Características de los sedimentos carbonáticos de la segunda transgresión del Triásico Medio (Ladiniense) en la zona central de la Cordillera Ibérica

• M. Pérez-Arlucea ^[1]
  1. [1] Universidad Complutense de Madrid

     Universidad Complutense de Madrid

     Madrid, España

     
• Localización: Revista de la Sociedad Geológica de España, ISSN 0214-2708, Vol. 4, Nº. 1-2, 1991, págs. 143-164
• Idioma: español
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• Resumen
  • español

    Durante el Triásico medio se produjeron dos transgresiones mayores en el Tethys, una que llegó a la parte occidental de la Cordillera Ibérica en el Anisiense superior y otra durante el Ladiniense. Ambas transgresiones se produjeron como consecuencia de la distensión cortical, con subsidencia térmica regional, acompañada de movimientos eustáticos del nivel del mar. La segunda transgresión se ha dividido en dos conjuntos litológicos bien diferenciados que corresponden a la Formación Dolomías de Tramacastilla y a la Formación Dolomías, Margas y Calizas de Royuela. La primera formación comprende facies típicas de rampa homoclinal carbonática, que evoluciona desde una rampa tipo fringing-bank a una rampa de tipo barrier shoal-complex con cinturones de facies más diferenciados. Se observa un progresivo aumento en la extensión horizontal de las facies de plataforma interna de baja energía a lo largo de la Formación Dolomías de Tramacastilla y Dolomías, Margas y Calizas de Royuela, lo que parece significar una disminución progresiva de la pendiente de las plataformas continentales. La segunda formación muestra un estilo cíclico diferente de la primera, con secuencias de menor orden de 2 a 6 m, que presentan influencia terrígena en la base y tendencia general a la somerización hacia el techo.

    Se pueden diferenciar dos Secuencias deposicionales iguales que las descritas en Cataluña por Calvet et al., 1990): Secuencia deposicional 1, que correspondería a la primera transgresión y Secuencia deposicional 2, que comprendería el Muschelkalk medio (lowstand systems tract) y el Muschelkalk superior (transgressive systems tract y highstand systems tract).

  • English

    During middle Triassic times two major transgressions (Ladinian and Anisian in age) which took place over the Tethys domain affected the western border of the Iberian Ranges. Both transgressions occurred as a consequence of a change in subsidence style, from tectonic origined subsidence (lower Triassic), associated to "Tardihercinian" fractures, to regional termic subsidence (middle Triassic). The deposits belonging to the middle Triassic transgression are analysed in this paper. Clasically, two characteristic litological units, later formally defined as the Tramacastilla Dolostone Fm. and the Royuela Dolostone, Marls and Limestone Fm., have been recognized. Several members, described for the first time in this paper, can be distinguished inside these formations. The Tramacastilla Dolostone Fm. is built up by typical facies of a carbonate ramp, which evolves, following a model presented here, from an extense homoclinal fringing-bank ramp type, with bioclastic deposits, to one in which facies belts are better developed, with low- and high-energy inner ramp facies and nodular, storm-influenced distal ramp facies. This evolution is perhaps due to a general gradient decrease of the platform; in fact, the upper part of the Tramacastilla Fm. shows a facies pattern distribution which seems to reflect an even lower gradient environment, with extense low-energy inner platform facies. The Royuela Fm. shows a similar evolution pattern, in this case complicated by cyclic fine-grained clastic inputs. Several laterally continuous shallowing-carbonating upwards sequences, 2-6 m thick, has been recognized.

    The wide distribution of this two major transgressions accross the Tethys domain suggests an eustatic origin, but the causes for cyclicity at different scales are difficult to identify because of the great variety of factors involved in the process (sea-level fluctuations, climatic changes associated with orbital variations -as the ones described by Milankovitch-, tectonic cycles, etc.) and the lack of good biostratigraphic markers. However, two first-order sequences (similar to the ones described by Calvet et al. (1990) for the Catalonian ranges) are here described: Depositional sequence 1, resulting from the first transgression (lowstand: upper Bundsandstein, only partially preserved; and highstand: lower Muschelkalk, represented by the Albarracin Dolostone and Marls Fm.); and depositional sequence 2, represented by the middle Muschelkalk (lowstand systems tract: Tramacastilla Fm. lower part; and highstand systems tract: upper part of the same unit plus the whole Royuela Fm.).