Marcelo G. Carrera


A detailed palaeoecological study with sponges and bryozoans was carried out in the Ordovician limestones of the San Juan Formation outcropping in the Precordillera of western Argentina. Ten localities were selected for the study along the basin. The San Juan Formation can be subdivided into four intervals according to the major percentages of sponges and bryozoans. These intervals are partially equivalent to brachiopod and conodont zones. An autoecologic study of the sponges and bryozoans is carried out, considering for each taxon its mode of life and the response to the physical conditions of the environment. The interrelations among species of the same group and between this one and other dominant groups such as brachiopods were also analyzed. On the basis of the recognition of the biovolume of each taxon, their influence in the assemblage could be determined. These studies were done at every locality and in the different sections of the formation, and four associations alternatively considered as biofacies or as communities were recognized. Association 1 (community or biofacies of Archaeoscyphia- Calatiumi represents the community present in shallow bioherms of the intemal ramp in a Trernadoc-Early Arenigian sea level rise. Association II (biofacies of Nexospongia-Nicholsonella¡ represents the association present in the skeletallimestones and tempestites of the mid ramp. Association III (biofacies of stromatoporoids) is found in the reefal and biostromal areas in the inner and middle ramp respectively, during regressive stages. Association IV (biofacies of "elongated" sponges, biofacies of Patellispongia and biofacies of roots-tufts) is developed in shallow and deep reaches of the distal ramp, respectively.Their position in the basin, their environmental conditions, and their bathymetry, were determined. Altogether they indicate a marked stability during the Arenigian and a sea-level drop for the Late Arenigian. The Early Llanvimian (Association IV) is characterized by a relative sea-level rise, although differences in bathymetry were found among the different studied localities. A deeper area toward the north (cerro Viejo region), a shallower area toward the south (Talacasto) and a general deepening toward the west (Las Aguaditas y Las Tunas) were determined.

Full Text:

 Subscribers Only