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Storm lag and related facies of the bioclastic limestones of the Eze-Aku Formation (Turonian), Nigeria
Sedimentary Geology, 30 (1981) 133--147
133
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
STORM LAG AND RELATED FACIES OF THE BIOCLASTIC LIMESTONES OF THE EZE-AKU FORMATION (TURONIAN), NIGERIA
INDRANIL BANERJEE
Department of Geology, University of Nigeria, Nsukka (Nigeria) (Received August 12, 1980 ; revised and accepted February 9, 1981 )
ABSTRACT Banerjee, I., 1981. Storm lag and related facies of the bioclastic limestones of the EzeAku Formation (Turonian), Nigeria. Sediment. Geol., 30: 133--147. The predominantly siliclastic Eze-Aku Formation (Turonian) located in the southeastern Nigerian Basin contains scattered lenses and layers of bioclastic limestone which show three distinct facies: Facies 1 --Beds (0.2--1 m thick) of sparite~cemented grainstones grading laterally into cross-bedded medium sandstones. Facies 2 - Lenses (1--2 m thick, 0.1--2 km wide) of grainstones and wackestones within fine bioturbated silty sandstones. Facies 3--Tabular units (1--2 m thick, 2--6 km w i d e ) o f wackestone within laminated black shale. Grainstones are interpreted as storm-lag deposits accumulated either at the margins of offshore bars (facies 1) or on the wide platform of bioturbated muddy sand located below the wave base (facies 2). Wackestones of facies 3 are believed to be deposited by high-density turbidity currents carrying shallow-water shells into the deeper basin. Wackestones of facies 2 are difficult to interpret but might have been deposited by smallscale storm-generated debris flows on a shallow platform.
INTRODUCTION
Accumulation of shell
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Fig. 1. General geology of the Eze-Aku Formation showing the shape of its outcrop, its structure, rock-types and the location of the more detailed geological maps (Figs. 2 and 6). Inset: Geological map of Nigeria showing the location of the area studied• Criss-cross: Frecambrian basement; blank: Cretaceous ; stipple : Tertiary.
(Turonian) occupies the Benue Trough which f o r m e d as an aulacogen (Olade, 1975) during the opening up of the Atlantic (Wright, 1968) and had been a co n tin u o us site o f deposition t h r o u g h o u t the Cretaceous. The EzeAku F o r m a t i o n is a 700 m succession o f shale, siltstone and sandstone with a shallow marine fauna o f ammonites, gastropods, pelecypods and foraminifera (Adeleye, 1975; Fayose and de Klasz, 1976; Arua and Rao, 1978; Fayose, 1978; Peters, 1978). At the s o u th er n end o f the Benue Trough near its eastern b o u n d a r y , the sediments have been folded into a series of shallow open northeast-trending folds constituting t he Abakaliki anticlinorium. The Eze-Aku sediments flank both sides o f the structure (Fig. 1) and the older Asu River G roup occupies the core. E x c e p t very close t o its b o u n d a r y with the Asu River Group, the Eze-Aku F o r m a t i o n is virtually u n d e f o r m e d .
135
The Eze-Aku Formation is a predominantly clastic sequence with no largescale development of carbonate banks or build-ups. In the entire southeast Nigerian basin only one patchy development of peritidal carbonates (Fayose, 1978) in the Calabar flank (Fig. 1) and some small oyster-reefs from the middle Benue valley (Peters, 1978) have been reported. The limestones studied range from grainstone to wackestone and consist mostly of fragments of pelecypods c o m m o n l y 0.5 to 10 mm in size b u t ranging up to 8 cm. O t h e r fossil fragments present are: gastropods, echinoids, caralline algae, brachiopods, foraminifera, bryozoa, ostracodes and vertebrate bones. The matrix varies from a dark-brown lime m u d to coarse sparite. Beds are sharp-based, massive with a crude grading in clast size and a rough fissility towards the top. An erosive base with low relief may be present. FACIES OF THE LIMESTONES
The bioclastic limestones of the Eze-Aku Formation exhibit the following three facies. Facies 1 The best example of this facies is in location 1 of Fig. 2 where a 2 m thick bioclastic grainstone overlies bioturbated siltstone and grades laterally to a pebbly coarse arkose (see Banerjee, 1980, fig. 9). The limestone is packed with unabraded or slightly abraded shell fragments, mostly of pelecypods cemented b y sparite. Four genera of pelecypods namely, Aphrodina, Cotaceramus, Mytiloides and Trigonarca and one genus of gastropods, Turritella have been identified. Other fragments belong to echinoids, brachiopods and vertebrate bone. The bed is massive ungraded with a random orientation of the clasts. The upper contact with the overlying pebbly arkose is sharp b u t the base is n o t well-exposed. Occurrences of this facies are more c o m m o n near the contact with the basement (loc. 3, 4 in Fig. 2) b u t become rarer away from it. The most diagnostic features of this facies are: (1) gradational relation with coarse cross-bedded sandstones; (2)grain-supported fabric; and (3) fauna of thick~helled bivalves. Facies 2 The most extensive lithology in the Eze-Aku Formation is a gray micaceous bioturbated silty very fine sandstone. The degree of bioturbation varies from 100% in burrow-mottled rocks to less than 50% in those rocks which show patches of ripple lamination. Enclosed within this sandstone, the limestones of facies 2 occur as lenses of varying size (0.2 to 1.5 m thick} with sharp contacts (Fig. 3a). Beds are massive b u t may show a crude fissility near the top. The base is always sharp and tocally erosive with low relief.
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Fig. 2. Geological map of an area (see Fig. 1 for location) close to the basement on the southeastern boundary of the Eze-Aku outcrop showing frequent bioclastic limestone lenses within bioturbated sandstone. (1), (2) and (3) = location of individual lenses•
Fig. 3. Field relationship of different facies of the Eze-Aku bioclastic limestones, a. Small lens of grainstone of facies 2. Note sharp upper and lower contacts with the bioturbated sandstone, b. Wackestones of facies 3. Tabular units within black shale at Nkalagu quarry (Band 1, Fig. 6). Note irregular erosive base (arrow) of the middle layer, c. Wackestones of facies 3. Multiple graded units of the basal limestone at Nkalagu quarry. Note massive basal part (b), very fissible laminated top (t) and undulating erosive bases (arrow) of the constituent units, d. Close-up view of the top laminated layer of 3c. Note the wavy bedding with alternate layers of dark shale and light carbonate shell hash.
