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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Lower Callovian-Middle Oxfordian Facies from the La Manga Formation Carbonate Ramp: A Forced Regression. Atuel Depocenter, Neuquén Basin, Mendoza Province, Argentina R.M. Palma 1 , D.A. Kietzmann 1 , J. Martín-Chivelet 2 , J.López-Gómez 2 , G.S. Bressan 1 1.Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pabellón II (1428); Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) Buenos Aires, Argentina 2. Departamento de Estratigrafía - Instituto de Geología Económica (CSIC-UCM), Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid, Spain The Neuquén Basin was a Mesozoic-Cenozoic retro-arc foreland basin that generated eastwards of the main Andean Cordillera between 36º S and 39º S, at the western margin of the South American platform, bounded by the magmatic arc to the west and by a tectonic foreland to the east. The foreland consisted of the Sierra Pintada belt to the northeast and the North Patagonia massif to the south (Fig.1 a). Four tectonic episodes of this basin development are described by Legarreta and Gulisano, (1989): 1- rifting (Upper Triassic-Lower Jurassic), 2- thermal subsidence (Lower Jurassic-Upper Cretaceous), 3- subsidence due to magmatic and loading (Upper Cretaceous-Lower Cenozoic) and 4- Andean tectonism (Lower Cenozoic -Early Qua- ternary). Much work has been conducted to regional knowledge of the Neuquén Basin (e.g. Legarreta and Gulisano, 1989; Legarreta and Uliana, 1996; among others). The Jurassic succession is part of the lower supersequence of Legarreta and Gulisano (1989) and includes the Precuyo, Cuyo and Lotena mesosequences. The Lotena Mesosequence, developed from Middle Callovian to Upper Oxfordian-Kimmeridgian times, consists of five depositional sequences and includes marine and continental siliciclastic facies (Lotena Fm.), marine carbonate deposits (La Manga Fm.), and evaporites (Auquilco Fm.) (Fig.1 b). Ac- cording to the new data the Lotena Mesosequence is extended to the Lower Callovian. From these, La Manga Formation reflects a widespread carbonate deposition through the basin during the Lower Callovian-Middle Oxfordian interval, in both shallow water mosaic-like environments, and the adjacent deeper-water settings. The unit records sea-level fluc- tuations and changes in the regional marine hydrogra- phic regime (Palma et al., 2007). In this study, La Manga Formation is analyzed at the so-called Atuel depocenter, where is 53 m thick and consists of two informal units (1 and 2), that respec- tively represent outer ramp (1), and intertidal to supratidal facies (2) (Fig. 1 c). The Unit 1 is 46 m thick and consists of peloidal-bioclastic wackestone-packstone with an abundant fauna of ammonites, some gryphaeids, echinoderm fragments, and oncolites, commonly inter- bedded with thinly laminated dark green marls and shales rich in organic matter (Palma et al., 2010). These 302 outer-ramp facies are arranged in meter-scale sedimentary cycles. According to the lack of storm or wave generated structures, the Unit 1 represents a low energy open marine setting. Lamination and organic matter content in the marls suggest that deposition was mainly below storm wave base in outer ramp environment (e.g. Burchette and Wright, 1992). A new biostratigraphy study includes ammonites, in the Unit 1, from the Lower Callovian to Middle Oxfordian (Riccardi, per. comm.). Near the base of Unit 1, a 11 cm thick condensed interval has been recognized. It consists of bioclastic wackestone and shows Thalassinoides-type burrows on the top. It has yielded Rehmannia cf. pau- cicostata (Tornq.) Homoeoplanulites sp. Rursiceras sp. (Upper Callovian-Lower Oxfordian) and Peltocera- toides sp. (Upper Callovian-Lower Oxfordian Pelto- ceratoides-Parawedekindia Zone). That level is overlaid by a Middle Oxfordian succession chara- cterized by Perisphinctes (Arisphinctes) sp. Perisphinctes (Kranaosphinctes) sp., Mirosphinctes sp (Bifurcatus Zone), Perisphinctes (?Antilloceras) cf. prophetae Gygi and Hill., and Perisphinctes (Dichotomoceras) sp., Perisphinctes (?Subdiscosphinctes) sp., and Peri- sphinctes (Kranaosphinctes) cf. Decurrens (Buck.) (Perisphinctes Araucanites Zone). The Unit 2 (7-12 m in thickness) consists of microbial stromatolites, intraformational breccias and discrete beds of flat-pebbles. Stromatolites are chara- cterized by wavy-wrinkled laminations. Subaerial exposure features including tepee structures, bird’s eyes, mud-cracks and sheet-cracks are common. Pala- leokarst surfaces are also present. Usually, microbial stromatolites are overlaid by breccia facies defining small-scale cycles. Breccia clasts show orientation from bedding – parallel to random, and imbrications also occur. Facies from Unit 2 are interpreted as deposited in upper intertidal-supratidal facies of Unit 2 with domi- nance of subaerial exposure. The transition between the outer ramp facies of Unit 1 and the overlying intertidal-supratidal facies of Unit 2 is sharp and reflect an interval of rapid decrease in accommodation causing forced regression. This can be interpreted to represent an abrupt fall in the relative sea-level during the end of the Middle Oxfordian in the Neuquén Basin, which probably precluded the deposi-
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 tion of Auquilco evaporites. Key words: Jurassic; Carbonate ramp; Forced regression; Neuquén Basin; Argentina Fig.1 a, Location map of the Neuquén Basin with indication of the Atuel depocenter; b, Jurassic stratigraphic framework of the Neuquén Basin at Atuel depocenter; c, simplified lithological log of the La Manga Formation at La Manga creek References: Burchette T.P., Wright, V.P. Carbonate ramp depositional systems, Sedimentary Geology, 1992, 79 (1): 3-57. Legarreta L., Gulisano C.A. Análisis estratigráfico de la Cuenca Neuquina (Triásico Superior-Terciario Inferior). In: Chebli G.A., Spalletti L. (ed). Cuen- cas Sedimentarias Argentinas, Tucumán: Sim- posio Cuencas Sedimentarias Argentinas, Serie Correlación Geo- lógica, 6, 1989 : 221-243. Legarreta L., Uliana M.A. The Jurassic succession in West central Argentina: Stratal patterns, sequences, and paleogeographic evolution. Palaeo- geography, Palaeoclimatology, Palaeoecology, 1996, 120: 303-330. Palma R.M., Kietzmann D.A., Bressan G.S., et al. Caracterización de la Materia Orgánica de la Formación La Manga (Oxfordiano), Mendoza. Bahía: IV Simposio Argentino Jurásico, 2010: 36. Palma R.M., López Gómez J., Piethé R.D. Oxfordian ramp system (La Manga Formation) in the Bardas Blancas area (Mendoza Province), Neuquén Basin, Argentina: Facies and depositional sequences. Sedimentary Geology, 2007, 195: 113-134. 303
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