Abstract
Tasman Sea guyots are former volcanic islands that have subsided an average of 166 fathoms since the Middle or Late Tertiary. These guyots have heights up to 2333 fathoms above the surrounding sea floor; their flat-topped platforms are at depths ranging from 50 to 500 fathoms below present sea level. Reef formations are found along the edges and on the tops of the guyots. Volcanic rocks dredged from these truncated undersea mountains are alkaline olivine basalts with large augite, olivine and plagioclase phenocrysts in a glassy matrix. Limestones dredged from the guyots are either hard, light brown, massive foraminiferal limestones or grey, porous limestones composed mainly of skeletal material. Using foraminiferal evidence we assign an age for the limestone accumulation of Late Pliocene or Early Pleistocene. Phosphorite is also present and is thought to be formed from the replacement of carbonate ions in calcareous material by phosphate ions. There is some evidence of direct precipitation of phosphatic material as coatings on many of the basalt samples.
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