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138
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Fig. 4. Cross-sections of limestone and associated sediments in different localities. 1. Section at l o c a t i o n (2), Fig. 2. S = s t o r m deposit; F = fairweather deposit. 2. Section of a limestone lens of facies 2 e m b e d d e d within b i o t u r b a t e d sandstone. 3. L i m e s t o n e bands within black shale at location (1) Fig. 6. N o t e layer o f calcareous concretion, erosive base of the limestones and the multiple nature o f the basal limestone. 4. Section o f limestone at l o c a t i o n (3), Fig. 6, in gray shale and siltstone interbedded. N o t e plant fragments in the overlying shale.
Amalgamation of two beds along an undulating sharp contact has been recorded (Fig. 4-2). No size grading or preferred orientation in the clasts have been found. In texture the limestones vary from grainstones to wackestones (Dunham, 1962). Average clast size varies f r o m 5 to 10 mm and the maximum is around 80 mm. Clasts range from almost unabraded to completely rounded forms (Fig. 5a) representing the following t w o faunal groups: (a) a shallowwater biota consisting of pelecypods, gastropods, echinoids, brachiopods and coralline algae; and (b) a deeper water biota consisting of thin-shelled bivalves and planktonic foraminifera. In summary, the diagnostic features of this facies are: (1) interstratification with bioturbated silty sandstones; (2) two contrasted fabrics, grainsupported and mud~supported; and (3) t w o contrasted faunas, one shallow water and the other deeper water. Facies 3
The Eze-Aku Formation shows a major facies change from dominant bioturbated sandstone to laminated black shale (Fig. 6) near the township of Nkalagu. Bioclastic limestones of facies 3 occur interbedded with the black shale as thick tabular units (Figs. 3b and 4-3). In the three quarries of the
ments mostly of pelecypods and pellets in a microsparite cement, b. Mixed fauna of facies 3 wackestone. Large pelecypod shell (P), Globigerina (G) and thin-shelled bivales (T) in a dark matrix o f lime mud. c. Grainstone of facies 1 showing pelecypod and gastropod shell.fragments, pellets and vertebrate bones. Note large vertebrate bone with a hollow centre and star-shaped outline (fish spine?) d. Wackestone of facies 3 with thick, and thin-shelled bivalves in lime mud. Subrectangular mud clast seen near middle right is believed
Fig. 5. Microscopic fabric of the Eze-Aku hioclastic limestones, a. Grainstone -- packstone of facies 2 showing subrounded shell frag¢D
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Nigerian Cement Company at Nkalagu, excellent exposures of a 30 m sequence of interbedded black shale and limestone reveal all the features of this facies. The exposed section shows six limestone beds of which the basal one is the thickest (6 m). It is in fact, a multiple unit (Fig. 3c) with four amalgamated units, each 1--1.5 m thick. Each unit starts with a sharp undulating erosive base followed by a massive basal part in which crudely graded clasts up to 6 cm in size float in a light-coloured m u d d y matrix. The top part is highly fissile and shows a crude wavy lamination (Fig. 3d). The limestones in the upper horizons are all single units 1--1.5 m thick. They have the same internal organisation as the constituent units of the basal limestone. The black shale containing the limestones is finely laminated, fissile, pyritiferous and contains starved ripples of very fine quartz sand with internal cross lamination. The black shale has yielded a microfauna of pelagic and aranecous foraminifera (Fayose and de Klasz, 1976; Peters, 1978). In some minor occurrences (loc. 2 and 3, Fig. 6) wackestones, otherwise similar in character to the Nkalagu limestones described above, occur within a sequence of interbedded gray shale and siltstone (Fig. 4-4). Their sedimentary setting is in between that of facies 2 and 3. Texturally, the limestone beds are all wackestones at the base and grade
141
to mudstones towards the top. The bioclasts represent as in facies 2, two faunal groups one representing a shallower and the other a deeper-water biota. The diagnostic features of this facies are: (1) Interstratification with black shale. (2) Extensive tabular units. (3) Distinct internal structural sequence with a sharp erosive base, massive basal unit and a laminated top unit. Crude vertical grading in clast size. (4) Mud-supported fabric. (5) Mixture of t w o distinct faunas, one shallow and one deep water. PETROGRAPHY
The Eze-Aku limestones vary in their fabric from grainstones to wackestones and in a few cases even to mudstones (Dunham, 1962). According to Folk's (1959) terminology they are biosparites, biomicrites and micrites. Shell clasts vary from very angular, unbraded to well-rounded grains (Fig. 5a). Pelecypods followed by gastropods contribute most of the larger shell fragments (Fig. 5b). Punctate echinoid plates, echinoid spines, fibrous shells of brachiopods, fragments of bryozoan and algal fragments constitute the rest of the coarse shell debris. The deeper water biota represented by tests of Globigerinids, and fragments of thin-shelled bivalves (Fig. 5b) make up the finer shell debris which also contain ostracodes. Closed valves of pelecypods and tests of gastropods are c o m m o n l y filled with a mixture of fine shell hash and lime mud. Vertebrate bones f o u n d only in the gralnstones of facies 1 and 2 are clear brown isotropic phosphatic grains, mostly well-rounded and circular b u t also oval and subrectangular in cross-section. They vary in size from 0.1 to 6 mm. A few show internal structures which in one case suggest a fish spine (Fig. 5c). A summary of the petrographic data is given in Table I. STORM-LAG DEPOSITS: THE GRAINSTONES
The Eze-Aku bioclastic limestones are coarse-grained exotic layers containing large clasts emplaced within fine-grained low-energy sediments. Episodic catastrophic events which may lead to such deposits are: storm, debris flow and turbidity currents. However, in nearshore marine sediments which the Eze-Aku represents, storm seems to be the c o m m o n e s t and the most probable agent of such deposition. A lag pavement of storm-generated, gravel-sized shell debris on the nearshore b o t t o m is expected to show a grainsupported fabric because storm e b b currents would normally winnow the m u d out. Observations from the recent sea also suggest that such layers are usually 1.5 to 2.0 m thick and typically overlie burrowed silt and sand (Kumar and Sanders, 1976). The grainstones of both facies 1 and 2 of the Eze-Aku limestone show
