Microfolding in the Castile and Todilto Evaporites, Texas and New Mexico

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The Castile laminated calcite-anhydrite of Eddy County, New Mexico, and Culberson County, Texas, and the Todilto laminated calcite of Valencia County, New Mexico, contain small-scale fold chains characterized by varying degrees of buckle shortening in successive laminae. These microfolds are associated with larger folds and have apparently resulted from compression generated by tectonism. In the Castile formation, anhydrite laminae, with an average thickness of about 1.1 mm, alternate with thinner calcite laminae (usually about 0.4 mm) which have thickened and thinned in the deformed zones to accommodate folding of the anhydrite laminae. Castile micro-folds generally have a wavelength of less than 1 cm and occur in hinge areas of “megafolds” with wave-lengths of about 0.5 to 2.5 m. The microfolds are cylindrical and have uniform axial trends. Particular zones in the Castile sequence were predisposed to microfolding, a tendency which apparently existed for tens of kilometers. In these zones, relatively thin anhydrite laminae commonly have been deformed by buckling, whereas thicker anhydrite laminae commonly have reacted to stress by increasing in thickness normal to bedding; laminae of intermediate thickness have been deformed by a combination of these processes. Thicker anhydrite laminae in some places have acted as struts which have prevented or reduced the degree of buckling of adjacent thinner laminae. The over-all style of microfolding has been determined usually by variability in the thickness of successive calcite and anhydrite laminae. In the Todilto limestone member, white laminae of finely crystalline calcite with thicknesses up to about 0.5 mm are intermittently present. These laminae commonly are micro-folded. Usually only a single lamina is involved in the microfolding, and adjacent dark colored, coarser crystalline limestone layers are unfolded. The folds, in profile, are of irregular pattern and usually have wavelengths between about 0.5 to 1.0 cm. In plan, the folds are composed of a complex array of miniature highs and depressions; no trend is apparent. The Todilto limestone at the locality studied apparently reacted to compressive stress by increasing in thickness normal to bedding as well as by buckling. The white, lithologically distinct laminae were subjected to buckle shortening, took up slack, and were microfolded in open spaces generated along bedding surfaces as layer-parallel shortening of adjacent dark limestone beds took place. Many microfolds in other laminated and thin-bedded sequences probably had origins analogous to those of the Castile and Todilto formations.