Petrographic differentiation of tectonically controlled cretaceous sedimentary cycles, southeastern Nigeria

Sedimentary Geology, 17 (1977) 235--245

235

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PETROGRAPHIC DIFFERENTIATION OF TECTONICALLY CONTROLLED CRETACEOUS SEDIMENTARY CYCLES, SOUTHEASTERN NIGERIA

MOMINUL HOQUE

Department of Geology, University of Nigeria, Nsukka (Nigeria) (Received October 30, 1975; revised and accepted April 28, 1976)

ABSTRACT

Hoque, M., 1977. Petrographic differentiation of tectonically controlled Cretaceous sedimentary cycles, southeastern Nigeria. Sediment. Geol., 17: 235--245. The Benue--Abakaliki trough and the Anambra basin of southern Nigeria were two major depocenters of sediments during the Cretaceous Period. The first depositional cycle of Albian to Santonian time was confined mainly to the Benue--Abakaliki trough and the lithic fill is characterised by feldspathic sandstone. The second sedimentary cycle, commencing after the Santonian tectonic episode and lasting till the Early Eocene, filled the Anambra basin and the small Afikpo syncline, and the lithic fill is distinguished by quartz arenite. These petrographic characteristics of sandstone units can be used as evidence of a major change in the sedimentary tectonism in this area, in recognising the stratigraphic boundary between the two sedimentary cycles, establishing correlation among subsurface well sections, and delineating margins of pre-Santonian and post-Santonian basins where other criteria are not reliable or readily available. The feldspathic sandstone of the first cycle is thought to be the product of high relief and rapid erosion of the Cameroun basement complex. The quartz arenites of the second cycle, largely generated in the same provenance, were the product of an intense chemical weathering of granitic basement rocks, both at the source and during a longer period of transportation to the Anambra basin, in a humid climate. These observations may have other palaeoclimatic significance.

INTRODUCTION

The s e d i m e n t a r y basin of southern Nigeria includes the B e n u e - - A b a k a l i k i folded belts, the Niger E m b a y m e n t , the A n a m b r a basin, the A f i k p o syncline a n d t h e N i g e r D e l t a ( F i g . 1). In t h i s p a p e r , t h e s o u t h e r n b a s i n has a m o r e res t r i c t e d g e o g r a p h i c e x t e n t t o i n c l u d e m a i n l y t h e areas l y i n g s o u t h a n d s o u t h east of the Niger--Benue confluence. Three m a j o r tectonic events and their resultant depositional cycles appear to d o m i n a t e the geologic history of southern Nigeria (Short and Stauble, 1967). Murat (1972) discussed the depositional history of the southern N i g e r i a s e d i m e n t a r y b a s i n in t e r m s o f t h r e e m a j o r t e c t o n i c p h a s e s w h i c h c o n -

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trolled the mo r phol ogy and the evaluation of the basin; he was thus able to establish a rational basis in explaining the facies pattern and the changes in basin m o r p h o l o g y in space and time, and their inter-relationship. Earlier attempts in establishing a biostratigraphic correlation on the basis of ammonites and foraminifers have been reviewed by R e y m e n t (1965). The geo m e t r y of lithic fill and the inter-relationship among units are, however, complicated by m a ny transgressive and regressive phases during each successive depositional cycle• Changes in thickness and facies are c o m m o n t h r o u g h o u t the basin. Problems o f stratigraphic correlation are also comp o u n d e d by paucity of good outcrops, lack of conspicuous key beds, and

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extensive tropical vegetation cover. This has led to creation of many formation names. Recognition of unconformities and delineation of geometry of the sedimentary basin therefore remained conjectural. This paper reviews the Cretaceous tectonic framework of the area and presents the petrography of sandstone units of several formations deposited during the first and the second sedimentary cycles. Division of the sandstones into quartz arenites and feldspathic types may constitute a simple criterion to recognise the unconformity brought about by a Santonian tectonism between the two depositional cycles; this may also help in delineating the basin geometry at successive stages of its evolution. TECTONIC SETTING

Tectonism in southern Nigeria probably started in Early Cretaceous time with the separation of Africa from South America and the opening of the Atlantic (Burke et al., 1972; Murat, 1972; Nwachukwu, 1972). The NE--SW trending Benue--Abakaliki trough is also thought to be the result of

