Geochemistry of shales of the Upper Cretaceous Hayang Group, SE Korea: Implications for provenance and source weathering at an active continental margin

Sedimentary Geology 215 (2009) 1–12

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Geochemistry of shales of the Upper Cretaceous Hayang Group, SE Korea: Implications for provenance and source weathering at an active continental margin Yong Il Lee ⁎ School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea

a r t i c l e

i n f o

Article history: Received 8 May 2008 Received in revised form 24 November 2008 Accepted 12 December 2008 Keywords: Cretaceous Nonmarine shales Provenance Geochemistry Source-rock mixing

a b s t r a c t Shales of the Upper Cretaceous Hayang Group in the Yeongyang Subbasin, southeastern Korea were deposited in fluvio-lacustrine environments as the Paleo-Pacific (Izanagi) Plate obliquely subducted beneath the Asian continent. Major and trace element compositions derived from shales analyzed from the Hayang Group reveal geochemical signatures that are different from Post-Archean average Australian Shale (PAAS), but are typical of igneous rocks found in the continental-margin island arc settings. For example, most major element concentrations are depleted relative to PAAS; among them MgO and MnO depletions are most pronounced in all shales. CaO enrichment is notable in some formations due to the presence of secondary calcite as dispersed micro-calcite nodules. Sc, Y, and Th concentrations are also slightly depleted relative to PAAS while chondrite-normalized REE data for shales from the Hayang Group show moderate to large uniform LREE enrichment but variable HREE depletions. In addition, they are more fractionated than PAAS, and have significant negative Eu anomalies. Moreover, the chemical index of alteration and A–CN–K relations comply that the source area was dominated by non-steady state weathering regimes indicative of active uplift along an active continental margin. These regions were punctuated by two periods of steady-state weathering or alternatively by enhanced recycling of sediments. The latter interpretation is in accord with the timing of exposure of a cherty sequence in the source area. Mixing calculations based on REE data suggest that the average shale in the Hayang Group was derived from sources composed of Precambrian granitic gneiss, arc basaltic rocks, Triassic Yeongdeok Granite in the subbasin, and the Mino accretionary complex of SW Japan with ratios around 43.7 (31.8–73.2):26.4 (15.8–36.7):15.4 (2.8–33.9):14.5 (0.0–33.8). However, these proportions change up sequence. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Chemical composition of shales provides important information on regional tectonic setting, provenance, weathering conditions, and sediment recycling (Roser and Korsch, 1988; McLennan et al., 1993; Cullers, 1995). Most older sedimentary rocks in the geologic record are composed of detritus with a significant and extended recycling history (Veizer and Jansen, 1979, 1985). Such recycling along a passive continental margin facilitates mixing and homogenization and uniform major, trace, and rare earth element compositions. However, in active tectonic settings sedimentary recycling processes are less efficient, and, as a result the chemical signature of some source rocks may dominate, resulting in REE patterns that differ from Post Archean average Australian Shale (PAAS). In such cases, source rock characteristics of first-cycle detritus or relatively immature sediment can be inferred from their geochemical compositions, and especially their REE chemistry and certain elemental ratios.

⁎ Fax: +82 2 871 3269. E-mail address: [email protected]. 0037-0738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.12.004

Sedimentary rocks deposited at active continental margins are associated with intra-island arc (e.g., western Alutian Islands) or continental-margin arc (e.g., South America) volcanism. In such settings sediments are transported down high stream gradients and commonly accumulate in forearc or intra-arc basins. Because of high relief significant recycling does not occur, and, as a result, sediments deposited in active arc settings retain the REE characteristics of their source rocks (Gromet et al., 1984; Gibbs et al., 1986; Condie and Wronkiewicz, 1990). If subduction is strongly oblique, then displaced terranes within the forearc region can complicate this relatively simple picture (McLennan et al., 1989, 1990). The Gyeongsang Basin is the largest Cretaceous nonmarine sedimentary basin in Korea (Fig. 1). During the Cretaceous, the eastern margin of the Asian continent, including the Korean Peninsula, was an Andean-type continental margin (Choi, 1986; Watson et al., 1987). At this time, older Permian to earliest Cretaceous accretionary complexes made up the eastern margin of the Asian continent (Taira et al., 1989; Isozaki, 1997), while further inboard a belt of Jurassic calc-alkaline arc granite dominated the paleogeographic setting (Isozaki, 1997). The oblique, northward subduction of the Paleo-Pacific (Izanagi) Plate beneath the Asian continent produced a number of Lower Cretaceous

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Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

Fig. 1. Geological map of the study area in the Yeongyang Subbasin (modified after Chang et al., 1990). Strike-slip faults in the Korean Peninsula are shown in the left inset.

pull-apart (transtensional) basins in East Asia (Watson et al., 1987; Okada and Sakai, 1993). The Gyeongsang Basin was developed as one of these nonmarine sedimentary basins (Lee, 1999). However, the earliest development of the Gyeongsang Basin is reinterpreted to be of rift origin associated with plume-related magmatism (Okada, 1999, 2000; Chang, 2002). Arc-related igneous activities and accretion were minimal during this early developmental phase (Maruyama et al., 1997). The Gyeongsang Basin is subdivided from north to south into the following three fault-bounded subbasins: the Yeongyang, Euiseong, and Milyang subbasins (Fig. 1). After deposition of early-stage basin fill, the initial plume-related basin expanded toward the east, resulting in the formation of the Yeongyang Subbasin (Fig. 1). The Yeongyang Subbasin contains many conglomerate beds with radiolaria-bearing

chert pebbles (Chang et al., 1990; Kamata et al., 2000; Mitsugi et al., 2001). There is no accretionary complex in the Korean Peninsula, and thus the presence of conglomerates containing radiolaria-bearing chert pebbles provides an important clue for understanding the tectonics and paleogeography of the East Asian continental margin. Based on radiolarian faunas from the chert pebbles, the possible source area was likely a Jurassic accretionary complex located in SW Japan (Chang et al., 1990; Kamata et al., 2000; Mitsugi et al., 2001). Using sandstone petrofacies, Lee and Kim (2005) reported that the tectonic setting of the Yeongyang Subbasin evolved up sequence from basement uplift to recycled orogen, to magmatic arc, and back to recycled orogen. The reoccurrence of the recycled-orogen provenance has been attributed to the oblique subduction of the Izanagi Plate, resulting in uplift of the accretionary complex. Based on the

