Allison Guyot

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Allison Guyot
Pacific Ocean laea location map.svg
Red triangle with thick white border.svg
Allison
Height1.5 kilometres
Summit area35 x 70 kilometres
Location
Group Mid-Pacific Mountains
Coordinates 18°16′N179°20′E / 18.26°N 179.33°E / 18.26; 179.33 Coordinates: 18°16′N179°20′E / 18.26°N 179.33°E / 18.26; 179.33 [1]
Geology
Type Guyot

Allison Guyot (formerly known as Navoceano Guyot) is a tablemount (guyot) in the underwater Mid-Pacific Mountains of the Pacific Ocean. It is a trapezoidal flat mountain rising 1,500 metres above the seafloor to a depth of less than 1,500 m, with a summit platform 35 by 70 kilometres wide. The Mid-Pacific Mountains lie west of Hawaii and northeast of the Marshall Islands, but at the time of their formation were located in the Southern Hemisphere.

Guyot An isolated, flat-topped underwater volcano mountain

In marine geology, a guyot, also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than 200 m (660 ft) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6.2 mi). Guyots are most commonly found in the Pacific Ocean, but they have been identified in all the oceans except the Arctic Ocean.

The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.

Pacific Ocean Ocean between Asia and Australia in the west, the Americas in the east and Antarctica or the Southern Ocean in the south.

The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east.

Contents

The tablemount was probably formed by a hotspot in the present-day Southern Pacific before plate tectonics moved it to its current location. Several hotspots, including the Easter, Marquesas and Society hotspots, may have been involved in the formation of the Mid-Pacific Mountains. Volcanic activity is dated to have occurred circa 111–85 million years ago and formed a volcanic island. Subsequently, carbonate deposition commenced as Allison Guyot subsided and eventually buried the island, forming an atoll-like structure and a carbonate platform. Among other animals, crocodilians lived on Allison Guyot.

Hotspot (geology) Volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Easter hotspot

The Easter hotspot is a volcanic hotspot located in the southeastern Pacific Ocean. The hotspot created the Sala y Gómez Ridge which includes Easter Island and the Pukao Seamount which is at the ridge's young western edge. Easter Island, because of its tectonomagmatic features, represents an end-member type of hotspot volcano in this chain.

Marquesas hotspot

The Marquesas hotspot is a volcanic hotspot in the central Pacific Ocean. It is responsible for the Marquesas Islands, a group of 12 volcanic islands and one of the five archipelagos of French Polynesia.

The platform emerged above sea level during the Albian and Turonian ages. It drowned about 99 ± 2 million years ago for unknown reasons; possibly a phase of renewed emergence damaged the reefs, or it was located in unfavourable waters. Later, pelagic sedimentation commenced on the seamount and led to the deposition of sediments including limestone, ooze and sand, which bear traces of climatic events and ocean currents.

The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0.9 Ma. The Albian is preceded by the Aptian and followed by the Cenomanian.

The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous epoch, or a stage in the Upper Cretaceous series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma. The Turonian is preceded by the Cenomanian stage and underlies the Coniacian stage.

Reef A bar of rock, sand, coral or similar material, lying beneath the surface of water

A reef is a bar of rock, sand, coral or similar material, lying beneath the surface of water. Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock outcrops, etc.—but the best known reefs are the coral reefs of tropical waters developed through biotic processes dominated by corals and coralline algae.

Name and research history

Allison Guyot is named after E.C. Allison, an oceanographer and paleontologist at the San Diego State College; [2] formerly it was named "Navoceano Guyot". [3] The name "Hamilton Guyot" has also been applied to Allison Guyot but is not correct; [4] Hamilton Guyot is a separate formation in the Mid-Pacific Mountains. [5] The seamount is the source of the Ocean Drilling Program [lower-alpha 1] drill core 865A, [8] which was bored into the summit platform of Allison Guyot [9] in 1992 [10] but did not reach the volcanic structure of the underwater mountain. [11] Two other cores 865C and 865B were obtained during the same operation; Allison Guyot is Site 865 Ocean Drilling Program. [8] These drill cores were part of a larger project to investigate and clarify the history of the flat-topped submarine mountains in the Pacific Ocean. [12]

