Limalok

Coordinates: 5°36′N 172°18′E / 5.6°N 172.3°E / 5.6; 172.3[1]
Source: Wikipedia, the free encyclopedia.

Limalok
A bathymetric map of Limalok; it lies southwest of Mili and has a roughly triangular shape.
Limalok is located in Marshall Islands
Limalok
Limalok
Location in the Marshall Islands
Summit depth1,255 metres (4,117 ft)
Summit area636-square-kilometre (246 sq mi)
Location
Group
Ratak Chain
Coordinates5°36′N 172°18′E / 5.6°N 172.3°E / 5.6; 172.3[1]
CountryMarshall Islands
Geology
TypeGuyot
Age of rockCretaceous

Limalok (formerly known as Harrie or Harriet) is a

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 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).

Limalok is formed 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

pelagic sediments; phosphate
accumulated in some sediments over time.

Name and research history

Limalok was formerly known as Harrie Guyot

drill cores from the oceans.[6][7] The proportion of material recovered during the drilling[8] was low, making it difficult to reconstruct the geologic history of Limalok.[9]

Geography and geology

Local setting

Limalok lies at the southernmost

Ratak Chain[11] in the southeastern Marshall Islands[12] in the western Pacific Ocean.[6] Mili Atoll is located 53.7 kilometres (33.4 mi) from Limalok,[3] with Knox Atoll in between the two.[13]

The relatively small[14] seamount rises from a depth of 4,500 metres (14,800 ft)[15] to a minimum depth of 1,255 metres (4,117 ft) below sea level.[16] The top of Limalok is 47.5 kilometres (29.5 mi) long[3] and broadens southeastward from less than 5 kilometres (3.1 mi) to more than 24 kilometres (15 mi),[13] forming a 636-square-kilometre (246 sq mi) summit platform.[17] The carbonate platform of Limalok crops out at the edges of the summit plateau.[10] Wide terraces[10] and numerous fault blocks surround the summit plateau;[18] some of the latter may have formed after the carbonate platform ceased growing.[19]

Mili Atoll and Limalok emerge from a common pedestal

seafloor is 152[20]–158 million years old,[21] but it is possible that Limalok rises from Cretaceous flood basalts[d] rather than the seafloor itself.[23] Volcanic sediments in the Eastern Mariana Basin may come from this seamount.[24]

Regional setting

Diagram of how an active volcano is accompanied by decaying inactive volcanoes that were formerly located on the hotspot but have been moved away
Illustration of how hotspot volcanoes work

The Pacific Ocean seafloor, especially the parts that are of

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.[27] Volcanic activity can occur even after the formation of the atoll or atoll-like[e] landforms, and during episodes where the platforms were lifted above sea level, erosional features such as channels and blue holes[f] developed.[30] The crust underneath these seamounts tends to subside as it cools and thus the islands and seamounts sink.[31]

The formation of many seamounts

lithospheric fractures secondary to hotspot activity were also involved.[42]

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.[43] About eight hotspots have formed a large number of islands and seamounts in that region, with disparate geochemistries;[44] the geological province has been called "South Pacific Isotopic and Thermal Anomaly" or DUPAL anomaly.[45]

Composition

Limalok has erupted

crystal fractionation processes appear to have been involved in the genesis of the magmas erupted by Limalok.[51]

Alteration of the original material has formed

hydrothermal alteration also exist on Limalok.[48]

Carbonate, clay,

diagenetic alteration,[57] meaning the carbonates have been chemically or physically modified after they were buried.[56] For example, aragonite, pyrite[58] and organic material were formed by alteration of living beings within the clays and limestones.[59]

Geologic history

←

Vertical axis scale: millions of years ago

Limalok is the youngest guyot in the Marshall Islands.

