Cretaceous- Tertiary crossroads of migration in the Sahel RICHARD MOODY & PETER SUTCLIFFE

A series of expeditions undertaken over the last three years by a team of geologists from Kingston Polytechnic and the British Museum (Natural History) has increased our understanding of the Cretaceous and Tertiary palaeogeography of Central West A frica. The latest expedition discovered a dinosaur graveyard of Lower Cretaceous (Wealden) age. Studies of these and other fossils within the sedimentary rocks of the Iullemmeden Basin in Niger and Mali have shed new light on the TransSaharan Seaway migration route during the Cretaceous and Tertiary periods.

T h e Sahel region, a broad east-west-trending zone to the south of the Sahara Desert, is the location of an extensive sedimentary basin con- taining Cretaceous and Tertiary strata. Named the Iullemmeden Basin, it extends from the Gao area of Mali through Niger into Chad (Fig. 1). Over the last two decades, it has yielded a large number of well-preserved dinosaur remains, which inspired a research team from Kingston Polytechnic and the British Museum (Natural History) to visit the region in 1988 (Fig. 2; and see Geology Today, v. 4, pp. 43 and 77, 1988).

For centuries, the Sahel towns of Agadez and Gao have been at the crossroads of migration and trade. Until recently, the desert crossing was restricted to Arab and African traders, European explorers or migrants often succumbing to the inhospitable climate and terrain, or to the sword. In some ways, this process of selection parallels that which took place during the Cretaceous and Tertiary along the length of a narrow and intermittent Trans-Saharan Seaway. Cretaceous-Tertiary faunas often show affin- ities both with those of the Tethyan realm to the north of the Sahara and with those of the South Atlantic area. The fossils present vary from complete dinosaur skeletons associated with freshwater sediments to fully marine invert- ebrate assemblages.

The oldest rocks examined crop out in the area around Agadez in central Niger. These consist of coarse, red conglomerates and sand- stones of high-energy braided-river origin. The most probable age of these unfossiliferous arkosic (feldspar-rich) sediments is earliest Cretaceous. They are overlain by finer red and green sandstones and siltstones typically arranged in fining-upwards cycles (cyclothems) resulting f r m deposition in a meandering river system. The cyclothems are capped by cal- careous fossil soils (calcrete palaeosols) with sparse plant debris.

Around the town of In Gall, the sandstone component of the succession decreases and the red siltstones alternate with thin white mark or ostracode-rich limestones. Ten kilometres east of In Gall, the red siltstones, which represent overbank floodplain deposits, contain numerous semi-articulated dinosaur remains, mainly of camarasaurid sauropod dinosaurs (which were herbivores) resembling forms from China and South America (Fig. 3). Clusters of bones indicate that whole carcasses, bloated after death, floated into overbank areas during flooding. Occasional shallow-channel deposits contain disarticulated vertebrate fossils includ- ing sharks, dinosaurs, crocodiles and lung fish. Ostracodes and wood debris also occur (Fig. 4). The capping white, calcareous sediments contain abundant ostracodes, together with non- marine bivalves, layered algal mats (stro-

Braided river: a river that divides into a network of branching and reuniting channels separated by islands andor sand bars.

Marl: a calcareous clay.

Ostracode: a type of small (usually less than 2 mm long) aquatic crustacean.

Fig. 1. Locality map showing the boundaries (hatched lines) of the Iullemmeden Basin. v

GEOLOGY TODAY Janualy-February 1990119

Fig. 2. David Atten- b borough and Richard Moody studying camaro- saurid dinosaur bones. (Photo: R. Ravenhill, Kingston Polytechnic.)

Fluviatile (or fluvial): resulting from the action of rivers.

Lacustrine: relating to lakes.

Argillaceous: composed largely of, or containing appreciable amounts of, clay minerals.

Scute: the horny or chitinous protective shell of the armadillo, turtle, crocodile, etc.

Fig. 3. Camarosaurid sauropod dinosaur bones, up to 1.5 m long, from Tiouraren, In Gall, Niger. v

matolites) and rootlet structures. The age of these freshwater, fluviatile-lacustrine sediments is Wealden (Lower Cretaceous).

