Origin of Slaty Cleavage in the Coeur d'Alene District, Idaho

Abstract

Deformational features in argillaceous rocks of the Belt Supergroup in the Coeur d9Alene district indicate that the slaty cleavage was produced by mechanical reorientation of original clays during an early period of folding. Furthermore, this regional folding event probably occurred soon after the deposition and burial of the younger units in the district. Regional folds and slaty cleavage are closely related geometrically. The cleavage is parallel to the axial planes of the folds in their hinges, but fans divergently along their limbs; that is, cleavage dips in the same direction as the bedding, but more steeply. Motion within the argillites during folding produced dimensional preferred orientations of detrital quartz and muscovite. Low-grade metamorphism produced crystallographic preferred orientations in quartz in the Prichard Formation but not in the shallower Wallace rocks. Metamorphism was synchronous or later than folding. In the Wallace Formation, injection structures of argillite intruded the sandy layers at the time of folding. The slaty cleavage was enhanced locally near the injection structures where the greatest amount of motion in the argillite occurred. Molar-tooth structures were folded both by compaction during burial and by shortening perpendicular to cleavage during folding. Mechanical reorientation of platy minerals is suggested by the following evidence: the strongest cleavages were formed in zones of greatest movement of argillite; a dimensional preferred orientation was produced mechanically in other detrital grains; the degree of recrystallization in the argillites was quite low; and there was no correlation between quality of cleavage and degree of recrystallization. The deformational environment is more difficult to deduce. The possibility that the environment was more diagenetic than metamorphic is suggested by the probable soft-sediment origin for the argillite injection structures, the lack of extensional deformation features in molar-tooth structures oriented parallel to the cleavage, and the presence of the bulk of small tight folds in the Wallace Formation rather than in deeper stratigraphic units.