Carboniferous basement rocks of northern Britain

Abstract

The study of Carboniferous basement rocks of northern Britain has concentrated on the dominantly clastic rock succession which unconformably overlies the Lower Palaeozoic basement and occurs beneath the lowermost Carboniferous limestones in areas surrounding the Vale of Eden. The clastic rocks exposed at Shap, the Mell Fells (Ullswater), Sedbergh and Cross Fell, mainly reflect fluvial deposition over an Upper Palaeozoic landscape which locally had considerable topographic relief. Within each clastic rock succession, which throughout the region varies in thickness from ca 4m to over 250m, overall largescale fining upward sequences document the evolution from gravel-dominated braided fluvial to sand-dominated braided fluvial deposits. This evolution is shown to be representative of the downstream transition from localised alluvial fans into a more extensive fluvial braidplain system, the latter also containing marked proximal to distal variations. Palaeocurrents are consistently toward the north and north-east throughout the region, demonstrating the existence of a broad northerly palaeoslope during clastic deposition. Proximal to distal relationships and clast provenance data for each area are used to predict fades variations across the Vale of Eden region and to reconstruct the Palaeogeography. Areas of high ground are shown to be underlain by deep-seated Caledonide granite plutons, evolution of the fluvial braidplain demonstrating that with time denudation of high ground developed fining upward sequences by the superposition of progressively finer grained sediment on coarse. The development of the braidplain system is related to post-intrusive uplift of the granites underlying the highest ground with deposition mainly taking place prior to marine limestones associated with the basal Carboniferous marine transgression. A comparison with the Upper Old Red Sandstone clastic succession of the Northumberland Border Basin is assessed.