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238 a pre-Albian rifting o f the African Shield prior to the opening of the South Atlantic (Uzuakpunwa, 1974). The first depositional cycle c o m m e n c e d in the Albian, if not earlier, and continued till the Santonian with a possible break during Cenomanian time. The only Cenomanian deposit, known as the Odukpani Form at i on ( R e y m e n t , 1955, 1965), occurs on Calabar flank. The absence of Cenomanian strata in the rest of the basin and the discordance in dip between the Albian Asu River Group and the overlying Turonian Eze-Aku Form at i on, reported at a n u m b e r o f localities in the Afikpo basin, might be indicative of a significant time gap with tectonic implications. Nwachukwu (1972) has reviewed all available evidence and concluded in favour of a minor Cenomanian folding phase. The basin m o r p h o l o g y and other tectonic elements as prevailed during the first sedimentary cycle (Albian to Santonian) is shown in Fig. 2. The second tectonic episode occurred during Santonian time and def o r med the Benue--Abakaliki trough into folded belts. This structural inversion was accompanied by the formation of a wider Anambra basin on the

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239

western flank and a smaller Afikpo basin on the southeastern flank of the anticlinorium (Fig. 3). The Santonian uplift resulted in the erosion of over 2000 m of sediments from the Abakaliki folded belts (Burke et al., 1972, p. 198). Much of the eroded material was deposited in the Anambra basin, which became the major depocenter during Campanian to Early Eocene time. The third sedimentary cycle c o m m e n c e d with the southward growth of the present Niger Delta perhaps in Late Eocene or Early Oligocene time. This cycle is outside the scope of this paper. STRATIGRAPHY AND PETROGRAPHY

Fig. 4 {based on Dessauvagie, 1975) shows the generalized geology of the area south and southeast of the Benue--Niger confluence. The stratigraphic relationship among the formations and the petrology of the sandstone units studied are shown in Fig. 5. The stratigraphic nomenclature used in this paper is based on R e y m e n t (1965). At least t w o sandstone samples, in a few cases as many as ten samples, were collected from different horizons of a formation. In a few cases, a published report on the petrography of a unit is used to arrive at a conclusion. But the sampling is not exhaustive enough to represent fully some of the very thick clastic units, such as the Asu River Group. In this report, feldspathic sandstone is defined as a sandstone with five or more percent feldspar (Pettijohn et al., 1972, p. 158). The Albian Asu River Group is dominantly shale with a few sandstone and limestone beds. The unit occupies the central part of the Abakaliki anticlinorium. Sandstone samples taken from the Abakaliki Shale {a formation) are feldspathic. The Mamfe Formation of the Mamfe Embayment, probably the time-equivalent of the Abakaliki Shale, is reported to be composed of massive arkosic sandstone {Reyment, 1965, p. 27}. The Cenomanian Odukpani Formation from north of Calabar consists of sandstone, shales and limestone. Dessauvagie {1972, p. 208) studied the type section and reported a 50 m thick arkosic sandstone at the base. The Turonian Eze-Aku Formation is composed of shales, sandstone and limestone. On the southwestern flank of the Abakaliki anticlinorium, the Eze-Aku Formation is conformably overlain by the Awgu Shale, and on the southeastern flank (in the Afikpo syncline) it is unconformably overlain by the Afikpo Sandstone. Sandstone samples collected from different horizons of the Eze-Aku Formation, including the uppermost Amaseri Sandstone Member, are feldspathic. The Amaseri Sandstone, however, contains much less feldspars ( a b o u t 8--10%) than other sandstone units of the Eze-Aku Formation {15--37%). The Bima Sandstone of the Benue valley, probably a time-equivalent of the Eze-Aku Formation {Barber, 1957) consists of coarseto very coarse-grained cross-stratified feldspathic sandstones (Jones, 1959). The Awgu Shale is present only on the western flank of the Abakaliki anticlinorium and is composed of shales with sandstones and minor lime-

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stones. Several samples from the Agbani Sandstone, a member of the Awgu Shale, have been studied; the sediment is thoroughly weathered and very friable and as a result, its petrographic character could not be ascertained

241

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Fig. 5. Stratigraphic sequence in the study area and the petrology of the sandstone units studied. positively. H o w e v e r , it appears to be d e f i c i e n t in, if n o t a l t o g e t h e r d e v o i d of, feldspars. T h e C a m p a n o - M a e s t r i c h t i a n E n u g u Shale and its lateral equivalents, t h e

