THE EXETER GROUP, SOUTH DEVON, ENGLAND - A CONTRIBUTION TO THE EARLY POST-VARISCAN STRATIGRAPHY OF NORTHWEST EUROPE

The lower part of the post-Variscan succession around Exeter, south De von, England, comprises some 800 m of breccias, with subordinate sands tones and mudstones, which rest upon Devonian and Carboniferous rocks folded during the Variscan Orogeny and are overlain, disconformably, b y the Aylesbeare Mudstone Group (Early Triassic?). These deposits comp rise the most westerly of the early post-Variscan successions preserve d onshore in northwest Europe and lie to the south of the Variscan Def ormation Front; they are assigned to the Exeter Group (new term). Geoc hronological and palaeontological studies, in conjunction with detaile d 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 pr e-dates intrusion of the nearby Dartmoor Granite (280 Ma). In the over lying, palynologically-dated, Late Permian sequence, older breccias co ntain clasts of the Dartmoor Granite aureole rocks, and younger ones c ontain clasts of that granite. The lower sequence occurs mainly within the Crediton Trough, an east-west trending, partly fault-bounded sedi mentary basin that probably formed by extensional reactivation of a Va riscan thrust. Breccias in this sequence formed largely on alluvial fa ns; the common occurrence of debris flows and a down-fan passage from gravity flows into fluvially deposited sediments is typical of deposit ion on semi-arid fans. The upper (Late Permian) sequence is more wides pread 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 ty ping. The stratigraphy revealed within the Exeter Group is broadly com parable with that recognized in the early post-Variscan Rotliegend suc cessions elsewhere in Europe. This similarity may, however, be decepti ve; the upper part of the Exeter Group may be coeval with the Zechstei n, and apparently correlatable major unconformities in the group and t he Rotliegend may reflect different events in the Variscan fold-belt a nd Variscan Foreland areas, respectively.