal metamorphic rock sources. The ZTR indices (74.0-94.4%) indicate mineralogically mature sediments.


Introduction
The Benue Trough is defined as an Intracontinental Cretaceous basin about 1000 km in length stretching in NE-SW direction and resting unconformably upon the Precambrian Basement (Benkhelil et al., 1989).Offodile, (1989) stated that geologically, the Benue Trough consists of a linear stretch of sedimentary basin running from about the present confluence of the River Niger and Benue to the northeast and bounded roughly by the Basement Complex areas north and south of the Benue River (Figure 1).There are several views about the evolution of this sedimentary basin but most agreed that the evolution of the Benue Trough is intimately connected to that of the South Atlantic ocean and that it is the failed arm of the Gulf of Guinea, South Atlantic and Benue Mesozoic triple junction whose center is occupied by the Niger Delta (Grant, 1971;Burke et al., 1972).The rift nature of the Benue Trough was first suggested by King (1950), but subsequently, several contradicting views were proposed (Freeth, 1990).Despite these diverse views, the most widely accepted is that proposed by Olade (1975), Burke (1976), Ofoegbu et al. (1990), andMurat (1972).
Though, some aspects of the sedimentology and stratigraphy of the pre-Santonian siliciclastic sequences within the Lower Benue Trough has been investigated by several authors (Reyment, 1965;Dessauvagie, 1972;Murat, 1972;Petters, 1983;Asseez, 1989;Kogbe, 1989;Benkhelil et al., 1989;Amajor, 1990;Obaje et al., 1999;Nton, 1999).However, the present study focuses on the holistic approach to field observations and sedimentological characterization of outcrop sections of the Awi Formation along Calabar-Ikom Expressway, Amaseri Sandstone in Amaseri Town, and Agbani Sandstone at Ugwueme and Ugwuokwute (Figure 2).The study provides detailed geology, lithologs, and lithologic descriptions of these sections, evaluate the textural and mineralogical characteristics of these sandstone units and ascertain their depositional environment, provenance and predicting the nature and state of the energy of the medium that transported and deposited them.delta complexes, with sediments having been derived from the Basement complex in areas of Ogoja while the Eze-Aku Shale may represent the prodelta clays and deeper water deposits associated with these complexes (Whiteman, 1980).The Eze-Aku Group grades into the marine Awgu Shales which consists essentially of grey-blue shales with subordinate limestone bands and calcareous sandstone.Agbani Sandstone which is the lateral equivalent of the Awgu Shales is made up of medium to coarse grained bioturbated and cross-bedded sandstones and laminated brownish clays.This depositional cycle was interrupted by the Santonian tectonic movement that folded and faulted the sediments in the trough.The Santonian folding was accompanied by extensive magmatic activity resulting in the formation of extrusive and intrusive igneous bodies which increase in amount and diversity from the Northeast to Southwest of the basin.Based on stratigraphic evidence, the total thickness of the entire sedimentary pile or depth to the Basement in the Lower Benue Trough as estimated from the thicknesses of individual units is given as 4.8 km (Burke et al., 1972;Ofoegbu et al., 1991).

Methodology
The methodology adapted in this work is divided into two sections; field observations and laboratory analysis.

Field Observations
Forty-one fresh sandstone (spot samples) were collected from four outcrops in three different localities.Samples collected from Location 1 (Calabar-Ikom Road) are representative of the Awi Formation (Figure 4) while samples from Location 2 (along Amaseri-Okposi road, close to Amaseri Crushed Stone Industry) are representative of Amaseri Sandstone (Figure 8).All samples collected from Locations 3 (Ugwueme) and 4 (Ugwuokwute) belong to the Agbani Sandstone unit (Figures 10 and 13).
It was observed in almost all the outcrops studied that most of their sedimentary structures have been destroyed or obliterated by weathering but those still identified were measured, described and photographed.

Laboratory Analysis
Laboratory studies include; granulometric analysis and petrographic studies involving thin sections and heavy minerals analyses.

