E.U. Durugbo
Department of Biological Sciences, Redeemer’s University, Mowe, Ogun State, Nigeria

The palynological analyses of ditch cuttings from two wells in the Western Niger Delta have revealed the provenance of Peridinales in the Middle-Late Miocene sections of the wells. These Middle-Late Miocene Niger Delta dinoflagellate cysts closely resemble those recovered from the Gulf of Mexico thus revealing their provenance in the South Atlantic. The commonest species were Selenopemphix spp., Lejeunecysta spp., Multispinula spp., with spot records of Apteodinium spp., Xandarodinium spp. Gonyaulacaceans recovered included such long ranging taxa as Spiniferites, Lingulodinium, Operculodinium, common oceanic Nematosphaeropsis labyrinthus but rare Impagidinium species.

Keywords: Peridinales, Western Niger Delta, Miocene, South Atlantic.

The Niger Delta belongs to the Southern Nigeria sedimentary basins whose depositional history commenced in the Early Cretaceous. It is located in the eastern corner of the Gulf of Guinea between longitude 4º - 9º E and latitude 4º - 9º   N. It occupies an area of about 75000 km2, and is composed of an overall regressive clastic sequence, which reaches a maximum thickness of 9,000 – 12,000 m (30,000 - 40,000ft).  It arose out of the failed rift formed from the separation of the South America from Africa during the Early Cretaceous (Burke et al., 1972). The sedimentary fill of the southern Nigeria sedimentary basin was controlled by three major tectonic stages together with epirogenic movements which led to major transgressive-regressive cycles (Murat, 1972). These tectonic events led to the displacement of the main basin axis and subsequently the emergence of three successive basins, viz: Abakiliki-Benue Trough, Anambra Basin and the Niger Delta with ages ranging from Albian –Lower Santonian, Upper Santonian – Lower Eocene, and Lower Eocene – Recent for the Abakiliki-Benue, Anambra and Niger Delta respectively. The Niger Delta Basin contains Cenozoic to Recent deposits emplaced in high energy constructive deltaic environments. The Tertiary Niger Delta (Figure 1) is a large arcuate delta of the destructive, wave dominated type. Its development has been dependent on the balance between the rate of sedimentation and the rate of subsidence. These Cenozoic formations are basically three which are the Benin Formation which is a predominantly sandstone sequence of continental delta origin which is up to 2000m thick and ranges in age from Oligocene to Recent. The Benin Formation is underlain by the paralic Agbada Formation which is a coarsening – upward sequence of alternating sandstones and shales of delta front and lower delta plain origin. This is the most petroliferous formation with a maximum thickness of 4000m , ranging from Eocene to Recent in age. The basal formation is the pro-delta Akata composed predominantly of overpressured marine shales. Its thickness is up to 6500m, with age ranging from Eocene to Recent (Short and Stauble, 1967; Oboh, 1992).

Dinoflagellates are protists (single celled organisms) which together with coccolithophores and diatoms make up the most important primary producers in the oceans. Though most dinoflagellates are marine, some do occur in freshwater lakes, rivers and bogs. They are generally classified in the division Dinoflagellata by Paleontologists. Majority of them have a complex life cycle, and many produce cysts during some part of this life cycle. These cysts can be recovered as fossils by the same techniques used to recover fossil pollen and spores from sediments as their organic walls are composed of a sporopollenin­ like material. The use of fossil dinocysts in biostratigraphy and paleoecology has been discussed in detail by such workers as Powell (1992), and Stover et al. (1996). Pioneer studies, primarily by palynologists working for petroleum companies, were based largely on subsurface sections.  Dinocyst biostrtaigraphy has been utilized successfully, especially in situations where calcareous microfossils are not abundant or well preserved such as in the clastic Mesozoic sediments of Alaska and the North sea.  Dinoflagellate cysts could contribute to a wider use of palynological schemes. The use of dinoflagellate cysts in combination with pollen and spores will produce meaningful biostratigraphical schemes which will be useful where other microfossils are scarce. There are few studies of dinocysts from the Gulf of Guinea and the Southern Hemisphere. Different authors have described assemblages from various ODP sites (McMinn et al., 2001; Brinkhuis et al.,2003) and the Antarctic shelf (Wrenn et al., 1998; Hannah et al., 1998).