single c o m p o n e n t
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none
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rare (5%)
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thin shelled bivalve, Globigerinids
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thin-shelled bivalve, Globigerinids
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double c o m p o n e n t
absent
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--
0--10% r o u n d e d sand
none
absent
present
present
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8 mm
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shallow-water
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60 m m
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80 m m
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Facies 1
3. M a x i m u m clast size
1. T e x t u r e (a) Dunham (b) F o l k
Properties
Petrography o f the limestones
TABLE I b-a t~
143
burrowed silty sandstone at the base. While grainstones of facies 2 are also overlain by similar rocks, those of facies 1 grade laterally to and are overlain by, cross-bedded coarse arkose of subtidal bar origin (Banerjee, 1980), indicating that they accumulated at bar margins. From a study of the Upper Jurassic coquinoid sandstones of Wyoming and Montana, U.S.A., Brenner and Davies (1973) presented a depositional model in which shell debris accumulating as lag pavements could be of the following three types: (1) Channel lag. Storms cut surge channels through tidal sand bars and transport shell debris along them. 'Channel lag' deposits left in these channels are characterized b y a deep erosive base and abundant cross-bedding. (2) Storm lag. At the leeward side of the channels the shell debris spreads o u t over the flanks of the bar and may extend as a sheet on the platform mud. (3) Swell lag. These are local concentrations of whole cells produced b y the passage of storm swells over the m u d d y platform. Facies 1 grainstones of the Eze-Aku are comparable to the 'storm lag' and facies 2 ones to the 'swell lag' deposits of the above model. GRAVITY FLOW DEPOSITS: WACKESTONES OF FACIES 3
The marked change of facies of the Eze-Aku sediments from a bioturbated gray siltstone with shelled infauna to a well-laminated black shale with planktonic foraminifera indicates a break of slope where shallow (0--50 m) aerobic water changes to deeper ( > 1 5 0 m) anaerobic water (Byers, 1977; Hallam, 1980). The sharp-based light-coloured massive, crudely graded beds of bioclastic limestones with laminated tops emplaced within the black shales are in overall character, somewhere in between the deposits of a high
144
deposition by a subaqueous gravity displacement process. The finer shell debris dominated by planktonic forminifera represent the indigenous fauna accumulating at the site of deposition. Fayose and de Klasz's (1976) analysis of the microfaunal assemblage in these beds revealed the following features of the in situ fauna: (1) dominance (70%) of planktonic foraminifers; (2) rarity o f calcareous benthic f o r m s ; a n d (3) low faunal diversity. All these features indicate low-productivity in an anoxic basin. Examples of such mixtures of a shallow-water 'displaced' fauna (Halimeda, corals and mollusc fragments} and deep-water indigenous fauna (planktonic foraminifera) have been found within bioclastic turbidites in the recent sea occurring at the edge o f carbonate banks (Bornhold and Pilkey, 1971). Carbonate gravity-flow deposits (Davies, 1977; Enos, 1977; Yurewicz, 1977) are typically present in carbonate slopes (Cook and Taylor, 1977; Moussa, 1977; Mcllreath, 1979) located at the margin of major carbonate build-ups. The setting for the Eze-Aku example, was different. Major carbonate build-up was absent and the limestones occur as thin beds within a siliclastic sequence. The gravity flows responsible for them, therefore, must have been small~cale rare events. PROBLEMATIC ORIGIN OF FACIES 2 WACKESTONES
The sedimentary setting of these wackestones is different from that of facies 3. They have a patchy location as lenses within highly bioturbated fine sandstones. Beds are massive, sharp-based and sharp-topped and the bioclasts show the same mixture of shallow and deep water fauna as in facies 3. Although the exact mechanism of emplacement of these beds remains unclear, an interesting comparison can be made with some m u d d y lenses of shell debris deposited by storms in the recent sea. Perkins and Enos (1968), while studying the geological effects of Hurricane Betsey noted that near the northern tip of the Andros Island, 'a lens o f soft lime m u d u p t o 8" t h i c k was d e p o s i t e d in t h e n e a r s h o r e m a r i n e e n v i r o n m e n t . I n c l u d e d in t h e s e s e d i m e n t s were shells, l a m i n a t e d clasts a n d f r a g m e n t s o f d o l o m i tic c r u s t e r o d e d f r o m a d j a c e n t b e a c h d e p o s i t s b y t h e e b b i n g s t o r m tide'.
They did n o t speculate on the actual mechanism by which these mud-supported shell debris were transported. It could be an example of storm-generated debris flow, because pressure-surges associated with a major storm can liquefy a mass of sediments (Middleton and Southard 1977, p. 8.5) which can develop into a debris flow. FACIES MODEL
The three facies of the bioclastic limestones of the Eze-Aku, indicate three specific environments. Facies 1 accumulated at the margin of offshore bars, facies 2 in a shoreface environment, and facies 3 in a basin.
145 I
(
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BAR SAND
1
GRAINSTONE
WACKESTONE Fig. 7. Paleogeographic reconstruction of the Eze-Aku sea and a facies model for the bioclastic accumulations H . T . = High tide level; L . T . = Low tide level.
The facies model presented in Fig. 7, shows an extensive shallow platform covered by bioturbated fine sand and mud accumulating below the wave base. Subtidal sand bars were responsible for the accumulation of facies 1 limestones at their margins. Patches of shell debris of facies 2 representing either a 'swell lag' of storm origin (grainstones) or a deposit of a small~scale storm-generated debris flow (wackestones) occur within the bioturbated sand. At the edge of the platform downslope movements of sediments into the euxinic basin beyond, occurred sporadically. Debris flows or highdensity turbidity currents carried a mixture of shell debris and lime m u d and emplaced facies 3 limestones within the basin mud.
