Les Pyrénées centrales et orientales au début du Paléozoique (Cambrien s.l.): évolution paléogéographique et géodynamique

Abstract

In order to debate of the early Paleozoic paleogeography, the repartition of the Hercynian blocks, today scattered around West-Mediterranean Sea. should be known. This is the case for the end of the Paleozoic (Fig. 1), but not for the beginning; Fig. 6 is drawn with the supposed repartition in the middle of the Carboniferous. In Central and Eastern Pyrenees and surrounding areas (Fig. 1), Upper Ordovician beds rest unconformably upon a thick (4–6 km), dominantly pelitic series known as Lower Paleozoic in the Eastern Pyrenees or Seo Formation in the Central Pyrenees. The metamorphic lower part of this series often lies over metagranilic orthogneisses, which are best interpreted as a Precambrian basement, Panafriean-Cadomian in age. By correlation with fossiliferous series of other areas, the Pyrenean Lower Paleozoic should be mainly Cambrian in age (ranging from Uppermost Proterozoic to Lowermost Ordovician). For the purpose of this paper, the complex lithostratigraphic succession of the Lower Paleozoic of the Eastern Pyrenees, with two groups and seven formations, could be summarized (Fig. 2) by a threefold division, from bottom to top: (i) a pelile-greywacke and carbonate unit, with a conspicuous plagioclasic component and a sodic composition (Uppermost Precambrian to Lowermost Cambrian?): (ii) a sandstone-pelite unit, with lithic sandstones, ending with a carbonate level, well developped in the Central Pyrenees (Lower Cambrian?): (iii) a mudstone-siltstone unit (Middle-Upper Cambrian?). Fossiliferous Lower Cambrian beds which outcrop at Terrades (south of the Eastern Pyrenees) could be a remnant of an allochthon unit which can be compared with the nappe-thrusts of the nearby Southern Montagne Noire. The pelite-greywacke and carbonate unit (Fig. 3) occurs only in the South-Eastern Pyrenees as a south to north transgressive platform bordering a basin extending southwards; not far south of Eastern Pyrenees, a volcanism of “intermediate” type supplied in plagioclasic clasts the greywackes and volcanoclastic deposits. Near the base of the sequence, a bimodal volcanism and synsedimentary faults reflect the extensional context of the basin initiation, the geochemistry of which has been related to back-arc setting. An acidic volcanism developped higher in the sequence (tufs and hypovolcanic bodies). Carbonate levels are numerous, particularly in the lower part of the unit. The upper part of the sequence is an oslistostrome made of polygenic intraformational conglomerates fed from the south: it outlines the transition to the next unit. The sandstone-pelite unit (Fig. 4) rests conformably on the previous one in the Eastern Pyrenees, and is unconformable upon the Precambrian basement to the north (North-Pyrenean massifs) and to the west (Central Pyrenees). It is characterized by arkosic lithic sandstones with clear quartz grains: they originated in the erosion of a granitic basement and/or acidic volcanic rocks. Coarseness of the sandstones and thickness (up to 2–4 km) of the unit increase from south-east to north and west. A carbonate upper level, well developped in the Central Pyrenees, can be correlated with Lower Cambrian limestones from the surrounding areas. The mudstone-siltstone unit (Fig. 5) is defined by the prevalence of mm- to cm- scale alternations of argillaceous mud and silt of a flyschoid type, representing a more basinal sedimentation. A carbonate level, the highest in the series, is intercalated in Ihe lower part ot the unit: above this level, deposits are very homogeneous and thiek (about 2 km). A poorly known formation with pelitcs and sandstones caps the muddy-silty unit: it could be Lower Ordovician in age. Thus, the Pyrenean domain shows the same depositional history as West-Mediterranean area: (i) first, a volcano-sedimentary platform or basin occurs, as in Central Spain. Eastern Pyrenees. Sardinia and axial zone of the Montagne Noire, but not further north; (ii) second, a silicoclastic platform spreads out. which becomes carbonated at the end: (iii) third. Ihe basin deepens and receives fine silicoclaslies. This evolution is not fully accounted for in recent synthesis of Pre-hercynian France or Spain, and it should appear useful for a better understanding of the south French Massif Central geological history.