Abstract: Sedimentation History of Late Carboniferous Basin, Cantabrian Mountains, North Spain

R. K. Park, R. H. Wagner

A Late Carboniferous tardigeosynclinal basin in northern Spain, 90 km long and 50 km wide and trending east-west, experienced considerable tectonic activity throughout its life span. Within the thick (12,000 m) sedimentary pile, at least four unconformities are evident. Marginal to shallow-marine environments predominated and rapid lateral facies changes are common, largely controlled by intrabasinal tectonics.

The western margin borders on a rising hinterland, and periodic uplift produced a dissected topography with scree slopes which then was infilled with coarse fluvial conglomerates. Basinward limestone conglomerate fans interfinger with sediments of more marine aspect.

During the first half of its existence, the eastern margin was delimited by a synsedimentary fault. While the fault remained active, a succession of coarsening-upward rhythmic units developed, infilling the basin sags which had been formed. These trough cross-bedded sands and silts contain abundant plant debris plus occasional seat earths and coals and are thought to represent mouth-bar and transgressive-barrier sequences of a deltaic complex.

The fault subsequently was transgressed by onlapping, nonrhythmic marine shales and one paralic coal measure interval, the whole sequence thinning eastward. Marine conditions prevailed throughout the basin long enough to permit the development of biogenic-carbonate banks. These are located over basin-floor swells and along the line of the synsedimentary fault. Their en-echelon distribution indicates continuing basin subsidence. Submarine debris flows represented by pebbly mudstones with flow textures were derived from these banks and from the carbonate platform on the east. These flows give a further indication of tectonic instability within the basin. Further, differential subsidence and overdeepening of isolated areas are responsible for locally developed turbidite facies.

The high rate of sedimentation is characteristic of a molasse trough which here abuts against an intermediate foreland, the Cantabrian block. Intrabasinal tectonics have influenced basin geometry and been responsible for basin sags, deep erosion of the hinterland and consequent supply of valley-fill conglomerates, slide limestones, and abrupt facies changes associated with a complex deltaic history.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA