(= million years) and 9 to 5 Ma (Reagan and Meijer,
1984). Rifting of the West Mariana Ridge created the
Mariana Trough and the Mariana Ridge. Volcanic activity
resumed $1.3 Ma in the active, northern, ridge. The main
volcanic basement of Guam resulted from the first volcanic phase and is evidenced in the middle Eocene Facpi
Formation ($200 m thick boninite lavals with some limestone infills) and the Eocene to early Oligocene Alutom
Formation ($400 m thick, tuffaceaous shales, sandstones,
volcanic breccias, lapilli conglomerates, pillow lavas,
tubidites, some calcareous shales in the Mahlac member,
and some coral-rich floats, Schlanger, 1964). Serious carbonate sedimentation began in the upper Oligocene/lower
Miocene as Maemong Limestone, a member of the
Umatac Formation which is of shallow-water origin and
consists of mud- to wackestones with coral (Porites and
Acropora) rudstones and intermixed volcanic detritus.
Reefal organisms such as corals, Tridacna, Halimeda,
and crustose algae are common, but ex situ and no reef edifice is known. This episode represents a shallowing with
carbonate precipitation around three volcanic highs on
Guam. The Miocene Bonya Limestone contains deeperwater facies with reef coral and coralline algae detritus
but no reefs are preserved, so deposition was likely on
a seaward slope between volcanic cones. The Miocene
Alifan Limestone contains mudstones that grade to sparry
packstones with well-preserved reef fauna and flora
(Porites, Acropora, and Halimeda). The smallest limestone unit on Guam is the Tortinian and Pontian Janum
Limestone consisting of deep fore-reef to basinal facies
(Siegrist and Randall, 1992). The Mio/Pliocene Barrigada
Limestone is a chalky foraminiferal-algal wackestone
with some corals (Porites, and Astraeopora). While essentially all limestones show reefal fauna among their components, no Miocene reefs crop out in Guam or anywhere in
the Mariana Islands (Siegrist and Randall, 1992).
The most widely distributed limestone is the PlioPleistocene Mariana Limestone, the dominant carbonate
on Guam, Rota, Aguijan, Tinian, and Farallon de
Medinilla (Siegrist and Randall, 1992; Randall, 1995)
and also of great importance on Saipan. It contains welldeveloped reefal and peri-reefal facies. On Guam, it can
reach a thickness of 175 m. Reef margin facies generally
crop out as the most seaward facies and coincide often
with the edges of terraces. Rocks are coralline and coralgal
boundstones, coarse grainstones, packstones, and
wackestones. Branching and massive corals occur in situ
and in growth position and Siegrist and Randall (1992)
report a rich fauna, reminiscent of the situation in modern
reefs (Acropora, Favia, Goniastrea, Leptoria, Platygyra,
Pocillopora, Porites, Stylophora, Symphyllia, and
Turbinaria). Detrital facies are common and contain benthic foraminifera, corals, mollusks, oysters, snails, and
Halimeda. The Mariana Limestone has thoroughly altered
porosity and mineralogy and displays many karst features
(Siegrist and Randall, 1992).
Rota shares many rock units with Guam but is primarily
(>90%) covered by carbonates, mainly Mariana
Limestone (Carruth, 2005; Figure 5). Near the shoreline,
exposures of the Milencatan Limestone occur (Sugawara,
1934; Kayanne et al., 1993 refer to it as Milencatan while
Bell and Siegrist (1988) refer to it as Mirakattan Limestone), which are an important sea-level indicator (Easton
et al., 1978; Randall and Siegrist, 1988; Bell and Siegrist,
1988; Siegrist and Randall, 1992; Kayanne et al., 1993;
Dickinson, 2000).
Saipan, the largest of the CNMI (48 sq. miles), consists
of an andesitic-dacitic volcanic core (Sankakuyama Formation) overlain by sandstones, conglomerates, and breccias (Hagman, Desinyama, Fina Sisu Formations) which
are capped by limestones covering about 95% of the
island’s surface (Figure 5). Volcanism on Saipan probably
ceased in the Oligocene, after which marine deposition
produced thick limestone sequences.