Granulometric Analysis
Nine (9) representative samples collected from the field were subjected to sieve analysis.Four (4) samples were collected from Awi Formation, one (1) sample from the top of Amaseri Sandstone and four (4) samples from several horizons in the Agbani Sandstone.These samples were dried, disaggregated and 100gm of each sample weighed.These samples were sieved using an automated Endecott's Sieve Shaker and a set of sieve meshes ranging from very coarse sand size (1 mm) to very fine sand size (75 µm).
The statistical parameters calculated were based on Folk and Ward (1957), Sahu (1964) and Friedman (1967).They include univariate parameters (mean size, skewness, sorting and kurtosis), bivariate and multivariate parameters.Moreso, the dip and azimuth of twenty five foreset planes of planar cross beds measured from the basal cross bedded section of the Agbani Sandstone were analyzed using variable constraints system by calculating the Mean Vector Azimuth (MVA) for the group of data collected from the field in order to ascertain the "mean vector" or specific flow direction of the transporting and depositional medium as well as paleocurrent pattern of the Agbani Sandstone.

Petrography
(a) Thin Section Petrography: Sixteen samples collected from different outcrop locations were selected for thin section petrography.Three samples were collected from Awi, two from Amaseri, and eleven from Agbani.Indurated samples were mounted directly after cutting while loosely consolidated were impregnated with epoxy resin before cutting and mounting on slides and polished to a thickness of approximately 0.03 mm.Sandstone classification was based on Folk (1968), paleotectonic and paleoclimatic settings were determined based on Dickinson et al. (1983), Dickinson and Suczek (1979) and Suttner et al. (1981) respectively.
(b) Heavy Mineral Analysis: Sixteen samples were used.Three belong to Awi Formation, two from Amaseri Sandstone while eleven belong to Agbani Sandstone.Dried samples were disaggregated, weighed (50 gm), and sieved through a 300 µm sieve mesh.10 gm of the sieved sample is pre-treated with dil.HCl to remove any carbonates and washed with water to remove any clay present.Only 5 gm of pre-treated sample was separated using bromoform in a separating funnel.The separated heavy minerals are then rinsed with acetone, allowed to dry, and then mounted on slides.Grains of both opaque and non-opaque minerals were counted using strips of masking tape mimicking the ribbon technique of Galehouse (1971).The counts of individual minerals identified are then tabulated and the ZTR indices for all samples determined based on Hubert (1962). www.ccsen

Field D Location 1
The log pl lies uncon Figure 5).The basal section of this outcrop is about 1.2 m thick, comprises of a brownish-orange, dirty, poorly sorted and coarse grained sandstone.At this base, there is oil seepage (Figure 11).h some white q 25 m thick lat rom left to rig hickness of 2.3 ng surfaces ap The mean size indicates the average kinetic energy (velocity) of the depositing agent (Sahu, 1964).Samples collected from the three formations, generally indicate high kinetic energy conditions of the depositing agent or medium, with a mild fluctuation to the low side accounting for the medium grain size.This is characteristic of fluvial environments.It is generally considered that sediments are usually more poorly sorted when they are closer to their source (provenance) and that mean size and sorting are hydraulically controlled so that in all environments, the finer the grain size, the better the sorting.This indicate that Awi Formation and Amaseri Sandstone are closer to their sources and haven't traveled very far, while samples from Agbani Sandstone indicate a relatively longer time of travel from their source (relatively farther from provenance).The poorly-moderately sorted nature of the sediments reflects an increased to moderate fluctuation of energy conditions of the depositional medium and hence, a very poor-moderate winnowing ability.Moreso, curves with excess fine material (a tail to the right) indicate coarse skewness and those with excess coarse material (a tail to the left) have fine to strongly fine skewness.Since the deposition of sediments is governed by the energy conditions of the transporting medium, a sudden drop in the energy of the depositing medium will cause the deposition of a higher percentage of sediments, coarser than the median size, thus making the sediments of the river fine to strongly fine skewed (Awasthi, 1970) as is seen in both Awi Formation and Agbani Sandstone.A gradual decrease in the energy of the depositional medium also causes the deposition of much finer grains.This condition therefore accounts for the near symmetrical nature of the sample representing Amaseri Sandstone (Figure 17e).
Frequency curves and histograms (Figure 17) for Awi Formation show a strong unimodal character for the basal units and a weak bimodal character for the overlying units.The only sample analyzed from Amaseri is of strong bimodal character, while three samples representing Agbani Sandstone display a strong unimodal character and one sample showing a bimodal pattern.

