Structural controls on Palaeozoic basin evolution and inversion in southwest Wales

Abstract

The tectonic evolution of SW Dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode. Movements along long-lived Palaeozoic extensional faults were reversed during Variscan compression, with the pre-existing faults accommodating the contraction in addition to the development of a thin-skinned thrust and fold system. Basin analysis of the pre-Variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults, thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements. Field examples on all scales illustrate active extension during the Palaeozoic. Structural cross sections through SW Dyfed demonstrate the interrelationship of the extensional and contractional deformations, with palinspastic restorations of the cross sections illustrating the complexity of the pre-Variscan template.

Process regime change on a Silurian siliciclastic shelf: controlling influences on deposition of the Gray Sandstone Formation, Pembrokeshire, UK

“…The inliers are exposed within the cores of anticlines related to the inversion of long-lived extension faults during the Variscan Orogeny (Dunne 1983;Powell 1987Powell , 1989.…”

“…The study area lay on the southern shelf of the Welsh Basin, being separated from correlative deep-marine environments to the north by the Welsh Borderland Fault System and the Benton Horst in Pembrokeshire (Figures 1A, 2;Dunne 1983;Powell 1989;Aldridge et al 2000;Cherns et al 2006). During Llandovery and Wenlock times, the Welsh Basin was subjected to transtension-related rifting (King 1994;Butler et al 1997), generating a series of half-grabens that accommodated sediment accumulation.…”

“…On the southern shelf, over 1000 m of Skomer Volcanic Group sediments and interbedded volcanic extrusives accumulated within the Skomer Sub-basin (Ziegler et al 1969;Bridges 1976; Thorpe et al 1989;Hillier 2000), being synchronous with rifting to the east in the Usk and Woolhope Sub-basins ( Figure 2A; Butler et al 1997). The Skomer Subbasin is presently defined by a series of inverted normal faults, namely the Wenall, Ritec and Benton Faults (Figure 2B,C;Powell 1987Powell , 1989. During a probable phase of thermal subsidence across the Skomer Sub-basin, the area was transgressed, with deposition of the mudstonedominated, open marine shales of the topmost Skomer Volcanic Group (Walmsley and Bassett 1976).…”

“…Bioturbation is generally Bassett et al 1992). (B) Presentday, simplified cross-section across southern Pembrokeshire (modified after Powell 1987Powell , 1989. (C) Restored section revealing pre-inversion, rifted geometry to Skomer Sub-basin.…”

Depositional environment and sequence architecture of the Silurian Coralliferous Group, Southern Pembrokeshire, UK

“…Internally, these blocks exhibit structural homogeneity, more specifically fold geometries and cleavage development, as recognized by Hancock et al (1981Hancock et al ( , 1982Hancock et al ( , 1983. The major WNW-ESE-trending faults were shown by Powell (1987Powell ( , 1989, and subsequently by Hayward and Graham (1989), to be intrinsically related to the inversion of preexisting extension faults. Two types of fault were recognized (Figures 3 and 4), namely those still demonstrating an extensional throw subsequent to inversion (the Benton, Wenall and Ritec Faults), and their corresponding footwall shortcuts (the Johnston and Musselwick Thrusts).…”