GEO LOGICAL AND GEO TECHNICAL ASSESSMENT OF SOME DERELICIT BARITE FIELDS  IN  TH E ABAKALIKI  BASIN SOUTH EASTER N NIGERIA

ABSTRACT

The proliferation  of abandoned  surface  excavations  in the Benue  Rift  not only contributes  to land degradation  and landscape disruption but also results in loss of economic deposits,  dearth of mining data and generation of unprotected  spoils with known potential to contaminate  ground and surface water sources.  This study employed field description and measurements, laboratory testing and numerical simulation techniques to evaluate some derelict barite fields in the rift with the aim of factoring geological and geotechnical issues that will enable the reclaiming  of untapped  reserves  and forestall the creation  of abandoned  excavations.  Field studies indicate a consistent rock sequence in which exposures of arkosic sandstone are overlain by profusely fractured  shale and both  rock types play host to barite  ore deposits,  brine ponds  and intrusives.  The area displays some major  lineaments which exhibit,  in order of decreasing magnitude, N-S ,NE-SW, NW-SE, and E-W structural styles.  Ore deposits occur in varied modes as disseminated  nodules,  strata-bound  deposits as well  as  in two  dominant  NW-SE  and N-S  trending  vein  sets with  steep  dips.  The modes  of occurrence, structural  styles  and lithofacies  associations  predispose  the ore  deposits  to manual  extraction  and vertical stripping  using  surface  excavations.  The  complex  and varied  geologic  setting  of ore  incidences  predicate significant errors in interpretation of their geophysical signatures and obtaining reliable ore quantity estimates. Ore  grades  vary widely  due to the varying  geologic  framework  and appear  to depend  on such factors  as mining  depth,  presence  of gangue  minerals  and  location  within  the  barite  fields.  Groundwater   flow  is constrained  in an unconfined  setting by the  fractured  and weathered  sections  of the lithologies,  driven by structural dips and topographic  gradient with the ores and intrusive rocks forming as seals.  The extraction of the permeable barriers by excavation inadvertently reverses and directs flow towards the excavation leading to groundwater  incursion and slope failures.  The use of diversion ditches,  sump and pump technique  and slope unloading  will  achieve  stable excavations.  Unreliable  reserve  estimates,  ore  grade migration,  groundwater invasion and the associated slope distress are the major impediments that render surface excavations  derelict. Optimum location of excavations should be preceded by exploratory drilling and analysis of groundwater flow regmme.

CHAPTER  1

INTRODUCTION

1.1 PREAMBLE

Globally,  an  estimate  of over  a hundred  million  people  including  men,  women  and children are engaged directly or indirectly  in artisanal abstraction of minerals and construction materials  in over fifty developing  countries  (Darby and Lempa, 2006). Despite the economic contribution  and social significance  of artisanal  mining,  it  commonly  attracts  critical scrutiny from government agencies,  major mining companies and environmental activists.  Such repulsive view follows from environmental  and health problems  that emanate from the processes  of ore extraction at small and artisanal  scale.  At such scale,  extraction protocols  are not systematic, which  invariably  predict  difficulty  in  implementation  of regulatory  framework  (Darby  and Lempa,  2006)  such  as  labor,  environment  and  safety  standards.  The  main  challenge  to  the regulators  is  the unorganized  nature of artisanal mining which often leads to oversights.   The consequences  of the unfortunate  situation are not only child labor and fatal accidents but also loss of deeper seated ore reserves and dearth of mining data.   These issues will become more aggravated at increasing  depth and size of surface excavations that artisans often adopt as the major extraction technique.  For instance,  there is a high safety,  economic and material stability risks often associated  with deepening  artisanal  surface  excavations.  Further,  artisanal  mining encourages the proliferation of derelict surface excavations and indiscriminate dumping of spoil heaps both which may not only result in land degradation and devaluation but also constitute a potential contamination threat to surface and groundwater sources. Unfortunately, these environmental and safety problems may continue to thrive in developing countries due, in part,

to the economic benefits  associated  with artisanal  mining and growing demand  for industrial minerals.

