Definition and Organization of Limestone-Marl Cycles in the Toarcian of the Northern and East-Central Part of the Iberian Subplate (Spain)

摘要

An analysis of the organization of the limestone-marl cycles of the Toarcian sediments located north and east of the Iberian Massif has been carried out. Sections representative of different paleogeographical environments located in the Asturian, Basque-Cantabrian and Iberian basins were selected. Four deepening and four shallowing sequences, with marked isochronism, have been recognized in the whole area. The Toarcian transgression included several deepening and shallowing episodes: the Lower Tenuicostatum deepening and shallowing sequences; the Tenuicostatum-Serpentinus deepening sequence; and the Upper Tenuicostatum shallowing sequence. Maximum bathymetric values were reached during the Serpentinus-Bifrons deepening sequence which is followed by the Bifrons-Variabilis shallowing sequence, bounded by a regional unconformity. The Thouarsense-Insigne deepening sequence, which represents a new flooding of the platforms, is followed by the Pseudoradiosa-Aalensis shallowing sequence which continues into the Aalenian. Isochronism is more evident in the tops of the deepening upward sequences, located in the upper part of the Tenuicostatum Zone, within the Bifrons Zone and in the upper part of the Insigne Zone. Results are homogeneous in sections having different evolutionary histories, suggesting that eustatic sea level change is the dominant factor in the generation of the sequences. In the area of research, tectonic activity does not impede recognition of the higher rank sequences.