A morphological classification of rock cleavage

Abstract

Existing classifications of rock cleavage are confused, with many overlapping terms and genetic connotations. Different names are commonly used by various observers to describe the same fabric. It is argued that rock cleavage should be classified initially on the basis of morphology only. The most useful measurable parameters are: (1) the spacing of cleavage domains; (2) the shape of cleavage domains; (3) microlithon fabric; and (4) the proportion of the rock occupied by cleavage domains. A simple descriptive scheme based on this morphology has a primary subdivision into continuous or spaced cleavage. Continuous cleavage exists where the cleavage results from the preferred orientation of platy minerals distributed evenly throughout the rock, or where the spacing of cleavage domains is so close that they cannot be resolved at the scale of observation. Continuous cleavage is further subdivided according to the mean grain size of the cleaved fabric. Spaced cleavage is subdivided into crenulation and disjunctive cleavage, depending on whether a pre-existing mechanical anisotropy is present. Crenulation cleavage can be further subdivided into discrete or zonal types depending on the relation of the cleavage domains to the crenulated fabric. Disjunctive cleavages can be divided into stylolitic, anastomosing, rough and smooth, depending on the shape of the cleavage domains. If cleavage morphology is measured routinely, it should be possible to define natural associations of particular morphologies (which may then be given specific names), and to examine change in morphology as a function of external gradients such as either total or incremental strain, rock type and metamorphic grade. A morphological classification of rock cleavage will greatly enhance communication, thus assisting in discussions about the genesis of particular rock fabrics.