Ocean Drilling Program Marine research program between 1985–2003

The Ocean Drilling Program (ODP) was a multinational effort to explore and study the composition and structure of the Earth's oceanic basins. ODP, which began in 1985, was the successor to the Deep Sea Drilling Project initiated in 1968 by the United States. ODP was an international effort with contributions of Australia, Germany, France, Japan, the United Kingdom and the ESF Consortium for Ocean Drilling (ECOD) including 12 further countries. The program used the drillship JOIDES Resolution on 110 expeditions (legs) to collect about 2000 deep sea cores from major geological features located in the ocean basins of the world. Drilling discoveries led to further questions and hypotheses, as well as to new disciplines in earth sciences such as the field of paleoceanography. In 2004 ODP transformed into the Integrated Ocean Drilling Program (IODP).

Geography and geology

Local setting

Allison Guyot is located in the equatorial Pacific Ocean, [1] part of the western Mid-Pacific Mountains. [13] The Mid-Pacific Mountains contain seamounts which were covered by limestones during the Barremian and Albian (circa 129.4 – circa 125 million years ago and circa 113–100.5 million years ago, respectively [14] ). [15] Hawaii lies due east and the Marshall Islands southwest; [16] Resolution Guyot is 716 kilometres northwest. [17]

Equator Intersection of a spheres surface with the plane perpendicular to the spheres axis of rotation and midway between the poles

An equator of a rotating spheroid is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid with the plane perpendicular to its axis of rotation and midway between its geographical poles.

Limestone Sedimentary rocks made of calcium carbonate

Limestone is a carbonate sedimentary rock that is often composed of the skeletal fragments of marine organisms such as coral, foraminifera, and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). A closely related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg(CO3)2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones.

The Barremian is an age in the geologic timescale between 129.4 ± 1.5 Ma and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch. It is preceded by the Hauterivian and followed by the Aptian stage.

The guyot [4] (also known as tablemount [18] ) has an outline resembling a trapezoid [13] and consists of two connected volcanic ridges facing north-northwest to east-northeast. [19] Its western parts may be a distinct volcano. [20] The surface platform has dimensions of 35 by 70 kilometres, [21] with an upwards-doming form 0.3–0.5 kilometres high, [22] and is covered by large sediment mounds; [23] the rim surrounding the platform lies at a depth of about 1,650 metres and there is evidence of former reefs. [21] The structure appears to consist of lagoonal sediments surrounded by a reef, [24] and the shallowest point of Allison Guyot lies at less than 1,500 metres (4,900 ft) depth below sea level. [25] Volcanic cones dot the eastern side of the summit plateau. [26] The seamount bears traces of slumping, [27] which on the southeastern side of Allison Guyot has removed part of the platform perimeter. [28]

Mound Artificial heaped pile of earth, gravel, sand, rocks, or debris

A mound is a heaped pile of earth, gravel, sand, rocks, or debris. Most commonly, mounds are earthen formations such as hills and mountains, particularly if they appear artificial. A mound may be any rounded area of topographically higher elevation on any surface. Artificial mounds have been created for a variety of reasons throughout history, including ceremonial, burial (tumulus), and commemorative purposes.

Lagoon A shallow body of water separated from a larger body of water by barrier islands or reefs

A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.

Slump (geology)

A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.

The seamount rises 1.5 kilometres [29] above the seafloor. Underneath Allison Guyot, the seafloor is about 130–119 million years old, [15] and a 128-million year-old magnetic lineation is located nearby. [30] The Molokai Fracture Zone forms a ridge which passes close to Allison Guyot and intersects with another ridge at the seamount. [31] Tectonically the seamount is part of the Pacific Plate. [4]

Regional setting

Diagram showing a cross section through the Earth's lithosphere (in yellow) with magma rising from the mantle (in red) Hotspot(geology)-1.svg
Diagram showing a cross section through the Earth's lithosphere (in yellow) with magma rising from the mantle (in red)

