Sedimentation and tectonics: the marine Silurian–basal Lower Old Red Sandstone transition in southwest Wales

Abstract

Both regional and localized tectonic events controlled deposition within the Wenlock and early Ludlow of SW Wales. Estuarine deposits within north-south-tending incised valleys dominate the youngest (Homerian) Gray Sandstone Group, valley incision being probably related to changing base-levels associated with Avalonia/Laurentian collision. Available accommodation space was outpaced by sediment supply, with the Red Cliff Formation (Late Ludfordian) defining a conformable transition from marine to Old Red Sandstone (ORS) deposition within the Marloes Peninsula. Sedimentation was dominated by fine-grained pedified siliciclastics, with subordinate fine-grained ephemeral sheet-flood sandstones. Local palaeocurrents indicate sediment transport from the south and west, though long-distance transport from a distant Laurentian provenance is assumed. A probable tectonically generated sequence boundary marks the base of petrographically distinctive, multi-storey pebbly sandstones of the Albion Sands Formation, deposited within the hangingwall valley of the active east-west-trending Wenall Fault. Sediment accommodation space was controlled by proximity to the tip-point of this important growth fault within the Lower ORS. Debris-flow-dominated fans, shed from both the hangingwall and footwall of the Wenall Fault, deposited the Lindsway Bay Formation, an exotic-clast conglomerate unit sourced predominantly from the south and west. It is uncertain as to whether movement along the Wenall Fault was caused by collision-related transtension, or rifting associated with the southern margin of Avalonia.

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

“…Marine Silurian rocks of the Powys Supergroup (Woodcock 1990) crop out within a number of small, mainly coastal inliers to the north and south of Milford Haven, southwest Wales ( Figure 1; Murchison 1839; De la Beche et al 1846;Cantrill et al 1916;Dixon 1921;Walmsley and Bassett 1976;Hurst et al 1978;Bassett 1982;Siveter et al 1989;Aldridge et al 2000;Hillier 2000Hillier , 2002Hillier and Williams 2004). 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.…”

“…LAE is dominated by bedsets of the tide-influenced IHS and tidal flat facies associations. The assemblage is equivalent to Valley Fill 5 of Hillier (2000) and Hillier and Williams (2004), and crops out to the north of Albion Sands ( Figure 6A) and at Sprat's Point in Lindsway Bay (SM 8423 0668, Hillier and Williams 2004). The base of the assemblage presumably also crops out in the inaccessible Watery or Victoria bays to the NW of Albion Sands.…”

“…The Welsh Basin ultimately became oversupplied with sediment and shoaled in a diachronous manner (Allen 1985a), with the transition into the continental Old Red Sandstone (ORS) being of Ludlow age in southern Pembrokeshire (Walmsley and Bassett 1976;Hurst et al 1978;Hillier and Williams 2004) to mid-Přídolí in the east (Bassett et al 1992). Sediment was possibly locally sourced from the southern Avalonian hinterland Pretannia (Cope and Bassett 1987), or from uplift created by Laurentian/ Avalonian collision to the west (Soper and Woodcock 1990;Woodcock et al 1996).…”

“…Lithofacies Assemblage E (LAE) is continuously exposed to the SE of Marloes Sands and comprises a logged thickness of 120 m ( Figure 6B; see graphic logs in Hillier and Williams 2004). Here, the base is defined by a marked sequence boundary cutting into the underlying shoreface deposits of LAD (SM 7895 0693).…”

“…Allen and Williams 1978;Hillier and Williams 2004;Veevers et al 2007), it is appropriate to downgrade its existing group status to that of a formation. Facies variability allows the GSF to be subdivided into five lithofacies assemblages that are present on the Marloes Peninsula (Table 2, Figure 6).…”