Reports

Report Number: 104
Year: 2004
 

Karst Features of Guam, Mariana Islands

This paper summarizes of the results from a two-year mapping survey undertaken to inventory and describe the karst of Guam in terms of the Carbonate Island Karst Model, a general theoretical model for the unique karst formed on small islands dominated by young (Cenozoic) limestone. Karst on Guam is found in two distinct physiographic provinces. The northern half of the island is an uplifted karst plateau formed on Pliocene-Pleistocene reef-lagoon deposits. In the south the karst is confined mostly to Miocene remnants on uplifted weathered volcanic terrain, except on the southeastern coast, where it is flanked by shallow fringing reef deposits contemporaneous with the limestone of the north. Across these two environments, the karst of Guam is remarkably diverse -exhibiting not only island karst features, but also classical karst features generally associated with continental karst in ancient limestone.

Because the fundamental motivation for the study was to better understand the karst aquifer of the island, the features are mapped and described in this paper in terms of five categories reflecting the sequence of water movement, namely (1) surface catchment features, (2) surface flow features, (3) closed depressions, (4) caves, and (5) discharge features. Surface catchment features include swallow holes, resurgences and artificial ponding basins and storm water injection wells, which are mostly placed in natural depressions where storm water collects to begin with. Surface flow features are associated with the older, more indurated limestone units of the south and the argillaceous limestone of the northern plateau and southeast coast adjacent to the volcanic terrain of the south. Surface flow features include streams originating on allogenic and autogenic catchments, insurgences, dry valleys, valley sinkholes, and contact and insurgence springs. Analysis of digital topographic maps revealed >1200 closed depressions deeper than 3 m on the plateau. Most are of primary depositional origin, but there are also numerous collapse, point recharge, and valley sinkholes. The largest and deepest sinkholes occur in cockpit karst in the interior of southern Guam. Caves on Guam include both vadose and phreatic caves. The former includes pit caves at the surface and stream caves along the bedrock-basement contact. The latter includes water table and halocline caves, and flank margin caves. Where water table caves form close enough to the surface, their ceilings may eventually collapse to form "banana hole" caves. Banana holes are rare on Guam, but have been found in the north. Lenticular voids that may be water table or halocline caves are exposed in the walls of deep quarries in the northern plateau. Flank margin caves are ubiquitous and well exposed everywhere along the limestone coasts. Discharge from the aquifer is via coastal springs and coastal seeps. Their distribution and density reflect the relative areas and shape of the catchments feeding them. Mapped discharge features include beach springs and seeps, reef springs and seeps, flowing fractures and caves, and submarine springs. The largest point discharge features in northern Guam are associated with fractures and caves, which are the characteristic discharge style in coastal areas occupied by sheer cliffs. Some high-level springs can be found in the interior, mostly rising at carbonate-noncarbonate contacts.

Author(s):
Danko TaboroŇ°i
John W. Jenson
John E. Mylroie