Chris Fergusson Research Interests

ODP Leg 190 – Nankai Trough – 21 May – 17 July 2000

Chris Fergusson and Leg 190 Shipboard Scientific Party

My application to join Leg 190 was successful and I was invited to join as a Shipboard Sedimentologist. Normally I consider myself a structural geologist with interests in mainly field based mapping of structures and regional geology. As I have always been interested in the sedimentology of deep-marine successions, particularly those associated with accretionary prisms, I gladly accepted the invitation to join Leg 190 with only minor misgivings about my new assignation. The scientific party for Leg 190 assembled in Guam from about 21 May 2000. My own travel required an interesting zig-zag trip with touchdowns at Brisbane, Nauru, Pohnpei (somewhere in Micronesia) before finally arriving in Guam at around 6 am on the 22 May. At the hotel we discovered the good news that the JOIDES Resolution was arriving early and that we would board the ship on Tuesday 23 May and depart at around 9 pm that night (just under a day early).

From Guam we had a 4-day transit to the first site. My first sight of the ship at the wharf was a surprise – it seemed smaller than when I last saw it in St. John's (Newfoundland) in 1993. Once on the ship we settled into a routine of seminars and familiarising ourselves with the core description laboratory. We spent much of the time waiting and for first-timers like myself adjusting to the rolling motion of the ship. The more experienced hands, however, considered that the seas were as close to dead calm as possible in the western Pacific. I am pleased to report that only once did I resort to swallowing one of those pink pills that the doctor left on her door. Towards the end of the transit we slowed down and collected some seismic data as we approached the first site.

The main objectives of Leg 190 were to:

1. test the distribution of structures and the role of diagenesis and fluid pressure in a transect across the Nankai accretionary prism off the Muroto Peninsula (Shikoku Island, Southwest Japan) including Site 808 (Leg 131),

2. to document spatial variations in structure and fluid pressure to test hypotheses of décollement development,

3. to understand the lateral variability of fluid flow in the accretionary prism, and

4. to contrast the stratigraphic and deformational framework along strike of the Nankai accretionary prism.

The first site (1173) was located in around 4800 m of water at the seaward edge of the Nankai Trough. The first full shift of core description was for me a day I will not forget in a hurry. It was possibly the busiest day of my life and required detailed description of turbidites while at the same time learning how to describe smear slides and plot the data onto computer drawn barrel sheets. Over the next few days we gradually caught up with all aspects of the core description and passed quickly out of the turbidites of the outer trench wedge into bioturbated muds of the upper Shikoku basin. These muds contained well-developed volcanic ash layers that disappeared in the lower Shikoku basin. At the base of the hole the near complete core recovery encountered so far degenerated dramatically and the hole bottomed in 2 cm of basalt basement. We were fortunate that recovery overall had been excellent (658 m from a hole depth of 734 m) and this had been achieved with the Advanced Piston Corer (APC) and the Extended Core Barrel (XCB) without the need for a changeover to Rotary Core Barrel (RCB) as initially planned. This saved us some time that would be put to good use later on. Some down-hole logging was then undertaken.

 

Chris Fergusson studying a core from the décollement, Site 1174. Photo taken by Ako Taira (Co-Chief of Leg 190).

In high spirits we then moved to the second site (1174) which was located around 3 km southeast of Site 808 and 11 km northeast of Site 1173. At this site we planned to drill through the proto-thrust zone at the toe of the accretionary prism and below the underlying basal décollement. From almost the start this site presented a challenge to the drillers. Thick loose sands of the axial trench wedge were soon encountered and the hole had to be abandoned at 74 mbsf (metres below sea floor) after the failure to pull up any more core. We were prepared for this eventuality and the next three days were spent waiting (reading time for me) as the drillers emplaced a drill-in-casing (DIC) system and started drilling with the RCB in Hole 1174B. More delays were experienced due to problems with the hole and the recovery of metal in parts of the core caused much speculation (had we found parts of a Pleistocene UFO - or was it from the drill assembly?). Once below the thick Pleistocene turbidites of the trench and outer trench the Shikoku basin mudstones were encountered again. In contrast to the first site recovery was much less (625 m for a depth of 1050 m). The highlight of the site was the highly comminuted rock recovered from the décollement at the base of the Nankai accretionary prism (effectively the plate boundary between the Philippine Sea Plate and the Eurasian Plate). Over 2 weeks was spent at this site in contrast to slightly more than half the time at the first site.