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

geochemical compositions of shales in the same sedimentary sequence studied by Lee and Kim (2005), this investigation intends to address the weathering conditions in the source area and the chemical characteristics of source rock. In addition, data derived from this study are compared with similar data obtained from coarse-grained sediments in the same sequence studied by Lee and Kim (2005). Provenanc information either from coarse-grained and fine-grained sediments may provide different interpretations on tectonic activity in the source areas in the active continental margin setting. Physically durable rocks tend to provide detritus generally larger than fine sand size and source signatures of these rocks will be accentuated more in coarse-grained sediments than in fine-grained sediments, Conversely, fine-grained sediments preferentially record less-resistant phases and their weathering products. Because information from both sediments is complementary, this aspect needs to be paid more attention when studying weathering history of recycled sediments, especially when the source area is suspected to be composed of fine-grained rocks such as fine-grained sedimentary rocks and low-grade metamorphic rocks, commonly found in the active continental margins. 2. Geological setting The Gyeongsang Basin is in unconformable contact with preCretaceous granitic rocks and Precambrian gneiss (Choi, 1985b). The Gyeongsang Supergroup, the basin fill, is divided into three groups based on the proportion of volcaniclastic and pyroclastic material. They are with decreasing age the Sindong (pre-volcanic), Hayang (sporadic volcanic), and Yucheon (climactic volcanic) groups (Chang, 1975). The Sindong and Hayang groups are composed of sandstone, shale, and minor amounts of conglomerate and marl deposited in a nonmarine environment. Following deposition of the Hayang Group, the Yucheon Group, comprised dominantly of volcanic rocks with subordinate volcaniclastic rocks, was deposited. Upper Cretaceous to Lower Tertiary granitic rocks, interpreted to be co-magmatic with the Yucheon volcanic rocks (Lee et al., 1987), then intruded the Hayang and Yucheon groups. Alluvial fans, fluvial plains and lakes dominated the basin during deposition of the Sindong Group (2000–3000 m thick) (Choi, 1985b) in the Aptian to Albian (Y.I. Lee et al., in press). The Gyeongsang Basin then expanded to the east by successive block faulting, resulting in the development of three subbasins (Fig. 1) with volcanism occurring intermittently within and outside the basin (Chang, 1975; Choi, 1985b; Chang, 1987). Basin expansion resulted in rapid subsidence, active volcanism, deposition of compositionally immature sediments, and the wide distribution of the Hayang Group on the crystalline basement (Yeongnam massif) (Choi, 1985b). The Hayang Group (1000–5000 m thick) is mainly composed of fluvio-lacustrine sandstone and shale with minor conglomerate (Choi, 1981; Um et al., 1983; Choi, 1985, 1986). In addition, intercalated extrusive rocks representing sporadic continental-margin arc volcanism are present. Different stratigraphic classifications are applied to the Hayang Group in each subbasin (Chang, 1987). Paleontological studies of Charophyta (Seo, 1985; Choi, 1987, 1989) and spores and pollen (Choi, 1985a) suggest that the Hayang Group was deposited during Aptian to Albian time. However, recent geochronological studies of tuff and other volcanic components within the Hayang Group reveal an Aptian to Campanian depositional age (Jwa et al., 2004; Kim et al., 2005; Lee et al., 2007). The Gyeongjeondong Formation, the lowermost stratigraphic unit of the study area, is part of the Sindong Group. It has very limited areal extent (Fig. 1) and its relationship to the development of the Yeongyang Subbasin is uncertain. In contrast, most Korean geologists believe that the basin fill of the Yeongyang Subbasin generally starts with the overlying Hayang Group. In the central part of the study area, sediments of the Hayang Group are unconformably in contact with pre-Cretaceous crystalline basement (i.e., the Yeongdeok Granite), and are intruded by the Upper Cretaceous Onjeong Granite. The 238 ±

