Devonian rift-related sedimentation and Variscan tectonics – new data on the Looe and Gramscatho basins from the resurvey of the Newquay District

The geological resurvey of the Newquay District (Geological Survey Sheet 346) has resulted in stratigraphical and structural revision. The Devonian successions form part of the Looe and Gramscatho basins and broadly young to the south throughout the area. Deposition of the green to purple mudstones and sandstones of the Whitsand Bay Formation (Dartmouth Group) had initiated by the latest Lochkovian and was conformably succeeded by the sandstones, mudstones and bioclastic limestones of the Bovisand Fomation (Meadfoot Group). The newly defined Trendrean Mudstone Formation (Meadfoot Group) is dated as mid-Emsian or younger on the basis of palynological studies. These three formations respectively record the transition from lacustrine/fluvial through shallow marine to outer shelf/slope depositional environments during Lower Devonian rifting and the development of the Looe Basin. The lowermost part of the Gramscatho Basin succession is represented by the undated Grampound Formation (Gramscatho Group) that has a faulted contact with the underlying Looe Basin succession. It predominantly comprises mudstone but includes sandstone-dominated ‘packets’ (Treworgans Sandstone Member) consistent with an outer shelf and/or slope depositional environment along the northern margin of the Gramscatho Basin. The conformably overlying Porthtowan Formation (Gramscatho Group) comprises mudstones and sandstone-mudstone couplets and is entirely deep marine. Variscan primary deformation (D1) resulted in isoclinal folding and an associated axial planar cleavage throughout both successions. The ‘Watergate Bay Antiform’ of earlier workers is discounted; the associated outcrop geometry of the Dartmouth and Meadfoot groups is thrust-controlled. D2 deformation is developed around Porth Joke (Looe Basin succession) and intensifies southwards towards the Gramscatho Basin, probably in response to the NNW thrusting of the northern ‘parautochthonous’ margin of the Gramscatho Basin over the southern margin of the Looe Basin. An anomalous 900 m wide zone of steeply dipping S2 cleavage around Penhale Point is interpreted as primarily reflecting reorientation by a large-scale southwards-verging monoformal F3 fold. The structural complexity within the boundary zone possibly reflects a pre-Devonian basement fault influence upon: (i) the transition from shelf to deep marine depositional environments during the Lower-Middle Devonian, (ii) Variscan thrust juxataposition (D1 and D2) of the Looe and Gramscatho basin successions, and (iii) D3 post-Variscan extensional reactivation and reorientation of earlier fabrics

Geology of the Newquay district : a brief explanation of the geological map Sheet 346 Newquay

This Sheet Explanation presents an account of the geology of the district covered by the Geological 1:50 000 Sheet 346 Newquay, published in 2012. The district is mainly underlain by Upper Palaeozoic rocks of Devonian age. These have undergone deformation in the Variscan Orogeny, a northward propagating mountain building event which commenced in the Devonian in this area, and are variably folded and faulted. Minor granite intrusions of latest Carboniferous to early Permian age occur near the coast and Cenozoic deposits are present locally around St Agnes, in the west. The regional strike in the district, and across the Cornish peninsula, is east–west, so coastal exposures afford a structural crosssection that is almost at right-angles to strike. Several notable early workers commented on aspects of the geology in the district, among them Borlase (1758), Conybeare (1817) and Sedgwick (1820), though the earliest detailed geological notes were published in 1839 by Sir Henry De la Beche, to accompany a geological map published in the same year. Further metalliferous lodes were added subsequently to De la Beche’s map by Sir W W Smyth (1858), and the work was republished in 1866. Survey at the same scale (1:10 560) by Clement Reid, J B Scrivenor and D A MacAlister was carried out around 1900–1905 and published in 1906 at a scale of 1:63 360. A reprint of the Newquay geological sheet, based predominantly on previous work, but converted to the modern 1:50 000 scale, was published in 1974

Geology of the Wellington district : a brief explanation of the geological map Sheet 311 Wellington

This Sheet Explanation provides a summary of the geology of the Wellington district, the area covered by 1:50 000 scale geological Sheet 311 (England and Wales). It also provides directions to further geological information about the district. The district includes parts of the counties of Somerset and Devon, within the local authority districts of Taunton Deane, South Somerset, Mid Devon and East Devon. The principal centres of population are the towns of Wellington and Chard, the southern outskirts of Taunton in the north and the western part of Ilminster in the east. Much of the district is occupied by the northern and central parts of the Blackdown Hills Area of Outstanding Natural Beauty (AONB), which was designated in 1991. The topography of the district is dominated by the Blackdown Hills (front cover), a dissected plateau that rises steeply to a maximum elevation of 315 m above Ordnance Datum south of the gently undulating Vale of Taunton Deane. In the northwest the ground rises to around 160 m. North and east of the Blackdown Hills complex, rivers such as the Isle and Tone flow north-westwards and west towards the Bristol Channel. Within the south of the Blackdown Hills, the rivers Culm, Axe, Otter and Yarty drain southwards towards the English Channel (Figure 1)

Geology of the Wellington district : a brief explanation of the geological map Sheet 311 Wellington

This Sheet Explanation provides a summary of the geology of the Wellington district, the area covered by 1:50 000 scale geological Sheet 311 (England and Wales). It also provides directions to further geological information about the district. The district includes parts of the counties of Somerset and Devon, within the local authority districts of Taunton Deane, South Somerset, Mid Devon and East Devon. The principal centres of population are the towns of Wellington and Chard, the southern outskirts of Taunton in the north and the western part of Ilminster in the east. Much of the district is occupied by the northern and central parts of the Blackdown Hills Area of Outstanding Natural Beauty (AONB), which was designated in 1991. The topography of the district is dominated by the Blackdown Hills (front cover), a dissected plateau that rises steeply to a maximum elevation of 315 m above Ordnance Datum south of the gently undulating Vale of Taunton Deane. In the northwest the ground rises to around 160 m. North and east of the Blackdown Hills complex, rivers such as the Isle and Tone flow north-westwards and west towards the Bristol Channel. Within the south of the Blackdown Hills, the rivers Culm, Axe, Otter and Yarty drain southwards towards the English Channel (Figure 1)