Calcareous Nannofossil, Palynological and Sedimentological data from the early Miocene Monte León Formation and the lowermost Santa Cruz Formation, southern Patagonia, Argentina

PÉREZ PANERA, J. P.; GUERSTEIN, G. R.; HEREDIA, M. E.; PARRAS, A.; GRIFFIN, M.; PAEZ, M. M.

Mendoza

Congreso; 4th International Paleontological Congress; 2014

International Paleontological Association - CONICET

The Monte León Formation and the basal beds of the Santa Cruz Formation record micropaleontological and palynological information. When integrated with sedimentological data, this information allows the refining of paleoenvironmental reconstructions (and related sea level changes) in southern South America during the early Miocene. The studied sections (Restinga Norte, Cabeza de León and Cerro Monte León) are exposed in the southeastern area of the Santa Cruz Province. They yield dinoflagellate cysts (dinocysts), pollen, spores, calcareous nannofossils, silicoflagellates, and diatoms. The reconstruction of depositional environments suggests a basal transgressive interval represented by ca. 3 m exposed at the base of the Restinga Norte and Cabeza de León sections. This interpretation agrees with the high abundance and moderate diversity of dinocysts indicating an inner-shelf to nearshore environment. The nannofossil assemblages show a moderate to low abundance of Coccolithus pelagicus, Reticulofenestra pseudoumbilicus pseudoumbilicus, R. pseudoumbilicus gelida, R. perplexa and Braarudosphaera bigelowii. The overlying ca. 30 m exposed along the coastal cliffs of the Restinga Norte and Cabeza de León sections reflect a shallowing-up facies trend. The nannofossil assemblages show low to high abundance and an increased diversity compared to the underlying beds. The first record of silicoflagellates in the Monte León Formation is Distephanus crux (sensu lato), recovered from the basal levels of the Cabeza de León section together with abundant planktonic diatoms. These assemblages could indicate the presence of cool water originated by an upwelling event, which agrees with high concentrations of protoperidinacean dinocysts. Towards the top of this section the dinocyst assemblages reflect a restricted marine environment represented by Lingulodinium hemicystum and Tuberculodinium vancampoae. The upper ca. 40 m exposed at the Cerro Monte León section show a drop in dinocyst abundance and an increase of Chlorophyceae and hydrophytic taxa. These changes may indicate an increase of freshwater influx related to the coastline progradation. Rare nannofossils (C. pelagicus) were recorded in only one level. Pollen records in the Restinga Norte section suggest the development of a forest dominated by both Podocarpaceae and Nothofagaceae, while in the upper part of the Cabeza de León and Cerro Monte León sections it includes mainly pollen of Podocarpaceae. Herb/ shrub vegetation is underrepresented as well as the hydrophyta and Pediastrum sp. The nannofosil assemblage suggests an early to middle Miocene age. This agrees with the dinocysts and with the Burdigalian age based on 87Sr/86Sr isotopes (approximately between 17.5 and 18.5 Ma) previously reported for the sections.