Argon-argon dating has yielded ages of 69.2[62] and 68.2 ± 0.5 million years ago on volcanic rocks dredged from Limalok.[63] Mili Atoll volcano is probably not much younger than Limalok.[64] During the Cretaceous Limalok was probably located in French Polynesia;[33] paleomagnetism indicates that Limalok formed at 15[65]–10 degrees southern latitude. Early limestones dredged from Limalok were considered to be of Eocene age (56–33.9 million years ago[2]) before earlier Paleocene deposits were discovered as well.[9]

Volcanism and first biotic phenomena

Limalok first formed as a

lava flows[41] with thicknesses reaching 1–7 metres (3 ft 3 in – 23 ft 0 in).[66] In addition, breccia[h][16] and pebbles encased within sediments occur.[52]

claystones[46] and laterites were also generated through weathering.[47] These deposits formed over a long time on an island that rose at least several metres above sea level[52] – the estimated time it took to generate the soil profiles obtained in drill cores is about 1–3 million years.[20] Thermal subsidence of the crust[33] and erosion flattened the seamount before carbonate deposition commenced on Limalok,[54] and it is possible that the growth of another volcano south of Limalok 1–2 million years after Limalok developed may be responsible for a southward tilt of the seamount.[64]

The soils on Limalok were colonized by vegetation

subtropical,[47] with an annual precipitation of less than 1,000 millimetres per year (39 in/year).[68]

Platform carbonates and reefs

The erosion of the volcanic island was followed after some time by the beginning of carbonate platform growth.[69] Sedimentation began in the Paleocene with one or two events in which the seamount was submerged;[13] the start of sedimentation has been dated to about 57.5 ± 2.5 million years ago.[70] After a Paleocene phase with open sea or back-reef conditions, lagoonal environments developed on the seamount during the Eocene.[71] It is possible that the platform periodically emerged above sea level, leading to its erosion.[54][72] 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.[28][73] Sea level rise at the Paleocene-Eocene transition may have triggered a transformation from a partially shielded platform to a true ring-shaped atoll.[74]

The carbonate platform reaches an overall thickness of 290 metres (950 ft) in one

isotope ratio recorded in the carbonates, implying there was little change to ocean pH at that time.[78]

The dominant living beings on Limalok were

oncoids[l] were contributed by algae and/or cyanobacteria.[82]

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.[83][84] Limalok drowned during the early-middle Eocene, soon after the start of the Lutetian,[54] 48 ± 2 million years ago.[70] It is the most recent carbonate platform in the region to submerge:[9] the similar platform at neighbouring Mili Atoll is still depositing carbonate.[85][86]

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.[87] 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.[88][89] 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.[90]

After the platform ceased growing, subsidence quickly lowered the tablemount below the

Hardgrounds[n][93] and iron-manganese crusts formed on the drowned platform[6] which contain Oligocene (33.9–23.02 million years ago[2]) sediments and planktonic fossils.[71] Some of the rocks underwent phosphatization[93] during three separate episodes in the Eocene and Eocene–Oligocene which may have been triggered by ocean upwelling events at that time.[94]

Until the Miocene, sedimentation on Limalok was probably hindered by strong currents.[95] Renewed significant sedimentation began at that point[71] 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 (23.3–5.333 million years ago[2]) and Pliocene–Pleistocene (5.333–0.0117 million years ago[2]),[6] reaching a cumulative thickness of 100–140 metres (330–460 ft).[96][71] Chemically, most of the sediments are calcite[97] and they often occur in rudstone or wackestone form.[98] Bivalves, echinoderms, foraminifera[98] and ostracods[o] are fossilized in the sediments,[96] which sometimes contain borings and other traces of biological activity.[98]

Notes

  1. ^ Between ca. 145 and 66 million years ago.[2]
  2. ^ Between 66 and 56 million years ago.[2]
  3. ^ 23.3–5.333 million years ago[2]
  4. lava flows.[22]
  5. ^ Whether the Cretaceous guyots were all atolls in the present-day sense is often unclear.[28]
  6. ^ Pit-like depressions within carbonate rocks that are filled with water.[29]
  7. ^ Asbolane, birnessite and buserite are found in the crusts.[53]
  8. ^ Volcanic rocks that appear as fragments.[67]
  9. atmospheric carbon dioxide levels and temperatures dramatically increased.[77]
  10. algae which deposit carbonates.[79]
  11. Nummulites.[13]
  12. ^ Pebble-like growths formed by cyanobacteria.[81]
  13. ^ The uppermost layers of water in the sea, through which sunlight can penetrate.[91]
  14. ^ In stratigraphy, hardgrounds are solidified layers of sediments.[92]
  15. ^ Ostracod taxa include Bradleya, various cytherurids, Eucythere, Krythe and Tongacythere.[96]

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Sources
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