The Impact of Stylolite Morphology on the Mechanical Strength of Limestones and Ultrasonic Coda-Waves

Abstract

Stylolites are rough dissolution surfaces ubiquitous in sedimentary rocks, which locally exercise a strong influence on porosity and cementation, resulting in a change in strengths and permeabilities of the host rock. Thus, these structures are important in many applications in geosciences. Yet, little research has been done to test the effect stylolites have on the strength of limestones, due to their seemingly obvious role as stress concentrators. Furthermore, there has been no research into whether the morphology of stylolites plays a role into this weakening, and to what extent. Ultrasonic monitoring using coda-wave decorrelation has shown to be effective in monitoring temporal changes in homogeneous rocks during loading tests in the laboratory. However, no research has shown that this technique can detect small changes in rock deformation for rocks with stylolites. To investigate the effect stylolites and their morphology has on the mechanical and acoustic behaviors of limestones experimentally, uniaxial compression tests with active source ultrasonic testing were performed on Treuchtlinger Marmor samples containing sedimentary stylolites and Meyrargues Limestone samples containing tectonic stylolites. Samples were grouped based on stylolite morphology using a novel classification
scheme utilizing the geometric features of 2D traces of the stylolites. Acoustic data was analyzed in conjunction with stress-strain curves, by utilizing coda-wave decorrelation, which allowed small perturbations such as microcracks and pore-collapse in the rocks to be detected. Additionally, X-ray micro-CT experiments were conducted on some samples pre and post-deformation, for greater insight into the weakening effect of stylolites. These experiments found that stylolites, for almost all samples, weaken the rock, however, this weakening effect depends on the degree of stylolite morphology. In these cores, the larger the stylolitized zone, the greater the strength of the rock, when compared to samples containing stylolites. Additionally, stylolite morphology impacts the frequency content of ultrasonic waves, whereby rougher stylolites attenuate higher frequencies during loading.