Geological Society of America Special Paper 404 2006 Karst of the Mariana Islands: The interaction of tectonics, glacio-eustasy, and freshwater/seawater mixing in island carbonates John W. Jenson Water and Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, 96923 Guam Thomas M. Keel Joan R. Mylroie John E. Mylroie Kevin W. Stafford Department of Geosciences, Mississippi State University, Mississippi State, Mississippi 39762, USA Danko Taboroši Laboratory of Geoecology, School of Environmental Earth Science, Hokkaido University, Sapporo, Japan Curt Wexel Water and Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, 96923 Guam ABSTRACT Insights from previous karst studies of the relatively simple- and stable-carbonate islands of the Caribbean and Western Atlantic have been applied to develop the carbonate island karst model (CIKM), a general model with which we can interpret the karst of more complicated islands in the Western Pacific. This paper summarizes the karst of the five southernmost Mariana Islands in order of increasing complexity. All exhibit complicated histories of tectonic uplift and subsidence overprinted by glacio-eustasy. Each, however, is distinct and can be described in total or by subunit in terms of the four idealized carbonate island types defined in the CIKM: (1) simple-carbonate island, (2) carbonate-cover island, (3) composite island, and (4) complex island. Aguijan is a simplecarbonate island, but contains a probable phreatic-lift cave draining a confined aquifer. Tinian illustrates application of the CIKM to subunits: the northern lowland is a simplecarbonate island area, while the southeastern ridge fits the carbonate-cover island category, and the central portion fits the composite-island category. Rota is a composite island grading laterally from the volcanic core into a carbonate-cover island, thence to a simple-carbonate island from the southwestern highland to the plains and terraces north and east. Northern Guam is a simple-carbonate island ringing a carbonate-cover section, which contains a small composite-island portion. Southern Guam exhibits composite- and complex-island features. Saipan is a complex island, where syndepositional volcaniclastic units interfingering with limestone are faulted to create isolated aquifers, including confined aquifers drained by phreatic-lift caves. Keywords: carbonate island karst model, eogenetic karst, flank margin cave, island karst, Mariana Islands, mixing-zone dissolution. 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, p. 129–138, doi: 10.1130/2006.2404(11). For permission to copy, contact editing@geosociety.org. ©2006 Geological Society of America. All rights reserved. 129 130 J.W. Jenson et al. INTRODUCTION In most respects, karst on large islands (e.g., the Mogote Karst of Puerto Rico or the Cockpit Karst of Jamaica) is indistinguishable from tropical continental karst, particularly in that the rock is out of reach of the coastal zone of freshwater/ seawater mixing and, therefore, is not affected by sea-level change. Karst on carbonate coasts and small islands, by contrast, reflects conditions and processes absent from interior settings. In particular, most carbonate coasts, and almost all carbonate islands are composed of limestones that have not been extensively compacted or cemented, retain much of their primary depositional porosity, and have never been isolated from the meteoric diagenetic environment. Vacher and Mylroie (2002, p. 183) thus described the terrain and the pore system developing in such rocks as “eogenetic karst,” which is consistent with Choquette and Pray’s (1970) definition of the “eogenetic” phase of early burial in carbonate-rock evolution. In spite of this common and unifying trait, however, carbonate islands exhibit a wide array of geomorphic forms and rock relationships. This diversity is apparent in the Mariana Islands, in which variations in primary depositional environments, glacioeustasy, and ongoing tectonic activity combine to produce a bewildering variety of forms and features. Of the 15 Mariana Islands (Fig. 1), the southern six (Guam, Rota, Aguijan, Tinian, Saipan, and Farallon de Medinilla) contain carbonate rocks that are present as a veneer over volcanic rocks (Cloud et al., 1956). Geologic mapping in the Marianas began prior to World War II by the Japanese (e.g., Sugawara, 1934; Tayama, 1936). During and after the war, the U.S. Geological Survey (USGS) compiled reports for Guam (Tracey et al., 1964), Tinian (Doan et al., 1960; Gingerich and Yeatts, 2000), and Saipan (Cloud et al., 1956). No USGS surveys were conducted on Rota, Aguijan, or Farallon de Medinilla. Guam, with important military installations and the largest population, has been the subject of significant research on its geology and hydrology (c.f. Mink and Vacher, 1997). Field research to sort out the unifying and distinguishing traits of island karst began in the Bahama Islands, which are tectonically stable, consist exclusively of young, unaltered limestone, and show the effects of mixing dissolution along the island margins (Mylroie and Carew, 1990). Subsequent work in Bermuda identified