Phytolith studies in Gran Barranca (central Patagonia, Argentina): the middle-late Eocene.

ZUCOL, A. F., BREA, M., & E. BELLOSI.

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia. R. Madden, A. Carlini, M. Vucetich and R. Kay (eds.).

Cambridge University Press.

Lugar: Cambridge; Año: 2009;

The sedimentary sequence of the southern escarpment of Lake Colhue-Huapi (Gran Barranca, Chubut Province, Argentina) is composed of the Sarmiento Formation and Rý´o Chico Group. At the type profile of the Sarmiento Formation at Gran Barranca (Profile MMZ), the sediments of the Gran Barranca Member form the lowest part of the Sarmiento Formation and are of middle Eocene age. Sediments of this member are analyzed for their phytoliths content and these results are compared with phytoliths assemblages from the underlying Koluel-Kaike Formation. A detailed phytolith analysis of the Gran Barranca Member reveals an abundance of phytoliths that allowed us to establish a phytolith zonation of the member. A basal zone (GBI) clearly similar to the Koluel-Kaike Formation assemblages indicates a savanna community; the middle zone (GBII) with different compositional characteristics described by the basal predominance of palms associated with grasses and dicots, that gradually decrease (principally palms phytoliths) with more abundant woody dicotyledons elements in the middle levels. The upper section of GBII is characterized by the first presence of open megathermic grasslands with the rising abundance of panicoid elements. The upper zone (GBIII) shows some changes in the floristic composition including an increase in aquatic herbs. Our results indicate the presence of C4 elements during the middle Eocene and explore the significance of this early record in light of its implications for the origins of grass-dominated ecosystems.