THE CARBONATE ISLAND KARST MODEL APPLIED TO ROTA (LUTA), COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS

Rota, the third largest island in the Commonwealth of the Northern Mariana Islands in the western Pacific, is tectonically active and composed of a core of Eocene volcanic rock mantled by Tertiary, Pleistocene, and Holocene limestone. Rota is classified as a Complex Island within the Carbonate Island Karst Model (CIKM) based on the presence of 1) exposed volcanic rocks and clay soils (the Sabana, the Talakhaya, and Santa Cruz areas), 2) limestone/volcanic interfingering (exposures near Gagani and Okgok), and 3) faulting. All the karst features predicted by the CIKM for a Complex Island are developed on Rota. Flank margin caves are well developed at Sagua Cave Complex, at Tonga Cave in Songsong Village, at As Matmos, in the cliffs below Taksunok, and at the Swimming Hole. The Swimming Hole is a sea level feature which is apparently a collapsed flank margin cave; analogous to the caletas of the Yucatan. The Reyes Cave Complex and Crab Hunter Cave are examples of flank margin caves in which the cave morphology was influenced by the dip (depositional) of the beds. Stream caves fed by allogenic catchment are developed in the Sabana area: Sabana Caves #1, #2, and #3. Contact springs at the limestone/volcanic contact are developed at the Water Cave, As Onan Spring, and along the coast near Okgok, adjacent to Black Cobble Cave. Sea level discharge from the freshwater lens occurs along the north coast between Tatachok and Teteto and at the Swimming Hole.

Although not directly predicted by the CIKM, caves formed along brittle fractures are also developed on Rota. Bonus Cave, Matan Cave, Deer Cave, Idle Threat Cave, Taisacan (Antigo) Museum Cave, and Pictograph Cave are mapped examples. Whereas flank margin caves tend to consist of circular, interconnected spaces, brittle fracture caves are quite linear. On Rota, they typically extend about 100 meters horizontally before ending abruptly. It is proposed that brittle fracture caves are not classic continental conduit caves but that they developed during past sea level high stands as fresh water (preferentially discharging from the island along fractures) formed zones of enhanced dissolution by mixing with sea water. Cave enlargement occurred as mixing zones worked headward in the fractures, not by overall dissolutional widening of the fracture as in continental caves.