Chronostratigraphy of the Chalk Group - Turonian Stage
The Turonian was first defined by d'Orbigny (1842), with the type area at Touraine, in northern France (Birkelund et al., 1984).
Definition of base of stage
The first occurrence of the ammonite Watinoceras devonense in the proposed boundary stratotype at Pueblo, Colorado, USA (Bengtson, 1996). In the UK, W. devonense has been recorded from SE Devon (Wright & Kennedy, 1981), but elsewhere the appearance of species of the inoceramid bivalve Mytiloides is a useful guide. In the absence of W. devonense, Gale (1996) defined the base of the Turonian at the top of the range of the ammonites Sciponoceras gracile and S. bohemicum anterior, since the disappearance of both of these species in Devon is coincident with the appearance of W. devonense.
Constituent Chalk Group lithostratigraphical units in the UK
Integrated southern England Chalk Group lithostratigraphy: Holywell Nodular Chalk Formation (except basal part), New Pit Chalk Formation, lower Lewes Nodular Chalk Formation
Northern England: Welton Chalk Formation (pars) & Burnham Chalk Formation (pars)
East Anglia: Middle Chalk (except basal part), Upper Chalk (pars)
N. Ireland: Turonian strata do not form part of the Chalk Group
North Sea: Herring Formation and Mackerel Formation (pars)
Lithostratigraphical marker-beds for the base of the stage
Southern England (except SE Devon): between Meads Marls 4 & 5 (Gale, 1996).
SE Devon: the Haven Cliff Neocardioceras Hardground (Jarvis & Woodroof, 1984)
Northern England: an argillaceous unit (upper Variegated Beds of Wood & Mortimore, 1995) above the Black Band s.s., at the base of the Welton Chalk Formation
East Anglia: the junction of a hard, nodular chalk bed and a hard, flaser-bedded chalk unit in the lower part of the Melbourn Rock of the Hitchin district (Hopson et al., 1996); no precise data for other parts of East Anglia
BENGTSON, P. 1996. The Turonian stage and substage boundaries. Bulletin de l'Institut Royal des Sciences Naturelles de Belgique, 66-Supp, 69-79.
BIRKELUND, T, HANCOCK, J M, HART, M B, RAWSON, P F, REMANE, J, ROBASZYNSKI, F, SCHMID, F, SURLYK, F. 1984. Cretaceous stage boundaries - Proposals. Bulletin of the Geological Society of Denmark, 33, 3-20.
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).
JARVIS, I & WOODROOF, P B. 1984. Stratigraphy of the Cenomanian and basal Turonian (Upper Cretaceous) between Branscombe and Seaton, S E Devon, England. Proceedings of the Geologists' Association, 95 , 193-215.
ORBIGNY, D', A. 1840-1843. Paléontologie française. Terrains Crétacés. 2. Gastéropodes. 456 pp. (Masson, Paris.)
WOOD, C J & MORTIMORE, R N. 1995. An anomalous Black Band succession (Cenomanian - Turonian boundary interval) at Melton Ross, Lincolnshire, eastern England, and its international significance. Berliner geowiss. Abh., Vol. 16, 277-287.
WRIGHT, C W & KENNEDY, W J. 1981. The Ammonoidea of the Plenus Marls and the Middle Chalk. Monograph of the Palaeontographical Society, London: 148 pp., 32 pls (Publ. No. 560, part of Vol. 134 for 1980).