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The evolution and turnover of the Ordovician sponge and bryozoan faunas in the Argentine Precordillera Evolucion y recambio de las faunas de poriferos y briozoos en el Ordovicico de la Precordillera Argentina


Ameghiniana, 343: 295-308
The evolution and turnover of the Ordovician sponge and bryozoan faunas in the Argentine Precordillera Evolucion y recambio de las faunas de poriferos y briozoos en el Ordovicico de la Precordillera Argentina
Cambro-Ordovieian rocks show the transition from nearshore carbonate bank through mixed carbonate-siliciclastic slope deposits to basinal clastics. In general, carbonate sedimentation on the platform stopped during the Early Llanvimian 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 Precordillera. 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 significant change and turnover of faunas. The Arenigian to Early Llanvimian Bahamian-type carbonates of the San Juan Formation 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 carbonatesiliciclastic 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 Precordillera there are successional changes in reef ecosystems from 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, siliciclastic influx and volcanigenic activity are analyzed from local and global viewpoints.


Accession: 038760775



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