Characterization of the Oxfordian coral-reef types, at Portada Covunco,Neuquen Basin.

BERESI, MATILDE; CABALERI, NORA; ARMELLA, CLAUDIA

Mendoza, Argentina

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologist

During the Oxfordian coral-reefs flourished at mid-palaeolatitudes on the Neuquén epicontinental shelf. The reef facies of the La Manga Formation extending along the southernmost part of the Sierra de la Vaca Muerta, at Portada Covunco, have been studied to identify and characterize different types of coral reefs. Sedimentological and paleoecological observation lead to the reconstruction of a coral reef tract in a shallowing-upward succession evidenced by the associated microfacies which show a coarsening upward trend from peloid mudstones to bioclastic wacke/pack/floastones and finally grain/rudstones. The fauna is dominated by scleractinian corals. Additional benthic faunal elements are siliceous sponges with calcareous preservation and coralline sponges, brachiopods, bivalves and some cidaroid echinoids. An encrusting and boring fauna composed by serpulids, bryozoans, and lithophagid bivalves occurs on the undersides of corals and, partly on sponges. Microbial crusts overgrow the sponges or the skeletal hardparts. Seven generic reef types are recognized in the vertical ecological zonation: 1) platy microsolenids (Microsolena?) associated to peloidal mudstone matrix; 2) reefal framestone of tall dense phaceloid colonies developed in pure carbonate muds; 3) mixed coral-siliceous sponge ( hexactinellids and “lithistids”) reef with echinoids in marly facies; 4) branching ramose coral (Actinastrea?) developed in marly facies; 5) coral- microbial crust reef dominated by thamnastero-meandroide massive colonies (Australoseris radialis?), ramose corals and brain corals, and some small siliceous sponges in microbial-peloidal packstones with dense micritic crusts; 6) platy- microsolenid corals associated with bioclastic wackestone with abundant and diverse skeletal material (cidarid echinoids, bivalves, gastropods, brachiopods, coralline sponges, bryozoans, serpulids, Tubiphytes); 7) tall dense phaceloid colonies (organ-pipe corals) developing among sand-shoal and coral debris channels. Such succession is repetitive. The microfacies model implies an oolitic shoal in the highest energy zone at the base of the coral facies.The intersticial material between corals is a bioclastic-pelecypods-wackestone matrix with serpulids. Towards the top, tabular-platy corals are replaced by tall, dense phaceloid colonies associated to lithoclastic-grainstones which point to shallower depositional conditions of this upper Jurassic coral reef. This reef facies can be remarkably usuful in local and regional stratigraphic correlations and for a better paleoenvironmental interpretation of the basin during this time. This reefal succesion can be correlated with the “global carbonate reef event”, which occurred widespread within the epicontinental seas bordering the northern Tethyan Ocean and the marginal basins of the young North Atlantic Ocean.