3

5.4 Ma Yeongdeok Granite is a medium-grained hornblende-biotite granitic rock (tonalite-granodiorite-granite) (Kim, 1997). The total thickness of the Hayang Group in the Yeongyang Subbasin is estimated to be about 4500 m (Chang et al., 1990). The Ullyeonsan Formation, the basal unit, is an alluvial fan deposit dominated by conglomerates and interbedded thin purple shales and arkosic sandstones. It rests unconformably on the pre-Cretaceous basement rock, and in the study area is well exposed (Fig. 1). The overlying Donghwachi Formation consists of sandstone, reddish silty shale, and conglomerate deposited in channel and floodplain settings. Characteristically, this formation contains up to 10 chert-pebble conglomerate beds. The overlying Gasongdong Formation comprises alternating layers of reddish siltstone and fine-grained sandstone with subordinate greenish gray shale, chert, and siliceous shale. Volcanism culminated during deposition of the Osipbong Formation which overlies the Gasongdong. This episode of volcanism consisted mostly of basaltic lava and tuff. It was accompanied by deposition of coarse alluvial conglomerates in its lower part. Pebbles reaching up to 8 cm in diameter are common in the alluvial conglomerates (Chang et al., 1990). The overlying Dogyedong Formation comprises reddish silty shale and sandstone. In the southern part of the study area it is greenish gray in color, but in the northern part it retains reddish color. The Gisadong Formation, 200–400 m thick, is composed of reddish shale with some caliches, along with sandstone and conglomerate beds 1 to 5 m thick. The conglomerates include many red and greenish gray chert pebbles, which are very similar to those observed in the Donghwachi Formation. The Dogyedong and Gisadong formations are very comparable to the Donghwachi and Gasongdong formations. However, paleocurrent data are more scattered in the former than in the latter (Chang et al., 1990). This observation suggests an increased sinuosity of the riverine system during deposition of the Dogyedong and Gisadong formations. Sedimentation in the Yeongyang Subbasin ended with the Sinyangdong Formation, a lacustrine deposit about 200 m thick. The Sinyangdong Formation is composed of dark gray to black shale with minor intercalated sandstone and conglomerate. 3. Methods Forty-eight unweathered shale samples were collected from the Hayang Group (Fig. 1). Collected specimens show a large variety in color and texture. Each collected sample was powdered in an agate motar, and fused-glass beads were prepared for major element analysis utilizing a Phillips PW 2404 X-ray fluorescence spectrometer with a rhodium X-ray source (Norrish and Hutton, 1969; Giles et al., 1995). Uncertainties in XRF analysis of major elements are ±5%. Each sample was analyzed three times, and the results were then averaged. Total Fe content is reported as Fe2O3. Loss on ignition (LOI) was measured by weighing before and after 1 h of calcination at 1000 °C. Trace element (Th, Sc, Y, and rare earth elements) concentrations were determined using a VG Elemental PQII Plus inductively coupled plasma-source mass spectrometer (ICP-MS). Analytical precision for trace elements is better than 5%. All major and trace elements were analyzed at the Korea Basic Science Institute. Carbon dioxide contents were measured using a CM5014 coulometer. In this paper, the median is used as the summary statistic because it provides a robust estimate of central tendency for datasets drawn from a population whose distribution pattern is unknown (Lister, 1982; Rock et al., 1987). 4. Results 4.1. Major elements Table 1 lists the major element concentrations of analyzed Hayang Group shales. Most samples in Table 1 have SiO2 contents between 50 and 70 wt.%. Sample 0401-6 from the Donghwachi Formation displays the largest SiO2 concentration (73.3%). Median SiO2 contents are 59.7%

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Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

Table 1 Major element concentrations (in wt.% oxide) for Hayang Group shales

Sinyangdong Formation

Gisadong Formation

Dogyedong Formation

Gasongdong Formation

Donghwachi Formation

Ullyeonsan Formation

a

Sample

SiO2

Al2O3

Fe2O3

MnO

CaO

MgO

K2O

Na2O

P2O5

TiO2

L.O.I

Total

CIAa

116-17 1005-1 1005-2 1005-3 1005-4 1005-5 median 116-13 116-19 114-9 114-17 0330-7 0330-9 0330-11 0330-21 median 114-2 114-5 116-21 116-30 116-36 115-35 median 115-15 115-20 median 0331-1 0331-12 0331-15 0401-2 0401-6 0401-8 0401-16 115-9 115-19 115-21 median 0401-20 0401-24 0401-26 115-3 median

61.55 66.44 67.14 63.66 65.14 64.44 64.79 71.53 52.03 71.39 63.43 58.42 70.50 71.29 63.94 67.22 66.27 60.35 68.67 67.37 66.31 64.05 66.29 61.07 69.54 65.31 59.04 62.80 63.17 71.66 73.31 67.42 67.35 70.12 66.84 60.77 67.10 49.12 67.75 56.62 62.85 59.73

16.99 13.84 12.94 17.98 15.99 17.77 16.49 15.72 13.42 9.94 17.22 10.06 13.00 12.91 15.17 13.21 14.43 17.49 13.64 15.65 16.06 17.75 15.85 14.64 12.48 13.56 17.70 14.62 15.71 15.10 13.91 16.40 14.33 15.54 16.26 17.03 15.63 10.70 15.96 14.35 18.08 15.15

5.68 4.33 5.09 2.82 3.86 3.75 4.09 3.12 5.71 3.53 5.74 4.12 3.26 4.15 5.29 4.13 4.34 6.18 4.60 5.55 6.05 6.19 5.80 6.25 3.53 4.89 6.73 5.69 5.23 4.33 3.85 7.10 4.69 4.52 5.03 6.66 5.13 5.07 5.20 4.74 5.41 5.13

0.34 0.11 0.26 0.12 0.02 0.04 0.11 0.01 0.15 0.04 0.01 0.15 0.01 0.07 0.02 0.03 0.03 0.02 0.03 0.07 0.03 0.09 0.03 0.07 0.04 0.05 0.04 0.06 0.03 0.00 0.01 0.08 0.04 0.00 0.00 0.03 0.03 0.18 0.01 0.03 0.03 0.03

2.17 7.12 0.84 0.44 1.28 0.45 1.06 0.39 9.24 4.69 1.51 8.59 1.85 2.57 3.20 2.88 2.72 2.26 1.93 0.36 0.39 0.36 1.16 3.28 4.03 3.66 1.51 3.89 3.14 0.09 0.36 0.25 3.56 0.45 0.24 2.51 0.98 15.54 0.44 8.51 1.49 5.00

1.68 1.49 1.18 1.04 1.20 0.91 1.19 0.65 2.86 0.41 1.02 3.36 0.90 0.61 0.85 0.88 1.27 1.32 1.61 1.18 1.17 1.32 1.30 2.23 0.47 1.35 1.49 1.41 1.26 0.45 0.45 0.42 0.29 0.41 0.31 1.09 0.45 0.79 0.72 0.75 1.08 0.77

