Lo-En

Coordinates: 10°07′N 162°48′E / 10.117°N 162.800°E / 10.117; 162.800[1]
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Lo-En
Bathymetry of Lo-En Guyot
Summit depth1,080 metres (3,540 ft)
Height4,561 ± 526 metres (14,964 ± 1,726 ft)
Summit area823 square kilometres (318 sq mi)
Location
Coordinates10°07′N 162°48′E / 10.117°N 162.800°E / 10.117; 162.800[1]
CountryMarshall Islands
Geology
TypeGuyot
Lo-En is located in Marshall Islands
Lo-En
Location in the Marshall Islands

Lo-En or Hess is an

Eniwetok
which rises above sea level, and Lo-En is almost connected to it through a ridge.

The seamount is formed by

pelagic sediments including limestone, some of which were later modified by phosphate
.

Name and research history

The name "Lo-En" is a reference to the

Geography and geology

Local setting

Lo-En lies within the northern

Ratak Chain, two seamount-and-island chains in the Marshall Islands.[8]

The seamount rises 4,561 ± 526 metres (14,964 ± 1,726 ft) above the seafloor

normal"[3] but with particular magnetization patterns that are different from the topography.[17]

No

barrier reef has been questioned,[20] and Lo-En has a 141.7–143.6-metre (465–471 ft) thick pelagic sediment cap.[21] It is possible that shallow water limestones exist at the margins of the platform, which were not drilled.[22]

Lo-En shares its volcanic edifice with Eniwetok;[23] a northern spur from Lo-En almost reaches Eniwetok[7] while another spur emerges in south-southeast direction[12] and is about 14.5 kilometres (9.0 mi) long.[24] Another seamount lies 50 kilometres (31 mi) farther west from Lo-En.[7] The seafloor underneath Lo-En is 113[1]–156.9 ± 5 million years old[9] and lies at a depth of more than 5 kilometres (3.1 mi).[7]

Regional setting

Illustration of how hotspot volcanoes work

The

corals and carbonate platforms.[26] It is not clear whether the Cretaceous guyots were atolls in the present-day sense[27] but many of these seamounts were, which today still exist. All these structures originally formed as volcanoes in the Mesozoic Ocean. First fringing reefs may have developed on the volcanoes, which then became barrier reefs as the volcano subsided and turned into an atoll. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms.[28] Sometimes volcanic activity occurred even after the formation of the atoll or atoll-like landforms, and during episodes where the carbonate platforms were lifted above sea level erosional features such as channels and blue holes developed.[29] The crust underneath these seamounts tends to subside as it cools and thus the islands and seamounts sink.[30]

Maps of the paths of some Pacific hotspots

The formation of many such seamounts

lithospheric extension.[35]

Candidate hotspots responsible for the formation of Lo-En are the

geochemical similarity to Lo-En.[37] In the case of Lo-En, volcanism on the Ogasawara Fracture Zone may also have contributed to its formation[8] considering that the seamount is much older than surrounding seamounts.[1]

Based on plate motion reconstructions, the region of the Marshall Islands was located in the region of 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.[38] About 8 hotspots have formed a large number of islands and seamounts in that region, with disparate geochemistries,[39] and that geological province has been called the "South Pacific Isotopic and Thermal Anomaly" or DUPAL anomaly.[40]

Composition

Lo-En has erupted

clinopyroxene, ilmenite, magnetite and plagioclase.[41] Strong alteration has occurred[43] and has given rise to calcite, carbonate, chabazite, clay, hematite, smectite and zeolite,[44][41] as well as palagonite.[15] Other rocks found on Lo-En are chalk, limestone, manganese crusts, phosphate[23] and sandstone.[6]

Geologic history

The geologic history of Lo-En seamount is relatively poorly known;

paleomagnetic data have been used to infer an age of 45–85 million years ago for the seamount.[46] It was located at about 25[47]–30 degrees southern latitude when it formed, south of the hotspots that are presently active in the Cook IslandsAustral Islands.[1] Eniwetok is about 36 million years younger than Lo-En.[48]

Volcanism and first biotic phenomena

A first phase of volcanic activity occurred 114 million years ago, and was followed by a second episode during the Campanian,[49] separated by as much as 30 million years.[50] The first episode has been dated at 111.6–114.0 million years ago[51] and was contemporaneous with volcanic activity at other western Pacific seamounts. The second episode may be responsible for the cones that emerge from or are embedded in sediments that cover Lo-En. Eniwetok was active 76.9 million years ago[15] and flexural effects from its growth may have influenced the second volcanic phase at Lo-En.[52] Potentially, the first episode was caused by the Macdonald hotspot and the second by the Rarotonga and Rurutu hotspots.[36]

The volcanic rocks encountered in drill cores consist of

Claystones indicate that weathered terrain occurred on Lo-En,[11] with subaerial alteration of volcanic rocks.[54]

