Logging Summary

Asian Monsoon

Expedition 346 Scientific Party

Figure 1. Bathymetry map of the Japan Sea/East Sea showing locations of the sites drilled during IODP Expedition 346 © IODP-USIO

Expedition 346 was the first scientific drilling expedition ever to focus exclusively on the climate system in the Sea of Japan/East Sea, dedicated mainly to the investigation of the East Asian monsoon system, and its influence on North Atlantic Ocean circulation and climate. This expedition offered the opportunity to explore the relationships between atmospheric, paleoceanographic, and sea level changes during at least the last 5 m.y. In particular, cores obtained during this expedition will be used to test the hypothesis that Pliocene-Pleistocene uplift of the Himalaya and Tibetan Plateau, and the consequent emergence of the two discrete modes of Westerly Jet circulation, caused the amplification of millennial-scale variability of the East Asian summer monsoon and winter monsoon and provided teleconnection mechanism(s) for Dansgaardâ-Oeschger cycles.

During International Ocean Discovery Program Expedition 346, the scientific vessel JOIDES Resolution drilled and cored nine sites from July to September 2013 (Figure 1). Seven of the sites were covering a wide latitudinal range in the Sea of Japan/East Sea in order to study and reconstruct surface water circulation, sea ice formation, deepwater convection and oxygenation, biological productivity of the surface ocean and Eolian dust flux related to the positioning of the atmospheric Westerly Jet circulation. Two sites were closely spaced off the East China Sea margin to explore high-resolution changes in Yangtze River discharge through reconstruction of sea-surface salinity. A more complete overview of the expedition operations and preliminary scientific results are available in the preliminary report.

Logging Tool and Operations

Figure 2. Graphic representation of thelogging operations during Expedition 346.

Downhole logging was carried out at four of the drill sites (Figure 2 and Table 1, below) with a standard suite of wireline logging tools, in order to complement the coring program by measuring in situ physical properties and to characterize the formation through intervals of incomplete core recovery.

 

 

Hole	Date of logging	Water depth (below rig floor)	Hole depth (below sea floor)	Maximum logged depth (below sea floor)	Tool Strings
U1423B	Aug 22, 2013	1796.8	249.1	251.1	PC, FMS-sonic
U1425A	Aug 30, 2013	1919	407.2	404.2	PC, FMS-sonic
U1427A	Sept 7, 2013	337	548.6	548.5	PC, FMS-sonic
U1430B	Sept 19, 2013	1082.9	275	272	PC, TC, FMS-sonic

 

Table 1. Logging operations summary, Expedition 346. PC= paleo-combo; TC= triple combo

 

Figure 3. Tool string combinations planned for IODP Expedition 346

The logging tool strings deployed during Expedition 346 are shown in Figure 3 and Table 1. The first tool string is the paleo combination (paleo combo), a variation of the triple combination (triple combo) in which the porosity tool has been replaced by a Magnetic Susceptibility Sonde (MSS). It measured, from top to bottom, resistivity, natural gamma radiation (NGR), density, and magnetic susceptibility. The paleo combo was run first at each logged site. The second tool string is the Formation MicroSanner (FMS)-sonic, which provides sonic velocities and FMS resistivity images of the borehole wall. In Hole U1430B, the tool strings were modified to maximize data acquisition in the lowest part of the hole. A short version of the paleo combo was run first to acquire NGR, density, and magnetic susceptibility. Another short tool string was run, including resistivity at the top and NGR at the bottom. The FMS-sonic was run as the third tool string.

During Expedition 346, in situ temperature measurements were made with the advanced piston corer temperature tool (APCT-3) in A holes. The APCT-3 temperature data were combined with measurements of thermal conductivity obtained from whole-core samples to obtain heat flow values.

Logging Results

We present here a summary of the logging data and some highlights from each site. The drill pipe was raised to ~80 m below seafloor prior to logging because of hole instability in shallow sediments. Logs were recorded only below this depth, in open hole. Gamma Ray was also acquired through pipe but with a strong attenuation of the signal. After logging was completed, logging data initially referenced to depth below the rig floor were shifted to a seafloor reference and depth-matched to remove offsets between different logging runs. The resulting depth scale, used for all data presented here, is meters below seafloor (mbsf). Because of good borehole conditions and generally low to moderate heave during downhole logging data acquisition, log data quality from Expedition 346 is generally very good.

