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= Limalok =
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Introduction
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Limalok (formerly known as Harrie or Harriet) is a
Cretaceous-Paleocene guyot/tablemount in the southeastern Marshall
Islands, one of a number of seamounts (a type of underwater volcanic
mountain) in the Pacific Ocean. It was probably formed by a volcanic
hotspot in present-day French Polynesia. Limalok lies southeast of
Mili Atoll and Knox Atoll, which rise above sea level, and is joined
to each of them through a volcanic ridge. It is located at a depth of
1255 m and has a summit platform with an area of 636 km2.
Limalok is formed by basaltic rocks and was probably a shield volcano
at first; the Macdonald, Rarotonga, Rurutu and Society hotspots may
have been involved in its formation. After volcanic activity ceased,
the volcano was eroded and thereby flattened, and a carbonate platform
formed on it during the Paleocene and Eocene. These carbonates were
chiefly produced by red algae, forming an atoll or atoll-like
structure with reefs.
The platform sank below sea level 48±2 million years ago during the
Eocene, perhaps because it moved through the equatorial area, which
was too hot or nutrient-rich to support the growth of a coral reef.
Thermal subsidence lowered the drowned seamount to its present depth.
After a hiatus lasting into the Miocene, sedimentation commenced on
the seamount leading to the deposition of manganese crusts and pelagic
sediments; phosphate accumulated in some sediments over time.
Name and research history
======================================================================
Limalok was formerly known as Harrie Guyot and is also known as
Harriet Guyot; Limalok refers to a traditional chieftess of Mile
Atoll. Limalok is one of the seamounts targeted during the Ocean
Drilling Program, which was a research program that aimed at
elucidating the geological history of the sea by obtaining drill cores
from the oceans. The proportion of material recovered during the
drilling was low, making it difficult to reconstruct the geologic
history of Limalok.
Local setting
===============
Limalok lies at the southernmost end of the Ratak Chain in the
southeastern Marshall Islands in the western Pacific Ocean. Mili Atoll
is located 53.7 km from Limalok, with Knox Atoll in between the two.
The relatively small seamount rises from a depth of 4500 m to a
minimum depth of 1255 m below sea level. The top of Limalok is 47.5 km
long and broadens southeastward from less than 5 km to more than 24
km, forming a 636 km2 summit platform. The carbonate platform of
Limalok crops out at the edges of the summit plateau. Wide terraces
and numerous fault blocks surround the summit plateau; some of the
latter may have formed after the carbonate platform ceased growing.
Mili Atoll and Limalok emerge from a common pedestal and are connected
by a ridge at 1.5 km depth. The seafloor is 152158 million years old,
but it is possible that Limalok rises from Cretaceous flood basalts
rather than the seafloor itself. Volcanic sediments in the Eastern
Mariana Basin may come from this seamount.
Regional setting
==================
The Pacific Ocean seafloor, especially the parts that are of Mesozoic
age, contains most of the world's guyots (also known as tablemounts).
These are submarine mountains which are characterized by steep slopes,
a flat top and usually the presence of corals and carbonate platforms.
These structures originally formed as volcanoes in the Mesozoic Ocean.
Fringing reefs may have developed on the volcanoes, which then were
replaced by barrier reefs as the volcanoes subsided and turned into
atolls. Continued subsidence balanced by upward growth of the reefs
led to the formation of thick carbonate platforms. Volcanic activity
can occur even after the formation of the atoll or atoll-like
landforms, and during episodes where the platforms were lifted above
sea level, erosional features such as channels and blue holes
developed. The crust underneath these seamounts tends to subside as it
cools and thus the islands and seamounts sink.
The formation of many seamounts including Limalok has been explained
with the hotspot theory, in which a "hot spot" rising from the mantle
leads to the formation of chains of volcanoes which get progressively
older along the length of the chain, with an active volcano at only
one end of the system, as the plate moves over the hotspot. Seamounts
and islands in the Marshall Islands do not appear to have originated
from simple age-progressive hotspot volcanism as the age progressions
in the individual island and seamount chains are often inconsistent
with this explanation. One solution to this dilemma may be that more
than one hotspot passed through the Marshall Islands, and it is also
possible that hotspot volcanism was affected by extensional
deformation of the lithosphere. For Limalok, geochemical evidence
shows affinities to the Rarotonga hotspot which is unlike the
geochemical trends in the other volcanoes of the Ratak Chain.
