Allison Guyot (formerly known as Navoceano Guyot) is a tablemount (

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

. It is a

trapezoid A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium (). A trapezoid is necessarily a convex quadrilateral in Eu ...

al 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 Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

and northeast of the

Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Intern ...

, but at the time of their formation were located in the Southern Hemisphere. The tablemount was probably formed by a hotspot in the present-day

Southern Pacific The Southern Pacific (or Espee from the railroad initials- SP) was an American Class I railroad network that existed from 1865 to 1996 and operated largely in the Western United States. The system was operated by various companies under the ...

before

plate tectonic Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

s moved it to its current location. Several hotspots, including the

Easter Easter,Traditional names for the feast in English are "Easter Day", as in the '' Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'') and Samuel Pepys''The Diary of Samuel ...

Marquesas The Marquesas Islands (; french: Îles Marquises or ' or '; Marquesan: ' (North Marquesan) and ' (South Marquesan), both meaning "the land of men") are a group of volcanic islands in French Polynesia, an overseas collectivity of France in t ...

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 Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

. Subsequently,

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

deposition commenced as Allison Guyot subsided and eventually buried the island, forming an

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gr ...

-like structure and a

carbonate platform A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms (usually micr ...

. Among other animals, crocodilians lived on Allison Guyot. The platform emerged above sea level during the

Albian 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 ...

and

Turonian 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 (million years ago). The Turonian is preceded b ...

ages. It drowned about 99 ± 2 million years ago for unknown reasons; possibly a phase of renewed emergence damaged the

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...

s, or it was located in unfavourable waters. Later,

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

sedimentation commenced on the seamount and led to the deposition of sediments including

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, ooze and

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

, which bear traces of climatic events and ocean currents.

Name and research history

Allison Guyot is named after E.C. Allison, an oceanographer and paleontologist at the

San Diego State College San Diego State University (SDSU) is a public research university in San Diego, California. Founded in 1897 as San Diego Normal School, it is the third-oldest university and southernmost in the 23-member California State University (CSU) system ...

; formerly it was named "Navoceano Guyot". The name "Hamilton Guyot" has also been applied to Allison Guyot but is not correct; Hamilton Guyot is a separate formation in the Mid-Pacific Mountains. The seamount is the source of the

Ocean Drilling Program 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 th ...

drill core A modern core drill is a drill specifically designed to remove a cylinder of material, much like a hole saw. The material left inside the drill bit is referred to as the ''core''. Core drills used in metal are called annular cutters. Core dr ...

865A, which was bored into the summit platform of Allison Guyot in 1992 but did not reach the volcanic structure of the underwater mountain. Two other cores 865C and 865B were obtained during the same operation; Allison Guyot is Site 865 Ocean Drilling Program. 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.

Geography and geology

Local setting

Allison Guyot is located in the equatorial

, part of the western Mid-Pacific Mountains. The Mid-Pacific Mountains contain seamounts which were covered by

s during the

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...

and Albian (circa 129.4 – circa 125 million years ago and circa 113–100.5 million years ago, respectively).

lies due east and the

southwest; Resolution Guyot is 716 kilometres northwest. The

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain (seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed . (also known as tablemount) has an outline resembling a trapezoid and consists of two connected volcanic ridges facing north-northwest to east-northeast. Its western parts may be a distinct volcano. The surface platform has dimensions of 35 by 70 kilometres, with an upwards-doming form 0.3–0.5 kilometres high, and is covered by large sediment

mound 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 ...

s; the rim surrounding the platform lies at a depth of about 1,650 metres and there is evidence of former

s. The structure appears to consist of

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...

al sediments surrounded by a reef, and the shallowest point of Allison Guyot lies at less than depth below sea level. Volcanic cones dot the eastern side of the summit plateau. The seamount bears traces of slumping, which on the southeastern side of Allison Guyot has removed part of the platform perimeter. The seamount rises 1.5 kilometres above the seafloor. Underneath Allison Guyot, the seafloor is about 130–119 million years old, and a 128-million year-old magnetic lineation is located nearby. The Molokai Fracture Zone forms a ridge which passes close to Allison Guyot and intersects with another ridge at the seamount. Tectonically the seamount is part of the Pacific Plate.

