Background

Horizontal seams, such as stylolites, are characteristic structures that can emerge during the compaction, diagenesis and lithification of carbonate sediments and that affect both hydraulic and mechanical properties of the rock (Toussaint et al. 2018). Stylolites can enhance the lateral conductivity of limestones (Heap 2018) and cause the accumulation of clay minerals in bands of low frictional strength. Stylolites are generally accepted to be caused by pressure dissolution and accompanying processes but many aspects are not well understood.

As stylolites represent the emergence of a localized pattern in a bulk compaction process, they are likely to be a product of a non-linear coupling between at least one compaction mechanism and a transport process. Bonnetier et al. (2009) proposed that stylolites form due to a mechanical instability that arises from the coupling of strain-driven dissolution and fluid-phase transport at a rough interface. They developed a theoretical model that predicts that stylolite surfaces generate self-affine roughness with two regimes of roughness exponent that is diagnostic of the stress regime. However, pressure dissolution is also coupled with local grain damage and stress (e.g. Croizé et al. 2010; Ben-Itzhak et al. 2016), as well as other compaction mechanisms. A role for secondary minerals, particularly clays, in the generation and growth of stylolites has been proposed (e.g., Renard et al. 2001; Macente et al. 2018) but may not be general.

Since natural samples provide a single temporal observation of a dynamic process, progress in understanding stylolite formation would be grealy aided by laboratory studies in which such features can be reproducibly generated as a function of mineral assembly, stress and solution chemistry in order to disentangle the coupled processes operating at the micro-scale. Although some laboratory studies have been attempted, (e.g. Zubtsov et al. 2005; Gratier et al. 2005), for minerals such as calcite, however, the very slow kinetics of the processes involved makes the dynamics of these systems extremely challenging.

As a feasibility test, a system composed of compacted nitratine (including a synthetic “oolite”), glass spheres (an insoluble phase) and clay was compacted in a mini-triaxial cell for ~2 hours in a NaNO₃-saturated solution. X-ray tomography visualized the resulting microstructure, which includes an insoluble particle indenting the soluble nitratine “oolite” (Fig. 1b), a feature that is also present in natural systems (Fig. 1a). Pressure dissolution induced indentations are highlighted with the red arrows.

Synthesis and Characterization of Nitratine Rock Analogs

Multiple forms of nitratine crystals and rock analogs can be fabricated. (1) Small euhedral crystals with controlled size can be obtained in a timeframe of a few days by crystallization from aqueous solution. Color-labeling the crystals could enable advanced quantitative optical imaging strategies as well. (2) Large monocrystals of high crystallinty, mimicking calcite {104} form, can be easily obtained by slow crystallization (seeded) from aqueous solution. These crystals can be used for e.g. single-contact experiments. (3) Given its low melting point and ease of crystallization, samples with a marble-like texture can be obtained from melting nitratine powder. This enables the fabrication of samples of a “synthetic marble analogue” with specific shapes by casting, to control the crystallinity by controlling the cooling rate, and to obtain the analogue of the pure “crushed marble” samples usually employed for studies involving calcite.

Replication of Pressure Dissolution Fabrics in the Laboratory

Mechanisms of Pressure Dissolution Initiation and Fabric Development

Time-lapse synchrotron X-ray tomography with a mini-triaxial cell (Voltolini et al. 2017) will provide detailed mechanistic insight into the initiation of pressure dissolution at the scale of individual grain–grain contacts and the coupling of multigrain deformation processes that give rise to compaction fabrics. ¼-inch diameter column samples allow for the best compromise of field of view and resolution, providing datasets where the quantitative analysis of fundamental parameters, such as evolution in shape/size and orientation of grain contacts, grain and pore space morphometry, anisotropy, etc. can be carried out. This kind of analysis provides a quantitative evaluation about variables such as the role of carbonate-clays interactions, or grains size distributions, which are fundamental to understand the mechanisms such as the local triggering of a pressure dissolution seam, or the generation of pressure dissolution-related rock fabrics (e.g. cleavage, pressure shadows, etc.).

References