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Expedition 367/368 summary
Conference proceeding   Open access

Expedition 367/368 summary

Hans Christian Larsen, Zhen Sun, Joann M Stock, Jian Zhimin, Carlos A Alvarez Zarikian, Adam Klaus, Jacopo Boaga, Stephen A Bowden, Anne Briais, Chen Yifeng, …
Proceedings of the International Ocean Discovery Program. Expedition reports, Vol.367/368
International Ocean Discovery Program
09/28/2018
DOI: 10.14379/iodp.proc.367368.101.2018
url
https://doi.org/10.14379/iodp.proc.367368.101.2018View
Published (Version of record) Open Access

Abstract

The primary objectives of International Ocean Discovery Program (IODP) Expedition 367/368 to the northern South China Sea (SCS) margin were to (1) examine its history of continental breakup and (2) compare it with other nonvolcanic or magma-poor rifted margins with the broader goal of testing models for continental breakup. A secondary objective was to further our understanding of the paleoceanographic and environmental development of the SCS and southeast Asia during the Cenozoic. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C and are located in the continent-ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include the following: - Determining the nature of the basement in crustal units across the COT of the SCS that are critical to constrain style of rifting, - Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, - Constraining vertical crustal movements during breakup, and - Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Site U1499 on Ridge A and Site U1500 on Ridge B were drilled during Expedition 367. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively, but based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In addition, Expedition 368 added two more sites on the OMH (Sites U1504 and U1505). Expedition 367/368 completed operations at six of the seven sites (U1499-U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing without coring to 990 m because of mechanical problems with the drilling equipment that prevented the expedition, after 25 May 2017, from operating with a drill string longer than 3400 m. New alternate Site U1504, proposed during Expedition 367, met this condition. Original Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already-drilled Site U1501. At Site U1499, we cored to 1081.8 m in 22.1 days with 52% recovery and then logged downhole data from 655 to 1020 m. In 31 days at Site U1500, we penetrated to 1529 m, cored a total of 1012.8 m with 37% recovery, and collected log data from 842 to 1133 m. At Site U1501, we cored to 697.1 m in 9.4 days with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m in Holes U1502A (758 m) and U1502B (921 m), set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10 3/4 inch casing. At Site U1503, we penetrated 995.1 m and set 991.5 m of 10 3/4 inch casing, but no cores were taken because of a mechanical problem with the drawworks. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Sites U1503 and U1505, all sites were drilled to acoustic basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment, but drilling options were severely limited from 25 May to the end of the expedition by the defective drawworks limiting deployment of drill string longer than 3400 m. At Site U1499, coring approximately 200 m into the interpreted acoustic basement sampled sedimentary rocks, possibly including early Miocene chalks underlain by Oligocene polymict breccias and poorly cemented gravels of unknown age comprising sandstone pebbles and cobbles. Preliminary structural and lithologic analysis suggests that the gravels might be early to late synrift sediment. At Site U1500, the main seismic reflector corresponds to the top of a basalt sequence at approximately 1379.1 m.
 Biostratigraphy Pacific Ocean Physical Properties Stratigraphy Tectonics Applied geophysics basalts basement Cenozoic chemostratigraphy continental margin crust downhole methods Expedition 367/368 extension tectonics Foraminifera geophysical methods geophysical profiles geophysical surveys igneous rocks International Ocean Discovery Program IODP Site U1501 IODP Site U1502 IODP Site U1503 IODP Site U1504 IODP Site U1505 lithostratigraphy magnetostratigraphy marine sediments metamorphic rocks microfossils nannofossils North Pacific Northwest Pacific paleomagnetism plate tectonics Quaternary rifted margins rifting schists sedimentary rocks sediments seismic methods seismic profiles South China Sea surveys Tertiary turbidite volcanic rocks well logs West Pacific

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