Palynology and paleoenvironment of the Cisuralian Vitiacua Formation in southern Bolivia

DI PASQUO, M.M.; GRADER, G.; BREEDLOVESTROUT, R.,

Lexington

Encuentro; 45 Annual Meeting of AASP (The Palynological Society) and CIMP (Commission Internationale de la Microflore du Paléozoïque Subcommissions),; 2012

University of Kentucky

The Vitiacua Formation was previously described comprising of three transgressive restricted marine cycles ranging from lower to upper Permain and Triassic age. Similar units were correlated into Peru and associated with widespread volcanism and extension. This study presents new stratigraphic and palynologic data of the Vitiacua Formation from three locations in southern Bolivia: the La Yesera area (West and Centre) and at Canaletas near Tarija. These outcrops overlie massive sandstones beds of the eolian to estuarine Cangapi Formation and are composed of thin grey and green silty micritic limestone beds that are interbedded with faintly laminated fissile shale with some silt and chert lenses.Overall thicknesses of the three sections are ca. 300 m. Eight productive samples (of 14 samples processed) yielded 62 species of terrestrial palynomorphs of which 17 are spores (11 trilete and 6 monolete), 42 are pollen grains (11 monosaccate and bisaccate non-striate, and 31 striate and costate), and 3 species of algae. The well preserved Yesera Centre assemblage (YCA, one sample) is defined by the presence of several species of Vittatina, Lunatisporites, Pakhapites and Lueckisporites virkkiae (Potonié and Klaus) Klaus together with other species of Hamiapollenites, Mabuitasaccites, Striomonosaccites, Striatoabieites, Striatopodocarpites, Weylandites, and Botryococcus brauni Kützing. The well preserved Yesera West assemblage (YWA, three samples) is characterized by abundant to frequent monolete (e.g., Polypodiisporites mutabilis Balme, Reticuloidosporites warchianus Balme, Thymospora rugulosa Mautino et al.) and trilete spores (Lundbladispora braziliensis (Pant and Srivastava) Marques Toigo and Pons emend. Marques Toigo and Picarelli, Convolutispora uruguaiensis Mautino et al.) with subordinate algae and pollen grains (e.g., Pteruchipollenites, Vitreisporites). The Canaletas assemblage (CA, four samples) was poorly preserved but contained scarce pollen grains including Lueckisporites virkkiae, Striatoabieites multistriatus (Balme and Hennelly) Hart, Pteruchipollenites indarraensis (Segroves) Foster, and Botryococcus brauni. The YCA and CA are correlated to the Lower Member Assemblage (mid Asselian to early Sakmarian) of the Copacabana Formation at Apillapampa in central Bolivia. However, the samples collected at Yesera West are similar to the overlying Copacabana Coal Member (early Sakmarian- Artinskian?). Diverse pollen grains (striate and non striate) of gymnospermous affinities are dominant in the YCA and CA, whereas notable pteridophytes, sphenophylls and lycopods are dominant in the YWA. These groups of plants characterized the terrestrial landscape of forests under a temperate climate belonging to the Glossopteris Flora during the Cisuralian in southern Bolivia. Algae such as Botryococcus indicate the development or input from fresh water environments (lakes and rivers). Calcareous muddy rocks, stromatolites, pyritization of palynomorphs in sample CICYTTP-Pl 333 (VI3) at Canaletas and abundant fish teeth suggest shelf marine conditions, although not clear is the extent of hyposaline conditions. Some stratigraphic interpretations suggest extensive deepwater marine environments. However a paucity of marine macrofauna, significant volcaniclastics, rare paleosols and terrestrial palynoassemblages suggest cyclic, restricted mud-prone sedimentation in a back arc or rift setting. These new stratigraphic data and palynology results from the lower Vitiacua Formation support correlative relationships and facies changes with respect to the upper Coal Member of the Copacabana Formation, in southern and central Bolivia. Radiometric data from interbedded tuffs in both places (Yesera and Canaletas) are under preparation.