Oxfordian reefal fancies of the Neuquén Basin, Argentina

BERESI, MATILDE; CABALERI, NORA G.; ARMELLA, CLAUDIA

Mendoza

Simposio; III Simposio Argentino del Jurásico; 2007

CRICYT/IANIGLA

During the Late Oxfordian, the Neuquén epicontinental shelf was the site of major coralline evolution and reef building. Reef forming corals are representative of the Phylum Coelenterata, Class Anthozoa, with dominance of the Order Scleractinia (stony or hard corals).  In the Neuquén Basin, these corals contributed with their skeletons to an extensive and impressive reef development, which extended along the southernmost part of the Sierra de la Vaca Muerta, at Portada Covunco. In the reefal  facies (bioherm), basically  three morphotypes of scleractinians are occurring: solitary calcareous cups; laminar plate coral colonies, which are flat and spread horizontally and on the top, branched corals with fingerlike colonies and a typical bifurcating pattern. In the coralline facies, these morphtypes are succeeding one to the other in an ecological zonation.  .Along the reef structures are present pelecypods, gastropods, echinoderms and algae. Bioerosion on the upper surface of the laminar corals was observed. Scleractinid corals  (Actinastraea, Columnastraea, Australoseris, among other genera) of the reefal facies had restrictive environmental requirements. Most of them could exist only in clear water of normal salinity, no deeper than about 50 m, and temperatures no lower than about 20ºC. On the top of the branched finger-corals a bioclastic wackestone with abundant skeletal thin-shelled pelecypods, equinoid spines, gastropods and worm tubes occur.. A few thousands of meters more to the bioherm, Northwest of Puesto Manzano Escondido, on the top of the Sierra de la Vaca Muerta, a reefal slope facies is exposed, where allochthonous specimens representing the three biohermal morphotypes mentioned, together with fragments of pelecypods, gastropods, scarce siliceous sponges, serpulids, algal crusts and different types of silicified bioclasts, are all embedded within a marly, grey matrix. A few tens of meters towards the East, a fine grained lagoonal facies, well exposed in a quarrel, can be observed.  Oxfordian coral reef facies can be remarkably useful in local and regional stratigraphic correlations and for a better paleoenvironmental characterization of the basin during this time.