Regional setting

The west central and south central Pacific Ocean

seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...

contains many guyots of the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

age that developed in seas shallower than typical of the present-day ocean. These are submarine mountains which are characterised by a flat top and usually the presence of

platforms that rose above the sea surface during the middle

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period. Many of these seamounts were formerly

atolls An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

, though there are some differences to present-day reef systems. All these structures originally formed as volcanoes in the Mesozoic ocean. The crust underneath these volcanoes tends to subside as it cools, and thus the islands and seamounts sink.

Fringing reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows direc ...

s may have developed on the volcanoes, which then became

barrier reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

s as the volcanoes subside and turn into atolls; these rims surround lagoons or

tidal flat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal fl ...

s. Continued subsidence offset by growth of the reefs led to the formation of thick carbonate platforms. 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 hole A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock ( limestone or coral reef). Their existence was discovered in the late 20th century by fishe ...

s developed. Eventually, these platforms drowned for reasons that are often unclear. 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, 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 Plate may refer to: Cooking * Plate (dishware), a broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...

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. The "South Pacific Superswell" is a region in the

at the present-day

Austral Islands The Austral Islands (french: Îles Australes, officially ''Archipel des Australes;'' ty, Tuha'a Pae) are the southernmost group of islands in French Polynesia, an overseas country of the French Republic in the South Pacific. Geographically ...

Cook Islands ) , image_map = Cook Islands on the globe (small islands magnified) (Polynesia centered).svg , capital = Avarua , coordinates = , largest_city = Avarua , official_languages = , lan ...

and

Society Islands The Society Islands (french: Îles de la Société, officially ''Archipel de la Société;'' ty, Tōtaiete mā) are an archipelago located in the South Pacific Ocean. Politically, they are part of French Polynesia, an overseas country of the ...

, where intense volcanic activity was underway during the Cretaceous, and is where the Cretaceous seamounts of the Mid-Pacific Mountains originated. The

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, Salas y Gómez Island and the Pukao Seamount which is at the ridge's young western ed ...

Marquesas hotspot The Marquesas hotspot is a volcanic hotspot in the southern Pacific Ocean. It is responsible for the creation of the Marquesas Islands – a group of eight main islands and several smaller ones – and a few seamounts. The islands and seamounts ...

and the Society hotspot may have been involved in the formation of the Mid-Pacific Mountains. After the Mountains had formed,

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

shifted them northwards to their present-day position. Allison Guyot appears to have formed in the same region.

Composition

One drill core on Allison Guyot has found a 136-metre-thick layer of

sediments, under which are 735-metre-thick limestones that formed in lagoons and might continue down for almost 600 metres. The limestone consists mostly of calcite with little

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

and occurs in the form of

bafflestone Bafflestone is a type of carbonate rock. The Dunham Classification (Dunham, 1962) of limestones Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of ...

grainstone Under the Dunham classification (Dunham, 1962) system of limestones, a grainstone is defined as a grain-supported carbonate rock that contains less than 1% mud-grade material. This definition has recently been clarified as ''a carbonate-dominated ...

packstone Under the Dunham classification (Dunham, 1962Dunham, R.J. (1962) Classification of carbonate rocks according to depositional texture. In: Classification of Carbonate Rocks (Ed. W.E. Ham), Am. Assoc. Pet. Geol. Mem., 1, 108–121.) system of limesto ...

peloid Peloid is mud, or clay used therapeutically, as part of balneotherapy, or therapeutic bathing. Peloids consist of humus and minerals formed over many years by geological and biological, chemical and physical processes. Numerous peloids are av ...

rudstone Rudstone is a type of carbonate rock. The Dunham classification (Dunham, 1962) did not consider grain size as a criterion for the description of carbonate lithologies. In an attempt to rectify this perceived deficiency, Embry & Klovan (1971) intr ...

and

wackestone Under the Dunham classification (Dunham, 1962Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Ed.), Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir. Amer ...

; ooliths have also been found. The carbonates are of biogenic origin, and fossils of dasyclads,

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

s, gastropods,

green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

e, molluscs,

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typi ...

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...

red alga Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...

rudist Rudists are a group of extinct box-, tube- or ring-shaped marine heterodont bivalves belonging to the order Hippuritida that arose during the Late Jurassic and became so diverse during the Cretaceous that they were major reef-building organisms ...

s and

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throug ...

s occur within the limestones; some of the fossils have partially dissolved and are thus poorly preserved. Remnants of crocodilians have been found within Aptian–Albian mudstones, together with fossils of fish and unidentified

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

s. The limestone is partly altered by karstification and

phosphatisation Phosphatic fossilization has occurred in unusual circumstances to preserve some extremely high-resolution microfossils in which careful preparation can even reveal preserved cellular structures. Such microscopic fossils are only visible under the ...

