CABALERI nora Graciela
congresos y reuniones científicas
Coral facies of the La Manga Formation at the Portada Covunco, Neuquén Basin
Buenos Aires
Congreso; XII Reunión Argentina de Sedimentología; 2008
Institución organizadora:
Asociación Argentina de Sedimentología
During the Late Oxfordian, the Neuquén epicontinental shelf was the site of major coralline evolution and reef building. The colonial scleractinid corals contributed with their skeletons to an extensive and impressive reef development in high latitudinal position suggesting an equilibrated climate. Shallow, warm and relatively clear water enabled the growth and spread of reef corals. They  extended along the southernmost part of the Sierra de la Vaca Muerta, about 400 m long and reach 8 m thick, in the Neuquén basin.  The reef coral facies of the La Manga Fm. at the Portada Covunco, were studied. The bioherms are basically composed by massive, platy, branching, and phaceloid scleractinian over solitary corals that built the framework accompanied by a variety of micro-encrusting and boring metazoans (algae, lithophagus bivalves, Tubiphytes and serpulids).The vertical successions of coral morphotypes represent a shallowing upward trend evidenced by a gradual transition from finger-like colonies to laminar, platy coral. In the associated microfacies a general coarsening upward trend from peloid mudstones to bioclastic wacke/pack/floastones and finally grain/rudstones is also detectable. The result of microfacies analysis show that about 40 % can be considered as coral framestones whereas about 60 % consist of peloid-bioclast-lithoclast-ooid  wackestones-packstones to grainstones. The microfacies model implies an oolithic sand shoal area (packstone ooidal -MF1-) in the highest energy zone and five small coral patch reefs (lens shape) with similar constructional and compositional reef types intercalate in a succession of limestones and marls, developed on the carbonate ramp. 1st stage of coral patch reef growth: peloidal mudstone (MF2) and a framestone of laminoid, platy (microsolenids?) coral (MF3); bioclastic-coral-calcarea sponges wackestones/packstones (MF4); calcareous marls with siliceous sponges and  scarce solitary corals accumulated in depressions; platy corals packstones  and bioclastic-dasycladaceas-packstones (MF5) at the top of the bioherm. 2nd stage: framestone of flabellate-meandriforme and platy coral colonies with microbial crusts growing on the top of the colonies (MF6) in bioclastic wackestones.  Intersticial material is a floastone with coral debris, bioclasts and siliciclstic grains (MF 7). Microbial-crusts point to low energy conditions for the top part of this coral patch reef. 5th stage: bafflestones of phaceloid colonies of dense, tall finger-like coral in calcareous marls (MF8); mudstones/fine bioclastic wackestones. Reefal debris are represented by platy coral rudstones with 5 % of siliciclastic grains. 6th stage: small algal-patch reef of phylloid algal boundstone (MF 9) in microbial-bioclastic wackestones with aligned bioclasts and dark algal laminae. Reefal boundstone show a fine-grained fabric with interwoven filaments forming crust up to several millimeters thick within a micritic matrix with differences in colorations. This alga point to very shallow conditions for the top most part of this patch  reef. Irregular channels are infill with bioclastic-serpulids-wackestone with oriented small fragments.7th stage is the more fully preserved coral reef within the area, characterized by low diversity of corals with dominance of tabular-platy coral which  is replaced upwards by  tall, dense, phaceloid colonies. Such succession repeat several times. In the middle part  of the reef grey calcareous marls with ramose corals, echinoids and small siliceous sponges occur. The intersticial material between corals is a bioclastic-pelecypods-wackestone matrix with serpulids.Towards the top of the reef tabular-platy corals and bioclastic-packstones and lithoclastic-grainstones point to more shallow conditions.