Middle to Upper Devonian (Givetian and Frasnian) shallow-water carbonates of Western Europe : facies analysis and cyclicity
The Middle and Upper Devonian (Givetian-Frasnian) shallow-water carbonate facies of western Europe were deposited as a large-scale transgressive succession over continental facies of the Old Red Continent. The transgression was in a northerly direction, reaching the southern Ardennes by the lower Eifelian and the Aachen area of Germany by the middle Givetian. Carbonate sedimentation continued through to the middle Frasnian, when a major pulse m relative sea-level rise drowned the platform. The carbonate platform had a complex internal structure, with three major palaeosettings. During the Eifelian, a storm-influenced homoclinal ramp existed over much of the Ardennes. Sedimentation was mostly open-marine in nature, with a protected back-ramp and tidal-Oats. The Givetian saw a transition from a ramp to shelf setting, with stromatoporoid reefs at the shelf edge providing protection for abroad shelf lagoon. The shelf had an ESE-WNW trend and extended from Boulogne (northern France) in the west to Aachen (western Germany) in the east. East of the river Rhine in the Rheinisches Schiefergebirge area of Germany, and in Southwest England, isolated carbonate complexes developed. These were positioned either on the shelf-edge, within the shelf or upon topographic highs within the basin. Synsedimentary tectonism and volcanism strongly influenced their development. As a result of their areal extent, lagoonal environments were studied extensively in shelf and isolated complexes. Palaeontology and sedimentology were used to identify 14 major microfacies within the lagoonal successions, which could be broadly categorised into four major groups. The semi-restricted subtidal microfacies group has a rich faunal assemblage which, although diverse, did not represent fully open-marine deposition. Sedimentation was entirely subtidal in nature. The restricted subtidal microfacies group is either characterised by monospecific fossil assemblages (chiefly molluscs or amphiporoids), or by macrofossil-poor facies. These facies represent poorly-circulated, subtidal environments which may have been subjected to flucmating salinities The intertidal microfacies group is characterised by fenestral limestones, which are commonly poorly-fossiliferous. Finally the supratidal microfacies group is typified by dolomudstones, microbial laminites and calcretes. A metre-scale cyclicity is prevelant in these lagoonal facies and two major types of cycle have been identified. Subtidal cycles show a decrease in circulation, decrease in diversity of organisms and increase in fluctuation of salinity upwards through the cycle. Peritidal cycles shallow upwards from a subtidal base through to an intertidal or supratidal cap. Subtidal cycles seem particularly common within the isolated carbonate complexes, yet both peritidal and subtidal cycles are identified in the shelf lagoon. The distribution of facies and cycles was controlled by a complex interaction of eustasy and differential subsidence. The setting (i.e., whether it was the shelf lagoon, or isolated carbonate complex) also influenced this distribution. Fischer plots were used successfully to correlate successions across the carbonate platform, and to identify areas of condensed or expanded sedimentation. Cycles were calculated to have a duration of approximately 42,000 years for the Upper Givetian. The magnitude of relative sea- level change was in the order of l-3m. The development of the metre-scale cyclicity is best explained by orbital forcing, yet this signature has been overprinted by autocyclic and tectonic noise. Third-order eustatic sea-level fluctuations were delineated by major marine transgressions, and a eustatic sea-level curve was established for the study area.