A history of cleavage and folding: An example from the Goldenville Formation, Nova Scotia

Temporal relationships between cleavage formation and folding in sedimentary rock sequences can vary from either phenomena preceding the other to both occurring simultaneously. Knowledge of this relationship, however, is required to understand fully the relationship of cleavage orientation to finite strain and the origin of cleavage refraction across competent-incompetent layers. Evidence from the Goldenville Formation provides an example of cleavage formed before folding by ∼50% pressure-solution removal of material, resulting in cleavage planes normal to bedding during active folding. This layer-parallel shortening was accompanied by excess pore pressure that in some cases, exceeded lithostatic pressure, causing formation of bedding-parallel quartz veins with crack-seal textures. Folds developed by a mechanism of differential flexural flow with no loss of coherence across bedding surfaces. En echelon veins formed sequentially across some beds during active folding and subsequently were deformed by various increments of bedding-parallel shear. After folds reached ∼80° limb dips, continued horizontal shortening was accommodated by additional pressure solution, but no recognizable new cleavage was formed. Because cleavage in the Goldenville Formation formed perpendicular to bedding before folding and was passively rotated and extended by bedding-parallel simple shear during fold amplification, the resulting cleavage planes cannot lie in the XY plane of the finite-strain ellipsoid. Differential shear between metagray-wacke and slate beds accounts for the observed cleavage refraction, and the bedding-cleavage angle provides a good estimate of the shearing across these beds during active folding. If cleavage can be shown to postdate folding in other examples, it is possible that cleavage planes will lie in the XY plane of the finite-strain ellipsoid.