Coral facies of the La Manga Formation at Portada Covunco, Neuquén Basin, Argentina.

55. BERESI M.S., CABALERI N.G. AND ARMELLA C.,

Mendoza

Congreso; XII Reunión Argentina de SEDIMENTOLOGIA,; 2008

Asoc. Sedimentológica

     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 (Matheos y Morsh, 1990; Legarreta, 1991). They extend along the southernmost part of the Sierra de la Vaca Muerta, about 400 m long, reaching a thickness of 8 m. The reef coral facies of the La Manga Fm at Portada Covunco, were studied. The bioherms are basically composed of 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 shows 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 suggests an oolithic sand shoal area (packstone ooidal - MF1-) in the highest energy zone and five small coral patch reefs (lens shaped) with similar constructional and compositional reef types that intercalate in a succession of limestones and marls, developed on a carbonate ramp. The First stage of coral patch reef growth is characterized by peloidal mudstone matrix (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; (MF5) platy corals packstones and bioclastic-dasycladaceas-packstones at the top of the bioherm.  The Second stage is characterized by a 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 siliciclastic grains (MF 7). Microbial-crusts point to low energy conditions for the top part of this coral patch reef.  In the Third stage there are 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. The Fourth stage is characterized by a small algal-patch reef of phylloid algal boundstone (MF 9) in microbial-bioclastic wackestones with aligned bioclasts and dark algal laminae. Reefal boundstone shows a fine-grained fabric with interwoven filaments forming crust up to several millimeters thick within a micritic matrix with differences in colorations. This alga points to very shallow conditions for the top-most part of this patch reef. Irregular channels are infilled with bioclastic-serpulids-wackestone with oriented small fragments. The Fifth 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 is repeated  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, bioclastic-packstones and lithoclastic-grainstones point to shallower depositional conditions. Legarreta, L. 1991. Evolution of a Callovian-Oxfordian carbonate margin in the Neuquén Basin of west-central Argentina: facies, architecture, depositional sequences and global sea-level changes. Sedimentary Geology, 70: 209-240 Matheos, S. and S. Morsh, 1990. Geochemistry and paleoecological aspects of  coral bearing limestones from the Late Jurassic at the southern end of  Sierra de la Vaca Muerta, Neuquen Basin, Argentina. Neues Jb. Geol. Palaeontology 181 (1-3):153-163 (Stuttgart, Alemania).      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 (Matheos y Morsh, 1990; Legarreta, 1991). They extend along the southernmost part of the Sierra de la Vaca Muerta, about 400 m long, reaching a thickness of 8 m. The reef coral facies of the La Manga Fm at Portada Covunco, were studied. The bioherms are basically composed of 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 shows 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 suggests an oolithic sand shoal area (packstone ooidal - MF1-) in the highest energy zone and five small coral patch reefs (lens shaped) with similar constructional and compositional reef types that intercalate in a succession of limestones and marls, developed on a carbonate ramp. The First stage of coral patch reef growth is characterized by peloidal mudstone matrix (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; (MF5) platy corals packstones and bioclastic-dasycladaceas-packstones at the top of the bioherm.  The Second stage is characterized by a 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 siliciclastic grains (MF 7). Microbial-crusts point to low energy conditions for the top part of this coral patch reef.  In the Third stage there are 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. The Fourth stage is characterized by a small algal-patch reef of phylloid algal boundstone (MF 9) in microbial-bioclastic wackestones with aligned bioclasts and dark algal laminae. Reefal boundstone shows a fine-grained fabric with interwoven filaments forming crust up to several millimeters thick within a micritic matrix with differences in colorations. This alga points to very shallow conditions for the top-most part of this patch reef. Irregular channels are infilled with bioclastic-serpulids-wackestone with oriented small fragments. The Fifth 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 is repeated  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, bioclastic-packstones and lithoclastic-grainstones point to shallower depositional conditions. Legarreta, L. 1991. Evolution of a Callovian-Oxfordian carbonate margin in the Neuquén Basin of west-central Argentina: facies, architecture, depositional sequences and global sea-level changes. Sedimentary Geology, 70: 209-240 Matheos, S. and S. Morsh, 1990. Geochemistry and paleoecological aspects of  coral bearing limestones from the Late Jurassic at the southern end of  Sierra de la Vaca Muerta, Neuquen Basin, Argentina. Neues Jb. Geol. Palaeontology 181 (1-3):153-163 (Stuttgart, Alemania).