Provenance of basal sediments in the South China Sea basin at IODP Expedition 367/368 Site U1499

Author(s): Robinson, Caroline; Krissek, Lawrence
International Ocean Drilling Program, Expedition 367 Scientists, Texas A&M University
Author Affiliation(s): Primary:
Ohio State University, School of Earth Sciences, Columbus, OH, United States
Volume Title: Geological Society of America, 2018 annual meeting & exposition
Source: Abstracts with Programs - Geological Society of America, 50(6); Geological Society of America, 2018 annual meeting & exposition, Indianapolis, IN, Nov. 4-7, 2018. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: IODP Expeditions 367/368 drilled the northern margin of the South China Sea (SCS) basin to investigate its rifting history. Recovery of the material at, and just below, acoustic basement was important for evaluating a magma-poor or a magma-rich rifting model for the basin. Instead of recovering igneous or continental basement rock below the hypothesized acoustic basement seismic reflector, however, the base of Site U1499 contained sandstone with polymict clasts and matrix-supported breccia, overlying sedimentary cobbles. Sixteen samples of fine to medium-grained sandstone cobbles from the base of Site U1499 were analyzed petrographically. The cobbles have an average framework composition of Q18F73L9, with limited variability. Plagioclase and K-feldspar grains are extremely weathered and altered to kaolinite and sericite with many oxide inclusions. The degree of alteration varies among feldspar grains within a sample; some grains still show twinning, while others are almost unrecognizable as feldspars. Monocrystalline quartz grains are subrounded to angular and show evidence of strain. Quartz grains contain abundant fractures, minimal overgrowth, and common inclusions of tourmaline, oxides, and needle-like minerals. Polycrystalline quartz (Qp) grains are subordinate (average Qp/Q ratio of 0.32) and vary from subangular to well rounded, which may indicate two sources: a first-cycle source proximal to Site U1499, and a source of recycled grains. Lithic composition in the cobbles averages Lv47Lm19Ls34, ranging 17-75% Lv, 0-60% Lm, and 0-83% Ls. Ls grains are mainly chert, with some sandstone and siltstone; Lm grains contain elongated quartz minerals and oriented mica minerals. In this study, Lv includes subequal abundances of volcanic and plutonic grains. Overall, the lithic assemblage indicates an arc and rifted continental margin provenance. For comparison, the plagioclase feldspar-rich composition of this unit suggests sources varying from continental block to dissected arc. As a result, petrographic analysis suggests that this unit was supplied from a relatively proximal source, potentially from granitoid basement rocks in southern mainland China. The cobble unit may have been deposited by a locally sourced gravity flow, based on its limited geographic distribution.
Year of Publication: 2018
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 06 Petrology, Sedimentary; Clastic rocks; Clastic sediments; Cobbles; Expedition 367; IODP Site U1499; International Ocean Discovery Program; Marine sediments; Mineral composition; North Pacific; Northwest Pacific; Pacific Ocean; Provenance; Sandstone; Sedimentary rocks; Sediments; South China Sea; West Pacific
Coordinates: N182434 N182434 E1155136 E1155135
Record ID: 2019026607
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States