Thin Section Petrography
The compositional framework of all the samples subjected to thin section study is summarized in Table 3. From the table, it can be seen that for all formations, both metamorphic and plutonic (common) quartz (Figure 20) are present in varying proportions and ratios.According to the classification scheme of Folk (1968), sandstones from Awi Formation are subarkosic arenites, while those representing Amaseri Sandstone are sublitharenites.Agbani Sandstone comprises of both quartz arenites and subarkosic arenites with a minor occurrence of sublitharenite.Samples from Awi Formation have total quartz ranging from 70 to 77 %, while those from Amaseri and Agbani Sandstones range from 72 to 76 % and 66 to 82 % respectively.Computed averages are 73.3%, 74%, and 74.4% respectively.These quartz grains are predominantly angular to subangular with a few subrounded to rounded grains.While the monocrystalline grains show dominantly straight extinction with point to point (tangential) contacts, the polycrystalline grains contain 2 to 10 crystals with similar sizes and show a moderate to strongly undulose extinction with sutured grain contacts.The ratio of monocrystalline to polycrystalline quartz for all samples is also shown in Table 3. Feldspars from Awi are both sodic and potassic (albite and microcline), with some degree of alteration and range from 7 to 11 %, that of Amaseri Sandstone, range from 3 to 4 % and include albite and orthoclase, while the feldspar from Agbani Sandstone are basically plagioclase and orthoclase it range from 1 to 12 %.The average values are 9.7%, 4%, and 5.45% respectively.The ranges and averages for rock fragments in the formations studied are as follows: Awi Formation, 2-3%; (2.7%), Amaseri Sandstone, 6-10%; (8%), and Agbani Sandstone, 1-9%; (2.7%).Both muscovite and biotite are present in very minute amounts and non-opaque heavy minerals such as zircon and tourmaline could be identified.Matrix is present in all samples studied averaging 5.2% for Awi Formation, 6.8% for Amaseri Sandstone, and 5.5% for Agbani Sandstone.Constituents of these matrices include clay minerals, quartz and silt size feldspars (Figure 20).Cement constitutes a significant fraction for some samples (especially those from Agbani Sandstone).It ranges from 4-7.5% for Awi Formation, 4-5% for Amaseri Sandstone, and 2-20% for Agbani Sandstone.Iron oxide, silica (quartz) and calcite were the three cement types identified.
Using the classification scheme of Folk (1974) for textural maturity, samples from Awi Formation and Amaseri Sandstone range from texturally immature to submature while those of Agbani Sandstone range from texturally immature to mature.Based on compositional maturity classification which involves the use of the ratio of quartz to that of feldspar and rock fragment put together, the compositional maturity of the sand bodies of Awi Formation and Amaseri Sandstone is submature.Although most of the samples from Agbani Sandstone are compositionally submature, a few are supermature.

Discussion
Provenance interpretations for the samples studied encompasses the use of both their textural characteristics and mineralogical compositions, which of course, reflect their source rock composition, climatic, as well as tectonic settings.

