Primary crenulation pencil cleavage

Abstract

Previous pencil cleavage models rely on either pressure solution or independent grain rotation as the principal shortening mechanism. In contrast, samples from the Rose Hill Formation and the Marcellus Shale from the Central Appalachians show a primary tectonic crenulation fabric dominated by domainal grain rotation in microfold limbs. The crenulations are identified in crossed-polarized light where they appear as alternating light and dark bands of optically aligned phyllosilicates in the microfold limbs. Relatively rare pressure solution developed concentrations of insoluble residue and thinned bedding laminae within microfold limbs. A systematic relationship of pressure solution to microfold limbs in more deformed samples indicates that microfolding preceded pressure solution. Shortening, in six samples without pressure solution, ranges from 9.2 to 20.4% and exhibits no direct relationship to the length/width ratios for the pencils. A new model for pencil cleavage begins with compacted shale having a strong bedding-parallel alignment of inequant grains. Compression causes microfolding, developing a primary crenulation pencil cleavage fabric. Pressure solution is then initiated within the microfold limbs and continues as a major shortening mechanism which overprints bedding to form planar cleavage.