The world energy demand will continue to be on the increase due to the increasing world population  and the unremitting  quest for industrialization.  This trend expectedly will not only stress  the  production  rate  of major  energy  sources  such  as  oil  and  gas  but  also  mount  a consequent  increase in demand for materials needed  in their exploitation. The availability and supply of project performance-enhancement  geomaterials are inadvertently  constrained to keep pace with their demand in extractive domains. For instance, as more oil and gas are required to combat the world energy situation,  more drilling fluid materials and additives such as bentonite and barite will be needed.  Barite (Bas0,)  is incorporated in drilling  fluids.  Its high specific gravity enables drilling mud to perform the vital function of equilibrating  formation pressures while drilling. The major aim is to mitigate wellbore problems which are known to be dependent on formation pressure gradients. Notable among such drilling challenges include lost circulation, stuck pipe,  blowout and borehole instability.  In addition, barite aids the mud in up-hole haulage of drill cuttings.   Beside the application in drilling,  barite ores are invaluable  in production  of barium based chemicals and X-ray contrast materials.

The importance ofbarite as an industrial mineral stems from its invaluable utilization as a major constituent   of drilling mud,  and in production of industrial wares such as glass, radiation shields as well as source for barium based-chemicals.  The efficient and economic use of barites in  any  of these  civil  and  industrial  applications  necessitates  that  the  ores possess  desirable qualities and properties  comparable  to generally acceptable  specification  standards  (Manning,

1995).  Furthermore,  its inherent properties and geologic disposition are pertinent considerations in reserve evaluation to justify safe and economic exploitation.

A thorough knowledge of the subsurface disposition and spatial distribution of ores is an important  consideration  in  the  intelligent  planning  and  design  of their  economic  and  safe exploitation strategy.  Although ore deposits may outcrop to the earth surface in some locations, field experience shows that significant and greater proportions of the ores are often masked and obscured  from direct  observation  by the  host rocks.  In ore mining,  therefore,  a precise  and adequate knowledge of the exposed and concealed ore dimensions and extent are often required. In that case,  various mineral exploration techniques  such as remote sensing (Harris and Copre,

2002),   geophysical   (Dobrin   and   Savit,   1988),   geotechnical   and   geochemical   methods (Foster,1993)  are usually adopted either as a single approach or in combination,  depending on the  complex   nature   of the  geologic   domain   and  the  exploration   target,   to  unveil  the characteristics  of the economic  mineral deposits  (Brock,1973;  Tanner  and Gibb,1979;  Dobrin and Savit,1988).

1.2 Literature Review

The  frequent  occurrence  and co-association  of base  metal  mineralized  zones,  igneous bodies  (Tertiary  volcanics  and  basic-intermediate   intrusives)  and  brine  ponds  in  the  Benue Trough (Figure 1.1) has been noted and reported by many authors including McConnell (1949), Farrington, (1952), Olade(1975),   and Offodile, (1989). These reports concluded that ore mineralization  can be ascribed to magmatic- hydrothermal  origin in a prevailing  mesothermal condition   during   the  pre-Turonian   times.   The   authors   also  believe   that   the  precursor hydrothermal  fluid was generated by the accompanying  Tertiary and Recent volcanics.  Akande et al (1989), apparently disagreeing slightly, based on fluid inclusion and stable isotope studies thought that expulsion ofbasinal brines due to sediment consolidation and overpressuring as well

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ig. 1.1. Distribution of lead-zinc-barite and salt mineralization along the Benue Trough (modified after Cratchley and Jones 1965). Inset sketch map of, Nigeria showing the Benue Trough.

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as gravity driven fluid flow are the major mechanisms that generated the metal bearing fluid that formed the ores in   fractures that were entrained  in the pre-Turonian  sediments.  Akande et al (1989) contended that the absence of the mineralized zones in post-Turonian sequences and their preponderance  in Albian  events  are clear pointers  to the  fact that  Cenomanian  tectonics  and sedimentation probably generated the fractures which served as both conduit for transfer of metal bearing fluids and hosts for the precipitated ore.  Ford (1989) disclosed that sulphides of zinc and lead which occur with subordinate copper in variety of lodes and veins in the Benue Trough are constrained within a relatively narrow strip of fractures that essentially trend in north-south  or northwest-southeast  direction in three main clusters.  These clusters have been delineated in the upper, middle and lower regions of the Trough.  Offodile,  1989, Akande et al (1989), Uma (1998) and Tijani  (2004) highlighted  that,  in the lower  Benue Trough (Abakaliki Basin), the isolated mineralized and brine pond locations are found at Ishiagu-Abakaliki-Ogoja  region.  In the middle Benue region,  it is located at Akwana-Arufu areas, while the region in the upper Benue is around Wase-Zurak-Gwana.  Offodile(1989),  Akande  et al(1989),  and Uma(1998)  working  separately agree that the ores are hosted in varied lithologies depending on the geographic location within the trough.  For example,  in the Abakaliki  Basin  (lower region),  gently dipping carbonaceous shales and siltstone  of the Asu River Group play host to the predominantly  Pb-Zn ores with notable  absence  of barites?  In the  middle  Benue  Trough,  Pb-Zn  sulphide  ores  are  found in silicified limestone sequences and barites are hosted in arkosic sandstones.  Both the limestone and  sandstone   units   belong   to   Asu-river   Group.   Akande   et   al(1989)   alluded   that   the predominance  and significant proportion of barite and fluorite in the mineralized regions of the middle Benue Trough and their lack in the lower  Benue Trough (Abakaliki Basin) seem to be related to variations in physiochemical  environment of ore formation.  Similarly,  in an attempt to

