Eye and sheath folds in turbidite convolute lamination: Aberystwyth Grits Group, Wales

Eye and sheath folds are described from the turbidites of the Aberystwyth Group, in the Silurian of west Wales. They have been studied at outcrop and on high resolution optical scans of cut surfaces. The folds are not tectonic in origin. They occur as part of the convolute-laminated interval of each sand-mud turbidite bed. The thickness of this interval is most commonly between 20 and 100 mm. Lamination patterns confirm previous interpretations that convolute lamination nucleated on ripples and grew during continued sedimentation of the bed. The folds amplified vertically and were sheared horizontally by continuing turbidity flow, but only to average values of about γ = 1. The strongly curvilinear fold hinges are due not to high shear strains, but to nucleation on sinuous or linguoid ripples. The Aberystwyth Group structures provide a warning that not all eye folds in sedimentary or metasedimentary rocks should be interpreted as sections through high shear strain sheath folds.

► Eye folds occur in turbidite convolute lamination viewed down the palaeoflow direction. ► The convolutions formed during bed deposition and are overturned down-flow by current shear. ► The eye folds are sections through non-cylindrical sheath folds due to this shear. ► Average shear strains are less than 1.0, lower than generally necessary to generate sheath folds. ► Instead, sheath folds nucleate on already sinuous current ripples lower in the turbidite bed.