Groups and Networks

Upper Cretaceous sediments belong to the Chalk Group. They are extensively preserved in the offshore basins all around the British Isles, while onshore they form the familiar chalk hills that extend from the Yorkshire and Lincolnshire Wolds southwards through the Chilterns to the North and South Downs and the Isle of Wight. They also occur beneath the Tertiary lavas in Antrim and locally in western Scotland. But it is probable that at one time deposition occurred across almost the whole of the British Isles, as landmasses became flooded during a continuing rise in global sea level through much of the Cretaceous. This reached a probable Phanerozoic peak during the Late Campanian.

In late Cretaceous times extensional activity continued to the west of Britain as the North Atlantic started to widen. But there is also evidence of three compressional phases that affected sedimentation from southern England to the North Sea, reflecting events much further south where the African Plate began to converge with the European Plate. But the main control on sedimentation was the predominantly arid Late Cretaceous climate. In the moderately deep, warm and clear seas biogenic oozes accumulated to form a remarkably pure limestone, the Chalk This is composed essentially of detritus from calcareous algae (coccolithophores), usually deposited on the sea floor via the faecal pellets of copepod crustaceans. The main clastic input is represented by the very thin but laterally extensive bentonitic marl bands that provide important marker horizons in the Chalk and reflect distant volcanic activity, probably in the North Atlantic. But more sandy sediments accumulated locally adjacent to the surviving landmasses.

The Chalk Group accumulated in two distinct provinces, a northern one with faunal affinities with North Germany, Scandinavia and the Russian Platform, and a southern province linked with the Paris Basin and Mediterranean Europe. There is an intermediate area across the buried Anglo-Brabant high.

Historically, the Chalk Group proved difficult to subdivided lithologically, because of its superficially homogenous nature. But for the onshore area a series of mappable lithological units are now distinguished for both northern and southern provinces, while there are many lithological markers (e.g. marl and flint bands, bioclastic horizons and small-scale sedimentary cycles) that show a remarkable lateral continuity and thus facilitate a detailed event correlation across the country. The more important flint and marl bands have been formally named. Units are also defined in the NW Scotland and Northern Ireland provinces. In the offshore basins, the Chalk Group has been most thoroughly studied in parts of the North Sea basin where five formations have been distinguished, which correlate closely or exactly with those recognised in northern England. The highest (Rowe) formation is recognised both offshore and onshore while the names of the other four unfortunately change at the present coastline.

Over much of England the highest chalks were eroded away during the Tertiary, so that the original extent of Maastrichtian sedimentation remains uncertain, but the successions in Northern Ireland preserve the youngest onshore chalks. But globally there was a Maastrichtian regression, so that within the British Isles the depositional area may have shrunk after the Late Campanian.

Further information:
Hopson, P.M. (2005). A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland with statements on the Chalk of Northern Ireland and the UK Offshore Sector. BGS Research Report RR/05/01.

Mortimore, R. (2011). A chalk revolution: what have we done to the Chalk of England? Proceedings of the Geologists’ Association 122, 232-297.

Mortimore, R.N., Wood, C.J. & Gallois, R.W., (2001). British Upper Cretaceous Stratigraphy, Geological Conservation Review Series, No. 23, Joint Nature Conservation Committee, Peterborough.