Cape Johnson Guyot

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]

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]

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]