REFERENCES Adeleye, D.R., 1975. Nigerian Late Cretaceous stratigraphy and paleogeography. Am. Assoc. Pet. Geol. Bull., 59: 2302--2313. Arua, I. and Rao, V.R., 1978. Ammonite evidence for the age of Nkalagu limestone, Anambra State, Nigeria. J. Min. Geol., 15: 47--48. Ball, S.M., 1971. The Westphalian Limestone of the Northern Midcontinent: A possible ancient storm deposit. J. Sediment. Petrol., 41: 217--232. Ball, M.M., Shinn, E.A. and Stockman, K.W., 1967. The Geologic effects of Hurricane Donna in South Florida. J. Geol., 75: 583--597. Banerjee, I., 1980. A subtidal bar model for the Eze-Aku sandstones, Nigeria. Sediment. Geol., 25: 291--309.
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Bornhold, B.D. and Pilkey, O.H., 1971. Bioclastic turbidite sedimentation in Columbus Basin, Bahamas. Geol. Soc. Am. Bull., 82: 1341--1354. Brenner, R.L. and Davies, D.R., 1973. Storm-generated coquinoid sandstone: genesis of high energy marine sediments from the Upper Jurassic of Wyoming and Montana. Geol. Soc. Am. Bull., 84: 1685--1698. Byers, C.W., 1977. Biofacies patterns in euxinic basins: a general model. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 5--17. Cook, H.E. and Taylor, M.E., 1977. Comparison of continental slope and shelf environments in the Upper Cambrian and Lowest Ordovician of Nevada. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 51--81. Crowell, J.C., 1957. Origin of pebbly mudstones. Bull. Geol. Soc. Am., 68: 993--1009. Davies, G.R., 1977. Turbidites, debris sheets, and truncation structures in Upper Paleozoic deep-water carbonates of the Sverdrup Basin, Arctic Archipelago. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 221--248. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. Am. Assoc. Pet. Geol. Mem., 1: 108--121. Enos, P., 1977. Tambara limestone of the Pozo Rico Trend, Cretaceous, Mexico. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog. Spec. Publ., 25: 273--314. Fayose, E.A., 1978. Depositional environments of carbonate, Calabar Flank, Southeastern Nigeria. J. Min. Geol., 15: 1--13. Fayose, E.A. and de Klasz, I., 1976. Microfossils of the Eze-Aku Formation (Turonian) at Nkalagu quarry, Eastern Nigeria. J. Min. Geol., ! 3 : 51--61. Folk, R.L., 1959. Practical petrographic classification of limestones. Am. Assoc. Pet. Geol. Bull., 43: 1--38. Friedman, G.M. and Sanders, J.E., 1978. Principles of Sedimentology. John Wiley, New York, N.Y., 792 pp. Hallam, A., 1980. Black shales. J. Geol. Soc., 137: 123--124. Howard, J.D. and Reineck, H.E., 1972. Georgia Coastal Region, Sapelo, Island, U.S.A.: Sedimentology and Biology. IV. Physical and biogenic sedimentary structures of the nearshore shelf. Senckenbergiana Marit., 4: 81--123. Kumar, N. and Sanders, J.E., 1976. Characteristics of shoreface storm deposits: modern and ancient examples. J. Sediment Petrol., 46 : 145--162. McUreath, I.A., 1979. Facies Models 12. Carbonate Slopes. In: R.G. Walker (Editor), Facies Models. Geoscience Canada. Reprint Ser., 1 : 133--143. Middleton, G.V. and Hampton, M.A., 1973. Sediment gravity flows: Mechanics of flow and deposition. In: G.V. Middleton and A.H. Bouma (Editors), Turbidites and Deepwater Sedimentation. Soc. Econ. Palaeontol. and Mineralogy, Pacific Sect., Los Angeles, California, pp. 1--38. Middleton, G.V. and Southard, J.B., 1977. Mechanics of sediment movement. Soc. Econ. Paleontol. Mineralogy., Short Course no. 3, Binghamton. Moussa, M.T., 1977. Bioclastic sediment gravity flow and submarine sliding in the Juana Diaz Formation, southwestern Puerto Rico. J. Sediment. Petrol., 47 : 593--599. Olade, M.A., 1975. Evolution of Nigeria's Benue Trough (aulacogen): A tectonic model. Geol. Mag., 112: 576--583. Perkins, R.D. and Enos, Paul, 1968. Hurricane Betsy in the Florida Bahama area -- Geologic Effect and Comparison with Hurricane Donna. J. Geol., 76: 710--717. Peters, S.W., 1978. Mid-Cretaceous paleoenvironments and biostratigraphy of the Benue Trough, Nigeria. Geol. Soc. Am. Bull., 89: 151--154. Powers, M.C. and Kinsman, B., 1953. Shell accumulations in underwater sediments and their relation to the thickness of the traction zone. J. Sediment. Petrol., 23 : 229--234.
147 Specht, R.W. and Brenner, R.L., 1979. Storm-wave genesis of bioclastic carbonates in Upper Jurassic epicontinental mudstones, east-central Wyoming. J. Sediment. Petrol., 49: 1307--1322. Wright, J.B., 1968. South Atlantic continental drift and the Benue trough. Tectonophysics, 6 : 301--310. Yurewicz, D.A., 1977. Sedimentology of Mississippian basin carbonates. New Mexico and West Texas -- The Rancheria Formation. In: H.E. Cook and P. Enos (Editors), Deepwater Carbonate Environments. Soc. Econ. Paleontol. Mineralog. Spec. Publ., 25: 203--220.
