Upper Albian to Lower Cenomanian Calcareous Nannofossil Biostratigraphy of the Proto-North Atlantic

Lower Cenomanian calcareous nannofossil assemblages from Deep Sea Drilling Project (DSDP) Sites 137 and 547 in the proto-North Atlantic Ocean were analyzed quantitatively to examine the fidelity of the widely used CC and UC calcareous nannofossil Zonal schemes. Datasets from Ocean Drilling Program (ODP) holes 1050C and 1052E (Blake Nose) and Tanzania Drilling Project (TDP) Site 24 were integrated into this dataset. Four biozones spanning the upper Albian through middle Cenomanian were determined using the method of unitary associations (UA). Data were also used from these sequences to generate a ranking and scaling (RASC) optimum sequence. A new reliability index method that uses binomial probabilities is proposed because the existing method does not work well for Mesozoic taxa due to patchy distribution and lower abundances. Three bioevents, Gartnerago stenostaurion - LAD, Lithraphidites eccentricum - FAD, and Staurolithites mutterlosei - LAD, were shown to be reliable markers for lower Cenomanian biostratigraphy based on the new reliability index values. This study also corroborates the lower Cenomanian FAD of Lithraphidites eccentricum, whereas the FAD of Cylindralithus scultpus and the LAD Zeugrhabdotus xenotus appear to be more effective as regional rather than global markers, due to discrepancies in superpositional relationships in these sections. A biostratigraphic framework improved the age model for DSDP Leg 79, Site 547 by documenting an expanded section across the Albian/Cenomanian boundary at Site 547 based on the LAD of Corollithion kennedyi. A revised age model for DSDP Leg 14, Site 137 now includes an older age into the upper Albian - middle Cenomanian. Site 547 had a higher diversity of holococcoliths than Site 137. Calculites anfractus, an important marker species, was proposed as a nannofossil marker species associated with the Global boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian; however, it does not occur globally. Controls on holococcolith distribution are poorly understood and are likely controlled by oceanic setting. C. anfractus may be controlled by these factors, and as such, is not a reliable nannofossil marker for the Albian/Cenomanian boundary. Advisor: David K. Watkin

Calcareous nannofossil response to Late Cretaceous climate change in the eastern Tethys (Zagros Basin, Iran)

14 pagesInternational audienceConiacian to Maastrichtian changes in calcareous nannofossil assemblages have been investigated in the eastern Tethyan Shahneshin section (central Zagros Basin, Iran). The nannofossil assemblages are mainly composed of Watznaueria spp. (avg. 54%), Retecapsa spp (avg. 7.9%), Cribrosphaerella ehrenbergii (avg. 7.7%) and Micula spp. (avg. 5.7%). Throughout the late Campanian, there is a trend to lower abundances in Watznaueria spp. together with increasing abundances of C. ehrenbergii and Arkhangelskiella cymbiformis, which are considered in this basin as the main cool-water taxa. Our results reveal that, despite a diagenetic impact on calcareous nannoflora, a number of primary paleoecological trends are preserved which depict well features of the progressive Late Cretaceous cooling. The first pronounced cooling episode occurs across the late Campanian to early Maastrichtian. The onset of pronounced cooling in the eastern Tethys appears to occur prior to the Campanian/Maastrichtian Boundary event (CMBE) δ13C negative excursion, in contrast with the Boreal realm where pronounced cooling only occurs in the early Maastrichtian, postdating the onset of the CMBE. The coincidence of this earlier cooling in the Zagros Basin with an interval characterized by a significant increase in benthic foraminifera suggests an amplified response of the assemblage due to a change to shallower environments. Hence, the late Campanian calcareous nannofossil assemblage turnover in central Zagros is either a response to an early cooling trend in the eastern Tethys or to sea-level fall or both. The mid-Maastrichtian warming and late Maastrichtian cooling episodes are also delineated in the nannofossil assemblage of Shahneshin and likely correlate with similar episodes in the Boreal Realm