A late ordovician/early silurian non-depositional slope and perched roll-over basin along the tywi anticline,mid wales

Ordovician stratigraphy between the Sugarloaf and the Wye valley is summarized with particular attention given to the area northeast of Abergwesyn. New data refine thickness estimates for the Cerriggwynion Formation, the Trembyd Formation (new), and the Cefn Cynllaith Formation (new). The major Bron Rhudd Fault is suggested to have been a synsedimentary down-to-basin fault which was later reactivated, folded, and suffered local reversal of throw. Roll-over into this fault controlled deposition of mass-flow conglomerates in a perched basin. During the late Ordovician-early Silurian the axis of the Tywi Anticline underlay a non-depositional slope which developed by northwest-directed progradation of a shelf margin, possibly accentuated by flexure over a basement fault zone with contemporaneous basin-down normal movement. The slope angle cannot be estimated from the geometry of the Ordovician because the contemporaneous throw on the Bron Rhudd Fault cannot be determined, but from the rate of Silurian onlap it was about 5 degrees.     

The stratigraphy of the Drummuck Group (Ashgill), Girvan

The Drummuck Group of the Girvan district (S. W. Scotland) comprises a varied sequence of marine siliciclastic sedimentary rocks some 350 m thick and ranging in age from early Cautleyan to late Rawtheyan (Ashgill Series). Deposition in an unstable slope environment is envisaged. These upper Ordovician rocks crop out in the centre of the Craighead inlier, north of the Girvan valley and are sporadically exposed and locally very fossiliferous. A detailed revision of the Drummuck rocks has permitted, for the first time, the formal lithostratigraphical division of the group. Four main units, in ascending order, the Auldthorns, Quarrel Hill, Lady Burn and South Threave formations are recognized and, within these, a number of smaller, useful divisions are defined. Detailed geological maps of the Drummuck outcrop are presented. Brachiopods numerically dominate the shelly faunas and all the divisions named are characterized by distinctive brachiopod assemblages. Preliminary correlations are discussed with selected upper Ordovician successions elsewhere but the emphasis of this study is placed upon the establishment of a stratigraphical framework within which future detailed research on the Drummuck faunas may be carried out.     

Conodont colour alteration indices (CAI) of Upper Ordovician limestones from the Iberian Peninsula

Conodont colour alteration index (CAI) data in Upper Ordovician rocks from several areas of the Variscan domain in the Iberian Peninsula indicate conditions ranging from diagenesis to low-grade metamorphism. In most of the areas, where studies using other indicators, such as illite crystallinity (IC) or where vitrinite reflectance are lacking, the CAI method has permitted a preliminary estimation of the metamorphic grade. In the Almadén syncline (Central-Iberian Zone), where IC studies are available, the thermal conditions inferred from CAI data agree with those obtained by the IC method. In the Puertollano–Almuradiel syncline, the thermal interval obtained primarily from fluid inclusions (270–370°C) overlaps considerably with that obtained from CAI data (180–340°C). In general, cleavage in rocks is present in anchizonal or epizonal conditions, whereas in diagenetic conditions with CAI 2.5, cleavage is scarce. The conodont texture changes with increasing metamorphism, and apatite recrystallisation appears in general with CAI 5. Variation of CAI values within a single sample and/or within short stratigraphic distances observed at several localities is due to hydrothermal activity.     

The ashgill rocks of the glyn ceiriog district. North wales

The Ashgill rocks of the Glyn Ceiriog district comprise a relatively thick sequence of mudstones and siltstones with subordinate sandstones and locally developed limestone members. Closely related deposits are found in the inliers of Cym-y-brain and Mynydd Cricor to the north. Correlation with other areas of Ashgill rocks suggests that the Dolhir Formation, of the Glyn Ceiriog district, can be dated as mid-Cautlcyan to Rawtheyan while the overlying Glyn Formation is high Rawtheyan to Himantian. In the inliers to the north, that part of the Cyrn-y-brain Formation which is exposed is dated as Rawtheyan and both the Plas Uchaf Formation (Cym-y-brain) and Cricor Formation (Mynydd Cricor) are taken to be Hirnantian. The succession is interpreted as a transgressive-regressive sequence in which shallow shelf mud-stones and siltstones with storm-sand and coquinid layers are overlain by shallow water shoreline deposits. Although the base of the Ashgill is not exposed in the inliers to the north, in the Glyn Ceiriog district it is seen to rest unconformably on older Ordovician strata; in all three areas the Ashgill is apparently conformably overlain by Lower Llandovery shales and sandstones.     

