Middle Triassic carbonates of Eastern Iberia (Western Tethyan Realm): A shallow platform model

Abstract

Much of the eastern sector of the Mesozoic Iberian basin belongs to the ‘Mediterranean type’ Triassic. This type is characterized by a Middle Triassic stratigraphic record comprised of two carbonate Muschelkalk units (lower and upper) separated by a detrital-evaporite middle Muschelkalk unit. In the present study areas, the two first marine flooding episodes of the Middle Triassic led to the deposition of extensive shallow-water carbonates on a block-controlled epicontinental platform in the lower and upper Muschelkalk units. These carbonates were part of third-order transgressive–regressive sequences. The two carbonate sequences display similar vertical facies and stratigraphic sub-unit arrangements. The base of the two sequences is marked by a (lower) carbonate-marl alternation, which was deposited in mixed tidal flat-to-lagoon settings. In the middle of the sequences, the carbonate character is predominant and facies associations mainly suggest shallow waters and high environmental variability. The change from a transgressive to regressive trend occurs in the middle of the sequences, although the position of the maximum flooding zone can vary from one to another stratigraphic section. The tops of the sequences show two different lithological assemblages, which are coeval and grade laterally to each other. One assemblage is a ‘carbonate terminal complex’ formed by tidal flat and sabkha deposits, which is predominant in the lower Muschelkalk unit. The other assemblage is a lagoon-peritidal (upper) carbonate-marl alternation, predominant in the upper unit. Together, the two sequences suggest a complex platform morphology with a mosaic facies distribution, which evolved from a not very high energy ramp-like profile (transgressive phases, TST) to a lower energy rimmed shelf, ending with tidal/lagoon environments (regressive phases, HST). Facies associations, however, suggest somewhat deeper depositional settings for the upper than lower Muschelkalk unit. Here we compare the two carbonate units in the study areas with those described in the literature from other Triassic basins of Eastern Iberia and the SW Germanic Basin, all forming part of the Peri-Tethys area of the Tethyan Realm.