4.20 2.70 2.90 4.10 3.17 3.32 3.25 3.92 3.79 1.80 4.52 2.08 3.13 2.41 3.91 3.46 3.12 4.63 3.23 3.58 4.09 4.20 3.83 3.22 2.65 2.94 4.91 3.95 4.07 3.19 3.08 3.02 3.28 3.33 4.25 4.88 3.64 2.91 4.50 3.94 4.63 4.22

0.30 0.24 0.61 1.03 1.12 0.12 0.46 0.94 0.78 2.46 0.96 1.03 1.54 0.98 1.36 1.00 2.50 1.03 1.92 2.76 1.83 0.94 1.87 0.69 2.35 1.52 1.25 1.05 1.79 1.92 2.26 1.31 0.82 1.67 1.14 1.07 1.28 0.66 1.70 1.34 1.07 1.20

0.26 0.20 0.09 0.11 0.12 0.18 0.15 0.12 0.10 0.10 0.13 0.07 0.08 0.08 0.11 0.10 0.14 0.14 0.10 0.10 0.18 0.12 0.13 0.18 0.07 0.12 0.13 0.11 0.10 0.05 0.10 0.08 0.21 0.15 0.11 0.09 0.10 0.10 0.13 0.11 0.13 0.12

0.64 0.47 0.37 0.69 0.62 0.65 0.63 0.65 0.58 0.44 0.70 0.41 0.48 0.46 0.61 0.53 0.62 0.74 0.56 0.64 0.70 0.77 0.67 0.76 0.45 0.61 0.68 0.65 0.58 0.52 0.52 0.86 0.63 0.72 0.68 0.69 0.66 0.40 0.63 0.56 0.60 0.58

6.37 1.06 7.55 6.85 6.23 6.91 6.61 3.30 11.91 5.89 5.05 12.04 4.61 4.29 5.22 5.14 4.69 5.80 4.06 3.21 3.49 4.70 4.37 7.95 5.21 6.58 7.48 6.14 5.62 2.74 2.52 3.23 5.19 3.24 3.06 5.86 4.22 14.71 3.21 9.53 4.74 7.14

100.18 100.01 98.97 98.83 98.76 98.54 98.90 100.32 100.56 100.70 100.27 100.31 99.37 99.81 99.68 100.29 100.13 99.96 100.35 100.45 100.29 100.48 100.32 100.35 100.81 100.58 100.94 100.37 100.69 100.05 100.36 100.16 100.40 100.14 97.90 100.68 100.36 100.17 100.24 100.46 100.10 100.20

75.46 73.38 70.35 72.98 68.62 80.98 71.66 71.34 71.39 58.79 70.12 71.86 68.71 71.33 67.11 70.72 61.72 69.57 65.03 63.69 66.44 73.03 65.74 75.98 58.34 67.16 70.52 70.88 68.10 69.06 64.37 73.76 68.90 68.53 70.27 67.59 68.98 61.15 66.72 54.20 69.78 63.93

CIA = [Al2O3/(Al2O3 + CaO⁎ + Na2O + K2O)] × 100 (Nesbitt and Young, 1982).

for the Ullyeonsan, 67.1% for the Donghwachi, 65.3% for the Gasongdong, 66.3% for the Dogyedong, 67.2% for the Gisadong, and 61.5% for the Sinyangdong shales. Al2O3 concentrations are high in the Ullyeonsan (15.2%), Donghwachi (15.6%), Dogyedong (15.7%), and Sinyangdong (17.0%) shales. A positive correlation is evident between Al2O3 and K2O, and Al2O3 and TiO2 in the Ullyeonsan (r = 0.98 and 0.90, respectively), Donghwachi (r = 0.72 and 0.55), Dogyedong (r = 0.92 and 0.98), and Gisadong (r = 0.94 and 0.94) shales (Fig. 2). The overall strong positive correlation between Al2O3 and K2O (r = 0.75) for the Hayang Group as a whole suggests the likelihood that both elements are associated with aluminous clay minerals such as illite. The positive correlation between Al2O3 and TiO2 (r = 0.66) may suggest residual enrichment of these elements as a result of source area chemical weathering or alternatively, as a result of sorting and dilution by SiO2. MgO contents show poor correlations with Al2O3 and Fe2O3 (Fig. 2). Shown in Fig. 3 are stratigraphic variations of median values of major elements normalized to PAAS (Taylor and McLennan, 1985). Relative to PAAS, MgO and MnO contents are depleted in all samples. Among the alkali elements, Na2O in most samples is slightly enriched, but in specimens from the Sinyandong it is depleted. Slight enrichment of Na2O in Dogyedong shales is probably due to the presence of albitized plagioclase, a mineralogical component commonly observed in associated sandstones (Lee and Kim, 2005). CaO enrichment relative to PAAS is noted in the Ullyeonsan, Gasongdong,

and Gisadong shales. Such enrichments are accounted for by the presence of secondary calcite in caliches, paleo-pedogenic features that are frequently observed in outcrop. After measuring CO2 contents this interpretation is supported by a strong correlation between total CaO contents and CaO in the carbonate fractions (r = 0.93). Depletion of both Na2O and CaO in Sinyangdong shales relative to PAAS is interpreted to reflect lesser plagioclase than in PAAS. 4.2. Trace elements Concentrations of trace elements of Hayang Group shales are provided in Table 2. Most Hayang Group shales are slightly depleted in Sc and Y relative to PAAS, whereas Th values are close to PAAS (Fig. 3). As shown in Fig. 4 all Hayang Group shales have similar chondritenormalized REE patterns except for one Sinyangdong sample (1005-2). Moreover, REE patterns reveal moderate to large LREE enrichment and HREE depletion [(La/Yb)n ratio = 7.5–20.8 (Table 2)] and display significant negative Eu anomalies (Eu/Eu⁎ = 0.49–0.80; av. = 0.64). The LREEs show a more uniform fractionation [(La/Sm)n = 3.13–4.58; av. = 3.81] than do the HREEs [(Gd/Yb)n =0.69–3.66; av. = 2.52]. Excluding Sinyang shales, the median values of total REE content of Hayang Group shales range from 125.4 ppm (Gisadong Formation) to 151.9 ppm (Ullyeonsan Formation). These values are lower than that of PAAS (165.0 ppm) because of the depleted HREEs. The Sinyangdong shales are