242 OweUi Sandstone and the Nkporo Shale have been studied. All sandstone samples are feldspar-free, and dominantly composed of mono- and polycrystalline quartz grains. These sandstones are referred to here as quartz arenites (Pettijohn et al., 1972, p. 158). The lateral equivalent of the Nkporo Formation in the Afikpo syncline is called the Afikpo Sandstone which is also composed of quartz arenite. Sandstone samples from the coal-bearing Mamu Formation {Lower Maestrichtian) collected from several localities, are found to be quartz arenite. The conspicuously cross-stratified Ajali Sandstone {Upper Maestrichtian) has been regionally mapped and sampled; its palaeocurrent direction has recently been evaluated {Hoque, 1977). The sandstones are composed of quartz arenites; the grains are mostly friable, fine to coarse, in places pebbly, poorly sorted, and subangular to subrounded. The mean sediment transport direction, based on statistical study of azimuthal readings of foreset planes of cross-stratified units, was to the west and southwest. The overlying Nsukka Formation is dominantly shale and sandy shale with thin coal seams and sandstone beds. The sandstones are composed of quartz arenites. The uppermost lithologic unit that may be included in the second sedimentary cycle is the Early Eocene Ameki Formation; its lateral equivalent, the Nanka Sand is composed of quartz arenite. DISCUSSION OF RESULTS An about 7000 m thickness of sediment was deposited during the first sedimentary cycle. Shales are the d o m i n a n t lithology, and feldspathic sandstones are its distinguishing petrographic feature. The palaeocurrent analysis based on measurements of cross-bedding in the Eze-Aku indicates an easterly source for the sediment. The significance of detrital feldspar in sandstones has been the subject of considerable literature. A drier climate which inhibits decomposition of feldspars and other labile particles was suggested to be the cause of survival and accumulation o f feldspar in sandstones {Mackie, 1899). High relief with consequent rapid erosion of a granitic mass was thought to be more important than arid climate in the production of feldspar-rich sands (Krynine, 1935). Strakhov (1969, pp. 86--96) has suggested that under humid climatic conditions and in a granitic drainage basin, active tectonism will give rise to feldspar-rich ( " p o l y m i c t " ) rocks, whereas passive tectonism will produce quartzrich {"oligomict") rocks. Pettijohn {1975, p. 219} has, however, concluded that the presence or absence of feldspar is the result of a balance between the rate of decomposition and the rate of erosion. The feldspathic sandstones of the first cycle could therefore be the product of high relief and rapid erosion of the Cameroun supracrustal basement complex located east of the Abakaliki sedimentary basin. The nearness of the Abakaliki sedimentary basin to the source region was an important factor in the rich accumulation of feldspar in sands {Fig. 2}.

243 Sediments over 4000 m thick were deposited during the second sedimentary cycle and about half of these are arenaceous clastics. The palaeocurrent pattern established for the Ajali Sandstone (Hoque, 1977) indicates an easterly source region for the sediment (Fig. 3). Burke et al. (1972) estimated that over 2000 m of sediment were eroded from the Abakaliki folded terrain, and the bulk of it was deposited in the Anambra basin. But this alone cannot account for the over 4000 m thick and largely coarser sediment of the second cycle. It is therefore suggested that the granitic complex of the Cameroun basement must have accounted for a large part of the sediment of the Anambra basin. The distinctive feature of these clastics is their high degree of mineralogical maturity w i t h o u t a corresponding textural maturity. Mineralogically and texturally mature sands, such as quartz arenites, are the characteristic sandstones of tectonically stable platform or cratonic areas (Pettijohn, 1975, p. 234). Gibbs (1967) reported a case from the Amazon basin where the suspended load of the tropical tributaries o f the Amazon was nearly free from feldspars. It appears that mineralogically mature sediment could form in a humid fluvio-deltaic environment. What is needed is an intense chemical weathering both at the source and during transportation, when feldspar and other labile particles will effectively be removed; this will impart the endproduct with a high degree of mineralogical maturity. Such a sediment is unlikely to attain textural maturity comparable to that of quartz arenites of a stable platform. The absence of feldspars in the sediment of the second cycle is therefore considered to be a result of profound chemical weathering of the Cameroun granitic complex at the source and during a longer period of transportation to the Anambra basin, in an environment very similar to the present-day humid climate. Perhaps one can argue on a possibility of a drier climate and lesser role of chemical weathering to account for the feldspathic sandstones of the first cycle, and a humid climate and d o m i n a n t role of chemical decomposition for the quartz arenites of the second cycle. This will call for a postulation on changes of palaeoclimatic conditions during the Late Cretaceous. This and other possibilities, such as changes in palaeolatitudes of the African plate with respect to trade-wind belts or the equator (Drewry et al., 1974; Briden et al., 1974) seem remote. A more detailed geological and geophysical study of the area would, however, be needed before a conclusive statement could be made in this regard. SUMMARY OF OBSERVATIONS AND CONCLUSIONS (1) Formations of the first sedimentary cycle are characterised by the presence of feldspathic sandstones, and those of the second sedimentary cycle by quartz arenites (Fig. 5). This petrographic differentiation can be used where other criteria are not reliable or readily available, not only to fix