The Matansa Limestone is an upper Eocene carbonate
bank deposit (Cloud et al., 1956). Saipan’s center is covered by the early Miocene Tagpochau Limestone,
a complex of calcareous clastic rocks. Saipan’s eastern
side is Pliocene Mariana Limestone, as found on Guam
and Rota. It is reefal with argillaceaous rubbly facies
(Carruth, 2003) and differs from the Tagpochau Limestone in the abundance of corals and the modern aspect
of its fossil assemblage. Mariana Limestone does not
occur along the entire western coast but disappears under
Tanapag lagoon (Figure 5). Thus, the structural control
of the lagoon rim (= barrier reef) is probably determined
by the course of the Mariana Limestone. Lagoon and the
coastal lowlands seaward of the Tagpochau Limestone
are probably fill, overlying the Mariana Limestone. The
Tanapag Limestone is a Pleistocene to Holocene raised
reef limestone, well indurated and porous with wellpreserved coral heads and mollusk shells (Cloud et al.,
1956). It formed as fringing reefs on an emerging surface
with maximum thickness <20 m. Its younger, Holocene,
parts are correlative to the Merizo Limestone of Guam
and the Milencatan Limestone of Rota (Dickinson, 2000)
and occur along the steep eastern shoreline of Saipan.
In Tinian, which is situated 5 km south of Saipan, the
oldest rocks belong to the Eocene Tinian Pyroclastics
(Doan et al., 1960), exposed in the central highlands and
the island’s southern section (Figure 5). The rest of Tinian
is covered by limestones. The same Miocene Tagpochau
Limestone as on Saipan is exposed on about 15% of the
island. Pliocene Mariana Limestone covers the majority
of Tinian ($80% of its surface and thickens from 0 to
150 m in all directions from near the Tinian Pyroclastics;
Doan et al., 1960; Gingerich, 2002). Like on Saipan, the
limestones on Tinian are heavily faulted and karstified.
Recent alluvium and colluvium reaches thicknesses up
to 13 m.
Farallon de Medinilla is the northernmost of the calcareous Marianas and is also mostly covered by Mariana
Limestone (Randall, 1995). North of Farallon de
Medinilla, the Mariana Islands bear no limestone caps
and are volcanically active. This active island arc stretches
from Anatahan to Uracas (Meijer and Reagan, 1981).
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MARIANA ISLANDS, CORAL REEF GEOLOGY
1984). Rifting of the West Mariana Ridge created the
Mariana Trough and the Mariana Ridge. Volcanic activity
resumed $1.3 Ma in the active, northern, ridge. The main
volcanic basement of Guam resulted from the first volcanic phase and is evidenced in the middle Eocene Facpi
Formation ($200 m thick boninite lavals with some limestone infills) and the Eocene to early Oligocene Alutom
Formation ($400 m thick, tuffaceaous shales, sandstones,
volcanic breccias, lapilli conglomerates, pillow lavas,
tubidites, some calcareous shales in the Mahlac member,
and some coral-rich floats, Schlanger, 1964). Serious carbonate sedimentation began in the upper Oligocene/lower
Miocene as Maemong Limestone, a member of the
Umatac Formation which is of shallow-water origin and
consists of mud- to wackestones with coral (Porites and
Acropora) rudstones and intermixed volcanic detritus.
Reefal organisms such as corals, Tridacna, Halimeda,
and crustose algae are common, but ex situ and no reef edifice is known. This episode represents a shallowing with
carbonate precipitation around three volcanic highs on
Guam. The Miocene Bonya Limestone contains deeperwater facies with reef coral and coralline algae detritus
but no reefs are preserved, so deposition was likely on
a seaward slope between volcanic cones. The Miocene
Alifan Limestone contains mudstones that grade to sparry
packstones with well-preserved reef fauna and flora
(Porites, Acropora, and Halimeda). The smallest limestone unit on Guam is the Tortinian and Pontian Janum
Limestone consisting of deep fore-reef to basinal facies
(Siegrist and Randall, 1992). The Mio/Pliocene Barrigada
Limestone is a chalky foraminiferal-algal wackestone
with some corals (Porites, and Astraeopora). While essentially all limestones show reefal fauna among their components, no Miocene reefs crop out in Guam or anywhere in
the Mariana Islands (Siegrist and Randall, 1992).
