The shallow and partly emergent margin of atolls
includes reef crest, reef flat (pavement), sand apron, and
marginal islands (Figure 1b). It is relatively narrow
(up to several hundred meters) as compared to the much
larger lagoon area. The margin is characterized by coral
reefs, which exhibit geomorphological and ecological
zonations (Ladd et al., 1950; Wiens, 1962, 234–243).
On the ocean side of the reef crest, which is often composed of an algal ridge in the Pacific, there is a fore-reef
slope that in many cases dips about 30
. The shallow
fore-reef slope is usually characterized by the spur and
groove system, a comb-shaped structure of ridges perpendicular to the reef crest that may have either constructional
or erosional origins (Gischler, 2010). The fore-reef slope
transitions to a break in slope and a steep wall several hundreds of meters high (James and Ginsburg, 1979). On the
lagoon side of the reef crest, a cemented pavement, occasionally with islands, and a sand apron occur. It is not
uncommon that a distinct sandy slope transitions to the
lagoon proper. Sand aprons are significant in that they
contribute to the lateral infilling of atoll lagoons by
lagoonward progradation (Purdy and Gischler, 2005).
Atoll margins are usually not continuous but feature several passes that connect the open ocean with the interior
lagoon. In many Pacific atolls, the passes are located on
the leeward sides (Wiens, 1962).
Atoll lagoons get up to 100 m deep; the average depth
amounts to some 36 m based on an analysis of the geomorphology of 295 atolls (Purdy and Winterer, 2001). The
same authors showed that atoll lagoon depths covary significantly with precipitation rates, thereby supporting the
contention that inherited karst relief from solution during
glacial sea-level lowstands is crucial in atoll development.
As compared to the atoll margin that exhibits accretion
rates of several meters per kyr, background sedimentation
within Quaternary atoll lagoons averages decimeters per
kyr (Purdy and Winterer, 2001; Purdy and Gischler,
2005). Atoll lagoons usually accommodate numerous
coral patch reefs. In the Maldives, lagoonal reefs as well
as some marginal reefs may form characteristic ringlike
structures called faroes. Their formation is largely attributed to the changing wind and current regime of the Indian
monsoon (Purdy and Bertram, 1993).
Sand and rubble islands termed cays are common on
both marginal and lagoonal atoll reefs. Beachrock is
a very common characteristic of sandy beaches on reef
islets (Gischler, 2007). Elongated cays on Pacific reef
margins are called motu, and the channels between them
hoa. Motus are in many cases relatively stable due to
underpinning by mid- to late Holocene (ca. 4–2 kyrs
BP) reef flat deposits that formed during higher-thanpresent sea-level (Dickinson, 2009). Human colonization
in the Pacific only started after high tide levels had fallen
below the upper level of these older Holocene deposits,
some 2,000 years ago (Dickinson, 2009).
Modern atoll sediments are usually coralgal grainstones
on marginal and lagoonal reefs. Atoll lagoon sediments
often include mollusk-rich and foraminifer-rich
wackestones and packstones as well as nonskeletal facies
(Gischler and Lomando, 1999; Gischler, 2006; and references therein). Atoll lagoon deposits have also been successfully used as archives of event sedimentation
(Klostermann et al. 2014).
Guyots
Guyots are seamounts with more or less flat tops that in
many cases lie in about 1–2 km water depth (Figure 1).
The tops typically are 3.0–4.5 km above the surrounding
seafloor and may exhibit reliefs of tens of meters. The
large majority can be found in the northwestern Pacific
at 8–27
N and 146–165
E (Hess, 1946; Hamilton,
1956; Menard, 1984; Wessel, 1997; Flood, 1999). There
are some 50,000 seamounts, many of them guyots, in the
Pacific Ocean, about 50 % of them not charted and
<1 % sampled for dating (Wessel, 1997). Some guyots
are also known from other oceans (e.g., Pratt, 1963,
1967). Their shape is circular to oval. Guyot size varies
and diameters may reach >200 km (Davies et al., 1972).
