ABSTRACT

The Iberian Pyrite Belt (South Portuguese Zone, Hercynian Chain), an important metallogenic province, has a volcanic-sedimentary origin. Overlying this volcanic succession is a thick sedimentary unit, the Culm Group, which represents postvolcanic Paleozoic sediments of the Iberian Pyrite Belt. It consists of three stratigraphic units: the Basal Shaly Formation (BSF), the Culm Facies Turbiditic Formation (CFTF), and the Shallow-platform Sandy Unit (SPSU). The BSF is represented by finegrained sediments of volcanic to nonvolcanic origin that mark the end of volcanism in the region, the reworking of volcanic products in a shallow-marine basin, and the beginning of autochthonous sedimentation of pelagic clay. It constitutes a depositional sequence sensu Mitchum et al. (1977). The d stribution, facies, and facies associations of the turbidites of the CFTF are related to the configuration of the basin, which controlled the mechanisms of deposition and distribution of the detritus from different source areas. The SPSU represents sediments eroded from the volcanic upland and redistributed over the shelf.

Detailed study of these three units and their relationships provides the means to define the geometry and evolution of the postvolcanic Carboniferous basin of the Iberian Pyrite Belt. The basin was roughly subdivided by the Paymogo and the Puebla de Guzman paleoridges. Together with the allochthonous Ossa-Morena Zone, north of the study area, they formed three topographic barriers that delineated two interconnected subbasins.

Sedimentation in the postvolcanic basin of the Iberian Pyrite Belt can be related to a model of oscillating sea level. Synorogenic characteristics of the Culm Group sediments support a tectonic origin for the sea-level oscillations, particularly since the sediments had already been affected by the first pulses of the Hercynian Orogeny.