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

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Fig. 2. Al2O3 vs. K2O, TiO2 and MgO concentrations and MgO vs. Fe2O3 plot of Hayang Group shales.

characterized by the greatest total REE contents (204.8 ppm), smallest negative Eu anomaly (Eu/Eu⁎ = 0.66) and flattest HREE patterns [(Gd/Yb) n = 1.42]. The overall characteristics of chondrite-normalized REE patterns derived from shales analyzed from the Hayang Group depends on the fractionation of the HREEs. Strongly fractionated HREEs relative to PAAS are likely due to the scarcity of HREE-carrying heavy minerals such as zircon and rutile (c.f., Götze, 1998). The large variation in HREEs (Fig. 4) is likely the result of the fractionation of dense minerals within the fluvial system represented by the Hayang (i.e., Singh and Rajamani, 2001). In contrast limited variation in (Gd/Yb)n ratios is evident in the lacustrine shales of the Sinyangdong Formation. 5. Provenance Fig. 5 shows the ratios of Th/Sc and Th/Yb in shales of the Hayang Group. The Th/Sc ratio is a very sensitive index of average provenance composition (Taylor and McLennan, 1985). For example, mafic to intermediate rocks tend to have significantly higher concentrations of Sc than do more silicic rocks. Hence, an average granodiorite will have a Th/Sc ratio similar to PAAS (~ 0.91) while model andesite has a ratio of 0.11 (Taylor and McLennan, 1985). The Th/Sc ratio in Hayang Group shales varies from 0.87 to 2.36, with an average of 1.26 (Table 2). The average value is much higher than that of PAAS. The Th/ Yb ratio is also a distinctive indicator of source rock and tectonic setting. In Hayang Group shales the Th/Yb ratio ranges from 3.54 to 13.36 and averages 7.47. It too is higher than that of PAAS (5.21). Trace element ratios greater than those of PAAS indicate that granitic clastic materials more silicic than average granodiorite dominate the shales of the Hayang Group. Moreover, stratigraphic variations of Th/Sc and Th/Yb ratios suggest that the proportions of clastic material more silicic than PAAS being delivered to the Hayang Group was

higher in the early rather than later development of the Yeongyang Subbasin. With the exception of the Donghwachi and some of the Gisadong Formation volcanic lithic fragments are present in associated sandstones of the Hayang Group (Lee and Kim, 2005). A block-uplift within a continental-margin island arc with an average granodiorite composition (see below) seems the most plausible source for the Hayang Group shales. This interpretation is supported by the La–Th–Sc plot where studied samples cluster within the continental-margin island arc field (Fig. 6) (Bhatia and Crook, 1986; McLennan et al., 1990). This result is not altogether surprising because the Izanagi Plate subducted obliquely beneath the eastern Asian continent, resulting in multiple large-scale left-lateral strike-slip movements (Lee, 1999; Okada, 1999, 2000). Transtensional releasing bands in such systems would provide the requisite block-uplift geometry. 6. Modeling of supply ratios of source rocks A wide variation in (La/Yb)n ratios in Hayang Group shales (Table 2) suggests that the sediments were derived from more than one common source, and that mixing was not very efficient during transport and deposition. Sandstone modal compositions of the Hayang Group indicate that coarse-grained sediments were derived from Precambrian basement, the Triassic Yeongdeok Granite, the Mino-Tamba terrane (an accretionary complex located in SW Japan), and volcanic rocks (Lee and Kim, 2005). In an effort to quantitatively constrain the relative contribution of these potential sources in each formation, five likely source lithotypes (end member: EM) were chosen for REE and Th/Sc (Table 3; Fig. 7) modeling. EM 1 is Precambrian gneissic-granitic rock in the Yeongnam massif located to the north of the Yeongyang Subbasin (Hong, 1992). EM 2 is the Triassic Yeongdeok Granite located in the central part of the study area (Kim, 2002). EM 3

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Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

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Table 2 Trace element and rare earth element concentrations (in ppm) for Hayang Group mudrocks Sample

La

Ce

Pr

Nd

Sm Eu Gd Tb

Dy Ho Er

Tm Yb Lu

Y

Sc

Th

Eu/Eu⁎ (La/Yb)n (La/Sm)n (Gd/Yb)n sum REE

Sinyangdong Formation 116-17 49.2 103.6 11.9 1005-1 40.1 73.1 8.3 1005-2 15.5 29.4 3.3 1005-3 46.8 91.1 10.1 1005-4 54.7 100.3 11.3 1005-5 45.5 86.1 9.8 median 46.2 88.6 10.0