 In the gulf of Guinea, the few records include those of Biffi and Grignani (1983) who studied Oligocene sediments from fifteen boreholes of subsurface Tertiary sediments in the Niger Delta which yielded rich dinoflagellate cyst assemblages characterized by abundant Peridinioids particularly Lejeunacysta, Phelodinium and Selenopemphix species. They described seven new species of Lejeunacysta (L. brassiensis, L. communis, L. pulchra, L. beninensis, L. globosa, L. lata, L. granosa); two new species of Phelodinium (P. nigericum and P. africanum) and one of Selenopemphix (S. warriensis). They further emendedsome species of Lejeunacysta.

Oboh et al., (1997) having worked on Oligocene to Early Miocene sediments of the ODP Leg 159 from the Côte d’ Ivoire – Ghana Transform Margin reported a cyclic pattern in the occurrences of gonyaulacalean and peridinialean forms. The peridinioid dominated sections were dominated by protoperidinioid cysts, while Spiniferites spp., and Hystrichokolpoma rigaudiae with spot records of Nematosphaeropsis labyrinthus were the commonest gonyaulacoids. There also appeared a cyclicity in the organic matter distribution which they opined controlled the abundance of dinocysts, calacareous and siliceous microfossils with all indicating changes in water mass and paleoproductivity. The occurrences of the pollen species Retibrevitricolporites obodoensis, Spirosyncolpites bruni and Magnolia spp. supported the age assignment. 

Sample preparation
A constant weight (30g) of each sample was initially given a hot (5%) Hydrochloric acid treatment to remove carbonates prior to complete digestion in 60% Hydrofluoric acid (HF) under a fume cupboard. Gentle agitation of the acid / sample mixture was carried out to aid acid digestion. The sample was further given a hot (10%) Hydrochloric acid (HCl) treatment and wet – sieved over a 5 - micro mesh polypropylene sieve.  The 5- micro sieve was used so that such small sporomorphs as Zonocostites ramonae were not washed off. The Branson Sonifier 250 was routinely employed during sieving to facilitate complete removal of silt and clay particles.   The residue was given controlled oxidation using concentrated (5%) Trioxo nitrate (V) acid (HNO3). The residues were prepared for study as strewn mounts using Loctite Corporation, U.S.A. One palynological slide was prepared for each sample/horizon. The slides were stained with Safranin O to enhance the study of dinoflagellate cysts. A 22 X 32 mm cover slip was used to mount the samples.

Sample analysis
A Leitz Dialux 20 EB binocular microscope was used for the sample analysis. All the species present were recorded. Coordinates of well preserved dinoflagellate cysts were aligned with the “England Finder” coordinates for easier references (Plate 1).

Species identification
 The different dinoflagellates and acritarchs species encountered were identified by comparison with the  monographs of Fauconnier et al., (2004), Rochon, et al.,(1999), Wrenn et al., (1986), Lentin and Williams(1973, 1980, 1989), Head and Wrenn (1992), Powell (1992), and Sluijs et al.,(2003). Unknown dinoflagellates species were sent to Prof. Niels Poulson of Geological Survey of Denmark and Greenland (GEUS) Laboratory in Denmark for proper identification.
The Motic 2.0 camera was used for the photomicrography of well preserved dinoflagellate cysts at 1000X.
The slides, residues, unprocessed samples, and duplicate prints are in the palynological collections of the Biological Sciences Department, Redeemer’s University, Mowe, Ogun State, Nigeria.

