Site 459: Mariana fore-arc ( DSDP).

Abstract

Site 459 is located on the E edge of a deep sediment pond immediately above (W of) the trench slope break. Hole 459 was abandoned when the lower half of the core barrel containing the mudline core dropped down the drill string and fishing attempts were unsuccessful. Hole 459A, a pilot hole for potential re-entry, was washed down to 87.0m; no cores were taken. In Hole 459B, a total of 691.5m was cored with a recovery rate of 26%. The upper 559.0m are sediments consisting mainly of vitric mud and ooze with ash layers, late to early Pleistocene in age, over a thick pile of turbidites of late Oligocene through middle Miocene age. These in turn overlie middle Eocene to early Oligocene claystones. Hiatuses are recognized for 3.0-10.0, 13.4-14.0, 30.0-34.5, and 40.0-42Ma. The sedimentary sequence shows that the Site 459 area was subjected to active vertical displacement and tensional stress in the late Oligocene to middle Miocene period. Below the sediments, 132.5m of fine- to medium-grained clinopyroxene-plagioclase pillow basalts, flows, and possible intrusives were cored. Two major chemical types divided into seven subtypes occur, all of them tholeiite and quartz tholeiite in composition. The stable paleomagnetic inclinations of the sediments become shallower deeper in the hole, indicating a northward movement of the site during the Cenozoic.-Authors