Palaeoenvironments of vertebrates on the southern shore of Tethys: The nonmarine Early Cretaceous of Tunisia

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Abstract

Through the Late Jurassic and Early Cretaceous, the southern shore of the Tethys Ocean migrated north and south over short distances. These vicissitudes are documented in the ‘continental intercalaire’, a long series of mainly non-marine sediments deposited across North Africa. A combined taxonomic, physical and chemical taphonomic study provides independent lines of evidence for reconstruction of palaeoenvironments within this marginal marine setting.

The Douiret, Chenini and Oum ed Diab formations from the Tataouine basin of southern Tunisia span the later part of the Early Cretaceous. Microvertebrates from four sites in these formations show different modes of physical abrasion, time averaging, and mixing, based on a taphonomic analysis using a combination of physical and chemical methods. The taxonomic composition of each assemblage, and trends in rare earth element (REE) compositions of bones, from each locality were used as independent lines of evidence to indicate differences in early depositional environments. The Jebel Boulouha assemblage (Douiret Formation) is interpreted as a terrestrial carbonate-rich environment with relatively little mixing. The Touil el Mra assemblage (Oum ed Diab Formation) suggests a marginal marine environment with some mixing of previously interred bones. The Oued el Khil assemblage (Chenini Formation) and the Oum ed Diab assemblage (Oum Ed Diab Formation) are more equivocal, suggesting mixed freshwater and marine influences. Interpreting salinity in marginal marine settings is difficult, and best attempted from multiple, independent lines of evidence. We suggest that REE geochemistry can contribute to palaeoenvironmental reconstruction when used in combination with other, independent physical, palaeontological and /or geochemical methods.

Introduction

The Late Jurassic and Early Cretaceous sedimentary successions of North Africa show evidence of many minor cycles of transgression and regression as the southern margin of the Tethys Ocean moved north and south. These were important times palaeogeographically, as the North Atlantic Ocean continued to open, as the South Atlantic Ocean began to open, and as land connections between Africa and Europe, perhaps across the Iberian Peninsula, waxed and waned.

This story is told in a long sequence of sediments termed loosely the ‘continental intercalaire’ (Lapparent, 1960) that are found across North Africa both north and south of the Sahara Desert. These successions range in age from Mid Jurassic to Mid Cretaceous, and they are largely non-marine, but with occasional minor marine transgressions, some consisting of limestones with ammonites, before the major worldwide late Cenomanian transgression. The ‘continental intercalaire’ has produced rich vertebrate remains from a dozen countries, from Morocco to Egypt, and from Sudan to Niger, and especially from Tunisia (Bouaziz et al., 1988, Benton et al., 2000, Cuny et al., 2004).

Hitherto, the sedimentary interpretation of many of the formations of the ‘continental intercalaire’ has been disputed, whether they were entirely non-marine or partially or fully marine (Lefranc and Guiraud, 1990). Geologists have cited evidence from sedimentary structures and fossils, but many of these are equivocal: for example, the discovery of shark fossils has been said to indicate that the deposits were marine, but several of these Mesozoic shark groups were almost certainly freshwater (Cuny et al., 2004). Without clear indications of depositional environment at a local level, it has proved hard to interpret the palaeogeography of southern Tethys.

Here, we present information from three formations that span the Aptian/Albian interval. Classic taxonomic and physical sedimentological parameters are presented, but these are then cross-tested using novel geochemical techniques. The aim of this paper is twofold: to demonstrate the value of rare earth elements in ancient bones and teeth in identifying time averaging and mixing, and in distinguishing marine and freshwater settings of ancient vertebrate deposits; and to use the new data to document some of the movements of the southern shore of Tethys in the Early Cretaceous.

Section snippets

Geological setting

The Late Jurassic and Early Cretaceous sediments of southern Tunisia were deposited during the filling of the Tataouine Basin, part of a major subsiding platform (Busson, 1967). The main vertebrate-bearing units are located on the Dahar plateau, which extends from the margin of the Saharan ergs in the west to the Jeffara plain in the east (Fig. 1). These Mesozoic rocks form a cliff line that runs for around 300 km from north to south through southern Tunisia and then curves to run from west to

Previous work

Palaeoecologists commonly use physical abrasion of bones and teeth as evidence for transport (Fiorillo, 1988, Lyman, 1994). Such methods are notoriously difficult to apply as many variables other than duration and distance of travel affect the degree of abrasion suffered by bone. For example, experiments documenting the effect of transport on bones and teeth show that the physical status of bone upon introduction to the transport system largely determines its response to abrasion (Argast et

Methods

The picked fossil material was also used for taxonomic analysis. Non-descript lumps of bone and bone fragments were set aside. Small bivalve and gastropod shells were counted. The identifiable vertebrate material consisted of isolated fish scales and fish vertebrae, as well as teeth of fishes and tetrapods. The teeth of sharks in particular are identifiable to genus and species in many cases (Cuny et al., 2004), while teeth of certain actinopterygians, such as Lepidotes and caturids, are robust

Previous work

Bone is composed of nanocrystals of calcium phosphate intimately associated with a protein matrix largely constructed from collagen. The small size of bone crystallites (∼ 20 × 40 × 5 nm, Weiner and Price, 1986) results in a high surface area/mass ratio and thus metals dissolved in circulating pore waters are readily sorbed onto bone crystallite surfaces. Bone crystallites have a high affinity for trace elements such as the rare earth elements (REE) (Koeppenkastrop and DeCarlo, 1992, Reynard et

Taphonomic processes

Combination of the physical, taxonomic, and chemical analyses allows the four sites to be distinguished, and points to major differences in burial conditions through time. These are outlined below, site by site. Time and space averaging is suggested by high levels of abrasion and possibly by the faunal content. Differing levels of variation in REE patterns in bones between sites suggest differences in the number of primary depositional sources, and thus also indicates variation in the extent of

Conclusions

Physical, taxonomic, and geochemical analysis of material from four study sites in the Early Cretaceous of Tunisia indicates these conclusions:

  • The Douiret Formation (Aptian; Jebel Boulouha) was deposited in essentially freshwater conditions. The succeeding Chenini Formation (early Early Albian; Oued el Khil) shows evidence for modest transgression. The Oum ed Diab Formation (late Early Aptian) shows a continuation of mixed conditions (Oum ed Diab), and then more fully marine conditions (Touil

Acknowledgements

We thank Chung Choi (Bristol) for help with geochemical analyses. We also thank our collaborators in collecting the material in Tunisia: Mohamed Ouaja (Tunis), Dorra Srarfi (Tunis), Eric Buffetaut (Paris), and Emmanuel Fara (Dijon). We thank David Grandstaff, Barbara Grandstaff, and an anonymous reviewer for their thorough and helpful comments on the MS. The work was funded in part by the National Geographic grant 6608-99 and a research grant from the Royal Society.

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