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ABSTRACT: Geochemical Characterization of Volcanoclastic Sediments in the Izu-Bonin Arc from Downhole Nuclear Measurements

P. K. Harvey, M. A. Lovell, P. A. Pezard

The Ocean Drilling program recently used downhole nuclear measurements to characterize the volcanoclastic sediments drilled in the Izu-Bonin Arc Region. Site 791 is located near the center of the Sumisu Rift, a backarc graben west of the Izu-Bonin island arc volcanoes Sumisu Jima and Tori Shima. Sites 792 and 793 are located in the forearc sedimentary basin, some 125 km and 170 km west of the trench axis, respectively. The first site was logged solely through pipe and consequently the data are comparatively poor. The other two sites, however, are logged primarily in open hole with correspondingly improved data quality. The sediments encountered are highly variable in terms of both sedimentary facies and chemical signatures. These reflect both the origin of the source mate ial and the processes of deposition. The in situ chemistry consequently provides characteristic signatures that enable individual units to be uniquely identified. The elemental signatures may be used in isolation as discrete markers or in association with each other through a series of numerical routines. Thus it is possible to map out the vertical stratigraphy of a site and to identify detailed changes in source/depositional processes. The diverse nature of the volcanoclastic sediments in the Izu-Bonin Arc is demonstrated. Particular reference is made to the inherent differences of both sediments and log responses in comparison with material encountered elsewhere in the Pacific by the Ocean Drilling Program.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990