Abstract

Pelagic and hemipelagic sediments deposited on the Zambales Ophiolite contain a nearly continuous depositional record of the original setting and emplacement history of this large ophiolite from the late Eocene through the Miocene. Pelagic limestone with thin ash layers (the Aksitero Formation) caps the volcanic complex of the ophiolite along its east flank. Calcareous nannofossil biostratigraphy of this limestone gives sedimentation rates of 3–5 m/m.y. from the late Eocene through the early Oligocene. Rates increase to 10 m/m.y. or more in the upper Aksitero Formation, where sandy turbidites appear in the middle and upper Oligocene sections. Lower Miocene mudstone, sandstone, and conglomerate of the Moriones Formation were deposited at much higher rates; this formation includes channels and debris-flow deposits characteristic of deep-sea fans. Oligocene sandstones are predominantly volcaniclastic, whereas sandstones in the lower Miocene section contain serpentine and other components derived from the ultramafic complex of the ophiolite. Sedimentary facies and sandstone composition show that the Zambales was deeply eroded by the early Miocene and probably first emerged above sea level in the middle or late Oligocene, only 10 to 15 m.y. after it formed as new ocean crust. A comparison of the Aksitero Formation with Deep Sea Drilling Project (DSDP) sites in the western Pacific suggests that the Zambales Ophiolite was originally part of a marginal basin and not an island arc.

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