COVES AND POCKET BEACHES: LITTORAL EROSION OF FLANK MARGIN CAVES

Coves and pocket beaches recently investigated on tropical, carbonate islands commonly represent remnants of pre-existing karst voids. Traditionally most cuspate coastlines in bedrock have been attributed to differential rates of littoral erosion along planes of weakness such as soft lithologies, faults, and fractures. Recent investigations in the Mariana and Bahama Islands have revealed that some cuspate coastlines result from coastal breaching and erosion of flank margin caves that developed at the edge of the freshwater lens where freshwater/saltwater mixing occurs. Evidence for spelean origin includes small alcoves and phreatic passages extending from the collapse features, ceiling remnants, bedrock pillars and true speleothems (as opposed to littoral speleothems). This phenomenon is similar to caleta development in Yucatan, Mexico, where discharging freshwater conduits mix with saltwater at their seaward margins, which increases local dissolution that induces conduit collapse that migrates inland to create a cove. The results presented here are the first documentation that diffuse freshwater flow alone can accomplish the same result.

The breaching of flank margin caves to create a cove has a direct influence on the diffuse flow pattern in the freshwater lens, as flow adjacent to the cove will now bend towards the cove for it is a closer discharge site to the sea. This excess flow from the sides of the cove will generate additional mixing, in a positive feedback effect that will widen the cove faster than it will deepen inland. Flow to the back of the cove remains constant, but as the sides widen, they continue to bend new flow lines to them. As a result, large bays will develop that are wide but not especially deep. Such a progression exists on Guam, where Haputo Bay represents the early stage of such bay development, Tumon Bay a mature form, and uplifted Sasajyan Bay a fossil form. Tumon Bay shows the largest springs are discharging near the bay sides, not at the bay center. Such bay enlargement removes evidence of the initiation of the feature as a breached flank margin cave or caves.