Carbonate Island Karst Model (CIKM)
Carbonate island karst is unique type of karst that develops in young, diagenetically relatively unaltered limestone units. It is highly distinct from classical types of karst that occur in old, diagenetically mature limestones. On Guam, karst forms reminiscent of classical karst can only be found in older limestones of central southern Guam and clay-rich limestones in the southern part of northern Guam plateau. The vast majority of karst on Guam, including practically the entire northern Guam plateau, can be described as carbonate island karst.
Carbonate island karst develops in young (Cenozoic) carbonates on islands and along coasts and is conceptually distinct from continental karst. Its development is partly controlled by the lithologic heterogeneity and high primary porosity of the host limestones, which tend to be young, diagenetically immature units, for which Vacher & Mylroie (2002) have proposed the term eogenetic karst. These rock qualities cause fresh water infiltration and percolation to be predominantly diffuse, obviating surface flow and subsurface conduit transport, which are typical of continental karst. Carbonate island karst is further defined by the effects of differential dissolution associated with the mixing zones of vadose water, phreatic groundwater, and marine water; the migration of the mixing zones in response to glacio-eustatic and tectonic variations in relative sea level; and the position of the contact between the carbonate platform and the underlying non-carbonate basement with respect to sea level (Mylroie and Jenson 2000).
The contact between the overlying carbonate rock, in which the karst terrain develops, and the underlying non-carbonate basement rocks that form the core of carbonate islands exerts important control on fresh water movement, conduit development, and fresh water interaction with marine water. Where the contact intersects the land surface, surface water from the higher-standing non-carbonate terrain dissolves sinkholes in the adjacent carbonate rock where it descends along the contact. In the vadose zone, stream caves develop along the contact as descending water follows the less permeable basement to the water table. Where the contact passes through the fresh water phreatic zone down to the fresh water-saltwater interface, it isolates the base of the fresh water lens from the influence of seawater. Based on the position of the contact relative to the fresh water-sea water interface and the island surface, carbonate islands can thus be classified into four ideal types (see diagram below):
1. Simple carbonate island karst forms where carbonate rocks extend from the surface to beneath the fresh water lens.
2. Carbonate cover island karst develops where non-carbonate basement rocks rise above sea level to partition the fresh-water lens but are not exposed at the surface.
3. Composite island karst is where non-carbonate rocks partition the lens and are exposed above the limestone surface; and
4. Complex island karst may exhibit characteristics of all three previous models and contain sedimentary units of intermediate or mixed carbonate-noncarbonate lithology and/or complex structurally modified stratigraphic relationships.
Diagram above from Jenson, J. W., Keel, T. M., Mylroie, J. R., Mylroie, J. E., Stafford, K. W., Taboroši, D. and Wexel, C. 2006, Karst of the Mariana Islands: The interaction of tectonics, glacio-eustasy, and freshwater/seawater mixing in island carbonates. In: Harmon, R. S. and Wicks, C. (eds.) Perspectives on karst geomorphology, hydrology and geochemistry – A tribute volume to Derek C. Ford and William B. White: Geological Society of America Special Paper 404: 129-138.
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While many islands can be described in terms of a single ideal type, northern Guam locally exhibits elements of each of the first three carbonate island karst environments (see diagram above). One percent of its land surface is occupied by the volcanic outcrops of Mt. Santa Rosa and Mataguac Hill, which protrude through the limestone plateau (composite island model). Beneath approximately 21% of the land surface, the basement extends above sea level, but is not exposed at the surface (Vann 2000) (carbonate cover island model). When the lowest long-term relative stillstand of 95 m below the modern sea level (based on submerged marine terraces mapped by Emery in 1962) is considered, an additional 37% of the northern Guam’s modern surface may also have fit the carbonate cover island model during the past. Finally, the basement is sufficiently deep under 41% of the plateau surface that it has probably not stood above sea level (simple carbonate island model) since the overlying limestone bedrock was deposited, assuming that the current level of tectonic uplift is the highest the island has experienced.
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