Post-volcanic

Lo-En was submerged in the Turonian[22] by about 112.8 ± 1.2 million years ago, long before surrounding seamounts and islands formed.[1] The absence of a barrier reef would have facilitated the erosion of Lo-En's summit.[52] It is possible that flexural loading by Eniwetok pushed Lo-En downward and thus prevented the formation of reefs on Lo-En during the Cretaceous when the seamount was uplifted by a hotspot.[55] It took about 2–7 million years from the end of volcanic activity until sedimentation began.[56]

During the Albian, limestones were emplaced on Lo-En.[18] These appear to reflect the formation of reefs[57] and a shallow carbonate bank, material from which has been dredged on the southern flank.[58] This platform drowned during the late Albian, similar to many other carbonate platforms of similar age,[59] and there is no evidence of mid-Cretaceous platforms.[48]

Starting in the Albian–Cenomanian,[10] pelagic limestone was deposited directly on the volcanic rocks during SantonianConiacian times; in light of fossil data it is likely that the seamount was not overly deep at that time.[60] Sediments of Cretaceous–Paleocene age reach thicknesses of less than 1 metre (3 ft 3 in).[23]

Starting in the Oligocene,[58][61] pelagic oozes were deposited on Lo-En. The previously emplaced limestones also underwent phosphate reactions as they were exposed on the seafloor,[60] and some of the sediments underwent reworking.[23]

nannofossils are:[60]

References

  1. ^ a b c d e f g Koppers et al. 1995, p. 538.
  2. ISSN 2331-1258
    .
  3. ^ a b Bryan et al. 1993, p. 388.
  4. TAMU
    . Retrieved 13 May 2019.
  5. ^ a b Watkins et al. 1995, p. 675.
  6. ^ a b c d Kurnosov et al. 1995, p. 475.
  7. ^ a b c d e Bryan et al. 1993, p. 390.
  8. ^ a b Koppers et al. 1995, p. 537.
  9. ^ a b Caplan-Auerbach, Duennebier & Ito 2000, p. 2687.
  10. ^ a b Erba et al. 1995, p. 884.
  11. ^ a b Larson et al. 1995, p. 922.
  12. ^ a b c Bergersen 1995, p. 566.
  13. ^ Bergersen 1995, p. 567.
  14. ^ a b Caplan-Auerbach, Duennebier & Ito 2000, p. 2689.
  15. ^ a b c Haggerty & Premoli Silva 1995, p. 938.
  16. ^ Bergersen 1995, p. 570.
  17. ^ Bryan et al. 1993, p. 395.
  18. ^ a b Erba, Premoli Silva & Watkins 1995, p. 157.
  19. ^ Erba et al. 1995, p. 873.
  20. ^ Bergersen 1995, p. 572.
  21. ^ Bogdanov et al. 1995, p. 747.
  22. ^
    ISSN 0096-3941.{{cite journal}}: CS1 maint: numeric names: authors list (link
    )
  23. ^ a b c d Watkins et al. 1995, p. 678.
  24. ^ Bergersen 1995, p. 569.
  25. ISBN 9781444304879. {{cite book}}: |journal= ignored (help
    )
  26. ^ Arnaud Vanneau et al. 1995, p. 819.
  27. , retrieved 2018-07-10
  28. ^ Pringle et al. 1993, p. 359.
  29. ^ Pringle et al. 1993, p. 360.
  30. ^ Larson et al. 1995, p. 916.
  31. ^ a b Koppers et al. 2003, p. 2.
  32. ^ Arnaud Vanneau et al. 1995, p. 833.
  33. ^ Pringle et al. 1993, p. 368.
  34. ^ Pringle et al. 1993, p. 299.
  35. ^ Koppers et al. 2003, p. 35.
  36. ^ a b Haggerty & Premoli Silva 1995, p. 939.
  37. ^ Koppers et al. 2003, p. 25.
  38. ^ Bergersen 1995, p. 561.
  39. ^ Koppers et al. 1995, p. 535.
  40. , retrieved 2018-07-14
  41. ^ a b c Koppers et al. 1995, p. 539.
  42. ^ Kurnosov et al. 1995, p. 476.
  43. ^ Kurnosov et al. 1995, p. 484.
  44. ^ Kurnosov et al. 1995, p. 479.
  45. ^ Larson et al. 1995, p. 928.
  46. ^ Bryan et al. 1993, p. 399.
  47. ^ Haggerty & Premoli Silva 1995, p. 941.
  48. ^ a b Bergersen 1995a, p. 605.
  49. ^ Larson et al. 1995, p. 917.
  50. ^ Bergersen 1995, p. 564.
  51. ^ Bergersen 1995a, p. 606.
  52. ^ a b Bergersen 1995, p. 577.
  53. ^ Haggerty & Premoli Silva 1995, p. 942.
  54. ^ Kurnosov et al. 1995, p. 487.
  55. ^ Bergersen 1995, p. 575.
  56. ^ Haggerty & Premoli Silva 1995, p. 947.
  57. ^ Haggerty & Premoli Silva 1995, p. 943.
  58. ^ a b Larson et al. 1995, p. 924.
  59. ^ Larson et al. 1995, p. 929.
  60. ^ a b c Erba, Premoli Silva & Watkins 1995, p. 158.
  61. ^ Bogdanov et al. 1995, p. 748.

Sources

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