Depositional Environments
Characteristics grain size distributions are governed largely by processes within the depositional environment (Cheel, 2005) and as such, properties like mean size, sorting, skewness and kurtosis, would cumulatively reflect these processes and provide a basis for interpreting ancient depositional environments.With that in mind, high energy conditions are thus considered to have prevailed dominantly during the deposition of the basal sand units of the Awi Formation but abruptly dropped during the deposition of the upper sand units.This energy drop is visible in the log motif of the Awi Formation which shows a fining-upward sequence (Figure 4).The drop in the velocity of the transporting medium stabilized during the deposition of the Amaseri Sandstone and at the lower units of the Agbani Sandstone, although further upwards on the stratigraphic sequence of the Agbani Sandstone, there are intermittent fluctuations between high and moderate energy conditions which is reflected by frequent increase and decrease in grain size as well as the degree of sorting.This is also visible in the log motif of the section where the samples were collected (initial fining-upward to subsequent coarsening upward) (Figure 10).Fining-upward sequences with basal lag (conglomeritic) within a single sand body as is the case for Awi Formation, implies a waning-current sequence, an upward decrease in fluid velocity within a channel, and typically characterizes meandering or tidal channels, alluvial point bars or deltaic distributaries (Catuneanu, 2002).But the cyclicity of this sequence in a single outcrop points to a play between rise and fall of eustatic sea level and basin subsidence at a local level and within a short period of time.The initial fining-upwards of the sandstone units of Agbani Sandstone would imply an upward deepening and a decrease in depositional energy (shoreline retreat) while the subsequent coarsing-upward sequence observed would depict an increase in depositional energy and shallowing upwards typical of a tidally influenced fluvial environments such as river mouth bar or delta front (Catuneanu, 2002).The overall implication is retrogradation and progradation of these clastic sediments.
Moreso, cumulative log probability curves of all samples collected (Figure 16a and 16b) show both two and three segment curves corresponding to suspension, saltation and traction fractions/loads of Visher (1969).While Awi Formation and Amaseri Sandstone are characterized by essentially two segment curves that are typical of fluvial environments (distributary channel and point bar), Agbani Sandstone is characterized by two and three segment log probability curves that indicate a fluvial to tidally influenced environment (channel and distributary mouth bar).From a bivariate plot of sorting against skewness by Friedman (1967) it was revealed that all sand samples analyzed are fluvial.The results from multivariate parameters revealed the sand bodies to be of fluvial deposits (Sahu, 1964).This further established the results from bivariate and univariate parameters.The variance value from paleocurrent analysis of Agbani Sandstone is 33.2 and it indicates that Agbani Sandstone was deposited in a fluvio-deltaic environment (Table 5) (Steinmetz, 1962).
Table 5. Paleocurrent analysis data for Agbani sandstone at Ugwueme