The low-energy red siltstone sequence is overlain by a series of coarse arkosic sandstones of Cenomanian age. They are the product of a braided-river system showing well-developed cross-bedding throughout the sequence. The early Cretaceous succession is almost entirely freshwater in character and shows that marine incursions (Fig. 5) had not yet reached into the interior regions of Central West Africa at this time.

The dawn of the late Cretaceous in the Iullemmeden Basin is marked by the first of four major marine transgressions (M1 to M4 in Fig. 5 ) . The first (Ml) took place at the end of the Cenomanian and lasted into the early Turonian. It is characterized by the appearance of Neo- lobites and other ammonites, which appear almost simultaneously in sediments of similar age in the Sergipe Basin of Brazil, in Nigeria, the Sahara, Spain and the Middle East, showing that

seaways existed between all these areas at this time.

Excellent outcrops of Cenomanian and Turonian strata occur in the Iguellala Mountains approximately 200 km north-north-east of Tahoua in central Niger. There, gypsiferous shales with sand and grit lenses fill fissures in the eroded surface of the Lower Cretaceous sand- stone formation (Tegama Sandstone). The lowest shales, which contain a varied bivalve fauna and rare nautiloids, are overlain by a sequence of interbedded shales, calcareous sandstones and grits with abundant echinoids (sea urchins) and ammonites. Among the latter group, the genus Nigericeras was a typical Trans- Saharan migrant which persisted into the early Turonian throughout this area.

Argillaceous limestones of late Turonian age outcrop between In Gall and Tahoua and contain a restricted fauna of cythereid ostracodes and small bivalves. The late Turonian-early Coniacian palaeogeography of the region is characterized by a second transgression (M2) with shallow, marine waters extending into eastern Niger and along the Gao trough. Throughout the area, the characteristic sedi- ments are chalky limestones with flints. In the Dammergou region on the south-western margin of the inhospitable TCnerC Desert, the so-called ‘Calcaires Blancs’ give way to sand- stones and shales with vertebrate remains. Bones of dinosaurs, crocodiles and fish and the shells of bivalves and gastropods are abundant in specific localities dated as Coniacian- Santonian. These remains usually occur in fluviatile shale- siltstone sequences such as that at Ibeceten. The overall evidence is of a regressive episode with an inland sea flanked by lakes fed by low-energy streams.

Poorly defined trace fossils and freshwater ostracodes are to be found in the mudstones and sandstones that overlie the Santonian deposits. It is apparent that the region was isolated from Tethys and the opening Atlantic at this time until the third marine transgression (M3) in the early Maast richtian.

Excellent outcrops of Maastrichtian shales and bone beds occur in the area of Kao, a. Touareg village 60 km north-east of Tahoua. The lower part of the sequence is unfossilifer- ous, but the bones of the carnivorous dinosaur Megalosaurus, crocodiles and fish are abundant 3-4 m above the base of the main outcrop. Cross-bedded sandstones indicate the presence of meandering channel systems within the lower sequence. These channels contain bone beds consisting of disarticulated fish vertebrae and spines, crocodile scutes and rare turtle bones. Phosphatic faecal pellets are abundant. Flood- plain deposits flank the meandering channels, and weathering has resulted in a badland topography (Fig. 6) with individual pinnacles of

20lGEOLOGY TODAY January-February 1990

JI/ SPARSE VEGETATION c& ARTICULATED BONES &@ PETRIFIED WOO0

'0' BONE BED @ SINGLE BONES OESSICATON CRACKS - FISH, FISH DEBRIS PEDOGENlClLACUSTRlNE CARBONATE CARBONATE SEDIMENT a CONCRETIONS

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0 U A T -

T E R T I

A R Y

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C R E T A C E 0 U S

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a BIVALVES

shale often surmounted by the shell of a large pelomedusid turtle.

Pelomedusid turtles are mostly marsh or coast dwellers, but large sea-going forms are recorded from Cretaceous and Tertiary deposits. It is likely that the Maastrichtian turtles of Niger are closely related to those of Brazil and North Africa, the Trans-Saharan Seaway enabling their migration both northwards and eastwards to the Middle East.

Bones of fish and mosasaurs (giant marine lizards) occur in abundance in the upper part of the Maastrichtian shale sequence around Kao. The mosasaurs, their remains commonplace in the Maastrichtian chalks of Niobrara (North America) and of Northern Europe (St Peters Mountain, Maastricht, Holland), are commonly known as 'Meuse Lizards'. They were voracious hunters that fed on fish and small reptiles.

