Journal of Cave and Karst Studies - ISSN 1090-6924
Volume 63 Number 1: 9-22 - April 2001

A publication of the National Speleological Society

Karst features of Guam in terms of a general model of carbonate island karst
John E. Mylroie, John W. Jenson, Danko Taborosi, John M.U. Jocson, David T. Vann and Curt Wexel

Abstract

This paper describes the karst of Guam in terms of the Carbonate Island Karst Model (CIKM), a general model for the unique karst carbonate islands. The CIKM recognizes several processes and conditions unique to carbonate islands: 1) enhanced dissolution at the surface, base, and margin of the fresh-water lens; 2) the history of both vertical migration and stable positioning of these zones according to glacioeustatic and tectonic changes in relative sea level; 3) the size of the island’s catchment area relative to its perimeter, which can vary with sea level change; 4) the hydrologic implications of the unique eogenetic environment of their young limestones; and 5) the position of the island’s carbonate-basement contact relative to sea level and the island surface over time.

Guam’s complex depositional and tectonic histories have endowed it with a unique legacy of karst features. The northern half 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. Most closed depressions are broad and shallow, probably reflecting original depositional morphology, although vertical-walled collapse and banana-hole type features are also present. 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 for three types of coastline around the northern plateau: (1) deeply scalloped embayments with broad beaches; (2) linear beaches fronted by fringing reefs; and (3) sheer cliffs with narrow or no benches, and only occasional small reefs. In the embayments, karst groundwater discharges by diffuse flow from numerous seep fields and as concentrated flow from springs along the beach. Seeps are found along the linear beaches, but we have not noted significant flow from springs or coastal caves. Along the cliff-dominated coast, karst groundwaters discharge in spectacular flows from dissolution-widened fractures, coastal caves, and submarine vents, most notably along the northwest coast.

The southern half of Guam is an uplifted volcanic highland containing a karst terrain on Mio-Pliocene limestone remnants in the interior. Because these units lie above the influence of the fresh water lens, sea water mixing, and sea level change, the karst is a classic tropical continental karst, with features that include contact springs issuing from well-developed caves, sinking streams with resurgences, and conical cockpit karst. Along the southeast coast, which is flanked by a continuous uplifted Pleistocene limestone unit, antecedent streams draining the interior have incised deep canyons. Dry valleys and large closed depressions in this unit appear to be associated with allogenic waters originating in the interior.

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