compare the hydrogeological characteristics of saline waters in the Benue Trough that often co• exist with the mineralized zones, Uma (1998) noted that some saline ponds occur without mineralized zone in their immediate  vicinity.  Specifically,  the author mentioned that the Ogoja brine field (Tijani et al,  1996)  exists without known occurrence of either Pb-Zn ore or barites. The absence of predominantly  barite lodes and/or veins in the lower  Benue Trough forms the main focus of the present investigation and report.  In addition,  the geotechnical problems facing ore extraction techniques will be evaluated.

In  exploitation  of ore  deposits  using  surface  excavation  mmmg  techniques,  there  is obvious need for stability of cut slopes and mine spoils/tailings  (Piteau,  1970; Golder.,  1972, Hawley and Stewart,  1986; MHSA,  1999; Miller, 2000; Nicholas and Sims, 2001,) due to safety and economic effects ofuncontrolled failures. The socioeconomic consequences of slope distress in mines are enormous. For example, unexpected mine slope failure may lead to challenges that include  loss of production,  excessive delays in addition to the burden of the removal of failed material  that  will  usually  incur  additional  cost  to  mine  operation  budget.  In  some  cases, unexpected and uncontrolled movement of slope materials may destroy equipment and endanger lives.  Yet,  in extreme cases, pit slope failure may result in total loss of ore reserves and prevent complete exploitation ofreserve in place. Abramson et al (1996) outlined several ways to reduce the  potential  hazards  often  associated  with  slope  failures  to  include  safe  and  economic geotechnical    design,    slope   stabilization    options,   monitoring    and   instrumentations.   In geotechnical  design to forestall the consequences  of slope quagmire, Wyllie and Mah (2008) suggest that slopes in open-pit mines should be configured with some factor of safety, usually in the range of 1.2  to 1.4.  Indeed,  it is acceptable for some insignificant  displacement  and partial

slope failure to occur during the life span of a mine.  Succinctly,  an optimized design slope is the one that fails soon after the end of mining operations.

The stability of natural or cut slopes is dependent on several factors which include  slope dimensions, geology, material strength and groundwater pressures (Hoek and Bray, 1980; Athanasiou-Grivas,  1980;  Call and Savely,  1990;  Stewart et al,  2000;  Atkinson,  2000).  Thus, these factors are expected to be the governing parameters  in pit slope design and analysis.  The design  and  analysis  of slopes  with  these  parameters  are  of two  categories,  namely;  limit equilibrium analysis and numerical  methods  (Brady and Brown,  1993). The limit equilibrium method evaluates the factor of safety of the slope in terms of the ratio of the mobilized shear strength to the destabilizing forces and invokes different procedures for use in the assessment of different  failure  modes  such  as  plane,  wedge,  toppling  and  circular  types.  In  contrast,  the numerical method examines the stresses and strains developed as a result of the presence of the slope and the stability of the slope is then evaluated by comparing the stresses in the slope with the rock/soil strength. In the design of cut slopes, there is usually little or no flexibility to modify the orientation and dimensions of the slope to suit the structural and stratigraphic dispositions of geologic materials encountered in the excavation (Barton,  1971; Flores and Karzulovic, 2000). In ore exploitation using open pit slopes,  for example,  the location of the pit is compelled to sit on the  ore  body  hence  the  ultimate  design  consideration  is  constrained  to  accommodate  the geological architecture that characterizes the ore reserve. In all cases, the common and efficient design approach for cut slopes is to determine the maximum safe cut face angle.  Ross-Brown (1973) recommends that the face angle should be made compatible with the planned maximum height of the pit. However,  the overall design process will always invoke  the delicate tradeoff between stability and economics  (Pierce et al., 2001).  In the final analysis,  the issue of slope