the role of noncarbonate rocks in contact with carbonates in the subsurface (Mylroie et al., 1995). Further study in tectonically uplifted islands, such as Isla de Mona, where caves formed prior to the onset of Quaternary glacio-eustasy, elucidated the contribution of the freshwater lens position to cave development (Frank et al., 1998). From these studies, Mylroie and Carew (1997, their Fig. 1) developed an initial model of distinct carbonate-island environments. In 1998, studies began in the Mariana Islands to apply observations and insights from the western Atlantic–Caribbean to the geologically more complex Mariana Islands. The primary result of this work is the carbonate island karst model, or CIKM (Mylroie et al., 2001), which provides a Figure 1. Location of the Mariana Islands and maps of each of the five carbonate islands discussed in the text, with important geologic features on each island listed. framework for defining and interpreting island karst (Fig. 2; Table 1). This paper briefly describes the Mariana Islands in terms of the CIKM. Landforms in Eogenetic Karst The unique landforms of eogenetic karst are illustrated in Figure 3 and summarized here in three categories: (1) surface features, i.e., epikarst and closed depressions, (2) subsurface features, or caves, and (3) interface features by which water enters or leaves the subsurface. Cave entrances of any type are, by definition, interface features. Intersections caused by retreat of cliffs and hillslopes, or by collapse, are also interfaces between the surface and subsurface environments. Small carbonate islands with eogenetic karst rarely develop extensive linear caves, rather only mixing chambers, since their high hydraulic conductivity promotes diffuse drainage around the entire perimeter. Because island perimeter only increases in direct proportion to the radius, while the catchment increases in proportion to the square of the radius, the capacity of an eogenetic karst island to drain itself efficiently by diffuse flow Karst of the Mariana Islands 131 swales. Faulting and folding can also create closed depressions. Though not excavated by karst processes, such depressions are drained by diffuse secondary dissolutional pathways, so that ponded water accumulates only during the most intense rainfall. Rapid infiltration precludes development of classic conical sinkholes. Instead, water is collected in the epikarst and lost to percolation, or is transmitted laterally to descend along vadose fast-flow routes (Jocson et al., 2002). Pit caves are expressed where fast-flow routes intersect the surface (Harris et al., 1995). Where allogenic recharge arrives from noncarbonate terrain, streams sink at the carbonate contact, providing a point recharge and creating large closed depressions drained by visible swallow holes. Transport of clastic sediment into depressions may armor their bottoms and cause the carbonate walls of the depression to retreat by lateral corrosion. Collapse of subsurface voids in eogenetic rock is common in carbonate islands, especially by upward stoping from voids formed at the carbonate/ noncarbonate contact. This collapse can create large sinkholes with widths and depths >50 m. Abandoned quarries and borrow pits resemble natural-appearing closed depressions on topographic maps, particularly since many, if not most, have been opened in pre-existing, natural depressions. Subsurface Features Simple-carbonate islands, on which recharge is exclusively autogenic, generally exhibit only two cave types. In the vadose zone, pit caves may form as small, closely spaced pits, smaller than a meter in diameter and only a few meters deep, filled with sediment, and commonly called soil pipes, or as large, open, vertical shafts up to 10 m in diameter, penetrating to 50 m or more. In the phreatic zone, dissolution voids form at the boundaries of the freshwater lens. Mixing of vadose and phreatic fresh water at the top of the lens forms water-table caves. Mixing of fresh water and seawater at the lens margin forms flank margin caves (Mylroie and Carew, 1990). Halocline caves have been hypothesized to form by mixing at the bottom of the lens (Mylroie and Carew, 1988), but conclusive evidence is yet to Figure 2. Boxes A–D detail the categories of the carbonate island karst be found. Vacher (2006) proposed the term “halophreatic” for model (CIKM) (see also Table 1). caves formed by mixing of fresh and salt water at the lens boundary. Decay of organic material trapped at the density interfaces of the lens boundaries, under either aerobic or anaerobic may nevertheless be overcome in larger islands (Vacher and conditions, may enhance dissolution (Bottrell et al., 1993). Mylroie, 2002). Therefore, conduits may develop to drain water Carbonate cover, composite, and complex islands may also through the relatively restricted perimeter. exhibit contact caves, which form where descending vadose water is intercepted above sea level by the less conductive baseSurface Features ment, so that it concentrates in a vadose stream descending The high primary porosity of eogenetic rock promotes very along the contact (Figs. 