Southern Region Chalk Group Lithostratigraphy: Traditional Classification - Glauconitic Marl

Originally named Chloritic Marl by Forbes & Ibbetson (1844), because chlorite was thought to be the mineral responsible for the distinctive green colouration of this bed. Jukes-Browne & Hill (1903) correctly pointed out that the colour is in fact due to abundant glauconite, and although they persisted to use the term Chloritic Marl, most modern usage has favoured the term Glauconitic Marl. Lithologically it is a sandy marl with abundant pelletal glauconite and common black, brown or buffish phosphatic nodules. It is typically strongly bioturbated and may contain beds of hard, spongiferous limestone. A phosphatised remanié fauna characterises the Glauconitic Marl, and includes terebratulid brachiopods, the bivalves Aucellina, Inoceramus anglicus and I. crippsi, gastropods and ammonites, the latter including particularly the heteromorphic genera Neostlingoceras and Idiohamites. This distinctive horizon ranges from a few tens of centimetres to several metres in thickness, and forms the basal bed of the Chalk Group, usually overlying an erosion surface, across much of southern England. However, the Glauconitic Marl is locally absent in Dorset (Bristow et al., 1997), where there is extensive condensation and omission of horizons over a structural high named the Mid Dorset Swell.

Macrofossil Biozonation: M. mantelli Zone, N. carcitanense Subzone

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.

FORBES, E & IBBETSON, L L D. 1845. On the Tertiary and Cretaceous Formations of the Isle of Wight. Report and Transactions of the British Association for the Advancement of Science, Vol. 144.

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.

see: marl