Slopes of guyots are concave and reach 20
inclination toward the top. Guyots have been repeatedly
investigated by dredging, seismics, and drilling. ODP
cruises 143 and 144 were especially devoted to the Pacific
guyots (Winterer et al., 1995; Haggerty and Premoli-Silva,
1995).
Ideal successions of guyot tops include the volcanic
(basaltic) edifice, a shallow-water atoll reef section with
elevated rims, and a package of pelagic ooze, usually
capped by a manganese-iron or phosphorite crusts
(Schlanger et al., 1987; Grötsch and Flügel, 1992; Jenkyns
and Wilson, 1999; Flood, 1999, 2001) (Figure 2). However, there are also a number of guyots that are purely volcanic, lack a reefal succession, or feature mixed volcanic
and limestone breccias (Jenkyns and Wilson, 1999).
The shallow-water reef deposits often are Cretaceous to
the Eocene in age. They may get as thick as 1.6 km as on
Resolution Guyot and contain clear indicators of a reefal
origin such as corals, rudist mollusks, calcareous sponges
(stromatoporoids), calcareous algae, larger foraminifera,
and nonskeletal grains including ooids (Grötsch and
Flügel, 1992; Jenkyns and Wilson, 1999; Flood, 1999,
2001). The majority of shallow-water reefs on guyots
drowned in the Cretaceous (Matthews et al., 1974; Wilson
et al., 1998; Jenkyns and Wilson, 1999). The age range of
pelagic oozes comprises the Cretaceous to the Quaternary.
The oozes get as thick as 150 m as on top of Allison
Guyot. They are characterized in many cases by stratigraphic gaps and the so-called ghost faunas, i.e., mixtures
of reworked foraminifera that comprise many biostratigraphical zones. The formation of ferromanganese crusts
is likely the result of microbial activity. Additional processes include phosphatization and aragonite dissolution
(Grötsch and Flügel, 1992). Collectively, these features
are characteristic of very low sedimentation rates and/or
nondeposition (omission), erosion, condensation of sediment, and dissolution (Hamilton, 1956; Lonsdale et al.,
304
GUYOT, ATOLL
includes reef crest, reef flat (pavement), sand apron, and
marginal islands (Figure 1b). It is relatively narrow
(up to several hundred meters) as compared to the much
larger lagoon area. The margin is characterized by coral
reefs, which exhibit geomorphological and ecological
zonations (Ladd et al., 1950; Wiens, 1962, 234–243).
On the ocean side of the reef crest, which is often composed of an algal ridge in the Pacific, there is a fore-reef
slope that in many cases dips about 30
. The shallow
fore-reef slope is usually characterized by the spur and
groove system, a comb-shaped structure of ridges perpendicular to the reef crest that may have either constructional
or erosional origins (Gischler, 2010). The fore-reef slope
transitions to a break in slope and a steep wall several hundreds of meters high (James and Ginsburg, 1979). On the
lagoon side of the reef crest, a cemented pavement, occasionally with islands, and a sand apron occur. It is not
uncommon that a distinct sandy slope transitions to the
lagoon proper. Sand aprons are significant in that they
contribute to the lateral infilling of atoll lagoons by
lagoonward progradation (Purdy and Gischler, 2005).
Atoll margins are usually not continuous but feature several passes that connect the open ocean with the interior
lagoon. In many Pacific atolls, the passes are located on
the leeward sides (Wiens, 1962).
Atoll lagoons get up to 100 m deep; the average depth
amounts to some 36 m based on an analysis of the geomorphology of 295 atolls (Purdy and Winterer, 2001). The
same authors showed that atoll lagoon depths covary significantly with precipitation rates, thereby supporting the
contention that inherited karst relief from solution during
glacial sea-level lowstands is crucial in atoll development.
As compared to the atoll margin that exhibits accretion
rates of several meters per kyr, background sedimentation
within Quaternary atoll lagoons averages decimeters per
kyr (Purdy and Winterer, 2001; Purdy and Gischler,
2005). Atoll lagoons usually accommodate numerous
coral patch reefs. In the Maldives, lagoonal reefs as well
as some marginal reefs may form characteristic ringlike
structures called faroes. Their formation is largely attributed to the changing wind and current regime of the Indian
monsoon (Purdy and Bertram, 1993).
