Southern Region Chalk Group Lithostratigraphy: Traditional Classification - Upper Chalk

The base of the Upper Chalk is traditionally taken at the base of an indurated feature-forming bed called the Chalk Rock (Jukes-Browne & Hill, 1904). However, this unit is practically undeveloped over much of southern England, so the base of the Upper Chalk in this area has traditionally been inferred using a combination of other lithological and faunal criteria. The Chalk Rock was placed at the base of the Sternotaxis plana Zone by Jukes-Browne and Hill (1904) (Bromley & Gale, 1982, Table 1), so the subjectively assessed base of this biozone became one of the criteria for defining the base of the Upper Chalk (Mortimore and Wood, 1986). At Dover, the base of the S. plana Zone coincided with a locally feature-forming succession of large flints, hard, nodular chalks and marl seams, named the 'Basal Complex' by Stokes (1977). Thus the upward development of regularly flinty chalk in the succession also became indicative of the Upper Chalk in southern England (Mortimore & Wood, 1986). In some parts of the country, the Chalk Rock is associated with a distinctive molluscan fauna, named the Reussianum Fauna (Woods, 1896, 1897), the identification of which gave a further means of defining the base of the Upper Chalk in the absence of the Chalk Rock.

However, recent stratigraphical studies (Bromley and Gale, 1982; Gale, 1996) have shown that where developed, the Chalk Rock comprises different suites of hardgrounds in different parts of the country, and that its total stratigraphical range is from the upper T. lata Zone to the middle S. plana Zone. Also, the Reussianum Fauna is only really distinctive of one of these hardground suites in the type area of the Chalk Rock, although in southern England, elements of this fauna probably occur over a broader stratigraphical interval. Thus, as historically applied, there is likely to be much lateral inconsistency in the definition of the base of the Upper Chalk in southern England.

In parts of Wiltshire (eg. Charnage Down [ST 836 328]), however, a highly condensed Chalk Rock, comprising a closely welded succession of the bottom, middle and upper hardground suites of Bromley & Gale's (1982) Chalk Rock 'Formation', does occur, and provides a conspicuous boundary to the base of the Upper Chalk hereabouts. The latter area is at the southern end of the Marlborough Downs / Berkshire Downs / Chilterns Region, where the Chalk Rock is most conspicuously developed.

Hard, nodular, regularly flinty chalk, with marl seams and hardgrounds characterises the lower part of the Upper Chalk, passing upwards into softer, flinty chalk, intervals of which are characterised by the occurrence of regularly developed marl seams. The fauna of the Upper Chalk is dominated by inoceramid bivalves and echinoids, although bryozoa, brachiopods, belemnites and crinoids are also locally common. Ammonites are generally very rare, except in the basal beds.

The Upper Chalk of the Southern Region is thicker than either the Lower or Middle Chalk. However, thickness differences between areas are influenced by the variable extent of Palaeogene erosion as well as stratigraphical changes. Up to 400 m of Upper Chalk is preserved in Hampshire, compared to 230 m in Sussex and 130 m in Kent (Rawson et al., 1978)

Macrofossil Biozonation: S. plana Zone, M. cortestudinarium Zone, M. coranguinum Zone, U. socialis Zone, M. testudinarius Zone, U. anglicus Zone, O. pilula Zone, G. quadrata Zone & B. mucronata Zone

Correlation: see Correlation with other Southern Region Chalk Group classifications

see Correlation with other UK Chalk Group successions

References

BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.

GALE, A S. 1996. Turonian correlation and sequence stratigraphy of the Chalk in southern England. In HESSELBO, S P & PARKINSON, D N (eds), Sequence Stratigraphy in British Geology, Geological Society Special Publication, No. 103, pp. 177-195.

JUKES-BROWNE, A J & HILL, W.1904. The Cretaceous rocks of Britain. Vol. 3 - The Upper Chalk of England. Memoir of the Geological Survey of the United Kingdom.

MORTIMORE, R N & WOOD, C J.1986. The distribution of flint in the English Chalk, with particular reference to the 'Brandon Flint Series' and the high Turonian flint maximum. In SIEVEKING, G de C & HART, M B. The scientific study of flint and chert. Cambridge University Press, Cambridge.

STOKES, R B. 1977. The echinoids Micraster and Epiaster from the Turonian and Senonian of England. Palaeontology, Vol. 20 (4), p. 805-821, 4 pls..

WOODS, H. 1896. The Mollusca of the Chalk Rock. Pt. 1. Quarterly Journal of the Geological Society of London, Vol. 53, 377-403.

WOODS, H. 1897. The Mollusca of the Chalk Rock. Pt. 2. Quart. Jl. geol. Soc., Lond., Vol. 53, 377-403.

See: marl, flint, hardground, Chalk Rock, Reussianum Fauna