National Museum and Galleries of Wales

Cardiff, United Kingdom

National Museum and Galleries of Wales

Cardiff, United Kingdom
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Hillier R.D.,National Museum and Galleries of Wales | Waters R.A.,National Museum and Galleries of Wales | Marriott S.B.,University of Bedfordshire | Davies J.R.,British Geological Survey
Sedimentology | Year: 2011

Distal environs of the Ludlow-age Trichrug Formation from south central Wales (UK) detail deposition on the outer fringe of an ephemeral debris flow-dominated alluvial fan. Debrites and subordinate sheetflood deposits are interbedded with sporadic thin sandstone-dominated heterolithic units deposited in shallow, ephemeral ponds in the axial valley. The latter slope wetland system fringed the permeable alluvial fan deposits, being maintained by streamflow, precipitation and ground water recharge. A prolonged high water table is indicated by low chroma intervals interpreted as the result of gleying. A variety of redoximorphic indicators, including colour mottling and ferricrete concretions, are evidence of iron mobilization and concentration associated with a seasonally fluctuating water table. The slope wetlands were colonized periodically by Skolithos-generating organisms, most likely to have been arthropods. The widespread occurrence of redoximorphic indicators and ferricrete contrasts markedly with calcrete Vertisols in penecontemporaneous continental deposits (Lower Old Red Sandstone) of the region. It is likely that the alluvial fan-toe area was the site of a wetland depression, which maintained a high, though fluctuating, water table. In contrast the alluvial channel reaches of the surrounding drainage network were significantly better drained, with floodplains prone to wetting and drying and Vertisol palaeopedogenesis. © 2010 The Authors. Journal compilation © 2010 International Association of Sedimentologists.

Morrissey L.B.,Nautilus | Hillier R.D.,National Museum and Galleries of Wales | Marriott S.B.,University of Bristol
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2012

Late Silurian to Early Devonian deposits from the Lower Old Red Sandstone Anglo-Welsh Basin preserve a diverse assemblage of trace fossils that collectively comprise an important record of early terrestrialisation. Five main palaeoenvironmental categories are represented: (1) perennial rivers, (2) ephemeral rivers, (3) floodplain lakes, (4) alluvial fan, and (5) marginal marine. Ichnocoenoses mainly reflect the activity of aquatic and/or semi-aquatic animals. Fluvial deposits preserve evidence for colonisation by eurypterids, fish and myriapods. Ephemeral river channel margins/splays preserve the most diverse ichnofauna composed of deposit feeders' burrows and trackways produced by arthropods feeding and foraging in channel-margin soft grounds that were colonised after fluvial flood events. High densities of deposit-feeding structures may indicate of explosive population growth in response to periodic, acyclical replenishment of nutrients in these aquatic settings. Ichnotaxa common to both perennial and ephemeral fluvial environments suggest that there might have been only one aquatic ecosystem. The degree to which the channel-margin ichnofaunas record a truly subaerial terrestrial arthropod signature is difficult to ascertain and most trace makers were probably semi-aquatic. Some terrestrial animals invaded channel-margin areas to feed and were also capable of inhabiting high-energy, alluvial fan environments. However the fans contain low diversity ichnological assemblages and were likely to have been inhabited by an opportunistic fauna. Possibly the only truly subaerial terrestrial ichnocoenosis is that of pedogenically modified mudrock with Scoyenia, which represents an early soil ichnofauna. The majority of ichnocoenoses described from continental palaeoenvironments could be classified under the Scoyenia ichnofacies. Marginal marine deposits contain an impoverished Skolithos-Cruziana ichnofacies typical of Silurian-Devonian brackish water assemblages. © 2012 Elsevier B.V.

Hillier R.D.,National Museum and Galleries of Wales | Morrissey L.B.,University of the West of England
Geological Journal | Year: 2010

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 storm-dominated 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 sub-basins 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. © 2009 John Wiley & Sons, Ltd.

Loughlin N.J.D.,University of Bristol | Hillier R.D.,National Museum and Galleries of Wales
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2010

The early Cambrian Caerfai Group of Pembrokeshire in Southwest Wales yields an intensely bioturbated Teichichnus-dominated ichnofabric, and provides evidence for the early evolution of infaunal organisms. Three units of the Caerfai Group comprise the focus of this study in sections exposed at Caerfai Bay. In the St. Non's Sandstone few primary sedimentary structures are observed, bedding being predominantly massive with subordinate parallel and current ripple cross-lamination. The topmost St. Non's Sandstone contains common soft-sediment deformation phenomena, and collectively deposition is believed to have taken place in the proximal subaqueous reaches of a steep-fronted delta system. Teichichnus is the dominant trace, with subordinate Planolites, Palaeophycus and possible Rhizocorallium.The overlying Caerfai Bay Shales were deposited from mass flows with evidence of downslope creep suggesting deposition on a steep delta front. Sandstone debrites are common, as are tuffaceous beds with evidence of reworking by bottom currents. Sandstone and tuff event beds contain common Skolithos and Arenicolites with less frequent Planolites. Interbedded mudstones contain infrequent Teichichnus.The Caerfai Bay Shales coarsen and thicken-upwards into the Caerbwdy Sandstone. Debrite sandstone beds dominate, and bed thickening upsection indicates progradation of the delta system. The dominant trace fossil within the unit is Teichichnus which is constrained to the top few centimetres of individual sandstone beds. Less common are Planolites and Palaeophycus.The size and abundance of Teichichnus in the Caerfai Group are similar to, or exceed those in contemporaneous sections in Avalonia and Baltica. Local environmental factors were probably the primary control on trace fossil distribution. Substrate and salinity are proposed as the main influences on ichnodiversity and size in the Caerfai Group. Teichichnus is most abundant in the St. Non's Sandstone, where the proximal reaches of the delta experienced reduced salinity and possibly higher oxygen levels. Burrow diameter progressively decreases upsection, and bioturbation becomes less pervasive in subsequent units of the Caerfai Group. This indicates that by the Cambrian Stage 2, significant advances into reduced salinity environments had occurred, along with the dispersal of the Teichichnus producer throughout the Avalonia-Baltica seaway. Correlation within Avalonia indicates that a transgression in the Cambrian Stage 2 initiated the deposition of the Caerfai Group marking the onset of the Welsh Basin. © 2010 Elsevier B.V.

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