The identity of the British Ordovician cystoid ‘Hemicosmites rugatus Forbes‘

New articulated specimens of the British Ordovician cystoid first mentioned under the name ‘Hemicosmites rugatus Forbes’ in 1848 allow its morphology and systematic affinities to be established for the first time. Despite being based on isolated plates, ‘H. rugatus Forbes’ has been reported from England and Wales, Belgium, France, Spain and Thailand, and has been attributed to four genera. It is characterized by a heteromorphic, circular stem, a theca with four basal, six infra‐lateral and eight lateral plates, all with sharp‐crested ridges running to plate angles, multiple arms in each ambulacrum and an extensive tegmen. These characters indicate it belongs in the family Caryocrinitidae and genus Caryocrinites. Caryocrinites rugatus had arms with lateral brachioles that folded in against the main arm axis, as in the type species C. ornatus Say. It is the first Ordovician species known to possess this character. Isolated plates show sorting and a few may represent other species of Caryocrinites s.l. Copyright © 2013 John Wiley & Sons, Ltd.     

Observations and speculations on the northeast Towy ‘axis’, mid-Wales

Late Ashgill and Llandovery sedimentation across the Towy ‘axis’ southeast of Rhayader is interpreted within the context of the rapid southeasterly thickening of the upper portion of the Ashgill best demonstrable in the Chwefru valley. This thickening sequence overlies deep-water middle-fan arenites and is interpreted as a northwest prograding shelf/slope association. The slope eventually became non-depositional and was onlapped by the basinal facies of the Llandovery during relative sea-level rise. The non-depositional slope and associated major slump-scars at the shelf-edge explain many of the non-sequences- in the region previously ascribed to erosion related to episodes of tectonism. The regional strain history is thus simplified.     

Ashgill trilobite faunas from the Llŷn Peninsula,North Wales,U.K.

The Crugan Mudstone Formation is formalized and the Dwyfor Mudstone Formation proposed for restricted Ashgill outcrops respectively near Llanbedrog and Llanystwmdwy in the Llŷn Peninsula. These mudstones contain brachiopods of the Foliomena fauna and a total of 26 determined genera of trilobites, one, Pseudosphaeroxochus represented by a new species, P. seabornei sp. nov. Elements of the trilobite fauna are used to demonstrate a Rawtheyan age for the mudstones. The generic assemblages both of brachiopods and of trilobites present in the mudstones indicate relatively deep water environments of deposition, that of the Dwyfor Mudstones probably being the deeper. The trilobites belong to what is here described as the Nankinolithus-Opsimasaphus association.     

Provenance of late Ashgill (Hirnantian) fine-grained sediments and pebbles in the Welsh Basin: A ND and SR isotope study

The isotope characteristics of interbedded fine-grained sedimentary rocks and pebbles from an Ashgill sequence in the Welsh Basin show that they have different provenance. Gondwana-derived continental crustal material dominates the Nd-Sr isotope signatures of the fine-grained rocks whereas the pebbles sample the Avalonian basement. Detritus from Gondwana and Avalonia accounts for the isotopic composition of most of the lower Palaeozoic sedimentary rocks of Southern Britain and this suggests that any contribution from the reworking of Lower Palaeozoic volcanic rock is minimal.     

Depositional and tectonic relationships of upper ordovician and lower silurian strata around dinas mawddwy,mid-wales

The field evidence for the lateral impersistence of the Garnedd-wen Formation (Hirnantian) and Derwen Formation (Aeronian) immediately northeast of Dinas Mawddwy is reviewed and the evidence for strike faulting as inferred by W. J. Pugh (1928) is reassessed. It is concluded that little if any strata are missing and that surfaces of non-deposition, rather than fault cut-out, better explain any strata that are locally abnormally thinned. However strike faulting is newly proven west of Dinas Mawddwy and accounts for much of the rapid apparent thickness variation in the Broad Vein (Rawtheyan) there. The newly constrained thickness estimates strongly imply that sedimentation was influenced by a basement fracture at Dinas Mawddwy. Structural reasoning and palaeocurrent evidence from the Garnedd-wen Formation suggest that this fracture probably parallels the Corris-Llangranog fracture to the southwest.     

The ecological,significance of variation in the generic composition of Rawtheyan (late Ordovician) trilobite faunas from North Wales,U.K.

Contemporaneous trilobite faunas from Rawtheyan rocks forming the local base to the Ashgill Series change in their generic composition along the outcrop between Bala and Corris. The change, although overall a very marked one, is progressive and is held to reflect original ecological control along a depth gradient between a locally elevated area around Bala and the deeper central part of the Welsh Basin. Along this intrabasinal slope three distinctive trilobite generic associations–the Ceraurinella-Encrinuroides Association, the Nankinolithus-Opsimasaphus Association and the Novaspis-cyclopygid Association–succeeded each other in gradually deeper water. Faunas of all three associations differ markedly from contemporaneous faunas of shallower shelf environments.