2 and 3). Such streams can mechanirapid infiltration, and the paucity of diagenesis ensures that cally erode, transport, or dissolve collapse material, and may outcrop-scale dissolutional morphology is controlled by pri- even meander to form large chambers. Large chambers can also mary rock characteristics. The typical result is a jagged, etched, form from mixing of vadose and phreatic fresh water at the and pinnacled karren. In their detailed report on the karren of stream junction with the freshwater lens. Such chambers may Guam, Taboroši et al. (2004a) proposed the term “eogenetic stope upward to form collapse sinkholes at the surface, as in karren” for this unique surface. Closed depressions are common Bermuda (Mylroie et al., 1995). In composite islands, allogenic in eogenetic karst, but many are inherited directly from the water originating on the noncarbonate terrain is likely to be original depositional morphology, such as lagoons and dune especially aggressive, forming extensive contact caves with 132 J.W. Jenson et al. Figure 3. Composite figure of a typical carbonate island in the Marianas and associated karst features as explained by the carbonate island karst model (CIKM). open entrances (the recharge caves of Fig. 3). As in carbonatecover islands, the junction of contact-cave allogenic water with the freshwater lens may produce large cave chambers (Mylroie et al., 2001). In complex islands, faulting and interfingering of carbonates with noncarbonate rock create a variety of pathways and barriers to flow. Vertical shafts may develop as insurgences along surface streams at fault contacts, and confined conditions may create phreatic-lift caves (Fig. 4) that bring water up and over the confining layer (Wexel et al., 2001). In addition to the caves described above, fissure caves also can develop along joints and faults (Stafford, 2003) in all island types (Fig. 3). In the Bahamas, bank-margin failure has produced blue holes along large fractures parallel to the bank margin. In the Marianas, tectonic forces and associated uplift have produced faults and joints and numerous terraces separated by high cliffs. Cliff failure creates large fractures inland and parallel to the cliff face, along which fissure caves have developed. Where such fractures penetrate to the freshwater lens, discharge is redirected parallel to the shoreline to emerge in coastal springs. Similar features have also been reported in the Bahamas (Whitaker and Smart, 1997) and Yucatan (Shaw, 2003). Lateral compression release from cliff retreat also produces fractures perpendicular to the cliff face in which fracture and pit caves can form. Coastal springs formed where perpen- dicular fractures intercept the coast are a common feature in the Marianas (Mylroie et al., 2001). Based on work on Rota Island, these discharge fissures are called “mixing-zone fracture caves” (Keel, 2005). Carbonate islands and coasts, as a result of glacioeustasy, uplift, and carbonate deposition and cementation, commonly form steep cliffs that can be unstable, leading to fractures that have origins independent of the tectonic grain of the region. Fissure caves are commonly found on carbonate islands and exhibit no pattern of preferential association with the different CIKM island types. Interface Features In autogenic regions of the four island categories, vadose water percolates diffusely through the bedrock matrix or concentrates in pit caves as vadose fast flow. Because of the high conductivity of the rock, surface flow and ponding occur only during the most intense storms. Composite- and complex-carbonate islands, on which allogenic surface waters form insurgences at the contacts with carbonate units, may exhibit sinking streams and associated features, such as blind valleys and large closed depressions. Coastal discharge emerges either as diffuse seepage or as concentrated spring flow on beaches and in the nearshore zone. In simple-carbonate islands with very young carbonate rock, Karst of the Mariana Islands 133 Cave entrances are passable connections between the surface and subsurface. Sinking streams frequently form contact caves large enough for human entry. Fissure caves are also commonly accessible to humans. Pit caves, water-table caves, and flank margin caves, on the other hand, form without obvious entrances. In water-table and flank margin caves, the water enters and leaves by diffuse flow. In all three cases, collapse may breach the void to the surface. Pit caves frequently are breached because the epikarst, where the top of the cave forms, is at most a few meters thick. Flank-margin caves are vulnerable to incision by slope or scarp retreat of only a few meters. Watertable caves will be expressed if they are large enough, or close enough to the surface, for collapse. In the lowland plains of the Bahamas, such roof failures are so common that breached water-table caves are called “banana holes” for the specialty crops that are often grown in them (Harris et al., 1995). Banana holes are rare in the Marianas, because the rapid uplift has precluded extensive development of shallow water-table caves. Table 2 