Southern Region Chalk Group Lithostratigraphy: Traditional Classification - Plenus Marls

The Plenus Marls comprise a succession of alternating greenish -grey marls and marly limestones, typically a couple of metres thick, but ranging from less than a metre to more than 10 metres thick (Rawson et al., 1978; Lake et al., 1987). Up to eight beds have been recognised, traceable across the UK and into France (Jefferies, 1963), though a complete succession is seldom developed owing to omission at erosion surfaces developed within the succession. A major burrowed erosion surface occurs at the base of the Plenus Marls, and is named the Sub-Plenus erosion surface.

The Plenus Marls usually have a well developed fauna, and are named after the belemnite Praeactinocamax plenus, although this characterises only a part of the succession. Major faunal changes in the succession coincide with erosion surfaces, so that particular fossil assemblages characterise particular intervals of beds. Thus Jefferies' (1963) Bed 1 contains large, gryphaeate specimens of the oyster Pycnodonte vesiculare and the brachiopod Orbirhynchia multicostata. The foraminifera Rotalipora cushmani becomes extinct at the top of Jefferies' (1963) Bed 3, a globally isochronous datum, and beds 4 to 6 are characterised by a macrofossil assemblage comprising Praectinocamax plenus and the bivalves Lyropecten Aequipecten arlesiensis and Oxytoma seminudum (Lake et al., 1987).

Macrofossil Biozonation: M. gelinianum Zone (pars)

Correlation: see Correlation with other Southern Region Chalk Group classifications

see Correlation with other UK Chalk Group successions

References

JEFFERIES, R P S.1963. The stratigraphy of the Actinocamax plenus Subzone (Turonian) in the Anglo-Paris Basin. Proceedings of the Geologists Association, Vol. 74, 1-33.

LAKE, R D, YOUNG, B, WOOD, C J & MORTIMORE, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey.

RAWSON, P F, CURRY, D, DILLEY, F C, HANCOCK, J M, KENNEDY, W J, NEALE, J W, WOOD, C J & WORSSAM, B C. 1978. A correlation of the Cretaceous rocks in the British Isles. Geological Society of London, Special Report No. 9, 70 pp..

See: marl