Wōdejebato (formerly known as Sylvania) is a Cretaceous guyot or tablemount in the northern Marshall Islands, Pacific Ocean. Wōdejebato is probably a shield volcano and is connected through a submarine ridge to the smaller Pikinni Atoll southeast of the guyot; unlike Wōdejebato, Pikinni rises above sea level. The seamount rises for to depth and is formed by basaltic rocks. The name Wōdejebato refers to a sea god of Pikinni.
It was probably formed by a hotspot in what is present-day French Polynesia before plate tectonics moved it to its present-day location. The Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. The first volcanic phase took place in the Cenomanian and was followed by the formation of a carbonate platform that quickly disappeared below the sea. A second volcanic episode between 85 and 78.4 million years ago (in the Campanian) led to the formation of an island. This island was eventually eroded and rudist reefs generated an atoll or atoll-like structure, covering the former island with carbonates and thus a second carbonate platform.
The second carbonate platform drowned about 68 million years ago (in the Maastrichtian), perhaps because at that time it was moving through the equatorial area which may have been too hot or too nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus, sedimentation commenced on the seamount and led to the deposition of manganese crusts and pelagic sediments, some of which were later modified by phosphate.
Name and research history
Wōdejebato is also written as Wodejebato. The name of the seamount comes from Wōdejebato, the name of the most feared and respected sea god of Pikinni Atoll. Wōdejebato was formerly called Sylvania, after the , a ship which was involved in its first mapping in 1946. The seamount was discovered in 1944, and was first investigated, using mainly seismic data, during Operation Crossroads (a nuclear bomb test). Later, several times rocks were dredged from the seamount and drill cores were taken; cores 873–877 of the Ocean Drilling Program are from Wōdejebato.
Geography and geology
Local setting
Wōdejebato lies within the Ralik Chain of islands and seamounts in the northern Marshall Islands, which consist of about three northwest-trending groups of islands of volcanic origin. Pikinni Atoll (formerly named Bikini) is located about southeast of the seamount.
The seamount lies at a depth of and is about long with a flat top that narrows southeastward from over to less than . The surface of the flat top slopes inward and is covered by small depressions and knobs with an average relief of about as well as ripple marks. The flat top is surrounded by a ridge, which has a width of and an average height of . On its northern and northeastern side, this ridge is in turn surrounded by another wide slightly raised ridge. The flat top has been interpreted as a lagoon surrounded by reefs which form the inner ridge; the outer ridge appears to be a pile of skeletal sand rather than a reef and may be a spit formed by reworked material. Small mounds, probably of biological origin, are found at the margins of the seamount.
The seamount is high above the sea floor and has an irregular shape, with spurs projecting from its circumference. These spurs have widths of and surface features that are distinct from those on the main flat top. The spurs appear to be rift zones, similar to these formed on Hawaii by dyke injection although some of the ridges at Wōdejebato may have a different origin. Wōdejebato appears to have four such ridges, which is more than is observed at Hawaii. One explanation is that the northwestern ridge is another seamount; another that Wōdejebato consists of more than one volcano although the relatively small size of the seamount would argue against this view. Wōdejebato's slopes descend rather steeply until, at depth, where they become more gentle, they are decorated with forms resembling cones and channels. Part of its southern flank, where there is a downdropped terrace, seems to have collapsed in the past. Another satellite volcanic cone lies north of Wōdejebato at a depth of . Wōdejebato contains a volcanic structure within a superficial sediment cap, and a free-air gravity anomaly has been observed on the seamount.