The benthic fauna of the 'Mosasaurus Shales' includes a variety of bivalves including oysters. Deep-bodied pycnodont fish armed with batteries of flat, crushing, palatal teeth, laminid and hybodont sharks, various bony fish and the ammonite Libycoceras are major components of the free-swimming fauna. The appearance of benthic bivalves and free-swimming ammonites indicates that normal marine conditions existed in the upper part of the Lower Maastrichtian sequence in Niger. The worldwide marine transgression of the early Maastrichtian is there- fore clearly manifested in the Iullemmeden Basin but probably (Fig. 7) followed a more westerly route than the earlier Cenomanian- Turonian incursion.

In both the Sokoto Basin of northern Nigeria and south-central Niger, the marine 'Mosa- saurus Shales' are overlain by a sequence of siltstones and mudstones. Apart from traces of roots, these terrestrial sediments are unfossi- liferous and mark the retreat of marine con- ditions during late Maastrichtian times.

d SHARK

Echinoids, bivalves and crocodile vertebrae are commonplace in the Palaeocene shales that locally overlie the non-marine sediments of the late Maastrichtian. The marine transgression of the Palaeocene (M4) is well documented, again following a more westerly route, that was no doubt structurally controlled. It results in the uniform distribution of ostracode faunas from the Nigerian coast across the Sahara into Tunisia and Libya.

The Lower Palaeocene shales give way to a relatively thick sequence of calcareous mud- stones and indurated bioclastic limestones. Effectively, these carbonate lithologies can be

STAGE

PLEISTOCENE

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PLIOCENE

MIOCENE

Early EOCENE

PALAEOCENE

MAASTRICHT I AN

C AMPA N I A N ~ ~~

SANTONIAN

CON\ AC I AN

TURONIAN

CENOMANIAN

WEALDEN

'Not to scale.

4 Fig. 4. Palaeoenviron- ments in the Wealden freshwater sediments of Tiouraren, In Gall, Niger.

Badland: stream-eroded and barren.

Benthic: relating to benthos, the marine life that inhabits the ocean floor.

Calcareous: containing calcium carbonate.

Indurated: hardened by pressure, cementation or heat.

Fig. 5. Sea and land fluctuations in the Cretaceous-Tertiary sediments of the Iullemmeden Basin. v

MARINE NON - MAR1 NE ENVIRONMENTS 1 ENVIRONMENTS

Hiatus

Diminishing shallow ssas Terrestrial -coastal

M4. Open to marginal marine

$ I

M3. Open marine 4 M2. Shallow marine

MI Marine

Terrestrial

Fluvla t ile, lacustrine

Braided rivers Fluvial, lacustrine Overbank floodplain Meandering rivers High-energy braided rivers

GEOLOGY TODAY January-February 1990121

Fig. 6. Typical badland F topography near Ibeceten, Central Niger.

Paper shales: shales that easily separate on weathering into thin layers suggesting sheets of paper.

Oolite: rock comprising small rounded carbonate particles that have grown by accretion around a nucleus.

Fermginous: relating to, or containing, iron.

Rhiioid: resembling a root.

Fig. 7. Early Maast- b richtian palaeogeography and the position of the Trans-Saharan Seaway (after Reyment, 1986b).

divided into three units, the lowest of which yields an abundance of bivalves, nautiloids, echinoids and vertebrates. The nautiloids include Cimonia landanensis and Cimonia sudanensis, the echinoids Schizaster sudanensis and Plesiolampas saharae. Cimonia is universally known throughout North Africa and the Middle East, and its presence in the Iullemmeden Basin indicates that migration routes across the Sahara persisted throughout the Palaeocene. The presence of pelomedusid turtles and dyrosaurid

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12IGEOLOGY TODAY January-February 1990

crocodiles similar to those of South America and Tunisia emphasize this point.

