Article: Freshwater algae from the Upper Triassic Cuyana Basin of Argentina: palaeoenvironmental implications

Diverse freshwater aquatic palynomorphs are present among rich terrestrial palynofloras from the upper part of the Potrerillos and Cacheuta formations outcropping in the Cuyana Basin at the southern extremity of the Precordillera of Argentina. The Potrerillos/Cacheuta sequence includes fluvial, deltaic and lacustrine facies deposited in the early Late Triassic fault-bounded syn-rift Cacheuta hemigraben. The phytoplankton described and illustrated consist of representatives of colonial chlorococcalean algae belonging to the Hydrodictyaceae (Plaesiodictyon) and Botryococcaceae (Botryococcus), a diverse group of Zygnemataceae zygospores, a freshwater dinoflagellate cyst (Bosedinia) and acritarchs (sphaeromorphs). Zygnematacean zygospores are represented by species of Gelasinicysta?, Lecaniella, Mougeotia, Ovoidites, Peltacystia and Schizocystia. Of these, Gelasinicysta? cuyanensis sp. nov. is newly described. Quantitative analysis of the palynofloras permits interpretation of changes in the local vegetation and phytoplankton communities controlled by changes in environmental setting. Two algal associations are recognized as belonging to different stages in the evolution of the basin. Diverse zygnematacean assemblages along with hydrodictyacean, botryococcacean and leiosphere/sphaeromorph algae are common in fluvial-deltaic environments of the upper part of the Potrerillos Formation and the lowermost part of the Cacheuta Formation. Higher in the sections (in the Cacheuta Formation) Botryococcus dominates, in association with amorphous kerogen, representing deposition in a relatively deep, quiet-water, lacustrine environment where anoxic conditions prevailed. In the Cacheuta hemigraben the lacustrine shales have average TOC values of 4 per cent (locally reaching 20 per cent), with some terrigenous components (Type II/III kerogen), but dominant amorphous, algal-like, organic matter (Type I/II kerogen). Oils derived from these source rocks are predominantly waxy.