Variable alluvial sandstone architecture within the Lower Old Red Sandstone, southwest Wales

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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.

Section: Tectonism On the Southern Welsh Basin Shelfsupporting

confidence: 81%

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

Hillier

Morrissey

2009

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“…Similar sediments have been described from the Middle Devonian of North Devon (Tunbridge , 1984. Within the Milford Haven Group in southern Pembrokeshire, Love and Williams silurian-lower ors transition, sw wales 243 (2000) and Williams and Hillier (2004) document sandstones with high width/thickness ratios as ephemeral through-flowing channels flanked by low-relief mudstone interfluves.…”

Section: Red Cliff Formationmentioning

confidence: 70%

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

Hillier¹,

Williams

2004

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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.

“…The four facies described above together with the associated interpretations (taken with the information on sandbody style from Williams and Hillier 2004) indicate a complex depositional environment for the Moor Cliffs Formation. In an earlier study, Marriott et al (in press) envisaged a dryland system similar to that of the Channel Country of central Australia, as a suitable analogue for the Rat Island Mudstone Member.…”

Section: A Model For Mudrock Depositional Systems In the Moor Cliffs mentioning

confidence: 98%

Mudrock deposition in an ancient dryland system: Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales, UK

Marriott

Wright

2004

Studies of the lowermost Old Red Sandstone of South Wales (Pridolian-Lochkovian) have revealed that the mudstones that constitute up to 80% of some of the formations have been deposited in a variety of environments. In addition there is a variety of lithofacies associations present in some mud-dominated units that suggests diverse mechanisms of deposition. This paper considers the Moor Cliffs Formation and examines four distinct mudrock facies.The overall depositional environment is considered to have been a multi-stage, multi-channel system, similar to a modern dryland river system, with mud-dominated, moderately sinuous ephemeral rivers that reworked muddy floodplain sediment over a proximal braidplain during seasonal flooding. The alluvial deposits were pedified to varying degrees as calcic Vertisols, which, due to seasonal wetting and drying, provided soil aggregates for reworking by wind and water as sand-or silt-sized pellets. Very infrequent flooding events (superfloods) resulted in extensive stripping of the floodplain surface and the introduction of allochthonous sand sheet deposits over wide areas in this normally sediment-starved system.Depressions (pans or waterholes) on the floodplain formed ephemeral lakes following floods, and acted as sediment traps for dust from the frequent dust storms occurring in the system.