Drilling-Induced Mineralization as a Formation Mechanism for Zebra Textures in Shale Cores

The interaction of stresses, fluid flow, and chemical reactions in sedimentary rocks controls basin evolution, fault behavior, and the performance of engineered subsurface systems. Yet, direct evidence for the timing and coupling of these processes remains elusive. Here, we provide evidence of calcite precipitation on the timescale of brittle failure in Campanian mudrocks potentially occurring during the drilling International Ocean Discovery Program (IODP) Hole U1581B in the Transkei Basin, offshore South Africa. Core samples recovered between depths of 850-980 m below seafloor exhibited zebra-like patterns formed by calcite veins cross-cutting a dark claystone matrix along with other drilling-induced core disturbances. The optical and scanning electron microscopy images indicate the zebra-like deformation structures are formed by syntaxial growth of calcite concurrent with the tensile and shear fracture development. Carbon and oxygen isotopic ratios from the calcite veins cluster outside the value range typical for natural marine carbonates deposited at seafloor. The values of strontium isotopic ratio scatter from the modern seawater to the Campanian carbonates. We proposed a scenario, supported by numerical simulations of drilling-induced core deformation and geochemical modeling, for the vein formation as the result of core decompression at elevated temperatures. The results of this study contribute to understanding in situ conditions and timescales for healing drilling-induced fractures by carbonate mineralization.