RINCÓN DEL BUQUE: A LARGELY UNEXPLORED RICH SANTACRUCIAN (EARLY MIOCENE) LOCALITY IN SOUTHERN PATAGONIA, ARGENTINA

BARGO, M.S.; VIZCAÍNO, S; KAY R. F; FERNICOLA J.C; RAIGEMBORN, M.S.; KRAPOVICKAS, V.; TOLEDO N.; MUÑOZ N.A.

Mendoza

Congreso; IV International Palaeontological Congress; 2014

The early Miocene deposits of the Santa Cruz Formation (SCF; Santacrucian Age), widely distributed in Southern Patagonia (Argentina), contain a rich assemblage of fossil mammals. The best-known fossiliferous exposures of SCF crop out along the Atlantic coastline of Santa Cruz Province, between Río Santa Cruz southward to Río Coyle and then to Río Gallegos. Localities south to Río Coyle haven been extensively studied in the last 20 years, but those to the north have received less attention. Isolated geological and paleontological studies for Cerro Observatorio and Monte León were performed in the 1990s, but a third major area, Rincón del Buque (= Media Luna), first reported in the 1920s has been neglected. This locality is of interest because it records the basal contact between the SCF and the underlying shallow marine Monte Léon Formation, a contact that is not exposed south to the Río Coyle, and because it is particularly rich in fossil vertebrates which are briefly mentioned in a short report in 1941. Since 2009, several field seasons to Rincón del Buque by the authors allow us to reconstruct the depositional paleoenvironments based on detailed stratigraphic profiles with sedimentological and ichnological data, and to make a large collection of vertebrates (~300 specimens). At Rincón del Buque there is a progradational succession, beginning with a marginal-marine-estuarine setting that transitions gradually to fluvial deposits. About 80% of the vertebrates were recovered in situ in two different tuff beds: a lower marine-bioturbated tuff immediately above the last oyster bed (the latter marking the top of the Monte León Formation), and a second tuffaceous bed at ~30 meters from the base deposited in a freshwater fluvial setting. The rest of the specimens come from surface collection at different levels. Specimens recorded belong to Calyptocephalellidae (Anura), Testudines, Phorusrhacidae (Aves), Paucituberculata and Sparassodonta (Marsupialia), Cingulata and Pilosa (Xenarthra), Toxodontia and Typotheria (Notoungulata), Litopterna, and Astrapotheria, Chinchilloidea, Cavioidea and Octodontoidea (Rodentia), and Primates. Most genera and/or species preliminarily identified have been previously recorded in the SCF, particularly in coastal localities south of Río Coyle. Rincón del Buque may hold the key to establishing details of faunal transitions in different areas where SCF crops out. On one hand connections are established between the lowermost levels north of Río Coyle and higher levels to the south, along the Atlantic coast, and on the other hand between the costal localities collectively and those of the Río Santa Cruz to the northwest.