planning and design in open pit mines is a complex procedure  that often involves at least two opposite requirements.   First, the economic interest implies higher slope angle which aims at reduction  in the amount  of waste  that may be generated  and working  time.  The second and converse requirement  is the safety of the mine which predicates  lower pit slope angle to ensure stability of the slope at every moment of exploitation.

Available  reports  show  that  two  previous  attempts  have  been  made  by  some  major mining companies to reclaim some of the abandoned surface excavations and probably discover sub-cropping  deposits.  The  first  is  two  investigations  conducted  and reported  separately  by Oladapo et al (2007) who employed combined gravity and resistivity techniques and Olowofela et al (2008) who used induced polarization method to quantify ore occurrences in the fields. Both concluded that ore quantity is uneconomical to attract major development.  In 2009, a second and another reserve estimation effort (Rao, 2010) utilized integrated magnetic, gravity and resistivity techniques to obtain quantitative data on the ore tonnage. The results, however, found the barite fields initially prospective at two main locations. As a consequence, two major open excavations were planned and executed. Unfortunately, the results of the excavation of the open pits recorded low  economic  output  leading  to  their  subsequent  abandonment.  Nevertheless,  estimates  of produced barite from the derelict fields over a three year period (conducted during the field work by the author) from the activities of the small-scale and artisanal excavations showed that, for the dry season peaks  alone,  a production  rate  of above  10,000  metric tonne per  week  could be attained.  This record  was obtained  based  on analysis  of data  from field visits  and the  local miner’s records of tolls.  Due to this production rate and the inconsistent results generated from the  geophysical  investigations,  the production  capacity  from the  barite  fields suggested  that either the  ore reserve might have been poorly quantified and undervalued or the understanding

of the geologic nature of ore occurrences was at best vague or both. However, the abandoned pits and many other active and inactive small-scale  and artisanal  excavations, despite their lethal environmental influences, provided a reliable page for geological and geotechnical evaluation of the economic potential of the barite fields as wells as factors that predicate the abandonment of the surface excavations.  The outcome of such study will be of immense  importance  in planning exploitation strategies ofuntapped reserves and reclamation of the derelict barite fields.

1.3 Statement of the Problem

The occurrence of barite ores in the Benue Trough has been earlier highlighted  in the geologic map ofNigeria by Cratheley and Jones (1965) but the paucity of data on their economic viability  in  terms  of ore  tonnage,  functional  industrial  properties,  mineral  variability  and potential  geotechnical  challenges  has constituted  a major  impediment towards their economic exploitation, utilization and commercialization. In fact, apart from the Azara field with estimated reserve of about 700, 000 tons (Akande et al,  1989) that is being mined by the Nigerian Mining Cooperation, other fields are at best being scavenged by unorganized  artisanal and small-scale miners.  In  such  non-regulated   and  un-organized   activities,  not  only  are  safety  standards jettisoned   and  compromised  they  are  also  bestowed  with  attendant  lethal  socio-economic impacts.

In all these fields in the Benue Trough where barite ores are presently being exploited by small and artisanal scale miners, exploitation techniques are essentially by  manual and vertical stripping using open excavated pits and mining is restricted to dry seasons due to pit flooding during the rain periods. In addition, most mine pits abandoned during the rains inevitably require intensive and expensive  maintenance  for mining operation  to commence  and continue  in the wake of the consecutive dry period.  Though the open pit mining technique that the artisans often

adopt is viable in some locations where barite ores crop out to the surface and limited within the competent  sandstones, the method  is  faced with tremendous  geotechnical  challenges  in other areas especially  where the overburden  thickness  is much,  in the range  of 10 to  50m.  These challenges obviously arise because the geologic materials that constitute the overburden must be outstripped to enable the exploitation of barites by open-pit mining method.  The stability of the resultant  soil heaps,  mine spoils and the excavated  pit slopes is ultimately  dependent  on the nature   and  geomechanical   characteristics   of the  ore-associated   rocks.   Unfortunately,   the instability of the overburden rock cuts and that of the mine tailing heaps pose serious challenges in the management of the mines. The instability of rock cuts in the mine pits is often exacerbated by excessive groundwater  ingress into the mines.  Groundwater  inundation becomes  imminent when production  and mine workings advance to depths that are often in the range of 10-15m below  the  surface.  Furthermore,  the  management  of groundwater  flooding  in  most  of the unorganized  mining operations  expectedly not only strains mining budgets but renders the pit derelict, most especially during the rains. These geotechnical problems, their consequent  cost implications and associated  risks have caused many artisans to relinquish  and abandon many surface excavations.