The west central and south central Pacific Ocean seafloor contains many guyots of the Mesozoic age that developed in seas shallower than typical of the present-day ocean. [32] These are submarine mountains which are characterised by a flat top and usually the presence of carbonate platforms that rose above the sea surface during the middle Cretaceous period. [33] Many of these seamounts were formerly atolls, [34] though there are some differences to present-day reef systems. [35] [36] All these structures originally formed as volcanoes in the Mesozoic ocean. [34] The crust underneath these volcanoes tends to subside as it cools, and thus the islands and seamounts sink. [37] Fringing reefs may have developed on the volcanoes, which then became barrier reefs as the volcanoes subside and turn into atolls; [34] these rims surround lagoons or tidal flats. [38] Continued subsidence offset by growth of the reefs led to the formation of thick carbonate platforms. [39] Sometimes volcanic activity continued after the formation of the atoll or atoll-like structure, and during episodes where the platforms rose above sea level erosional features such as channels and blue holes [lower-alpha 2] developed. [41] Eventually, these platforms drowned for reasons that are often unclear. [33]

The formation of many such seamounts has been explained with the hotspot theory, which describes the formation of chains of volcanoes that get progressively older along the length of the chain, [42] with an active volcano only at one end of the system. This volcano lies on a spot of the lithosphere heated from below; as the plate moves the volcano is moved away from the heat source and volcanic activity ceases, producing a chain of volcanoes that become older away from the currently active one. [43]

The "South Pacific Superswell" is a region in the Southern Pacific at the present-day Austral Islands, Cook Islands and Society Islands, where intense volcanic activity was underway during the Cretaceous, and is where the Cretaceous seamounts of the Mid-Pacific Mountains originated. The Easter hotspot, Marquesas hotspot and the Society hotspot may have been involved in the formation of the Mid-Pacific Mountains. After the Mountains had formed, plate tectonics shifted them northwards to their present-day position. [15] Allison Guyot appears to have formed in the same region. [11]

Composition

One drill core on Allison Guyot has found a 136-metre-thick layer of pelagic sediments, under which are 735-metre-thick limestones that formed in lagoons [9] and might continue down for almost 600 metres. [17] The limestone consists mostly of calcite with little dolomite [44] and occurs in the form of bafflestone, [45] grainstone, packstone, peloid, rudstone and wackestone; [46] [47] ooliths have also been found. [48] The carbonates are of biogenic origin, [49] and fossils of dasyclads, [50] echinoderms, [19] gastropods, green algae, [50] molluscs, [22] ostracods, [46] oysters, [51] red algae, [19] rudists and sponges occur within the limestones; [50] some of the fossils have partially dissolved and are thus poorly preserved. [52] Remnants of crocodilians have been found within Aptian [lower-alpha 3] –Albian mudstones, together with fossils of fish and unidentified vertebrates. [53] The limestone is partly altered by karstification and phosphatisation, and manganese has accumulated in the upper layers. [50]

Basalts occur in the form of cobbles [47] and sills within the limestones. [54] These basalts define an alkali basalt suite [55] and contain clinopyroxene, feldspar, ilmenite, plagioclase, pyroxene, spinel and titanomagnetite. They probably also contained olivine but the basaltic rocks sampled are so heavily altered that no olivine is left. [56] [57] The basalts are typical of intraplate volcanism [58] and their geochemistry shows evidence that fractional crystallisation and mixing between different magmas were involved in their genesis. [59] The component minerals have often been completely altered to calcite, clays, gypsum, hematite, quartz and other, unidentified minerals, [60] either when exposed above sea level or through hydrothermal fluids when the sills formed. [61] The formation of the sills led to the hardening and hydrothermal alteration of the surrounding sediments. [54]

Clays are found both within the limestones [54] and in layers between the carbonates. [49] They consist of berthierine, chlorite, feldspar, hydromica, illite, kaolinite, mica, quartz, serpentine, smectite and possible zeolite. [62] [63] [64] The clays were in part derived from lateritic soils that developed on the volcanic island before they were completely buried in carbonates, [65] and in part formed in settings with limited water exchange during lagoonal stages. [66] Dolomite, gypsum and pyrite co-occur with some clays, [67] and claystones [lower-alpha 4] have been found in some places. [46] Mudstones with evidence of animal burrows [69] and containing amber, glauconite, organic material including plant debris and pyrite have also been encountered; [53] pyrite indicates that anoxic environments existed on Allison Guyot. [9]

Black shale and coal form layers in one drill core. [47] The lower limestones contain substantial amounts of organic material that originated from terrestrial settings, [70] and remnants of animal burrows [71] and plant roots have been found [51] in many layers of the platform. [19] The clays and mudstones are rich in organic material. [49] Most of this organic material appears to come from plants [72] but some material has been attributed to algae. [73] Cells and tracheids can be found in the plant remnants. [74]

Geological history

Eniwetok Atoll today. Allison Guyot may have resembled Eniwetok in the past. Enewetak Atoll - 2014-02-10 - Landsat 8 - 15m.png
Eniwetok Atoll today. Allison Guyot may have resembled Eniwetok in the past.

