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Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea
Quaternary International xxx (2015) 1e11
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Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea Jang-Jun Bahk*, In-Kwon Um, Bo-Yeon Yi, Dong-Geun Yoo Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, 305-350, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Cyclic variations of density and gamma-ray logs achieved from the two drill sites of UBGH2-1_1 and UBGH2-1_2 in the western slope of the Ulleung Basin were investigated to reveal their paleoceanographic implications and establish cyclostratigraphy of the well logs. Integration of the core, log, and seismic data from the two sites indicates that the sediments of the drilled intervals mostly consist of hemipelagic muds, with a few sporadic mass transport deposits intercalated only in the downslope site of UBGH2-1_1. Comparison with variations in sediment composition suggests that ratios of terrigeneous to biogenic materials which were modulated by orbital-scale climate changes should be responsible to the well-log variations. The established cyclostratigraphy of the well logs based on correlation with marine oxygen isotope records indicates such climatic modulation of the sedimentation in the western slope of the Ulleung Basin has persisted at least since 2.6 Ma, consistent with the variation reported from the Oki Ridge. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Paleoceanography Cyclostratigraphy Well logs Ulleung Basin East Sea
1. Introduction In addition to oil and gas reservoir characterization, well logs of marine sediment sequences have been widely applied to paleoceanographic and cyclostratigraphic studies (e.g., deMenocal et al., s et al., 1999; Gorgas and Wilkens, 2002; Paulissen and 1992; Barthe Luthi, 2011) because most of the well logs are primary affected by sediment porosity that are closely related to mineralogical and grain-size compositions which in turn can vary significantly with cyclic changes of climate and sea-level (Goldberg, 1997). Well-logs are particularly useful for time-series analysis to reveal orbital and sub-orbital cyclicity by providing continuous data with uniform sampling rates, which are often impossible or hard to achieve with discrete core sections. However, before applying the well logs to paleoceanographic and cyclostratigraphic studies, site-specific relations between log variables and sediment properties should be clarified by correlation of core and log data. Moreover, presence of event beds and significant hiatus or changes in sedimentation rates has to be examined to ensure consistency in environmental
* Corresponding author. E-mail address: [email protected] (J.-J. Bahk).
conditions and relatively uniform thicknessetime relationship (Weedon, 2003). Density and gamma-ray logs achieved by logging-while-drilling (LWD) from the two drill sites in the western lower slope of the Ulleung Basin during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) show fairly consistent cyclic variations with each other in most of the log intervals (Ryu et al., 2012). To reveal paleoceanographic implications of the cyclic variations of the logs and establish cyclostratigraphic framework, we first examined which sediment properties are responsible for the cyclic variations of the logs using available core data. Comparisons of the logs and the core data with the seismic section across the drill sites provided further control of the intervals suspected as event beds such as mass transport deposits. The gamma-ray logs devoid of the event beds were converted to time series mainly based on correlations with the well logs from Site ODP 798 from the Oki Ridge which have established ageedepth relations before 1 Ma and LR04 stack of global benthic d18O records (deMenocal et al., 1992; Lisiecki and Raymo, 2005). The time-series gamma-ray logs evidence that, for the first time, orbital-scale cyclic modulation of ratios of terrigeneous to biogenic materials had been persistent during the entire Pleistocene and rates and changes of hemipelagic sedimentation in the western and the southern margins the East Sea had been nearly identical.
http://dx.doi.org/10.1016/j.quaint.2015.08.023 1040-6182/© 2015 Elsevier Ltd and INQUA. All rights reserved.
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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2. Geological and oceanographic settings The East Sea (Japan Sea) is a semi-enclosed marginal sea that is connected to the North Pacific and adjacent seas through four shallow and narrow straits (water depths of 12e140 m; Fig. 1). The Tsushima Warm Current, a branch of the warm saline Kuroshio Current, enters the East Sea through the Korea Strait and flows out through the Tsugaru and Soya straits. A cold (<1 C), highly oxygenated (5e6 ml/l) water mass that originates from winter cooling of surface water in the northern part occurs below the water depths of 200e300 m (Kawamura and Wu, 1998; Kim et al., 2002). Significant changes in oceanographic conditions have been expected in response to the glacio-eustatic sea-level fluctuations which resulted in a sea-level drop of as much as about 120 m during the last glacial maximum. Tada et al. (1999) suggested that the modulation of the volume and character of the surface water inflow through the Korea Strait associated with glacio-eustatic sea level changes played a key role in the changes in deep-water ventilation and surface productivity. According to their hypothesis, the East Sea
was nearly isolated by a sea-level drop of more than 90 m during the glacial maximum periods. During such periods, low-salinity surface water developed because of excess precipitation and caused strong density stratification of the water column, resulting in euxinic deep water and low surface productivity (Tada et al., 1999). Together with the changes in oceanic circulations and surface productivity, variations in influx of Asian aeolian dust to the East Sea also have been suggested in both orbital and sub-orbital timescales (Irino and Tada, 2002; Nagashima et al., 2007). The Ulleung Basin in the southwestern part of the East Sea was opened in the Late Oligocene to Early Miocene by crustal extension accompanied with southward movement of SW Japanese Islands (Tamaki et al., 1992; Yoon and Chough, 1995). At the end of the Middle Miocene (11e12 Ma), basin closure was caused by the northward collision of the Bonnic Arc with central Japan (Chough and Barg, 1987). The basin closure led to uplift of the southern margin and basin-wide deposition of mass transport deposits (MTDs) which were evolved from frequent slope failures along the southern slope during the latest Neogene (Lee and Suk, 1988; Lee
Fig. 1. Bathymetric map of the Ulleung Basin (contours in meter), showing the locations of drill sites UBGH2-1_1 and UBGH2-1_2. Also shown is the location of ODP Site 798 and shallow sills and straits of the East Sea in the inset. Box in the inset indicates the location of the enlarged area. Modified from Lee et al. (2004).
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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et al., 2001). Since the Pleistocene, the deeper parts of the basin have been dominated by distal turbidites and hemipelagites as the depocenter of the MTDs rapidly retreated to the southern margin (Lee and Suk, 1988; Lee et al., 2001). The presence of gas hydrates in the Ulleung Basin has been suggested based on seismic indicators, such as bottom simulating reflectors (BSR), seismic chimneys, high amplitude reflections within the gas hydrate stability zone (GHSZ), acoustic blanking, enhanced reflections below the BSR, and seafloor gas-escape features (Yoo et al., 2013 and references therein). During the UBGH2, Sites UBGH2-1_1 and UBGH2-1_2 in the western lower slope of the basin were drilled to test enhanced dipping reflections below the BSR (Fig. 2; Ryu et al., 2012). In Site UBGH2-1_1, both LWD and coring were conducted, while in Site UBGH2-1_2, 1.2 km apart from the UBGH2-1_1 to the upslope, only LWD well logs were achieved (Fig. 2; Table 1). Unlike most other UBGH2 drill sites from the basin plain, the recovered sediments from Site UBGH2-1_1 are generally devoid of reworked sediments other than in a few limited intervals of MTDs, making them feasible for further paleoceanographic studies. In this paper, we focus on cyclostratigraphic and paleoceanographic implications of the well-log and core data from the two sites. Gas hydrate occurrences in the cores from Site UBGH21_1 were already detailed in Bahk et al. (2013) and evaluation of gas hydrate petroleum systems by integration of core, log and seismic data will be dealt with elsewhere.
Table 1 Summary of LWD and coring holes in the Sites UBGH2-1_1 and UBGH2-1_2. Site
Phase
Hole
Latitude Longitude
36 150 04.400 N 130 030 56.500 E Coring UBGH2_1_1B 36 150 04.800 N 130 030 56.500 E UBGH2-1_1C 36 150 04.800 N 130 030 56.600 E UBGH2-1_1D 36 150 04.800 N 130 030 56.500 E UBGH2-1_2 LWD UBGH2_1_2A 36 150 04.400 N 130 030 10.300 E
UBGH2-1_1 LWD
UBGH2-1_1A
Water TD (mbsf) depth (mbss) 1534
281
1529
8
1529
8
1529
216
1499
380
The water depths were measured with the length of seafloor touching drill pipe; mbss ¼ meter below sea-surface; mbsf ¼ meter below seafloor; TD ¼ termination depth.
