The climatic, eustatic and tectonic controls on the Mid Carboniferous (Visean and Namurian) strata of Northumbria, England

Abstract

The Mid Caiboniferous (Viséan and Namurian) Yoredale cycles of Northumbria were deposited as a result of glacio-eustatic fluctuations, arising from waxing aid waning of Gondwanan ice sheets in the southern hemisphere. Each cycle contains a variety of Uthofacies, generally comprising carbonate platform lithofacies deposited during the transgressive systems ttact, followed by deltaic or marine shoreline lithofocies, deposited during the highstand, lowstand and falling stage systems tract. There may or may not be a transgressive shoreline lithofacies present at the top of each cycle. Although there is a general pattern to the composition of each Yoredale cycle, carbonate platform lithofacies are more dominant in the south of the area due to a close proximity to the main marine source to the south-west. Likewise, deltaic and marine shoreline lithofacies are more common in the north of the area due to a close proximity to the main sedimentary source to the north-east The duration of each cycle has been calculated as approximately 200,000 years resulting in their classification as fourth-order cycles. Within each Yoredale cycle, the components of a sea-level curve have been identified indicating their formation was directly influenced by fluctuations in relative sea- level. By using Fischer plots, third-order cycles have also been identified and it is inferred that composite eustasy was տ operation, resulting from Gondwanan glaciation. Evidence for climate change is abundant throughout the Mid Carboniferom. By using palaeosoils and lithological evidence for climate change has been recognised. A major arid phase at the Asbian I Brigantian boundary has been identified by the presence of calcretes, red flmial sediments, a decrease both the amount of coal the amount of fine siliciclastic material within each cycle. This change in climate can be correlated with other areas of similar palaeolatitude indicating that this change was global. Stable isotope analysis reveals little or no information regarding Mid Carboniferous palaeoclimate and I or palaeoceanogiaphy. It has been possible to identify major post-depositional influences on the Mid Caiboniferous strata of Northumbria. Both the Weardale Granite and the Whin Sill Complex appear to have generated large amounts of hydrothermal fluids, both during their emplacement and in the case of the Weardale Granite, after еmplacement. This has led to the obliteration of the original isotopic composition of the marine limestones. Tectonic activity associated with the Variscan orogeny began in the Late Devonian, but was still active in Northumbria during the Viséan. The resulting extensional tectonics had a profound affect on sedimentation. During the Viséan, active extension was still ongoing, with the syn-rift phase lasting until the end of the Asbian period. The result of this was a series of E-w trending sedimentary basins with wedge-shaped geometry of sediments. The intervening blocks subsided at a slower rate due to underlying buoyant granite masses compared to the Caledonian basement rock of the basinal areas. Differential subsidence ended in the Namurian and the post rift-phase gradually took over from the start of the Brigantian period onwards. This resulted in uniform deposition, with localised intrabasinal faulting. The initial objectives for this thesis were to assess not only the effects of climate, eustasy and tectonics on the Mid Caibontferous strata of Northmnbria, but also to look at the clastic and carbonate interactions within the classic Yoredale cycles. The vast amount of previously unpublished information that has been obtained from the rocks especially with regards to climate change has meant that the clastic-carbonate interaction study was abandoned. In an area that has been studied for over two centuries due to it mineral wealth it is perhaps surprisng to find that there is still much work to be gleaned from such a classic area of British geology.