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Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
STORM LAG AND RELATED FACIES OF THE BIOCLASTIC LIMESTONES OF THE EZE-AKU FORMATION (TURONIAN), NIGERIA
INDRANIL BANERJEE
Department of Geology, University of Nigeria, Nsukka (Nigeria) (Received August 12, 1980 ; revised and accepted February 9, 1981 )
ABSTRACT Banerjee, I., 1981. Storm lag and related facies of the bioclastic limestones of the EzeAku Formation (Turonian), Nigeria. Sediment. Geol., 30: 133--147. The predominantly siliclastic Eze-Aku Formation (Turonian) located in the southeastern Nigerian Basin contains scattered lenses and layers of bioclastic limestone which show three distinct facies: Facies 1 --Beds (0.2--1 m thick) of sparite~cemented grainstones grading laterally into cross-bedded medium sandstones. Facies 2 - Lenses (1--2 m thick, 0.1--2 km wide) of grainstones and wackestones within fine bioturbated silty sandstones. Facies 3--Tabular units (1--2 m thick, 2--6 km w i d e ) o f wackestone within laminated black shale. Grainstones are interpreted as storm-lag deposits accumulated either at the margins of offshore bars (facies 1) or on the wide platform of bioturbated muddy sand located below the wave base (facies 2). Wackestones of facies 3 are believed to be deposited by high-density turbidity currents carrying shallow-water shells into the deeper basin. Wackestones of facies 2 are difficult to interpret but might have been deposited by smallscale storm-generated debris flows on a shallow platform.
INTRODUCTION
Accumulation of shell
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Fig. 1. General geology of the Eze-Aku Formation showing the shape of its outcrop, its structure, rock-types and the location of the more detailed geological maps (Figs. 2 and 6). Inset: Geological map of Nigeria showing the location of the area studied• Criss-cross: Frecambrian basement; blank: Cretaceous ; stipple : Tertiary.
(Turonian) occupies the Benue Trough which f o r m e d as an aulacogen (Olade, 1975) during the opening up of the Atlantic (Wright, 1968) and had been a co n tin u o us site o f deposition t h r o u g h o u t the Cretaceous. The EzeAku F o r m a t i o n is a 700 m succession o f shale, siltstone and sandstone with a shallow marine fauna o f ammonites, gastropods, pelecypods and foraminifera (Adeleye, 1975; Fayose and de Klasz, 1976; Arua and Rao, 1978; Fayose, 1978; Peters, 1978). At the s o u th er n end o f the Benue Trough near its eastern b o u n d a r y , the sediments have been folded into a series of shallow open northeast-trending folds constituting t he Abakaliki anticlinorium. The Eze-Aku sediments flank both sides o f the structure (Fig. 1) and the older Asu River G roup occupies the core. E x c e p t very close t o its b o u n d a r y with the Asu River Group, the Eze-Aku F o r m a t i o n is virtually u n d e f o r m e d .
135
The Eze-Aku Formation is a predominantly clastic sequence with no largescale development of carbonate banks or build-ups. In the entire southeast Nigerian basin only one patchy development of peritidal carbonates (Fayose, 1978) in the Calabar flank (Fig. 1) and some small oyster-reefs from the middle Benue valley (Peters, 1978) have been reported. The limestones studied range from grainstone to wackestone and consist mostly of fragments of pelecypods c o m m o n l y 0.5 to 10 mm in size b u t ranging up to 8 cm. O t h e r fossil fragments present are: gastropods, echinoids, caralline algae, brachiopods, foraminifera, bryozoa, ostracodes and vertebrate bones. The matrix varies from a dark-brown lime m u d to coarse sparite. Beds are sharp-based, massive with a crude grading in clast size and a rough fissility towards the top. An erosive base with low relief may be present. FACIES OF THE LIMESTONES
The bioclastic limestones of the Eze-Aku Formation exhibit the following three facies. Facies 1 The best example of this facies is in location 1 of Fig. 2 where a 2 m thick bioclastic grainstone overlies bioturbated siltstone and grades laterally to a pebbly coarse arkose (see Banerjee, 1980, fig. 9). The limestone is packed with unabraded or slightly abraded shell fragments, mostly of pelecypods cemented b y sparite. Four genera of pelecypods namely, Aphrodina, Cotaceramus, Mytiloides and Trigonarca and one genus of gastropods, Turritella have been identified. Other fragments belong to echinoids, brachiopods and vertebrate bone. The bed is massive ungraded with a random orientation of the clasts. The upper contact with the overlying pebbly arkose is sharp b u t the base is n o t well-exposed. Occurrences of this facies are more c o m m o n near the contact with the basement (loc. 3, 4 in Fig. 2) b u t become rarer away from it. The most diagnostic features of this facies are: (1) gradational relation with coarse cross-bedded sandstones; (2)grain-supported fabric; and (3) fauna of thick~helled bivalves. Facies 2 The most extensive lithology in the Eze-Aku Formation is a gray micaceous bioturbated silty very fine sandstone. The degree of bioturbation varies from 100% in burrow-mottled rocks to less than 50% in those rocks which show patches of ripple lamination. Enclosed within this sandstone, the limestones of facies 2 occur as lenses of varying size (0.2 to 1.5 m thick} with sharp contacts (Fig. 3a). Beds are massive b u t may show a crude fissility near the top. The base is always sharp and tocally erosive with low relief.
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Fig. 2. Geological map of an area (see Fig. 1 for location) close to the basement on the southeastern boundary of the Eze-Aku outcrop showing frequent bioclastic limestone lenses within bioturbated sandstone. (1), (2) and (3) = location of individual lenses•
Fig. 3. Field relationship of different facies of the Eze-Aku bioclastic limestones, a. Small lens of grainstone of facies 2. Note sharp upper and lower contacts with the bioturbated sandstone, b. Wackestones of facies 3. Tabular units within black shale at Nkalagu quarry (Band 1, Fig. 6). Note irregular erosive base (arrow) of the middle layer, c. Wackestones of facies 3. Multiple graded units of the basal limestone at Nkalagu quarry. Note massive basal part (b), very fissible laminated top (t) and undulating erosive bases (arrow) of the constituent units, d. Close-up view of the top laminated layer of 3c. Note the wavy bedding with alternate layers of dark shale and light carbonate shell hash.
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138
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Fig. 4. Cross-sections of limestone and associated sediments in different localities. 1. Section at l o c a t i o n (2), Fig. 2. S = s t o r m deposit; F = fairweather deposit. 2. Section of a limestone lens of facies 2 e m b e d d e d within b i o t u r b a t e d sandstone. 3. L i m e s t o n e bands within black shale at location (1) Fig. 6. N o t e layer o f calcareous concretion, erosive base of the limestones and the multiple nature o f the basal limestone. 4. Section o f limestone at l o c a t i o n (3), Fig. 6, in gray shale and siltstone interbedded. N o t e plant fragments in the overlying shale.