1.4 Objectives and Scope

The aim of this study  is to evaluate  geological  and geotechnical  factors that will enable the reclamation  of untapped reserves (if any) in some remote barite fields around Gabu,  Alifokpa, and  Oshina  areas  in  order  to  inhibit  the  proliferation   of abandoned  surface  excavations. Specifically, the aim is to assess ore reserves  and grade variability  in order to determine the economic potentials of the abandoned fields as well as to investigate the potential geotechnical

problems  that render  surface excavations  derelict  to enable the design of safe and economic abstraction strategy.  In order to attain this aim, the objectives are to:

•    establish ore grade and mineral variability.

•     ascertain lithologic, stratigraphic and structural characteristics of ore host rocks.

•     indicate untapped ore reserves.

•    assess geomechanical and hydrogeologic properties of ore host rocks

•     conduct hydro-mechanical stability analysis and design of the excavation slopes.

1.5 The Study Area

The Benue  Trough  is  an elongate  intra-cratonic  basin  (see Figure. 1. 1 ), underlain  by a thick succession of Cretaceous  sedimentary rocks deposited on undulating basement  and punctuated by economic  ore veins,  volcanic  and intrusive rocks. The basement  rocks  are exposed  in the south-eastern and north-western  extremities of the trough where their peaks generally coincide with the major hydrologic boundaries.  The principal area of investigation,  Gabu/Alifokpa barite field, falls within the Southern part of the Trough and is located in Yala Local Government Area of Cross River State.  It is bounded by latitudes 645N  and 657N,  and longitudes  840E  and

855E (Figure 1.2) and spans an approximate area of about 250km.

The topography which is part of the major regional land form, the Cross River plains (Bygott and Money,  1975)  shows a gradual ascent from the plains to the south to highlands in the central north (see Figure 1.2).  Whereas the northern part of the area is characterized by gentle sloping high lands that can attain up to 300m to 500m above mean sea level,  the terrain in the southern part of the area is dominated by low lands that are well below 300m above mean sea level. In terms of the climate,  the area falls within the geographical  region that experiences two major distinct seasons namely, the rainy and dry seasons (Duze and Ojo, 1993). Balogun (2000) noted

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that  the  climate  of the  area  is  controlled  by  the  seasonal  movement  of the  inter-tropical convergence zone (ITCZ) that leads to contrasting dry and wet seasons. The dry season which extends from November to February,  is characterized by high mean temperatures  of about 32C (Gates,  1978)  with period  of December  to January witnessing  some large diurnal variation  in temperature  due to the harmattan  during which temperatures  may fall below 28C. The mean temperatures may vary widely from the month of January reaching a maximum about May.

The rainy season usually commences from the month of March and ends about October. Rainfall is heaviest during the months of June to September, with dry spell of two to three weeks duration in late July to early August  (Monamu,  1975). The annual total rainfall  in the region ranges between 1375mm and 2560mm (Gates,  1978).   The area is drained by some major rivers and their tributaries (see Figure 1.2). These major rivers and their tributaries show dendritic and occasionally trellis drainage patterns and empty into the Cross River drainage system. Whereas some of the streams are perennial, others are ephemeral and usually dry up during the dry season. They  commonly  originate  as  surface  flows  from  springs  that  indent  the  relatively  higher elevations from the north towards the southern part of the area.  Igbozuruike,(1975)  attributed the drainage pattern to structural inequalities in rock hardness/texture, recent diastrophism and geologic/geomorphic  history.  Worthy of note among the rivers is the closeness of some of the main rivers  and their  tributaries  to  some of the known  barite  reserves.  The rivers  and their tributaries expectedly influence the groundwater dynamics around the mines.

