FIRST CALCAREOUS NANNOFOSSIL RECORD FROM LA RAMADA BASIN, SAN JUAN PROVINCE, ARGENTINA

CHUAMEIL RODRÍGUEZ, MICAELA; MOREL, LUCIANO; HOQUI, MARTÍN

San Rafael

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2024

The Cordillera Principal area in San Juan province, Argentina, experienced a marine transgression during the Jurassic, registered in its south-western sector in the La Ramada Basin. Nevertheless, unlike the Neuquén Basin and other Jurassic basins in the Chilean sector where marine transgression began in the Upper Triassic-early Lower Jurassic, La Ramada Basin behaved as a topographic high during this period, allowing the ingression from Pacific waters not until the middle-late Pliensbachian. In this context, the first deposited marine unit was the Los Patillos Formation, lithologically characterized by carbonate rocks with intercalations of calcareous sandstones, tuffaceous sandstones and bioclastic limestones. The formation is assigned to the upper Pliensbachian–lower Callovian, based on the ammonite, brachiopod, and bivalve faunas. Overlapping is the La Manga Formation, characterized by mudstones and packestone limestones, and calcareous breccias, assigned to the late Oxfordian according to its Peltoceras and Perisphinctes fauna. We present the first record of calcareous nannofossil from La Ramada Basin. Sections from the Río de Los Patos and Arroyo Las Flores were analysed. The associations are overall well preserved, with high specific richness. At Río de Los Patos, samples from Los Patillos Formation were barren, preventing age assignment. Sediments from the La Manga Formation yielded few calcareous nannofossils towards the top of the section, represented by Cyclagelosphaera aff. margerelii, Lotharingus hauffii, Watznaueria barnesiae, and W. britannica, and would correspond to the lower Bajocian–Bathonian interval. At Arroyo Las Flores, the lower part of Los Patillos Formation was barren, while the middle and upper part were fertile for nannofossils. The assemblages are dominated by Watznaueria britannica, accompanied by Ansulaspahera helvetica, Cyclagelosphaera aff. margerelli, W. barnesiae, W. fossacincta, W. ovata, and Zeugrhabdotus erectus. This preliminary analysis allows to assign a lower Callovian age for Los Patillos Formation, consistently with the previous ammonite record in this unit of Rehmannia paucicostata andPerisphinctes sp. The studied samples will be stored in the INGEO repository, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan.