EARLY DIAGENESIS OF DEEP-SEA SEDIMENTS ON INCOMING PLATES: EXAMPLES FROM THE IZU-BONIN TRENCH AND THE SUNDA TRENCH

Microfabrics in deep-sea sediments prior to incorporation in an accretionary prism give us important visual information for deformation fabric analyses. Analyses of primary sedimentary fabrics and compaction processes of deep-sea sediments are a key to understanding formation processes of the decollement zone. This paper discuss the microfabric development processes of unconsolidated hemipelagic and pelagic clayey sediments during early diagenesis from deposition, burial compaction-cementation, and deformation by means of magnetic measurements, microfabric observations and physical property measurements using drilling core samples collected from the Izu-Bonin Trench (ODP Leg 185 Site 1149A, about 200 m long) and the Sunda Trench (IODP Exp. 362 Site U1480, about 1500 m long).

The main objective of this paper is to understand the microfabric changing processes in the pelagic/ hemipelagic sediments from several centimeters to several hundreds meters in depth during long-term burial compaction. The burial compaction processes have been mainly studied using consolidation tests in laboratories for a short-term, although, sediments are consolidated gradually during geologic time as an age effect. Microfabric changing processes by age effects are not well understood. Pelagic/ hemipelagic sediments are the best examples for long-term compaction study, because they have mostly constant sedimentation rates through geologic time and homogeneous structures and components.

The samples for microfabric observation were treated by freeze-drying and embedding methods in order to avoid volume shrinkages by air-drying. The microfabrics in the pelagic/ hemipelagic sediments are mainly constituted of clay aggregations (ped) and clay linkages (connector). Large pores between the peds are supported by connectors at shallow depths. These are constructed mainly of the clay flakes in edge-to-edge (EE) and edge-to-face (EF) contacts. In contrast, the large pores between the peds are closed, and the size of small pores within the peds decreases with burial depth. The contacts of clay flakes change from EE and EF to face-to-face (FF) with wavy lamina. Thus, the microfabrics change downward from random fabrics to wavy shaly fabrics. The waviness represents probably a relict of such change of peds from EE to FF contacts.

Session No. 115

T227. Ophiolite and Ocean Plate Stratigraphy Records in Modern and Ancient Accretionary and Collisional Orogenic Belts in the Circum Pacific Region I

Monday, 23 October 2017: 8:00 AM-12:00 PM

Tahoma 4 (The Conference Center)

Geological Society of America Abstracts with Programs. Vol. 49, No. 6
doi: 10.1130/abs/2017AM-296938

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