STRUCTURALLY CONTROLLED EOGENETIC KARST: MARIANA ISLANDS

Eogenetic karst development associated with brittle failure has been identified on Aguijan, Rota, and Tinian, Mariana Islands. Structural control of karst development is common in continental settings, where the rock exhibits low porosity and is highly fractured. Eogenetic karst is generally associated with caves formed by mixing zone dissolution, primarily independent of structural controls, in rocks that have never been buried beyond the range of meteoric diagenesis and retain high primary and syngenetic porosities. Recent investigations show that zones of brittle failure can control cave development in eogenetic rocks, independent of mixing zone dissolution, by providing zones for the preferential flow of fluids through the subsurface, similar to kluftkarren or fissure cave development observed in continental settings.

Zones of brittle failure in the Mariana Islands include: faults produced from active tectonic uplift; fractures resulting from bank-margin and smaller-scale scarp failures; and joints produced from passive isostatic subsidence or associated with regional tectonism. Three types of cave development have been observed in association with these zones of brittle failure: (1) Cave development near faults form features at moderate to steep angles along the dip that extend laterally along the strike as a result of dissolution of the planar surface and collapse of the hanging wall. (2) Cave development along fractures near parallel to coastlines and scarps, produced from bank-margin failure, form laterally extensive features that can extend near vertically to more than 40 meters depth. (3) Cave development perpendicular to coastlines and inland scarps, with discernable fractures or joints in the floors and ceilings, represent active or paleo-discharge features that may exhibit horizontal widening from the addition of mixing zone dissolution. These failure zones provide planar surfaces for the movement of fluids both vertically and laterally, creating fast flow routes through the subsurface that may act as point source recharge features, focused discharge features, and can distort the freshwater lens morphology.