Chlorococcalean algae from the Triassic Los Rastros Formation, Argentina

MANCUSO, A.C; OTTONE, E.G.

Barcelona

Congreso; IV International Limnological Congress; 2007

Universitat de Barcelona

Lacustrine strata of the Triassic Los Rastros Formation at Río Gualo section (La Rioja province) and Ischigualasto Provincial Park section (San Juan province), Ischigualasto- Villa Unión Basin, yield a distinctive palynological assemblage of miospores and Chlorococcalean algae. The miospore association is characterized by a relative abundance of corystosperm pollen grains with subordinate inaperturates (Araucariaceae?), diploxylonoid disaccates (Podocarpaceae?), spores of pteridophytes, lycophytes and sphenophytes, monocolpates, monosaccates and striate pollen grains. The phytoplankton is represented by the Chlorococcalean Botryococcus and Plaesiodictyon. Botryococcus/ Plaesiodictyon ratio at the base of the sequence and the local presence of abundant microscopic, ill preserved filaments of probable algal affinity would presumably reflect a milieu of moderate eutrophic conditions. The presence of structureless mass of Botryococcus and coenobia of Plaesiodictyon lacking any dehiscent slit in its cells would indicate stressed conditions or restricted access to oxygen during deposition. B/ P ratio through the middle and upper part of the sequence, should indicate a change of the lacustrine system into a body with oligotrophic conditions. The presence of structureless mass of Botryococcus together with well preserved, simple or globular colonies compound colonies showing branching processes or botryoidal form would reflect an increase of oxygen content.