3
3. Material and methods 3.1. LWD and coring operations The UBGH2 consists of the first LWD phase and the second coring phase (Ryu et al., 2012). The LWD was conducted using Schlumberger's GeoVision, TeleScope, EcoScope and SonicVision to the termination depths of 281 and 380 m below seafloor (mbsf) at Sites UBGH2-1_1 and UBGH2-1_2 (Table 1). In Site UBGH2-1_1, coring was performed at three holes UBGH2-1_1B, UBGH2-1_1C, and UBGH2-1_1D that are located approximately 10 m north from the LWD hole UBGH2-1_1A (Table 1). To retrieve undisturbed core sediments near the seafloor, coring tools were deployed from about 2 m above the seafloor at the coring holes of UBGH2-1_1B and UBGH2-1_1C, and then landed on the seafloor at the UBGH2-1_1D hole to continuously core from 2 to 216 mbsf (Ryu et al., 2012). The coring was conducted using Fugro Hydraulic Piston Corer, Fugro Corer, and Fugro Rotary Corer for non-pressurized cores and Fugro Pressure Corer and Fugro Rotary Pressure Corer for pressurized cores. 3.2. LWD data and matching with seismic reflections For characterization of changes in sediment porosity and composition, we chose gamma-ray and density logs of the EcoScope which were sampled at 6 in intervals. For the matching of the depths between core and log data, the datum of LWD log depth was shifted from the rig floor to the seafloor, identifying the step changes in the gamma-ray logs indicating the seafloor. Comparison of the LWD density logs and the core-derived densities in reference to the common datum of seafloor shows overall good correlations in Site UBGH2-1_1 (Fig. 3). Multi-channel seismic data across the drill sites were acquired in 2005 by the Korea Institute of Geoscience and Mineral Resources (KIGAM) on the R/V Tamhae II as part of the exploration of gas hydrate in the Ulleung Basin. A 240-channel (3000-m long) streamer recorded shots from a 1035-in3 (2000 psi) six air-gun array. The hydrophone group interval and shot spacing were 12.5 and 25 m, respectively, yielding 60-fold coverage. The sampling interval and recording length are 1 ms and 7 s, respectively. Main frequency of the seismic data was set on 60 Hz, yielding 10 m vertical seismic resolution at 2500 m/s seismic velocity. Matching of well-log characteristics with seismic reflections was performed by the following seismic-to-well tie procedures: (1) calibrate the sonic and density log for valid reflectivity series; (2) extract statistical wavelet; (3) construct synthetic seismogram from
Fig. 2. Seismic profile crossing the drill sites UBGH2-1_1 and UBGH2-1_2 superimposed with LWD gamma-ray logs. BSR ¼ bottom simulating reflector, MTD ¼ mass transport deposit.
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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Fig. 3. Summary of distributions of sedimentary facies, core and LWD log data from the Site UBGH2-1_1. From the left, each column represents the follows: 1) LWD log depth in meters below the seafloor (mbsf), 2) borehole plan for the coring holes UBGH2-1_1C and UBGH2-1_1D (D ¼ drilling, H ¼ Fugro Hydraulic Piston Corer, C ¼ Fugro Corer, R ¼ Fugro Rotary Pressure Corer, P ¼ Fugro Pressure Corer, T ¼ formation temperature), 3) core recovery (indicated in black), 4) mean grain size in reversed F scale, 5) sedimentary facies (for facies codes in color, see Table 2; arrows on the left indicate locations of thin tephra layers), 6) gamma-ray log (GR) (API ¼ American Petroleum Institute gamma ray unit), 7) density log (RHOB), 8) bulk density of cores from the gamma attenuation method (GRA) and the moisture and density measurements (MAD), 9) opal-A contents from XRD analysis, 10-13) Ti intergrals, Br/Ti and Ca/Ti intergrals ratios from the XRF scan. Blue cross-hatching indicates intervals of mass transport deposits. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the convolution of the well-log reflectivity series and initial wavelet; (4) correlate the seismogram from well log to tie the seismic data; (5) extract a new wavelet using the correlated well logs; (6) repeat the log correlation until the correlation is greater than 0.7 which is reliable ratio for most seismic data analysis procedures (Hampson-Russell, 2007). The correlations for the UBGH21_1 and UBGH2-1_2 are high (0.762 and 0.745, respectively). 3.3. Core data The retrieved cores were processed for bulk density measurements by a gamma attenuation method and X-ray imaging using Geotek's MSCL-S and MSCL-XCT systems, respectively. The core sections subsequently split into working and archive halves for visual core description, digital color imaging with Geotek's MSCL-CIS, and moisture and density (MAD) sampling which were done normally twice per each 1.5-m-long subsection with varying intervals. MAD measurements to determine moisture content,
grain density, bulk density, and porosity were conducted onboard following the methods of the ODP Technical Note 26 (Blum, 1997). Grain-size analysis was conducted onshore using a laserscattering particle analyzer (Microtrac S3500) for both MAD samples and squeeze cakes remaining after onboard pore-water extraction. Mineral compositions including opal-A were also analyzed onshore for selected MAD samples by X-ray powder diffraction (XRD) and a computer software (SIROQUANT) based on Rietveld quantification method. For the XRD analysis, the Philips X'pert MPD diffractometer was used with Cu anode in conditions of 40 kV and 20 mA. Concentration of selected elements, given in peak area integrals in X-ray spectra, were measured onshore using an ITRAX XRF core scanner with 2 cm step size, 10 s count times, 30e40 kV and 38e50 mA X-ray on the archive half core sections. The original XRF scan data were filtered with 5-point moving average after removing abnormal peaks due to the cracks of the sediment surface.
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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3.4. Stratigraphic correlation and constructing ageedepth relations For stratigraphic correlation between the log data, we used AnalySeries which can interactively adjust depth scale of the logs based on tie points set up by graphic correlations (Pailard et al., 1996). It also simultaneously provides correlation coefficients as a measure of similarity between the adjusted logs. To construct ageedepth relations, tie points were set up between logs with depth scale and time series logs or marine d18O records. Assuming constant sedimentation rate between the tie points (“Linage” option in AnalySeries), the original depth scale of a log was converted to age scale (Pailard et al., 1996). 4. Cyclic variations in the well logs In Site UBGH2-1_1, LWD gamma-ray and density logs show an exponential increase from the seafloor to about 10 mbsf, reflecting consolidation of the surface water-rich sediments (Fig. 3). Below this depth, the two LWD logs generally exhibit a coherent cyclic variation to the termination depth with each other within ranges from 17 to 119 API for gamma-ray log and from 1.1 to 1.8 g/cm3 for density log. Polynomial regression of the two log variables shows fairly high correlation (R2 ¼ 0.47) (Fig. 4A) and spectral analysis in depth scale demonstrates coherent periodicity in length between the two logs which seems conspicuous at 64.1 m, 32.6 m, 16.8 m, 13.3 m, and 8.8 m (Fig. 4B). In addition to the coherency within each site, the cyclic log variations are highly correlative between Sites UBGH2-1_1 and
UBGH2-1_2. Adjusting LWD log depths based on correlated gamma-ray log variations between the sites indicates that in most parts of the correlated intervals, the gamma-ray log values are nearly identical (Fig. 5). However, it is also obvious that in a few narrow intervals of ~23e33, 70e71.5, 84e86.5, 91.6e95.5, and 131e132 mbsf in Site UBGH2-1_1, there are significant deviations in the log values between the sites (Fig. 5). 5. Sediment properties responsible to the well-log variations 5.1. Sedimentary facies The core sediments from Site UBGH2-1_1 mostly consist of clayey silt with mean grain sizes from 7.8 to 5.64 (avg. 6.74) (Fig. 3), except a few intercalated tepthra layers of pumice lapilli and ash. They are classified into 4 sedimentary facies based on sedimentary structures observed from the digital color and X-ray images: bioturbated mud (BM), crudely laminated mud (CLM), mud-clast mud (MCM) and obliquely layered mud (OLM) (Table 2). Other than the four primary facies, deformed sediments due to the flow-in phenomenon during the piston coring are observed in the lower parts of some cores.
Table 2 Summary of interpretations of sedimentary facies. Facies
Color codesa
Interpretation
Mud-clast mud (MCM)
Hemipelagites formed under welloxygenated bottom water condition (Bahk et al., 2000) Hemipelagites formed under poorly oxygenated bottom water condition (Bahk et al., 2000) Slide/slump or debris flow deposits (Tripsanas et al., 2008)
Obliquely layered mud (OLM)
Slide/slump or debris flow deposits (Tripsanas et al., 2008)
Tephra (T)
Fallout tephra
Flow-in mud
Deformation during piston coring
Bioturbated mud (BM) Crudely laminated mud (CLM)
a
Fig. 4. (A) Correlation between gamma-ray (GR) and density (RHOB) log values from the Site UBGH2-1_1. (B) Power spectra of GR and RHOB logs in meter scale.
5
Color codes for sedimentary facies in the Fig. 3.
The BM facies predominates in the entire coring intervals and is generally characterized by mottling by circular and oval-shaped burrows (Figs. 3 and 6A). The facies is usually dark olive in color with a variation from dark brown (in the web version) to light gray (Fig. 6A). The CLM facies is characterized by poorly defined laminae without systematic vertical change in both clarity and thickness (Fig. 6A). The facies units are 1e43 cm in thickness and dark brown to light brown (in the web version) in color (Figs. 3 and 6A). They alternate with the BM with sharp to diffuse gradational boundaries (Figs. 3 and 6A). The MCM facies comprises variously colored mud clasts which were tightly welded to each other and range from 1 to 15 cm in diameters (Fig. 6B). The OLM facies is represented by olive to light grey mud which is characterized by inclined internal layers or bedding (Fig. 6C). Their occurrence is confined within discrete three zones of the entire cored interval: 33.0e33.4, 70.8e93.5, and 127.8e130.3 mbsf (Fig. 3). The BM and CLM facies had been suggested to be formed by hemipelagic sedimentation under relatively well and poorly oxygenated bottom-water conditions, respectively (Bahk et al., 2000). Preservation of lamination in the CLM facies was ascribed to suppressed bioturbating macrofauna under poor oxygenation
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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Fig. 6. Photographs of selected core sections showing typical examples of sedimentary facies. For facies codes in the right of photographs, see Table 2.
