FRESH-WATER LENS ANISOTROPY AND FLANK MARGIN CAVE DEVELOPMENT, FAIS ISLAND, FSM

Fais Island, Yap State, Federated States of Micronesia, is an uplifted carbonate atoll 1.2 km wide and 2.9 km long with a maximum elevation of 28 m, located 220 km west of the island of Yap in the Western Pacific. Observation during negative spring tides shows that modern fresh-water lens discharge is concentrated where high-relief cliffs extend seaward beyond the beach and reef flats. Fresh-water flow from the beaches and reef flats is small to insignificant. Surface surveys indicate that flank margin caves are also concentrated in these headlands, and are conspicuously absent in the vertical cliffs that are inland of beach and reef flat areas. It has been demonstrated in Bermuda and the Bahamas that fresh-water discharge is slowest in the beaches and related young carbonate rocks, and enhanced in older rocks, which have experienced the longest contact with the fresh-water lens over numerous glacioeustatic sea-level highstands. The original porosity in the pre-Holocene carbonate rocks of Fais Island has been re-arranged into high-permeability flow systems by repeated exposure to the fresh-water lens. By extending past the lower permeability beaches and reef flats, the older headlands act as preferred high-permeability flow routes to conduct water from the lens to the sea. The outer boundary of the fresh-water lens therefore exhibits a high degree of anisotropy. The preferential location of flank margin caves in these headlands indicates that the current anisotropic flow regime was also active in the past, promoting mixing-zone dissolution in the distal margin of the fresh-water lens under the flank of the enclosing headland rock mass. At the same time, flank margin cave development between headlands was diminished by the lack of fresh-water lens discharge in those areas. A large closed-contour depression containing a fresh-water pool was initially reported to be a collapse sinkhole or blue hole, but field investigation interpreted it as a well dug and terraced in Holocene sands which infill a reentrant in a paleo-sea cliff. The low relative permeability of these sands creates a more substantial fresh-water lens than is available elsewhere on the island.