INGEOSUR   20376
INSTITUTO GEOLOGICO DEL SUR
Unidad Ejecutora - UE
artículos
Título:
Late Miocene Continental and Marine Palynological Assemblages from Patagonia
Autor/es:
BARREDA, GULER, PALAZZESI
Revista:
Developments in Quaternary Sciences
Editorial:
Elsevier
Referencias:
Lugar: Amsterdan; Año: 2008 vol. 11 p. 343 - 350
ISSN:
1571-0866
Resumen:
During Miocene, Patagonian vegetation structure became closer to that of the present day. Early Miocene times were marked by the initial rise of modern families in Patagonia belonging to Asteraceae (sunflower family) and Poaceae (grasses), in a general context of the southern forests (Nothofagaceae, Podocarpaceae)(Barreda, 1993, 1996). By Middle-Late Miocene times shrubby-herbaceous groups increased consistently in abundance and diversity with widespread arid-adapted communities by the latest Miocene (Guler et al., 2001; Palazzesi and Barreda, 2004). Last demises of some megathermal elements were also recorded at this time. Several forcing factors may have influenced Miocene vegetation trends to their current establishment. Particularly important was the development of the Antarctic Circumpolar Current, which would have prevented equatorial currents from penetrating into southern Polar Regions. The result was the thermal isolation of Antarctica and the development of the major ice caps on this continent by the Late Miocene. This process caused an abrupt sea level drop, and a general decrease in moisture availability. Based on global deep-sea oxygen isotope record, after the Middle Miocene climatic optimum a long trend towards cooling conditions was established (Zachos, 2001). The mounting building episodes (diastrophic phases) are also thought to have had significant impact on the global climate, having contributed to a general trend of increasing aridity and development of arid-adapted communities. The pattern of changes in continental configuration and ocean circulation coupled with reductions in atmospheric CO2 concentrations, led to a decline in global temperatures all through the Neogene. Thus, inland areas became progressively arid and wide regions of shorelines were exposed due to the sea level fall. Palynological records from the Patagonian Late Miocene times are limited. Pollen is particularly susceptible to oxidation, and arid landscapes tend to leave poorer records. Hence, Late Miocene paleoenvironments were not particularly suitable for fossil pollen preservation. However, some data are available from estuarine and shallow marine deposits of northeastern Patagonia. The data here analyzed came from outcrop samples from Barranca Final, Río Negro and Puerto Madryn Formations at northeastern Patagonia, representing the southernmost deposits of the “Entrerriense” sea (Guler et al., 2002; Guler, 2003; Palazzesi and Barreda, 2004). Puerto Madryn Formation was deposited about 10 Ma ago considering the Sr87/Sr86 age based on pectinid shells (Scasso et al., 2001).This date coincides with a previous K/Ar determination of a tuff level from the upper part of the marine sequence (Zinsmeister et al., 1981). The Barranca Final Formation was Middle-Late Miocene ranged on the basis of foraminifera stratigraphic distributions (Malumian, 1998). Vertebrate studies from high levels of the upper Member of the Río Negro Formation suggested an Early Pliocene Montehermosense Mammalian age for this deposits. Other data came from cutting samples of boreholes drilled in the Colorado Basin, penetrating Barranca Final Formation (Gamerro and Archangelsky, 1981; Quattrocchio and Guerstein, 1988; Guerstein, 1990a-c; Guerstein and Quattrocchio, 1988; 1991; Guerstein et al., 1995; Guerstein and Guler, 2000; Guerstein and Junciel, 2001; Guler et al., 2001). These palynofloras are difficult to date with precision (cutting samples), though they are adequate to compare among other palynological data. In spite of the scarcity of suitable pollen data, the main trends of the Late Miocene Patagonian vegetation are sketched in this chapter.