Middle and Late Cretaceous planktonic foraminifera of the Western Mediterranean area

Abstract

Planktonic foraminifera from sections in predominantly pelagic and hemipelagic sediments of Late Albian to Late Maastrichtian age in southern Spain, central Tunisia and northern Italy were studied. Their vertical distribution allows for the recognition of 21 zones. They are correlated with the calcareous nannofossil zonation of Verbeek (1977), which was essentially based on the same material. Recent data from stratotypes allow for a rather detailed correlation with the chronostratigraphic scale. An improved calibration of the planktonic biozonations and the Middle and Late Cretaceous magnetostratigraphic scale is proposed. Late Aptian, Late Albian and Cenomanian reversals are documented with micropaleontological evidence from the actual cores. The systematics and phylogeny of several groups of Cretaceous planktonic foraminifera are discussed. The subdivision of post-Cenomanian globotruncanids into genera, as now widely accepted, is shown to be unrealistic from an evolutionary point of view. The genus Globotruncana Cushman is shown to be polyphyletic. Turonian and younger globotruncanids can be subdivided into a number of natural groups by means of conservative diagnostic features, which remain constant throughout the ranges of the groups. Generic features appear to be progressive and develop analogously in each of the groups. The evolution of Middle and Late Cretaceous planktonic foraminifera appears to be closely related to the occurrence of two major crises in the oxygenation of the World's ocean waters. Entirely new associations of keeled forms appear immediately after these anoxic events, which occurred during most of the Aptian and Albian and at the Cenomanian-Turonian boundary. A third event, close to the Santonian-Campanian boundary, is thought to be due to fundamental changes in the oceanic circulation pattern in relation with the opening history of the Atlantic Ocean. The Cretaceous-Tertiary boundary extinction is briefly discussed. Its explanation must be essentially different from that of the other three major events. Taxonomic notes are made on the most important species. New generic names for apparently natural groups have not been introduced and incorrect names for important zonal markers have not been altered as this would hamper the comparison between the zonal scheme as proposed here and previous ones.