8 research outputs found
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
The processes and products of submarine erosions along the southeastern margin of the Southern Welsh Basin : a response to Telychian tectonism
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