Amalgamation of two beds along an undulating sharp contact has been recorded (Fig. 4-2). No size grading or preferred orientation in the clasts have been found. In texture the limestones vary from grainstones to wackestones (Dunham, 1962). Average clast size varies f r o m 5 to 10 mm and the maximum is around 80 mm. Clasts range from almost unabraded to completely rounded forms (Fig. 5a) representing the following t w o faunal groups: (a) a shallowwater biota consisting of pelecypods, gastropods, echinoids, brachiopods and coralline algae; and (b) a deeper water biota consisting of thin-shelled bivalves and planktonic foraminifera. In summary, the diagnostic features of this facies are: (1) interstratification with bioturbated silty sandstones; (2) two contrasted fabrics, grainsupported and mud~supported; and (3) t w o contrasted faunas, one shallow water and the other deeper water. Facies 3
The Eze-Aku Formation shows a major facies change from dominant bioturbated sandstone to laminated black shale (Fig. 6) near the township of Nkalagu. Bioclastic limestones of facies 3 occur interbedded with the black shale as thick tabular units (Figs. 3b and 4-3). In the three quarries of the
ments mostly of pelecypods and pellets in a microsparite cement, b. Mixed fauna of facies 3 wackestone. Large pelecypod shell (P), Globigerina (G) and thin-shelled bivales (T) in a dark matrix o f lime mud. c. Grainstone of facies 1 showing pelecypod and gastropod shell.fragments, pellets and vertebrate bones. Note large vertebrate bone with a hollow centre and star-shaped outline (fish spine?) d. Wackestone of facies 3 with thick, and thin-shelled bivalves in lime mud. Subrectangular mud clast seen near middle right is believed
Fig. 5. Microscopic fabric of the Eze-Aku hioclastic limestones, a. Grainstone -- packstone of facies 2 showing subrounded shell frag¢D
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Nigerian Cement Company at Nkalagu, excellent exposures of a 30 m sequence of interbedded black shale and limestone reveal all the features of this facies. The exposed section shows six limestone beds of which the basal one is the thickest (6 m). It is in fact, a multiple unit (Fig. 3c) with four amalgamated units, each 1--1.5 m thick. Each unit starts with a sharp undulating erosive base followed by a massive basal part in which crudely graded clasts up to 6 cm in size float in a light-coloured m u d d y matrix. The top part is highly fissile and shows a crude wavy lamination (Fig. 3d). The limestones in the upper horizons are all single units 1--1.5 m thick. They have the same internal organisation as the constituent units of the basal limestone. The black shale containing the limestones is finely laminated, fissile, pyritiferous and contains starved ripples of very fine quartz sand with internal cross lamination. The black shale has yielded a microfauna of pelagic and aranecous foraminifera (Fayose and de Klasz, 1976; Peters, 1978). In some minor occurrences (loc. 2 and 3, Fig. 6) wackestones, otherwise similar in character to the Nkalagu limestones described above, occur within a sequence of interbedded gray shale and siltstone (Fig. 4-4). Their sedimentary setting is in between that of facies 2 and 3. Texturally, the limestone beds are all wackestones at the base and grade
141
to mudstones towards the top. The bioclasts represent as in facies 2, two faunal groups one representing a shallower and the other a deeper-water biota. The diagnostic features of this facies are: (1) Interstratification with black shale. (2) Extensive tabular units. (3) Distinct internal structural sequence with a sharp erosive base, massive basal unit and a laminated top unit. Crude vertical grading in clast size. (4) Mud-supported fabric. (5) Mixture of t w o distinct faunas, one shallow and one deep water. PETROGRAPHY
The Eze-Aku limestones vary in their fabric from grainstones to wackestones and in a few cases even to mudstones (Dunham, 1962). According to Folk's (1959) terminology they are biosparites, biomicrites and micrites. Shell clasts vary from very angular, unbraded to well-rounded grains (Fig. 5a). Pelecypods followed by gastropods contribute most of the larger shell fragments (Fig. 5b). Punctate echinoid plates, echinoid spines, fibrous shells of brachiopods, fragments of bryozoan and algal fragments constitute the rest of the coarse shell debris. The deeper water biota represented by tests of Globigerinids, and fragments of thin-shelled bivalves (Fig. 5b) make up the finer shell debris which also contain ostracodes. Closed valves of pelecypods and tests of gastropods are c o m m o n l y filled with a mixture of fine shell hash and lime mud. Vertebrate bones f o u n d only in the gralnstones of facies 1 and 2 are clear brown isotropic phosphatic grains, mostly well-rounded and circular b u t also oval and subrectangular in cross-section. They vary in size from 0.1 to 6 mm. A few show internal structures which in one case suggest a fish spine (Fig. 5c). A summary of the petrographic data is given in Table I. STORM-LAG DEPOSITS: THE GRAINSTONES
The Eze-Aku bioclastic limestones are coarse-grained exotic layers containing large clasts emplaced within fine-grained low-energy sediments. Episodic catastrophic events which may lead to such deposits are: storm, debris flow and turbidity currents. However, in nearshore marine sediments which the Eze-Aku represents, storm seems to be the c o m m o n e s t and the most probable agent of such deposition. A lag pavement of storm-generated, gravel-sized shell debris on the nearshore b o t t o m is expected to show a grainsupported fabric because storm e b b currents would normally winnow the m u d out. Observations from the recent sea also suggest that such layers are usually 1.5 to 2.0 m thick and typically overlie burrowed silt and sand (Kumar and Sanders, 1976). The grainstones of both facies 1 and 2 of the Eze-Aku limestone show
single c o m p o n e n t
7. Fauna
none
none
c o m m o n (5--10%)
11. Glauconite
12. Microstylolite
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--
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rare (5%)
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rare
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common
