Southern Region Chalk Group Lithostratigraphy: Bristow et al. (1997) - Middle Chalk
The Middle Chalk comprises the Holywell Nodular Chalk overlain by the New Pit Chalk (Bristow et al., 1997). The base of the Middle Chalk follows the traditional scheme in being marked by the Melbourn Rock, an indurated, feature-forming unit, originally described in the northern Chilterns (Hill & Jukes-Browne, 1886) and redefined by Mortimore (1986a) for application to southern England successions. However, Bristow et al. (1997) downwardly revised the top of the Middle Chalk compared to traditional classifications, placing the upper boundary immediately below the upward occurrence of hard nodular chalk belonging to the Lewes Nodular Chalk Member.
In the Isle of Wight, the Middle Chalk, with its base traditionally placed at the base of the S. plana Zone, is c. 45 -60 m thick, and comprises hard, nodular, gritty, inoceramid shell-rich chalk with marl seams in the lower part, overlain by firm, blocky white chalk with marl seams, capped by a bed of intensely indurated chalk overlain by hard, nodular chalk (White, 1921). The lowest c. 1.8-2.4 m of Middle Chalk in the Isle of Wight have previously been referred to as Melbourn Rock (old survey memoir ref), although it does not really compare with the indurated character of this unit at the stratotype locality (White, 1921). The bed of intensely hard chalk in the higher part of the Middle Chalk is the Spurious Chalk Rock, and since its base is coincident with the upward development of hard, nodular chalk, this horizon marks the base of the Upper Chalk sensu Bristow et al. (1997) (see above).
Macrofossil Biozonation: M. gelinianum Zone (pars), N. juddii Zone, Mytiloides spp. Zone, T. lata Zone (pars)
Correlation: see Correlation with other Southern Region Chalk Group classifications
see Correlation with other UK Chalk Group successions
References
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.
HILL, W & JUKES-BROWNE, A J. 1886. The Melbourn Rock and the Zone of Belemnitella plena from Cambridge to the Chiltern Hills. Quarterly Journal of the Geological Society of London, Vol. 42, 216-31
MORTIMORE, R N.1986a. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97(2), 97-139.
See: Melbourn Rock (traditional classification), Melbourn Rock (Mortimore, 1986a), Holywell Nodular Chalk, New Pit Chalk, Lewes Nodular Chalk, Spurious Chalk Rock.