The vegetation is typically mangrove overlapping into the rain forest belt (Gates,  1978) and is characterized  by scattered  tress with low  covering shrubs and grassland  (Iloeje,  1965; Igbozurike,   1975).   The  vegetation   is  generally   controlled   by  geology,   climate   and  the distribution of rivers in the area.  Areas that are underlain by unconsolidated sandstone and shale

are  covered  by giant  green trees  and plants,  while  areas that  are underlain  by  consolidated sediments are characterized  by grasses and shrubs. Tall trees and evergreen plants  follow the trending pattern  of the major  and minor river channels in the area.  Surfacial soils in the area classify as interior zone oflaterite soils following the classification scheme oflloeje, (1965). The soils  are  deeply  ferruginized  with  colour  grading  from  dark  grey to  mottled  red.  They  are generally sticky when wet with some zones of alluvium.

The  Benue  Trough  is  a major  rift  structure  of the  mega  West  African  Rift  System (WARS) and has been conveniently  subdivided  into three main geographic  domains, namely; upper, middle and lower sections as described in Tattam, (1943), Offodile, (1989), Akande et al,(1989) and Uma(1998).   The lower Benue Trough encompasses the Abakaliki Basin and other adjoining sub-basins such as Mamfe embayment and Ikang Trough. The Benue Trough, which is an intra-plate rift sub-basin of the West African Rift System,  is the major source of base metal ores  in Nigeria  and other African  countries  (see Figure  1.1; Odukwe,  1990).  In the trough, barium-lead-zinc- copper sulphide and fluoride ores are known to occur in sandstones, shales and limestones  of the  Asu-River   Group  (Farrington,   1952;  Grant,   1971;  Burke  et  al,   1972; Whiteman,   1982;  Akande  and  Abimbola,   1987;  Orazulike,   1994).   The  structural   styles, lithofacies  association  and  stratigraphic  dispositions  of the  ore  deposits  have  led  to  their description as endogenetic ores that exist either as anatomizing swarms of veins or concordant strata-bound mineral flats (Nwachukwu,  1972, Ezepue,  1984). The sedimentological processes, stratigraphic  framework and tectonics that dominated the Cretaceous  geodynamic evolution  of the Benue Trough  and the prevalent  theories  on the genesis of the base  metal deposits  bear largely on the predominant role of hydrothermal processes entrained by igneous systems such as mafic intrusions  and tertiary volcanics.  Known typical world-class  barite  occurrences that are

being exploited by open pit mining techniques  in the trough are found in fields at Ibi,  Zurak, Azare,  Awe,  Gabu,  Oshina  and  Tarka,  which  are  all restricted  to  the  central  and  southern portions of the trough.

The origin, stratigraphic and structural relationship between sulphide ores and their host rocks  in  the  Benue  Trough  are  intricately  linked  to  the  evolution  of the  trough  about  the Cretaceous times when viewed from the regional perspective. The trough is one of the sub-basins of the mega West African Rift System and is about 80-150km wide and 800km long (Tijani,  et al,1996).  It extends in the NE-SW direction from the Niger delta in the Gulf of Guinea to Chad basin in the interior of the West African Pre-Cambrian  shield.  It has often been described as an elongate partly  fault-bounded  depression  filled up by about  6000m thick  marine  and  fluvio• deltaic  sediments.  The  sediments  are thought  to have been  compressionally  or  extensionally folded in a non-orogenic  shield environment  (Wright,  1976; Ugwuonah and Obiora,  2008).   In the  southern  (lower)  region  are  many  structural  and  depositional  elements  that  include  the Abakaliki anticlinorium, and the Afikpo and Anambra synclines. These structural elements inadvertently control and dominate the geologic evolution and litho-stratigraphic  architecture of the southern part of the trough.

Several published  and unpublished  reports  on the tectonic  evolution  and stratigraphic history of the Benue Trough exist which predicate that the basin may have been widely studied, reviewed and discussed. The discussion that follows attempts to synthesize the presentations  of Reyment (1965); Murat (1970); Grant (1971); Burke et al, (1972); Olade (1975);Wright,(1976); Offodile  (1989); Uzuakpunwa  (1980);  Hoque  and Nwajide  (1984);  Benkhelil  (1989);  Kogbe (1989); Ojo (1992); Akande and Mucke (1989), Tijani et al, (1996) and Akande (1999). These authors agree that the application  of Y-shaped  triple rift model (RRR) to the break-up  of the

Afro-Brazilian plate, in early Cretaceous times, best explains the final configuration of the Benue Trough. They believe that the Benue Trough originated as an aulacogen consequent upon the separation of African and South American plates sequel to the opening of Southern Atlantic in the early Cretaceous.  The tectonic and the accompanying  depositional processes  led to the on• land reactivation of the equatorial  oceanic  fracture zones,  particularly  the chain and charcort, which probably generated successive tensional and compressional stresses in the basement rocks as well as the overlying sedimentary successions.

