The Nankai Accretionary Prism off the southwest coast of Japan

Emily Comer

Nankai Accretionary Prism off the southwest coast of Japan
Nankai Accretionary Prism off the southwest coast of Japan

My project is a study of sand sized sediment from the Nankai Accretionary Prism off the southwest coast of Japan. In this project I am trying to find the provenance of this sediment and what the compositional balance of the material is. The Nankai Accretionary Prism is an accumulation of sediment that formed as the Philippine Plate subducted beneath the Eurasian Plate. This deepwater sediment was collected in 4 borehole sites during Expedition 316 of the NanTroSEIZE Project, an extensive project taking place over the next few years that will investigate tectonic activities and diagenesis processes off the coast of Japan.

All of my samples were collected as a mix up clay and sand. Because I am only studying sand sized sediment, these samples were wet-sieved with a 62μ screen (62μ being the size separating sand and silt sized material). Next these samples were thin sectioned and stained for feldspar identification. After preparing all the samples, I began point-counting in order to identify the composition. 35 samples were point-counted, and the data was put into ternary diagrams giving the composition of the sediment.

Some conclusions of my project are that all of my samples fall within the lithic arkose or feldspathic litharenite category of Folk’s classification and though many of these samples are very similar in composition, the difference between the sites is significant and could be the result of difference in provenance. Older sediment was found to be more lithic rich indicating a decrease in volcanic activity as subduction progresses. Also, an interesting correlation between grain size and composition was found, finer grained sediments tended to contain more feldspar and sedimentary lithics than coarser grained sediment.

Honors Advisor:

Dr. Kitty Milliken