Southern Region Chalk Group Lithostratigraphy: Bristow et al. (1997) - Holywell Nodular Chalk
The Holywell Nodular Chalk, c. 25-50 m thick, mostly equates with the concept of the Holywell Chalk of Mortimore (1986a), comprising hard, nodular, shelly chalk, with regularly developed marl seams and the positive feature-forming Melbourn Rock at the base. However, following stratigraphical revisions to the unit proposed by Mortimore & Pomerol (1996), the top of the Holywell Nodular Chalk is downwardly revised (from Malling Street Marl 1) to coincide with the Gun Gardens Main Marl (= Lulworth Marl of Gale, 1996), although in the field this horizon is more readily recognised by the upper limit of Mytiloides-rich shell detrital chalk. Apart from the 6 distinctive paired Meads Marls at the base, Mortimore & Pomerol (1996) recognised that there was a much more extensively developed marl seam stratigraphy comprising the successively overlying Holywell Marls (17 recognised) and Gun Gardens Marls (16 recognised) than was previously detailed by Mortimore (1986a). As so defined, the Holywell Nodular Chalk of Bristow et al. (1997) excludes more massive bedded chalk that formed the top of the Holywell Chalk of Mortimore (1986a), these beds belonging to the lower New Pit Chalk of Bristow et al. (1997). Flints are locally developed in a few successions in Dorset (Mortimore & Pomerol, 1987) and the northern Chilterns (Shephard-Thorn et al., 1994, Hopson et al., 1996).
Macrofossil Biozonation: Mytiloides spp. Zone
Correlation: see Correlation with other Southern Region successions
see Correlation with other UK successions
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.
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.
HOPSON, P. M., ALDISS, D. T. & SMITH, A. 1996. Geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England & Wales).
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.1996. A revision of Turonian litho- and biostratigraphy in the Anglo-Paris Basin. Mitteilungen aus dem Geologisch-Paläontologischen Institut der Universitat Hamburg, Vol. 77, 423-441.
SHEPHARD-THORN. E. R., MOORLOCK, B. S. P., COX, B. M., ALLSOP, J. M. & WOOD, C. J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England & Wales).
See: Holywell Chalk (Mortimore, 1986a), marl, Gun Gardens Main Marl, New Pit Chalk (Bristow et al., 1997), flint