In wells BA and BB, out of the 66 and 70 recovered dinoflagellate cysts, 16 and 19 were Peridinales making up 24% and 27% respectively (Plate 1). The commonest species were Selenopemphix nephroides, S. warriensis, S. coronata, S. quanta, S. armata, Selenopemphix spp., Lejeunecysta lata, L. communis, L. diversiforma, Lejeunecysta spp., Multispinula quanta, M. minuta, Xandarodinium sp. Brigantedinium spp., Protoperidinium spp., with spot records of Apteodinium sp., Cribroperidinium sp., and Trinovantedinium sp.

Age determination
The middle Miocene age is indicated by the continuous occurrence of Crassoretitriletes vanraadshooveni towards the base of the two wells. Normally the base occurrence of this spore marker species delineates the Middle /Early Miocene boundary (P720 of Evamy et al., 1978) in the Niger Delta, these occurred below the tops of the of the N9/N8 (Globorotalia peripheroacuta /Eponides eshira) foraminiferal zones defined by the FDO Eponides eshira and LDO Orbulina universa dated 15.0Ma (Blow, 1969, 1979; Berggren et al., 1995).
                                                      PLATE 1



























                                                               PLATE 1

All figures at X1000 unless otherwise stated

1. Lejeunacysta lata Biffi and Grignani (1983)      2. Lejeunacysta sp. Well BA (3790-3880) O26/2
         Well BA (3880-3970)X21/2       
3. Xandarodinium sp. cf. X. xanthum Reid 1977.  4. Selenopemphix armata Bujak (Well BA 4870-4960)
    Well BA(4320-4410) W33/4                                     S43/1            
5. Selenopemphix coronata Biffi and Grignani (1983)    6. Selenopemphix nephroides (Benedek,1972Well BB(3970-4060) P42/3     emend. Bujak et al., 1980) Well BA(3690-3780) K33/1
7. Selenopemphix sp. E of Duffield and Stein,1986   8. Lejeunacysta communis (Biffi and Grignani,1983
Well BB (5500-5590) K33/1                                 Reid,1974) Well BA(4050-4140) L30/4
9. Apteodinium sp. (Well BA 11520-11610)N43/4     10. Selenopemphix quanta (Bradford)Matsouka
    (Well BA 10900-11780) M34/2
11.  Lejeunacysta diversiforma (Bradford) Artzner   12. Multispinula quanta (Well BA 4050-4140) J44/1                                                                                  and Dorhofer 1978 (Well BA 10890-10980)W37/4                    


13. Selenopemphix nephroides  Benedek                  14. Brigantedinium cf. cariacoense(Wall) Reid
       Well BA(3690-3780) K33/1                                    1977 Well BB (900-10990) P38/2

15.   Protoperidinium sp.  (Well BA 2160-2250) J50/1    16. Lejeunacysta cf. hyalina.
                                                                                             (Well BA (4590-4680) W41/1
17.   Trinovantedinium? cf. capitata.                        18. Multispinula quanta Bradford 1975
         Well BB(5050-5140) L27/1                                    Well BA(10980-111070) L23/4
         19.   Nematosphaeropsis labyrinthus)                       20. Leiosphaeridia sp.
       Well BB(5050-5140)Q22/3                                    Well BB(2160-2250) J50/1


These common records of peridinoids in the Middle Miocene of the Niger Delta could have resulted from the wet climate and dominantly estuarine to deep marine environments inferred for the sediments. The dominance of mangrove and such wet climate indicators as freshwater swamp, brackish water swamp and palm species in association with the records of the oceanic dinoflagellate cysts Nematosphaeropsis labyrinthus and Impagidinium spp. at some intervals in the wells had accounted for this. This concurs with Edet and Nyong (1993) who posited that regular peridinoid cysts could represent periods of reduced salinities following increased dilution of marine waters by continental runoff. Biffi and Grignani(1983) had documented Oligocene ages for these Lejeunacysta spp,  and Selenopemphix species such as S.  warriensis, S. coronata etc, their occurrence up to the Middle Miocene as reported here need to be investigated further as they off promise of being age diagnostic species. Conclusively, the recovery of most of the species documented in the Gulf of Mexico by Duffield and Stein (1986) reveals the provenance of these species in the South Atlantic and their acme during the Middle Miocene.


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