Turonian 3

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In basinal areas, a nodular bed just below New Pit Marl 1 represents a sequence boundary, above which is a shelf margin wedge equating with the marly chalk interval at the top of the New Pit Chalk Formation (New Pit Marls and Glynde Marls) (Gale, 1996). In Dorset, the Isle of Wight and parts of Wiltshire the formation of the Ogbourne Hardground was contemporaneous with the shelf margin wedge (Gale, 1996). This hardground forms the sequence boundary in these proximal areas and represents a major sedimentary hiatus, equating with 20-40 m of succession in basinal areas (Gale, 1996). The Ogbourne Hardground equates with the Spurious Chalk Rock of some historical accounts (Bromley & Gale, 1982).

In areas where the Ogbourne Hardground is developed, it is usually closely overlain by the Pewsey Hardground, and this is interpreted as a transgressive surface, locally onlapping the lowstand wedge formed in the basin (Gale, 1996). Above this, the succession in Dorset and the Isle of Wight becomes basinal (Gale, 1996), and the sequence stratigraphical interpretation of this interval has not been described. However, in areas that remained at the basin margin, such as Wiltshire and the Chilterns, the sequence is terminated by the Fognam Farm Hardground, underlain some distance below by a marl that is usually identified as the Fognam Marl (Gale, 1996). The Fognam Marl has not been consistently identified in sections, at least two different marl seams having been given this name. Thus although in many cases the Fognam Marl equates with Southerham Marl 1 of the basinal succession, locally it equates with Glynde Marl 1 (Gale, 1996). The Fognam Farm Hardground forms the boundary with the overlying sequence (Turonian 4).

References[edit]

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.

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