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Old Red Sandstone (old + red_sandstone)
Selected AbstractsEnigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, WalesGEOLOGICAL JOURNAL, Issue 5 2006Richard G. Thomas Abstract The Lower Devonian (Lochkovian-Emsian) Cosheston Group of south Pembrokeshire is one of the most enigmatic units of the Old Red Sandstone of Wales. It consists of a predominantly green, exceptionally thick succession (up to 1.8,km) within the red c. 3,km-thick fill of the Anglo-Welsh Basin, but occupies a very small area (27,km2). Four formations,Llanstadwell (LLF), Mill Bay (MBF), Lawrenny Cliff (LCF) and New Shipping (NSF),group into lower (LLF,+,MBF) and upper (LCF,+,NSF) units on stratigraphical and sedimentological criteria. Two palynostratigraphic associations (Hobbs Point and Burton Cliff) are recognised in the LLF. Overall, the Cosheston succession comprises a fluvial, coarsening-upward megasequence, mostly arranged in fining-upward rhythms. It is interpreted as the fill of an east-west graben bounded by faults to the north and south of the Benton and Ritec faults, respectively. Both ,lower Cosheston' formations were deposited by east-flowing, axial river systems draining a southern Irish Sea landmass. Drainage reversal, early in the deposition of the LCF, resulted in ,upper Cosheston' lateral, SW-flowing rivers which carried predominantly second- and multi-cycle detritus. The ,lower Cosheston' is characterized by an abundance of soft-sediment deformation structures, probably seismically triggered by movements along the graben's northern bounding fault. A minimum average (, mesoseismic) earthquake recurrence interval of c. 4000,yr is estimated for the MBF. This and the correlative Senni Formation of south-central Wales form a regionally extensive green-bed development that represents a pluvial climatic interval. Copyright © 2006 John Wiley & Sons, Ltd. [source] Sedimentation and tectonics: the marine Silurian,basal Lower Old Red Sandstone transition in southwest WalesGEOLOGICAL JOURNAL, Issue 3-4 2004Robert D. Hillier 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. Copyright © 2004 John Wiley & Sons, Ltd. [source] Variable alluvial sandstone architecture within the Lower Old Red Sandstone, southwest WalesGEOLOGICAL JOURNAL, Issue 3-4 2004Brian P. J. Williams Abstract Sandstone bodies within the Lower Old Red Sandstone (ORS) in southern Pembrokeshire exhibit variability in architecture, sediment grade and composition both spatially and temporally. Four architectural styles are observed, namely decimetre- to metre-thick sheets, metre-thick multi-storey amalgamations, inclined-heterolithic units and ribbon geometries. Sandstone bodies in the Freshwater East Formation are sheet-like, heterolithic units several metres thick. An association with lingulids and wave ripples alludes to a marine influence, possibly estuarine tidal flats or storm washovers. Within the Moor Cliffs Formation, the most common sandstone bodies are centimetre- to metre-thick sheets with high width-to-depth ratios. Fine-grained sandstones represent sheet-flood deposition on unconfined, planar surfaces, whereas coarser-grained sandstones constitute distinctive amalgamations of discrete flood events, reflecting either a change in provenance or tectonic influence. Clear incision of coarse-grained, multi-storey units within the Inter-Tuff Moor Cliffs Formation reflects a change in relative sea-level, possibly tectonically induced. The base of the Conigar Pit Sandstone Member (CPSM) is marked by a distinctive, exotic-clast conglomerate defining the base to heterolithic, lateral-accretion bedsets and sandstone sheets. This association defines a significant influx of coarse-grained sediment post-Chapel Point Calcrete formation, an interval of presumed topographic stability across the Anglo-Welsh Basin. This influx must reflect rejuvenation of source regions, with changes in base-level reflecting either eustatic or tectonic influences. Commonly observed in the CPSM are fine-grained, inclined-heterolithic bedsets recording deposition by highly sinuous rivers with flashy discharge. Up-sequence within the CPSM are metre-thick, multi-storey amalgamations of predominatly trough cross-stratified medium- to coarse-grained sandstone. It is likely that these units are genetically related to contemporaneous decimetre-thick sandstone sheets, the latter being ,splay' events marginal to the main channel axis. The interbedding of multi-storey sandstones and fine-grained laterally accreted units reflects changes in provenance, slope and/or climate. Thickness variations within the Lower ORS detail significant thickening of all units northward into the Benton Fault. It seems likely that this thickening reflects variable accommodation space development associated with active growth along this and other WNW,ESE-trending faults, and migration of channel belts toward the footwall. Copyright © 2004 John Wiley & Sons, Ltd. [source] A trace fossil assemblage from fluvial Old Red deposits (Wood Bay Formation; Lower to Middle Devonian) of NW-Spitsbergen, SvalbardLETHAIA, Issue 2 2004MAX WISSHAK From the fluvial Old Red Sandstone (ORS) of the Lower to Middle Devonian Wood Bay Formation (NW-Spitsbergen), a diverse trace fossil assemblage, including two new ichnotaxa, is described: Svalbardichnus trilobus igen. n., isp. n. is interpreted as the three-lobed resting trace of an early phyllocarid crustacean (Rhinocarididae). Cruziana polaris isp. n. yields morphological details that point towards a trilobite origin. This occurence of presumably marine trace makers in a fluvial red bed sequence raises the question of whether we are dealing with marine ingressions that are not sedimentologically expressed, with homeomorphy, or with an adaptation of marine groups to non-marine environments. [source] | ||||||||||