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ABSTRACT: Chemostratigraphy of a Forearc: Preliminary Results from Insitu Downhole Nuclear Measurements in the Isu-Bonin Arc Region

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

The Izu-Bonin intra-oceanic arc is a product of the subduction of Pacific lithosphere since the Eocene. Leg 126 of the Ocean Drilling Program drilled 19 holes at seven sites during April-June 1989. Of these, Sites 792 and 793 are located in the forearc region where variable thicknesses of sediments cover the basement. The former is located in the western half of the Izu-Bonin forearc sedimentary basin about 170 km west of the Izu-Bonin trench axis while the latter is in the center, some 125 km west of the trench. Hole 729E penetrated 803 m of sediments and 82.9 m of basement, while Hole 793B penetrates to a total depth of 1682 mbsf, the upper 1403.9 m of which are in the sediment section. Both holes were characterized by the use of downhole nuclear logging measurements th t provide effectively continuous quantitative geochemical data for the section logged. Despite the necessity to log the uppermost sections of the holes through pipe and casing, respectively, the data quality is generally high. It is thus possible to assess both the variations in geochemistry within the sedimentary units and the basement intervals, as well as the broader nature of the incompletely recovered sections of the hole. The variations in the chemical signatures of the different formations consequently allow the development of a detailed and more complete chemostratigraphy for the two sites than that observed solely through recovered core. These are presented and should enable a more detailed analysis of the evolution of the region.

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