The role of initial fabric on strain determination from deformed ellipsoidal objects

Abstract

Twenty-one samples of undeformed conglomerates, sandstones, tuffs, oolites, and pisolites from Precambrian to Pleistocene in age were collected from a variety of geologic environments. Each sample was sectioned on three mutually-perpendicular faces with one face parallel to bedding. Selected samples were also sectioned at 60 and 30 degrees to the bedding plane. The shape and orientation of at least 100 particles were measured on each section. Particle shape ranges from nearly circular in some samples to highly eccentric in others. Sections parallel with bedding have essentially random fabrics (no preferred orientation of particles). Sections not parallel with bedding have a preferred orientation of particles along the bedding trace but particles with all orientations do exist. The fabric distributions on the polar plot (Elliott, 1970) are often irregular in shape and the initial circle point lies well within the distribution in all cases. Transformation equations for the final shape and orientation of an individual particle using known shapes and orientations of the initial particle and the strain ellipse are used to construct the resultant fabrics after homogeneous simulated strains of several directions and magnitudes. The resultant fabrics can be used as a test of utility and accuracy of strain determination methods. A simple algebraic method of strain determination is found to be fairly accurate. If the initial fabrics measured in this study are representative, the accuracy of the method can be increased with the use of an error function, introduced herein.