The Great Cyclothem of northern England
The Great Cyclothem is comprised of the Great Limestone and equivalents at the base, its upper part being formed by the Coal Sills Group and equivalents which immediately overlie the limestone. The detailed variations in this thin but widely persistent group of strata in its 6,500 square miles area of development in Northern England are considered and evaluated primarily in order to assess the conditions of deposition, sedimentary history and palaeogeography of the region during formation of the cyclothem. Critical palaeontological evidence is presented to show that the cyclothem lies at the very base of the Upper Carboniferous succession of Northern England, the base of the Great Limestone and equivalents being taken as the Viséan-Namurian junction. A detailed stratigraphical and palaeontological consideration of the Great Limestone and equivalents is made. Separate detailed studies are also presented of the stratigraphy, petrology, sedimentary structures and palaeontology of the overlying Coal Sills Group and equivalents, these beds consisting of alternating shales, sandstones and coals with thin lenticular marine bands and a restricted chert development. An analysis of the above information is made and it is postulated that the cyclothem's basal limestone was laid down in a widespread shallow clear sea, the sea being eventually encroached upon by deltaic sediments consisting essentially of material now represented by the shales, sandstones and coals. The deltaic conditions built out from land to the north and north-west, the sediment source consisting predominantly of pre-existing sediments. Evidence exists to show that the deltaic conditions were subject to local and regional regressions during formation of the cyclothem, thus giving rise to the latter's composite nature, each sheet sandstone with its overlying coal seam representing the final stage in each phase of delta building. The control which the trough and block features of Northern England exercised upon the overall thickness of terrigenous sediments deposited is clearly illustrated and the complete wedging out of terrigenous sediments to the south-east is demonstrated. In the deltaic sediments the courses of prominent in filled river channels occupying washouts have been delimited. The restricted formation of chert is related to the deposition of primary colloidal silica under marine conditions, the required silica-rich waters having been fed into the specific area by a contemporaneous river "channel" whose course is delimited. The mechanism which controlled the formation of the rhythmic sequence of the cyclothem is discussed, and it is concluded that the major role was played by tectonic activity which involved uplift in the source area followed by erosion to base level, with complementary diastrophic subsidence in the area of deposition; the rhythmic tectonic activity is related to the pulsating movements of the Sudetic period which heralded the Hercynian orogeny. In an appendix the widespread and marked devolatilization of the coal seams on the Alston Block is discussed and tentatively related to greater than normal heat-flow from the pre-Carboniferous "Weardale granite". The first recorded occurrence in Great Britain of ovoidal bodies called nigger- heads is reported from these heat affected coals.