Stratigraphy, petrography and dispersion of the lower Permian syn-eruptive deposits in the Viar Basin, Spain

This paper presents a stratigraphic, petrographic and sedimentologic analysis of felsic syn-eruptive deposits of the southernmost Permian basin in Europe (Viar Basin), an inner-orogenic, transtensional half-graben located between two major geotectonic zones of the Iberian Variscan Belt, SW Spain. The syn-eruptive deposits of the Viar Basin represent a mappable lithostratigraphic unit, that is herein named the Los Canchales Formation. This unit forms a wedge-shaped body bounded by alluvial–fluvial deposits (red layers) that are in gradational contact. The stratigraphic architecture of the Los Canchales Formation is complex with frequent facies changes and interfingering among pyroclastic rocks, resedimented–volcaniclastic rocks and sedimentary rocks. Abundant fossil flora is also recorded.A wax–wane cycle of volcanic activity is recognized during the syn-eruptive period. Two short-lived volcanic episodes were recorded including ignimbrites and a possible phreatomagmatic event. Explosive volcanism generated pyroclastic flow, surge and ash-fall deposits, in cases collapsing the drainage network. They also modified the vegetation and created a new depositional setting with four successive sedimentary environments: vegetated floodplain, ephemeral sandy braidplain, palustrine environment and again vegetated floodplain. Input of pyroclasts followed a NW to SE longitudinal–axial dispersal pattern.