Scientific highlights include records of cyclic sedimentation through time, which seems to relate to Milankovitch cycles. Regular high-amplitude cyclic swings are often observed in the gamma ray, resistivity and density logs, with several orders of cycles varying from one to several meters in thickness. With the exception of ash and dolomite layers, intervals with high gamma ray values, high density, and high resistivity generally reflect an increase in terrigenous clay content relative to diatom-rich intervals. Conversely, the intervals with gamma ray, density, and resistivity low values correlate in cores with diatom-rich intervals.

Site U1423

Figure 4. U1423B downhole logs (Caliper, Spectral Gamma Ray, and borehole FMS image) and corresponding logging and lithological units.

Coring at Site U1423 in the northeastern part of the Sea of Japan/East Sea penetrated a 250 m thick succession of Pliocene to Holocene clay, silty clay, and diatomaceous ooze with discrete foraminifer-bearing clay levels. Volcaniclastic material represents a minor component throughout the sediment succession, except in tephra layers where it is the dominant component.

Downhole measurements were made in Hole U1423B to a total depth of 251 m mbsf using the Paleo-combo and the FMS-Sonic tool strings (Figure 2, Table 1). The entire logged interval was assigned to one logging unit (LU1, Figure 4). FMS images were of excellent quality because of the good borehole conditions and sea state during logging operations. The combination of logs closely reflects lithological changes in the recovered cores, including ash layers. Logging Unit LUI has been divided into two subunits (LUIa, from the base of the drill pipe to ~124 mbsf; LUIb: from 124-250 mbsf) on the basis of changes in character of gamma ray and density logs. The base of logging Subunit LUIa correlates with a distinct change in log characteristics at ~ 124 mbsf, which approximates the lithostratigrahic boundary between lithological subunit IIA and IIB. It reflects a change downhole to a more diatomaceous rich lithology characterized by lower values in total and spectral natural gamma radiation, likely reflecting the abundance of non radioactive elements (diatoms and other siliceous components) within lithologic Unit IIB. Preliminary inspection has also revealed apparent cyclicity in some parts of the section that will require further study.

Site U1425

Figure 5. U1425A downhole logs (Caliper, Spectral Gamma Ray, and borehole FMS image) and corresponding logging and lithological units.

Coring at Site U1425 in the central part of the Sea of Japan/East Sea penetrated 427 m of Miocene to Holocene clay, silty clay, diatomaceous ooze, and claystone with numerous discrete tephra (i.e., volcanic ash) layers.

Downhole wireline log measurements were made in Hole U1425B to ~403 mbsf using the Paleo-combo tool and FMS-Sonic tool strings. Each logging tool string was run twice in the hole to ensure the quality of the logging data. The logged interval was divided in three Logging Units (LU1: from the base of the drill pipe to 244 mbsf; LU2: from 244-338 mbsf, and LU3: 338 mbsf to the bottom of the hole) (Figure 5). The combination of logs closely reflects the lithological changes in the recovered cores, including dolomite and ash layers. Preliminary inspection of the data has also revealed apparent cyclicities in the logs collected by the two logging tool strings below ~244 m WSF, mainly reflecting variation in diatom content relative to terrigenous clays. The cyclic nature of the sediment record at intervals of ~8-10 m is especially well expressed in both the gamma ray and density logs. These cyclicities are also observed on the FMS resistivity data. Conductive intervals in the FMS images tend to reflect intervals enriched in diatoms whereas resistive intervals reflect relative high terrigenous clay content. In the lower part of the Hole, a shift toward higher density and resistivity is observed ~340 mbsf, corresponding to the diagenetic boundary from biogenic opal-A to opal-CT.

Site U1427

Figure 6. U1427A downhole logs (Caliper, Spectral Gamma Ray, and borehole FMS image) and corresponding logging and lithological units.