Reconstructions of the area's geological history suggest that the
first hotspot to pass by Limalok was the Macdonald hotspot 9585
million years ago, followed by the Rurutu hotspot and the Society
hotspot 7565 million years ago. The Rarotonga and especially the
Rurutu hotspots are considered to be the most likely candidates for
the hotspot that formed Limalok. However, some paleogeographical
inconsistencies indicate that lithospheric fractures secondary to
hotspot activity were also involved.
From plate motion reconstructions, it has been established that the
Marshall Islands were located in the era now occupied by present-day
French Polynesia during the time of active volcanism. Both regions
display numerous island chains, anomalously shallow ocean floors and
the presence of volcanoes. About eight hotspots have formed a large
number of islands and seamounts in that region, with disparate
geochemistries; the geological province has been called "South Pacific
Isotopic and Thermal Anomaly" or DUPAL anomaly.
Composition
=============
Limalok has erupted basaltic rocks, which have been classified as
alkali basalts, basanite and nephelinite. Minerals contained in the
rocks are apatite, augite, biotite, clinopyroxene, olivine, nepheline
and plagioclase, and there are ultramafic xenoliths. Shallow crystal
fractionation processes appear to have been involved in the genesis of
the magmas erupted by Limalok.
Alteration of the original material has formed calcite, chlorite,
clay, iddingsite, montmorillonite, zeolite, and a mineral that could
be celadonite. Volcanogenic sandstones and traces of hydrothermal
alteration also exist on Limalok.
Carbonate, clay, manganese phosphate crust materials and mudstones
have been found in boreholes or have been dredged from the seamount.
The carbonates take various forms, such as grainstone, packstone,
limestone, rudstone and wackestone. Porosity is usually low owing to
cementation of the deposits, a process in which grains in rock are
solidified and pores filled by the deposition of minerals such as
calcium carbonate. The carbonate rocks show widespread evidence of
diagenetic alteration, meaning the carbonates have been chemically or
physically modified after they were buried. For example, aragonite,
pyrite and organic material were formed by alteration of living beings
within the clays and limestones.
Geologic history
======================================================================
Limalok is the youngest guyot in the Marshall Islands. Argon-argon
dating has yielded ages of 69.2 and 68.2±0.5 million years ago on
volcanic rocks dredged from Limalok. Mili Atoll volcano is probably
not much younger than Limalok. During the Cretaceous Limalok was
probably located in French Polynesia; paleomagnetism indicates that
Limalok formed at 15-10 degrees southern latitude. Early limestones
dredged from Limalok were considered to be of Eocene age (5633.9
million years ago) before earlier Paleocene deposits were discovered
as well.
Volcanism and first biotic phenomena
======================================
Limalok first formed as a shield volcano. The volcanic rocks were
emplaced as lava flows with thicknesses reaching 1 -. In addition,
breccia and pebbles encased within sediments occur.
Soils formed on the volcano through the weathering of volcanic rocks,
reaching a thickness of 28.6 m; claystones and laterites were also
generated through weathering. These deposits formed over a long time
on an island that rose at least several metres above sea level - the
estimated time it took to generate the soil profiles obtained in drill
cores is about 13 million years. Thermal subsidence of the crust and
erosion flattened the seamount before carbonate deposition commenced
on Limalok, and it is possible that the growth of another volcano
south of Limalok 12 million years after Limalok developed may be
responsible for a southward tilt of the seamount.
The soils on Limalok were colonized by vegetation that left plant
cuticle and woody tissues; angiosperms including palms, ferns and
fungi with an overall low diversity developed on the volcano.
Organisms burrowed into the soils, leaving cavities. The climate was
probably tropical to subtropical, with an annual precipitation of less
than 1000 mm/year.
