THE CONSEQUENCES OF FRACTURE FLOW IN A COASTAL KARST AQUIFER: A CONCEPTUAL MODEL

Fracture flow in karst systems has traditionally been viewed as occurring in the epigenic fresh-water realm, or in deep hypogenic systems involving acidic or thermal waters. These fracture flow systems are considered intermediate between slow matrix flow at one boundary condition, and rapid conduit flow at the other boundary condition.

In eogenetic karst aquifers, where fracture or conduit flow occurs, the matrix is the major aquifer storage component. Matrix evolution by dissolution creates touching-vug flow systems that may then become the dominant water transport mechanism. Coastal carbonate aquifers are dominated globally by eogenetic conditions in tropical locations, as the carbonates are proximal to their depositional environment. As a result, fracture influences are subdued. Fractures can become more important as diagenetic maturity of the host carbonate proceeds and occludes matrix and touching vug permeability, or if the fractures create an efficient preferential flow path within the matrix. Tortuosity within touching-vug flow regimes results in a transition to fracture flow as recharge-to-discharge distance increases.

In telogenetic coastal carbonates, such as in New Zealand or the Croatian coast and islands, mixing dissolution is restricted to scattered fractures and bedding planes. However, when fracture density increases, mixing dissolution occurs across a volume of rock, as opposed to scattered planes, and flank margin caves develop that may approach the cave pattern associated with mixing zone dissolution in eogenetic coastal carbonates. In New Zealand, the largest and best developed flank margin caves occur in highly tectonized teleogenetic limestones. In Croatia, paleotalus breccias of telogenetic limestones form the largest and best developed flank margin caves. In Mallorca, the transition from micritic lagoonal limestones to porous reef limestones results in dissolution pathways that follow joints in the former, and are anastomatic in the latter, indicating a switch from fracture to matrix control based on facies.