Geology of the country around Llanilar and Rhayader : memoir for 1:50 000 geological sheets 178 and 179 (England and Wales)

This memoir describes the geology of a corridor across central Wales extending from Cardigan Bay in the west, across the Cambrian Mountains, to Llandrindod Wells in the east. As such, it provides a detailed, east–west transect across the central part of the Lower Palaeozoic Welsh Basin onto the western edge of the Midland Platform. The exposed basin fill is dominated by a variety of mudand sand-dominated turbidite systems of latest Ordovician to early Silurian age. Using data from the mapping, and palaeontological and sedimentological studies, the memoir represents the first detailed sedimentary architecture for this part of the basin. It details the characteristics of each turbidite system and discusses the depositional processes involved. Factors governing the evolution of the various systems include the direction and rate of sediment supply, eustasy, and intrabasinal growth faulting resulting from the reactivation of basement structures. Similar, but structually dislocated, turbidite systems of mid- to late-Ordovician age are exposed near the basin margin. Mid-Ordovician to early Silurian shelf sequences, including the richly fossiliferous mudstones and volcanic tuffs of the Ordovician Builth Inlier, are also described. Computer modelling of the regional gravity and aeromagnetic databases has provided east–west profiles across the district, and located the position and approximate depths of several of the basement faults. The structural history of the region, dominated by the low-grade metamorphism and deformation of the basin fill during the Acadian Orogeny (early to mid- Devonian), receives comprehensive treatment. Several of the late stage faults contain lead, zinc and silver ore; the disused mines form part of the Central Wales Mining Field. The geological setting, mineralogy and genesis of these mineralised veins is described. Glacial deposits are widespread across the district, and include tills derived from the local Welsh ice and the Irish Sea ice sheet

Synchronous very low-grade metamorphism, contraction and inversion in the central part of the Welsh Lower Palaeozoic Basin

A metamorphic map (white mica crystallinity) based on 663 data points has been generated for the recently published British Geological Survey 1:50 000 geological sheets 178 (Llanilar) and 179 (Rhayader). Epizone, upper anchizone, lower anchizone and sub-anchizone are delineated. Metamorphic cross-sections are drawn and superimposed on structural cross-sections. Assumptions made in order to draw metamorphic cross-sections are: (1) the anchizone spans 100°C; (2) the metamorphic field gradient was 36°Ckm−1; (3) metamorphic surfaces were initially generated parallel to the contemporary ground surface. Restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults, development of back-thrusts, footwall short-cuts and limited growth of precursor folds had taken place. Metamorphism continued locally during later stages of the contractional phase as a strain-related phenomenon. Finally, tensional faulting followed the cessation of folding and metamorphism