Palynological interpretation of the palaeoenvironments of Miocene strata of the well Igbomotoru-1, Niger delta

Palynological and lithological results have been employed in interpreting the depositional environment of strata penetrated by the Igbomotoru-1 well at the interval 1123–3583 metres. The environment of deposition is largely transitional with marine influence occurring only at the base of the studied sequence. Zonocostites ramonae is frequent to abundant in most samples: this indicates a high influence of mangrove swamp vegetation in the environment at the time of deposition. The absence or rarity of this species at some horizons within the studied interval is taken as evidence of deposition in a more continental environment or of a swamp forest area composed of non-Rhizophora type trees. The nature of the organic matter points to a source area near the basin of deposition for the sediments.

The most comprehensive published work to date on the palynology of the Niger delta was carried out by Germeraad et d. (1968). The work involved the study of palynomorph assemblages of Tertiary sediments in some parts of tropical South America, Africa and Asia. Its primary purpose was to arrive at a better interpretation of the stratigraphy of those sedimentary basins where other means of correlation have failed. Other published works on the palynology in this area, covering the Late Cretaceous to Recent, are largely concerned with descriptions of palynomorphs and palynological zonations. However, some of them incorporated environmental interpretation in their studies (see van Hoeken-Klinkenberg, 1964Clarke, 1966;Clarke & Frederiksen, 1968;Knaap, 1971;Sowunmi, 1973Sowunmi, , 1981Legoux, 1978;Jan du Ch@ne & Salami, 1978;Jan du Ch@ne et a/., 1978 andBiffi &Grignani, 1983). This study of sidewall samples from Igbomotoru-1 well in the Niger delta, aims at using both palynological and lithological data to interpret environmental conditions in the area of deposition a s well as highlighting the problems often encountered in environmental reconstruction.

SAMPLE SITE AND PREPARATION
Well Igbomotoru-1 is located at latitude 431'40.3"N and longitude 603'29.6"E within the Niger delta (Fig. 1). It was drilled to a total depth of 4325 metres by the Shell Petroleum Development Company of Nigeria. The sediments vary from coarse clastics with occasional lignite streaks (0-1590m) to finer clastics (15904325m). They have been grouped into four informal lithostratigraphical units designated A to D (Fig. 2) which were delineated on the basis of gamma ray log and sidewall sample descriptions (Oboh & Salami 1989). These units correspond to the Benin and Agbada Formations which consist of continental sands and transitional sand and shale sediments described by Short & Stauble (1967). Three informal palynological subzones which are subdivisions of the Forty-one sidewall samples were collected from slialy horizons from 1123-3583m depth which has been dated as Middle to Upper Miocene on the presence of cetain foraminifera and palynomorphs (Oboh & Salami 1989). Samplesof about 258 were prepared by standard palynological techniques, including oxidation in nitric acid for a few minutes, and the unsieved residues were strew-mounted on slides. The total area under the cover slip of each slide was systematically studied under a Leitz Orthomat -W binocular microscope and identification was attempted for as many forms as possible. Colouration and size of organic matter present were also determined.

ENVIRONMENTAL RECONSTRUCTION
A. Limitations toreconstruction of environment of deposition The main limitations which arise when reconstructing environments of deposition in deltaic sediments, like those in the Niger delta, come from the problems of contamination, and the interpretationof the data collected. Palynologists work with two types of samples: surface and subsurface samples. The latter can be further divided into three classes: (a) conventional cores, (b) sidewall cores and (c) ditch-cuttings. Both surface and subsurface samples are susceptible to contamination during collection, which in turn has negative effects on stratigraphical interpretations. Contamination may also occur during sample preparation from air-or water-borne pollen and spores or human error. Of the three sample types derived from oil wells, conventional cores are the most reliable because they are the least likely to be contaminated by drilling mud or caving of rock strata, whilst the ditch-cuttings are the least reliable for these reasons. In a situation in which ditch-cutting have to be used, the sampling interval may have to be increased to eliminate the problems due to caving. A sampling interval of 20-30m is usually appropriate in such cases. Adequate care must be taken to see that sidewall cores are not contaminated by extraneous matter.
It is important to bear in mind whilst making palaeoecological deductions that climatic conditions and latitudes have changed through geological time (Germeraad et nl., 1968, Smith rt nl., 1981. Despite the factthat theNigerdelta hasadvanced seaward since its inception, its palaeocoastline in the Middle to Upper Miocene was not substantially different from the present position (see Fig. 3), and the present climatic conditions are considered similar to those of the Miocene (Weber, 1971). Since this delta has remained sufficiently close to the equatorial region, the polar advances of the Pleistocene have had a minimal effect on the flora, but the eustatic drop in sea level has resulted in the advance of the coastline (Fig. 3). This minimal effect of the Pleistocene is substantiated by the nearly uniform nature of the palynofloras in thedeltaic sediments since Miocene and Pliocene times (Germeraad et a/., 1968). Several pollen and spore types found in these sediments appear similar to those produced by the present vegetation zones and can therefore be used as indicators of past environments based on recent ecological information.

