EVOLUCION y RECAMBIO DE LAS FAUNAS DE PORÍFEROS y BRIOZOOS EN EL ORDOVÍCICO DE LA PRECORDILLERA ARGENTINA

Abstract

"The Argentine PrecordilIera consists of a thick Palaeozoic sedimentary succession, where Cambro-Ordovician rocks show the transition from nearshore carbonate bank through mixed carbonate-silicicIastic slope deposits to basinal clastics. In general, carbonate sedimentation on the platform stopped during the Early Llanvirnian due to regional drowning. However, there are a few occurrences of Middle and Upper Ordovician carbonates, located on topographic highs which developed in the western margin of the central PrecordilIera. The transition between the carbonate bank to the mixed carbonate-siliciclastic deposits is represented by the deposition of graptolitic black shales and mudstones. The shifting environmental conditions brought about a signiftcant change and tumover of faunas. The Arenigian to Early LIanvirnian Bahamian-type carbonates of the San Juan Fonnation are dominated by lithistid sponges and the black shales and mudstones of the Los Azules Formation contain graptolites, brachiopods, trilobites and very few sponges located in carbonate remnants (Archaeoscyphia biofacies). The Caradocian carbonate siliciclastic deposits of the Las Aguaditas Formation are dominated by bryozoans and can be regarded as nontropical, foramol or bryomol carbonates. The possible biotic or abiotic and local or global factors involved in this change are analyzed considering the autoecologic characteristics of both sponges and bryozoans. In the PrecordilIera there are successional changes in reef ecosystems frorn algal-sponge-dominated assemblages in the Early Ordovician to bryozoan dominated associations in the Middle Ordovician. Abiotic factors such as sea level fluctuations, climatic cooling, nutrient supply, silicicIastic influx and volcanigenic activity are analyzed frorn local and global viewpoints. The volcanic activiry in the Early Llanvirnian (K-bentonites in Los Azules Formation), the effect of a sea level rise, and the global climatic cooling by this time, enhanced by the movement of the PrecordilIera towards high latitudes in the Caradocian, are postulated as the main factors involved in the disappearance ofthe sponges and the development to a diversified bryozoan fauna."