provides a summary of cave types as found by field work to date in the Mariana Islands. ISLAND KARST FEATURES OF THE MARIANA ISLANDS Figure 4. The map (A) and entrance (B) to Kalebera Cave, Saipan, a phreatic-lift cave that released waters confined at depth by volcaniclastic units. The carbonate Mariana Islands are discussed here in order of increasing complexity. Only Farallon de Medinilla is excluded, because its remote location and use as a munitions training area by the military has prevented field work. These islands illustrate how individual islands typically exhibit, in different regions, each of the different CIKM categories (Fig. 2). Islands are obvious geographic entities, but their geology can range from the very simple (e.g., Bahamas) to the very complex (e.g., Saipan), with commensurately variable hydrology. The four CIKM island types can in some cases be applied to entire islands, but more typically, carbonate islands typically are an amalgam of more than one type. The CIKM is best viewed as a series of idealized island types, while islands are physical entities that exhibit the idealized types in varying proportions, depending not only on geology, but on the current sea-level position. Aguijan seepage is the predominant means of discharge. Dissolution voids formed at previous sea-level stillstands, however, can redirect groundwater flow to form springs at the coast. In the Marianas, ubiquitous fractures along cliff-dominated shorelines intercept diffuse flow in the lens and deliver it to coastal springs, as noted already. In addition, concentrated flow may be directed to the coast along carbonate/noncarbonate contacts. In all settings, discharge sites reflect the integration of current and pre-existing flow paths associated with prior sea levels. In the Marianas in particular, tectonic sea-level change has overprinted glacio-eustatic changes and introduced fractures and rock deformations that further influence lens discharge. Aguijan (also known as Agujan, Aguiguan, or Goat Island, Fig. 1, item 9) is uninhabited. The only geologic studies of the 7.2 km2 island are Tayama’s (1936) and Stafford’s (2003). The terrain of Aguijan (and the adjacent islet, Naftan Rock) is built entirely on carbonate rocks, which are considered to be Miocene and Pliocene-Pleistocene in age (Stafford, 2003). Given that the island must rest on the same Eocene volcanic edifice as adjacent Tinian, Aguijan is either a simple-carbonate island or a carbonate-cover island, depending whether the core stands above sea level, as Stafford (2003) suggests. The island consists of three terraces at 0–50 m, 50–100 m, and 100–150 m. It is surrounded by vertical cliffs with no beaches and has no 134 J.W. Jenson et al. surface streams. Sea-level springs, if present, are too small to be observed by boat. Flank-margin caves are abundant, often in horizons that indicate sea-level stillstands (Fig. 5), with some large chambers of impressive size. Complex, interconnected flank margin caves are not known. Only one water-table cave, Dove Cave, is known. Numerous cliff-perpendicular fissure caves exist. They penetrate up to 100 m into the cliff and exhibit widened intervals that correlate with sea-level stillstands. These fissures are fossil springs that drained a past, higher-standing freshwater lens (Fig. 6). Of special interest is Liyang Atkiya, the largest single cave on Aguijan, which consists of a large entrance chamber that becomes a linear passage extending northeast for 75 m before branching into many small passages, reminiscent of boneyard (i.e., dissolutional fretwork) in continental caves. The ceiling is dissolutional, with scallops that indicate previous conduit flow upgradient, toward the entrance. The configuration suggests noncarbonate rocks exist beneath the floor rubble, providing a lithologic barrier that forced fresh water to flow upgradient toward the paleocoastline, with mixing dissolution forming the Figure 5. Flank-margin cave horizons on Tinian (A) and Aguijan (B) Islands. These cave horizons indicate a period of sea-level, and hence freshwater lens, stability prior to further uplift. Karst of the Mariana Islands broad entrance chamber. Alternatively, the volcanic core of the island may have still been warm when the freshwater lens formed, so that the cave reflects dissolution from thermal waters rising within the island, cooling with subsequent increase in water aggressivity, and discharging at the coast. Application of the CIKM to Aguijan thus suggests aspects of geology not visible on the surface. Tinian Tinian (Fig. 1) covers 102 km2 and reaches a maximum elevation of 187 m. An unpublished report by Doan et al. (1960) remains the most detailed geologic study, while recent work has focused on the freshwater lens (Gingerich and Yeatts, 2000) and karst (Stafford, 2003). High-angle faults separate Tinian into distinct physiographic provinces, as shown in Figure 1 (items 6—northern lowland; 7—north-central highland, central plateau, and median valley; and 8—southeastern ridge). Two carbonate units, of Miocene through Pleistocene age, overlie an Eocene volcanic core. Holocene limestones