Southern Region Chalk Group Lithostratigraphy: Traditional Classification - Chalk Marl

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The Chalk Marl consists of a succession of rhythmically alternating soft, typically decimetre to metre scale marls and harder spongiferous marly limestones, the limestones typically weathering proud as a series of 'ribs' in quarry and coastal exposures. In thin section the marl-limestone alternations comprise foraminiferal/calcisphere biomicrite and argillaceous biomicrite respectively (Ditchfield and Marshall, 1989), and are pervasively bioturbated with the trace fossils Thalassinoides, Planolites and Chondrites (Gale, 1995). In common with the whole of the Lower Chalk in southern England, flint is absent from the Chalk Marl.

The marls and limestones form distinctive couplets that are traceable across Europe, and form the basis for a detailed cyclostratigraphy (Gale, 1995). The rhythmic alternation of lithologies that is characteristic of the Chalk Marl is thought to be caused by regular climatic oscillations that accord with the Milankovitch Cycle (Gale, 1995). Enhanced organic productivity in warmer phases enriched the ambient marl deposition with calcite, and formed marly limestone; lower organic productivity in cooler phases produced less indurated marls (Ditchfield & Marshall, 1989). Some of the beds are characterised by distinctive macro-faunas, and are traceable over a wide geographical area, and these have been variously named, and include, in ascending order, the 'Aucellina Beds' (Wood, 1996), the Crippsi Limestone (Wood, 1996), the Dixoni Limestone (Wood, 1996) and the Tenuis Limestone (Bristow et al., 1995).

In southern England the Chalk Marl shows a gradational boundary with the overlying Grey Chalk. The absence of a consistent definition for the top of the Chalk Marl, makes regional thickness comparisons misleading. Jukes-Browne and Hill (1903) first pointed out that that the Chalk Marl was a difficult unit to consistently define, and as such, was rather unsatisfactory as a stratigraphical unit. In Sussex, Lake et al. (1987) recorded that the Chalk Marl varied from 22m to over 40 m thick. Rawson et al. (1978) placed the top of the Chalk Marl at the base of a bed rich in the brachiopod Orbirhynchia mantelliana, and as so defined, recorded that it was 10 to 20 m thick in Hampshire and Sussex, and 30 to 42 m thick in Kent. In parts of Dorset, much of the Chalk Marl is condensed out over the Mid Dorset Swell.

The Chalk Marl is probably the most fossiliferous part of the Chalk Group, and is particularly rich in sponges, brachiopods, inoceramid bivalves, and echinoids, and ammonites are quite common and important for biozonation.

Macrofossil Biozonation: M. mantelli Zone, M. dixoni Zone, C. inerme Zone, A. rhotomagense Zone (pars)

Correlation: see Correlation with other Southern Region Chalk Group classifications

see Correlation with other UK Chalk Group successions

References

DITCHFIELD, P & MARSHALL, J. 1989. Isotopic variation in rhythmically bedded chalks: Paleotemperature variation in the Upper Cretaceous. Geology, Vol. 17, 842-845.

GALE, A S. 1995. Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe. In HOUSE, M R & GALE, A S (eds.), Orbital Forcing Timescales and Cyclostratigraphy, Geological Society Special Publication, No. 85, pp. 177-197.

JUKES-BROWNE, A J & HILL, W.1903. The Cretaceous rocks of Britain. Vol. 2 - The Lower and Middle Chalk of England. Memoir of the Geological Survey of the United Kingdom.

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: M. mantelli Zone, M. dixoni Zone, C. inerme Zone, A. rhotomagense Zone, cyclostratigraphy, marl