Ordovician palaeogeography of the southern and eastern flanks of the Harlech Dome, Merionethshire, North Wales

Ridgway, J.

doi:10.1002/gj.3350110203

Geological Journal

1976

DOI: 10.1002/gj.3350110203

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Ordovician palaeogeography of the southern and eastern flanks of the Harlech Dome, Merionethshire, North Wales

J. Ridgway

Abstract: The Arenig to Caradoc palaeogeographic evolution of the Harlech Dome region is interpreted from a study of the volcanic and sedimentary successions on the eastern and southern flanks of the Dome. Acid ignimbrite volcanism is shown to have been associated with periods of uplift and intermediate to basic volcanism with periods of subsidence. The evolution of the region can be compared to that of an island arc.Gaol.

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Cited by 12 publications

References 46 publications

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A re‐examination of the type Arenig Series

Zalasiewicz

1984

Geological Journal

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The stratotypc of the Arenig Series (Ordovician System) and its boundaries are critically re‐examined. The lithostratigraphy and biostratigraphy of the Tremadoc, Arenig, and Llanvirn age rocks at Arenig Fawr, North Wales that were constructed by Feamsides in 1905 are amended. The Arenig Series at Arenig is represented by the Camedd Iago Formation which was established by Lynas (1973) in the Migneint area. Examination of the sparse graptolite faunas indicates that the type Arenig Series is incomplete, only the extensus Zone having been recognized with certainty. There is a possible unconformity at the base and a probable unconformity at the top, and on present knowledge the succession cannot be correlated precisely with what are considered to be Arenig sequences elsewhere. It therefore may prove necessary to redefine the Arenig Series at a more useful type section. It is suggested that the Carmarthen district may provide a suitable stratotype.

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A re‐examination of the type Arenig Series

Zalasiewicz

1984

Geological Journal

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Age and controls of ironstone deposition (Ordovician) North Wales

Trythall¹,

Eccles²,

Molyneux

et al. 1987

Geol. J.

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Post-Arenig ironstones and associated sediments of North Wales have yielded Upper Llanvirn, Llandeilo and Lower Caradoc acritarch assemblages. The occurrence of most ironstones coincides with a eustatic regression and subsequent transgression, and the emergence to the northwest of the Irish Sea Landmass, the probable source of the iron. The majority of ironstone deposition within the basin was restricted to short-lived local submarine rises most likely produced by synsedimentary faulting.

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Tectonic evolution of the Bala Lineament in the Welsh Basin

Fitches

Campbell

1987

Geol. J.

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The Bala Lineament is one of several NE-SW major linear fault zones traversing the Lower Palaeozoic Welsh Basin. Recent published and unpublished information relating to the lineament is synthesized and an evolutionary model is proposed.Geophysical data suggest it was initiated as a strike-slip structure, probably in late Precambrian times. Unlike the other lineaments, it had no separate Lower Palaeozoic identity; during that period, sedimentation was strongly influenced by contemporary faulting but there is no reason to suspect that deformation was confined to a long, narrow, NE-SW belt or that neighbouring regions were not similarly affected. Moreover, the lineament is not distinguishable from adjoining regions by anomalous end-Caledonian strains.In the Phanerozoic, the lineament became a recognizably discrete tectonic zone during or after the end-Caledonian regional deformation. Then, cleavage and folds along part of the lineament were rotated, and overprinted by a local cleavage, in a broad NE-SW dextral shear zone. This ductile event perhaps denotes the onset of simple shear deformation which affected Carboniferous sedimentation in northeast Wales and culminated in major strike-slip faulting. These faults, which dominate the geological maps of this part of Wales and include the Bala Fault itself, are essentially Variscan structures; their sense and amount of displacement remains uncertain.Previously underemphasized N-S Lower Palaeozoic faults are shown to be as important as the more familiar NE-SW structures in influencing sedimentation patterns along the lineament. It is suggested that the Lower Palaeozoic combination of N-S and NE-SW structures is widespread in the Welsh Basin and is due to distributed regional sinistral shear. Both sets of structures were reactivated repeatedly. In Cardigan Bay, Mesozoic-Tertiary sedimentary basins subsided on these structures, whilst the 1984 Lleyn earthquake and other historical seismicity denote neotectonic activity on the same structures.

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Ordovician palaeogeography of the southern and eastern flanks of the Harlech Dome, Merionethshire, North Wales

Cited by 12 publications

References 46 publications

A re‐examination of the type Arenig Series

A re‐examination of the type Arenig Series

Age and controls of ironstone deposition (Ordovician) North Wales

Tectonic evolution of the Bala Lineament in the Welsh Basin

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