42.5 29.1 11.7 35.6 39.6 35.1 35.3

8.3 5.3 2.3 6.2 6.9 6.9 6.5

1.8 1.0 0.5 1.1 1.2 1.4 1.1

9.4 3.4 1.5 3.9 4.5 4.5 4.2

1.2 0.7 0.4 0.8 0.9 1.0 0.9

5.9 3.5 2.2 4.3 4.5 4.9 4.4

1.0 0.7 0.5 0.9 0.9 1.0 0.9

2.9 2.2 1.7 2.7 2.7 2.9 2.7

0.4 0.3 0.3 0.4 0.4 0.4 0.4

2.3 2.0 1.8 2.6 2.6 2.7 2.5

0.3 0.3 0.3 0.4 0.4 0.4 0.4

27.4 12.6 19.2 0.61 17.9 9.4 9.4 0.73 13.1 7.4 9.4 0.80 21.9 8.8 15.2 0.68 22.2 7.2 15.5 0.64 24.6 10.6 14.0 0.76 22.1 9.1 14.6 0.71

13.61 12.67 5.53 11.82 13.79 10.90 12.25

3.40 4.33 3.81 4.31 4.58 3.80 4.06

3.41 1.41 0.69 1.28 1.48 1.42 1.42

Gisadong 116-13 116-19 114-9 114-17 0330-7 0330-9 0330-11 0330-21 median

sum LREE

sum sum Ce HREE MREE anomaly

240.73 170.10 71.22 206.89 230.81 202.64 204.76

217.26 156.84 62.65 190.95 213.90 184.75 187.85

23.48 13.26 8.57 15.94 16.91 17.89 16.43

27.62 14.71 7.33 17.19 18.83 19.67 18.01

0.95 0.89 0.92 0.93 0.90 0.91 0.91

Formation 28.7 56.0 30.0 56.0 21.8 43.6 28.4 59.6 23.0 49.5 25.9 47.7 24.4 48.5 33.0 71.1 27.2 52.8

6.7 6.9 5.1 7.1 5.5 6.1 5.6 7.6 6.4

23.7 24.0 19.4 25.5 19.5 21.7 19.8 26.9 22.7

4.3 4.8 3.5 4.6 4.0 4.2 3.5 5.5 4.2

0.9 1.1 0.7 0.9 1.1 0.9 0.7 1.2 0.9

4.3 5.3 4.1 5.0 5.2 4.7 3.9 5.9 4.8

0.5 0.7 0.4 0.6 0.8 0.6 0.4 0.7 0.6

2.7 3.8 2.7 3.5 3.9 3.0 2.8 3.6 3.2

0.6 0.7 0.4 0.6 0.8 0.5 0.5 0.6 0.6

1.7 2.1 1.3 2.0 2.1 1.6 1.5 2.0 1.8

0.2 0.3 0.1 0.2 0.3 0.2 0.2 0.2 0.2

1.6 1.9 1.1 1.9 2.0 1.6 1.4 1.8 1.7

0.2 0.3 0.1 0.2 0.3 0.2 0.2 0.2 0.2

13.0 18.0 13.5 16.8 20.5 15.2 14.3 18.9 16.0

7.6 11.2 5.4 12.0 10.3 8.7 8.3 13.4 9.5

10.7 10.2 6.1 12.6 10.2 10.3 10.2 11.8 10.3

0.64 0.68 0.59 0.59 0.74 0.64 0.57 0.63 0.63

11.91 10.02 12.42 9.90 7.51 10.72 11.50 11.88 11.11

3.83 3.62 3.53 3.55 3.27 3.53 4.00 3.45 3.54

2.33 2.32 3.07 2.29 2.21 2.52 2.39 2.77 2.36

132.05 137.97 104.49 140.25 117.97 118.76 113.38 160.28 125.40

120.27 122.76 94.14 126.16 102.60 106.51 102.60 145.24 113.39

11.78 15.21 10.35 14.09 15.37 12.25 10.78 15.04 13.17

13.27 16.44 11.94 15.26 15.77 13.79 11.74 17.48 14.52

0.90 0.87 0.92 0.93 0.98 0.85 0.92 1.00 0.92

Dogyedong Formation 114-2 28.0 55.0 114-5 36.6 75.2 116-21 24.5 48.4 116-30 23.9 42.9 116-36 33.5 63.0 115-35 34.8 62.9 median 30.7 58.9

6.5 8.3 5.6 5.4 7.9 7.3 6.9

23.4 29.7 19.6 18.8 27.6 25.3 24.4

4.6 5.6 3.6 3.3 5.2 4.6 4.6

1.0 1.1 0.8 0.8 1.2 1.0 1.0

5.0 6.3 4.1 3.6 5.6 5.4 5.2

0.6 0.7 0.5 0.5 0.7 0.7 0.6

3.5 4.5 2.8 2.4 3.6 3.7 3.6

0.6 0.7 0.5 0.7 0.7 0.8 0.7

1.9 2.4 1.6 1.4 2.0 2.2 2.0

0.2 0.2 0.2 0.2 0.3 0.3 0.2

1.7 2.2 1.4 1.3 1.8 2.2 1.8

0.2 0.2 0.2 0.2 0.3 0.3 0.2

18.0 24.7 13.7 11.2 15.4 19.2 16.7

9.6 14.6 8.5 10.7 12.1 13.8 11.4

10.2 12.7 9.5 9.7 13.4 16.6 11.4

0.62 0.58 0.67 0.67 0.66 0.64 0.65

10.48 10.89 11.04 11.60 12.04 10.23 10.96

3.47 3.73 3.90 4.18 3.66 4.30 3.81

2.46 2.44 2.43 2.31 2.62 2.08 2.44

132.40 173.68 114.23 105.38 153.33 151.66 142.03

118.54 156.50 102.69 94.99 138.38 136.01 127.27

13.86 17.18 11.54 10.39 14.96 15.66 14.41

15.34 18.87 12.50 11.23 17.00 16.24 15.79

0.91 0.96 0.92 0.84 0.86 0.88 0.89

9.8 11.6 0.68 7.4 10.4 0.67 8.6 11.0 0.67

13.42 18.12 15.77

3.82 3.65 3.74

2.95 3.66 3.31

143.22 129.89 13.33 15.52 0.90 127.90 118.21 9.70 12.48 0.94 135.56 124.05 11.51 14.00 0.92

