REDISCOVERY OF PALEOCENE MACROFLORAS IN CENTRAL PATAGONIA, ARGENTINA

IGLESIAS WILF JOHNSON ZAMUNER MATHEOS CUNEO

Denver

Congreso; Geological Society of America Annual Meeting; 2007

Geological Society of America

In contrast to the abundant North American record, confirmed Paleocene macrofloras from South America are scarce. We report a sequence of four well-preserved Paleocene compression floras, all either new or investigated for the first time in 70 years, from central Patagonia, Argentina. The floras come from three early/middle Paleocene levels of the Salamanca Formation and one level of the overlying, middle Paleocene Peñas Coloradas Formation in the San Jorge Basin. Their floristic, taphonomic, and sedimentological characteristics reveal distinct types of paleocommunities reflecting varying marine influence. From the Salamanca Fm., the Palacio de los Loros flora derives from infilling of high-sinuosity fluvial channels as well as oxbow lakes. This assemblage preserves angiosperm fruits, flowers, and seeds; ferns, and conifer leaves, cones, and seeds. At a slightly lower level, in the nearby Ormaechea Petrified Forest, is found a rich flora of angiosperm leaves as well as ferns, and conifer leaves and seeds, from a delta plain environment. Lower in the formation, in fully marine beds from a tidal flat, possibly mangrove environment, the Rancho Grande flora preserves a completely distinct, coastal plant community of low diversity, featuring small, thick-petioled leaves with low-rank venation. From the Peñas Coloradas beds, the Las Flores flora is currently the only compression assemblage known, and its deposition stems from a high energy fluvial environment related to a rapid fall in base level. It preserves a diverse and distinct assemblage of angiosperm leaves and flowers, as well as ferns and insects. These data represent the first stratigraphically controlled, quantitatively collected sequence of Paleocene floras in South America and provide critical information about plant diversity and composition between the end-Cretaceous event and the onset of extremely high plant diversity in Patagonia by the early Eocene. The samples reveal considerably greater species richness than was previously known from Paleocene Patagonia, including more than 43 species of angiosperm leaves. All except the unusual Rancho Grande sample are more rich than U.S. Paleocene analogs, pushing back the timeline for the origins of high South American floral diversity.