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Lower Cretaceous

Chronostratigraphy Lower Cretaceous sediments occur across the eastern and southern parts of the UK and adjacent offshore basins, but are missing over most of Wales, the Pennines, NW England and Scotland. It is generally accepted that for most of Early Cretaceous time these areas were mainly emergent, following a significant sea-level fall that commenced very late in the Jurassic. The depositional basin had a long pre-Cretaceous history, often originating in the Permian or Triassic. During the Cretaceous they were affected by continuing tectonism, initially in the North Sea area (Late Cimmerian movements). But as tectonism waned there, rifting and accompanying uplift commenced to the west of the British Isles.

For much of Early Cretaceous time the Irish, Welsh and Anglo-Brabant landmasses formed an effective barrier between basins to the north and south. To the north, marine sedimentation continued from the late Jurassic through the whole of Early Cretaceous time, though in the present-day onshore area of eastern England the Lower Cretaceous sequence is generally only about 100 m thick and includes numerous breaks. In north Norfolk and Lincolnshire it consists of a lithologically varied sequence (sandstones, ironstones, mudrocks and impure limestones) that accumulated over the East Midlands Shelf. In the Cleveland Basin (North Yorkshire) the condensed Speeton Clay Formation (latest Berriasian to Early Albian) provides the only onshore glimpse of the much thicker sequence of mudrocks that accumulated over most of the North Sea Basin.

In contrast, to the south of the Anglo-Brabant to Irish landmasses, the fall in sea level resulted in a 20 million year interval of non-marine sedimentation (Purbeck and Wealden groups) from the Western Approaches and Celtic Sea basins eastward as far as the Weald and Paris basins. The Purbeck Group spans the Jurassic-Cretaceous boundary though only the lowest few metres are of Jurassic age. It consists of calcareous and argillaceous sediments deposited under predominantly arid conditions, while the Wealden sands and mudrocks represent a more humid intervals. Thin quasi-marine horizons occur throughout this non-marine sequence, at least some representing brief spillovers of seawater from the north, via an intermittent connection round the western margin of the Anglo-Brabant landmass often referred to as the “Bedfordshire Straits”.

Very early in the Aptian marine transgression commenced in these southern basins, reflecting the beginning of a global rise in sea level that continued for much of the later Cretaceous. As sea levels continued to rise much of the Anglo-Brabant landmass was flooded so that by Mid Albian times there was a continuous area of marine sedimentation extending from the northern North Sea to the southern and south-western basins. This led to the widespread deposition of reddish-coloured, marly mudrocks (Rødby Formation and equivalents) over much of the North Sea area, thin, condensed marly red limestones (Hunstanton Formation) across eastern England and grey clays (Gault Clay Formation) in the more southerly basins. In latest Albian times uplift to the west of Britain began to influence sedimentation, with the spread of sands (Upper Greensand Group) eastwards across much of southern England. But by the beginning of Late Cretaceous times clastic input had effectively ceased except adjacent to local highs, and deposition of pure coccolith limestones (Chalk Group) commenced over most of the onshore and offshore area.

Further information
Rawson, P.F. (2006). Cretaceous: sea levels peak as the North Atlantic opens. In Brenchley, P.J. & Rawson, P.F. (Eds). The Geology of England and Wales (2nd edition), pp. 365-393. London: The Geological Society.

Hopson, P M Wilkinson I P and Woods M A (2010). A Stratigraphical framework for the Lower Cretaceous of England. BGS Research report RR/08/03.