FLUVIOKARST ON A SMALL CARBONATE ISLAND: GUAM, MARIANA ISLANDS

Karst of carbonate islands is distinct from that of continental settings. The islands’ young limestones, which are lithologically heterogeneous and retain high primary porosity, tend to cause diffuse recharge and preclude significant surface flow and subsurface conduit flow. On Guam, however, complex geologic histories have locally resulted in the development of unique karst features, including typical contact karst and fluviokarst landforms such as sinking streams, blind valleys, ponors, swallets, a border polje, dry valleys, valley dolines, uvalas, losing streams, through valleys, a gorge, stream caves, natural bridges, exsurgences, and resurgences. Most of these have previously not been reported from an oceanic carbonate island.

Guam consists of two physiographic provinces: northern Guam, a Plio-Pleistocene uplifted limestone plateau with two volcanic inliers; and southern Guam, a volcanic highland with several limestone outliers. In northern Guam, which generally lacks surface drainage, the volcanic inliers provide allogenic catchment areas. They contain deeply incised ephemeral blind valleys, which terminate at the surface contact with the surrounding limestone and feed into basement conduits. In the southern part of the plateau, low matrix porosity of the local argillaceous limestone has resulted in the development of genuine fluviokarst. The former surface drainage network is evidenced by deeply incised dry valleys and strings of valley dolines and uvalas. Due to low hydraulic gradients and extensive alluvial deposits, two allogenic and two autogenic streams continue to flow at the surface.

In southern Guam, the karst of limestone outliers is largely shaped by allogenic water from the surrounding volcanic terrain. The highest areas of southern Guam are capped by a Miocene reef unit which contains exsurgence springs and outflow caves at the surface contact with the underlying volcanic units. In the southeast, a narrow Pleio-Pleistocene limestone belt separates the volcanic highlands from the ocean and contains six allogenic through streams, a deep gorge, and abandoned stream caves. In the central basin of southern Guam, Miocene limestone outcrops are entirely surrounded by volcanic terrain and contain cockpit karst with allogenic stream caves and resurgences.