Outstanding examples of small-scale folds in the Castile formation are of considerable interest to structural geologists because, owing to the low-strain condition of the rock (∼25% shortening), they afford a view of folds that have not yet formed pervasively throughout the multilayer. Some folds have long, along-the-layer continuity involving few of the layers of the multilayer sequence. Others, predominantly in the hinge zone of larger folds (wavelength ≳1 m), have more predominant components of across-the-layer folding. Combinations of these components result in dendriform patterns of distribution of folds and possible zones of interference between systems.
One of the most striking features of the Castile folding is the presence of planar layers between folded layers, creating internal fold systems. Even more striking is that competent layers both thicker and thinner than folded layers remain planar. We believe that current fold theories for the onset of folding, which are based on the premise of folding perturbations being identically unstable, cannot adequately explain this observation. A published theory is discussed that incorporates both gravity and surface-tension effects and offers an explanation for this observation. It suggests that folding in the Castile was likely a phenomenon akin to a Kelvin-Helmholtz instability, that is, a critical rate of strain must be exceeded before folding occurs.