CONICET | Buscador de Institutos y Recursos Humanos

A well preserved assemblage of organic-walled microfossils is described from theAlicia and Cerro Negro formations of the La Providencia Group. The microbiota isdominated by Leiosphaeridia species (L. minutissima, L. tenuissima, L. crassa and L.jacutica) and comprises colonial sphaeroids (Synsphaeridium sp.) and filamentous microfossils of the genus Siphonophycus. The assemblage is assigned to the Late Ediacaran Leiosphere Palynoflora (LELP), which is corroborated by the occurrence in the overlying unit of the discoidal fossil Aspidella. The distribution of microfossils within the La Providencia Group is mainly controlled by the distance to the shore and thus environmental factors. This is indicated by the abundance and size of filaments, which tend to occur near to the paleoshore. Furthermore, large leiospheres also occur in shallower facies together with filaments, in accordance with existing models for acritarch ecology and distribution in the Proterozoic marine successions. The absence of acanthomorphic acritarchs is thus interpreted as a real evolutionary trend, and not as the result of preservational or environmental biases. Regarding the preservation of microfossils, a higher Thermal Alteration Index for the Alicia Formation, compared with older and younger units, is interpreted as the result of hydrothermal overprint and/or contact metamorphism by basic dikes.