Critical Review of Criteria for Distinguishing Structures Developed Before and After Lithification
Paul F. Williams
The factors influencing the morphology of folds and other structures are numerous and complex, and the morphologies are correspondingly variable. Therefore, the degree of lithification that existed at the time of deformation cannot be distinguished from the morphology of the structure.
Various criteria have been proposed for distinguishing between postlithification and prelithification structures. Truncation of folds by erosional surfaces is commonly cited as evidence for soft-sediment deformation, but the same morphology is common in hard-rock folds. Axial-plane cleavages have been cited as evidence for a hard-rock origin, but are known also from soft-sediment folds. There may, however, be some differences in the details of cleavage morphology. Melanges are particularly difficult because they commonly seem to combine prelithification and postlithification structures, but deformed quartz veins and clast shape may provide clues concerning the degree of lithification at the time of deformation. For example, some structures described as diapiric melange can be shown to deform quartz veins and must therefore be hard-rock structures. Breaking up of alternating sandstone and shale such that the sandstone forms angular blocks floating in a shale matrix indicates hard-rock deformation.
Perhaps the most definitive criterion for prelithification deformation is the presence of trace fossils that cut the deformation structure. For a hard-rock origin, deformation of individual grains or veins, in a manner consistent with the form of the structure, is considered the most reliable criteria, although it must be used with care.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.