5.2. Facies distribution and sedimentary environment
Fig. 5. Graphic correlations between gamma-ray (GR) logs of the Sites UBGH2-1_1 and 2-1_2 with reference to the Site UBGH2-1_1 LWD log depth. Dotted lines indicate graphic tie points. Arrows indicate selected tie points with mbsf depths in the Site UBGH2-1_1. For complete list of the tie points, see Table A.1. Shading denotes intervals with significant deviations.
states (Bahk et al., 2000). The inclined or distorted internal beddings of OLM facies and the matrix-to clast-supported mud clasts of MCM facies have been reported from various kinds of mass transport deposits (MTDs) such as slides, slumps, and debris flow deposits and ascribed to internal shearing and disintegration during the mass transport processes (Piper et al., 1985; Tripsanas et al., 2008). In the Ulleung Basin, such MTDs were frequently identified in the cores from the southern slopes where acoustically transparent MTD lobes are widespread (Lee et al., 2004; Scholz et al., 2012).
Predominance of the BM facies intercalated with the CLM facie suggests that sediments recovered from Site UBGH2-1_1 were mostly deposited by hemipelagic settling in varying bottom-water oxygenation state and could be correlative with the Unit I of Tada (1994) which was identified from the sites of ODP Leg 127/128. The sporadic occurrence of the MCM and the OLM facies indicates that the normal hemipelagic settling was interrupted by downslope mass transport by submarine slope failures in certain periods. Such interruptions should be carefully assessed before applying the log variation to paleoceanographic and cyclostratigraphic studies. The MTDs are also identified in the seismic profile as acoustically chaotic units which wedge out upslope across Site UBGH2-1_1 in the intervals of 2.130e2.141 and 2.178e2.200 s TWT (Fig. 2). The MTD intervals in the seismic section correspond to 20.4e32.3 and 60.4e74.8 mbsf in depths based on the timeedepth relation and roughly coincide with the upper two occurrence zones of the facies MCM and OLM. Moreover, as Site UBGH2-1_2, 1.2 km upslope from Site UBGH2-1_1, seems devoid of acoustically chaotic MTD units in the seismic profile, the significant deviations of correlated LWD gamma-ray log values at Site UBGH2-1_1 from Site UBGH2-1_2 (Fig. 5) are attributed to interruption by MTDs. The significantly deviated intervals in LWD gamma-ray log correlations generally agree with those defined by the MCM and OLM facies and acoustically chaotic units (Figs. 2,3 and 5). The discrepancy in the MTD intervals defined by sedimentary, log, and seismic evidences is mainly attributed to resolution difference in each type of data. The air-gun multichannel seismic data with about 10 m vertical resolution could not identify thin MTD deposits interbedded with hemipelagic sediments. Considering the resolution difference, we adopted three MTD intervals to encompass both the MCM and OLM facies occurrences and the LWD gamma-ray log deviations: 24e34, 70e95, 127e132 mbsf (Fig. 3).
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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Fig. 7. Correlations between sediment compositions and density values from the Site UBGH2-1_1. (A) Ti intergrals and density (RHOB) log, (B) Br/Ti intergrals ratio and density (RHOB) log, (C) Ca/Ti intergrals ratio and density (RHOB) log, (D) biogenic opal contents (Opal-A) and bulk density measured by the moisture and density method (MAD).
5.3. Correlations between sediment composition and log variations Gamma-ray logs have been often used for estimation of sand to shale ratio in reservoir exploration on the assumption that gamma ray is largely controlled by abundance of radioactive clays (Rider, 1995). Density logs also can be related to sand to mud ratio because in unconsolidated marine sediments like the drilled interval of Site UBGH2-1_1, sandy sediments have lower porosity and higher density than muddy sediments (Kominz et al., 2011). The cyclic variations in the LWD gamma-ray and density logs at Site UBGH2-1_1, however, cannot be attributed to changes in sand to mud ratio because the recovered sediments are dominated by fine silt throughout the drilled interval with stable mean grain sizes mostly between 5.8 and 7.24 (Fig. 3). This implies that changes in sediment compositions other than grain size should be responsible to the cyclic variations in the LWD logs. To determine the most likely compositional changes that are responsible to the LWD log variations, we examined elemental compositions of Ti, Ca, and Br obtained by the XRF scan which have been adopted as representatives of detrital particles (Ti), biogenic calcites (Ca), and marine organic matters (Br) (Croudace et al., 2006; Thomson et al., 2006; Ziegler et al., 2008, Fig. 3). As Si associated with biogenic silica cannot be differentiated from those of silicate minerals by the XRF scan, we used opal-A contents from XRD analysis instead (Fig. 3). SEM images of selected opal-A-rich sediments from the UBGH2-1_1 revealed that they are dominated by intact or broken diatom frustules (Bahk et al., 2013). The measured XRF integrals of Br and Ca were divided by those of Ti to normalize effects from changes in terrigeneous flux (Fig. 3).
Regression of the Ti contents and the LWD density values shows a positive correlation (R2 ¼ 0.50) between the two variables, suggesting the variations in the LWD density are generally related to relative contribution of detrital particles over biogenic matters (Fig. 7A). Regression of the elemental contents representative of biogenic matters reveals that variations of Br/Ti ratio are highly negatively correlated with the LWD density values (R2 ¼ 0.59) while variations of Ca/Ti do not show significant correlations (Fig. 7B and C). Biogenic opal contents are highly variable between 13.7% and 55.9% and demonstrate high negative correlations with density values measured using MAD samples (Fig. 7D). The correlations between the elemental compositions and density values suggest that variations in the ratio of detrital to biogenic inputs which were associated with changes in fluxes of biogenic silica and organic matters are mainly responsible to the LWD density variations. Negative correlation between biogenic opal and sediment bulk density has been indicated in other drill sites in the East Sea such as Sites UBGH2-2_1, UBGH2-6 and ODP 798 (deMenocal et al., 1992; Bahk et al., 2013). As biogenic opal and organic matters are non-radioactive as well as low in density, variations in their contents may have caused coherent changes in both LWD gamma-ray and density logs. 6. Paleoceanographic implications and cyclostratigraphy of the well-log variations The East Sea is supposed to have received significant aeolian dust flux from the deserts and Loess Plateau in China. The present dust flux to the East Sea is estimated to be 0.7e4.3 g/cm2/ky, which
Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023
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Fig. 8. Graphic correlations between gamma-ray (GR) logs of the Sites UBGH2-1_2 and ODP 798 before (A) and after (B) depth-scale adjustment based on the tie points denoted by blue dotted lines. Orange curves are filtered gamma-ray logs with a moving average width of 6 m to aid the graphic correlations. For the Site ODP 798, logs from the upper cased hole above 90 mbsf were not used. Note that the horizontal axis scales of the gamma-ray logs were reversed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
is more than 30 times greater than the flux to the central North Pacific (0.0013e0.0453 g/cm2/ky) (Suzuki and Tsunogai, 1987). The past 530-ky sediment record from the northwest Pacific demonstrates that variations in aeolian dust flux are highly correlative to both marine d18O record and loess-soil stratigraphy in China, suggesting greater dust flux and source area aridity during glacial times (Hovan et al., 1989). Aeolian dust flux record reconstructed from the ODP Site 797 in the Yamato Basin of the East Sea also indicates higher values by a factor of 4 during glacial maximums of MIS 2 and 6.2 (Irino and Tada, 2002). Biogenic opal contents of the late Quaternary sediments in the East Sea also exhibit orbital-scale cyclicity with higher values more than a factor of 4 during interglacial maxima such as MIS 1 and 5.5 (Khim et al., 2007, 2008, 2012; Kido et al., 2007). The high biogenic opal contents generally accompany higher organic carbon contents and have been attributed to enhanced surface productivity and/or increased abundance of large warm-water diatoms which were associated with strong influx of the Tsushima Warm Current through the Korea Strait during the sea-level high stands (Khim et al., 2007, 2008, 2012; Kido et al., 2007). In contrast, the sediments during glacial maxima are generally characterized by minimal contents of both biogenic opal and organic carbon and have been attributed to reduced surface productivity during the sealevel low stands due to the developed density stratification in the water column (Khim et al., 2007, 2008, 2012; Kido et al., 2007). The orbital-scale cyclic variations of aeolian dust and biogenic opal contents revealed from the late Quaternary sediments in the
East Sea suggest that they should be one of the primary control factors which were responsible for the variations in the LWD logs from Sites UBGH2-1_1 and UBGH2-1_2. deMenocal et al. (1992) also found a striking similarity between the variations of the gamma-ray log from Site ODP 798 in the Oki Ridge and the marine d18O record from the North Atlantic during 1.0e2.8 Ma. They ascribed the similarity to climatically-modulated variations in aeolian dust and biogenic opal sedimentations in the East Sea. As paleomagnetic or biostratigraphic datums are currently not available from Site UBGH2-1_1, we tried graphic correlation with the gamma-ray logs from Site ODP 798 to establish cyclostratigraphy for the LWD logs. For the graphic correlation, the ODP 798 gammaray log between 94.7 and 291.8 mbsf, which was already fitted to about 1.0e2.6 Ma of the marine d18O record by deMenocal et al. (1992), was compared to the lower part of LWD gamma-ray log from Site UBGH2-1_2, assuming the same climatic modulations of sedimentation have generally worked in the both sites (Fig. 8). The significant correlation coefficient of 0.44 between the graphically fitted gamma-ray logs demonstrates that this assumption is valid for a first approximation, though considerable unexplained variance due to local differences in sedimentation also exists. The gamma-ray log of the UBGH2-1_2 was converted to time series by assigning ages to the tie points with the gamma-ray log of Site ODP 798 before 1.5 Ma and graphic tuning with the LR04 stack of global benthic d18O records after 1.5 Ma (deMenocal et al., 1992; Lisiecki and Raymo, 2005) (Figs. 8 and 9). Because deMenocal et al. (1992) deployed the marine d18O record from Site 607 in the North
