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

Resumo

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.).