MAGNETIC FABRIC ANALYSES OF DEEP-SEA SEDIMENTS: IDENTIFYING DEPOSITIONAL PROCESSES AT IODP EXPEDITION 333 SITES C0018 AND C0011, NANKAI TROUGH AND SHIKOKU BASIN

Hemipelagic/pelagic mud and mudstone sediments were collected from Sites C0018 and C0011 off the coast of southeastern Japan during Integrated Ocean Drilling Program Expedition 333. Site C0018 is located in the Pleistocene-Holocene accretionary wedge in the Nankai trough and Site C0011 is located on the northwestern flank of the Kashinozaki Knoll on the northern rim of the Shikoku basin (Miocene-Holocene section). 168 samples from C0018 and 243 samples from C0011 underwent AMS measurements in order to characterize the depositional processes within the Shikoku Basin. These sediments will also be used to determine the effects of compaction and slumping on the paleomagnetic signal recorded in the sediments as well as creating a revised magnetostratigraphy at both sites.

Depositional mechanisms (gravity, current, grain collision and viscous suspension) can be distinguished using q- ß diagrams: magnetic lineation/foliation (q) versus the imbrication of the magnetic foliation plane (ß). The q-ß diagrams revealed that depositional processes at Sites C0018 and C0011 are dominated by slope gravity and current deposition. Scatter outside the regions of slope gravity and current deposition indicate disturbed sediments (drilling disturbance, slumps, etc). Sediments from site C0011 are less disturbed and indicate relatively quiet deposition away from the accretionary wedge in the Shikoku Basin and provide a reference for a normal, non-slumped sedimentary section. Trends in the degree of anisotropy and the inclination of the minimum susceptibility with depth suggest zones of quiet deposition punctuated by zones of disturbed sediment at C0018. The AMS ellipsoid trends toward oblate with increasing depth in undisturbed sediments- this is expected in a normally-consolidating section, and will be used to evaluate the effects of burial and compaction on the paleomagnetic inclination.

The larger zones of disturbed sediment recognized in this study correlate well with preliminary Exp 333 data indicating multiple mass transport deposits at site C0018, each varying in size. Smaller disturbances were poorly defined in this study due to the sampling interval. The magnetic fabric analyses were successful in distinguishing and characterizing depositional events at Sites C0018 and C0011.