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Fig. 9. Graphic correlations of gamma-ray logs from the Sites UBGH2-1_1, UBGH2-1_2, ODP 798, and LR04 stack of benthic d18O records (Lisiecki and Raymo, 2005) in age scale since 2.6 Ma. Note that the horizontal axis scales of the logs and LR04 stack were reversed. Dotted black lines indicate graphic tie points between the logs (see also Figs. 5 and 8), dotted red lines tie points with LR04 stack. Tie points in the logs of Sites UBGH2-1_1 and UBGH2-1_2 are available in the Table A.1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Atlantic to construct ageedepth relation of the gamma-ray log of Site ODP 798, we refined it with LR04 stack instead for this study. The correlation coefficients between the LR04 stack and time-series gamma-ray logs of Sites ODP 798 and UBGH2-1_2 since 2.6 Ma are 0.42 and 0.36, respectively. The time-series gamma-ray log for Site UBGH2-1_2 includes significant deviation from the LR04 stack in the uppermost part from the seafloor to ~15 mbsf, which was caused by exponential increase of gamma-ray log due to consolidation, regardless of climatic modulation of sedimentation (Fig. 9). If the uppermost part is excluded, the correlation coefficient increases to 0.43. Finally, the LWD gamma-ray log from Site UBGH21_1 was also converted to time series by assigning ages to the graphic tie points with that from Site UBGH2-1_2, based on the established age-scale for Site UBGH2-1_2 (Figs. 5 and 9). The results of the graphic correlations indicate that the LWD gamma-ray log
data from Sites UBGH2-1_2 and UBGH2-1_1 can go back to 2.54 Ma at 293 mbsf and 2.12 Ma at 269 mbsf with significant confidence, respectively. The ageedepth relations based on the tie points between each log show nearly identical linear sedimentation rates of 11.5 cm/ky for Sites ODP 798 and UBGH2-1_2 (Fig. 10). The linear sedimentation rate of Site UBGH2-1_1 has a bit higher value of 12.7 cm/ky, which is ascribed to more susceptibility of this site to downslope transport of reworked materials. The time-series gamma-ray logs from the Oki Ridge and this study also demonstrate that the orbital scale climatic modulation of hemipelagic sedimentation in the East Sea had been persistent throughout the Pleistocene (Fig. 9). In spite of the overall similarity, significant deviations remain in shorter terms between the time-series gamma-ray logs and LR04 stack of global benthic d18O records after graphic tuning (Fig. 9).
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of the ratios of terrigeneous to biogenic material inputs had been persistent throughout the Pleistocene in the East Sea and rates and changes of hemipelagic sedimentation in the southern (Site ODP 798) and the western (this study) margins of the East Sea had been nearly identical.
Table A. 1 Depth and age of graphic tie points in the gamma-ray logs of the Sites UBGH2-1_1 and UBGH2-1_2 since 2.6 Ma. Tie points for the Site UBGH2-1_1 were set up based on graphic correlation with the UBGH2-1_2 log and those for the Site UBGH2-1_2 were based on correlations with ODP 798 log before 1.5 Ma and with LR04 stack after 1.5 Ma.
Fig. 10. Age-depth relations for the Sites UBGH2-1_1, UBGH2-1_2, and ODP 798 based on the tie points in the Fig. 9.
Other than the deviation due to consolidation effect in the uppermost about 15 mbsf interval, they could be generally ascribed to time lags or modulation by sub-orbital scale variations in surface productivity and/or aeolian dust input. However, confirmation of the regional significance of such deviation and distinction of causes for each case seem to warrant further study of independent proxies for surface productivity and aeolian dust and comparison with other recent drilling results from the East Sea.
2-1_1 depth (mbsf)
Age (ka)
2-1_1 depth (mbsf)
Age (ka)
2-1_2 depth (mbsf)
Age (ka)
0.2 6.1 9.6 12.1 17.8 19.6 24.0 41.2 41.8 44.0 49.5 54.0 57.5 65.0 69.3 75.3 82.0 104.6 109.0 111.6 116.2 119.2 121.5 125.6 127.1 133.4 137.4 140.2 142.1 145.0 151.4 155.9 159.8 162.3
0.4 11.1 40.8 69.2 103.3 119.9 132.1 215.6 223.2 239.6 274.5 309.4 335.0 387.0 405.3 433.8 498.0 621.5 649.3 670.5 701.8 725.5 752.4 782.2 795.6 836.3 872.5 910.1 927.2 966.8 1042.7 1107.7 1143.0 1161.7
177.5 183.9 186.6 195.0 203.0 206.9 214.2 216.5 220.8 227.5 230.6 233.3 237.0 243.1 248.9 252.3 254.4 259.4 269.0
1281.5 1345.8 1389.4 1465.3 1513.4 1551.5 1598.9 1613.0 1635.0 1664.0 1691.9 1713.9 1749.4 1809.7 1864.4 1900.2 1935.2 2038.9 2116.4
5.4 12.6 17.5 29.6 43.0 53.6 59.0 69.3 87.1 115.4 125.3 130.2 133.4 150.0 163.2 176.1 189.6 207.3 215.7 225.1 235.5 246.0 279.9 286.6 293.1
11.1 82.0 123.0 221.9 342.3 434.2 540.2 631.2 800.8 1125.4 1198.1 1228.0 1247.2 1441.0 1515.8 1600.1 1664.4 1809.8 1897.5 2036.5 2115.1 2175.2 2406.7 2487.5 2543.5
7. Conclusions The LWD gamma-ray and density logs from Sites UBGH2-1_1 and UBGH2-1_2 in the western lower slope of the Ulleung Basin show coherent cyclic variations throughout the entire drilled intervals. Distribution of sedimentary facies, inter-site log variations, and comparison with seismic profile collectively indicate that the sediments are pre-dominated by hemipelagic mud, intercalated with a few sporadic MTD units only in the downslope site of UBGH2-1_1. Correlations of variations in the sediment compositions and the LWD logs reveal that the cyclicity in the LWD logs mainly represents climatic modulations of the ratios of terrigeneous to biogenic material inputs in a way that higher aeolian dust inputs and lower surface productivity prevailed during the glacial maximums and vice versa during the interglacial maximums. Based on the paleoceanographic implications of the LWD log variations, the LWD gamma-ray log from Site UBGH2-1_2 which is devoid of the MTDs was correlated with the LR04 stack of global benthic d18O records to establish the ageedepth relation. The correlation was underpinned by comparison with the gamma-ray log from Site ODP 798 which had been already fitted to marine d18O records from 1.0 to 2.8 Ma. The time-series LWD gamma-ray logs based on the established ageedepth relations indicate that climatic modulations
Acknowledgements Samples and data for this study were acquired by onboard and post-cruise analyses of the 2nd Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2), which was funded by Ministry of Knowledge Economy of Korea (MKE) under management of Gas Hydrate Research and Development Organization (GHDO) of Korea. We greatly thank the onboard scientific party and technical staff of UBGH2 for their support at sea. This research is a contribution of ‘Studies on Gas Hydrate Resource Assessment and Reservoir Characterization’ funded by Ministry of Science, ICT and Future Planning, Korea. This research is also a part of the project titled ‘International Ocean Discovery Program’ funded by the Ministry of Oceans and Fisheries, Korea. References Bahk, J.J., Chough, S.K., Han, S.J., 2000. Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan). Marine Geology 162, 459e477. Bahk, J.J., Kim, D.H., Chun, J.H., Son, B.K., Kim, J.H., Ryu, B.J., Torres, M.E., Riedel, M., Schultheiss, P., 2013. Gas hydrate occurrences and their relation to host
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Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea Jang-Jun Bahk*, In-Kwon Um, Bo-Yeon Yi, Dong-Geun Yoo Petroleum and Marine Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, 305-350, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
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Cyclic variations of density and gamma-ray logs achieved from the two drill sites of UBGH2-1_1 and UBGH2-1_2 in the western slope of the Ulleung Basin were investigated to reveal their paleoceanographic implications and establish cyclostratigraphy of the well logs. Integration of the core, log, and seismic data from the two sites indicates that the sediments of the drilled intervals mostly consist of hemipelagic muds, with a few sporadic mass transport deposits intercalated only in the downslope site of UBGH2-1_1. Comparison with variations in sediment composition suggests that ratios of terrigeneous to biogenic materials which were modulated by orbital-scale climate changes should be responsible to the well-log variations. The established cyclostratigraphy of the well logs based on correlation with marine oxygen isotope records indicates such climatic modulation of the sedimentation in the western slope of the Ulleung Basin has persisted at least since 2.6 Ma, consistent with the variation reported from the Oki Ridge. © 2015 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Paleoceanography Cyclostratigraphy Well logs Ulleung Basin East Sea
1. Introduction In addition to oil and gas reservoir characterization, well logs of marine sediment sequences have been widely applied to paleoceanographic and cyclostratigraphic studies (e.g., deMenocal et al., s et al., 1999; Gorgas and Wilkens, 2002; Paulissen and 1992; Barthe Luthi, 2011) because most of the well logs are primary affected by sediment porosity that are closely related to mineralogical and grain-size compositions which in turn can vary significantly with cyclic changes of climate and sea-level (Goldberg, 1997). Well-logs are particularly useful for time-series analysis to reveal orbital and sub-orbital cyclicity by providing continuous data with uniform sampling rates, which are often impossible or hard to achieve with discrete core sections. However, before applying the well logs to paleoceanographic and cyclostratigraphic studies, site-specific relations between log variables and sediment properties should be clarified by correlation of core and log data. Moreover, presence of event beds and significant hiatus or changes in sedimentation rates has to be examined to ensure consistency in environmental
* Corresponding author. E-mail address: [email protected] (J.-J. Bahk).