B. Methods of interpretation
Four features have been employed in this environmental interpretation. They are: 1 . Association of some environmentally restricted marker species such as Cvnssoretitriletes uniiraadslzooz)eiii, Magnnstriatites hozcrardi and Pachydermites diederixi.

4.
Lithological characters of the strata.
C. Results of environmental study The distribution of some of the palynomorphs from the Igbomotoru-1 well samples is shown in Fig. 2. The various parameters employed in the interpretation of the environment of deposition are discussed as follows: 1. Environmental marker species. Many of the palynomorphs are closely similar in morphology to recent species so that assumptions can be made about their ecological requirements (Germeraad et a/., 1968). For example, species indicative of coastal plain environments include Cvnssoretitriletes zmirandshoozwii (a climbing fern of coastal swamp forest), Mnpnstrintites hozclnrdi (a small fern of open freshwater habitats), Pnclz!/dermites rliederiii (an angiosperm of coastal swamps) and Zoiiocostites rnmoiine (the mangrove group). All four species are present throughout most of the sampled sequence although M. Iiozmrrliis rather intermittent and rare. This last species, according to Germeraad et 01. (1968), is dispersed mainly by water and the spore is likely to occur most frequently in inland environments whereas the distribution of Z . rniiioiiae tends to be coastal and offshore marine. Therefore, the fairly high levels of Z . rniiioiine compared with the infrequent occurrenceofM. hozinrdisuggests that the sequence recorded here represents mainly a coastal environment of deposition or a marine situation where Z. rniiioiine has been transported offshore before deposition.
2.Percentageof Zonocostites ram0nae.Z. rniiioiincisadistinctive pollen type found in extant genera of mangroves notably Rhizophorn (Germeraad et d., 1968). Muller (1959), after study of recent deposition of pollen in and around the Orinoco delta, showed that R/iizophorn produced abundant pollen which can be blown by wind over a wide geographical area. High perceiktges of NiizoyIiorn pollen occur close to the source and at some distance offshore in marinedeposits. Inshore these high percentages (>50%) are due to high concentrations of the Rhizophora pollen; further offshore they are due to low total pollen counts. Therefore, its quantitative distribution makes it a useful species for environmental interpretation (Muller & Caratini, 1977).
Z . ranionae is frequent to abundant throughout most of the sequence described here, suggesting the presence of Rhizopkorn type mangrove swamp in the area. In the studied interval, the percentage varies from about 2% to more than 40% (Fig. 2). The absence or rarity of this species from some horizons indicates that deposition of those sediments occurred somedistance from the mangrove edge.
Above approximately 1600m the lithological analysis indicates a more continental influence which would fit with the advance of the delta front suggested by Short & Stauble (1967). This is also suggested by the intermittent presence of the freshwater fern M. hnzcnrdi.
Between 2680m and about 3000m the decrease in Z . r a m r i m could also indicate a more inland environment. However, Crassoretitriletes zmrandshooveiii and Verrircii tosporites irsiiierisis are very common over this interval. Both species are considered tobeclimbingferns (Cermeraad etnl., 1968) and their abundance indicates a swamp forest area composed of non-Rhizopliorn type trees. The occurrence of M . hozonrdi again suggests some freshwater influence and the presence of more open vegetation. Unfortunately the sampling intervals in this lower part are too great to determine the extent of this vegetation change very accurately .
Marine organisms are rare in the studied interval. This indicates that marine influence is very slight. This is further evidence that the high percentages of Z. ranioiine mentioned abovearedue todepositionclose tosource, i.e., coastal deposition rather than the offshore transportation and concentration found by Muller (1959).

