The Nature of Deformation in Experimentally Deformed Calcitecemented Sandstones

Abstract

<p> Stress-shortening data and the micro- and macro-fabrics or experimentally deformed calcite-cemented graywacke were analyzed in order to understand the nature of deformation in calcite-cemented rocks. Forty four room temperature triaxial experiments were conducted on the Blairmore sandstone in the range of 2.1 - 19.8 percent shortening, 1 - 2600 bars confining pressure, at strain rates on the order of lo-4/second. Under these condition, the normal transition from longitudinal fracture, at low confining pressure, to limited homogeneous flow, at high confining pressure is observed. The strength ( σ1, - σ3 ) increases from 2. 3 kilobars (kb) at 1 kb confining pressure (op), to 5.9 kb at 2.6 kb cp. At low confining pressures, deformation takes place primarily by brittle fracture of the calcitee At high confining pressures, the calcite deforms primarily by twinning, and the sand grains deform by fracturing parallel to σ1. The transition in deformational behavior of this rock is similar to the transition observed in orthooalcite rocks. However, in contrast to orthocalcite rocks, (1) strength is enhanced, (2) ductility is markedly reduced, end (3) the brittle-ductile transition is suppressed to much higher confining pressures. The behavior of this rock is analyzed by considering a two-phase model material consisting of a dispersion of strong, brittle particles within a weak and/or ductile matrix. Principles developed through the study of particulate reinforced composite engineering materials indicate that the mechanical behavior of these materials is dependent upon the mechanical interaction of the two phases. The sand grains act to constrain flow of the ductile matrix. Concurrently, plastic deformation within the matrix results in the development of stress concentrations within the nearly rigid sand grains. These enhanced stresses may result in the initiation and propagation of fractures within the sand grains. Propagation of the fractures is parallel to the maximum principal stress and the fractures characteristicly develop in this manner. Although the inherent physical properties of the individual phases determine the strain development within the individual phases, it is the interaction of the two phases which determines the unique behavior of the composite material. This behavior in turn controls the strength of the material and exerts an important influence on the development of the deformation fabric. The two-phase model not only provides insight into the behavior of calcite-cemented sandstones and other analogous rocks, but also makes it possible to predict how these rocks will behave when factors such as temperature and strain rate are permitted to vary. </p>