Geology, Fossil Wood and Phytolith assemblages from the upper Paleocene-Eocene? of Central Patagonia, Argentina.

RAIGEMBORN, M.S.; BREA, M.; ZUCOL, A.; MATHEOS, S.

Bilbao, España

Otro; Climate and Biota of the Early Paleogene; 2006

Departament of Stratigrphy and Paleontology, Faculty of Science and Technology

In central Patagonia (Golfo San Jorge basin, Chubut, Argentina), the Río Chico Group (Upper Paleocene-Eocene?) outcrops. This group is composed by three units, from the eldest to the youngest: the Peñas Coloradas, Las Flores and Koluél Kaike formations (Legarreta and Uliana, 1994). This sequence represents a meandering fluvial system which was deposited during the Late Paleocene Thermal Maximum (LPTM) and Early Eocene Climatic Optimum (EECO). In this context, we define the paleoxylological assemblages, found in the Peñas Coloradas Formation in Punta Peligro and Cerro Abigarrado profiles, and phytolith assemblages from the Las Flores formation in the Gran Barranca profile. These data were used to reconstruct the climatic condition that happened during the Early Paleogene in central Patagonia. The outcroppings from the Peñas Coloradas Formation are characterized by having gray, yellowish and reddish epiclastic and volcaniclastic sandstones and silstones, carrying in the lower levels fragments of fossil wood. Clay minerals composition is represented by smectite. The Las Flores Formation is dominated by grayish fine tuffaceous materials showing faint evidence of pedogenesis and minor ash-fall deposits. Clay minerals are represented by kaolinite and smectite. This unit has a rich fossil mammal fauna (SALMA Itaboraiense, sensu Bond et al, 1995), and a noticeable abundance and diversity of phytoliths. One conifer and three angiosperm taxa were assigned, two of which are new additions to the Chubut paleoflora records. The wood flora is integrated by Podocarpoxylon mazzonii (Podocarpaceae), Weinmannioxylon multiperforatum (Cunoniaceae) and two new genus of Styracaceae and Araliaceae (Brea et al, 2004). The phytolith assemblages are characterized by the abundance of dicotyledons phytolith and palm phytolith, associated with Poaceae, Cyperaceae and Podostemaceae phytolith. Sponge siliceous spicules, Chrysostomataceae cycts and diatoms have also been found in these assemblages. Nine families and one Order were recognized: Arecaceae, Chrysobalanaceae, Mimosaceae, Magnoliaceae, Annonaceae, Burseraceae, Zingiberales, Poaceae, Cyperaceae y Podostemaceae (Zucol et al., 2005). In this way, the reconstruction of the vegetation based on palaeobotanical records and sedimentary deposits indicated a warm and humid climate. The palaeovegetation would have conformed humid tropical and subtropical forests, one dominated by dicotyledoneous woody and the other by palms.