Lower Callovian-Middle Oxfordian facies from the La Manga Formation carbonate

PALMA, R.M.; KIETZMANN, D.A.; MARTÍN CHIVELET, J; LÓPEZ GÓMEZ, J.,; BRESSAN, G.S.

Sichuan

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

Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences

The Neuquén Basin is a typical retro-arc 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. 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 sequences are part of the lower supersequence of Legarreta and Gulisano (1989) and include three mesosequences: Precuyo, Cuyo and Lotena. The Lotena Mesosequence consists of five depositional sequences that include marine and continental facies (Lotena Fm.) carbonate deposits (La Manga Fm.), and evaporites (Auquilco Fm.). The Lotena Mesosequence developed from Middle Callovian to Late Oxfordian-Kimmeridgian times. During the Callovian-Oxfordian times, in the Neuquén Basin a widespread carbonate deposition resulted in a vertical and lateral mosaic of shallow-water facies, adjacent to deep-water, calcareous deposits, which are known as La Manga Formation, which  recorded 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 occurs across the Atuel depocenter. In the studied area the deposits of the La Manga Formation (53 m) have been divided into two informal rock units (1 and 2), representing outer ramp (1), and upper intertidal-supratidal environments (2). The Unit 1 consists of peloidal-bioclastic wackestones-packstones with an abundant fauna of ammonites, some gryphaeids, echinoderm fragments, and oncolites. These facies appear interbedded with thinly laminated dark green marls and shales rich in organic matter (Palma et al. 2010). Smaller-scale cyclic arrangements are also recorded within meter-scale cycles. A new biostratigraphy study includes ammonites, in the Unit 1, from the Lower Callovian to Middle Oxfordian (Riccardi per. comm.). Detailed microstratigraphic data allows the recognition of a bioturbated bioclastic wackestone (11 cm in thickness) which it is interpreted as a condensed interval. According to the lack of storm or wave generated structures, Unit 1 represent a low energy open marine setting. Lamination and organic matter contents in the marls suggest that deposition was mainly below storm wave base in an outer ramp environment (Burchette and Wright 1992). The Unit 2 (between 7 and 12 m in thickness) consists of microbial stromatolites, breccias and flat-pebbles. Stromatolites are characterized by wavy-wrinkled laminations, recording subaerial exposure (tepee structures, bird’s eyes, mud cracks and sheet cracks). Commonly, microbial stromatolites underlyed breccia facies. Breccia clasts show orientation from bedding–parallel to random and occasional imbrications. Some levels of flat-pebbles, many of which are highly tabular, are characteristics. Facies from Unit 2 are interpreted as upper intertidal-supratidal in association with subaerial exposure. Some peritidal cycles are capped by a paleokarst surface. The sharp contact between upper intertidal-supratidal facies (Unit 2) and underlying outer ramp facies (Unit 1) represent regressive surface of erosion produced during falling stages of sea-level (forced regression).