Flow of fresh seawater onto the tidal flats and salinas of the south end of the island of Bonaire takes place through permeability conduits in the underlying Pleistocene terrace limestone and emerges as numerous springs. Reflux of some heavy brine probably is taking place continuously because of gravitational instability. In addition, evidence suggests the existence of a major annual reflux event in early summer that causes a significant loss of heavy brine. This annual event is controlled as follows: the total pressure of the brine exerted at the level of an impermeable clay layer within the Holocene sediments is greater than the pressure exerted by the seawater at equivalent level. This annual return flow of brine moves through some of the same permeability conduits tha supply fresh seawater through most of the year. Dolomite formed by such a hydrological system should be confined to permeability paths such as faults or dissolution channels under an evaporitic sediment in older rock. This pattern of rock-selective or permeability-controlled dolomitization by downward flow of brine through older rocks may be common in the geologic record.

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