Chlorococcalean algae from the Triassic Los Rastros Formation, Argentina

MANCUSO, ADRIANA CECILIA; OTTONE, EDUARDO GUILLERMO

Barcelona, España

Congreso; 4º International Limnogeology Congress; 2007

The Triassic Ischigualasto-Villa Unión Basin is an extensional basin located in the western margin of Gondwana; its origin is associated with the pre-breakup of Pangea during the earliest Triassic. Lacustrine strata of the Triassic Los Rastros Formation consist of several coarsening-upward cycles of black shales, siltstones and sandstones.The succession, at Río Gualo section (La Rioja province) and Ischigualasto ProvincialPark section (San Juan province), 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. The low 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. The high 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 globular or compound colonies showing branching processes or botryoidal form would reflect an increase of oxygen content. The Chlorococcalean development could have promoted, in the course of time, that nutrients initially available were consumed, and a subsequent increase of the oligotrophic conditions of the environment. Geological data and variations in phytoplankton content indicate that the lacustrine system probably evolved from a stretcht of freshwater with eutrophic conditions, into a body with oligotrophic conditions through the middle and upper part of succession.