IANIGLA   20881
INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Chlorococcalean algae from the Triassic Los Rastros Formation, Argentina
Autor/es:
MANCUSO, ADRIANA CECILIA; OTTONE, EDUARDO GUILLERMO
Lugar:
Barcelona, España
Reunión:
Congreso; 4º International Limnogeology Congress; 2007
Resumen:
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.