Pressure dissolution and cleavage in the Martinsburg Shale ( USA).

Abstract

The Martinsburg Shale in Pennsylvania, Maryland, W Virginia, and Virginia has undergone considerable deformation, and a pronounced cleavage has formed. Graptolites of known undeformed dimensions were passively deformed with the enclosing rock and provide quantifiable strain gauges. They record maximum shortening perpendicular to cleavage but are little distorted parallel to cleavage. At large bedding/cleavage angles (70-90o) shortening is 50-70%. As bedding/cleavage angles decrease, this apparent shortening in the bedding plane decreases according to the sine of the bedding/cleavage angle, while dimensions parallel to cleavage remain unchanged. Where bedding and cleavage are nearly parallel on limbs of folds, the apparent shortening decreases to nearly zero, yet no evidence for extension was found in any direction. These observations are consistent with material volume loss from the rock of approx 50%, parallel to cleavage planes but are not compatible with constant-volume deformation models. This material loss was caused by pressure dissolution of calcite and silicate minerals, and provides an example of cleavage formed in shales almost entirely by pressure dissolution.-Authors