Stratigraphic-tectonic relations in Spain's Cantabrian Mountains; fan delta meets carbonate shelf

Stratigraphic patterns indicate that the Hercynian orogenic belt of northwestern Spain underwent an important reorganization in the latest Carboniferous Period. The passive margin of the Cantabrian foreland basin, initially far removed from southwestern mountains, was the early site of a major carbonate platform. This distant margin, however, experienced a late phase of deformation directed from the north. Limestones of the passive platform then became the thrust-faulted source area for three successive fan deltas, which prograded across the remaining carbonate shelf. The fan deltas are comprised of conglomerates and coarse sandstones of several areally restricted Upper Carboniferous formations. Their distribution identifies a local subbasin in the Picos de Europa Province, and paleocurrent data confirm a new northern source of sediment. These coarse-grained deposits are interpreted to have formed in direct response to tectonic uplift along a nearby thrust-fault system. The stratigraphic succession spans a 7 Ma interval from latest Moscovian to Late Kasimovian time and defines three separate tectonic episodes: late Myachkovskian (304 Ma), early Chamovnicheskian (300 Ma), and early Dorogomilovskian (298 Ma). Rather than advancing basinward, though, the fan deltas shifted laterally during this period. They become younger in an eastward direction, that is, parallel to the structural grain. The underlying subregional unconformity, likewise related to thrust faulting, also becomes younger to the east. The thrust-fault system apparently propagated laterally by means of three consecutive tectonic episodes, which led in turn to the sideward displacement of sediment source area and fan-delta depocenter.