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

Abstract

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.