The abundance of echinoids in the top two limestone units is indicative of normal salinity and open marine conditions. Above the lime- stones, paper shales, calcareous mudstones with chamosite oolites and pelletal phosphates are the dominant lithologies. The paper shales exfoliate like the yellowed pages of an abandoned book and are usually barren, although decalcified bivalves and opalized teeth and vertebrae are occasionally present. The vertebrate remains are mostly from porbeagle sharks and it is probable that small individuals ventured into, and perished in, marginal marine environments. Simple vertical burrows are commonplace in the paper shales, but within the oolitic ferruginous mudstones the burrow systems are much more intensive (Fig. 8). In some localities they are of the branched trace-fossil burrow Thalassinoides, but elsewhere the traces are more comparable with the rhizoid systems found in mangrove swamps. The remains of ‘ghost’ shrimps, bony fishes (including several species of pycnodont fish), turtles and dyrosaurid crocodiles are the main components of the fauna from these sediments. One of these is Pycnodus bowerbankii, a large bony fish known from the London Clay of the Isle of Sheppey. Migration via the Trans-Saharan Seaway was therefore still a possibility during Eocene (Lower Ypresian) times. Disarticulated bones of catfish, lung fish, sea snakes, marsh turtles and crocodiles are concentrated to form thick bone beds. Rare beds with the mammal Moerifherium are also recorded. The association of these vertebrates indicates shallow coastal environ- ments. The crocodile species are again similar to those found in the Eocene (Ypresian) phosphates of Morocco and Tunisia and to species found in South America. On this evidence, it would appear that a migration route still existed across the Sahara during the Lower Eocene, the migrants ‘hopping’ between slowly diminishing inland seas.

Throughout the Iullemmeden Basin, the phosphate-shale sequence is overlain by sedi- ments that mark the advent of terrestrial conditions and the final withdrawal of the sea from the region. These non-marine sediments are dated as Mio-Pliocene and Pleistocene, and a major stratigraphic hiatus is inferred during the late Eocene and throughout the Oligocene. It is evident from the numerous ferruginous oolitic intervals that outcrop that lacustrine environ- ments similar to those of the present Lake Chad occurred at various t i e s . Climatic variations occurred in the Plio-Pleistocene. A higher rain- fall (pluvial regime) illustrated by the weather- ing (Fig. 9) of many of the older limestones throughout the region is typical of this climatic change.

The study of the Cretaceous and Tertiary sediments and fossils has yielded much new information on the palaeogeographic evolution of Central West Africa. Of key importance is the role of the Trans-Saharan Seaway in allowing the passage and spread of many fossil species throughout the Tethyan and South Atlantic areas. Detailed work on local outcrops has confirmed that direct correlations can be made between the presence or absence of the seaway and global sea-level changes. These changes are well documented throughout the Cretaceous and Palaeocene, but the question of the persistence of the seaway as a migration route into and beyond the Ypresian is still debatable.

Suggestions for further reading Kogbe, C. A. 1981. Cretaceous and Tertiary of

the Iullemmeden Basin in Nigeria (West Africa). Cretaceous Research, v. 2 , pp. 129-186.

Reyment, R. A. 1986a. Transgressional maxima of the late Cretaceous in the western Tethys. Proceedings of the International Symposium on Shallow Tethys 2. Wagga Wagga, pp. 303-308.

Reyment, R. A. 1986b. Spanish and North African region of Tethys in the late Cre- taceous. Proceedings of the Intemarional Symposium on Shallow Tethys 2 . Wagga Wagga, pp. 309-317.

Reyment, R. A. & Reyment, R. E. 1980. The Palaeocene Trans-Saharan Transgression and its Ostracod fauna. In: Geology of Libya (eds M. J . Salam and M. T. Busrewil), v. 1, pp. 245-254. Academic Press, London.

4 Fig. 8. Intensive bio- turbation in the Eocene oolitic limestones of In Fargas, eastern Mali.

Fig. 9. Weathering of Palaeocene limestones, near In Fargas, eastern Mali. 'I

Richard Moody is Professor of Geology in the School of Geological S c k e s , Kingston Polytechnic, and the Direcror of Kingsron Geological Services.

Perer Sutclqfe is Principal Lecrurer in Geology in the School of Geological Sciences, Kingston Polytechnic.

In future issues of Geology Today Carboniferous trilobites: the beginning of the end by Bob Owens Peter Harvey

Evolution of Earth's atmosphere and its geological impact by Jane Francis by Steve Kershaw

Radiocarbon dating by accelerator mass spectrometry by Rupert Housley

Borehole images of the subsurface crust by Mike Lovell, Phillipe Pezard and

Polar fossil forests

PLUS our regular items

GEOLOGY TODAY January-February 1990123