Search

Trace fossils from the Middle Devonian Caherbla Group of the Dingle Peninsula, southwest Ireland, record a diverse arthropod fauna inhabiting a hot-arid intracontinental rift setting. Aeolian dunefield and coeval fluvial interdune deposits interfinger spatially and temporally with alluvial fan sedimentary rocks. Three distinct trace fossil assemblages are recognized. The Taenidium-Scoyenia ichnocoenosis occurs in alluvial fan and fluvial channel deposits, and includes the large backfilled burrow Taenidium, interpreted as eoarthropleurid aestivation chambers. The Rusophycus-Protichnites ichnocoenosis, composed of arthropod trackways and surface pits, occurs in an interdune ponded area that was susceptible to ephemeral fluvial flow, with Rusophycus showing preferred orientation into the oncoming palaeocurrent. Both the Taenidium-Scoyenia and Rusophycus-Protichnites ichnocoenoses are assignable to the globally recurring continental Scoyenia ichnofacies. They are clearly substrate-controlled and moisture-related due to the ephemeral nature of the fluvial system. The Palmichnium-Entradichnus ichnocoenosis occurs in aeolian dune deposits, and includes Palmichnium, attributed to large stylonurid eurypterids, and Diplichnites, attributed to eoarthropleurids. These trackways represent the activities of dune pioneers that left their fluvial habitat to forage for detritus. Interface burrows (Entradichnus, Palaeophycus) were also constructed by arthropods moving just under the sand surface and vertical burrows (Cylindricum, Pustulichnus) were made by arthropods digging downward. Trace preservation in the aeolian environment was probably enhanced by heavy nocturnal dew-fall or light rain. The Palmichnium-Entradichnus ichnocoenosis is assigned to the globally recurring aeolian Octopodichnus-Entradichnus ichnofacies. This aeolian facies, and associated ichnofauna described herein, represents the oldest development of a unique erg system in the Old Red Sandstone (Devonian) of the southern British Isles, and one of the oldest and most diverse aeolian ichnofaunas to be reported worldwide. Copyright © 2011 John Wiley & Sons, Ltd.

Triassic dryland fluvial systems occur infilling extensional and post-rift basins across northwest Europe. These systems were deposited by streams draining off the major basin margin catchments of Greenland, Fennoscandia, the Scottish Highlands and the remnant Variscan mountains. Fluvial drainage was dominantly endorheic in character, and terminated in playa, aeolian dune, sabkha or marsh settings. Whilst the ambient basin climate fluctuated through varying levels of aridity and humidity, the dispersal of sand into these basins was critically dependant on catchment run-off from the major, basin flanking regions. In the Early Triassic the Tethyan monsoon drove seasonal precipitation over the Variscan mountains which transported sediment northward into arid dune fields and playa. In addition, far-travelled fluvial systems supplied sediment from distant, wetter catchments. Through the Middle and Late Triassic the effects of the Tethyan monsoon apparently weakened, and sand-prone fluvial systems draining off the Variscan mountains were more limited in areal extent, but expanded during pluvial episodes. However, Greenland and Fennoscandia maintained sediment supply from the north, with perennial run-off from Fennoscandia able to maintain marshes and levels of vegetation cover not encountered elsewhere in the region and to episodically drive major exorheic drainage systems into the Tethys Sea. Published climatic simulations are consistent with these observations, which indicate that Greenland, and particularly Fennoscandia, had precipitation levels higher than the regions to the south with a positive precipitation: evaporation balance. Copyright © 2009 John Wiley & Sons, Ltd.

The lower part of the Old Red Sandstone in the Dingle Penisula has been previously assigned to one lithostratigraphic group (Dingle Group) despite marked variations in sedimentary facies. However the apparently oldest non-marine sequence in the northwest of the peninsula has sedimentary and lithological attributes that contrast strongly with those of the late Silurian-early Devonian Dingle Group to the south. This northern sequence, here renamed the Smerwick Group, evolved independently of the Dingle Group in a separate basin of deposition. Field relationships between the two groups in the north of the peninsula are interpreted as indicating that the Smerwick Group overlies, with angular unconformity, a normal Dingle Group succession. Similarly, it is argued that the Smerwick Group overlies, with angular unconformity, the Dingle Group in the northwest of the peninsula, but there the Dingle Group is attenuated, represented only by a conglomerate unit some 10 m thick. In the absence of biostratigraphic evidence the age of the Smerwick Group is poorly constrained. Nevertheless, we propose a tectonic model that suggests that the Smerwick Group evolved within a small extensional half-graben on the northern margin of the Munster Basin. This model accounts for the stratigraphic and structural relationships observed, and implies that the Smerwick Group is of Late Devonian age.

Trace fossils of Beaconites type are found abundantly and widely in the Old Red Sandstone magnafacies of the British Isles, particularly in fluviatile deposits, where they occur in or closely associated with channel sandstones. Because of their giant size, however, terminations to the sediment-filled burrows are almost impossible to detect, and it has hitherto been uncertain whether the Beaconites organism dwelt in or near active rivers, or merely colonized the sandy deposits at their abandoned sites. At Llanstephan in southwest Wales B. antarcticus is preserved in a laterally accreted sandstone, a unique example of the trace fossil being truncated by an erosion surface cut during the sideways growth of a presumed point bar. The Beaconites organism is therefore thought to have lived in and near active river channels, where a moist environment was provided.

