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; D.A. KIETZMANN; J. MARTÍN-CHIVELET; J.LÓPEZ GÓMEZ,; , G.S. BRESSAN

Sichuan

Congreso; The 8th International Congress on the Jurassic System,; 2010

Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences

The Neuquén Basin is a typicalretro-arc foreland basin that developed to the east of the Cordillera Principal between 36ºS and 39ºS. It was developed at the west margin of South American platform and limited by a magmatic arc to the west and 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.1a). 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 Quaternary). 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.  he 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.1b). 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 fluctuations and changes in the regional marine hydrographic regime (Palma et al., 2007).  Well-exposed transgressive and regressive sequence of the La Manga Formation Formation occurs across the Atuel depocenter. In the studied area the deposits of the La Manga Fm. (53 m) have been divided into two informal rock units (1 and 2), that respectively represent outer ramp (1), and intertidal to supratidal facies (2) (Fig. 1c). 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 interbedded with thinly laminated dark green marls and shales rich in organic matter (Palma et al., 2010). These outer-ramp 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