Horizon Guyot is a presumably Cretaceous guyot (tablemount) in the Mid-Pacific Mountains, Pacific Ocean. It is an elongated ridge, over long and high, that stretches in a northeast-southwest direction and has two flat tops; it rises to a minimum depth of . The Mid-Pacific Mountains lie west of Hawaii and northeast of the Line Islands.
It was probably formed by a hotspot, but the evidence is conflicting. Volcanic activity occurred during the Turonian-Cenomanian eras 100.5–89.8 million years ago and another stage has been dated to have occurred 88–82 million years ago. Between these volcanic episodes, carbonate deposition from lagoonal and reefal environments set in and formed limestone. Volcanic islands developed on Horizon Guyot as well and were colonised by plants.
Horizon Guyot became a seamount during the Coniacian-Campanian period. Since then, pelagic ooze has accumulated on the seamount, forming a thick layer that is further modified by ocean currents and by various organisms that live on the seamount; sediments also underwent landsliding. Ferromanganese crusts were deposited on exposed rocks.
Name and research history
The seamount is named after the research vessel and is also known as Horizon Ridge, Horizon Tablemount, Gora Khorayzn and . During the Deep Sea Drilling Project, the drill cores called Site 44 and Site 171 were taken on Horizon Guyot in 1969 and 1971, respectively; a further drill core was obtained north of the seamount at Site 313 in 1973. This seamount is the best studied seamount of the Mid-Pacific Mountains and more is known about its morphology than any other seamount of the Mid-Pacific Mountains.
Geography and geology
Local setting
Horizon Guyot lies west of Hawaii and is part of the Mid-Pacific Mountains. Unlike conventional island chains in the Pacific Ocean, the Mid-Pacific Mountains feature an oceanic plateau with guyots (also known as tablemounts) which become progressively younger towards the east. Other guyots in the Mid-Pacific Mountains are Sio South, Darwin, Thomas, Heezen, Allen, Caprina, Jacqueline, Allison and Resolution. South of Horizon Guyot, deep water in the "Horizon passage" leads into the Line Islands and Horizon Guyot is sometimes considered to be a member of that chain.
The seamount rises - to a minimum depth of –, and is a ridge wide and over long; Horizon Guyot is the largest seamount in the Mid-Pacific Mountains. It trends in a southwest-northeast direction with an orientation matching that of other structures in the region such as fracture zones on the seafloor. Faulting has been observed on the western side of the seamount.
Two summit platforms lie on the ridge. The eastern one is the larger of these platforms and the western oval-shaped platform lies close to the western end of the ridge. These platforms are relatively flat and are surrounded by a slope break beyond which the guyot falls off steeply to the surrounding abyssal plain. This appearance characterises Horizon Guyot as a guyot although the elongated shape is unlike that of most guyots in the region which have one circular summit platform. At the margin of the platform, lie terraces which are up to wide and up to high and that discontinuously surround the summit platform; the flat surfaces of the terraces may be former fringing reefs. On the eastern summit platform there are buried terraces beneath the sediment cover.
Sediment layers cover almost the entire summit of Horizon Guyot, and consist mainly of sand, with clay and silt making up a minor part. Features on the sedimented seafloor are flat areas, hummocks, ripples and sediment waves. Seismic transects have revealed a relief of about in the underlying basement and the presence of a central peak. Material obtained in drill cores includes chalk, chert, hyaloclastite, limestone, ooze and sandstone; basalt and chert outcrop in some places. In certain areas boulders and cobbles cover the seafloor; ferromanganese crusts cover exposed rocks.
The seamount shows evidence of repeated mass failures; including hummocky terrain, scarps and slump blocks, which are on average thick. Landsliding is probably triggered by earthquakes; after the failure the landslides either stay coherent and do not travel far but some advance quickly and far. Talus blocks up to in size cover the seafloor around Horizon Guyot.