Palaeogeography, Palaeoclimatology, Palaeoecology
The end of the Messinian salinity crisis: Evidences from the Chelif Basin (Algeria)
Introduction
An intense debate rose in the middle of the 1990's on how the Messinian salinity crisis ended and how marine conditions were restored at the onset of Pliocene in the Mediterranean basins. This debate examined whether the late Messinian conditions changed to lacustrine settings before the abrupt marine reflooding in the earliest Zanclean, or whether marine conditions were restored earlier, either episodically or definitely, during the upper Messinian.
During more than 30 years, a lot of studies carried out both in marginal and deep offshore Mediterranean basins provided a great amount of data indicating that the salinity crisis ended by generalised low salinity conditions, the so-called Lago–Mare, that was suddenly interrupted in the earliest Zanclean by an exceptionally fast marine transgression. This change of the regional water budget that started during deposition of the upper evaporites and climaxed after the end of the evaporite deposition, was reported everywhere in land basins from Spain to Cyprus (Decima and Wezel, 1973, Ruggieri and Sprovieri, 1974, Ruggieri and Sprovieri, 1976, Sturani, 1978, Casati et al., 1978, Ciaranfi et al., 1978, Colalongo et al., 1978, Vismara-Shilling et al., 1978, Cita and Colombo, 1979, Roep and van Harten, 1979, Orszag-Sperber and Rouchy, 1979, Rouchy et al., 1980, Geerlings et al., 1980, Cita et al., 1980, Giammarino et al., 1984, Orszag-Sperber et al., 1989, Fortuin et al., 1995, Cipollari et al., 1999, Bonaduce and Sgarrella, 1999, Gliozzi, 1999, Orszag-Sperber et al., 2000, Rouchy et al., 2001, Rouchy et al., 2003, Bassetti et al., 2006, Cosentino et al., 2006, Orszag-Sperber, 2006, Pierre et al., 2006, Rouchy and Caruso, 2006). Fresh water dilution was also reported in deep sea cores, from the Algero–Balearic, Tyrrhenian, Ionian, Cretan and Levantine basins (Lawrence, 1973, Cita et al., 1978, Schrader and Gersonde, 1978, Pierre and Rouchy, 1990, Blanc-Valleron et al., 1998, Pierre et al., 1998, Iaccarino and Bossio, 1999, Iaccarino et al., 1999a, Iaccarino et al., 1999b, Marsaglia and Tribble, 1999, Pierre et al., 2006). The most striking feature pointed out by many studies, beside the absence or scarcity of in-situ marine fossils and the common occurrence of hyposaline dwellers, was the presence of Paratethyan immigrants whose ecological significance is still under discussion.
By contrast, several authors proposed, from regional studies, that the Lago–Mare corresponded to very local freshwater settings along the shores of the Mediterranean sea and that marine conditions would have been definitely restored during the upper Messinian (Butler et al., 1995, Riding et al., 1998, Braga et al., 2006). These authors considered the presence of assemblages of planktonic foraminifera as being in-situ living organisms whereas most of the preceding authors interpreted them as reworked. Recently, Carnevale et al. (2005) reported the occurrence of otoliths of marine fishes within the Lago–Mare deposits of Tuscany, which would strengthen the marine hypothesis. Due to the presence, at some levels within the Lago–Mare deposits, of a dinocyst (Galeacysta etrusca) associated to calcareous nannoplancton belonging to the NN11 biozone from the Dacic Basin, Clauzon et al. (2005) assumed that episodes of sea level highstands occurred coevally in the Mediterranean and Paratethys allowing for free exchanges between the two realms during the upper Messinian.
The Chelif Basin in northern Algeria displays a complete sedimentary record of the Messinian salinity crisis and of the Miocene–Pliocene transition (Anderson, 1936, Perrodon, 1957, Rouchy, 1982). The evaporites (only calcium-sulfate deposits) grade upward into typical Lago–Mare reaching locally thickness of several hundreds of meters. The Miocene–Pliocene transition is exposed in many spectacular sections among which we have selected five sites in marginal (Djebel Touaka, Saouria, El Ghomri) and central (Djebel Meni, Oued el Aicha) areas of the basin (Fig. 1). These sections were measured and sampled during field investigations realised in 1979 and 1986.
Section snippets
Geological background
The Chelif Basin is a gutter-shaped basin elongated ENE–WSW over about 260 km long and 35 km wide between the sabkha of Oran to the west and El Asnam city to the east (Fig. 1), so that it represented, with the Central Sicilian Basin, one of the largest Messinian peripheral sub-basins. The basin is an intramountainous post-nappes basin located in the Tellian Atlas, which is a complex chain constituted by alpine folded and thrusted units, composed of Mesozoic to lowermost Miocene deposits, which
Methodology
In order to correlate the paleoenvironmental changes at the Miocene/Pliocene boundary (MPB) through the Mediterranean we have previously defined a methodology (Pierre et al., 2006) that was also used for the investigations in the Chelif Basin. The analytical approach combines sedimentology, biostratigraphy, and stable isotopes of carbonates allowing thus a reconstruction of the paleoenvironmental changes and an estimation of the rate of fresh waters inputs relative to marine water influxes.
Geological setting
The section is exposed on the Djebel Touaka, a hill located on the southeastern margin of the basin and on the edge of the Tessala mountain (Geological map at the 1/50 000, no. 182 of Saint-Denis-du-Sig) (Fig. 1), 1.5 km south to the Sig city and 2 km NE of the Sig River along which a continuous sedimentary succession of lower Messinian deposits is cropping out. In the Djebel Touaka as well as in the Sig River, the Messinian sedimentary succession starts with blueyish marls interbedded with
The late Messinian paleoenvironments
The uppermost Messinian deposits of the Chelif basin in Algeria exhibit a wide range of lithologies dominated by clastic deposits i.e., siltstones, sandstones, conglomerates, with locally, as in the Saouria section, sandy carbonates and finely laminated stromatolitic limestones. In the deeper areas of the basin (Djebel Meni–Abreuvoir and Oued-el-Aicha), sedimentary successions, exclusively composed of siltstones with interbeds of sandstones, are more homogeneous and reach a greater thickness up
Conclusions
The Chelif Basin with well exposed and continuous sections through the Miocene–Pliocene transition, is one of the key areas to improve our understanding of how the Messinian salinity crisis ended and how the marine conditions were restored into the Mediterranean in the Early Pliocene times. Five sections representative of marginal and central parts of the basin have been selected and investigated in detail for mineralogy, microfaunal assemblages, and stable isotope composition of the carbonate
Acknowledgements
We acknowledge Anne-Marie Brunet for sample processing, Vincent Rommevaux for thin sections, Marie-José Urrutiaguer for stable isotope analyses, Christiane Chancogne for the SEM observations, Maria-Elena Gargano for the help in the foraminifera picking, Agate Cambreleng for the illustration of the paper, Pierre Clément for X-ray diffraction assistance. Elsa Gliozzi and Silvia Iaccarino are gratefully acknowledged for their thorough revision, critical comments and suggestions. This study is a
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