The stratigraphy and structure of the Devonian rocks around Liskeard, east Cornwall, with regional implications

Abstract

A formal stratigraphy based on new macrofaunal evidence is proposed for a little-known area of slates, thin limestones, and volcaniclastic rocks of Lower Devonian to early Upper Devonian age around Liskeard, between South Devon and north Cornwall. Open shallow marine shelf conditions persisted from the Emsian until the onset of basinal sedimentation in the Frasnian, with the local development of pillow lavas and pillow fragment breccias during Eifelian and Givetian times. Regional stratigraphic correlations identify a Middle Devonian axial ridge which runs from Tor Bay to Bodmin, and separates a redefined Trevone (north Cornwall) Basin from the south Cornish Basin. A moderate to steeply dipping slaty cleavage (Dl) dips southward throughout the area and is associated with early E–W major folds, inferred to be north- or upward-facing, which plunge gently east. A penetrative, closely spaced crenulation cleavage (D2) is developed in a zone, >0.7 km thick, which trends E–W and dips at 15º to 30º south. This fabric, previously misidentified as the D1 slaty cleavage, is associated with major D2 folds which have near horizontal, east–west trending axes; structures of this age and geometry have not been reported from elsewhere in the region. The Portwrinkle Fault has been traced NW–SE across the area and passes west of Liskeard; it shows a large dextral strike-slip displacement and may have developed early in the history of the thrust belt.