Northern Guam geospatial information server
Karst sinkholes in northern Guam
A fundamental part of drainage in karst terrains are closed-contour depressions generally referred to as sinkholes. This term is quite broad and imprecise, and bears different connotations in different fields. From the point of view of geomorphology, a sinkhole is any of many types of natural and relatively prominent depressions or holes in the surface topography caused by the removal of soil or bedrock, often both, by water and/or gravity. From the engineering perspective, a sinkhole is the site of gradually descending or suddenly collapsing land surface; this is sometimes due to excessive load placed by construction activities and represents a significant geologic hazard that can lead to extensive damage to structures and loss of human life. In hydrology and hydrogeology, sinkholes are key aquifer recharge features that focus the inflow of surface and shallow subsurface water and direct it through subterranean preferential pathways to the underlying groundwater body.
Many of the closed depressions in northern Guam are of primary origin, but also many are true karst sinkholes. At least four categories of karst sinkholes have been noted:
1. collapse sinkholes, formed by sudden collapse of subsurface voids;
2. point recharge sinkholes, formed by ephemeral allogenic streams coming off the flanks of Mt. Santa Rosa and Mataguac Hill and "sinking" underground as soon as they reach limestone at the edge of volcanic terrain;
3. valley sinkholes, found as beads-on-a-string series of holes within the dry valleys in the argillaceous limestone at southern end of the plateau, where former streams became gradually diverted to the subsurface; and
4. solution sinkholes, which form by gradual and localized lowering of the land surface due to slow removal of soluble material by focused inflow of storm water.
The first three types are not pervasive in northern Guam and are easily identifiable. The fourth type, solution sinkholes, are spread throughout northern Guam, have great hydrologic significance, yet are not easily distinguished from simple depositional depressions. The strongest indicators of a solutional origin, and thus true karst character of a depression, is the presence of one or more swallow holes, openings that lead to underground preferential pathways for water (somewhat analogous to a drain in a tub). In northern Guam, swallow holes are rarely visible on the sinkhole floor. Instead, they are generally covered by deposits of alluvium, thick soil and residual debris accumulated by storm water flow. Furthermore, determining the origin of any particular depression in northern Guam can be complicated by anthropogenic alteration of the surface, including filling for construction and urban development, reshaping into ponding basins, and quarrying, all of which are common. It is important to investigate any depression of interest in the field and thoroughly examine whether it exhibits sinkhole morphology before concluding anything about its origin.
To appreciate the true significance of karst sinkholes in the drainage system of northern Guam (and karst in general) it is important to understand the way they form and convey water from surface to the subsurface. Before we proceed, we should note here that a depression/sinkhole of dissolutional origin is technically known as solution doline, but for the sake of simplicity, we will refer to it as solution sinkhole. A solution sinkhole forms in topographic lows where ponding of water over time dissolves away sufficient amount of soluble rock (limestone and others) to cause the land surface to descend. Solution sinkholes are shaped as broad, gently sloping depressions that have been deepened and expanded over geologic time as storm water converged across the limestone surface into what originally must have been a much more modest depression and dissolved the soluble limestone surface of the depression downward and outward. Concurrently with this processes, topmost part of the bedrock within the sinkhole is intensely weathered to create a highly permeable zone known as the epikarst (or the subcutaneous zone). Though the amount of weathering and permeability of epikarst decreases with depth, its numerous pathways converge downward to solution channels, fissures, and systems of interconnected voids that conduct water from the sinkhole downward. These, in turn, converge on high-capacity conduits that convey the water deep underground through the surrounding bedrock of far lower permeability. In other words, a solution sinkhole is a special type of sinkhole, which forms not from sudden collapse (“sinking”) of the surface into a void below, but from slow dissolution of the surface downward by focused inflow of storm water. It drains the surrounding landscape by collecting surface water and conveying it to subsurface pathways that lead to the aquifer below. Sinkholes are, therefore, highly significant not only in geological terms, but also as hydrologic features. Solution sinkholes that overlay the Northern Guam Lens Aquifer represents the aquifer's key sites of concentrated recharge.
As a solution sinkhole grows over time, and water percolates through the overburden soils into the limestone formation below, part of the soil and sedimentary residue is transported underground. However, much remains and thick aprons of sediment typically accumulate in the bottom of the sinkhole. For that reason, sinkholes are often floored by thick layers of sediment and support the growth of vegetation. Most of this sediment is fine-grained and infills and coats the porous limestone surface at the bottom of the sinkhole. That inhibits the infiltration of storm water during intense storms. However, it also redirects flowing surface water and ponded water into particular locations within the sinkhole's floor where large and sudden inflow can be accommodated. These sites are known as swallow holes and lead into high-capacity vertical shafts that descending storm water has dissolved into the underlying bedrock and enlarged over geologic time. Such shafts typically converge downward into trunk shafts that descend all the way to the water table and thus provide routes for rapid drainage of sheeted and ponded water. Below is a textbook example of how flow paths typically converge to high-capacity vertical shafts within a solution sinkhole.