17°08′N 177°15′W / 17.133°N 177.250°W / 17.133; -177.250^[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.^[2] 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.^[3] The seamount was first described in a 1946 publication.^[4] Both Hess and Cape Johnson were discovered during the same cruise^[5] and Cape Johnson Guyot is the type locality of guyots.^[6]

Geography and geology

The seamount lies in the Mid-Pacific Mountains^[7] on their southern side and is a submarine mountain with a flat top that rises^[8] over 3,000 metres (10,000 ft)^[9] to a depth of 1,692 metres (5,551 ft)^[10][11]-1,778 metres (5,833 ft).^[12] 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);^[13] it is characterized by a limestone dome on the summit, buried beneath sediments; in turn a volcanic hill is buried within the limestone dome.^[14] The top of the seamount has a hummocky appearance which has been interpreted as a sediment cover^[15] and its southeastern sector has a bank-like shape that resembles that of an atoll.^[16] Cape Johnson Guyot is considered to be of Middle Cretaceous age^[17] with an age of 120 million years reported^[18] and shallow-water fossils were emplaced on it at that time.^[19]

Apatite,^[20] basaltic sandstone containing hypersthene,^[21] clay,^[22] limestone, manganese crusts,^[23] manganese oxide, phosphorite^[24] and lithified carbonates have been found on Cape Johnson Guyot;^[25] some carbonates of biogenic origin have been altered by apatite.^[26] Globigerina ooze is also found on the seamount^[27] and can reach substantial thickness; such accumulations might be formed by ocean currents.^[28] Similar rocks have been found at other guyots of the Mid-Pacific Mountains.^[29]

Biology

During the Cretaceous, corals and rudists lived on Cape Johnson Guyot^[30] and fossils have been dated to 91-112 million years ago;^[31] some corals^[32] and rudists are of Albian to Cenomanian age.^[33] Fossils of anthozoa,^[34] gastropods, reef-building hexacorals,^[35] pelecypods, stromatoporoids^[36] and Tridacna were also found.^[37] The Cretaceous fossils^[38] Actinostroma pacifica,^[39] Astrocoenia dietzi,^[40] Brachyseris montemarina,^[41] the caprinid rudists Caprina mediopacifica, Caprina mulleri^[42] - including a detailedly described holotype of the latter^[43] - and Cardita sp.,^[44] Lophosmilia fundimaritima,^[45] Montastrea menardii,^[46] Nerinea sp.^[47] and Tiarasmilia casteri occur on Cape Johnson Guyot.^[48] Caprina mulleri was also found on other Mid-Pacific Mountains.^[49] About 300 species of extant foraminifera have been found on the seamount as well,^[50] with additional fossil foraminifera^[51] including Paleocene-Eocene specimens that were redeposited by ocean currents.^[52] Finally, a cetacean bone of undetermined age has also been found on Cape Johnson Guyot.^[53]