This paper reports on the biostratigraphic, paleobiogeographic and paleoecologic aspects of palynoflora recovered from sedimentary rocks preliminarily assigned to the Lago Colhué Huapi Formation, in the Golfo San Jorge Basin. The present palynological assemblage includes Maastrichtian marker species of the austral Proteacidites/Nothofagidites Province such as Quadraplanus brossus and Tubulifloridites lilliei. Typical species of the Maastrichtian paleotropical Palmae Province, Buttinia andreevi and Gabonisporis vigourouxii, are also recognized in the palynoflora. These facts indicate the mixed character of the paleoflora and the prevalence of transitional climatic conditions at the paleolatitude of the Chubut Province in the uppermost Cretaceous. The palynoflora composition suggests the local existence of a warm and low energy freshwater body.

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a b s t r a c tThe freshwater ferns (Salviniales) are well represented in the Maastrichtian deposits of Cerro de los Fragmentos in the headwaters of the Río Chico, Golfo San Jorge Basin. The fossil material of Salviniaceae includes complete megaspore apparatuses with attached microspore massulae and dispersed megaspores, float systems and microspore massulae of Azolla. The new species Azolla colhuehuapensis displays distinctive morphological characters including relative small megaspore apparatuses with easily detached floats and usually attached eglochidiate massulae. The Marsileaceae are represented by the megaspore Molaspora lobata, microspores of Crybelosporites and vegetative remains of Marsileaceaephyllum sp. The assemblage also includes zygospores of the green filamentous algae Zygnemataceae, spores of Marchantiophyta and Monilophyta (Schizaeales), megaspores of Isoetalean affinity and Palmtype angiosperm pollen grains. All these organic microfossils and the associated palynomorphs indicate the presence of a freshwater environment where abundant water ferns were developing and reproducing.

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