Comparison of deformation mechanics for two different carbonates: oolitic limestone and laminites

Carbonate rocks form under a range of conditions which leads to a diverse rock group. Even though carbonatesare overall mineralogically simple, the solid-space distribution ranges from simple compositions such as ooliticlimestones to highly complex networks of pores and solids as seen in coquinas. Their fundamental mechanicalbehaviour has been identified to be like clastic rocks (Vajdova 2004, Brantut, Heap et al. 2014). However itis very likely that this observation is not true for more complex carbonates. Triaxial tests were performed oncylindrical samples of two different carbonates; a) oolitic limestone (Bicqueley quarry, France) and b) laminite(Ariripe basin, Brazil). The samples were deformed under confining pressures of 8, 12 and 20MPa, and 20, 30 and40MPa, respectively. All tests were stopped as soon as peak load was observed to preserve as many deformationcharacteristics as possible. Photographs of the samples were taken before and after deformation to allow surfaceanalysis of deformation features. Additionally, samples were analysed post-deformation with X-ray tomography(XRT) (using the Zeiss XRadia XRM 520 at the 4D Imaging Lab at Lund University). The 3D tomography imagesrepresent the post-deformation samples’ density distribution, allowing detailed, non-destructive, 3D analysis ofthe deformation features that developed in the triaxial testing, including the complex geometries and interactionsof fractures, deformation bands and sedimentary layering. They also provide an insight into the complexity ofdeformation features produced due to the carbonate response.Initial results show that the oolitic limestone forms single shear bands almost the length of the sample, exhibitingsimilar characteristics to sandstones deformed under similar conditions. These features are observed for all threeapplied loads. The laminate sample deformed at the lowest confining pressure exhibits compactive features.However, the laminite samples deformed at the two higher confining pressures both show highly complex fracturenetworks comprising open fractures and fracture propagation. This suggests that the laminate changes fromcompactive to dilational responses over the selected confining conditions. The XRT analysis indicates that a morecomplex fracture distribution could be linked to rock component properties e.g. grain size and composition. Forthe laminite these are variable with the layers. This is in agreement with field observations of laminite microfabrics(Calvo, Rodriguez-Pascua et al. 1998). Additionally, the typical grain size of the laminate (m) is much smallerthan the oolitic limestone (mm), which suggests that fracture network complexity can also be linked to bulksystem complexity i.e. pore & grain network.These deformation experiments show that, as previously observed, oolitic limestones seem to behave similarly tosandstones. However this observation is not true for laminites and it is very likely that more complex carbonateswill develop even more complicated deformation behaviour. It is therefore necessary to systematically testdifferent carbonate rocks to understand the impact of geometry and composition, as well as the interplay with thepore network.