conditions and relatively uniform thicknessetime relationship (Weedon, 2003). Density and gamma-ray logs achieved by logging-while-drilling (LWD) from the two drill sites in the western lower slope of the Ulleung Basin during the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) show fairly consistent cyclic variations with each other in most of the log intervals (Ryu et al., 2012). To reveal paleoceanographic implications of the cyclic variations of the logs and establish cyclostratigraphic framework, we first examined which sediment properties are responsible for the cyclic variations of the logs using available core data. Comparisons of the logs and the core data with the seismic section across the drill sites provided further control of the intervals suspected as event beds such as mass transport deposits. The gamma-ray logs devoid of the event beds were converted to time series mainly based on correlations with the well logs from Site ODP 798 from the Oki Ridge which have established ageedepth relations before 1 Ma and LR04 stack of global benthic d18O records (deMenocal et al., 1992; Lisiecki and Raymo, 2005). The time-series gamma-ray logs evidence that, for the first time, orbital-scale cyclic modulation of ratios of terrigeneous to biogenic materials had been persistent during the entire Pleistocene and rates and changes of hemipelagic sedimentation in the western and the southern margins the East Sea had been nearly identical.
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2. Geological and oceanographic settings The East Sea (Japan Sea) is a semi-enclosed marginal sea that is connected to the North Pacific and adjacent seas through four shallow and narrow straits (water depths of 12e140 m; Fig. 1). The Tsushima Warm Current, a branch of the warm saline Kuroshio Current, enters the East Sea through the Korea Strait and flows out through the Tsugaru and Soya straits. A cold (<1 C), highly oxygenated (5e6 ml/l) water mass that originates from winter cooling of surface water in the northern part occurs below the water depths of 200e300 m (Kawamura and Wu, 1998; Kim et al., 2002). Significant changes in oceanographic conditions have been expected in response to the glacio-eustatic sea-level fluctuations which resulted in a sea-level drop of as much as about 120 m during the last glacial maximum. Tada et al. (1999) suggested that the modulation of the volume and character of the surface water inflow through the Korea Strait associated with glacio-eustatic sea level changes played a key role in the changes in deep-water ventilation and surface productivity. According to their hypothesis, the East Sea
was nearly isolated by a sea-level drop of more than 90 m during the glacial maximum periods. During such periods, low-salinity surface water developed because of excess precipitation and caused strong density stratification of the water column, resulting in euxinic deep water and low surface productivity (Tada et al., 1999). Together with the changes in oceanic circulations and surface productivity, variations in influx of Asian aeolian dust to the East Sea also have been suggested in both orbital and sub-orbital timescales (Irino and Tada, 2002; Nagashima et al., 2007). The Ulleung Basin in the southwestern part of the East Sea was opened in the Late Oligocene to Early Miocene by crustal extension accompanied with southward movement of SW Japanese Islands (Tamaki et al., 1992; Yoon and Chough, 1995). At the end of the Middle Miocene (11e12 Ma), basin closure was caused by the northward collision of the Bonnic Arc with central Japan (Chough and Barg, 1987). The basin closure led to uplift of the southern margin and basin-wide deposition of mass transport deposits (MTDs) which were evolved from frequent slope failures along the southern slope during the latest Neogene (Lee and Suk, 1988; Lee
Fig. 1. Bathymetric map of the Ulleung Basin (contours in meter), showing the locations of drill sites UBGH2-1_1 and UBGH2-1_2. Also shown is the location of ODP Site 798 and shallow sills and straits of the East Sea in the inset. Box in the inset indicates the location of the enlarged area. Modified from Lee et al. (2004).
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et al., 2001). Since the Pleistocene, the deeper parts of the basin have been dominated by distal turbidites and hemipelagites as the depocenter of the MTDs rapidly retreated to the southern margin (Lee and Suk, 1988; Lee et al., 2001). The presence of gas hydrates in the Ulleung Basin has been suggested based on seismic indicators, such as bottom simulating reflectors (BSR), seismic chimneys, high amplitude reflections within the gas hydrate stability zone (GHSZ), acoustic blanking, enhanced reflections below the BSR, and seafloor gas-escape features (Yoo et al., 2013 and references therein). During the UBGH2, Sites UBGH2-1_1 and UBGH2-1_2 in the western lower slope of the basin were drilled to test enhanced dipping reflections below the BSR (Fig. 2; Ryu et al., 2012). In Site UBGH2-1_1, both LWD and coring were conducted, while in Site UBGH2-1_2, 1.2 km apart from the UBGH2-1_1 to the upslope, only LWD well logs were achieved (Fig. 2; Table 1). Unlike most other UBGH2 drill sites from the basin plain, the recovered sediments from Site UBGH2-1_1 are generally devoid of reworked sediments other than in a few limited intervals of MTDs, making them feasible for further paleoceanographic studies. In this paper, we focus on cyclostratigraphic and paleoceanographic implications of the well-log and core data from the two sites. Gas hydrate occurrences in the cores from Site UBGH21_1 were already detailed in Bahk et al. (2013) and evaluation of gas hydrate petroleum systems by integration of core, log and seismic data will be dealt with elsewhere.
Table 1 Summary of LWD and coring holes in the Sites UBGH2-1_1 and UBGH2-1_2. Site
Phase
Hole
Latitude Longitude
36 150 04.400 N 130 030 56.500 E Coring UBGH2_1_1B 36 150 04.800 N 130 030 56.500 E UBGH2-1_1C 36 150 04.800 N 130 030 56.600 E UBGH2-1_1D 36 150 04.800 N 130 030 56.500 E UBGH2-1_2 LWD UBGH2_1_2A 36 150 04.400 N 130 030 10.300 E
UBGH2-1_1 LWD
UBGH2-1_1A
Water TD (mbsf) depth (mbss) 1534
281
1529
8
1529
8
1529
216
1499
380
The water depths were measured with the length of seafloor touching drill pipe; mbss ¼ meter below sea-surface; mbsf ¼ meter below seafloor; TD ¼ termination depth.
