MECHANICAL LAYER EVOLUTION IN CARBONATE AQUIFERS

Carbonate aquifers in fold-thrust belt settings often have low matrix porosity and permeability, and thus fracture and fault zones with high porosity and permeability dominate groundwater-flow pathways. Methods from sedimentology and structural geology are combined to understand the evolution of fracture-controlled flow pathways and to determine their spatial distribution in case examples from the Apennines and Albanides. Bed-parallel, pressure-solution surfaces (PS1) are identified as a fracture type that influences fragmentation in peritidal and basinal carbonate, and upon shearing provides a major flow pathway in fold-thrust belt carbonate aquifers. Stratigraphic analysis and fracture mapping demonstrates that depositional setting plays a critical role in PS1 localization and spacing. In the peritidal and basinal environments, solution seams form in mud-rich intervals, with the resulting pressure-solution surface geometry influenced by sedimentary geometry (i.e., stacked fining-upward cycles, burrows, planar laminations). Peritidal strata have more closely spaced PS1 than basinal strata, however, in both facies types laterally continuous PS1, upon shearing, behave as mechanical layer boundaries. As layer-parallel slip increases to accommodate shear strain in the fold-thrust belt, additional PS1 behave as mechanical layer boundaries. Where stratigraphy is known, this understanding of sedimentological control on PS1 localization can aid development of geologically based fracture-flow models.