Site U1503

Abstract

International Ocean Discovery Program (IODP) Site U1503 is located at 3868 meters below sea level (mbsl) near the top of the structural high named Ridge C. Ridge C is the most seaward ridge of the three margin-parallel Ridges A, B, and C that characterize the lower continental slope underlain by thin (5–7 km) crust. Ridge C is believed to represent at least partly, if not fully, igneous crust and hence the completion of continental breakup along this margin segment of the northern South China Sea (SCS). A key operational objective of Site U1503 was to sample the lowermost ~300 m of sediments on top of basement to constrain the age and subsidence history of the crust at this location, the timing of normal faulting, and the environment of the early half-graben fill. The most important goal was to sample the igneous stratigraphy to at least 100 m below the basement. Because of a rig floor equipment failure (drawworks), we abandoned the site after installing casing in Hole U1503A to 991.5 m. Deep, representative sampling of the basaltic material at this site would have provided an important reference frame for breakup modeling. With an estimated 1640 m sediment thickness overlying basement, obtaining basement samples and log data at this site represented a challenging operation.