CARBONATE ISLAND KARST OF SAIPAN

Saipan, in the western Pacific Ocean, is part of the Commonwealth of the Northern Mariana Islands (CNMI). It is a 125 square km composite island with a central highland of Eocene and Oligocene volcanic and volcanic-derived sedimentary rocks interfingering with, and mantled by, limestones of Late Eocene to Pleistocene age. Tectonic deformation, primarily as high-angle normal faults, has created separate carbonate aquifer compartments on the island. The island perimeter, and inland scarps, contain abundant flank margin caves at specific horizons, indicating sea-level stillstands as a result of tectonic uplift and glacio-eustatic interaction. Some very large voids at higher elevations may represent cave development prior to the glacio-eustasy of the Pleistocene, allowing mixing-zone dissolution in the freshwater lens to proceed for a longer time at a given position. Stream caves have developed on the carbonate flanks of the volcanic core as a result of allogenic recharge from the volcanics. Several springs and resurgent streams are noted; however, the typical discharge from the aquifer compartments is either below current sea level, or is distributed along the coast as diffuse flow. The northern Saipan plain has numerous collapse caves indicating significant dissolutional void development at depth, followed by progradational collapse towards the land surface. Kalabera Cave is an abandoned lift tube, with a rise of 55m, which developed to release water confined in an aquifer compartment by normal faults along strike, and by interfingering volcanic sediments down dip. Analysis of caves and karst features, both active and fossil, by use of the Carbonate Island Karst Model (CIKM) can illuminate the aquifer characteristics of the various compartments, and serve to assist water resource development for the island population.