Microperforate planktonic foraminifera from the late Maastrichtian-Danian of Bajada del Jagüel, Neuquén, Argentina

NÁÑEZ, C., ARZ, J.A., ARENILLAS, I. Y MALUMIÁN, N.

Concepción

Simposio; International Symposium on Foraminifera FORAMS 2014; 2014

Microperforate planktonic foraminifera characterize the late Maastrichtian and Danian of the Bajada del Jagüel section, Neuquén Basin, a shallow marginal basin at mid southern latitudes. The late Maastrichtian is characterized by the abundance of the triserial genus Guembelitria, represented by G. cretacea Cushman, G. blowi Arz, Arenillas and Náñez and G. dammula Voloshina. This genus absolutely dominates the planktonic assemblages in the last two meters of the Maastrichtian, except for a small peak of biserial species at ca. 40cm below the Cretaceous/Paleogene (K/Pg) boundary. The associated benthic assemblages are relatively diverse, suggesting stable conditions. The first meter of Danian sediments, including the K/Pg boundary bed, contains foraminiferal assemblages of a very low p/b ratio, where planktonics are represented by the serial genera Guembelitria and Woodringina/ Chiloguembelina. Planktonic species indicative of the Parvularugoglobigerina eugubina biozone were not found. Towards the upper part of this first meter, biserial planktonic foraminifera (Woodringina/ Chiloguembelina) increase in abundance relative to triserials and are associated with a low diversity benthic assemblage dominated by Antarcticella pauciloculata (Jenkins). This small opportunistic species endemic to southern high latitudes and apparently of polar origin, was originally assigned a planktonic habit, and later regarded as a benthic species. At Bajada del Jagüel, it abruptly appears just after the K/Pg boundary. This assemblage characterized by Antarcticella and planktonic biserials occurs in a horizon with diffuse lamination suggesting deficiency in oxygen content. An acme of biserial planktonics has been described elsewhere in the Danian but local factors cannot be disregarded. Upwards in the section, p/b ratio increases and planktonic assemblages become more diverse suggesting a relative increase in sea level. They include abundant low trochospiral, normal perforate taxa; among microperforate taxa, Globoconusa daubjergensis (Brönnimann) is dominant; serial planktonic foraminifera are also common, including ?Guembelitria? spp. and Trochoguembelitria alabamensis (Liu and Olsson). These planktonic assemblages are associated with the cosmopolitan Midway-type benthic assemblage. Thus the section provides opportunity to analyze the paleoecology and evolution of the wall texture of microperforate planktonic foraminifera in the late Maastrichtian-Danian from southern mid latitudes.