Albian-Cenomanian flora changes in Southern Patagonia, Argentina.

ALBA ZAMUNER, DANIEL POIRÉ, ARI IGLESIAS, CONSTANZA KOEFOED, AUGUSTO VARELA

Bonn, Germany

Congreso; 12Th International Palynological Congress - 8Th International organisation of Paleobotany Conference; 2008

Albian-Cenomanian floral changes in southern Patagonia, Argentina   Alba Zamuner1,3, Daniel Poiré2,3, Ari Iglesias2,3, Constanza Koefoed1, Augusto Varela2,3   1División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de la Plata. Paseo del Bosque S/Nº. 1900. La Plata. Argentina 2Centro de Investigaciones Geológicas. 1 Nº 644. La Plata. Argentina 3Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina.  Here we describe patterns of floral evolution within the Austral Basin, southern Patagonia, South America during the lower Upper Cretaceous. During this epoch the basin was dominated by marine sedimentation. However two lithological units, the Piedra Clavada Fm (Albian) and Mata Amarilla Fm (Cenomanian-Coniacian?) contain well preserved litoral to continental deposits. Within the Mata Amarilla Fm, flood-plain sediments contain a characteristically diverse angiosperm-dominated Cenomanian flora. Until now the Piedra Clavada Fm, which would provide the opportunity to understand the transition of vegetation and taxa from middle Cretaceous to Cenomanian floras lacked reported fossils. However, here we describe new taphofloras, corresponding to the uppermost levels of the Piedra Clavada Fm. This unit predominantly comprises yellowish sandstones, dark pelites, heterolithic facies and scarce conglomerates containing petrified woods. Three plant levels are present. The lower one, formed in a pro-delta environment, has ferns, gymnosperms but few angiosperms.  A middle level, formed on deltaic plains contains angiosperms, gymnosperms, ferns and sphenophytes, preserved as impressions, casts and compressions. Based on abundance data, the flora was dominated by sphenophytes and Polypodiaceae with gymnosperms and angiosperms as subordinate elements. Sphenophytes (Equisetites sp) are represented by vegetative organs including articulated branches, stems and rhizomes in life position. Polypodiaceae include a new species that is frequently preserved with fertile fronds possessing one file of round sori, lacking indusium, either side of the midvein. The secondary floral elements, gymnosperms, include conifers and cycads. Scarce angiosperms are recognized including laurophyll, myrtophyll, pentalobaphyll and acrodromophyll morphotypes. The upper level formed in subaquatic channels within the deltaic plain. It preserves impressions, almost exclusively of angiosperms of the same morphotypes as the middle level.  Conifers are subordinate elements. Although there are no strong differences in sedimentary facies between the two upper levels of the Piedra Clavada Fm, the taphofloras show a notable shift in floral dominance and presence/absence of certain groups. Angiosperm leaf morphotypes from the upper levels of the Piedra Clavada Fm are similar to those of the Mata Amarilla Fm. Absence of discordances between these lithologic units probably allows a floristic continuity within angiosperms promoting diversification and dominance of this group through the Cenomanian.