3
3. Material and methods 3.1. LWD and coring operations The UBGH2 consists of the first LWD phase and the second coring phase (Ryu et al., 2012). The LWD was conducted using Schlumberger's GeoVision, TeleScope, EcoScope and SonicVision to the termination depths of 281 and 380 m below seafloor (mbsf) at Sites UBGH2-1_1 and UBGH2-1_2 (Table 1). In Site UBGH2-1_1, coring was performed at three holes UBGH2-1_1B, UBGH2-1_1C, and UBGH2-1_1D that are located approximately 10 m north from the LWD hole UBGH2-1_1A (Table 1). To retrieve undisturbed core sediments near the seafloor, coring tools were deployed from about 2 m above the seafloor at the coring holes of UBGH2-1_1B and UBGH2-1_1C, and then landed on the seafloor at the UBGH2-1_1D hole to continuously core from 2 to 216 mbsf (Ryu et al., 2012). The coring was conducted using Fugro Hydraulic Piston Corer, Fugro Corer, and Fugro Rotary Corer for non-pressurized cores and Fugro Pressure Corer and Fugro Rotary Pressure Corer for pressurized cores. 3.2. LWD data and matching with seismic reflections For characterization of changes in sediment porosity and composition, we chose gamma-ray and density logs of the EcoScope which were sampled at 6 in intervals. For the matching of the depths between core and log data, the datum of LWD log depth was shifted from the rig floor to the seafloor, identifying the step changes in the gamma-ray logs indicating the seafloor. Comparison of the LWD density logs and the core-derived densities in reference to the common datum of seafloor shows overall good correlations in Site UBGH2-1_1 (Fig. 3). Multi-channel seismic data across the drill sites were acquired in 2005 by the Korea Institute of Geoscience and Mineral Resources (KIGAM) on the R/V Tamhae II as part of the exploration of gas hydrate in the Ulleung Basin. A 240-channel (3000-m long) streamer recorded shots from a 1035-in3 (2000 psi) six air-gun array. The hydrophone group interval and shot spacing were 12.5 and 25 m, respectively, yielding 60-fold coverage. The sampling interval and recording length are 1 ms and 7 s, respectively. Main frequency of the seismic data was set on 60 Hz, yielding 10 m vertical seismic resolution at 2500 m/s seismic velocity. Matching of well-log characteristics with seismic reflections was performed by the following seismic-to-well tie procedures: (1) calibrate the sonic and density log for valid reflectivity series; (2) extract statistical wavelet; (3) construct synthetic seismogram from
Fig. 2. Seismic profile crossing the drill sites UBGH2-1_1 and UBGH2-1_2 superimposed with LWD gamma-ray logs. BSR ¼ bottom simulating reflector, MTD ¼ mass transport deposit.
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Fig. 3. Summary of distributions of sedimentary facies, core and LWD log data from the Site UBGH2-1_1. From the left, each column represents the follows: 1) LWD log depth in meters below the seafloor (mbsf), 2) borehole plan for the coring holes UBGH2-1_1C and UBGH2-1_1D (D ¼ drilling, H ¼ Fugro Hydraulic Piston Corer, C ¼ Fugro Corer, R ¼ Fugro Rotary Pressure Corer, P ¼ Fugro Pressure Corer, T ¼ formation temperature), 3) core recovery (indicated in black), 4) mean grain size in reversed F scale, 5) sedimentary facies (for facies codes in color, see Table 2; arrows on the left indicate locations of thin tephra layers), 6) gamma-ray log (GR) (API ¼ American Petroleum Institute gamma ray unit), 7) density log (RHOB), 8) bulk density of cores from the gamma attenuation method (GRA) and the moisture and density measurements (MAD), 9) opal-A contents from XRD analysis, 10-13) Ti intergrals, Br/Ti and Ca/Ti intergrals ratios from the XRF scan. Blue cross-hatching indicates intervals of mass transport deposits. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the convolution of the well-log reflectivity series and initial wavelet; (4) correlate the seismogram from well log to tie the seismic data; (5) extract a new wavelet using the correlated well logs; (6) repeat the log correlation until the correlation is greater than 0.7 which is reliable ratio for most seismic data analysis procedures (Hampson-Russell, 2007). The correlations for the UBGH21_1 and UBGH2-1_2 are high (0.762 and 0.745, respectively). 3.3. Core data The retrieved cores were processed for bulk density measurements by a gamma attenuation method and X-ray imaging using Geotek's MSCL-S and MSCL-XCT systems, respectively. The core sections subsequently split into working and archive halves for visual core description, digital color imaging with Geotek's MSCL-CIS, and moisture and density (MAD) sampling which were done normally twice per each 1.5-m-long subsection with varying intervals. MAD measurements to determine moisture content,
grain density, bulk density, and porosity were conducted onboard following the methods of the ODP Technical Note 26 (Blum, 1997). Grain-size analysis was conducted onshore using a laserscattering particle analyzer (Microtrac S3500) for both MAD samples and squeeze cakes remaining after onboard pore-water extraction. Mineral compositions including opal-A were also analyzed onshore for selected MAD samples by X-ray powder diffraction (XRD) and a computer software (SIROQUANT) based on Rietveld quantification method. For the XRD analysis, the Philips X'pert MPD diffractometer was used with Cu anode in conditions of 40 kV and 20 mA. Concentration of selected elements, given in peak area integrals in X-ray spectra, were measured onshore using an ITRAX XRF core scanner with 2 cm step size, 10 s count times, 30e40 kV and 38e50 mA X-ray on the archive half core sections. The original XRF scan data were filtered with 5-point moving average after removing abnormal peaks due to the cracks of the sediment surface.
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3.4. Stratigraphic correlation and constructing ageedepth relations For stratigraphic correlation between the log data, we used AnalySeries which can interactively adjust depth scale of the logs based on tie points set up by graphic correlations (Pailard et al., 1996). It also simultaneously provides correlation coefficients as a measure of similarity between the adjusted logs. To construct ageedepth relations, tie points were set up between logs with depth scale and time series logs or marine d18O records. Assuming constant sedimentation rate between the tie points (“Linage” option in AnalySeries), the original depth scale of a log was converted to age scale (Pailard et al., 1996). 4. Cyclic variations in the well logs In Site UBGH2-1_1, LWD gamma-ray and density logs show an exponential increase from the seafloor to about 10 mbsf, reflecting consolidation of the surface water-rich sediments (Fig. 3). Below this depth, the two LWD logs generally exhibit a coherent cyclic variation to the termination depth with each other within ranges from 17 to 119 API for gamma-ray log and from 1.1 to 1.8 g/cm3 for density log. Polynomial regression of the two log variables shows fairly high correlation (R2 ¼ 0.47) (Fig. 4A) and spectral analysis in depth scale demonstrates coherent periodicity in length between the two logs which seems conspicuous at 64.1 m, 32.6 m, 16.8 m, 13.3 m, and 8.8 m (Fig. 4B). In addition to the coherency within each site, the cyclic log variations are highly correlative between Sites UBGH2-1_1 and
UBGH2-1_2. Adjusting LWD log depths based on correlated gamma-ray log variations between the sites indicates that in most parts of the correlated intervals, the gamma-ray log values are nearly identical (Fig. 5). However, it is also obvious that in a few narrow intervals of ~23e33, 70e71.5, 84e86.5, 91.6e95.5, and 131e132 mbsf in Site UBGH2-1_1, there are significant deviations in the log values between the sites (Fig. 5). 5. Sediment properties responsible to the well-log variations 5.1. Sedimentary facies The core sediments from Site UBGH2-1_1 mostly consist of clayey silt with mean grain sizes from 7.8 to 5.64 (avg. 6.74) (Fig. 3), except a few intercalated tepthra layers of pumice lapilli and ash. They are classified into 4 sedimentary facies based on sedimentary structures observed from the digital color and X-ray images: bioturbated mud (BM), crudely laminated mud (CLM), mud-clast mud (MCM) and obliquely layered mud (OLM) (Table 2). Other than the four primary facies, deformed sediments due to the flow-in phenomenon during the piston coring are observed in the lower parts of some cores.
Table 2 Summary of interpretations of sedimentary facies. Facies
Color codesa
Interpretation
Mud-clast mud (MCM)
Hemipelagites formed under welloxygenated bottom water condition (Bahk et al., 2000) Hemipelagites formed under poorly oxygenated bottom water condition (Bahk et al., 2000) Slide/slump or debris flow deposits (Tripsanas et al., 2008)
Obliquely layered mud (OLM)
Slide/slump or debris flow deposits (Tripsanas et al., 2008)
Tephra (T)
Fallout tephra
Flow-in mud
Deformation during piston coring
Bioturbated mud (BM) Crudely laminated mud (CLM)
a
Fig. 4. (A) Correlation between gamma-ray (GR) and density (RHOB) log values from the Site UBGH2-1_1. (B) Power spectra of GR and RHOB logs in meter scale.
5
Color codes for sedimentary facies in the Fig. 3.
The BM facies predominates in the entire coring intervals and is generally characterized by mottling by circular and oval-shaped burrows (Figs. 3 and 6A). The facies is usually dark olive in color with a variation from dark brown (in the web version) to light gray (Fig. 6A). The CLM facies is characterized by poorly defined laminae without systematic vertical change in both clarity and thickness (Fig. 6A). The facies units are 1e43 cm in thickness and dark brown to light brown (in the web version) in color (Figs. 3 and 6A). They alternate with the BM with sharp to diffuse gradational boundaries (Figs. 3 and 6A). The MCM facies comprises variously colored mud clasts which were tightly welded to each other and range from 1 to 15 cm in diameters (Fig. 6B). The OLM facies is represented by olive to light grey mud which is characterized by inclined internal layers or bedding (Fig. 6C). Their occurrence is confined within discrete three zones of the entire cored interval: 33.0e33.4, 70.8e93.5, and 127.8e130.3 mbsf (Fig. 3). The BM and CLM facies had been suggested to be formed by hemipelagic sedimentation under relatively well and poorly oxygenated bottom-water conditions, respectively (Bahk et al., 2000). Preservation of lamination in the CLM facies was ascribed to suppressed bioturbating macrofauna under poor oxygenation
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Fig. 6. Photographs of selected core sections showing typical examples of sedimentary facies. For facies codes in the right of photographs, see Table 2.
