The Exeter Group, south Devon, England: a contribution to the early post-Variscan stratigraphy of northwest Europe

Abstract

The lower part of the post-Variscan succession around Exeter, south Devon, England, comprises some 800 m of breccias, with subordinate sandstones and mudstones, which rest upon Devonian and Carboniferous rocks folded during the Variscan Orogeny and are overlain, disconformably, by the Aylesbeare Mudstone Group (Early Triassic?). These deposits comprise the most westerly of the early post-Variscan successions preserved onshore in northwest Europe and lie to the south of the Variscan Deformation Front; they are assigned to the Exeter Group (new term). Geochronological and palaeontological studies, in conjunction with detailed geological mapping, show that the constituent formations comprise a lower (Late Carboniferous(?)-Early Permian) sequence separated from an upper (Late Permian) sequence by an unconformity which represents an hiatus with a duration of at least 20 m.y. The lower sequence contains volcanic rocks dated at between 291 and 282 Ma (Early Permian) and pre-dates intrusion of the nearby Dartmoor Granite (280 Ma). In the overlying, palynologically-dated, Late Permian sequence, older breccias contain clasts of the Dartmoor Granite aureole rocks, and younger ones contain clasts of that granite. The lower sequence occurs mainly within the Crediton Trough, an east-west trending, partly fault-bounded, sedimentary basin that probably formed by extensional reactivation of a Variscan thrust. Breccias in this sequence formed largely on alluvial fans; the common occurrence of debris flows and a down-fan passage from gravity flows into fluvially deposited sediments is typical of deposition on semi-arid fans. The upper (Late Permian) sequence is more widespread but includes similar deposits overlain, at the top of the Exeter Group, by aeolian dune and interdune deposits. Correlation within the laterally variable facies associations which comprise these sequences has been achieved using a combination of sedimentary facies analysis, sedimentary geochemistry, and petrographical and geochemical clast typing. The stratigraphy revealed within the Exeter Group is broadly comparable with that recognized in the early post-Variscan Rotliegend successions elsewhere in Europe. This similarity may, however, be deceptive; the upper part of the Exeter Group may be coeval with the Zechstein, and apparently correlatable major unconformities in the group and the Rotliegend may reflect different events in the Variscan fold-belt and Variscan Foreland areas, respectively.

Heavy-mineral, mineral-chemical and zircon-age constraints on the provenance of Triassic sandstones from the Devon coast, southern Britain

“…However, magnetostratigraphy suggests that the base of the OSF in the outcrops of south Devon corresponds to a slightly younger level (Hounslow & McIntosh, 2003). Late Permian red-beds underlying the Aylesbeare Mudstone Group in the western part of the Wessex Basin clearly document the unroofing of the Permian (Variscan) granites of SW England in the Early and Middle Permian (Edwards et al, 1997). The BSPBF is composed of orthoconglomerates and subordinate sandstones, representing the deposits of a braidplain or wet alluvial fan (Smith, 1990;Smith & Edwards 1991).…”

“…8). The absence of Early Permian zircons in the BSPBF and OSF indicates that the Cornubian massif to the west had ceased to supply clastics into the western Wessex Basin by the mid-Triassic, even though it supplied clastic material during the Permian (Edwards et al, 1997). It also appears that the unconformity that separates the Sherwood Sandstone and Aylesbeare Mudstone Groups was not associated with sufficient uplift to rework the underlying Permian, at least in the south Devon area.…”

“…The scarcity of Late Carboniferous and Early Permian zircons in the Devon coast succession suggests that the catchment area did not extend into regions of Armorica that include widespread Variscan granites, and therefore implies that the source was more proximal, probably in the English Channel and possibly extending into the Channel Islands, the La Hague peninsula and the northern Brittany coast, where Variscan granites are comparatively minor (Brun et al, 2001;Sansom et al, 2005). Furthermore, the Early Permian Dartmoor granite is only some 25 km away from the Devon outcrop, and is known to have been unroofed in the Late Permian since it supplied a significant amount of clastic detritus to the Permian Exeter Group that underlies the Early-Middle Triassic Sherwood Sandstone Group and Early Triassic Aylesbeare Mudstone Group (Edwards et al, 1997). Derivation from the Cornubian area immediately to the west therefore also seems unlikely.…”

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

“…Southwest England occupied a lower plate position during late Devonian convergence; continental collision initiated in the earliest Carboniferous and Variscan shortening migrated northwards through the passive margin (Franke, 2000;Shail and Leveridge, 2009). The youngest deformed successions are late Carboniferous (Moscovian) in age (Edwards et al, 1997). Regional metamorphism of all Devonian-Carboniferous successions seldom exceeds very low grade (epizone-anchizone) (Warr et al, 1991).…”

“…Radiometric data indicate that lamprophyres, basalts and granites are contemporaneous (e.g. Thorpe, 1987;Edwards et al, 1997;Dupuis et al, 2015).…”