Radiometric dating has been performed on some of the volcanic rocks. Potassium–argon dating on the sills has yielded ages of 102 ± 6 million years ago and 87 ± 3 million years ago, while argon–argon dating also on the sills produced ages of 111.1 ± 2.6 million years, [54] 111.2 ± 1.2 million years ago and 104.8 ± 0.8 million years ago. [9] Other ages from the sills are about 110.7 ± 1.2 million years ago and 104.9 ± 2.0 million years ago. [75] Rocks dredged from the slopes of Allison Guyot have yielded ages of 101.2 ± 0.8 million years, [9] 102.7 ± 2.7 million years and 85.6 ± 1.3 million years ago. [75] Overall, the volcano is considered to be at least 111 million years old [75] and volcanic activity probably spanned 30 [26] –25 million years and several stages. [29]

Both the sills and the dredged rocks were probably erupted after the main shield stage [29] and they may constitute a late stage of secondary volcanism; [11] two or three separate stages might have taken place, including one which formed a secondary cone on the eastern side of Allison Guyot. This may indicate that the seamount passed over more than one hotspot. [75] The volcano of Allison Guyot was apparently already partially eroded when the secondary volcanism took place. [55] Paleomagnetic data taken from limestones show that Allison Guyot developed in the Southern Hemisphere, at a latitude of about 11.2° ± 2.0° south. [76]

Emergent phase

Aerial image of the rim of Bikini Atoll. Allison Guyot during its carbonate platform stage may have resembled Bikini. Sikorsky SH-3G Sea King from Helicopter Combat Support Squadron 1 in flight during an aerial radiation survey over Bikini Atoll in November 1978.jpg
Aerial image of the rim of Bikini Atoll. Allison Guyot during its carbonate platform stage may have resembled Bikini.

Allison Guyot began as a volcanic island [9] with a relief of perhaps 1.3 kilometres. [19] Located in equatorial waters suited to the deposition of carbonate platforms, [77] limestone [9] platform grew on the guyot [77] as it rapidly subsided during the Albian. [78] Eventually, the seamount became an atoll. Volcanic rocks cropped out for some time before they were buried into the carbonates, [9] and weathering products of the volcanic rocks accumulated in the limestones. [79] The islands were covered by vegetation, and vegetation cover decreased over time as the volcanic edifice sank. [72] The climate was probably humid and runoff was intense. [80]

The platform contains lagoon and swamp environments, [9] with water depths not exceeding 10 metres, [77] and at some stage contained sand shoals and islands formed by storms as well. The interior was not protected from the sea [77] and the sector of the platform that was investigated by drill cores apparently became increasingly accessible to it over time. [19] The inner platform had a quiet muddy water setting; [81] generally Allison Guyot at that time resembled the present-day Bikini and Eniwetok atolls in terms of morphology when Allison Guyot was emergent. [82]

The carbonate deposits indicate sea level changes following orbital cycles [83] consistent with Milankovitch forcing; [77] parts of the platform occasionally rose above sea level. [84] At some point, karst environments existed on Allison Guyot and are probably the reason for the irregular surface of the summit platform [85] and the presence of sinkholes; there are clear indications of about 200 metres of emergence. [86]

Bivalves [87] including rudists, [81] corals, echinoderms, foraminifera, green algae, hydrozoans, red algae and sponges have been found in the platform's deposits. [88] Rudists were at that time important reef builders [89] and together with sponges colonised the margin of the platform. [77] Among the rudist species discovered on Allison Guyot is Requienia cf. migliorinii . [81] Teeth of crocodilians have been found on the seamount. [53] Its 110 million years old [90] remnants are the oldest known crocodilians in the region of the Pacific Ocean. They indicate that such species lived within the lagoon of Allison Guyot and may give clues about the history of Pacific animals and their dispersal. [53]

