A correlation of the Uppermost Albian and the Cenomanian of South-West England

Abstract

In south-west England, the uppermost Albian and the Cenomanian show considerable lateral variation when compared with the Gault, Upper Greensand and Lower Chalk succession of south-east England. From Warminster as far south as Melbury Down, west of Salisbury, there is a continuous succession from Albian to Cenomanian, with some Cenomanian Upper Greensand. In the valley of the Stour, Cenomanian greensands have disappeared, and the top of the Upper Greensand is of Upper Albian Stoliczkaia dispar and Mortoniceras (Durnovarites) perinflatum Sub-zone age as far west as Evershot. A few kilometres south of Evershot, a quartzose sandstone, the Eggardon Grit, appears. At some localities, this grit yields Lower Cenomanian ammonites. The Eggardon Grit and the underlying Chert Beds appear just north of the line of the Hooke Valley, and thicken considerably in a south-westerly direction. These relationships can be interpreted as the result of a major north-west to south-east ‘structure’, the Mid-Dorset swell of Drummond ( in Smith & Drummond, 1962). Westward, the top of the Upper Greensand is highly variable, with boulder beds, sandy limestones and phosphatic conglomerates, the latter yielding rich Lower Cenomanian ammonite faunas. Over most of south-west England the base of the Chalk is a phosphatic conglomerate known as the Chalk Basement Bed. This is diachronous. When traced south-westward, it yields phosphatised and unphosphatised faunas indicating progressively higher horizons in the Middle and Upper Cenomanian. Bed C, of Actinocamax plenus Marls, Metoicoceras gourdoni Zone age, the base of the Middle Chalk on the Devon Coast, is the obvious extension of this Basement Bed.