IODP Site U1427 is in the southernmost part of the Sea of Japan/East Sea. The site is situated on the outer margin of the southeast-northwest–trending continental shelf ~35 km from the northern coast of Honshu Island. Coring at Site U1427 penetrated ~548 m of early Pleistocene to Holocene sediments dominated by clayey silt and nannofossil- or biosiliceous-rich clayey silt. Shell fragments of shallow-water origin are observed throughout the sedimentary succession.

Downhole wireline log measurements were made in Hole U1427A to 548.5 mbsf using the Paleo-combo and the FMS-Sonic tool strings. The logs do not show major steps in the base levels and the entire logged interval was assigned to one Logging Unit (LU1) corresponding to lithostratigraphic Unit A (Figure 6). Preliminary inspection of the data revealed cyclicities that mainly reflect variations in biogenic content relative to terrigenous clays and are consistent with lithological changes in the recovered cores. Intervals with high gamma ray values, high density, and high resistivity generally reflect terrigenous clay rich intervals. Such intervals possibly correspond to glacial stages characterized by lowered productivity. In Hole U1427A, the FMS images also reveal numerous resistive and conductive intervals, with thicknesses ranging from several tens of centimeters to a few meters.

Site U1430

Figure 7. U1430B downhole logs (Caliper, Spectral Gamma Ray, and borehole FMS image) and corresponding logging and lithological units.

IODP Site U1430 in the southwestern part of the Sea of Japan/East Sea is on the southern upper slope of the eastern South Korean Plateau, which bounds the northern margin of the Ulleung Basin. Coring at Site U1430 penetrated ~274 m of middle Miocene to the Holocene sediments dominated by clayey silt, silty clay, nannofossil ooze, diatomaceous ooze, claystone, and sandstone. Numerous discrete tephra layers occur throughout the sedimentary sequence, especially within the upper 50 m. Site U1430 will provide a continuous slow-sedimentation record that is ideal to study the long-term history of dust provenance and flux changes since 10 Ma.

Downhole measurements were made in Hole U1430B to a total depth of ~272 mbsf. The paleo combo wireline logging tool string was split into two shorter strings to maximize data acquisition in the lowest part of the hole. The logged interval was divided in two Logging Units (LU1, from the pipe entrance to ~244 mbsf; LU2, from ~244 mbsf to the bottom of Hole U1430B) (Figure 7). In Logging Unit LU1, the log data mainly reflects variation in diatom content relative to terrigenous clays, and matches lithological changes throughout the section. Preliminary examination of the data revealed apparent high frequency cyclicity in the FMS images. There is a distinct change in log characteristics at ~244 m WSF, which correlates closely with a change downhole to indurated deposits. The core recovery is low in this interval and the good quality of the downhole logs and borehole images should allow us to refine the lithology within the core gaps.

As observed at previous IODP Expedition 346 sites, conductive intervals (light color in the FMS image in Figure 7) in Logging Unit LU1 generally correlate with low gamma ray, low density, and low resistivity logs. Conversely, more resistive intervals generally correlate with higher values in the gamma ray, bulk density, and resistivity logs. This relationship can be interpreted in terms of the relative abundance of clay/diatom in the sediment, with clay having higher K and Th contents and relatively greater density than diatom-rich sediment.

The good FMS resistivity data quality allows the borehole formation resistivity to be interpreted at several scales. With the exception of ash and indurated layers, conductive intervals in the FMS images tend to reflect intervals enriched in diatoms, whereas resistive intervals reflect relatively high terrigenous clay content. In LU2, below ~240 mbsf, the FMS images are characterized by high resistivity (light colors in the FMS image in Figure 7, which reflects the increase in clay content in lithologic Subunits IIIB and IIIA as well as the presence of cemented intervals, in agreement with the high density, velocity, and resistivity observed within this unit.

Temperature Measurements

In situ sediment temperature data were measured during deployments of the APCT on Expedition 346. The geothermal gradient at the Exp. 346 sites varies from 70 to 140 °C/km, a large range that reflects tectonic structures and processes in the area.

---

Johanna Lofi: Logging Staff Scientist, Géosciences Montpellier - UMR 5243 - CC 060 - Bat. 22, Université de Montpellier 2, Place E. Bataillon, 34095 Montpellier Cedex 05, France


---