Foraminifers and Calcareous Dinoflagellate Cysts as Proxies for Deciphering Sequence Stratigraphy, Sea-Level Change, and Paleoceanography of Cenomanian–Turonian Hemipelagic Sediments in Western Europe
Kai-Uwe Gräfe, Jens Wendler, 2003. "Foraminifers and Calcareous Dinoflagellate Cysts as Proxies for Deciphering Sequence Stratigraphy, Sea-Level Change, and Paleoceanography of Cenomanian–Turonian Hemipelagic Sediments in Western Europe", Micropaleontologic Proxies for Sea-Level Change and Stratigraphic Discontinuities, Hilary Clement Olson, R. Mark Leckie
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Assemblages of planktic and benthic foraminifers were studied from the northern continental margin of the Late Cretaceous Iberian Plate (Basco-Cantabrian Basin, BCB). A cross section from inner-ramp carbonates containing large benthic foraminifers (section Sobrón) to hemipelagic marl–limestone alternations (Villasana section) and pelagic claystones and marlstones (Galarreta–Gordoa section and Urbasa-2 borehole) allows the study of foraminiferal parameters. These parameters are plankton/benthos ratio (p/b ratio), foraminiferal number, species richness, abundances of agglutinated and calcareous benthic foraminifers, heterogeneity, keeled and non-keeled (globular) planktic foraminiferal morphogroups, and biofacies distribution of benthic foraminifers. Foraminiferal parameters are studied with respect to relative sea-level changes in the Cenomanian–Turonian.
Ratios of planktic to benthic foraminifers and abundances of benthic foraminifers match long-term (4–8 Myr) sea-level changes in the BCB. Foraminiferal numbers and p/b ratios are highest at maximum flooding surfaces and low at sequence boundaries. Abundances of calcareous benthic foraminifers can increase during prograding highstand systems tracts. Abundances of agglutinated benthic foramini-fers increase during prograding lowstand systems tracts and are lowest during maximum transgression. Species richness is often highest in transgressive systems tracts. Keeled planktic foraminiferal morphogroups are concentrated in the highstand systems tract and at the maximum flooding surfaces in outer-shelf depositional realms. However, there is no unique solution for a sequence stratigraphic interpretation from foraminiferal parameters alone. Often, there is more than one candidate for the interpretation of a sequence stratigraphic surface from p/b ratio or foraminiferal numbers.
These results were compared to the analysis of foraminiferal biofacies from the Escalles section (Paris Basin, northern France). The cyclic hemipelagic marls and gray chalks are composed mainly of coccoliths, foraminifers, and calcareous dinoflagellate cysts. Parameters like the p/b ratio did not match existing sequence stratigraphic interpretations. Instead, it can be shown that the precession-controlled cyclicity is explained by a paleoceanographic model. This model proposes stratified water masses during deposition of marls and mixed high-productivity water masses during deposition of chalks. Sea-level changes are indicated by (1) the foraminiferal number, (2) the total abundance of planktic foraminifers, and (3) by the abundance of Pithonella ovalis, which all increase with transgressive systems tracts and decrease in highstand systems tracts.
Four curves with p/b ratios from logged sections in the BCB and in the Paris Basin and from one borehole in the BCB were converted to time series and then stacked together. This stacked p/b ratio curve was converted by paleoslope modeling in the BCB to a proxy for sea-level change in Cenomanian to Turonian times. Three periods of sea-level rise in the Cenomanian are recognized on this proxy (97.5 Ma, 95.2 Ma, 93.8 Ma). In the Turonian, two major sea-level rises are identified (91.2 Ma, 90.1 Ma) together with two major sea-level falls (around 92 Ma, 89.5 Ma). This proxy predicts no major sea-level falls in the Cenomanian. Instead, the proxy sea-level rises from datum plane at the Albian–Cenomanian boundary to + 30 m (minimum estimate)/+ 50 m (maximum estimate) in the Early Turonian. The sea-level proxy has a value of +10 m above the datum plane at the Turonian–Coniacian boundary. Positive excursions of stable carbon isotope data match strong sea-level rises or falls in the Middle Cenomanian, the Late Cenomanian, the Middle Turonian, and the Late Turonian, but not in the Early Cenomanian.