Pseudodiscordante gelaagdheid in gedeformeerde gesteenten van wisselende competentie

Discordant bedding is being reported with increasing frequency from rocks of almost any grade of metamorphism. It is, usually without much discrimination, cited as evidence of sedimentary origin or even as an indication of the sequence of deposition. In the present paper examples are given of discordant structures simulating both deltaic and torrential cross lamination but on closer inspection obviously due to differential stretching and variable orientation of cleavage in a thinly laminated quartzphyllite which has been isoclinally folded and subsequently flattened in the bedding plane. Microfabric analysis of micaceous cleavage planes can be a valuable aid in the distinction between discordant structures of sedimentary and deformational nature, and the more so the higher their degree of recristallisation and neomineralisation provided that these processes are strictly nimetic. Workers in the metamorphic field are cautioned to be on the alert for similar features of pseudo discordant bedding which is undoubtedly common in flattened and sheared rocks of variable competence such as the phyllonites and granulites, many of which have been derived from igneous rocks with a primary planar structure. No reliable recognition of top- and bottomsets can be made where discordant bedding has been affected by deformation along existing planes of weakness such as the lamination surfaces.