Mean vector Azimuth = Tan -1 [(∑ Sin A) / (∑ Cos A)]
Where ∑ Sin A = 0.2599 Ternary plots for framework components of sandstones sandstone classification (quartz, feldspar and rock fragment) (Folk, 1968) revealed that the Awi Formation are subarkosic arenites while those of Amaseri Sandstone are sublitharenites.Samples from Agbani Sandstone plotted within the subarkosic; sublitharenite and quartz arenite fields (Figure 22).It was observed that the quartz arenites of Agbani Sandstone are essentially products of leaching, alteration and possibly longer distance of transport from provenance.This is reflected in the nature of grain (Figure 20e).
The predominance of angular to subangular grains in most samples studied is suggestive of near source (Folk, 1974) and this together with the high percentage (>5%) of clay matrix accounts for the textural immaturity to submaturity observed in all samples from Awi and Amaseri.Agbani Sandstone on the other hand, is comprised of texturally immature to mature sand bodies that are alternating.The implication of this is that, there are possibly two alternating sources of sediment supply.Compositionally, the sand deposits of Awi Formation and Amaseri Sandstone are submature while those of Agbani Sandstone range from submature to supermature.The discrepancy in levels of maturity texturally and mineralogically can be attributed to factors such as a humid paleoclimate since it is of common knowledge that humid climates produce sands with higher proportion of quartz and lower proportions of labile components such as feldspar and lithic fragments.This is as a result of rapid weathering and as such, mineralogical maturity is facilitated at the expense of textural maturity.The Mean Vector Azimuth (MVA) from the paleocurrent analysis is 179Az (Table 5).This value and the paleocurrent pattern reflected in the rose diagram (Figure 19) for Agbani Sandstone suggests an essentially southerly direction of current flow at the time of deposition, while the provenance was from the north.
Considering the above, together with proximity, textural and mineralogical properties of the sandstone in question, the Northeastern (Obudu Plateau) and/or Southeastern (Oban Massif) Basement Complexes would be the most probable source(s).The presence of both plutonic and metamorphic quartz in all samples studied is indicative of a basement terrain source, while the absence of volcanic rock fragments in all samples suggests a non-volcanic source (Moorehouse, 1959).The presence and amount of labile minerals such as feldspars in samples representative of Awi Formation and some of Agbani Sandstone, is suggestive of a primary rather than a reworked source (Pettijohn et al., 1973).Average ratios for monocrystalline to polycrystalline quartz for Awi Formation, Amaseri and Agbani Sandstone is given as; 4:5, 2:5, and 18:23 respectively.These ratios indicate a relatively higher concentration of polycrystalline quartz in the total quartz fraction (Table 3).Although in the quartz arenites, the monocrystalline quartz with straight extinction dominates, the more common variety of the polycrystalline quartz is the stretched metamorphic quartz which is indicative of shearing or straining in the absence of recrystallization.This probably suggests that these rocks (especially Agbani and Amaseri Sandstones) have been affected by some level of stress greater than that which would have being caused by just overburden pressure (Santonian Orogeny?).The presence of polycrystalline quartz made up of more than 5 individual grains and strong undulosity points to a metamorphic source for these formations.The QFRF (quartz, feldspar and rock fragment) ternary diagram of Suttner et al. (1981) was used for delineating paleoclimatic settings.Although all samples from Awi and Agbani plotted and were evenly spread within the plutonic humid and metamorphic humid fields, samples from Amaseri plotted only in the metamorphic humid field (Figure 23).This implies that the Oban Massif and the Northeastern basement were subjected to humid climatic conditions during the Lower to early Upper Cretaceous times and this interpretation is consistent with that of Hoque (1976).
QFRF ternary plot by Dickinson (1982) used for typing provenances and inferring paleotectonic settings revealed that all the sampled sandstone units were sourced from a Craton Interior with the exception of Amaseri Sandstone which plotted in the Quartzose Recycled field (Figure 24).The QFRF ternary plot of Dickinson and Suczek (1979), show that thesandstone deposits of Awi Formation are products of a continental block provenance while Amaseri and Agbani Sandstone are sourced from both continental block and recycled orogen provenances (Figure 25).The fact that most of these sandstone units were derived from a continental block provenance would explain why they are mostly texturally immature.This is because, in the setting proposed, two tectonic sources are possible.If the continental block has recently split as a result of continental rifting, sands shed from fault bounded uplifts of the continental basement will accumulate in the troughs not far from their source and as such, will not undergo much transportation.Moreso, the rapid erosion from the high relief areas would give rise to sandstones that are quartzo-feldspathic (arkoses).But in the case of the sandstones under study, they were either probably subjected to a humid climate which altered the mineralogy to give rise to subarkoses instead of arkoses as would be expected or the sands were derived from high topographic areas located long distances from the depositional areas and as such, the sands are more quartz rich, showing a higher degree of mineralogical maturity.The inferred paleotectonic settings seem more likely to be correct since it corresponds with the tectonic history and setting of the Benue Trough as proposed by Olade (1975).It is therefore pertinent to point out that the change in sandstone type or difference in composition of sandstones as one moves from older (Awi Formation) to younger (Agbani Sandstone) formations is due to factors such as nature of provenance relief, distance of transport, rates of sedimentation, diagenesis and leaching since all the sandstones are believed to be derived from a common source (Obudu Plateau and/or Oban Massif).The number of heavy mineral species identified in all samples studied is given as follows: 11 (Awi Formation), 9 (Amaseri Sandstone) and 9 (Agbani Sandstone).The heavy mineral assemblages present in all samples studied, show that zircon, tourmaline, rutile, hornblende, garnet and staurolite are common.Mineral species such as Kyanite are peculiar to Awi Formation and Agbani Sandstones, apatite was found only in a single sample from Awi Formation, absent from Amaseri Sandstone and abundant in samples from Agbani Sandstone.Sillimanite is rare in Awi Formation and Agbani Sandstone but is common in Amaseri Sandstone.Topaz was found only in Awi Formation and Amaseri Sandstone but completely absent from Agbani Sandstone.Spinel (hercynite) and Zoisite were found only in Agbani Sandstone and Awi Formation respectively.The presence of zircon, topaz and apatite points to acid igneous rocks as source, while tourmaline, rutile, hornblende, and garnet indicate a combination of both acid igneous rocks and contact to regionally metamorphosed rocks as sources.Minerals such as sillimanite, staurolite, kyanite, zoisite and spinel (hercynite) are typical pointers to solely metamorphic rocks as source (Mange & Maurer, 1992).As already stated in the heavy mineral descriptions, most of the zircon, tourmaline and rutile identified are euhedral and prismatic indicating closeness to source area.According to Mange and Maurer (1992), zircons of arkoses are mostly sharp euhedral crystals or euhedral grains with rounded terminations (Figure 21 A, B, and C) but well rounded forms are not common.
The ZTR indices calculated for all samples show that they are mineralogically mature (74.1-94.4%).A ternary plot adapted from Hubert (1962) for normalized values for zircon, tourmaline and rutile showed most samples plotting closer to the tourmaline apogee and this reflects the abundance of tourmaline in almost all samples (Figure 26).