cover <1% of the island surface area. Weathered volcanic rocks crop out on ~1% of the island surface. Surface karst development is extensive as epikarst and constructional closed depressions. Dissolutional depressions have only been identified in the north-central highland, where perennial streams developing on volcanic terrains supply allogenic water to the adjacent carbonate/noncarbonate contact. In the subsurface, flank margin caves are abundant, but few water-table caves have been identified. Numerous cliffs contain extensive horizons of small, breached flank margin caves (Fig. 5). The largest solitary flank margin cave, Liyang Dangkolo, exceeds 1300 m2 in area (Fig. 7). Although differential uplift prevents correlation of cave horizons, a north-central highland transect shows at least three previous stillstands. Fissure caves, generally associated with faults, margin failures, and compression-release structures, are abundant and well Figure 6. Insect Bat Cave, Aguijan, with map (A) and interior (B). This cave is perpendicular to the cliff line and represents a fossil resurgence. The wider parts of the fissure indicate times of relative lens stability, as the lithology is fairly uniform. 135 expressed on Tinian. Two typical caves, Plunder Cave and Water Cave, align closely with regional faulting and show horizontal widening from collapse and additional mixing dissolution. Caves associated with margin failures on Tinian are the best developed in the Marianas, with narrow passages over 100 m long, extending over 40 m deep, and intersecting the freshwater lens, as at Masalok Fracture Cave. Fissure caves associated with compression release (i.e., joints) extend inland, near-perpendicular to scarps for over 30 m and show horizontal widening from the mixing of fresh and saline water associated with discharge. Two caves, Gecko Cave and Cetacean Cave, currently discharge fresh water at sea level, while others, such Figure 7. Dangkulo Cave (Liyang Dangkulo), Tinian, with map (A) and interior (B), showing the irregular cluster of chambers found in flank margin caves. 136 J.W. Jenson et al. as Chiget Fracture Cave, are located above sea level and appear to be paleodischarge features. Contact caves are limited, with only two features, identified as recharge caves in association with dissolutional closed depressions. Interface features, other than cave entrances, are limited on Tinian. Only one pit cave has been positively identified. Highly fractured regions contain numerous small-scale crevices and fissure caves that act as vadose fast-flow routes. Discharge features include both seeps and springs, with 17 sites identified. More certainly exist, however, as less than half of the coastline was investigated at low tide because of strong surf and steep scarps. Because of the scarcity of carbonate sand beaches (<5% of the coastline), springs are the dominant freshwater discharge features. Most springs are associated with fissure caves and faulted province boundaries. Tinian is separated into hydrologically distinct, faultbounded provinces, which variously display each of the first three CIKM ideal island types: the northern lowland fits the simple-carbonate island model, the southeastern ridge fits the carbonate-cover island model, and the three central provinces together fit the composite-island model. Rota Rota (Fig. 1) covers ~96 km2 and reaches an elevation of 496 m, the highest in the carbonate Mariana Islands. No geologic map has yet been compiled. Sugawara (1934) published a M.S. thesis that focused on geomorphology and depositional facies. Keel et al. (2004) and Keel (2005) reported on the limited geological literature about Rota, and on the 120 caves that have been mapped to date. Rota consists of a volcanic core covered by limestones that are no doubt contemporaneous with those of the nearby islands. High-angle normal faults common on nearby islands, while probably present on Rota, are not reported. On the Sabana, at the summit of the island, the volcanic core is exposed over ~0.25 km2 (smaller “NC” on the Fig. 1 Rota map). This volcanic outcrop provides allogenic water that sinks at the limestone contact producing three known contact caves, of which Summit Cave is the largest. On the south side of the Sabana, discharge converges inside the Water Cave at ~350 m elevation along the contact of the overlying limestone with the volcanics. The volcanic outcrop below the Water Cave (large “NC” on the Fig. 1 Rota map) contains the only surface streams on Rota. Along the coast of Rota are several small volcanic outcrops, including some that show interfingering of limestone with volcanic rock. These exhibit springs perched above sea level. Many sea-level springs and seeps are also present. Flank-margin caves have been found on Rota from sea level to 350 m elevation. The best example is the 800-m-long Sagua Cave Complex, located on the SW coast. There are extensive continuous horizons of remnant flank margin caves at several locations around the island (e.g., As Matmos, I Koridot, Tachok, and Taksunok). Rota has a greater known concentration of fissure caves than any of the other islands