Gasongdong Formation 115-15 31.4 60.3 7.1 25.2 4.7 1.1 5.3 0.7 3.2 0.6 1.6 0.2 1.5 0.2 15.6 115-20 27.0 56.5 6.6 22.9 4.2 0.9 4.2 0.5 2.3 0.4 1.1 0.1 1.0 0.1 8.4 median 29.2 58.4 6.9 24.1 4.5 1.0 4.7 0.6 2.7 0.5 1.4 0.2 1.2 0.2 12.0 Donghwachi Formation 0331-1 28.3 56.0 0331-12 29.8 66.1 0331-15 33.4 63.7 0401-2 24.7 45.7 0401-6 27.1 52.1 0401-8 27.8 59.3 0401-16 35.5 63.9 115-9 22.0 44.4 115-19 32.7 63.4 115-21 39.5 78.9 median 29.0 61.3

6.6 6.8 7.5 5.8 6.2 7.0 7.9 5.3 7.2 9.0 6.9

24.5 24.2 26.1 20.2 22.6 26.7 28.0 18.5 24.5 31.6 24.5

4.5 5.0 4.8 3.7 4.1 5.1 5.3 3.2 4.1 5.7 4.6

0.9 1.1 1.0 0.8 0.8 1.0 1.2 0.7 1.1 1.3 1.0

5.1 5.7 5.2 4.0 4.2 5.3 6.1 3.2 4.4 6.3 5.2

0.6 0.6 0.6 0.5 0.4 0.6 0.7 0.4 0.5 0.8 0.6

3.3 3.8 3.3 2.4 2.5 3.0 3.5 1.6 1.8 4.0 3.2

0.6 0.7 0.6 0.4 0.4 0.5 0.6 0.3 0.3 0.9 0.5

1.9 2.1 1.9 1.3 1.2 1.6 1.7 0.9 1.0 2.1 1.7

0.2 0.3 0.2 0.1 0.1 0.2 0.2 0.1 0.2 0.3 0.2

1.7 1.9 1.8 1.2 1.1 1.5 1.5 0.9 1.0 2.0 1.5

0.2 0.3 0.2 0.1 0.1 0.2 0.2 0.1 0.2 0.3 0.2

17.2 18.4 16.4 10.9 10.8 14.1 16.4 6.8 7.5 18.5 15.3

14.1 11.7 12.1 9.8 11.9 11.8 11.2 8.3 10.7 11.6 11.7

0.57 0.66 0.63 0.61 0.58 0.61 0.67 0.65 0.79 0.66 0.64

11.05 9.99 12.36 13.32 15.45 11.67 15.59 16.11 20.83 12.55 12.93

3.64 3.41 3.96 3.80 3.83 3.13 3.86 3.89 4.55 4.01 3.85

2.61 2.48 2.51 2.81 3.13 2.89 3.48 3.10 3.63 2.61 2.85

134.27 148.19 150.39 110.98 122.81 139.82 156.16 101.64 142.36 182.66 141.09

120.71 132.96 136.61 100.93 112.74 126.94 141.79 94.13 133.06 165.92 129.95

13.56 15.24 13.78 10.06 10.07 12.87 14.38 7.51 9.30 16.75 13.22

14.94 16.87 15.51 11.78 12.28 15.48 17.32 9.40 12.15 18.87 15.21

0.91 1.04 0.89 0.85 0.90 0.95 0.85 0.92 0.92 0.93 0.92

Ullyeonsan Formation 0401-20 36.6 54.4 0401-24 34.2 69.5 0401-26 32.9 62.4 115-3 30.1 63.3 median 33.5 62.8

7.4 8.2 7.5 7.0 7.5

26.0 28.5 26.1 23.5 26.0

5.1 5.5 5.1 4.1 5.1

1.1 0.9 0.9 0.7 0.9

6.6 5.9 6.2 4.6 6.1

0.9 0.6 0.8 0.5 0.7

5.9 3.6 4.4 2.8 4.0

1.2 0.6 0.7 0.4 0.6

3.4 1.7 2.2 1.4 2.0

0.4 0.2 0.3 0.1 0.2

2.8 1.6 2.0 1.3 1.8

0.4 0.2 0.3 0.2 0.2

42.7 8.4 9.8 16.0 9.6 15.1 21.0 9.8 13.7 12.3 10.5 18.0 18.5 9.7 14.4

0.57 0.51 0.49 0.52 0.52

8.51 13.99 10.80 14.45 12.39

4.11 3.59 3.66 4.17 3.89

2.00 3.16 2.67 2.92 2.79

152.12 161.15 151.69 140.26 151.90

130.56 146.81 134.93 128.82 132.74

21.55 14.33 16.77 11.44 15.55

20.81 17.10 18.11 13.26 17.60

0.73 0.93 0.88 0.97 0.91

represents the average composition of clastic rocks in the chert-clastic sequence in the Mino accretionary complex, SW Japan (Joo et al., 2007), while EM 4 is the average composition of Triassic chert from the same complex (Shimizu et al., 2001). Clastic and chert end members derived from the Mino complex were treated separately. Such treatment was justified because in coarse-grained sediments in the Yeongyang Subbasin radiolaria-bearing chert grains formed the only evidence of their derivation from the Mino accretionary complex. EM 5 is Chaeyaksan basaltic rock (Kim et al., 1999). It represents the last phase of basaltic volcanism in the Gyeogsang Basin as well as the initial stage of continental-margin arc volcanic activity (Kim et al., 2000). Pr, Gd, Tb, Ho, Er, Tm and Lu were not reported for the Chaeyaksan basaltic

