MIDDLE  PALEOCENE TO EARLY  MIOCENE PAL YNOSTRA TIGRAPHY OF SEDIMENTS IN BENDE UMUAHIA  AREA NIGER DELTA  BASIN SOUTHEASTERN NIGERIA.

ABSTRACT

Cenozoic sediments outcrop in the Niger Delta Basin.  Deposits in the Basin thicken southward (down-dip) but thin northward (up-dip) around Bende    Umuahia area where approximately 260 m thick exposure was logged from base to top.  Sieve analysis and petrography were carried out to establish the depositional  settings of sediments while palynological  analysis was undertaken  in order to identify palynomorphs  and establish their biozonations,  determine and correlate age of sediments,  ascertain their palaeoenvironrnents  of deposition,  and evaluate kerogen quantity and maturation.   Five   lithological   units   were   encountered.   They   include   carbomaceous   shale, fossiliferous  limestone,  sandstone,  mudstone  and  coal.    A  total  of 65  sporomorphs  and  51 dinoflagellate species were identified and described.  Six informal palynological assemblage zones labeled (A- F) were established across the study area,  which comprise:  Zone A Scabratriporites simpliformis-Bombacidites    annae   zone,    Middle    Paleocene;    Zone   B   Foveotricolporites crassiexinus-Mauritidiites  crassiexinus  zone,  Late Paleocene;  Zone C Striatopollis  catatumbus• Momipites    africanus    zone,     Early    Eocene;     Zone    D    Margocolporites     umuahiaensis• Gemmastephanocolporites  brevicolpites   zone,   Middle   Eocene;   Zone   E   Cicatricosisporites dorogensis-Perfotricolpites   nigerianus  zone,   Late  Eocene;     and  Zone  F  Verrucatosporites usmensis-Magnastriatites howardii zone,  Oligocene-Early Miocene.  Three main palynofacies (A, B,  and C) were recognized  based  on the significant  changes  in the particulate  organic  matter constituents of the kerogen samples.  Age determination and correlations were achieved based on selected index  palynomorph  assemblages.  Imo Formation was dated Middle Paleocene     Early Eocene  age  based  on  the  following  assemblage:  Proxapertites  operculatus,   Scabratriporites simpliformis,  Bombacidites annae,  Foveotricolporites  crassiexinus and Striatopollis  catatumbus. Ameki Formation was assigned Early to Late Eocene age with the following species, Monoporites annulatus,     Momipites    africanus,     Proxapertites     cursus,    Retistephanocolpites    williamsi, Margocolporites  umuahiaensis,  Spinizonocolpites echinatus,  and Retibrevitricolpite triangulatus. Ogwashi  Formation  was  dated  Oligocene-Early   Miocene   age  based  on   Verrucatosporites usmensis,    Magnastriatites    howardii,    Retibrevitricolporites  protrudens,    R.    obodoensis,    R. ibadanensis,  and Chenopodipollis dispersus.  Palynomorphs of environmental value indicated that Imo Formation was deposited in a marginal marine setting ranging from estuary at the base, upper to lower shoreface towards the middle,  and tide-dominated  estuary at the top.  Ameki Formation ranged  from  open  marine  at  the  base  to  beach/  fluvial  towards  the  middle  and  estuarine• lagoon/marine at the top,  while Ogwashi Formation was deposited between freshwater upper to brackish water lower deltaic plain within the tropical rainforest.

CHAPTER ONE: INTRODUCTION

Palynology  is an important  multidisciplinary  branch of paleontological  sciences.  Its applications include   determining   relative   ages,   reconstructing   palaeoenvironmental    and   palaeoclimatic conditions,  identifying and correlating rock sequences, estimating the degree of thermal maturation of sediments, and evaluating hydrocarbon source rock potential. In addition, palynology provides a good, relatively  inexpensive replacement  proxy with a reasonable  degree  of accuracy  for some more  expensive  organic  geochemical  analyses,   such  as  vitrinite  reflectance  (Ro  % ),  thermal alteration index (TAI), total organic carbon (TOC), and Rock-Eval Pyrolysis. Fossil palynomorphs exist in a variety of depositional environments, which make them very useful where other types of environmentally  restricted  microfossils  (e.g.,  foraminifera,  ostracodes  and  calcareous nannoplakton) are lacking in sedimentary rocks.

