Recent developments in Carboniferous geology: a critical review with implications for the British Isles and N.W. Europe

Abstract

Significant increases in our understanding of Carboniferous geological and geographical processes, including plate tectonics, palaeomagnetism, climatology and sea level changes have occurred in recent years. Further advances will increasingly depend on the accurate determination of radiometric ages for the boundaries of the major Carboniferous stratigraphie subdivisions. The recent 39 Ar/ 40 Ar dating of sanidines from European Silesian tonsteins holds out great hopes that structural, igneous and metamorphic events dated by radiometric methods can be better correlated with stratigraphie events defined by goniatite zonation. Palaeomagnetic and tectonic studies in the European Hercynides have established that the Upper Palaeozoic geological evolution of the British Isles took place to the north (present coordinates) of an active micro-plate collision zone along the Galician-Brittany-Massif Central line. Lithospheric stretching of the British/Irish Hercynian ‘foreland’ in the Lower Carboniferous was followed by a belt of north-migrating crustal shortening which disrupted the thermal sag phase of extensional subsidence in northern Britain from Westphalian C times onwards. Backstripped subsidence curves for north British Carboniferous basins indicate that subsidence may have occurred in response to lithospheric thinning of up to 50%. The proposal that there was crustal extension and limited seafloor spreading between Greenland and Scotto-Scandinavia along the Rockall/Faroes line during Carboniferous times is discussed and it is suggested that strike-slip tectonics, known to have been active in Maritime Canada may have played a more important role. Radiometric studies of detrital zircons reveal that the nature of the sourcelands for the huge amounts of Carboniferous detritus in the northern British Isles changed little during the course of the period. They were dominated by outcrops of post-Archaean sediments, minor Archaean basement and abundant Caledonian granitoids with little evidence for Proterozoic crustal growth in the hinterlands. A combination of Mid-Carboniferous climatic change, to a more humid regime, and granite/gneiss terrane unroofing, substantially explains the flushing-out of huge amounts of feldspathic detritus in the Namurian. This Carboniferous climatic change itself must have been influenced by the growth of the E-W Hercynian mountain chain and the accompanying fusion of Gondwanaland with Pangea. Some palaeomagnetic evidence also exists for latitudinal shift at this time. Perhaps the most important influence was the early Namurian expansion of the great Gondwanan ice centre. Waxing and waning of this on a Milankovich time scale dominated Silesian sea level changes and facies evolution. Many late Dinantian and Silesian ‘minor’ sedimentary cycles are probably of glacio-eustatic origins, but there seems little evidence that supposed Dinantian and Namurian mesothemic cycles have such an origin. There are, in fact, increasing doubts as to the actual existence of these particular cycles.