Organic matter.
Theaccumulationand degradationof organic matter is related to surface conditions in the depositional environment and to diagenetic changes. Hence the presence or absence and abundance of different organic components can be directly related to lithofacies (Batten, 1981). It is also important to separate the characteristics that derive from the nature of the primary material of the sediments from those induced by subsequent diagenesis, differential migration and metamorphism (Staplin, 1969, p.50). The organic matter in the samples recorded here consists of plant cuticles and wood fragments of a complete range of size from small to large. A long distance of transportation would have effected the sorting of the sizes as finer fractions tend to be deposited further offshore (Nwachukwu & Barker, 1985). The varied range in these Nigerian samples indicates short distance transport, i.e. the source area for the sediments is the surrounding environment, and deposition in an area of variable energy where both coarse and finer particles settle out.
The colours of the organic matter types were determined, although organic matter colouration was not used in environmental interpretation,. The fragments of cuticles and wood are a mixture of colours, ranging from yellow to black. Normally, organic matter colour gets darker with increasing depth of burial. Therefore, the wide range of colour of organic fragments in these samples probably suggests that there has been reworking and subsequent oxidation in the paralic sequence. Additionally, the use of nitric acid for oxidation may have contributed to the wide range of colours in our samples. Reworking waseasily established for someof thepalynomorphs studied, especially when their ages were known. For example, a specimen of Cingdatisporites cf. ornatus, an Upper Cretaceous spore, was found in one assemblage. However, the occurrence of Cicatricosisporites dorogensis in these Miocene sediments, above the Oligocene range reported by Germeraad et al. (1968), was moredifficult todismissas reworkingas thesporesoccurred in several samples (see Fig. 2) and wereextremely well preserved. A common feature observed on many palynomorphs was corrosion; this results from oxidation and may indicate a degree of reworking of deltaic sediments. The plant debris in the sediments are terrestrially-derived types I1 and 111 organic matter (Bustin, 1988).

4.
Lithology. Lithological studies of the borehole have dealt with the nature of the accessory minerals associated with the sediments (Oboh, 1987;Oboh & Salami 1989). Pyrite, mica flakes, limonite stains, lignite streaks and heavy minerals were occasionally encountered in the sand bodies. The shales commonly contained pyrite and mica flakes and occasionally lignite, carbonaceous materials and glauconite. Of these accessory minerals, pyrite and glauconite are environmentally significant.
The common occurrence of pyrite in the shale bodies confirms the reducing conditions under which they were deposited. Glauconite is a typically marine mineral and although it was present in small amounts within unit D, its occurrence decreased from the base of the sequence. This shows that the sediments at the base of the succession are of transitional nature from marine influenced conditions.

CONCLUSIONS
Palynological and lithological evidence has beenused to interpret the environment of deposition of the studied interval (1123-3583 metres). The range of size of organic matter (especially cuticles and wood fragments) indicates that the source of sediment is near the basin of deposition, although the colour range and presence of corroded palynomorphs show a degree of reworking. The marine influence, indicated by small amounts of glauconite within the shales of unit D, is strongest towards the base of this unit. The rarity of dinoflagellates in the sequence, however, shows the lack of marine depositional environments throughout the rest of the sampled section.
The frequent to abundant occurrence of Z. ramoiiae in most parts of the sequence suggests the presence of Rhizophora type mangrove swamp in the area. However, the percentages of Z.
rainonae obtained in the present study seldom reach the very high levels quoted by Muller (1959) for the modern vegetation of the Orinoco delta environment.
The absence or rarity of Z. ramonae at some horizons in the Peregrinipollis iiigericus and Clauamoriocolpites sp. subzones suggests two periods of long term variation in the mangrove vegetation during the time of deposition. The interval of low frequency of Z. rnmoiiae in the Middle Miocene corresponds with high percentages of climbing ferns indicative of swamp or marsh habitats (Germeraad et al., 1968). Such changes could be due to fluctuation in the species making up the coastal swamp forestoranadvanceof thedelta frontcreatinga more freshwater influence in the area of deposition. Towards the top of the studied sequence, in the Upper Miocene, Z. ranlome and the other pollen types become intermittent and the sedimenary environment more continental. This supports the view of Short & Stauble (1967) that the delta front advanced towards the end of the Miocene as deltaic sediments built up.