The next two sites (1175 and 1176) were located at shallower water depths (around 3000 m) and the holes penetrated beneath the inner trench slope of the Nankai Trough. At Site 1175 we drilled into a lower trench slope basin and into the underlying accretionary prism. This hole produced unexpected results including an exceptionally young section with the base of the hole at 439 m in upper Pliocene sediments including pebbly mudstone and sandstone. Both sites were marred by low recovery especially at depth and both were stopped at depths of around 440 mbsf with only 51% recovery at Site 1176. The lack of recovery prevented detailed work on physical properties, geochemistry and caused some consternation in parts of the core description laboratory. Nevertheless documentation of spectacular soft-sediment deformation related to slumping down the trench slope in the upper parts of both holes was a highlight. Another significant outcome was the relatively younger than expected ages of sedimentation at both sites that indicated an extremely rapid growth rate of the Nankai accretionary prism.

From here we moved around 140 km to the southwest to drill a reference site at the southeast end of the western transect that includes Sites 297, 298, 582, and 583 from previous deep-sea drilling legs. Site 1177 was located in a water depth of around 4800 m and we initially drilled down to 300 mbsf without any attempted recovery of core. A total penetration of 840 mbsf was achieved with recovery of 283 m. This was certainly low but in contrast to the two previous sites recovery was fairly consistent down the hole giving a reasonable idea of the stratigraphy, physical properties and geochemistry down to basement. Nearly 2 m of basalt basement was recovered from the base of the hole - considerably more impressive than that recovered at Site 1173. The main feature of this site was the presence of a Miocene siliciclastic turbidite unit. One feature of these turbidites was the abundance of woody plant material that indicated derivation from a landmass, most likely ancient Southwest Japan. This turbidite succession was not encountered at Sites 1173 and 1174 in the eastern Muroto transect – it represents part of a major submarine fan that extended out across the northern Shikoku basin.

Last "core-on-deck", Site 1178, Saturday 15th July 2000.

 

We had now completed drilling at the five planned sites for Leg 190 yet it was only the 5 July and 12 days still remained. The Co-Chiefs had suggested that a new site be drilled between Sites 808 and 1176 rather than drill any of the available alternate sites. Permission was sought and eventually obtained to drill this new site. At this stage the forces of nature intervened to change our plans. Throughout June the weather had generally been warm but rain was common, by the end of June the rainy season had passed and we had clear skies. In early July we became aware of typhoon Kirogi which had developed somewhere near the Philippines and started moving northwards. It was not to change its trajectory for some time. On July 6 we had to WOW (wait on weather) at the new proposed site back on the Muroto transect and the next day the captain decided to move the ship away from Kirogi which eventually continued on towards Tokyo. So much time had been lost waiting for Kirogi to pass that it was now no longer feasible to drill at the proposed new site. The fall-back position was to drill at an alternate site (1178) on the inner trench slope to the northwest of Sites 1175/1176 in the Muroto transect.

Site 1178 was located in 1741 m water depth and two holes were drilled to a total depth of 680 mbsf with a recovery of 506 m of core. At this stage of the leg it was becoming more difficult to maintain enthusiasm given that we had already been on the ship for seven weeks. Nevertheless this site proved to be just as, if not more, interesting than the other sites. This site has muddy and turbidite units from the Quaternary back to the early Miocene with an upper slope section separated by a major discontinuity from the underlying accretionary prism. One feature of the accretionary prism was the development of a bedding-parallel fissility and a bedding-oblique foliation. The Kuro Shio (an ocean current), one of the potential difficulties encountered during ocean drilling in the Nankai Trough, was particularly strong at this site. The final core for Leg 190 came aboard at around 6 am 15 July and all core description was completed by 9 am.

