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

Abstract

Shallow marine deposits comprising the Silurian Gray Sandstone Formation (GSF) exhibit pronounced process regime changes through time. The formation was deposited on the southern shelf of the Lower Palaeozoic Welsh Basin (UK), and conformably overlies the Coralliferous Formation. The basal Lithofacies Assemblage A (of Sheinwoodian age) is dominated by a storm-dominated process regime, comprising shoreface and offshore shelf facies associations. The overlying Lithofacies Assemblage B records a mixed process regime, with units being deposited under both storm-and tide-influenced conditions. Tidal-influence prevailed during deposition of the overlying Lithofacies Assemblage C, with proximal to distal facies variations across a significant tide-influenced river delta being observed. A return to stormdominated shoreface conditions is seen in the succeeding Lithofacies Assemblage D. Lithofacies Assemblage E (Homerian age) records the return of a tide-influenced river delta to the area, prior to the conformable transition into the overlying Old Red Sandstone (ORS) Red Cliff Formation (of Ludlow age). Northward thickening of the formation across southern Pembrokeshire into the Musselwick Fault indicates a tectonic control on sedimentation, the formation infilling accommodation space developed in an intra-shelf half-graben. Recurring changes in process regime from storm-to tide-influenced sedimentation may be related to the onset and subsequent cessation of tidal resonance in subbasins across the shelf area which itself was probably controlled by episodic tectonism. It is proposed that the Coralliferous and Gray Sandstone formations comprise the newly erected Marloes Group.

Late Silurian event stratigraphy and facies of South Wales and the Welsh Borderland, United Kingdom

“…Their occurrence in this environment is well documented from Phanerozoic and Palaeozoic examples (Mata & Bottjer, 2009), a prerequisite for formation appearing to be low levels of bioturbation due to environmental stress (Hagadorn & Bottjer, 1997, 1999; Pflűger, 1999). A similar lithofacies is observed in the Wenlock age Gray Sandstone Formation, where salinity variations associated with freshwater plumes are interpreted as leading to impoverished infaunas and subsequent matground colonization of the sea bed (Hillier & Morrissey, 2010). Nicholls (2019) also identifies such features in deep water deposits of western mid Wales formed during the period of environmental flux associated with the Late Ordovician glacioeustatic lowstand.…”

“…The lithofacies represents deposition in terminal, delta-top distributary channels (e.g. Olariu & Bhattacharya, 2006; Charvin et al 2010; Hillier & Morrissey, 2010; Li et al 2011). Stratification is dominated by unidirectional, tractional bedforms of in-channel bars.…”

The Silurian Transgression of a Palaeoshoreline: The Area between Old Radnor and Presteigne, Welsh Borderlands

“…In addition, there is the possibility of a stratigraphic gap within the lower Telychian. This stratigraphic gap likely correlates with an episode of tectonic uplift and subaerial exposure within Pembrokeshire (approximately 150 km to the SW of the study area) and a rerouting of sediment influx to the Welsh Basin resulting in the deposition of thick turbidite successions (e.g., the Devil's Bridge Formation) [36,37]. This tectonic event may be the cause of the angular unconformity between the Folly Sandstone 2 and 7 for the location of the thin sections.…”

“…Both the Folly Sandstone and Cowleigh Park formations are unconformably overlain by younger Telychian sediments, suggesting the regional exposure of much of the intervening shelf. The Aeronian-Telychian unconformity is associated with uplift and has been linked to a pulse of regional tectonism [36,37].…”

Alluvial fan and wetland interactions: evidence of seasonal slope wetlands from the Silurian of south central Wales, UK

“…The Midland Platform and its western extension in South‐west Wales were themselves subjected to periodic intra‐shelf faulting and localized basin development (Butler et al. , 1997; Hillier, 2000; Hillier & Williams, 2004; Hillier & Morrissey, 2010). The study area is located within the south‐western part of the Myddfai Steep Belt, a faulted south‐east facing monocline between the Pontesford Lineament and Church Stretton Fault, both elements of the WBFS (Fig.…”