Dolomite-Evaporite Relations on Pacific Islands
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概要
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Recent work on the dolomite problem has led several workers to suggest that dolomite found on, or in the subsurface of, islands and atolls in the Pacific Ocean is genetically linked to the diagenetic effect of Mg-rich residual brines produced by evaporation of sea water. This paper is intended to serve as an additional link between the suggestion and the geologic evidence for dolomitization through evaporation. A number of small, low islands such as Jarvis, Malden, Starbuck, McKeans, Sydney, and Enderbury in the Equatorial Dry Zone of the south central Pacific Ocean are characterized by small to ephemeral, hypersaline lagoons and salt and gypsum deposits. Development of evaporite deposits on these islands is a function not only of climate but of island size and topography which determines the degree of sea water inflow. Dolomite has not yet been reported from these islands but conditions on them are so similar to conditions described on Bonaire, a site of modern dolomite formation, that its apparent absence may only be due to a lack of detailed mineralogic work. A study of pre-Recent dolomite in the Pacific was made of localities where some information on the lateral as well as the vertical distribution of dolomite is known or could be inferred. Such localities include Eniwetok Atoll, Kita-Daito-Jima, Nauru, Ocean, Atiu, Niue, and Man-gaia Islands and several islands in the Lau Group of eastern Fiji. These ancient sites of dolomitization have in common with their probable modern analogues in the Equatorial Dry Zone small size, they are generally less than 6 miles in diameter, and basin-like topography or paleotopography. This topography was for some islands inherited from an original atoll-like form; for others it was produced by post-emergence subaerial solution. Thus emergence, or cyclic emergence and submergence, may have been necessary for the development of restricted lagoons within which significant evaporation of sea water could have taken place. Closely timed conjunctions of tectonic, eustatic, and climatic effects then may be preconditions for dolomite formation.
- 東北大学の論文
- 1965-12-10