244

the stratigraphic boundary between the two sedimentary cycles, but also as addtional evidence for a major tectonism in this part of the country. (2) In subsurface mapping, the petrographic differentiation will be of particular use in establishing a reliable correlation among well sections. (3) The Santonian tectonism separated the Anambra basin as a major sedimentary basin from the earlier Benue--Abakaliki trough. The distinctive petrographic character of sandstone units of these two basins can be used in delineating the basin morphology both at surface and in subsurface sections. (4) The feldspathic sandstone of the first cycle is considered to be the product of high relief and rapid erosion of the Cameroun basement complex and the nearness of the depositional basin to the source region. The quartz arenite of the second cycle, largely brought in from the same source area, was a product of a humid climate and intense chemical weathering at the source and during transportation. As a note of caution, it must, however, be pointed out that sampling of sandstones units has not been exhaustive. Detailed geological and geophysical studies on a regional scale are needed before plate tectonic or other palaeoclimatic implications of the observations are understood and a conclusive statement can be made in this regard. ACKNOWLEDGEMENTS

I gratefully acknowledge the financial support from the Staff Travelling and Subsistence Vote of the Department of Geology, University of Nigeria, Nsukka. I also wish to thank Dr. S. O. Nwachukwu for many helpful discussions on the tectonics of southern Nigeria, and Mr. G. I. Onyegbula and Mr. C. O. Iweh for draughting the diagrams and preparing the typescript.

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245 Hoque, M., 1977. Petrology and palaeogeography of the Ajali Sandstone. J. Min. Geol., 14: in press. Jones, G.P., 1959. Sedimentary structures in the Bima Sandstone. Rec. Geol. Surv. Niger., 5--14. Krynine, P.D., 1935. Arkose deposits in the humid tropics, a study of sedimentation in southern Mexico. Am. J. Sci., 29: 353--363. Mackie, W., 1899. The feldspars present in sedimentary rocks as indicators of the conditions of contemporaneous climate. Trans. Edinburgh Geol. Soc. Proc., 7: 443--468. Murat, R.C., 1972. Stratigraphy and palaeogeography of the Cretaceous and Lower Tertiary in southern Nigeria. In: T.F.J. Dessauvagie and A.J. Whiteman (Editors), African Geology. Univ. Ibadan, Nigeria, pp. 251--266. Nwachukwu, S.O., 1972. The tectonic evolution of the southern portion of the Benue Trough, Nigeria. Geol. Mag., 109: 411--419. Pettijohn, F.J., 1975. Sedimentary Rocks. Harper and Row, New York, N.Y., 3rd ed., 628 pp. Pettijohn, F.J., Potter, P.E. and Siever, R., 1972. Sand and Sandstones. Springer, New York, N.Y., 618 pp. Reyment, R.A., 1955. The Cretaceous Ammonoidea of southern Nigeria and the southern Cameroons. Bull. Geol. Surv. Niger., 2 5 : 1 1 2 pp. Reyment, R.A., 1965. Aspects of the Geology of Nigeria. Ibadan Univ. Press, Nigeria, 145 PP. Short, K.C. and Stauble, A.J., 1967. Outline of geology of Niger Delta. Bull. Am. Assoc. Pet. Geol., 51: 761--779. Strakhov, N.M., 1969. Principles of Lithogenesis, 2. Oliver and Boyd, Edinburgh, 609 pp. (English ed.). Uzuakpunwa, A.B., 1974. The Abakaliki pyroclastics -- eastern Nigeria: new age and tectonic implications. Geol. Mag., 111: 65--70.