Northern Guam geospatial information server

Karst in northern Guam

Karst is a geological phenomenon that develops in limestone and other soluble rocks through the dissolving action of water. It is a collective term for the surface landscape and subsurface features that form in such rocks and include characteristic landforms such as caves, sinkholes, and springs. Typical of karst terrains is that existence of surface waters is limited and there are few to no rivers or lakes. Drainage is largely subterranean, through underground conduits and other voids.

Guam’s complex depositional and tectonic histories have endowed it with a unique legacy of karst features. The northern Guam is almost entirely a karst landscape. It is a Pleistocene karst plateau in Plio-Pleistocene limestone units that exhibit all the characteristic karst features of carbonate islands, from the simplest to the most complex. The epikarst is similar to that on other carbonate islands. Many of the closed depressions are broad and shallow, probably reflecting original depositional morphology, although there are numerous sinkholes of undoubtedly dissolutional origin, deep vertical-walled collapse sinkholes, and shallow banana-hole-type collapse features. Caves include a few pit caves, some of which are very deep. Traversable stream caves occur where the limestone-basement contact is exposed on the flanks of volcanic outcrops. The most abundant caves are flank margin caves, which can be found all along the modern coast, and which occupy distinct horizons in the faces of the cliff line surrounding the plateau. Discharge features have been documented throughout the coastline around the northern plateau, but are more numerous and voluminous on the Philippine Sea side.

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