The most widely distributed limestone is the PlioPleistocene Mariana Limestone, the dominant carbonate
on Guam, Rota, Aguijan, Tinian, and Farallon de
Medinilla (Siegrist and Randall, 1992; Randall, 1995)
and also of great importance on Saipan. It contains welldeveloped reefal and peri-reefal facies. On Guam, it can
reach a thickness of 175 m. Reef margin facies generally
crop out as the most seaward facies and coincide often
with the edges of terraces. Rocks are coralline and coralgal
boundstones, coarse grainstones, packstones, and
wackestones. Branching and massive corals occur in situ
and in growth position and Siegrist and Randall (1992)
report a rich fauna, reminiscent of the situation in modern
reefs (Acropora, Favia, Goniastrea, Leptoria, Platygyra,
Pocillopora, Porites, Stylophora, Symphyllia, and
Turbinaria). Detrital facies are common and contain benthic foraminifera, corals, mollusks, oysters, snails, and
Halimeda. The Mariana Limestone has thoroughly altered
porosity and mineralogy and displays many karst features
(Siegrist and Randall, 1992).
Rota shares many rock units with Guam but is primarily
(>90%) covered by carbonates, mainly Mariana
Limestone (Carruth, 2005; Figure 5). Near the shoreline,
exposures of the Milencatan Limestone occur (Sugawara,
1934; Kayanne et al., 1993 refer to it as Milencatan while
Bell and Siegrist (1988) refer to it as Mirakattan Limestone), which are an important sea-level indicator (Easton
et al., 1978; Randall and Siegrist, 1988; Bell and Siegrist,
1988; Siegrist and Randall, 1992; Kayanne et al., 1993;
Dickinson, 2000).
Saipan, the largest of the CNMI (48 sq. miles), consists
of an andesitic-dacitic volcanic core (Sankakuyama Formation) overlain by sandstones, conglomerates, and breccias (Hagman, Desinyama, Fina Sisu Formations) which
are capped by limestones covering about 95% of the
island’s surface (Figure 5). Volcanism on Saipan probably
ceased in the Oligocene, after which marine deposition
produced thick limestone sequences.
The Matansa Limestone is an upper Eocene carbonate
bank deposit (Cloud et al., 1956). Saipan’s center is covered by the early Miocene Tagpochau Limestone,
a complex of calcareous clastic rocks. Saipan’s eastern
side is Pliocene Mariana Limestone, as found on Guam
and Rota. It is reefal with argillaceaous rubbly facies
(Carruth, 2003) and differs from the Tagpochau Limestone in the abundance of corals and the modern aspect
of its fossil assemblage. Mariana Limestone does not
occur along the entire western coast but disappears under
Tanapag lagoon (Figure 5). Thus, the structural control
of the lagoon rim (= barrier reef) is probably determined
by the course of the Mariana Limestone. Lagoon and the
coastal lowlands seaward of the Tagpochau Limestone
are probably fill, overlying the Mariana Limestone. The
Tanapag Limestone is a Pleistocene to Holocene raised
reef limestone, well indurated and porous with wellpreserved coral heads and mollusk shells (Cloud et al.,
1956). It formed as fringing reefs on an emerging surface
with maximum thickness <20 m. Its younger, Holocene,
parts are correlative to the Merizo Limestone of Guam
and the Milencatan Limestone of Rota (Dickinson, 2000)
and occur along the steep eastern shoreline of Saipan.
In Tinian, which is situated 5 km south of Saipan, the
oldest rocks belong to the Eocene Tinian Pyroclastics
(Doan et al., 1960), exposed in the central highlands and
the island’s southern section (Figure 5). The rest of Tinian
is covered by limestones. The same Miocene Tagpochau
Limestone as on Saipan is exposed on about 15% of the
island. Pliocene Mariana Limestone covers the majority
of Tinian ($80% of its surface and thickens from 0 to
150 m in all directions from near the Tinian Pyroclastics;
Doan et al., 1960; Gingerich, 2002). Like on Saipan, the
limestones on Tinian are heavily faulted and karstified.
Recent alluvium and colluvium reaches thicknesses up
to 13 m.
Farallon de Medinilla is the northernmost of the calcareous Marianas and is also mostly covered by Mariana
Limestone (Randall, 1995). North of Farallon de
Medinilla, the Mariana Islands bear no limestone caps
and are volcanically active. This active island arc stretches
from Anatahan to Uracas (Meijer and Reagan, 1981).
668
MARIANA ISLANDS, CORAL REEF GEOLOGY