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

Abstract

The Lower Silurian siliciclastic Coralliferous Group is shown to have been deposited in an intra-shelf position 10-15 km south of the palaeogeographic shelf-break of the Welsh Basin. After a phase of thermal subsidence related to the development of the predominantly Llandovery Skomer Volcanic Group, the shelf basin was transgressed. This transgression was punctuated by an episode of tectonic uplift in southern Pembrokeshire, resulting in subaerial exposure of the shelf and a significant basinward shift in sedimentary environments. Erosion and sediment bypass ensued, with coarse-grained low-sinuosity fluvial channels transporting sediment to the northerly Welsh Basin, where significant submarine fans developed. During the early Telychian, renewed transgression took place, with lowstand gravels being ravined and reworked into parasequences of the transgressive systems tract. These thin, coarse-grained parasequences record deposition within high-energy wave-dominated shoreface/inner shelf environments. Further coastal onlap resulted in the closing down of significant coarse-grained sediment supply, with the remaining Coralliferous Group being dominated by wave-influenced silts, mud-shales and thin sandstones comprising the highstand systems tract.

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

“…This event correlates with a switch from lateral to axial sediment influx into the basin (Wilson et al 1992;Woodcock et al 1996;Davies et al 1997), being possibly related to the initial docking of Eastern Avalonia with Laurentia (Soper and Woodcock 1990). Shelf exposure occurred in southwest Pembrokeshire at this time (Hillier 2002), with a lowsinuosity fluvial network feeding lowstand submarine fans in the northerly Welsh Basin. Subsequent transgression of the shelf region resulted in a thin, coarse-grained interval being deposited in a wave-dominated shoreface (Hillier 2002), or fan delta environment (the Renney Slip Member of Veevers et al 2007).…”

“…Dunne, personal communication 2004). As such, it is reasonable to postulate that the Musselwick Fault exerted a control on deposition, rather than the Wenall Fault as previously thought (Hillier 2000(Hillier , 2002.…”

“…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.…”

“…Shelf exposure occurred in southwest Pembrokeshire at this time (Hillier 2002), with a lowsinuosity fluvial network feeding lowstand submarine fans in the northerly Welsh Basin. Subsequent transgression of the shelf region resulted in a thin, coarse-grained interval being deposited in a wave-dominated shoreface (Hillier 2002), or fan delta environment (the Renney Slip Member of Veevers et al 2007). A shallow-marine siliciclastic wedge (the Deadman's Bay Member of the Coralliferous Formation, and the overlying GSF) prograded northward across the Skomer Sub-basin during late Llandovery and Wenlock times (Sanzen-Baker 1972;Hillier 2000).…”

“…It is likely that growth within the Skomer Sub-basin was synchronous during latest Llandovery times, extending well into the Wenlock. Tectonic uplift and exposure of the southern limb of the Marloes-Lindsway Anticline occurred during latest Aeronian through to earliest Telychian times (Hillier 2002), and correlates with a switch from lateral to axial sediment influx into the basin (Wilson et al 1992;Woodcock et al 1996;Smith 2004). Such tectonism was probably related to the initial docking of Eastern Avalonia with Laurentia (Soper and Woodcock 1990).…”