Lower Cretaceous calcareous nannoplankton from the Neuquén Basin, Argentina

Abstract

We present nannofossil analyses from five new sections of the Neuquén Basin, which together provide a relatively continuous stratigraphic succession ranging from Tithonian to Hauterivian (Vaca Muerta to Agrio Formations). The sections provide an opportunity to study nannofossils from a relatively little-studied area, which nevertheless represents an important marginal basin and a relict of the single, largest Mesozoic marine habitat, the Pacific Ocean. Our aims were, firstly, to generate biostratigraphic data in order to test existing ammonite-based age-models and, ultimately, to improve stratigraphic correlations between the Argentinian sections and global chronostratigraphic standards; and, secondly, to collect palaeogeographic information in order to improve our understanding of Mesozoic nannoplankton distributions and ecology. The sections yielded a continuous nannofossil succession, with assemblages of moderate preservation, abundance and diversity. Although many standard biostratigraphic index-species, of both Boreal and Tethyan affinities, were absent, we were able to use age-diagnostic taxa to provide a relatively robust stratigraphic framework (the first and last occurrences of Clepsilithus maculosus, Cruciellipsis cuvillieri, Eiffellithus primus, Eiffellithus windii, Eiffellithus striatus, Lithraphidites bollii and Nannoconus? ligius). Throughout the sections, there is remarkably good agreement between the correlation suggested by the nannofossils, based largely on globally distributed species, and ammonites, which are largely endemic to the basin. The Vaca Muerta and Upper Agrio Member correlations are identical, or within one or two ammonite zones. The Lower Agrio Member correlations are similarly close, but the occurrence of wide-canalled nannoconids suggests slightly younger, latest Valanginian to early Hauterivian, ages for much of the member. The Neuquén nannofossil assemblages are rather low in diversity and suggest that the basin supported a restricted nannoflora, dominated by shelf-adapted taxa ( Watznaueria, Micrantholithus, Nannoconus). Biogeographically distinctive taxa, whether Tethyan or Boreal-Austral, are limited to a few rare and sporadically distributed examples but two new species of Crucibiscutum may prove to be Austral in affinity. The presence of nannoconids consistently from the late Valanginian indicates the presence of a good, shallow-marine connection with Tethys via the Hispanic Corridor seaway, but their low abundance and rather sporadic occurrence suggests they were probably at the limits of their ecological ranges. Four new nannofossil species are described: Crucibiscutum trilensis, Crucibiscutum neuquenensis, Calculites suturus and Nannoconus quadricanalis.