The Devonian Old Red Sandstone Ridgeway Conglomerate Formation crops out in Pembrokeshire, SW Wales. It was deposited as part of a dryland alluvial fan, axial fluvial valley deposystem. It conformably overlies the mid Lochkovian Freshwater West Formation and probably predates deposition of the Lower Cosheston Group Mill Bay Formation indicating an Early Devonian (latest Lochkovian to earliest Pragian) age, rather than a Middle Devonian age as suggested by previous workers. It therefore represents the youngest preserved formation of the Milford Haven Group south of the Ritec Fault. The Formation thickens drastically into the Ritec Fault, indicating its control on sedimentation. The half-graben topography initiated deposition of a hangingwall alluvial fan that was sourced from a southerly Lower Palaeozoic/Precambrian provenance within the present-day Bristol Channel. The Formation is heterolithic in nature, with deposits on the fan reflecting a mixture of processes. Conglomerates were deposited primarily by laterally extensive sheetfloods, and as bars in low-relief, laterally accreted channels. Sandstones were also predominantly deposited by sheetfloods. Gritty mudrocks in comparison demonstrate deposition by cohesive debris flows. The fan prograded northward and interfingered with a low-gradient, high-sinuosity fluvial channel system dominated by inclined and non-inclined heterolithic stratification. Thinly laminated mudstone and sandstone interbeds were deposited in ephemeral fan-toe and axial valley lakes that may have developed during sub-humid climatic episodes. The lacustrine heterolithic association has associated matgrounds and possible ‘algal roll-up’ structures. Calcretized peetee structures and root traces comprise a lake margin calcrete association. Fan gravels prograded into the axial fluvial valley during periods of increased sediment flux that may represent semi-arid conditions and/or episodes of tectonic activity. Calcretes of varying development were established in both the fan and axial valley zones. Calcretes with lower stages of development are more proximal to the Ritec Fault reflecting decreased soil residence times and high deposition rates within the axial valley. More strongly developed soil profiles on the fan may indicate sequence boundaries associated with low sediment flux, or increased soil residence time due to active fan-channel migration (the pedofacies concept). Groundwater calcretes have sharp-based and layer-bound calcrete profiles. Gully-bed cements are locally developed within the fan gravels. Copyright © 2007 John Wiley & Sons, Ltd.

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.

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.

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.

Two typical representatives of the Ediacaran biota, Ediacaria booleyi n.sp. and Nimbia occlusa have been found in deep-water turbidites within the Upper Cambrian Booley Bay Formation at Booley Bay in Co. Wexford, Eire. The examples of E. booleyi were transported over short distances by the turbidity currents and then impacted, in some instances at high angles, into the ocean floor muds where they formed moulds but, as their bodies decayed, these were filled by casting sand from the overlying turbidite. They preserved fine detail during transportation and impaction, thereby indicating the existence of a rigid body wall. The three-dimensional nature of their preservation allows reconstruction, which shows that Ediacaria had a discoid form with prominent relief, particularly on its dorsal side, which was divided into three main concentric zones with thin radial features, more evident at the periphery. The ventral side had lower relief, with alternate areas of coarse and fine concentric markings and numerous fine radial lines, some also thicker at the periphery. Nimbia occlusa are disc-like bodies of low relief and occur in profusion on one sole. Some show a circular outline indicating that they were preserved flat on the bedding plane, but others occur as semicircular reliefs oriented in the palaeocurrent direction. This indicates that they too were moved by the turbidity current, but their abundance and excellent preservation indicate that transport distances were short. There is no evidence in either genus for the coelenterate characteristics of a mouth and a two-layered body wall enclosing a single cavity. This, and the evidence for a rigid outer wall, adds to recent doubt on the concept of the world-wide Ediacaran fauna as dominated by soft-bodied coelenterates.

The Red Marls are renamed the Milford Haven Group (thickness approximately 2 · 4 km) and divided between the Red Cliff, Albion Sands, Lindsway Bay, Sandy Haven and Gelliswick Bay Formations, in ascending order. The lowest three, totalling approximately 230 m in thickness, are largely restricted to the Marloes area, where they succeed the marine Silurian rocks sharply and with a distinct facies change, indicative of a possibly substantial stratigraphical break. Northwards the Red Cliff and Albion Sands Formations are overlapped, the Lindsway Bay Formation in the Winsle area resting disconformably on marine Silurian rocks older than the youngest of the marine Silurian beds exposed at Marloes. The Sandy Haven and Gelliswick Bay Formations are more widely distributed and closely resemble the Red Marls of the Pembroke Peninsula and the main outcrop north of the Benton Fault. They are Gedinnian in age on the basis of ostracoderms, but the Gelliswick Bay Formation may range into the Siegenian. The thick Cosheston Group, which grades up from the Gelliswick Bay formation, probably represents the remainder of the Siegenian and may possibly be partly Emsian in age. The character of the Milford Haven Group lends no credence to the idea that allochthonous blocks are assembled in southwest Dyfed as the result of Variscan movements.