in the Marianas. As Matan, KnuckleBone Cave, Deer Cave, and Liyang Apaka, all perpendicular to inland scarps, are outstanding examples of fissure caves on Rota. No caves associated with margin failure have been found on Rota. Rota is a composite island that grades laterally away from the volcanic core into a carbonate-cover island, thence to a simple-carbonate island from the southwestern highland to the plains and terraces to the north and east. Recognition of cave type, especially contact caves, can assist in determining the underlying geology for islands such as Rota, since no geologic map exists. Guam Guam is the largest of the Mariana Islands, with 550 km2 surface area, and is divided by a major fault into two equalsized provinces (Fig. 1). The report by Tracey et al. (1964) is still the central reference on the island’s geology. Taboroši et al. (2003, 2004b, 2005) provided a comprehensive description of the island’s karst features. The northern province is a low-relief island karst plateau (Fig. 1, item 13) on Pliocene-Pleistocene reef-lagoon deposits, surrounded by sheer 60–180 m coastal cliffs. The south is a dissected west-facing volcanic cuesta (Fig. 1, item 17) with an uplifted limestone unit on its eastern flank (Fig. 1, item 15) that is contemporaneous with the cliff-forming unit in the north and stands 60–70 m above sea level. Remnants of Miocene limestones occupy the interior basin of the south (Fig. 1, item 16) and cap the ridge of the cuesta, including the island’s highest peak at 406 m. The karst is remarkably diverse, exhibiting not only island karst, but also classic features of mature tropical continental karst (cones and cockpits) in the southern interior. The northern plateau exhibits characteristic island karst features, except in the argillaceous limestone at the southern end of the plateau adjacent to the higher volcanic terrain of the south. The argillaceous limestone includes sinkholes, blind valleys, and disappearing streams. These features may reflect clastic armoring of the limestone surface by sediments shed from the nearby volcanic highlands. Northern Guam locally exhibits each of the first three carbonate island karst environments (Fig. 2). One percent of the surface is a composite-island terrain occupied by two minor outcrops of volcanic basement that protrude through the limestone plateau (“NC” and item 14 of the Fig. 1 Guam map). Contact caves are found on the flanks of each. Beneath ~21% of the land surface, unexposed basement extends above sea level. At the lowest long-term relative stillstand of 95 m below the modern sea level, an additional ~37% of northern Guam’s modern surface may also fit the carbonate-cover island category. Finally, the basement is sufficiently deep under 41% of the plateau surface that it has not stood above sea level since the overlying limestone bedrock was deposited, assuming that the current level of tectonic uplift is the highest the island has experienced. This portion of the island therefore fits the simple-carbonate island model. Caves include some pit caves open at the surface (Fig. 8), stream caves along bedrock-basement contacts, dissolution voids Karst of the Mariana Islands reflecting ancient lens positions, a single documented water-table cave, and flank margin caves along the coast. Abandoned stream caves are found in the cliffs of the southeastern coast. Discharge from the northern plateau is via coastal springs and coastal seeps. Mapped discharge features include beach springs and seeps, reef springs and seeps, flowing fractures and caves, and submarine springs. The largest point-discharge volumes in northern Guam are associated with fractures and caves, which are the characteristic discharge features in coastal areas occupied by sheer cliffs. High-level springs can be found in the interior, mostly rising at carbonate/noncarbonate contacts. Application of the CIKM to Guam has been useful to assess water resources, which are increasingly challenged by economic development. Saipan Saipan (Fig. 1) has an area of 124 km2. The most recent definitive geologic study was by Cloud et al. (1956), from which the geomorphic and geologic descriptions are summarized here. Saipan, containing heterogeneous syndepositional volcanic and limestone units, with widespread synchronous and subsequent structural modification, is the most complex carbonate island studied to date. The first three CIKM categories are all seen as subunits on the island but proved insufficient to describe the full complexity of the island. Saipan thus provided the basis for defining and adding the complex-island category to the CIKM (Fig. 2). Saipan consists of a core of volcanic rock dipping to the east, enveloped by late Eocene and younger limestones, with interlayered andesitic Oligocene volcanics. Cloud et al. (1956) divided the island into geomorphologic areas, shown in Figure 1, with numerous subsections. The northern three-quarters of the island is dominated by an axial upland (Fig. 1, item 1) with northern and southern limestone terraces. The southern terrace culminates at Saipan’s maximum elevation of 474 m on Mount 137 