Turonian correlation and sequence stratigraphy of the Chalk in southern England

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Abstract
A stratigraphical framework for the Turonian Chalk of southern England is described using lithostratigraphy, marker beds, cyclostratigraphy and biostratigraphy. For the Early and Mid-Turonian succession a basin-wide resolution of correlation of about a metre can be achieved. Sequences are identified from various criteria, which include bed-by-bed correlation along transects from platform to basin to provide a precise picture of sediment geometry in Turonian Chalk. Hardgrounds develop towards the platform margins and it is possible to identify some as lithified and commonly glauconitized sequence boundaries. For example, the Ogbourne Hardground, which represents a major Mid-Turonian sea-level fall, rests erosively on highstand chalks. Transgressive surfaces are also represented by hardgrounds; these onlap onto the basin margins and commonly show complex sedimentary histories and phosphatization. Transgressive events are marked by brief positive excursions in δ 13 C, detectable in basinal chalks. During the major Mid-Turonian sea-level lowstand, a thick ‘shelf margin wedge’ developed in the basin. A total of three complete and two partial sequences are identified in the Turonian of southern England. These compare well in number and biostratigraphical position with sequences described from a Turonian succession in Tunisia, which is an order of magnitude thicker.