Southern Region Chalk Group Lithostratigraphy: Bristow et al. (1997) - Portsdown Chalk
The Portsdown Chalk follows the lithostratigraphical concept of Mortimore (1986a), and comprises white, flinty chalk with common marl seams. It probably exceeds 60 m in the Isle of Wight (Wood, WH/95/32R, WH/95/31R; Bailey et al., 1983), although Bristow et al. (1997) reported about half this thickness beneath the Tertiary erosion surface in Dorset. The lower part is characterised by common inoceramid bivalve shell fragments, and finely comminuted shell debris in both chalk and marl (Bristow et al., 1997). A distinctive belt of typically orangey-brown skinned Zoophycos flints, named the Précy Zoophycos by Mortimore & Pomerol (1987), occur near the base of the Portsdown Chalk and in the top of the underlying Spetisbury Chalk. The base of the Portsdown Chalk is a negative topographic feature at the base of an escarpment, apparently formed by the upward change to less flinty chalk, and approximately coincident with the Portsdown Marl of Mortimore (1986a) (Bristow et al., 1997). The Portsdown Chalk is the youngest mappable subdivision of the Chalk Group recognised by Bristow et al. (1997), and includes in its topmost part the Studland Chalk of Gale et al. (1987).
Macrofossil Biozonation: top G. quadrata Zone & B. mucronata Zone (pars)
Correlation: see Correlation with other Southern Region successions
see Correlation with other UK successions
BAILEY, H W, GALE, A S, MORTIMORE, R N, SWIECICKI, A & WOOD, C J. 1983. The Coniacian-Maastrichtian stages of the United Kingdom, with particular reference to southern England. Newsletters on stratigraphy, Vol. 12, 29-42
BRISTOW, C. R., MORTIMORE, R. N. & WOOD, C. J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol. 108, 293-315.
MORTIMORE, R N.1986a. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97(2), 97-139.
MORTIMORE, R N & POMEROL, B. 1987. Correlation of the Upper Cretaceous White Chalk (Turonian to Campanian) in the Anglo-Paris Basin. Proceedings of the Geologists' Association. Vol. 98 (2), 97-143.
WOOD, C.J. 1995. Notes on the macrofossil and macrofossil biostratigraphy of the traditional Upper Chalk macrofossil zones of southern England with particular reference to the zonal concepts of Rowe, Brydone and Gaster. British Geological Survey Technical Report, WH/95/31R
WOOD, C. J. 1995. Observations on the lithostratigraphical classification of the Blandford, Tarrant, Spetisbury, Portsdown and Studland chalks, Upper Chalk, southern England. British Geological Survey Technical Report, WA/95/32R.
See: Portsdown Chalk (Mortimore, 1986a), Précy Zoophycos, Portsdown Marl