Gisbornian (Caradoc) graptolites from New South Wales,Australia: systematics,biostratigraphy and evolution

The biodiversity record of graptolites from the Cheeseman's Creek Formation, considered herein as late Gisbornian (Caradoc) in age, has been substantially increased to fifteen taxa, including the new species Dicellograptus praemorrisi sp. nov. and Climacograptus vandenbergi sp. nov. Some of the records have global correlative significance enabling us to identify the wilsoni Biozone ( = calcaratus Biozone of eastern Australia). Several evolutionary lineages have been recognized:

• 1 Dicellograptus moffatensis (Carruthers, 1858) → D. praemorrisi sp. nov → D. morrisi Hopkinson, 1871
• 2 Glossograptus hincksi Hopkinson, 1872 → Glossograptus? sp.
• 3 Climacograptus bicornis (J. Hall, 1847) → C. vandenbergi sp. nov. → C. lanceolatus VandenBerg, 1990.

Copyright © 2001 John Wiley & Sons, Ltd.     

Historical valley alluviation in mid-Wales

Recent developments on a floodplain reach of the Afon Ystwyth are analysed with the aid of five sets of maps and ten of air photographs covering the period since 1800. The area has been subjected to around 3 m of aggradation and then incision in association with historic metal mining. Analysis of sediment texture and metal concentrations allows subdivision of the mining-associated sediments; C/M plots and shape analyses allow a distinction between proglacial, mining aggradation, and wandering river phases. The distinctive style of aggradation, involving overbank gravel splays and interfingering fines, and the lagged response of channel and sedimentation, relate largely to alterations in sediment supply rates. This has relevance for the interpretation of Quaternary fills and river terraces and for contemporary river management.     

A late Ordovician (Hirnantian) karstic surface in a submarine channel,recording glacio‐eustatic sea‐level changes: Meifod,central Wales

The growth and decay of the end‐Ordovician Gondwanan glaciation is globally reflected by facies changes in sedimentary sequences, which record a major eustatic fall and subsequent rise in the Hirnantian Stage at the end of the Ordovician. However, there are different reported estimates of the magnitude and pattern of sea‐level change. Particularly good evidence for end‐Ordovician sea‐level change comes from a sequence at Meifod in central Wales, which has a karstified limestone unit within a channel incised into marine shelf sediments. Pre‐glacial (Rawtheyan) mudstones have a diverse fauna suggesting a mid‐to‐deep‐shelf water depth of c. 60 m. The channel, 20 m deep, was incised into these mudstones and partially filled with a mixture of fine sand and detrital carbonate. The taphonomy of bioclasts and intraclasts indicates that many had a long residence time on the sea floor or suffered diagenesis after shallow burial before being resedimented into the channel. The presence of carbonates on the Welsh shelf is atypical and they are interpreted as having accumulated as patches during a minor regression prior to the main glacio‐eustatic fall. Comparison of the carbon stable‐isotopic values of the bioclast material with the global isotopic record confirms that most of the material is of Rawtheyan age, but that some is Hirnantian. The resedimented carbonates lithified rapidly and formed a limestone, several metres thick, in the deepest parts of the channel. As sea‐level fell, this limestone was exposed and eroded into karstic domes and pillars with a relief of over 2 m. The overall, glacio‐eustatic, sea‐level fall is estimated to be in excess of 80 m. A succeeding sea‐level rise estimated to be 40–50 m is recorded in the laminated crust that mantles the karstic domes and pillars. The crust is formed of encrusting bryozoans, associated cystoids, crinoid holdfasts and clusters of the brachiopod Paromalomena, which is normally associated with mid‐shelf environments. Fine sands buried the karst topography and accumulated to fill the channel. In the sandstones at the base of the channel there is a Hirnantia fauna, while in the sandstones high in the channel‐sequence there is cross‐stratification characteristic of mid‐shoreface environments. This would indicate a fall of sea‐level of c. 30 m. The subsequent major transgression marking the end of the glaciation is not recorded at the Meifod locality, but nearby exposures of mudstones suggest a return to mid‐to‐deep‐shelf environments, similar to those that prevailed before the Hirnantian regression. The Meifod sequence provides strong evidence for the magnitude of the Hirnantian sea‐level changes and by implication confirm larger estimates for the size of the ice sheets. Smaller oscillations in relative sea‐level seen at Meifod may be local phenomena or may reflect eustatic changes that have not been widely reported elsewhere. Copyright © 2005 John Wiley & Sons, Ltd.     