The sedimentary  sequences have been subjected  to several tectonic events in the Cenomanian (Nwachukwu,  1992;  Ojo,  1992),  Turonian and Santonian (Uma and Lohnert,  1992).  During the Santonian,  such crustal  instability  is believed  to have been accompanied  by wide spread and spectacular  magmatism,  folding,  and   faulting which resulted  in the creation of the relatively elevated Abakaliki anticlinorium (in the  Southern Benue Trough) flanked by two synclines; the Anambra to the west and Afikpo to the east. The Abakaliki anticlinorium then became a positive geomorphic feature and sedimentation was transferred  laterally to the adjoining synclines.  The sedimentary sequences of the lower Benue Trough (the Abakaliki Basin) summarized in Table 1 and cartographically  represented  in Figure  1.3,  record the first tectono-sedimentary  cycle that inundated  the southeastern Nigeria  rift basins.  The sedimentary  cycle which probably  started with  the  Aptian-Albian   transgression   deposited  the  oldest  marine  with  shelf environment sediments  designated  as  the  Asu  River  Group  (ARG).  The  ARG  consists  of mainly  thick laminated  shales,   arkosic  sandstones,   and  subordinate  limestones  and  later  with  associated volcanic intrusions  and pyroclastics.  The ARG represents the first cycle of shallow marine and brackish  water  terrigenous  elastic  sediments  that  lie unconformably  over the Precambrain  to Lower Paleozoic Basement Complex rocks. The transgressive sequence of the Albian is overlain

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Figure 1.3: Regional  stratigraphic sequences in Southeastern Nigeria (Modified from geologic map of Nigeria, 1994).

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Table   1:     Stratigraphic   Sucessions   of  the   Lower-Upper  Cretaceous   Southern  Benue

Trouglht– Ab:akali1ki and Afi1kKpo B; asm”

s (Hoque and Nwap·uide, 1985,’ OJi0, 1990)

Age                                                                                         Southern   Benue  Trough  (Abakaliki

and Afikpo Basins)

Santonian-Coniacian               Awgu Fomation

e       nz

Turonian                                 Eze-Aku Group

(Keana,      Makurdi,      Agala      And

4

=-        c                                 Cenomanian                           Amaseri Formation)

~     •

<

=-        4                                                                                          –                    –

d                                                              —

u Odukpani Formation   Lower Albian Asu River Group   Cretaceous   Aptian   Ogoja Sandstone .     – Pre-Cambrian   Basement  Complex

4e

in  some  localities  by  regressive   sedimentary  packages  tagged  Odukpani  Formation.   The formation consists of continental to marginal marine  facies made up of dark grey shales with subordinate  units  of mudstones  and limestones.  In the Turonian,  the  following  transgressive cycle is dominated by the Benue-Trans Sahara Seaway (Tethian) and accounts for the deposition of extensive fossiliferous black to grey shales, designated as the Ezeaku Group which  consists of marine  shales  with  subordinate  limestone  (Benkhelil,  1989),  as well  as  several  sand bodies ranging from fluvial to marine (Hoque and Nwajide, 1985). In the Coniacian, the Awgu Group sediments were deposited,  overlying the Turonian  facies.  The Coniancian deposition  followed from sediment subsidence and shift in deposition axis westwards due to continued increase in sea level and accommodation. It consists of mainly thick shales with sandy unit interbeds. The basin, in the Santonian records an extensive and intense tectonics accompanied by magmatic processes that  led  to  folding,  faulting  and  uplifting  of older  sequences  and  transfer  of depositional processes to the adjoining rifts of the Anambra and Afikpo basins.  In addition to Santonian,  and perhaps   other   earlier   deformation   phases,   the   tectono-sedimentary   processes   generated hydrothermal systems for formation, concentration and precipitation ofbase metal ore (lead-zinc, and fluorite-barite) bodies hosted mainly in the intensely deformed ARG units.

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