Cape Johnson Guyot

17°08′N 177°15′W^[1]

Cape Johnson Guyot is a guyot in the Pacific Ocean, more precisely in the Mid-Pacific Mountains, and the type locality of guyots. It is of middle Cretaceous age and a number of fossils have been dredged from it.

Name

Cape Johnson Guyot is also known as Cape Johnson Seamount or Cape Johnson Tablemount.^[3] The guyot was named by Harry Hammond Hess, after his ship the USS Cape Johnson; Hess had also named the kind of flat-topped seamount "guyot" and another seamount was named after Hess himself.^[4] The seamount was first described in a 1946 publication.^[5] Both Hess and Cape Johnson were discovered during the same cruise^[6] and Cape Johnson Guyot is the type locality of guyots.^[7]

Geography and geology

The seamount lies in the Mid-Pacific Mountains^[8] on their southern side and is a submarine mountain with a flat top that rises^[9] over 3,000 metres (10,000 ft)^[10] to a depth of 1,692 metres (5,551 ft)^[11][12]-1,778 metres (5,833 ft).^[13] The flat top has an oval shape and a surface area of 6 by 12 nautical miles (11 km × 22 km; 6.9 mi × 13.8 mi);^[14] it is characterized by a limestone dome on the summit, buried beneath sediments; in turn a volcanic hill is buried within the limestone dome.^[15] The top of the seamount has a hummocky appearance which has been interpreted as a sediment cover^[16] and its southeastern sector has a bank-like shape that resembles that of an atoll.^[17] Cape Johnson Guyot is considered to be of Middle Cretaceous age^[18] with an age of 120 million years reported^[19] and shallow-water fossils were emplaced on it at that time.^[20]

Apatite,^[21] basaltic sandstone containing hypersthene,^[22] clay,^[23] limestone, manganese crusts,^[24] manganese oxide, phosphorite^[25] and lithified carbonates have been found on Cape Johnson Guyot;^[26] some carbonates of biogenic origin have been altered by apatite.^[27] Globigerina ooze is also found on the seamount^[28] and can reach substantial thickness; such accumulations might be formed by ocean currents.^[29] Similar rocks have been found at other guyots of the Mid-Pacific Mountains.^[30]

Biology

During the Cretaceous, corals and rudists lived on Cape Johnson Guyot^[31] and fossils have been dated to 91-112 million years ago;^[32] some corals^[33] and rudists are of Albian to Cenomanian age.^[34] Fossils of anthozoa,^[35] gastropods, reef-building hexacorals,^[36] pelecypods, stromatoporoids^[37] and Tridacna were also found.^[38] The Cretaceous fossils^[39] Actinostroma pacifica,^[40] Astrocoenia dietzi,^[41] Brachyseris montemarina,^[42] the caprinid rudists Caprina mediopacifica, Caprina mulleri^[43] - including a detailedly described holotype of the latter^[44] - and Cardita sp.,^[45] Lophosmilia fundimaritima,^[46] Montastrea menardii,^[47] Nerinea sp.^[48] and Tiarasmilia casteri occur on Cape Johnson Guyot.^[49] Caprina mulleri was also found on other Mid-Pacific Mountains.^[50] About 300 species of extant foraminifera have been found on the seamount as well,^[51] with additional fossil foraminifera^[52] including Paleocene-Eocene specimens that were redeposited by ocean currents.^[53] Finally, a cetacean bone of undetermined age has also been found on Cape Johnson Guyot.^[54]