TINIAN, CNMI: A CARBONATE ISLAND KARST MODEL EVALUATION

Tinian is a Composite Island as classified by the Carbonate Island Karst Model (CIKM) and is the second largest island in the Commonwealth of the Northern Mariana Islands (CNMI). It rises to a maximum elevation of 187 meters with a surface area of 102 square kilometers that is composed of Eocene volcanic rocks cropping out in the north-central highland, which are mantled by Tertiary and Pleistocene carbonate rocks. High angle normal faults, resulting from tectonism, have divided the island into five main physiographic regions that can be evaluated separately: the northern lowland and median valley fit the Simple Carbonate Island model; the central plateau and southeastern ridge fit the Carbonate-Cover Island model; and the north-central highland fits the Composite Island model. Eogenetic karst, expressed in a wide variety of forms, is common throughout Tinian. Extensive horizons of flank margin cave development, which indicate previous sea-level stillstands where mixing zone dissolution occurred, exist along coastal cliffs and inland scarps (Suicide Cliffs). Large flank margin caves have developed near Unai Dangkolo that are laterally extensive and contain individual chambers up to 10 meters in height. Point source recharge features from allogenic waters on volcanic outcrops form closed depressions at non-carbonate/carbonate contacts. Coastal discharge features include diffuse discharge (Unai Dangkolo) and several joint controlled, point discharge stream caves (Unai Masalok). Fractures formed by cliff margin failure and faulting have produced extensive karst zones with partially roofed karst features over 500 meters long (Laderan Masalok and Puntan Vicente). Pit cave development on the southern margin of the island has produced vadose fast flow routes. Additional features may represent horizontal stacking of banana-holes that have undergone collapse by the upward stoping of lower chambers. Evaluation of the cave and karst features, both active and fossil, provides insight into the hydrogeologic properties of the island that can assist in managing water resources from currently developed wells. In addition, it can be used to identify regions where parabasal waters may provide sites for future wells in order to produce sustainable, potable water.