Staircase fractures in microbialites and the role of lamination related mechanical anisotropy: The example of the Acquasanta Terme travertine deposits (central Italy)

This study describes a peculiar, yet common type of fracture showing a staircase trajectory, which forms in rocks with moderately weak planar anisotropies. The staircase fracture trajectory is given by alternating fracture segments oriented parallel to (LaP) or at an angle (ramp) with respect to the lamination/layering. The analyses has been accomplished on travertines, which are continental microbial/hydrothermal deposits having a typical poorly stratified yet strongly laminated texture. In these rocks, porosity and permeability have a high across-lamination variability and are mostly controlled by an interconnected and locally corroded array of permeable layers, fractures, and faults. Structural analysis integrated with analytical modeling provided a conceptual model of staircase fracture localization as a function of the travertine lamination dip. Lamination-parallel fracture segments localize within the porous laminae, mostly at the interface with tight laminae. Ramp-type fracture segments cut the lamination, connecting lamination parallel segments. Two types of staircase fracture can be modeled. The first group develops in sub horizontal to gently dipping deposits (lamination dip < 30 degrees) corresponding to low-energy depositional environments. The second group relates to staircase fractures developed in moderately to steeply dipping laminations (lamination dip > 30 degrees) and corresponds to high-energy environments. Major discoveries of hydrocarbon have been recently made in continental (lacustrine) microbial carbonates in the Brazilian South Atlantic margin, some of which exhibit a texture similar to those usually observed in travertines. Understanding of the lacustrine carbonates is still at an early stage. Given that in modern rift settings, vent-related thermal (travertine) and nonthermal (tufa) carbonates are a major component, the proposed conceptual model of staircase fracture localization contributes to the preparation of a model for the potential occurrence of high-permeability pathways in hydrocarbon and geothermal microbial reservoirs.