Le Paléozoïque anté-varisque de France, contexte paléogéographique et géodynamique

Abstract

The pre-Variscan Palaeozoic successions of France and Belgium are reviewed in the areas which offer the best preseved sedimentary and fossil record (Brabant, northern France, Ardenne, Armorican Massif, Aquitaine, Montagne Noire, Pyrenees). Special attention is paid to the proper sedimentary and faunal characteristics of these regions, to their pre-orogenic evolution, to their respective affinities and to their probable location in the pre-Variscan palaeogeography. Brabant, northern France and Ardenne were parts of East Avalonia. Their Palaeozoic evolution is marked both by Caledonian tectonic events of Late Ordovician and latest Silurian age and by Variscan tectonic events during the Late Carboniferous. The Cambrian, Ordovician and Silurian "pre-Caledonian" periods were characterized by thick terrigenous sedimentation where argillaceous and silty deposits with pelagic faunas predominated; the palaeoenvironments varied through time but most frequently corresponded to distal shelf, deep shelf or even basin conditions. Volcanic activity occurred during the Ordovician and Early Silurian. Following the Caledonian events, the Early Devonian transgression resulted in a "pre-Variscan" period characterized by a different palaeogeographic setting. These regions were located at the southern border of the Old Red Sandstone Continent and corresponded to a marine epicontinental shelf which subsided considerably in its southern part. Shelf conditions were maintained throughout the Devonian and Early Carboniferous and were characterized by argillaceous and carbonate deposits. A strong terrigenous influx, related to Variscan events in the south, began in the early Namurian and developed during the Namurian and the Westphalian with paralic coal-bearing deposits. Tangential tectonics occurred during the Westphalian. The rest of France, from the Armorican Massif to the Pyrenees, was located on the opposite margin of the Rheic Ocean and belonged to the North Gondwanan marine shelf: During the Cambrian, distensive conditions prevailed with localized areas of active subsidence and volcanic activity. Marine sedimentation became general in the Early Ordovician and continued up to the latest Devonian or Early Carboniferous. There were no tectonic events during this period and the sedimentary hiatuses occurring close to the Ordovician-Silurian boundary have no tectonic significance; they apparently record the influence of the development of the African glaciation. The Ordovician shelf deposits were followed by Silurian condensed euxinic black-shale sedimentation. An almost continuous marine sedimentation developed during the Devonian and the first syn-orogenic deposits appeared in the Early Carboniferous. The main Variscan tectonic events developed during the Early Carboniferous in southern France and during the Westphalian in the Armorican Massif. The existence of the mid-European Rheic Ocean during the Early Ordovician is still a matter of debate, but we are convinced that the processes opening this ocean probably started as early as the Middle or Late Cambrian. During Ordovician times, the North Gondwanan Province was located in high latitudes and the Rheic Ocean reached its maximum width in the Early Silurian. The appearance of carbonate deposits in Late Silurian and Early Devonian times indicates that the North Gondwanan regions had moved into lower latitudes. During the Devonian, increasing faunal affinities between East Avalonia and North Gondwana illustrate the progressive narrowing of the Rheic Ocean which later closed during the Variscan uniting of the two provinces. All the North Gondwanan regions maintained strong sedimentary and faunal affinities throughout the Palaeozoic period, which clearly excludes the development of a wide ocean within this province. However, two types of regions can be distinguished: - the Mid-North Armorican Domain and its Central Iberian counterpart are characterized by inner shelf deposits where neritic faunas predominate. Arenaceous near-shore sedimentary deposits are common in the Ordovician. In the Devonian deposits, the abundance of spores and plant fragments most probably indicates the relative proximity of emerged land areas. - regions of southern France show more argillaceous deposits and, in the Devonian, carbonate-dominated sediments; benthic faunas indicate more distal outer shelf environments with abundant pelagic fauna. We propose a southern location for the pre-Variscan position of the MidNorth Armorican Central Iberian Domain. This clearly differs from previous reconstructions, where the northern position adopted was at variance with the proximal inner shelf characteristics of the sedimentation in these regions, and has consequences for the arrangement of the North Gondwanan regions. Using this southern position, the South Armorican Ocean, for which the Silurian-Devonian evolution ended with the so-called Ligerian orogeny, appears as the southern part of the mid-European Rheic Ocean. This palaeogeographical model is in some ways overschematized as it does not give any detail of the evolution of the South Brittany and Massif Central regions. However, these regions, where the Variscan structures, metamorphism and plutonism are best seen, correspond to the active margin in our reconstruction.