Fracture network evolution in laminites

Laminites are rather complex carbonate rocks, deposited in lacustrine environments that typically consist of laminae of precipitated carbonate particles (Figure 1). To better understand the influence of these characteristics on deformation behaviour, the mechanical response needs to be studied systematically, especially as most current understanding appears to be limited to field observations. Triaxial compression tests were performed on 3 cylindrical laminite samples (Ariripe Basin, Brazil) to investigate their mechanical response and the evolved deformation geometries relative to the rock properties and loading conditions. Even though the lamination angle is not constant, all plugs were cut so that horizontal laminations are dominant. The samples were deformed under triaxial conditions with confining pressures of 20, 30 and 40 MPa and all tests were stopped just after reaching peak stress. Post-deformation X-ray tomography (XRT) was carried out to allow detailed, non-destructive, 3D analysis of the deformation features, including the fracture network geometries. The laminite sample deformed at 40MPa confining pressure provisionally appears to exhibit compactive features. However, the laminite samples deformed at 30 and 20 MPa confining pressures both show complex fracture networks comprising (benchtop) open fractures and shear fractures. Quantification of dilation and compaction features are still on-going. This suggests that the laminite changes from dilation-dominant to compactive -dominant responses between 30 and 40MPa confining pressures. XRT analysis shows that the overall strike of the fracture network remains constant and independent of layering. The image analysis of the post-deformation plug photographs shows that the angle of fracture dip changes at lamination boundaries. All angles are measured clockwise from the top surface of the plug. Shear fracture dip ranges between 65° and 90°, whereas the now-open fractures range between 79° and 86°. Example XRT images are shown in Figure 1. They indicate that the observed fracture distribution variation could be linked to characteristics such as particle size, layer composition and layer boundary effects. This is in agreement with field observations of laminite microfabrics (Calvo et al., 1998). These deformation experiments show that laminites develop complex deformation features and fracture relationships. Further systematic testing is required to improve the understanding in relation to carbonate composition, structure and pore-solid networks.

Calvo et al., 1998. Sedimentology, 45, 279-292