Tagpochau. Toward the north, the axial upland is separated by a volcanic ridge that includes Mount Achugau (234 m), while the northern limestone plateau rises to 255 m before ending abruptly at the Banadero Cliffs, which drop more than 180 m to the platform below. From the axial upland, a series of limestone benches, separated by scarps, drop successively to the sea in 10 or 12 major, discontinuous marine terrace units. The western coastal plain (Fig. 1, item 2) consists of carbonate sands and volcanic outwash, whereas low to high cliffs of limestone and locally intermixed volcanics form the coastal boundary elsewhere. Significant seeps or springs are absent along this coastline. The central area of Saipan has extremely complex geology and hydrology. Individual hydrologic units can be quite small and discreet. The term “complex island” was developed to address the highly variable interaction of structure, lithology, sea level, and hydrology as found in central Saipan. Broad platforms of Pleistocene limestones are found to the north, south, and east of the uplands, where the island generally fits the simple-carbonate island and carbonate-cover categories. Numerous flank margin caves are exposed along the coast, and there are several large sinkholes near the terrace edges. A number of springs flow from the eastern side of the Donni Hills (Fig. 1, item 3) area, and several others to the north. These drain a band of exposed volcanic rock on the eastern flank of the upland, which fits the composite-island model. Numerous streams in this east-central region traverse a broad band of the Donni Sandstone Member of the multilithic Miocene Tagpochau Formation, and the argillaceous and rubbly facies of Pleistocene Mariana Limestone. Several sinking streams are located in the east-central region. A contact cave, Liyang Falingun Hanum, is located to the northeast of the volcanic ridge of the axial uplands. Another contact cave, Big Scary Cave, is found to the northwest of Mt. Tagpochau, with fault-controlled vertical (80 m) and lateral (180 m) development. Volcanic spurs and outcrops extend to the south and east from Mt. Tagpochau, and there is evidence of perched aquifers. Kalabera Cave (Liyang As Teo, Fig. 4) appears to have been a phreatic-lift tube (Wexel et al., 2001), through which water traveled down the bedding plane eastward, confined laterally and downdip by volcaniclastic units, before intersecting fractures that allowed upward release. Kalabera extends 55 m vertically, with ~300 m of passage predominantly parallel to the escarpment and strike of the beds. This cave’s existence, as with Liyang Atkiya on Aguijan Island, provides clues to subsurface geology not available from surface reconnaissance. Because of the complex geology, rugged terrain, limited access, and difficulties in coordinating with landowners, field work has been limited on Saipan, and fewer caves are recorded here (Table 2) than elsewhere in the Mariana Islands. SUMMARY Figure 8. Amantes Pit, Guam, with map (A) and entrance (B), a pit cave that once conducted water as a vadose fast-flow route from the surface to the freshwater lens. Study of the carbonate islands of the Marianas Archipelago shows that the complexity of island karst is greater than previously observed or reported. As a result, the carbonate island 138 J.W. Jenson et al. karst model has been extended to include the effects of tectonism and syndeposition of carbonate and noncarbonate rock on the production of karst flow paths and features on carbonate islands. Observations from the Marianas Islands also demonstrate the utility of the CIKM for classifying islands into subdivisions that reflect distinct geomorphic and hydrologic environments (Fig. 2). ACKNOWLEDGMENTS We thank the Water and Environmental Research Institute of the Western Pacific, University of Guam, and Mississippi State University for their support. Funding by the State Water Resources Research Institute Program, U.S. Geological Survey, and by the Guam Hydrologic Survey was essential. The manuscript was improved by the very useful comments of reviewers S. Bacchus and P. Smart. Thanks also are due to J.L. Carew, H.G. Siegrist, H.L. Vacher, the many local government officials who supported us, the cavers who assisted us, and the many landowners who gave us access. REFERENCES CITED Bottrell, S.H., Carew, J.L., and Mylroie, J.E., 1993, Bacterial sulphate reduction in flank margin environments: Evidence from sulphur isotopes, in White, B., ed., Proceedings of the 6th Symposium on the Geology of the Bahamas: Port Charlotte, Florida, Bahamian Field Station, p. 17–21. Choquette, P.W., and Pray, L.C., 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: American Association Petroleum Geologists Bulletin, v. 54, p. 207–250. Cloud, P.E., Jr., Schmidt, R.G., and Burke, H.W., 1956, Geology of Saipan, Mariana Islands, Part 1. General geology: U.S. Geological Survey Professional Paper 280-A, 126 p. Doan, D.B., Burke, H.W., May, H.G., Stensland, C.H., and Blumenstock, D.I., 1960, Military geology of Tinian, Mariana Islands: U.S. Government Printing Office, Washington, D.C., Chief of Engineers, U.S. Army, 