Subsidence of the Daito Ridge and Associated Basins, North Philippine Sea1
Published:January 01, 1979
Atsuyuki Mizuno, Yoshihisa Okuda, Shozaburo Niagumo, Hideo Kagami, Noriyuki Nasu, 1979. "Subsidence of the Daito Ridge and Associated Basins, North Philippine Sea", Geological and Geophysical Investigations of Continental Margins, Joel S. Watkins, Lucien Montadert, Patricia Wood Dickerson
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Cenozoic history of the Daito Ridge and two associated abyssal basins is preliminarily discussed, based on a shipboard near-trace monitor record section from a multichannel seismic reflection survey across the region and on dredge-sample data from the ridge area.
On the Daito Ridge, middle Eocene Nummulites-bearing deposits, shown by a dipping high-frequency pattern on the profile, accumulated in a shallow sea environment on a basement that included the Daito Metamorphic Rocks. Volcanic activity around the time of deposition is suggested by dredged rocks from the Daito north peak. The Daito Ridge has undergone a change in environmental conditions from neritic to pelagic and has subsided to a depth of 1.5 km below sea level from the middle Eocene through the Quaternary, resulting in the deposition of a transparent veneer on the main peak.
The acoustic sequence in the Minami-Daito Basin is divided into Units A through E, in descending order. Of them, Units C and D are partly correlative with the middle Eocene deposits on the ridge and include the earlier deposits. They represent archipelagic apron sedimentation on the slope and basin boundary area when the top of the ridge was extensively uplifted. Possibly related to the submergence of the Daito Ridge, a new sedimentary cycle characterized by turbiditic sedimentation (Unit B) started after the cease of apron sedimentation, and was followed by pelagic deposition (Unit A). A similar feature is observed in the Kita-Daito Basin and in the valley on the Daito Ridge.
Two types of post-opening intrusive activity are observed in the abyssal basins. One is indicated by Unit E, characterized by stratified layers and diffractions and having a smooth upper surface. It represents the anomalously smooth oceanic crust of the basin and resulted from repeated intrusion of sills after or during the deposition of Units C and D. Another is indicated by the acoustic basement with rugged upper surface and dense diffractions in the Kita-Daito Basin. It represents the rugged oceanic crust of the basin, which was likely formed by extensive intrusion of dikes after or during the deposition of Unit B.
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Geological and Geophysical Investigations of Continental Margins
Knowledge of continental margins advanced rapidly during the 1970s. Multichannel seismic reflection whose cost formerly restricted its use largely to the immediate vicinity of shallow-water prospects has become more common in deeper waters. The use of the technique by government and academic groups helped solve basc structural and evolutionary problems of rocks of the deeper offshrore. Better sources and more sophisticated processing yielded better and deeper resolution of the data. To better disseminate new knowledge of continental margins, AAPG held three meetings in 1977 to review the current status of knowledge. The papers presented at those meetings are contained in this volume. There are 32 chapters divided into the following sections: Rifted Margins; Convergent Margins; Small Basin Margins; and Resources, Comparative Structure, and Eustatic Changes in Sea Level.