Report Number: 89
Year: 1999

Karst Geology and Hydrology of Guam: a Preliminary Report

The karst geology of Guam is being inventoried and studied in detail for the first time. Previous work on carbonate islands in the Atlantic and Caribbean suggests that a systematic model can be developed to describe the fundamental geologic features that control the movement of natural waters, both fresh and marine, through the rock. The depositional, glacio-eustatic, and tectonic histories of carbonate islands give rise to a unique combination of karst features, which in turn determine characteristic aquifer properties for limestone units in carbonate islands. This report documents initial work during the first year of a two-year project aimed at developing a general Carbonate Island Karst Model (CIKM) by incorporating observations from Guam into the current conceptual models of island karst environments, which have been largely derived from observations of Atlantic-Caribbean islands. Differences in climate and tectonic history between the Indo-Pacific and Atlantic-Caribbean have produced features unique to each. Comparative study of islands from the two environments provides clues from each that will help resolve questions raised in the other and contribute to the development of a comprehensive general model. The broad categories of karst features on Guam that are being surveyed and classified according [to] their hydrologic properties include the epikarst, closed depressions and caves. Along with meteorological variables, particularly the temporal and spatial distribution of rainfall, these features determine the rate of surface infiltration, aquifer storage, and transport rates and paths. Aquifer discharge points have been mapped on the northwest coast of Guam, from Tumon Bay to Double Reef, and continue to be studied. Observations of the spatial and temporal distribution of recharge provide important clues regarding aquifer properties and the hydrologic characteristics of the terrain. Work to date has focused on compiling a comprehensive inventory of karst features on Guam and comparing them with analogous features on carbonate islands elsewhere. Specific related questions and supporting work include resolving the origin of the cliff face notches making former sea level stillstands, characterizing spring hydrography through water temperature measurements, conducting petrographic and paleomagnetic analyses to gain insight into uplift rates and other key historical questions, and re-examining previous interpretations of lithology on the island. Research priorities are listed for these, as well as subsequent studies.

John E. Mylroie
John W. Jenson
John M. U. Jocson
Mark A. Lander