5.2. Facies distribution and sedimentary environment
Fig. 5. Graphic correlations between gamma-ray (GR) logs of the Sites UBGH2-1_1 and 2-1_2 with reference to the Site UBGH2-1_1 LWD log depth. Dotted lines indicate graphic tie points. Arrows indicate selected tie points with mbsf depths in the Site UBGH2-1_1. For complete list of the tie points, see Table A.1. Shading denotes intervals with significant deviations.
states (Bahk et al., 2000). The inclined or distorted internal beddings of OLM facies and the matrix-to clast-supported mud clasts of MCM facies have been reported from various kinds of mass transport deposits (MTDs) such as slides, slumps, and debris flow deposits and ascribed to internal shearing and disintegration during the mass transport processes (Piper et al., 1985; Tripsanas et al., 2008). In the Ulleung Basin, such MTDs were frequently identified in the cores from the southern slopes where acoustically transparent MTD lobes are widespread (Lee et al., 2004; Scholz et al., 2012).
Predominance of the BM facies intercalated with the CLM facie suggests that sediments recovered from Site UBGH2-1_1 were mostly deposited by hemipelagic settling in varying bottom-water oxygenation state and could be correlative with the Unit I of Tada (1994) which was identified from the sites of ODP Leg 127/128. The sporadic occurrence of the MCM and the OLM facies indicates that the normal hemipelagic settling was interrupted by downslope mass transport by submarine slope failures in certain periods. Such interruptions should be carefully assessed before applying the log variation to paleoceanographic and cyclostratigraphic studies. The MTDs are also identified in the seismic profile as acoustically chaotic units which wedge out upslope across Site UBGH2-1_1 in the intervals of 2.130e2.141 and 2.178e2.200 s TWT (Fig. 2). The MTD intervals in the seismic section correspond to 20.4e32.3 and 60.4e74.8 mbsf in depths based on the timeedepth relation and roughly coincide with the upper two occurrence zones of the facies MCM and OLM. Moreover, as Site UBGH2-1_2, 1.2 km upslope from Site UBGH2-1_1, seems devoid of acoustically chaotic MTD units in the seismic profile, the significant deviations of correlated LWD gamma-ray log values at Site UBGH2-1_1 from Site UBGH2-1_2 (Fig. 5) are attributed to interruption by MTDs. The significantly deviated intervals in LWD gamma-ray log correlations generally agree with those defined by the MCM and OLM facies and acoustically chaotic units (Figs. 2,3 and 5). The discrepancy in the MTD intervals defined by sedimentary, log, and seismic evidences is mainly attributed to resolution difference in each type of data. The air-gun multichannel seismic data with about 10 m vertical resolution could not identify thin MTD deposits interbedded with hemipelagic sediments. Considering the resolution difference, we adopted three MTD intervals to encompass both the MCM and OLM facies occurrences and the LWD gamma-ray log deviations: 24e34, 70e95, 127e132 mbsf (Fig. 3).
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Fig. 7. Correlations between sediment compositions and density values from the Site UBGH2-1_1. (A) Ti intergrals and density (RHOB) log, (B) Br/Ti intergrals ratio and density (RHOB) log, (C) Ca/Ti intergrals ratio and density (RHOB) log, (D) biogenic opal contents (Opal-A) and bulk density measured by the moisture and density method (MAD).
5.3. Correlations between sediment composition and log variations Gamma-ray logs have been often used for estimation of sand to shale ratio in reservoir exploration on the assumption that gamma ray is largely controlled by abundance of radioactive clays (Rider, 1995). Density logs also can be related to sand to mud ratio because in unconsolidated marine sediments like the drilled interval of Site UBGH2-1_1, sandy sediments have lower porosity and higher density than muddy sediments (Kominz et al., 2011). The cyclic variations in the LWD gamma-ray and density logs at Site UBGH2-1_1, however, cannot be attributed to changes in sand to mud ratio because the recovered sediments are dominated by fine silt throughout the drilled interval with stable mean grain sizes mostly between 5.8 and 7.24 (Fig. 3). This implies that changes in sediment compositions other than grain size should be responsible to the cyclic variations in the LWD logs. To determine the most likely compositional changes that are responsible to the LWD log variations, we examined elemental compositions of Ti, Ca, and Br obtained by the XRF scan which have been adopted as representatives of detrital particles (Ti), biogenic calcites (Ca), and marine organic matters (Br) (Croudace et al., 2006; Thomson et al., 2006; Ziegler et al., 2008, Fig. 3). As Si associated with biogenic silica cannot be differentiated from those of silicate minerals by the XRF scan, we used opal-A contents from XRD analysis instead (Fig. 3). SEM images of selected opal-A-rich sediments from the UBGH2-1_1 revealed that they are dominated by intact or broken diatom frustules (Bahk et al., 2013). The measured XRF integrals of Br and Ca were divided by those of Ti to normalize effects from changes in terrigeneous flux (Fig. 3).
Regression of the Ti contents and the LWD density values shows a positive correlation (R2 ¼ 0.50) between the two variables, suggesting the variations in the LWD density are generally related to relative contribution of detrital particles over biogenic matters (Fig. 7A). Regression of the elemental contents representative of biogenic matters reveals that variations of Br/Ti ratio are highly negatively correlated with the LWD density values (R2 ¼ 0.59) while variations of Ca/Ti do not show significant correlations (Fig. 7B and C). Biogenic opal contents are highly variable between 13.7% and 55.9% and demonstrate high negative correlations with density values measured using MAD samples (Fig. 7D). The correlations between the elemental compositions and density values suggest that variations in the ratio of detrital to biogenic inputs which were associated with changes in fluxes of biogenic silica and organic matters are mainly responsible to the LWD density variations. Negative correlation between biogenic opal and sediment bulk density has been indicated in other drill sites in the East Sea such as Sites UBGH2-2_1, UBGH2-6 and ODP 798 (deMenocal et al., 1992; Bahk et al., 2013). As biogenic opal and organic matters are non-radioactive as well as low in density, variations in their contents may have caused coherent changes in both LWD gamma-ray and density logs. 6. Paleoceanographic implications and cyclostratigraphy of the well-log variations The East Sea is supposed to have received significant aeolian dust flux from the deserts and Loess Plateau in China. The present dust flux to the East Sea is estimated to be 0.7e4.3 g/cm2/ky, which
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Fig. 8. Graphic correlations between gamma-ray (GR) logs of the Sites UBGH2-1_2 and ODP 798 before (A) and after (B) depth-scale adjustment based on the tie points denoted by blue dotted lines. Orange curves are filtered gamma-ray logs with a moving average width of 6 m to aid the graphic correlations. For the Site ODP 798, logs from the upper cased hole above 90 mbsf were not used. Note that the horizontal axis scales of the gamma-ray logs were reversed. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
is more than 30 times greater than the flux to the central North Pacific (0.0013e0.0453 g/cm2/ky) (Suzuki and Tsunogai, 1987). The past 530-ky sediment record from the northwest Pacific demonstrates that variations in aeolian dust flux are highly correlative to both marine d18O record and loess-soil stratigraphy in China, suggesting greater dust flux and source area aridity during glacial times (Hovan et al., 1989). Aeolian dust flux record reconstructed from the ODP Site 797 in the Yamato Basin of the East Sea also indicates higher values by a factor of 4 during glacial maximums of MIS 2 and 6.2 (Irino and Tada, 2002). Biogenic opal contents of the late Quaternary sediments in the East Sea also exhibit orbital-scale cyclicity with higher values more than a factor of 4 during interglacial maxima such as MIS 1 and 5.5 (Khim et al., 2007, 2008, 2012; Kido et al., 2007). The high biogenic opal contents generally accompany higher organic carbon contents and have been attributed to enhanced surface productivity and/or increased abundance of large warm-water diatoms which were associated with strong influx of the Tsushima Warm Current through the Korea Strait during the sea-level high stands (Khim et al., 2007, 2008, 2012; Kido et al., 2007). In contrast, the sediments during glacial maxima are generally characterized by minimal contents of both biogenic opal and organic carbon and have been attributed to reduced surface productivity during the sealevel low stands due to the developed density stratification in the water column (Khim et al., 2007, 2008, 2012; Kido et al., 2007). The orbital-scale cyclic variations of aeolian dust and biogenic opal contents revealed from the late Quaternary sediments in the
