THE ACE OF TIT3 ?TIGER DELTA (IZST AAFRICA) -

The Age of the Niger Delta (West Africa) R. A. REYMENT, Sweden

ABSTRACT

Recent investigations on the biostratigraphy and palaeobiogeography of the south- ern Atlantic region indicate tha t the first incursion of the sea in Nigeria took place from the south during late Middle Albian time, in the most northerly extension of the southern Atlantic rift. The gradual post-Middle-Albian rotational(?) movement of South America away from Africa finally left, in the Lower Turonian, a 200-km-wide gap in the crust that began to fill rapidly with sediments. This marks the initiation of the Niger Delta, a delta with a tectonic origin. The delta has, since the Turonian, grown about 200 km outward from the original coastline. The drift interpretation of the origin of the delta carries with i t the implication that the sediments flooring the outer delta basin cannot be older than Lower Turonian.

LNTRODUCTION A GLANCE a t the map of West Africa shows that the coastline of the Gulf of Guinea is dominated by the outward-expanding Niger Delta. In palaeogeogra- phical reconstruction of western Africa, i t is not always taken into account that this outgrowth of the coastline is a relatively recent occurrence, although this was demonstrated by Reyment (1956), and more recently by Stoneley (1966).

The growing belief in the likelihood of the South Atlantic ocean having originated by the agency of continental drift f i ts in with the geological facts surrounding the history of the delta. Following on from this, and in wider per- spective, these facts must be understood if the developments of the West African and opposite South American margins are to be lucidly interpreted.

The conclusions presented in this paper result from 20 years of study of the Cretaceous and Tertiary geology of West Africa and more recent studies in South America.

THE FIRST MARINE TRANSGRESSION IN THE NIGERIAN SEDIMENTARY BASIN

The oldest, reliably dated marine sediments in the Nigerian sedimentary basin are Middle Albian in age (Reyment, 1955, 1957, 1965). They have only been found outcropping in the middle Benue Valley, where they contain a typical as- sociation of oxytropidoceratids and dipoloceratids. What preceded this incursion of the sea is still "shrouded in mystery". Unfortunately, the geologists who have worked in Nigeria have shown very slight interest in the pre-marine Cretaceous sediments, which is a reflection of the poor state of our knowledge on the strati- graphic relationships of the Benue Valley, on the southern side of which, a t least, fair exposures of terrestrial sediments, underlying the marine Albian, occur

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Authors' addresses are given a t the back of this book.

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(Reyment, 1965). I t is also unresolved as to whether the Nigerian marine se- quence began during the Middle Albian, or somewhat earlier in the Cretaceous. The Upper Aptian, the time a t which the f i rs t dateable marine transgressions took place in the southern Atlantic rift, between Angola and Alagoas (Brazil)- Gabon, can be considered as a lower limit for the age of the delta. All of these sequences begin with thick salt deposits of Aptian age. I t is now known that the opening of the Atlantic r i f t had affected the Alagoas-Sergipe-Gabon area by the Upper Aptian, but the events that occurred in the Lower Albian north of this region still remain to be elucidated. TWO possibilities are sugge.sted: first, that the rift continued to open gradually during the Lower Albian, during which time the sea entered the Nigerian basin via the Benue graben and attained its maximum extension during the Middle Albian. The second hypothesis is that the opening processes marked time during the Lower Albian and then suddenly continued during Middle Albian. From what we now know of the rates of pro- cesses of drift mechanisms, the former hypothesis seems to be more likely than the latter. This implies that if the first hypothesis is true, progressively younger deposits of Lower Albian age must line the margins of the rift, in a narrow zone, from Alagoas to the Nigerian Basin. Moreover, the older part of the Ni- gerian sedimentary basin, the distal half of the "Benue urgraben", must be floored by sediments of late Lower Albian to early Middle Albian age.

THE ORIGIN OF THE DELTA CRADLE I t now seems highly likely that West Africa became definitely separated

from South America during the later part of the Lower Turonian. Thus, as the basement rocks of the Pernambuco section (NE Brazil) moved away from the Nigerian basement, a gap some 200 km wide was left in the crust. As I demon- strated graphically in a recent publication (Reyment, 1969), the subtriangular region between Natal and Recife in northeastern Brazil f i ts snugly into this gap in the crystalline basement between the Benin and Calabar flanks of the Niger Delta.

After the gap had been formed, sediments began to accumulate in this large, though structurally weak, basin. The oldest of the sediments in this part of the Nigerian Basin must be Lower Turonian in age, if the foregoing argument is essentially correct.

THE EVOLUTION OF THE DELTA The evolution of the Niger Delta proper must, as a result of the above argu-

ments, be dated from the late Lower Turonian. The deepest indentation of the middle Turonian coastline lay some 200 km inland from the present outermost point of the delta, which clearly indicates that the over-all outgrowth of the coast since the birth of the delta is of this order. The maximum thickness of se- diments in the Niger Delta is in excess of 5000 m, according to estimates based on geophysical work (cf. Stoneley, 1966).

The present-day symmetrical deltoid shape of the delta may possibly reflect the form of the crustal gap left by the departure of the Pernambuco basement. The drift interpretation of the origin of the South Atlantic ocean bears with it the corollary of a tectonic origin of the Niger Delta.

SOME CONCLUDING OBSERVATIONS The stringent application of the ideas presented in this paper to the geology

of the continental margin of West Africa and northeastern Brazil leads to some

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'TABLE 1 -- Estimates of the ages of the oldest marine sediments i n the immediate offshore zones of central West Africa and northeastern Brazil

Coastal stretch Coastal stretch (West Africa) Age estimate (NE Brazil) Age est imate Northern Gabon Upper Aptian Sergipe-Alagoas Upper Aptian Cameroun Lower Alhian South Pernambuco Lower Albian Eastern Nigeria Lower Albian North Pemambuco Middle Albian Vestern Nigeria Lower Turonian Rio Grande do Norte Lower Turonian

interest ing conclusions concerning t he ages of t h e oldest sediments of mar ine origin in t h e immediate offshore zones of these coastlines. My est imates f o r parts of these coastlines a r e summarized in Table 1. These age est imates depend entirely on t h e validity of the d r i f t hypothesis.

REFERENCES Reyment, R. A., 1955. The Cretaceous Ammonoidea of Nigeria and the Southern Came-

roons. Bull. Geol. Surv. Nigeria, 25, 112 p. , 1956. On the stratigraphy and palaeontology of Nigeria and the Cameroons,

British West,,Africz. Geol. Foren. Stockh. Forh., 78, p. 17-96. , 1957. Uber einige wirbellose Fossilien aus Nigerien und Kamerun, West-

afrika. Palaeontographica, 109, p. 41-70. ------, 1965. Aspects of the geology of Nigeria. Ibadan University Press, 145 p.

, 1969. Ammonite biostratigraphy, continental drift, and oscillatory trans- gressions. Nature, 5215, p. 137-140.

Stonely, R., 1966. The Niger Delta region in the light of the theory of continental drift. Geol. Mag., 103, p. 385-397.

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