Pencil structure and strain in weakly deformed mudstone and siltstone

Weakly deformed mudstone and siltstone (Middle Ordovician Knobs Formation) of the Appalachian Valley and Ridge Province, south-western Virginia, U.S.A. show strain-dependent transitions between bedding fissility, pencil structure and cleavage. Pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain (Z) ranges between 9 and 26% shortening (assuming a plane strain and constant volume). Where strains are less, bedding fissility dominates. Pencil fragments are defined by intersecting fracture sets subparallel to the pre-existing bedding fissility and cleavage. Their long axes are both parallel to the bedding-cleavage intersection and to the inferred Y axis of the tectonic strain ellipsoid. Pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post-tectonic stress relaxation. Pencils show variations in size and shape depending on lithology (grain size and composition), degree of initial clay preferred orientation, degree of cleavage development, type of cleavage, total bulk strain and degree of strain homogeneity. Their shape (l/w) however is a direct measure of total Y/Z strain since strain determinations from chlorite pressure-fringes on framboidal pyrite within the pencil fragments give

$\text{(}\text{Y}\text{Z}\text{) = 0.913 + 0.019 (l/w)}$

. Pencil structure is therefore a potential strain marker in weakly deformed rocks.