Burial stylolites favouring Mg-rich fluid ingress and fluid-rock interaction: petrophysical variations during regional dolomitization in the Lessini Mountains (Southern Alps, Italy)

Rock petrophysical properties, including porosity and permeability, are fundamental factors in regulating fluid ingress, flow and fluid-rock interaction across various length scales and tectonic settings. The composition of fluids, and their modes of ingress into- and reaction with host rocks, in turn influence bulk rock properties. Therefore, fluids can significantly alter rock rheology, potentially modifying their density, long-term viscosity, porosity and permeability. Mineralizing fluids may also change the rock composition, heal fractures, and promote strain hardening within active fault systems, potentially impacting the seismic cycle.

To better understand these processes and the governing background conditions, it is particularly useful to investigate the pathways for fluid flow, especially in rocks with low primary porosity and permeability, which would normally hinder significant fluid circulation. In micritic, horizontally-bedded carbonates, for example, vertical fluid flow is generally limited far away from tectonic fractures, whereas lateral, bed-parallel circulation is favoured exploiting laterally continuous planar anisotropies (e.g., bed-bed interfaces), to progressively permeate the succession. Secondary anisotropies, such as pressure-solution seams (e.g., stylolites), may represent other interesting features for fluid flow, being at times very abundant in limestone. However, they are typically considered to reduce the permeability and porosity of host rocks due to (i) the cementation and precipitation of dissolved materials in pores within the immediately surrounding rock and (ii) the accumulation of insoluble, fine-grained and low-permeability material on their surfaces. Recent studies in carbonates challenged this assumption, demonstrating that burial stylolites can be preferential pathways for karst dissolution.

We present here new data on the petrophysical properties of Cretaceous micritic limestone from the Lessini Mountains, Veneto, Italian Southern Alps, where much of the exposed, generally sub-horizontal Mesozoic carbonate succession underwent pervasive dolomitization during Eocene extensional tectonics and the onset of the Venetian Volcanic Province magmatism. Petrographic analyses, Hg-porosimetry, and He-pycnometry were applied to assess the effects of Mg-rich fluids, probably connected with the volcanic environment, on micritic limestone. Preliminary results indicate that burial stylolites developed in low porosity limestone are selectively dolomitized, with dolomitization seams 1-10 mm thick, suggesting pervasive bedding-parallel fluid flow. Dolomitization occurred also along vertical fractures and in fracture meshes of mosaic breccias, suggesting across-bedding Mg-rich fluid circulation associated with normal faults accommodating.

Dolomitization significantly increased rock density from ~2.65 g/cm³ in the pristine micritic limestone to ~2.9 g/cm³ in the fully dolomitized rock. Additionally, pore size, porosity, and the capillary threshold pressure of Hg injection change gradually but substantially from the limestone to the dolomite, with median pore sizes increasing from ~0.012 μm to ~0.33 μm, porosity from ~3.2% to 21.8%, and capillary threshold values decreasing from ~5140 to ~35 PSI.

These results demonstrate that, under specific conditions, stylolites can actually serve as effective pathways for fluid ingress/migration and thus promote fluid-rock interaction in rocks characterized by overall low porosity and permeability (e.g., micritic limestone). Furthermore, we show that dolomitization significantly modified the petrophysical rock properties, further enhancing fluid ingress and likely promoting fracturing and brecciation by changing rheology, with consequences for deformation localization and partitioning during later tectonic activity.