Platform carbonates and reefs
===============================
The erosion of the volcanic island was followed after some time by the
beginning of carbonate platform growth. Sedimentation began in the
Paleocene with one or two events in which the seamount was submerged;
the start of sedimentation has been dated to about 57.5±2.5 million
years ago. After a Paleocene phase with open sea or back-reef
conditions, lagoonal environments developed on the seamount during the
Eocene. It is possible that the platform periodically emerged above
sea level, leading to its erosion. It is not clear if the platform
took the form of an atoll, or of a shallow platform shielded on one
side by islands or shoals, similar to the present-day Bahama Banks.
Sea level rise at the Paleocene-Eocene transition may have triggered a
transformation from a partially shielded platform to a true
ring-shaped atoll.
The carbonate platform reaches an overall thickness of 290 m in one
drill core. Drill cores in the platform show variations between
individual carbonate layers that imply that parts of the platform were
submerged and emerged over time while the platform was still active,
possibly because of eustatic sea level variations. Furthermore, the
platform was affected by storms which redeposited the carbonatic
material. The deposition of the platform lasted about 10 million
years, spanning the Paleocene-Eocene Thermal Maximum (PETM). Drill
core evidence shows that the PETM had little impact on carbonate
deposition at Limalok despite a decrease in the δ13C isotope ratio
recorded in the carbonates, implying there was little change to ocean
pH at that time.
The dominant living beings on Limalok were red algae that occupied
many ecological niches and formed rhodoliths. Other lifeforms were
bivalves, bryozoans, corals, echinoderms, echinoids, foraminifera,
gastropods, molluscs and ostracods. Species and general composition
varied over time, leading to different species being found in
different parts of the platform. Red algae were important early
colonizers, and algal mats and oncoids were contributed by algae
and/or cyanobacteria.
Drowning and post-drowning evolution
======================================
A carbonate platform is said to 'drown' when sedimentation can no
longer keep up with relative rises in sea level, and carbonate
deposition stops. Limalok drowned during the early-middle Eocene, soon
after the start of the Lutetian, 48±2 million years ago. It is the
most recent carbonate platform in the region to submerge: the similar
platform at neighbouring Mili Atoll is still depositing carbonate.
The drownings of carbonate platforms such as Limalok, MIT,
Takuyo-Daisan and Wōdejebato appear to have many causes. One is a sea
level drop resulting in the emergence of much of the platform; this
reduces the space that carbonate-forming organisms have to grow upward
when sea levels again rise. A second factor is that these platforms
were not true reefs but rather piles of carbonate sediment formed by
organisms; these constructs cannot easily out-grow sea level rises
when growing on a constrained area. Two final key factors are the
passage of the platforms through nutrient-rich equatorial waters which
cause the overgrowth of algae that hampered the growth of reef-forming
organisms, and global temperature extremes that may overheat the
platforms especially when close to the equator; present-day coral
bleaching events are often triggered by overheating and Limalok and
the other seamounts were all approaching the equator when they
drowned. In the case of Limalok and some other guyots, paleolatitude
data support the notion that approaching the equator led to the demise
of the platforms.
After the platform ceased growing, subsidence quickly lowered the
tablemount below the photic zone, where sunlight can still penetrate.
Hardgrounds and iron-manganese crusts formed on the drowned platform
which contain Oligocene (33.923.02 million years ago) sediments and
planktonic fossils. Some of the rocks underwent phosphatization during
three separate episodes in the Eocene and Eocene-Oligocene which may
have been triggered by ocean upwelling events at that time. The
Palaeocene-Eocene thermal maximum and accompanying ocean acidification
left no recognizable traces in Limalok's carbonate pile.
Until the Miocene, sedimentation on Limalok was probably hindered by
strong currents. Renewed significant sedimentation began at that point
after the drowning of Limalok, with sediments consisting mainly of
foraminifera and other nanofossils. Some of the sediments were
reworked after deposition. At least two layers formed during the
Miocene (23.35.333 million years ago) and Pliocene-Pleistocene
(5.3330.0117 million years ago), reaching a cumulative thickness of
100 -. Chemically, most of the sediments are calcite and they often
occur in rudstone or wackestone form. Bivalves, echinoderms,
foraminifera and ostracods are fossilized in the sediments, which
sometimes contain borings and other traces of biological activity.
License
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Original Article: http://en.wikipedia.org/wiki/Limalok
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