Characteristics of a Seamount Apron in the South China Sea, Hole U1431E, IODP Expedition 349

Monday, 30 January 2017
Marina/Gretel (Hobart Function and Conference Centre)
Kelsie Anne Dadd, University of Sydney, Sydney, Australia and Anthony A P Koppers, Oregon State University, Corvallis, OR, United States
Abstract:
International Ocean Discovery Program (IODP) Expedition 349 Site U1431 is located near the relict spreading ridge in the East Sub-basin of the South China Sea. Site U1431 is close to a seamount and holes intersected its volcaniclastic apron. The apron is approximately 380 m thick and sandwiched between non-volcaniclastic units that represent the background sedimentation. Underlying the volcaniclastic package is sandstone, siltstone, and claystone with minor intervals of volcaniclastic breccia. The sandstone, siltstone and claystone occur in fining-upward cycles that are interpreted as turbidite sequences. Late Miocene clay overlying the volcaniclastic apron comprises nannofossil ooze, silt, and sandy silt interpreted as turbidite and hemipelagic deposits that accumulated at abyssal water depths.

The volcaniclastic sequence includes graded beds of sandstone, siltstone, and claystone intercalated with intervals of volcaniclastic breccia. The breccia beds are up to 4.8 m thick. The breccia is typically massive to weakly graded and poorly sorted with angular to subangular basaltic clasts, as well as minor reworked subrounded calcareous mudstone, mudstone, sandstone and basalt clasts. There are rare foraminifers in most beds. The base of many beds has reverse grading. The volcaniclastic beds were most likely deposited as high-concentration turbidite flows on a relatively steep slope in the abyssal plain environment.

Vitroclasts in the breccia and sandstone range from blocky to bubble-wall in shape and massive to amygdaloidal and aphyric to porphyritic in microstructure. All vitroclasts are angular to irregular indicating little shape modification during transport. Crystals of plagioclase and clinopyroxene are abundant as single crystals and in vitroclasts. Other mineral grains include olivine, amphibole, biotite and accessory apatite and Fe-Ti oxides. The glass is alkalic in composition and ranges from basanite to trachyte with some vitroclasts of rhyolite composition.

Beds in the seamount sequence range in age from 8.03 ± 0.02 to 9.36 ± 0.06 Ma (2σ) based on 40Ar/39Ar geochronology of plagioclase, hornblende and biotite separated from the base layers of individual turbidite beds. This indicates an eruption duration of at least 1.3 Myr for the seamount apron section.

<< Previous Abstract | Next Abstract >>