From this site there was a short transit of around one day to the final destination of Yokohama and we docked at around 4 pm on 16 July. After spending the final night on the ship we left for down-town Yokohama and I caught a bus to Narita Airport and flew back to Sydney that night. I arrived back in Wollongong at around 9 am on 18 July and gave my first lecture for the session later that morning at 11.30 am. For me Leg 190 was a truly exciting scientific experience – given the vast resources and the company of an excellent scientific and technical team ably lead by Co-Chiefs Greg Moore and Ako Taira (Staff Scientist Adam Klaus) one constantly felt privileged to be present. The science was particularly exciting and the results have major implications for the development of accretionary prisms and the formation of continental crust in general.

 

Drilling tower of the JOIDES Resolution.

Underwood M. B. and Fergusson C. L. 2005. Late Cenozoic evolution of the Nankai trench-slope system: evidence from sand petrography and clay mineralogy. In Hodgson D. M. and Flint S. S. eds Submarine Slope Systems: Processes and Products. Geological Society, London, Special Publications, 244, 113-129.

Fergusson C. L. 2003. Provenance of Miocene-Pleistocene turbidite sands and sandstones, Nankai Trough, Ocean Drilling Program Leg 190. In Mikada H., Moore G. F., Taira A., Becker K., Moore J. C. and Klaus A. eds Proceedings of the Ocean Drilling Program, Scientific Results, 190/196 [Online]:
http://www-odp.tamu.edu/publications/190196SR/205/205.htm

Underwood M. B., Moore G. F., Taira A., Klaus A., Wilson M. E. J., Fergusson C. L., Hirano S., Steurer J., and Leg 190 Shipboard Scientific Party, 2003. Sedimentary and tectonic evolution of a trench-slope basin in the Nankai subduction zone of southwest Japan. Journal of Sedimentary Research 73, 589–602.

Screaton E., Saffer D., Henry P., Hunze S. and Leg 190 Shipboard Scientific Party 2002. Porosity loss within the under-thrust sediments of the Nankai accretionary complex: implications for overpressures. Geology 30, 19–22.

Moore G. F., Taira A., Klaus A., Becker L., Boeckel B., Cragg B. A., Dean A., Fergusson C. L., Henry P., Hirano S., Hisamitsu T., Hunze S., Kastner M., Maltman A. J., Morgan J. K., Murakami Y., Saffer D. M., Sanchez-Gomez M., Screaton E. J., Smith D. C., Spivack A. J., Steurer J., Tobin H. J., Ujiie K., Underwood M. B. and Wilson M. 2001. New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of the Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems 2, 10.129/2001GC000166.
http://gcubed.org/

Moore G. F., Taira A., Klaus A., Becker K., Becker L., Boeckel B., Cragg B. A., Dean A., Fergusson C. L., Henry P., Hirano S., Hisamitsu T., Hunze S., Kastner M., Maltman A. J., Morgan J. K., Murakami Y., Saffer D. M., Sanchez-Gomez M., Screaton E. J., Smith D. C., Spivack A. J., Steurer J., Tobin H. J., Ujiie K., Underwood M. B. and Wilson M., 2001. Deformation and fluid flow processes in the Nankai Trough Accretionary Prism. Proceedings of the Ocean Drilling Program, Initial Reports, 190. Also available at: http://www-odp.tamu.edu/publications/190_IR/190ir.htm

Wilson, M.E.J., Hirano, S., Fergusson, C.L., Steurer, J., and Underwood, M.E., 2003. Data report: Sedimentological and petrographic characteristics of volcanic ashes and siliceous claysontes (altered ashes) from Sites 1173, 1174, and 1177, Leg 190. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 190/196 [Online]:
http://www-odp.tamu.edu/publications/190196SR/204/204.htm

Initial Reports Leg 190
http://www-odp.tamu.edu/publications/190_IR/190ir.htm

Scientific Results Leg 190
http://www-odp.tamu.edu/publications/190196SR/190196sr.htm

 

 

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