Volcanic and tectonic controls of lacustrine and alluvial sedimentation in the Stephanian coal-bearing sequence of the Malpàs-Sort Basin, Catalonian Pyrenees

The infill of a small Variscan basin shows a complex succession of lacustrine and alluvial sediments whose pattern of distribution through time appears to have changed in response to contemporaneous volcanism. Following initial deposition of pyroclastic breccias and extrusion of lavas, small lakes were established on the floor of the basin. Their infills were probably locally controlled by relief on top of the lavas but the pattern was disrupted by phases of mass movement from the basin margin which emplaced debris flows and a sedimentary mélange containing large blocks of limestone and led to extensive soft-sediment deformation. This complex sequence of events culminated in the emplacement of a widespread ash-flow bed. An alluvial plain, established during a period of rather uniform subsidence, was rapidly drowned to give an extensive lake whose progradational infill led to re-establishment of an alluvial plain. The deposition of a widespread airfall tuff coincided with the onset of differential subsidence during which major channel sandstones were stacked in association with poorly drained palaeosols in the east of the area, whilst a mudstone-dominated sequence characterized by well-drained palaeosols developed in the west. A return to more uniform subsidence appears to have coincided with the cessation of volcanic activity.