Glacial activity and paraglacial landsliding in the Devensian Lateglacial: evidence from Craig Cerrig-gleisiad and Fan Dringarth,Fforest Fawr (Brecon Beacons), South Wales

The tongue-shaped mass of debris and associated ridges on the cirque floor below Craig Cerrig-gleisiad, Brecon Beacons National Park is important and controversial because it has been attributed to more than one glacier advance during the Late Devensian. A new origin is proposed involving landslide development from the collapse of part of the western headwall followed by a single phase of glacier development in the Loch Lomond Stadial (Younger Dryas), which reworked the landslide sediments. Evidence for this landslide, which provides useful criteria for differentiating moraines formed by small glaciers from landslides, lies in tension cracks, backward-tilted blocks and bedrock joints dipping out of the western headwall, together with lateral levées, upstanding termini and angular clasts with only occasional, indistinct striae on the tongue-shaped mass, which is interpreted as a flowslide. Glacier reworking of debris in the upper part of the Cwm Cerrig-gleisiad landslide is indicated by subparallel ridges rising to 20 m above the cirque floor containing abraded clasts (16-32% striated). This interpretation is supported by a comparison with the morphological and sedimentary characteristics of a neighbouring landslide at Fan Dringarth, where no glacier developed in the Loch Lomond Stadial. The existence of paraglacial landsliding has significant palaeoenvironmental implications leading to: (1) erroneously large estimates of equilibrium line depression ($Δ$ELA) in the Loch Lomond Stadial; (2) consequent underestimates of summer palaeotemperatures and/or overestimates of the contribution of wind-drifted snow to glacier accumulation; and (3) larger moraines than usual and overestimation of the efficacy of glacial erosion because of antecedent processes.     

Jointing and minor tectonics of the N. Crop of the South Wales coalfield between kidwelly and black mountain,S. Wales

Joints, minor faults and mineral veins were recorded at 334 stations between Kidwelly and Black Mountain, S. Wales. A systematic pattern occurs with joint sets striking at 360°, 340°, 290°, 250°, and 210°. Minor faults and shear planes exhibit both strike and dip-slip displacements. The jointing is considered to have formed quite early during folding, and both joints and faults can be attributed to a principal compression acting almost N-S and subparallel to bedding. Phases of dilation are recorded by sets of mineral veins. All the deformational structures are considered to be of Armorican age.     

Chadian faunas and flora from dyserth: Depositional environments and palaeogeographic setting of Viséan strata in northeast wales

New floral and faunal data from the oldest Dinantian limestones (Foel Formation) in the Dyserth area, suggest that these sediments are of Chadian age, rather than the Asbian age concluded by earlier workers. The basal late Chadian limestones rest conformably on Dinantian Basement Beds of ?Chadian age or older. The initial inundation of St. George's Land occurred during Chadian times, when shallow-water marine limestones accumulated in the Dyserth area and further to the south, together with terrestrially derived siliciclastics, containing drifted plant fragments. Periodically, a restricted hypersaline lagoonal environment was established but an open marine, neritic environment with abundant stenohaline fauna prevailed in this area. These Chadian sediments accumulated on the proximal part of a carbonate ramp and are presumed to have passed laterally downslope into deeper water basinal facies with Waulsortian buildups of the Irish Sea Basin. In the later Arundian, a carbonate ramp–to–platform transition occurred, with widespread deposition of shallow-water carbonates. In the Asbian this platform developed a rimmed margin, with buildups forming a linear belt between platform and basin. An almost complete Chadian to Brigantian Lower Carboniferous sequence can now be recognized in North Wales. This succession is comparable with the shelf succession in south Cumbria on the northern margin of the Irish Sea Basin.