1.1      Location and Accessibility

The study area covers approximately 235 km and is delimited by latitudes 5″27 ‘N and

540’N,  and  longitude  7″25’E  and  743’E.  The  area  lies  within  the  Niger  Delta  Basin  in southeastern Nigeria,  (Fig.1.1).  The surrounding towns include  Ohafia,  Umuasua  and Ovim on the north towards Okigwe;  Okwe on the south towards Obot-Ndom  forest reserve;  Ozu-Abam and Ndi-Okorie  on the east; and Umuokpara  on the western part. The Umuahia     Bende road which runs from east to west is the major  access route to the outcrops  in the area, (Fig.  1.2). Umuahia     Uzuakoli  road  also provides  access to other exposures  especially  to the northern parts.

1.2      Geography:

1.2.1   Relief and Drainage

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Fig.  1.1: Geologial sketch map of Nigeria showing the Cenozoic successio’n of the Niger Delta Basin and

location of the study area (Modified after Murat, 1972)

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Fig.  1.2:  Location and accessibility map showing sample locations  and drainage system of study area.

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The  Obudu   Plateau   and  Oban   Massif  along  the  eastern   frontier   (Fig.   1.3)  have   a maximum   altitude  of 2000m  (6,600ft)  Among  the  other  major  relief features  is the  Enugu  – Okigwe  escarpment,  a NS trending  lineament,  which  turns  east  at Uturu  and  southeast  to terminate   around  Arochukwu   (Ofomata,   1975).  It  forms  the  watershed   of streams   draining westward  to the Niger,  to the east into Cross River Basin,  south to the Atlantic,  and north to the Benue River.

The study  area is characterized  by coastal plains  and ridges,  valleys,  and highlands.  The flood plains  of Imo  River,  Inyang  River,  and Igwu  River  are the  lowest points.  The Imo  River which is the principal river with its tributaries,  the Oyivo, Nkpa  and Ahi,  flow southeastward into the Atlantic  Ocean (Fig.  1.2 and Fig.  1.3).

1.2.2    Climate and Vegetation

Climate:

The  area  falls  within  the  equatorial  climatic  zone  with  rainfall  and  humidity  declining from the coast inland, (Fig.1.4)  and characterized  by two climatic regimes, the rainy and dry.  The rainy  season begins  in March  and ends in October  with little break  in August  usually  referred  to as ‘August  Break’.  The total annual rainfall  decreases  from 2000mm  in the south to  1900mm  in the north.  The dry season which  lasts for four mouths begins  in November and ends in February. The hottest  period  starts from January  to March with mean annual temperature above 27C. The relative  humidity  is usually  high  throughout   the  year,  reaching  its  maximum  peak  during  the rainy season when values above 80% are recorded.

Vegetation

The vegetation  in the area is considered  part oflowland tropical rainforest zone which is the

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Fig.1.3. Relief map of southeastern Nigeria (after Ofomata, 1975)

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Nsukka

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Figure 1.4: Climatic Regions of southeastern Nigeria (after Inyang,  1975)

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dominant  natural  vegetation  in  most  parts  of the  southern  Nigeria,  (Fig.  1.5).  However,  the southern  part  has rich  evergreen  trees,  and gradually  gives way northwards  to the rainforest  – savannah,  and ecotone where trees are interspersed  with grasses especially the bamboo  species. The palm appears to be the most important economic tree in the area.

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Figure 1.5: Natural Regions of southeastern Nigeria (after Igbozurike, 1975)

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1.3      Objective of Present Work

This work was undertaken in order to;

1.            establish depositional settings of sediments,

11.            identify palynomorphs and establish their biozonations,

111.            determine and correlate age of sediments,

1v.      ascertain the palaeoenvironrnents of deposition, and v.      evaluate kerogen quantity and maturation.

1.4      Scope of Work and Statement of Problem

The  scope  of this  work  ranged  from  Middle  Paleocene  to  Early  Miocene  intervals, covering the Imo,  Ameki,  and Ogwashi Formations.  The problem of stratigraphic demarcations and  location  of the  above  formations  in  the  southern  Nigerian   sedimentary  basins,  their palaeoenvironrnents  of deposition,  and  the  palynological  evidence  of potential  hydrocarbon mature source rock of the Ameki Formation, were addressed using organic-walled microfossils.

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