Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe

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Abstract
Basinal successions of Cenomanian age in Western Europe are dominated by pelagic and hemipelagic sediments, and display conspicuous primary bedding cyclicity (couplets) which is attributed to climatically-controlled variations in carbonate productivity. Within single basins individual rhythmic couplets are widely identifiable and enable basin-wide decimetre-scale correlation. A high resolution ammonite biostratigraphy (aided by inoceramid bivalves) is used to correlate successions in separate basins, and allows a comparison of couplet numbers to be made. Couplet numbers are similar across Western Europe, in spite of an order of magnitude thickness variation, and the carbonate: clay ratios of beds and groups of beds are persistent and diagnostic of particular levels. Additionally, beds with distinctive ichnofabrics (e.g. abundant dark Chondrites ) are regionally extensive in the basins of northern Europe. It has proved possible to construct a composite cyclostratigraphy for the Cenomanian, graduated by 212 precession units (mode at 21 ka), which indicate a duration for the stage of 4.45 Ma. Corroboration of this cyclochronology comes from radiometric dating; the Middle Cenomanian basal A. rhotomagense Zone to the Upper Cenomanian basal N. juddii Zone contain 107 precession couplets in Western Europe, giving a duration 2.24 Ma. The equivalent biostratigraphic interval in the Western Interior Basin of the USA recently yielded Ar-Ar dates of 2.2 Ma from sanidines in bentonites The dominance of the precession cycle in the mid-Cretaceous is in keeping with results from climate sensitivity modelling.