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. [91] Carbonate sedimentation on Allison Guyot ended during late Albian times, [85] about 99 ± 2 million years ago, at the same time as at Resolution Guyot. [92] By Turonian times (93.9  89.8 ± 0.3 million years ago [14] ), pelagic sedimentation was prevailing on Allison Guyot. [93] On both Allison and Resolution Guyots, the drowning was preceded by an episode where the platform rose above the sea; [94] possibly it was this emergence and the following submergence which terminated carbonate deposition and prevented it from beginning again. [95] Such emergence and drowning has been recorded at carbonate platforms of that age around the world and may be the consequence of tectonic events across the Pacific Ocean, [86] culminating in the uplift of a part thereof. [77] At that time, a last phase of volcanic activity on Allison Guyot generated several cones on its eastern part. [96] The evidence for this theory is not conclusive, [97] and another theory holds that the drowning of Allison Guyot occurred when it moved through equatorial waters, where upwelling increased the amount of nutrients available, [98] hampering the growth of platforms. [lower-alpha 5] [99] The waters might also have been too hot to support the survival of reef builders, as happens in present-day coral bleaching events. [100]

About 160 metres [lower-alpha 6] of pelagic sediment [17] in the form of sand, ooze [101] and pelagic limestone accumulated on Allison Guyot; pelagic limestone is of Turonian to Campanian (83.6 ± 0.2  72.1 ± 0.2 million years ago [14] ) age while the oozes and sands were deposited starting in the early Paleocene (66–56 million years ago [14] ). [84] In drill cores, the ooze has a sandy, watery habitus owing to the prevalence of fossil foraminifera in the sediment. [13] The pelagic sediments have been bioturbated [lower-alpha 7] in some places [103] and modified by sea currents, which have formed the large mound of pelagic sediment. [23] In drill cores, the ooze overlies Cretaceous shallow-water limestones, [104] which were modified by phosphatisation and manganese accumulation. [50] As plate tectonics moved Allison Guyot northward, its surrounding water masses changed, as did the properties of the pelagic cap. [85] Slumping of the platform occurred during the Cenozoic (the last 66 million years). [14] [27]

The pelagic ooze bears evidence of the Paleocene–Eocene thermal maximum, [lower-alpha 8] including temporary dissolution of carbonates, changes in the isotope ratios of carbon in sediments on Allison Guyot [106] and changes in foraminifera [107] and ostracod fossils found in the ooze. The latter underwent a major extinction during the Paleocene–Eocene Thermal Maximum on the seamount and took a long time to recover. [108]

Sea currents have altered the pelagic deposits by removing smaller particles. In particular deposits from warmer periods have been altered in this way on Allison Guyot, perhaps because warmer climates increased hurricane activity and thus the energy available in sea currents or deep-sea circulations shifted. [109] Furthermore, pauses in the sedimentation or episodes of slowdown have been identified. [110]

Notes

  1. The Ocean Drilling Program was a multinational research program that aimed at elucidating the geological history of the sea by obtaining drill cores from the oceans [6] and lasted from 1983 to 2003. [7]
  2. Pit-like depressions within carbonate rocks that are filled with water. [40]
  3. Between circa 125 and 113 million years ago [14]
  4. Clays that became solid rocks. [68]
  5. Increased nutrient levels favour plankton growth and reduce the amount of sunlight available to symbiotic organisms in the platform builders. [99]
  6. Some of which was later probably eroded away. [17]
  7. Animals have stirred, mixed and otherwise modified the sediments. [102]
  8. The Paleocene–Eocene Thermal Maximum was an episode of extreme global warmth about 55.5 million years ago, during which temperatures rose by about 5–8 °C. [105]

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Ujlān volcanic complex is a group of seamounts in the Marshall Islands. The complex consists of the seamounts Ļajutōkwa, Ļalibjet, Likelep, Ļotāb and Ujlān which with a minimum depth of 1,250 metres (4,100 ft) is the shallowest part of the complex; sometimes Ujelang Atoll is also considered to be a part of the complex; Eniwetok atoll and Lo-En seamount form a cluster together with this volcanic complex.

Vlinder Guyot

Vlinder Guyot is a guyot in the Western Pacific Ocean. It rises to a depth of 1,500 metres (4,900 ft) and has a flat top covering an area of 40 by 50 kilometres. On top of this flat top lie some volcanic cones, one of which rises to a depth of 551 metres (1,808 ft) below sea level. Vlinder Guyot has noticeable rift zones, including an older and lower volcano to the northwest and Oma Vlinder seamount south.

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Sources