Sedimentation across the Tywi Lineament, mid Wales

Abstract

Along the southeast edge of the Welsh Basin, faults control sedimentation throughout the Lower Palaeozoic. Two important fault zones in the Llandovery area are the Myddfai Steep Belt and the Tywi Lineament, both of which show several phases of movement that are reflected by subtle changes of sedimentary facies. The Myddfai Steep Belt consists of a relatively simple fault system that defined a palaeoshoreline in Ashgill time, only becoming fully overstepped by a basal Wenlock transgression. The Tywi Lineament, which lies further offshore, is a more complex structure. Facies variations across the lineament were controlled by a fine balance between the sedimentation and fault displacement rates, with the fault tip lines concealed beneath a blanket of shelf sediment. Coarse elastics were resedimented locally in response to rapid displacement on fault strands. Less dram atic fault movements are shown by in situ liquefaction of mudstones, together with a variety of small-scale soft sediment deformation features. The Tywi Lineament was active from at least Ashgill to Pridoli time, and was probably originally an extensional fault, reactivated in a transpressive stress regime. Evidence for possible oblique slip during Ashgill time is found elsewhere along the Lineament.