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.
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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
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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
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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.
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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
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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.
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