Sand and rubble islands termed cays are common on
both marginal and lagoonal atoll reefs. Beachrock is
a very common characteristic of sandy beaches on reef
islets (Gischler, 2007). Elongated cays on Pacific reef
margins are called motu, and the channels between them
hoa. Motus are in many cases relatively stable due to
underpinning by mid- to late Holocene (ca. 4–2 kyrs
BP) reef flat deposits that formed during higher-thanpresent sea-level (Dickinson, 2009). Human colonization
in the Pacific only started after high tide levels had fallen
below the upper level of these older Holocene deposits,
some 2,000 years ago (Dickinson, 2009).
Modern atoll sediments are usually coralgal grainstones
on marginal and lagoonal reefs. Atoll lagoon sediments
often include mollusk-rich and foraminifer-rich
wackestones and packstones as well as nonskeletal facies
(Gischler and Lomando, 1999; Gischler, 2006; and references therein). Atoll lagoon deposits have also been successfully used as archives of event sedimentation
(Klostermann et al. 2014).
Guyots
Guyots are seamounts with more or less flat tops that in
many cases lie in about 1–2 km water depth (Figure 1).
The tops typically are 3.0–4.5 km above the surrounding
seafloor and may exhibit reliefs of tens of meters. The
large majority can be found in the northwestern Pacific
at 8–27
N and 146–165
E (Hess, 1946; Hamilton,
1956; Menard, 1984; Wessel, 1997; Flood, 1999). There
are some 50,000 seamounts, many of them guyots, in the
Pacific Ocean, about 50 % of them not charted and
<1 % sampled for dating (Wessel, 1997). Some guyots
are also known from other oceans (e.g., Pratt, 1963,
1967). Their shape is circular to oval. Guyot size varies
and diameters may reach >200 km (Davies et al., 1972).
Slopes of guyots are concave and reach 20
inclination toward the top. Guyots have been repeatedly
investigated by dredging, seismics, and drilling. ODP
cruises 143 and 144 were especially devoted to the Pacific
guyots (Winterer et al., 1995; Haggerty and Premoli-Silva,
1995).
Ideal successions of guyot tops include the volcanic
(basaltic) edifice, a shallow-water atoll reef section with
elevated rims, and a package of pelagic ooze, usually
capped by a manganese-iron or phosphorite crusts
(Schlanger et al., 1987; Grötsch and Flügel, 1992; Jenkyns
and Wilson, 1999; Flood, 1999, 2001) (Figure 2). However, there are also a number of guyots that are purely volcanic, lack a reefal succession, or feature mixed volcanic
and limestone breccias (Jenkyns and Wilson, 1999).
The shallow-water reef deposits often are Cretaceous to
the Eocene in age. They may get as thick as 1.6 km as on
Resolution Guyot and contain clear indicators of a reefal
origin such as corals, rudist mollusks, calcareous sponges
(stromatoporoids), calcareous algae, larger foraminifera,
and nonskeletal grains including ooids (Grötsch and
Flügel, 1992; Jenkyns and Wilson, 1999; Flood, 1999,
2001). The majority of shallow-water reefs on guyots
drowned in the Cretaceous (Matthews et al., 1974; Wilson
et al., 1998; Jenkyns and Wilson, 1999). The age range of
pelagic oozes comprises the Cretaceous to the Quaternary.
The oozes get as thick as 150 m as on top of Allison
Guyot. They are characterized in many cases by stratigraphic gaps and the so-called ghost faunas, i.e., mixtures
of reworked foraminifera that comprise many biostratigraphical zones. The formation of ferromanganese crusts
is likely the result of microbial activity. Additional processes include phosphatization and aragonite dissolution
(Grötsch and Flügel, 1992). Collectively, these features
are characteristic of very low sedimentation rates and/or
nondeposition (omission), erosion, condensation of sediment, and dissolution (Hamilton, 1956; Lonsdale et al.,
304
GUYOT, ATOLL