A southern, shallow marine, Cretaceous-Paleogene boundary: Bajada del Jagüel section, Neuquén Basin, Argentina

NÁÑEZ, C.; PARRAS, A.; HANSEN, H. J.; CONCHEYRO, A.; ALONSO, S.; LOJAN, S.; PIRES, M.

Saskatoon

Simposio; Geological Association of Canada and Mineralogical Association of Canada Annual Joint Meeting. Special Session 22: The palynology and micropaleontology of boundaries; 2002

Geological Association of Canada and Mineralogical Association of Canada

The Bajada del Jagüel section is situated at southern mid latitudes in a rather shallow shelf setting. The section exposes more than 30 m Late Maastrichtian-Danian marine mudstones of the Jagüel Formation. Approximately at mid section, there is a 10 cm thick, rusty loose rhyolitic ash-bed, which is regarded as the base of the Danian. Late Maastrichtian age is indicated by Nephrolithus frequens, Micula murus and Rugoglobigerina spinosa. With the exception of the uppermost part, the Maastrichtian is almost devoid of megafossils and has scattered and small-size bioturbation structures. The abundance of low diversity, biserial planctonic foraminifera and dominance of buliminiid species, in some horizons suggest abnormal, marine, dysaerobic conditions. Keeled planktonic foraminifera were recovered only of one horizon, about 5 m from the base of the section; it is interpreted as the deepest environment of the Late Maastrichtian sediments. Approximately one meter below the rusty ash-bed there is a horizon with the oyster Pycnodonte vesicularis, suggesting oxic bottom conditions. Last occurrence of Maastrichtian foraminifera such as Rugoglobigerina and Planoglobulina are at 5 m and 3 m below the ash-bed, respectively. The interval between these last occurrences and the sandy layer yielded a planktonic foraminiferal assemblage almost exclusively composed of Guembelitria cretacea. Nannofossils from the same interval are of late Maastrichtian age. The K-P boundary is tentatively situated at the base of the ash-bed, where the Danian planktic species Chiloguembelina morsei and Biantholithus sparsus first appear. The ash-bed is overlain by one meter thick mudstones witch frequently are laminated, devoid of megafauna, and with scarce planktonic foraminifera, mainly G. cretacea. These earliest Danian beds are assigned to the NP1 Biozone of nannofossils and tentatively to P0/Pα Biozones of planktonic foraminifera. The pattern of magnetic susceptibility of the sediment suggests that the section is incomplete with respect to latest Maastrichtian. Organic stable carbon isotopic ratios across the K-P boundary demonstrated a distinct negative anomaly. Clay mineralogy suggests relatively warm and humid conditions during the latest Maastrichtian, and cooler and/or drier during the earliest Danian