, and

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

has accumulated in the upper layers.

Basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

s occur in the form of cobbles and sills within the limestones. These basalts define an

alkali basalt Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...

suite and contain

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

, pyroxene, spinel and titanomagnetite. They probably also contained

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

but the basaltic rocks sampled are so heavily altered that no olivine is left. The basalts are typical of intraplate volcanism and their geochemistry shows evidence that fractional crystallisation and mixing between different

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

s were involved in their genesis. The component minerals have often been completely altered to calcite,

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...

, hematite,

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

and other, unidentified minerals, either when exposed above sea level or through

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

fluids when the sills formed. The formation of the sills led to the hardening and hydrothermal alteration of the surrounding sediments. Clays are found both within the limestones and in layers between the carbonates. They consist of berthierine, chlorite, feldspar, hydromica,

illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...

, kaolinite, mica, quartz, serpentine,

smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

and possible

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

. The clays were in part derived from lateritic soils that developed on the volcanic island before they were completely buried in carbonates, and in part formed in settings with limited water exchange during lagoonal stages. Dolomite, gypsum and

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

co-occur with some clays, and claystones have been found in some places. Mudstones with evidence of animal burrows and containing

amber Amber is fossilized tree resin that has been appreciated for its color and natural beauty since Neolithic times. Much valued from antiquity to the present as a gemstone, amber is made into a variety of decorative objects."Amber" (2004). In ...

glauconite Glauconite is an iron potassium phyllosilicate ( mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Gre ...

, organic material including plant debris and pyrite have also been encountered; pyrite indicates that

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

environments existed on Allison Guyot.

Black shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

form layers in one drill core. The lower limestones contain substantial amounts of organic material that originated from

terrestrial Terrestrial refers to things related to land or the planet Earth. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...

settings, and remnants of animal burrows and

plant root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...

s have been found in many layers of the platform. The clays and mudstones are rich in organic material. Most of this organic material appears to come from plants but some material has been attributed to

alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

e. Cells and

tracheid A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms use another type of tracheary element, called vessel elements, to transport water through th ...

s can be found in the plant remnants.

Geological history

Enewetak Atoll - 2014-02-10 - Landsat 8 - 15m

Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...

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 Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measurements, while the newer ...

also on the sills produced ages of 111.1 ± 2.6 million years, 111.2 ± 1.2 million years ago and 104.8 ± 0.8 million years ago. Other ages from the sills are about 110.7 ± 1.2 million years ago and 104.9 ± 2.0 million years ago. Rocks dredged from the slopes of Allison Guyot have yielded ages of 101.2 ± 0.8 million years, 102.7 ± 2.7 million years and 85.6 ± 1.3 million years ago. Overall, the volcano is considered to be at least 111 million years old and volcanic activity probably spanned 30–25 million years and several stages. Both the sills and the dredged rocks were probably erupted after the main shield stage and they may constitute a late stage of secondary volcanism; 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. The volcano of Allison Guyot was apparently already partially eroded when the secondary volcanism took place.

Paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

data taken from limestones show that Allison Guyot developed in the Southern Hemisphere, at a

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

of about 11.2° ± 2.0° south.

Emergent phase

Sikorsky SH-3G Sea King from Helicopter Combat Support Squadron 1 in flight during an aerial radiation survey over Bikini Atoll in November 1978

Allison Guyot began as a

with a

relief Relief is a sculptural method in which the sculpted pieces are bonded to a solid background of the same material. The term '' relief'' is from the Latin verb ''relevo'', to raise. To create a sculpture in relief is to give the impression that th ...

of perhaps 1.3 kilometres. Located in equatorial waters suited to the deposition of

s, limestone platform grew on the guyot as it rapidly subsided during the

. Eventually, the seamount became an atoll. Volcanic rocks cropped out for some time before they were buried into the carbonates, and

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement) ...

products of the volcanic rocks accumulated in the limestones. The islands were covered by vegetation, and vegetation cover decreased over time as the volcanic edifice sank. The climate was probably humid and

runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock marke ...

