Sampling the unconformity, Shipboard scientists Tao Jiang, Yujin Kitamura, Arito Sakaguchi and Co-Chief Scientist, Gaku Kimura (Japan) >>Expedition 316 Photo Gallery

IODP NanTroSEIZE Stage 1:
Expedition 316: NanTroSEIZE "Shallow Megasplay and Frontal Thrusts"
Weekly Report #1
Date 27/Dec/2007

The Expedition 316 began since December 19th immediately after the completion of previous cruise. Chikyu was on site and Hole C0004C was tagged at 5:35 AM December 20th.

Site C0004 is located along the slope of the accretionary prism landward of the inferred intersection of the megasplay fault zone with the seafloor. The purpose of drilling at this site is to core and sample the materials comprising the youngest sediments in the slope basin, material underlying these sediments (inferred to be older slope basin sediments overlying deformed and uplifted older accretionary prism material). The main target of the site is to core and sample across the megasplay fault zone at a depth of approximately 300 meters below seafloor (mbsf), and to core down to 400 mbsf in the underthrust materials identified in 3-D seismic interpretation.

Coring with HPCS (Hydraulic Piston Core Sampling System) commenced at that time and continued to a depth of 89.22 mbsf. Three APCT3 (Advanced Piston Corer Temperature) measurements were taken at the same time as collection of cores C0004C-3H, 6H, and 9H. ESCS (Extended Shoe Coring System) coring commenced at 22:13 on December 20th in Hole C0004C, and continued to a depth of 127.22 mbsf. Due to poor core quality, HPCS coring was resumed at 04:47 AM on December 21, and 3 cores (C0004C-16H-18H) were collected. The last HPCS (C0004C-18H, 133.18-135 mbsf) core also included an APCT3 run. At 07:32 AM on December 21, due to failure to achieve full penetration using the HPCS, the coring assembly and drill pipe was run out of the hole, and the hole was completed.

Repositioning to site C0004D began at 13:00 December 21, 2007, in parallel with making up the RCB assembly and running pipe to the seafloor. Hole C0004D was tagged at 00:30 December 22, 2007, and washing down to 100 mbsf commenced. RCB (Rotary Core Barrel) coring commenced at 12:25 December 22, 2007. During RCB coring, several instances of plugged bit throat and plugged bit nozzles were inferred from coring results and pressure readings, and bit de-plugging was conducted to clear the problem. Variations in RPM (revolutions per minute) and drilling fluid pressure/flow rate were used to attempt to improve recovery, and cleaning the bit by increasing flow after each core was implemented. Additionally, due to recovery problems, short cores were taken on every coring run below 238 mbsf (C0004D-21R).

TD of 400 mbsf was reached at 23:34 December 25th, POOH (pull out of hole) and completing the hole with high density fluid was carried out. Tripping the drill string and breaking down the bit assembly commenced.

Coring in holes C0004C and D recovered recent sediments from the slope basin deposits, crossed an unconformity into older slope sediments and then passed into the accretionary prism.

Core recovery was excellent during HPCS and ESCS coring, however the ESCS cores were determined to be too disturbed by drilling to meet the scientific needs of the expedition. Recovery was 100-105% in Hole C0004C, down to a depth of 135 mbsf. Core recovery during RCB operations was variable primarily due to the difficulty of rotary coring in highly fractured materials that make up the shallow accretionary prism. As a result of increasing lithification, combined with excellent work by the Core Technicians, Drillers, and Operations personnel, recovery in the lower section of Hole C0004D improved remarkably. Recovery between 100-242 mbsf ranged from 0-60%, averaging about 30% or less. Recovery between 242-400 mbsf ranged from 20-100% and averaged approximately 70%.

Based on limited recovery, the accretionary prism material is highly deformed and fractured. Near the base of the prism, two age reversals were crossed, indicating thrust faulting. These cores have been held back from splitting in preparation for detailed examination. Underthrust (younger) sediments exhibit decreasing deformation with depth below the splay fault system.

A program including piston coring, downhole temperature measurement, and rotary coring was designed to accomplish the coring, and a comprehensive sampling plan was developed to provide a full spectrum of scientific analyses of these materials. Sampling, description and measurement for interstitial pore water, microbiology, micropaleontology, mineralogy, sedimentology, structural geology, hydrological and geotechnical research, physical properties investigation, paleomagnetism, organic and inorganic chemistry of both solids and fluid were planned for all collected cores.

On December 26th, seafloor acoustic navigation beacons were recovered, the vessel was repositioned to site C0006C, and preparations for testing of the underwater television camera (for drilling at depths greater than 3000 meters water depth) commenced, and are underway as of the writing of this report.