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none
10. Mudclasts
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none
9. Feldspar
0--20% r o u n d e d sand
10--20% angular silt
thin shelled bivalve, Globigerinids
gastropod, brachiopod
0--40%, angular silt to r o u n d e d sand
thin-shelled bivalve, Globigerinids
p e l e c y p o d , gastropod, echinoid, brachiopod, coralline algae, bryozoa
double c o m p o n e n t
absent
micrograded and loaded laminae
--
0--10% r o u n d e d sand
none
absent
present
present
pelecypod, echinoid, b r y o z o a coralline algae ostracode
none
double c o m p o n e n t
--
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8 mm
1:4
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Top
8. Quartz
(b) deep-water
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present
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shallow-water
none
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(a)
none
4. Vertical gradation in clast size
60 m m
1:1
wackestone biomicrite
Base
Facies 3
80 m m
1:0--1:1
1 :0
60 m m
2. G r a i n / m u d
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Facies 2
grainstone biosparite
Facies 1
3. M a x i m u m clast size
1. T e x t u r e (a) Dunham (b) F o l k
Properties
Petrography o f the limestones
TABLE I b-a t~
143
burrowed silty sandstone at the base. While grainstones of facies 2 are also overlain by similar rocks, those of facies 1 grade laterally to and are overlain by, cross-bedded coarse arkose of subtidal bar origin (Banerjee, 1980), indicating that they accumulated at bar margins. From a study of the Upper Jurassic coquinoid sandstones of Wyoming and Montana, U.S.A., Brenner and Davies (1973) presented a depositional model in which shell debris accumulating as lag pavements could be of the following three types: (1) Channel lag. Storms cut surge channels through tidal sand bars and transport shell debris along them. 'Channel lag' deposits left in these channels are characterized b y a deep erosive base and abundant cross-bedding. (2) Storm lag. At the leeward side of the channels the shell debris spreads o u t over the flanks of the bar and may extend as a sheet on the platform mud. (3) Swell lag. These are local concentrations of whole cells produced b y the passage of storm swells over the m u d d y platform. Facies 1 grainstones of the Eze-Aku are comparable to the 'storm lag' and facies 2 ones to the 'swell lag' deposits of the above model. GRAVITY FLOW DEPOSITS: WACKESTONES OF FACIES 3
The marked change of facies of the Eze-Aku sediments from a bioturbated gray siltstone with shelled infauna to a well-laminated black shale with planktonic foraminifera indicates a break of slope where shallow (0--50 m) aerobic water changes to deeper ( > 1 5 0 m) anaerobic water (Byers, 1977; Hallam, 1980). The sharp-based light-coloured massive, crudely graded beds of bioclastic limestones with laminated tops emplaced within the black shales are in overall character, somewhere in between the deposits of a high
144
deposition by a subaqueous gravity displacement process. The finer shell debris dominated by planktonic forminifera represent the indigenous fauna accumulating at the site of deposition. Fayose and de Klasz's (1976) analysis of the microfaunal assemblage in these beds revealed the following features of the in situ fauna: (1) dominance (70%) of planktonic foraminifers; (2) rarity o f calcareous benthic f o r m s ; a n d (3) low faunal diversity. All these features indicate low-productivity in an anoxic basin. Examples of such mixtures of a shallow-water 'displaced' fauna (Halimeda, corals and mollusc fragments} and deep-water indigenous fauna (planktonic foraminifera) have been found within bioclastic turbidites in the recent sea occurring at the edge o f carbonate banks (Bornhold and Pilkey, 1971). Carbonate gravity-flow deposits (Davies, 1977; Enos, 1977; Yurewicz, 1977) are typically present in carbonate slopes (Cook and Taylor, 1977; Moussa, 1977; Mcllreath, 1979) located at the margin of major carbonate build-ups. The setting for the Eze-Aku example, was different. Major carbonate build-up was absent and the limestones occur as thin beds within a siliclastic sequence. The gravity flows responsible for them, therefore, must have been small~cale rare events. PROBLEMATIC ORIGIN OF FACIES 2 WACKESTONES
The sedimentary setting of these wackestones is different from that of facies 3. They have a patchy location as lenses within highly bioturbated fine sandstones. Beds are massive, sharp-based and sharp-topped and the bioclasts show the same mixture of shallow and deep water fauna as in facies 3. Although the exact mechanism of emplacement of these beds remains unclear, an interesting comparison can be made with some m u d d y lenses of shell debris deposited by storms in the recent sea. Perkins and Enos (1968), while studying the geological effects of Hurricane Betsey noted that near the northern tip of the Andros Island, 'a lens o f soft lime m u d u p t o 8" t h i c k was d e p o s i t e d in t h e n e a r s h o r e m a r i n e e n v i r o n m e n t . I n c l u d e d in t h e s e s e d i m e n t s were shells, l a m i n a t e d clasts a n d f r a g m e n t s o f d o l o m i tic c r u s t e r o d e d f r o m a d j a c e n t b e a c h d e p o s i t s b y t h e e b b i n g s t o r m tide'.
They did n o t speculate on the actual mechanism by which these mud-supported shell debris were transported. It could be an example of storm-generated debris flow, because pressure-surges associated with a major storm can liquefy a mass of sediments (Middleton and Southard 1977, p. 8.5) which can develop into a debris flow. FACIES MODEL
The three facies of the bioclastic limestones of the Eze-Aku, indicate three specific environments. Facies 1 accumulated at the margin of offshore bars, facies 2 in a shoreface environment, and facies 3 in a basin.
145 I
(
8 0 Km BREA~ IN SLOPE
o iooi (m)
EUXINIC SEDIMENT ~BOTURBATE SAND8, SJLT ~ (~), ~),(~)- FACIESOF BIOCLASTIC LIMESTONES
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BAR SAND
1
GRAINSTONE
WACKESTONE Fig. 7. Paleogeographic reconstruction of the Eze-Aku sea and a facies model for the bioclastic accumulations H . T . = High tide level; L . T . = Low tide level.