Conclusion
Generally, the field observations such as sedimentary structures and grain size analysis used for this study have establish the siliciclastic deposits of the Lower Benue Trough as products of a fluvial to tidally influenced fluvial (fluvio-deltaic) environments.These deposits are sandstone units of Awi Formation, Amaseri Sandstone and Agbani Sandstone.The kinetic energy of the transporting medium of these sediments based on univariate parameters such as mean size, sorting, skewness and kurtosis has been established to be high to moderate during the time of deposition.Sedimentary structures such as planar cross beds and ripple laminations have also been used to explain the nature of the stream power at the time deposition.Thin section petrography was effectively used to classify these siliciclastics into subarkosic arenites (Awi Formation and Agbani Sandstone), sublitharenites (Amaseri and Agbani Sandstones), and quartz arenites (Agbani Sandstone) derived mainly from a continental block provenance under humid climates.It was also observed, that these sediments become texturally immature-submature-mature as one moves from the oldest (Awi) formation to the youngest (Agbani).This has been attributed to several factors already stated in this report.The compositional framework of the rocks under study has also pointed to a basement terrain as source and in this case it is considered that the Northeastern (Obudu Plateau) and Southeastern (Oban Massif) are the most likely sources.Heavy mineral studies from the present study have shed more light on the closeness of these siliciclastic deposits to their source area and their high mineralogical maturity.This study has therefore been able to establish the depositional environment, provenance and nature of the pre-Santonian siliciclastic deposits of the Lower Benue Trough based on their sedimentological characteristics.

Figure 1 .
Figure 1.(a) Map of Africa showing Nigeria; (b) Map showing Nigerian basement complex and sedimentary basins.Note the trend and structure of the Benue Trough (BT)

Figure 2 .
Figure 2. (a) Map of Nigeria showing sedimentary basins with the southeastern sedimentary basins outlined with a box.(b) Geologic map of southeastern Nigeria showing the lower Benue trough and outcrop locations of the present study (Modified afterOkezie, 1984) Figure 5.

Table 4 .
Data presentation for heavy mineral identified in samples collected from Awi, Amaseri and Agbani Sandstones