was intense. The platform contains lagoon and swamp environments, with water depths not exceeding 10 metres, and at some stage contained sand shoals and islands formed by storms as well. The interior was not protected from the sea and the sector of the platform that was investigated by drill cores apparently became increasingly accessible to it over time. The inner platform had a quiet muddy water setting; generally Allison Guyot at that time resembled the present-day

Bikini A bikini is a two-piece swimsuit primarily worn by women that features two triangles of fabric on top that cover the breasts, and two triangles of fabric on the bottom: the front covering the pelvis but exposing the navel, and the back coverin ...

and

Eniwetok Enewetak Atoll (; also spelled Eniwetok Atoll or sometimes Eniewetok; mh, Ānewetak, , or , ; known to the Japanese as Brown Atoll or Brown Island; ja, ブラウン環礁) is a large coral atoll of 40 islands in the Pacific Ocean and with i ...

atolls in terms of morphology when Allison Guyot was emergent. The carbonate deposits indicate sea level changes following

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

al cycles consistent with Milankovitch forcing; parts of the platform occasionally rose above sea level. At some point, karst environments existed on Allison Guyot and are probably the reason for the irregular surface of the summit platform and the presence of

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are locally also known as ''vrtače'' and shakeholes, and to openi ...

s; there are clear indications of about 200 metres of emergence. Bivalves including rudists,

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and ...

s, echinoderms,

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

, green algae,

hydrozoan Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialize ...

s, red algae and sponges have been found in the platform's deposits. Rudists were at that time important reef builders and together with sponges colonised the margin of the platform. Among the rudist species discovered on Allison Guyot is '' Requienia cf. migliorinii''. Teeth of crocodilians have been found on the seamount. Its 110 million years old 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.

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. Carbonate sedimentation on Allison Guyot ended during late Albian times, about 99 ± 2 million years ago, at the same time as at Resolution Guyot. By

times (93.9 – 89.8 ± 0.3 million years ago), pelagic sedimentation was prevailing on Allison Guyot. On both Allison and Resolution Guyots, the drowning was preceded by an episode where the platform rose above the sea; possibly it was this emergence and the following submergence which terminated carbonate deposition and prevented it from beginning again. 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, culminating in the uplift of a part thereof. At that time, a last phase of volcanic activity on Allison Guyot generated several cones on its eastern part. The evidence for this theory is not conclusive, and another theory holds that the drowning of Allison Guyot occurred when it moved through equatorial waters, where

upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...

increased the amount of nutrients available, hampering the growth of platforms. The waters might also have been too hot to support the survival of reef builders, as happens in present-day

coral bleaching Coral bleaching is the process when corals become white due to various stressors, such as changes in temperature, light, or nutrients. Bleaching occurs when coral polyps expel the zooxanthellae ( dinoflagellates that are commonly referred to as a ...

events. About 160 metres of pelagic sediment in the form of sand, ooze and pelagic limestone accumulated on Allison Guyot; pelagic limestone is of Turonian to

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

(83.6 ± 0.2 − 72.1 ± 0.2 million years ago) age while the oozes and sands were deposited starting in the early

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

(66–56 million years ago). In drill cores, the ooze has a sandy, watery habitus owing to the prevalence of fossil foraminifera in the sediment. The pelagic sediments have been bioturbated in some places and modified by sea currents, which have formed the large mound of pelagic sediment. In drill cores, the ooze overlies Cretaceous shallow-water limestones, which were modified by phosphatisation and manganese accumulation. As plate tectonics moved Allison Guyot northward, its surrounding water masses changed, as did the properties of the pelagic cap. Slumping of the platform occurred during the Cenozoic (the last 66 million years). The pelagic ooze bears evidence of the Paleocene–Eocene thermal maximum, including temporary dissolution of carbonates, changes in the

isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

s of

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

in sediments on Allison Guyot and changes in foraminifera and ostracod fossils found in the ooze. The latter underwent a major

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

during the Paleocene–Eocene Thermal Maximum on the seamount and took a long time to recover. 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 A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...

activity and thus the energy available in sea currents or deep-sea circulations shifted. Furthermore, pauses in the sedimentation or episodes of slowdown have been identified.

Notes

References

Sources

* * * * * * * * * * * * * * * * * * * * * * {{Mid-Pacific Mountains Albian Cretaceous volcanoes Former islands Volcanoes of the Pacific Ocean Guyots