The facies model presented in Fig. 7, shows an extensive shallow platform covered by bioturbated fine sand and mud accumulating below the wave base. Subtidal sand bars were responsible for the accumulation of facies 1 limestones at their margins. Patches of shell debris of facies 2 representing either a 'swell lag' of storm origin (grainstones) or a deposit of a small~scale storm-generated debris flow (wackestones) occur within the bioturbated sand. At the edge of the platform downslope movements of sediments into the euxinic basin beyond, occurred sporadically. Debris flows or highdensity turbidity currents carried a mixture of shell debris and lime m u d and emplaced facies 3 limestones within the basin mud.
REFERENCES Adeleye, D.R., 1975. Nigerian Late Cretaceous stratigraphy and paleogeography. Am. Assoc. Pet. Geol. Bull., 59: 2302--2313. Arua, I. and Rao, V.R., 1978. Ammonite evidence for the age of Nkalagu limestone, Anambra State, Nigeria. J. Min. Geol., 15: 47--48. Ball, S.M., 1971. The Westphalian Limestone of the Northern Midcontinent: A possible ancient storm deposit. J. Sediment. Petrol., 41: 217--232. Ball, M.M., Shinn, E.A. and Stockman, K.W., 1967. The Geologic effects of Hurricane Donna in South Florida. J. Geol., 75: 583--597. Banerjee, I., 1980. A subtidal bar model for the Eze-Aku sandstones, Nigeria. Sediment. Geol., 25: 291--309.
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Bornhold, B.D. and Pilkey, O.H., 1971. Bioclastic turbidite sedimentation in Columbus Basin, Bahamas. Geol. Soc. Am. Bull., 82: 1341--1354. Brenner, R.L. and Davies, D.R., 1973. Storm-generated coquinoid sandstone: genesis of high energy marine sediments from the Upper Jurassic of Wyoming and Montana. Geol. Soc. Am. Bull., 84: 1685--1698. Byers, C.W., 1977. Biofacies patterns in euxinic basins: a general model. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 5--17. Cook, H.E. and Taylor, M.E., 1977. Comparison of continental slope and shelf environments in the Upper Cambrian and Lowest Ordovician of Nevada. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 51--81. Crowell, J.C., 1957. Origin of pebbly mudstones. Bull. Geol. Soc. Am., 68: 993--1009. Davies, G.R., 1977. Turbidites, debris sheets, and truncation structures in Upper Paleozoic deep-water carbonates of the Sverdrup Basin, Arctic Archipelago. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog., Spec. Publ., 25: 221--248. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. Am. Assoc. Pet. Geol. Mem., 1: 108--121. Enos, P., 1977. Tambara limestone of the Pozo Rico Trend, Cretaceous, Mexico. In: H.E. Cook and P. Enos (Editors), Deep-water Carbonate Environments. Soc. Econ. Paleontol. Mineralog. Spec. Publ., 25: 273--314. Fayose, E.A., 1978. Depositional environments of carbonate, Calabar Flank, Southeastern Nigeria. J. Min. Geol., 15: 1--13. Fayose, E.A. and de Klasz, I., 1976. Microfossils of the Eze-Aku Formation (Turonian) at Nkalagu quarry, Eastern Nigeria. J. Min. Geol., ! 3 : 51--61. Folk, R.L., 1959. Practical petrographic classification of limestones. Am. Assoc. Pet. Geol. Bull., 43: 1--38. Friedman, G.M. and Sanders, J.E., 1978. Principles of Sedimentology. John Wiley, New York, N.Y., 792 pp. Hallam, A., 1980. Black shales. J. Geol. Soc., 137: 123--124. Howard, J.D. and Reineck, H.E., 1972. Georgia Coastal Region, Sapelo, Island, U.S.A.: Sedimentology and Biology. IV. Physical and biogenic sedimentary structures of the nearshore shelf. Senckenbergiana Marit., 4: 81--123. Kumar, N. and Sanders, J.E., 1976. Characteristics of shoreface storm deposits: modern and ancient examples. J. Sediment Petrol., 46 : 145--162. McUreath, I.A., 1979. Facies Models 12. Carbonate Slopes. In: R.G. Walker (Editor), Facies Models. Geoscience Canada. Reprint Ser., 1 : 133--143. Middleton, G.V. and Hampton, M.A., 1973. Sediment gravity flows: Mechanics of flow and deposition. In: G.V. Middleton and A.H. Bouma (Editors), Turbidites and Deepwater Sedimentation. Soc. Econ. Palaeontol. and Mineralogy, Pacific Sect., Los Angeles, California, pp. 1--38. Middleton, G.V. and Southard, J.B., 1977. Mechanics of sediment movement. Soc. Econ. Paleontol. Mineralogy., Short Course no. 3, Binghamton. Moussa, M.T., 1977. Bioclastic sediment gravity flow and submarine sliding in the Juana Diaz Formation, southwestern Puerto Rico. J. Sediment. Petrol., 47 : 593--599. Olade, M.A., 1975. Evolution of Nigeria's Benue Trough (aulacogen): A tectonic model. Geol. Mag., 112: 576--583. Perkins, R.D. and Enos, Paul, 1968. Hurricane Betsy in the Florida Bahama area -- Geologic Effect and Comparison with Hurricane Donna. J. Geol., 76: 710--717. Peters, S.W., 1978. Mid-Cretaceous paleoenvironments and biostratigraphy of the Benue Trough, Nigeria. Geol. Soc. Am. Bull., 89: 151--154. Powers, M.C. and Kinsman, B., 1953. Shell accumulations in underwater sediments and their relation to the thickness of the traction zone. J. Sediment. Petrol., 23 : 229--234.
147 Specht, R.W. and Brenner, R.L., 1979. Storm-wave genesis of bioclastic carbonates in Upper Jurassic epicontinental mudstones, east-central Wyoming. J. Sediment. Petrol., 49: 1307--1322. Wright, J.B., 1968. South Atlantic continental drift and the Benue trough. Tectonophysics, 6 : 301--310. Yurewicz, D.A., 1977. Sedimentology of Mississippian basin carbonates. New Mexico and West Texas -- The Rancheria Formation. In: H.E. Cook and P. Enos (Editors), Deepwater Carbonate Environments. Soc. Econ. Paleontol. Mineralog. Spec. Publ., 25: 203--220.