Representation of Nannoplankton Assemblage Succession Throughout Cretaceous or Tertiary Boundary in the P Well Section, Santos Basin, Brazil



The massive change in calcareous nannoplankton assembalages throughout Cretaceous/Tertiary boundary (65.5 M.a.) have been illustrated by several authors. The diverse and abundant assemblage disappears suddenly above the Cretaceous/Tertiary boundary, with estimate of a 92% decrease in diversity (Bown, 1991) and a 99% abundance reduction (Pospichal, 1994). This event is related to the most dramatic environmental changes in the Earth’s history due to the catastrophic events, including meteorite impact (Chicxulub) and volcano eruption (Deccan) occurred at the end of Cretaceous. The succeeding age was a time of rapid evolution of nannoplankton, there is evident that about 25 new genera occurred during +12.5 m.y. of Paleocene time interval that evolved from 10 to 12 survivors (Bown, 2005). A quantitative method analysis of nannoplankton throughout Maastrichtian to Paleocene of “P” well section, Santos Basin, Brazil indicates that the nannoplankton assemblages abruptly decrease in diversity and abundance and mostly change in species composition. The various complex shapes of species at Maastrichtian are also underwent change to simple plain shapes at Paleocene. The sedimentary section ranges from the top of zone CC23 (Sissingh Zonation, 1977) to NP9 (Martini Zonation, 1971). It is bounded by the Last Occurrence (LO) of Tranolithus pachelosus at the base and Fasciculithus tympaniformis at the top. The biostratigraphic discontinuity characterized by the absence of zone CC26 to NP4 is indicator for the presence of an unconformity at K/T boundary within analyzed section. The Cretaceous nannoplankton assemblages are dominated by Genera Watznaueria, Micula, Arkhangelskiella, Cribrosphaerella, Eiffellithus, Predicosphaera and Retecapsa, whilst the paleocene assemblages are dominated by Genera Toweius, Ericsonia and Coccolithus. Survivor Cretaceous species recovered into Tertiary sediment consist of Braardosphaera bigelowii, Biscutum melaniae, Neocrepidolithus neocrassus, Placozygus sigmoides, Cyclagelosaphaera reinhardtii, Markalius inversus and Scapolithus fossilis. Rare Cretaceous nannoplankton reworked specimens are present, including Micula decussata and Watznaueria barnesae.


Author : Panuju