An Early Cenomanian (Late Cretaceous) maximum flooding bioevent in NW Europe: Correlation, sedimentology and biofacies

The Early Cenomanian (mid-Mantelliceras dixoni Zone) Schloenbachia/Inoceramus virgatus Event of NW Europe is recorded in five calcareous ‘marl’-limestone couplets rich in inoceramid bivalves of the Inoceramus virgatus plexus. The 1.3–6.8m thick beds represent a bundle of precession couplets inferred to have been deposited during a short-term cycle of sea-level change triggered by the 100kyr short eccentricity cycle within the Milankovitch periodicities. A burrowed omission surface terminates the Schloenbachia/Inoceramus virgatus Event in most places. Rock accumulation rates vary in different sections between 13 and 68Bubnoff units (mm/kyr) for the complete 100kyr cycle with extreme values ranging from 10 to 100Bubnoffs at shorter (i.e., 20kyr precession couplet) scales. Taphonomic and microfacies trends suggest deposition below fair-weather wave base with frequent to sporadic reworking of the sea-floor by storm waves for proximal and medial sections. The distal sections were close to, or even slightly below, maximum storm wave base. The biofacies of the Schloenbachia/Inoceramus virgatus Event is strongly dominated by inoceramid bivalves (65–85%). The two representatives of Inoceramus virgatus, I. virgatus virgatus Schlüter and I. virgatus scalprum Böhm, are not geographic subspecies as they co-occur in the same beds. However, proximal morphs tend to be more strongly ribbed and flatter whereas distal ones are more inflated and smoother, representing ecophenotypes of one polymorphic species. Taphonomic observations and shell morphology suggest that the inoceramids lived as epibyssate recliners. The low species richness and fossil content of the event strata (without the inoceramid bivalves) are attributed to low fluxes of organic matter to the sea-bed during the maximum flooding interval of Cenomanian depositional sequence Ce III. The inoceramids of the virgatus plexus are inferred to have been very effective filter feeders flourishing under nutrient-depleted, low-energy conditions. The Schloenbachia/Inoceramus virgatus Event is an example of a maximum flooding bioevent, forming a widespread marker dominated largely by autochthonous biota.