Biostratigraphy of the Liu Malal section (Maastrichtian-Danian), Neuquén Basin, Argentina

CONCHEYRO, A.; PIRES, M.; CASADÍO, S.; PARRAS, A.

Buenos Aires

Simposio; Simposio Paleógeno de América del Sur; 1996

South American Regional Commitee on Paleogene Stratigraphy

First results about nannoflora and megafossils of the Liu Malal section, Neuquén Basin, are performed. The section, 112 m thick, comprises fine grained sandstones, silstones, claystones, and sparse grainstones deposited in outer to inner-shelf environments and they belong to the Jagüel and Roca Formations, previously assigned to a Late Cretaceous-Paleocene age. We recognize two nannofossil and megafossil assemblages in the Liu Malal section, both poorly preserved. The first assemblage is diverse, through their nannofossils results Late Maastrichtian in age, CC26 Zone (Sissingh, 1977). The megafossils comprise: Rastellum clarae, Pterotrigonia (Rinetrigonia) windhauseniana, Exogyra mendozana and Ostrea wilckensis, and they are representatives of the Jagüel Formation. The second assemblage contains at the base abundant Micrantholithids and Thoracosphaerids and, progressively replaced by Toweius africanus and Neochiastozygus modestus to the top. This nannoflora indicates a Danian age, NP1-NP3 Zones (Martini, 1971). Among the megafossils are frequent: Odontogryphaea burckardti, Venericardia iheringi, Turritella aff. T. malaspina and they characterize the Roca Formation. An anomalus Hauterivian reworked nannoflora is detected in the Danian samples and monitored through counting. This unusual association is probably due to tectonism or uplift of Cretaceous rocks on the western flanks of the basin, a sea-level fall during the latest Maastrichtian-earliest Danian, unroofing Cretaceous rocks in the provenance, or to a combination of both factors. Liu Malal section represents an almost complete record of Maastrichtian and Danian sediments in the Neuquén Basin. It could be correlated in its lower part with Barda Baya section (Maastrichtian), and in the upper part with Cerros Bayos section (Danian) which crop out to the south in the La Pampa province. In the absence of diagnostic nannofossils, the K/P boundary cannot be precisely located in the studied area.