East Sea suggest that they should be one of the primary control factors which were responsible for the variations in the LWD logs from Sites UBGH2-1_1 and UBGH2-1_2. deMenocal et al. (1992) also found a striking similarity between the variations of the gamma-ray log from Site ODP 798 in the Oki Ridge and the marine d18O record from the North Atlantic during 1.0e2.8 Ma. They ascribed the similarity to climatically-modulated variations in aeolian dust and biogenic opal sedimentations in the East Sea. As paleomagnetic or biostratigraphic datums are currently not available from Site UBGH2-1_1, we tried graphic correlation with the gamma-ray logs from Site ODP 798 to establish cyclostratigraphy for the LWD logs. For the graphic correlation, the ODP 798 gammaray log between 94.7 and 291.8 mbsf, which was already fitted to about 1.0e2.6 Ma of the marine d18O record by deMenocal et al. (1992), was compared to the lower part of LWD gamma-ray log from Site UBGH2-1_2, assuming the same climatic modulations of sedimentation have generally worked in the both sites (Fig. 8). The significant correlation coefficient of 0.44 between the graphically fitted gamma-ray logs demonstrates that this assumption is valid for a first approximation, though considerable unexplained variance due to local differences in sedimentation also exists. The gamma-ray log of the UBGH2-1_2 was converted to time series by assigning ages to the tie points with the gamma-ray log of Site ODP 798 before 1.5 Ma and graphic tuning with the LR04 stack of global benthic d18O records after 1.5 Ma (deMenocal et al., 1992; Lisiecki and Raymo, 2005) (Figs. 8 and 9). Because deMenocal et al. (1992) deployed the marine d18O record from Site 607 in the North
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Fig. 9. Graphic correlations of gamma-ray logs from the Sites UBGH2-1_1, UBGH2-1_2, ODP 798, and LR04 stack of benthic d18O records (Lisiecki and Raymo, 2005) in age scale since 2.6 Ma. Note that the horizontal axis scales of the logs and LR04 stack were reversed. Dotted black lines indicate graphic tie points between the logs (see also Figs. 5 and 8), dotted red lines tie points with LR04 stack. Tie points in the logs of Sites UBGH2-1_1 and UBGH2-1_2 are available in the Table A.1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Atlantic to construct ageedepth relation of the gamma-ray log of Site ODP 798, we refined it with LR04 stack instead for this study. The correlation coefficients between the LR04 stack and time-series gamma-ray logs of Sites ODP 798 and UBGH2-1_2 since 2.6 Ma are 0.42 and 0.36, respectively. The time-series gamma-ray log for Site UBGH2-1_2 includes significant deviation from the LR04 stack in the uppermost part from the seafloor to ~15 mbsf, which was caused by exponential increase of gamma-ray log due to consolidation, regardless of climatic modulation of sedimentation (Fig. 9). If the uppermost part is excluded, the correlation coefficient increases to 0.43. Finally, the LWD gamma-ray log from Site UBGH21_1 was also converted to time series by assigning ages to the graphic tie points with that from Site UBGH2-1_2, based on the established age-scale for Site UBGH2-1_2 (Figs. 5 and 9). The results of the graphic correlations indicate that the LWD gamma-ray log
data from Sites UBGH2-1_2 and UBGH2-1_1 can go back to 2.54 Ma at 293 mbsf and 2.12 Ma at 269 mbsf with significant confidence, respectively. The ageedepth relations based on the tie points between each log show nearly identical linear sedimentation rates of 11.5 cm/ky for Sites ODP 798 and UBGH2-1_2 (Fig. 10). The linear sedimentation rate of Site UBGH2-1_1 has a bit higher value of 12.7 cm/ky, which is ascribed to more susceptibility of this site to downslope transport of reworked materials. The time-series gamma-ray logs from the Oki Ridge and this study also demonstrate that the orbital scale climatic modulation of hemipelagic sedimentation in the East Sea had been persistent throughout the Pleistocene (Fig. 9). In spite of the overall similarity, significant deviations remain in shorter terms between the time-series gamma-ray logs and LR04 stack of global benthic d18O records after graphic tuning (Fig. 9).
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of the ratios of terrigeneous to biogenic material inputs had been persistent throughout the Pleistocene in the East Sea and rates and changes of hemipelagic sedimentation in the southern (Site ODP 798) and the western (this study) margins of the East Sea had been nearly identical.
Table A. 1 Depth and age of graphic tie points in the gamma-ray logs of the Sites UBGH2-1_1 and UBGH2-1_2 since 2.6 Ma. Tie points for the Site UBGH2-1_1 were set up based on graphic correlation with the UBGH2-1_2 log and those for the Site UBGH2-1_2 were based on correlations with ODP 798 log before 1.5 Ma and with LR04 stack after 1.5 Ma.
Fig. 10. Age-depth relations for the Sites UBGH2-1_1, UBGH2-1_2, and ODP 798 based on the tie points in the Fig. 9.
Other than the deviation due to consolidation effect in the uppermost about 15 mbsf interval, they could be generally ascribed to time lags or modulation by sub-orbital scale variations in surface productivity and/or aeolian dust input. However, confirmation of the regional significance of such deviation and distinction of causes for each case seem to warrant further study of independent proxies for surface productivity and aeolian dust and comparison with other recent drilling results from the East Sea.
2-1_1 depth (mbsf)
Age (ka)
2-1_1 depth (mbsf)
Age (ka)
2-1_2 depth (mbsf)
Age (ka)
0.2 6.1 9.6 12.1 17.8 19.6 24.0 41.2 41.8 44.0 49.5 54.0 57.5 65.0 69.3 75.3 82.0 104.6 109.0 111.6 116.2 119.2 121.5 125.6 127.1 133.4 137.4 140.2 142.1 145.0 151.4 155.9 159.8 162.3
0.4 11.1 40.8 69.2 103.3 119.9 132.1 215.6 223.2 239.6 274.5 309.4 335.0 387.0 405.3 433.8 498.0 621.5 649.3 670.5 701.8 725.5 752.4 782.2 795.6 836.3 872.5 910.1 927.2 966.8 1042.7 1107.7 1143.0 1161.7
177.5 183.9 186.6 195.0 203.0 206.9 214.2 216.5 220.8 227.5 230.6 233.3 237.0 243.1 248.9 252.3 254.4 259.4 269.0
1281.5 1345.8 1389.4 1465.3 1513.4 1551.5 1598.9 1613.0 1635.0 1664.0 1691.9 1713.9 1749.4 1809.7 1864.4 1900.2 1935.2 2038.9 2116.4
5.4 12.6 17.5 29.6 43.0 53.6 59.0 69.3 87.1 115.4 125.3 130.2 133.4 150.0 163.2 176.1 189.6 207.3 215.7 225.1 235.5 246.0 279.9 286.6 293.1
11.1 82.0 123.0 221.9 342.3 434.2 540.2 631.2 800.8 1125.4 1198.1 1228.0 1247.2 1441.0 1515.8 1600.1 1664.4 1809.8 1897.5 2036.5 2115.1 2175.2 2406.7 2487.5 2543.5
7. Conclusions The LWD gamma-ray and density logs from Sites UBGH2-1_1 and UBGH2-1_2 in the western lower slope of the Ulleung Basin show coherent cyclic variations throughout the entire drilled intervals. Distribution of sedimentary facies, inter-site log variations, and comparison with seismic profile collectively indicate that the sediments are pre-dominated by hemipelagic mud, intercalated with a few sporadic MTD units only in the downslope site of UBGH2-1_1. Correlations of variations in the sediment compositions and the LWD logs reveal that the cyclicity in the LWD logs mainly represents climatic modulations of the ratios of terrigeneous to biogenic material inputs in a way that higher aeolian dust inputs and lower surface productivity prevailed during the glacial maximums and vice versa during the interglacial maximums. Based on the paleoceanographic implications of the LWD log variations, the LWD gamma-ray log from Site UBGH2-1_2 which is devoid of the MTDs was correlated with the LR04 stack of global benthic d18O records to establish the ageedepth relation. The correlation was underpinned by comparison with the gamma-ray log from Site ODP 798 which had been already fitted to marine d18O records from 1.0 to 2.8 Ma. The time-series LWD gamma-ray logs based on the established ageedepth relations indicate that climatic modulations
Acknowledgements Samples and data for this study were acquired by onboard and post-cruise analyses of the 2nd Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2), which was funded by Ministry of Knowledge Economy of Korea (MKE) under management of Gas Hydrate Research and Development Organization (GHDO) of Korea. We greatly thank the onboard scientific party and technical staff of UBGH2 for their support at sea. This research is a contribution of ‘Studies on Gas Hydrate Resource Assessment and Reservoir Characterization’ funded by Ministry of Science, ICT and Future Planning, Korea. This research is also a part of the project titled ‘International Ocean Discovery Program’ funded by the Ministry of Oceans and Fisheries, Korea. References Bahk, J.J., Chough, S.K., Han, S.J., 2000. Origins and paleoceanographic significance of laminated muds from the Ulleung Basin, East Sea (Sea of Japan). Marine Geology 162, 459e477. Bahk, J.J., Kim, D.H., Chun, J.H., Son, B.K., Kim, J.H., Ryu, B.J., Torres, M.E., Riedel, M., Schultheiss, P., 2013. Gas hydrate occurrences and their relation to host
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Please cite this article in press as: Bahk, J.-J., et al., Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea, Quaternary International (2015), http://dx.doi.org/10.1016/j.quaint.2015.08.023