RELATIONSHIPS OF STRATIGRAPHY, LITHOLOGY AND FOLDING

Folds develop in layered stratigraphic sequences undergoing contraction because of effective viscosity contrasts among different lithologies. In a typical sequence undergoing first deformation, the most viscous (competent) lithologies are usually sandstones or limestones, whereas less viscous (incompetent) lithologies are mudstones and evaporites. This contribution investigates how the arrangement of competent and incompetent lithologies in stratigraphic sequences controls the size, mechanics and geometry of the folds.

Much of the large body of research on folding has focused on buckling of single layers, for example revealing dominant wavelength-thickness relationships to viscosity ratios. However, most natural folds are multilayered, affecting sections of stratigraphy. Models of multilayer folding have mostly concentrated on regular alternations of competent and incompetent layers that can be considered as statistically anisotropic media. Anisotropic media fold more readily than isotropic media, and so regularly laminated or layered rocks will experience stronger folding than uniform massive beds. However, the strength and scale of folding also depends on the embedding medium, and rocks will rarely have a semi-infinite soft embedding medium of the kind assumed in many models. Application of current theories and models to nature are also made difficult, because we do not have direct information on viscosity ratios between competent and incompetent rocks. Some recent work suggests that the orders-of-magnitude viscosity ratios assumed in early fold models are too high, and the effective competent/incompetent viscosity contrasts among many sedimentary rocks may be nearer to 10 than 100. A competent/incompetent viscosity ratio of 10 in alternating layers is sufficient to generate significant folding in a multilayer.

Simple rules are sought for the relationships of folds to stratigraphy, focusing on three questions. (1) What controls the wavelengths of folds in stratigraphic sequences? (2) How important is the viscous anisotropy of multilayered competent/incompetent rocks? (3) Can a knowledge of the statigraphy indicate the mechanism of folding, to predict the deformation patterns in different lithologies? The answers are important, geologically and economically.