Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe

Abstract

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.