Inversion and exhumation of the St. George's Channel basin, offshore Wales, UK

Abstract

The western UK basins of the Irish Sea have provided one of the best natural laboratories for investigating the causes and consequences of intracratonic uplift and erosion (exhumation). To date, the emphasis has been on igneous underplating as the chief process driving their exhumation. In this paper, we demonstrate that tectonic inversion -the shortening of formerly extensional basins and reactivation of their constituent faults -dominated the exhumation of the St. George's Channel basin, offshore Wales. Based on mapping of an extensive 2D seismic grid, evidence is presented for at least two major inversion episodes in the Late Cretaceous and the Neogene, plus minor shortening during the Eocene. Inversion was distinctly noncoaxial, especially during the Neogene when coeval transpression and transtension was focused at discrete bends and stepovers on the basin-bounding St.George's, Bala and Northwest Flank faults. That the principal mechanism driving these uplift episodes was inversion (as opposed to igneous underplating) is corroborated by analysis of thermal history data (apatite fission track and vitrinite reflectance). They reveal late Cretaceous and Neogene geothermal gradients that were comparable with the present day i.e. no significant increase in basal heatflow. Sonic velocity profiles logged in hydrocarbon boreholes constrain the minimum thickness of the eroded section which varies between c.1000m in the centre and c.2240m at the margins of the basin. Given the strength of evidence for tectonic inversion in the St. George's Channel basin, our favoured model invokes superimposition of the effects of inversion and igneous underplating to account for the complex exhumation history of the St. George's Channel basin in particular, and the western UK basins in general. KEYWORDS:ABBREVIATED TITLE: Inv. & Exh. of the SGCB 2 2 Intracratonic basinsadjacent to 'passive' ocean margins undergo uplift in response to a diverse range of processes related chiefly to i) epeirogeny (i.e uplift of broad regions of continental interiors), and ii) intraplate shortening and tectonic inversion. Sedimentary basin inversion, the compressional or transpressional reactivation and shortening of formerly extensional basins, can have a marked influence on the structure and evolution of the basin fill. It follows that inversion has major implications for the exhumation of petroliferous basins, where the attendant cooling and lithostatic pressure release affects hydrocarbon generation and retention directly (e.g. Dore et al. 2002).Reconstructing the tectonic history of the western UK basins is hampered by the difficulty in recognizing and discriminating the superimposed effects of epeirogeny and inversion. This paper reports on the results of recent mapping of an extensive seismic dataset imaging the SE margin of the St. George's Channel basin (SGCB; Fig. 1). The focus of this mapping has been to determine the geometry, kinematics and timing of deformation associated with the Cretaceous-Cenozoic inversion of the NW European 'A...

Evidence for kilometre-scale Neogene exhumation driven by compressional deformation in the Irish Sea basin system

“…This episode is recorded by seismic reflection data and AFTA results from the Central Irish Sea Basin (Duncan et al 1998;Green et al 2001a) and SW Wales (Williams et al 2005). In contrast to the en echelon CBB, the main depocentre of the SGCB appears to have been unaffected by this episode.…”

“…The Mesozoic rocks of the CBB and SGCB are overlain, often with marked unconformity, by Cenozoic sediments which attain a thickness of 0.52 km in the CBB at Mochras, and locally exceed 1.5 km in the SGCB (Tappin et al 1994;Williams et al 2005). The Cenozoic succession at Mochras comprises interbedded mudstones and siltstones with subordinate lignites and several thick conglomeratic units towards the base ( Fig.…”

“…The sediments from both basins are indicative of deposition in nonmarine, alluvialdominated environments (Dobson & Whittington 1987;Tappin et al 1994). In terms of its relation to regional fault systems, Williams et al (2005) interpreted the SGCB as a large transtensional basin located at a releasing stepover on a major NW-SE trending Mesozoic-Cenozoic strike-slip fault system comprising the Codling and Sticklepath Faults. Dextral displacements along this fault system also led to the formation of Eocene-Oligocene transtensional basins containing similar deposits in SW England, Central Irish Sea Basin and Northern Ireland (Izatt et al 2001;Cunningham et al 2004;Ziegler 2004).…”

“…Dextral displacements along this fault system also led to the formation of Eocene-Oligocene transtensional basins containing similar deposits in SW England, Central Irish Sea Basin and Northern Ireland (Izatt et al 2001;Cunningham et al 2004;Ziegler 2004). In the southern Irish Sea, the transfer of displacement between the Sticklepath and Codling faults was accommodated by normal movements along the Mochras-St. George's Channel-Bala fault system (Turner 1997;Holford et al 2005b;Williams et al 2005). The CBB has the form of a southeasterly deepening half-graben downfaulted against the Lower Palaeozoic Welsh Massif following normal displacements along the Mochras Fault (Tappin et al 1994) ( Fig.…”

“…Using a combination of seismic reflection, compaction and palaeothermal data Williams et al (2005) identified three important phases of exhumation which affected the SGCB and its margins during the Cretaceous-Cenozoic. Firstly, up to 2.5 km of section was removed from the NW and SE margins of the basin during early…”