CONICET | Buscador de Institutos y Recursos Humanos

The calcareous nannofossils (CN), a valuable protist algae group, have demostrated their utility in biostratigraphic studies allowing to precise the age of mesozoic marine sequences. During the Cretaceous, the nannofossil associations have had, in general, a worldwide distribution, with some cosmopolitan species, and other more endemics, depending of their paleogeographic situation and latitude. In the last decade many studies on CN have been carried out in Argentina with the aim to establish stratigraphic correlations with southern mesozoic basins and global chronostratigraphic standards. Particularly, investigations have been concentrated in Cretaceous basins as Neuquina, Austral, Malvinas and Larsen Basins (Patagonia and Antarctica, respectively), as is summarized here. In the Neuquina Basin, the Lower Cretaceous calcareous nannofossil associations comprise the early Berriasian-late Hauterivian interval, between the NJK to CC5a Bizones. The main recorded bioevents are: FO and Cruciellipsis cuvillieri, FO and LO Eiffellithus windii, FO and LO Eiffellithus striatus, FO and LO Clepsilithus maculosus, FO Nannoconus bucheri and FO Nannoconus circularis, FO Lithraphidites bollii, and FO and LO Nannoconus ligius. All these assemblages are mainly composed by nannofossils and nannoconids with Tethyan affinities that indicate sporadic seaway connections during that time. Upper Cretaceous nannofossils are also well distributed in this basin. The marine sedimentites contain a good preserved assemblage compose of Nephrolithus frequens and Cribrosphaerella daniae among the reliable species. Both markers indicate an Upper Maastrichtian age, more precisely, the CC26 Biozone. The K/P boundary is recognized with CN, and this Upper Cretaceous nannofossils show an austral distribution, due to the presence of some high latitude markers. In the Austral Basin, 15 wells are studied and the recorded CN contains Diadorhambus rectus, a good Valanginian-Hauterivian marker. The FO Radiolithus planus with the FO de Braarudosphaera africana and Braarudosphaera bigelowi indicate Aptian- Albian ages. The diagnostic FO Eprolithus floralis suggest a middle Albian age. The Turonian is not identified and the FO Calculites obscurus indicates the Santonian-Campanian boundary. The FO Arkhangelskiella cymbiformis confirm the CC22 Biozone (late Campanian) and ocurrences of Cribrosphaerella daniae, Nephrolithus corystus and Nephrolithus frequens confirm a late Maastrichtian age (CC 26 Biozone). In the Malvinas Basin, 60 samples from Merluza x-1, Calamar x-1 and Orca x-1 wells have been studied, and it is possible recognize an important CN assemblage ranging in age from the Aptian-Albian to Maastrichtian. The FO Eprolithus floralis is identified and it allow to determine de CC7 Biozone (lower Albian); the FO and LO Hayesites albiensis define the CC8 and CC9 Biozones (middle and upper Albian). An important unconformity has been detected in this basin and it range from the uppermost Albian to lower Turonian. The base of CC12 Biozone is recongnized through the FO. Eiffellithus eximius. The upper Coniacian and the Santonian (CC15 and CC16 Biozones) are determined through the FO Lucianorhabdus cayeuxii, Calculites obscurus and the LO E.floralis, respectively. Campanian is defined with LO Bukryaster hayii, Calculites obscurus and Reinhardtites anthophorus, and the upper Maastrichtian is precised with the FO and LO Nephrolithus frequens. In the Larsen basin, Antarctica, calcareous nannofossils are recovered from the southern area of James Ross and Seymour Islands. Associations are partially dissolved, and they show low diversity and abundance. Taking account the occurrences of Misceomarginatus pleniporus, Monomarginatus quatternarius, Biscutum coronum, and Biscutum magnum, an Upper Campanian-Lower Maastrichtian age is determined, between the Biscutum coronum and Biscutum magnum Biozones for high austral latitudes. Nannofossils can be included in the "Falkland Plateau Province", and they show a good correlation with other Campanian-Maastrichtian nannofossils from Northern Patagonia, South Atlantic and Southern Oceans.

enviar mensaje