149 p. Frank, E.F., Mylroie, J., Troester, J., Alexander, E.C., and Carew, J.L., 1998, Karst development and speleologenesis, Isla de Mona, Puerto Rico: Journal of Cave and Karst Studies, v. 60, p. 73–83. Gingerich, S.B., and Yeatts, D.S., 2000, Ground-water resources of Tinian, Commonwealth of the Northern Mariana Islands: U.S. Department of the Interior Water-Resources Investigations Report 00-4068, 2 sheets. Harris, J.G., Mylroie, J.E., and Carew, J.L., 1995, Banana holes: Unique karst features of the Bahamas: Carbonates and Evaporites, v. 10, p. 215–224. Jocson, J.M.U., Jenson, J.W., and Contractor, D.N., 2002, Recharge and aquifer responses: Northern Guam Lens Aquifer, Guam, Mariana Islands: Journal of Hydrology, v. 260, p. 231–254, doi: 10.1016/S0022-1694(01) 00617-5. Keel, T.M., 2005, The caves and karst of Rota Island, Commonwealth of the Northern Mariana Islands [Master’s thesis]: Mississippi State, Mississippi State University, 206 p. Keel, T.M., Mylroie, J.E., and Jenson, J.W., 2004, Caves and karst of Rota (Luta) CNMI: Water and Environmental Institute of the Western Pacific, University of Guam, Mangilao, Technical Report 105, 105 p. Mink, J.F., and Vacher, H.L., 1997, Hydrogeology of northern Guam, in Vacher, H.L., and Quinn, T., eds., Geology and hydrogeology of carbonate islands: Developments in Sedimentology, v. 54, p. 743–761. Mylroie, J.E., and Carew, J.L., 1988, Solution conduits as indicators of late Quaternary sea level position: Quaternary Science Reviews, v. 7, p. 55–64, doi: 10.1016/0277-3791(88)90093-5. Mylroie, J.E., and Carew, J.L., 1990, The flank margin model for dissolution cave development in carbonate platforms: Earth Surface Processes and Landforms, v. 15, p. 413–424. Mylroie, J.E., and Carew, J.L., 1997, Land use and carbonate island karst, in Beck, B.F., and Stephenson, J.B., eds., The engineering geology and hydrogeology of karst terranes: Brookfield, A.A. Balkema, p. 3–12. Mylroie, J.E., Carew, J.L., and Vacher, H.L., 1995, Karst development in the Bahamas and Bermuda, in Curran, H.A., and White, B., eds., Terrestrial and shallow marine geology of the Bahamas and Bermuda: Geological Society of America Special Paper 300, p. 251–267. Mylroie, J.E., Jenson, J.W., Taboroši, D., Jocson, J.M.U., Vann, D.T., and Wexel, C., 2001, Karst features of Guam in terms of a general model of carbonate island karst: Journal of Cave and Karst Studies, v. 63, p. 9–22. Shaw, C.E., 2003, Controls on the movement of ground water in the northeastern Yucatan Peninsula: Examples from Yal Ku Lagoon and North Akumal: Field Trip to the Caribbean Coast of the Yucatan Peninsula, Salt Water Intrusion Conference, 30 March–2 April 2003: Merida, Yucatan, Universidad Nacional Autónoma de México and Universidad Autónoma de Yucatán, p. 23–32. Stafford, K.W., 2003, Structural controls on megaporosity in eogenetic carbonate rocks: Tinian, CNMI [Master’s thesis]: Mississippi State, Mississippi State University, 340 p. Sugawara, S., 1934, Topography, geology and coral reefs of Rota Island: Tokyo, Pacific Geological Surveys, Military Geology Branch, United States Geological Survey, 59 p. Taboroši, D., Jenson, J.W., and Mylroie, J.E., 2003, Zones of enhanced dissolution and associated cave morphology in an uplifted carbonate island karst aquifer, northern Guam, Mariana Islands: Speleogenesis and Evolution of Karst Aquifers, v. 1, www.speleogenesis.info, 16 p. Taboroši, D., Jenson, J.W., and Mylroie, J.E., 2004a, Karren features in island karst: Guam, Mariana Islands: Zeitschrift für Geomorphologie, N.F., v. 48, p. 369–389. Taboroši, D., Jenson, J.W., and Mylroie, J.E., 2004b, Karst features of Guam, Mariana Islands: Water and Environmental Research Institute of the Western Pacific, University of Guam, Mangilao, Guam, Technical Report 104, 28 p. Taboroši, D., Jenson, J.W., and Mylroie, J.E., 2005, Karst features of Guam, Mariana Islands: Micronesia, v. 38, p. 17–46. Tayama, R., 1936, Geomorphology, geology, and coral reefs of Tinian Island together with Aguijan and Naftan Islands: Sendai, Japan, Institute of Geology and Paleontology, Tohoku University, 72 p. Tracey, J.I., Jr., Schlanger, S.O., Stark, J.T., Doan, D.B., and May, H.G., 1964, General geology of Guam: U.S. Geological Survey Professional Paper 403-A, 104 p. Vacher, H.L., 2006, Keynote address: Plato, Archimedes, Ghyben Herzberg, and Mylroie, in Davis, R.L., and Gamble, D.W., eds., Proceedings of the 12th Symposium on the Geology of the Bahamas and Other Carbonate Regions: San Salvador, Bahamas, Gerace Research Center, p. ix–xix. Vacher, H.L., and Mylroie, J.E., 2002, Eogenetic karst from the perspective of an equivalent porous medium: Carbonates and Evaporites, v. 17, p. 182–196. Wexel, C., Jenson, J.W., Mylroie, J.R., and Mylroie, J.E., 2001, Carbonate island karst of Saipan: Geological Society of America Abstracts with Programs, v. 33, no. 6, p. A-6. Whitaker, F.F., and Smart, P.L., 1997, Hydrogeology of the Bahamian archipelago, in Vacher, H.L., and Quinn, T.M., eds., Geology and hydrogeology of carbonate islands: Amsterdam, Elsevier, p. 183–216. MANUSCRIPT ACCEPTED BY THE SOCIETY 22 SEPTEMBER 2005 Printed in the USA