10.8 11.7 10.5 5.7 8.1 12.1 11.1 3.8 4.6 13.3 10.6

rocks by Kim et al. (1999). As a result, the concentrations of these elements were adopted from the average Phanerozoic basalt composition reported in Condie (1993). Estimation of the relative contribution of source materials required to generate the compositions of shales in the Hayang Group was accomplished through simple least-squares mixing calculations (Le Maitre, 1981). The mixing results are summarized in Table 4. Fig. 8 shows the chondrite-normalized REE patterns of median shale compositions along with the results of mixing calculations for each formation. Based on the mixing calculations for each formation, the variations in detritus delivered to the Hayang Group in the Yeongyang Subbasin are reconstructed in Fig. 9A.

Fig. 3. Spiderplots of median values (with ranges in gray) of major element and trace element concentrations of Hayang Group shales normalized against Post-Archean average Australian shale (PAAS; Taylor and McLennan, 1985).

8

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

Fig. 4. Chondrite-normalized rare earth element plots for Hayang Group shales. PAAS plotted for reference. Note large negative Eu anomaly and fractionated LREEs and HREEs.

The Precambrian gneissic-granitic rock (EM 1) was the most dominant source rock for shales of the Hayang Group (av. 43.7%; Fig. 9B). With the exception of the Ullyeonsan Formation, it generally supplied between 32% and 47% of the clastic detritus in the various formations making up the Hayang Group. For the Ullyeonsan Formation it supplied 73%. This result is in agreement with petrographic observations indicating that sandstones contain large amounts of sand-sized alkali feldspar and plagioclase presumably derived from a basementuplift tectonic setting (Lee and Kim, 2005). The second most important source was basaltic rock (EM 5). It contributed, on average about 26.4%, but its formation by formation contribution varied from 16% to 42%. This result is beyond expectation because only a small number of sandstone samples in the Ullyeonsan, Donghwachi, and Gasondong formations contain basaltic volcanic rock fragments (Lee and Kim, 2005). In contrast, basaltic rock contributed about 16 to 29%

Fig. 5. Variations of Th/Sc and Th/Yb ratios in Hayang Group shales. Note all ratios are greater than those of PAAS.

of the clastic material in shales contained within the Ullyeonsan, Donghwachi, and Gasongdong formations. In addition, basaltic rock contributed 19% to 42% of the clastic detritus in shales deposited in the Dogyedog, Gisadong and Sinyangdong formations. This result is consistent with point-count data derived from coarse-grained sediments in these formations (Lee and Kim, 2005). The largest contribution of basaltic clastic materials occurred during the deposition of the Dogyedong shales (Fig. 9A).

Fig. 6. La–Th–Sc plot of Hayang Group shale samples in this study (diagram after Bhatia and Crook, 1986) compared to upper crust and model andesite compositions (Taylor and McLennan, 1985). A: oceanic island arc; B: continental island arc; C: active continental margin; D: passive margin.

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

The Triassic Yeongdeok Granite also contributed significant amount of sediments to shales in the Hayang Group (av. 15.4%). Its contribution is notable for shales in the Donghwachi, Gasongdong, and Sinyangdong formations. In contrast, the contribution of granitic detritus to the Ullyeonsan Formation was not significant (Fig. 9B). Nonetheless, the widespread occurrence of clastic detritus derived from the Yeongdeok Granite suggests that it formed a paleogeographic high throughout the deposition of the Hayang Group. Both clastic and chert debris derived from the Jurassic Minoterrane contributed material to the Donghwachi, Gisadong, and Sinyangdong formations. Mino-terrane clastic rocks (EM 3) were a significant contributor of debris to the Gisadong shales, but their influence on the composition of shales within the Donghwachi and Sinyangdong formations was less significant. In contrast, very minor amounts of sedimentary rock fragment (generally less than 5% of the rock), possibly of Mino-terrane origin have been reported from sandstones in the Hayang Group (Lee and Kim, 2005). Based on the occurrence of abundant radiolaria-bearing pebbles and sand-sized clasts in the Donghwachi and Gisadong formations, the Mino-Tamba accretionary complex located in SW Japan have been interpreted as the source of this material. However, the results of mixing calculations imply that ~ 20% of the composition of shales in the Donghwachi and Sinyangdong formations was contributed from a chert sequence. In contrast, shales in the Gisadong Formation contain only a 9% contribution from chert. The mixing calculations apparently do not show well the source characteristics inferred from pebble and sandsized sediments (Lee and Kim, 2005). This disparity may be due to the fact that chert grains are resistant to comminution, and thus their presence may lead to overestimation of their significance in sourcearea terranes. Nevertheless, mixing calculations indicate that both clastic and chert debris derived from the Mino terrane makes up about 30% of the composition of shales in the Donghwachi and Gisadong formations. Shales in the Sinyangdong Formation also record a relatively important contribution from the Mino terrane. The results of this study indicate that source rock signatures are differently recorded in conglomerates and sandstones, and shales. The presence of chert clasts in conglomerates and sandstones provides firm evidence of chert in the source area, and the presence of volcanic rock fragments in the Dogyedong and Gisadong formations has a similar

Table 3 Mixing end menbers (EM) for mixing calculations EM1a La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu sum REE Eu/Eu⁎ (La/Yb)n (La/Sm)n (Gd/Yb)n

EM2b

EM3c

EM4d

EM5e

34.90 72.71 7.68 30.25

30.62 55.82 6.21 20.10

31.65 69.13 7.84 27.58

4.33 9.59 1.09 4.25

37.45 69.20

5.88 0.66 4.47 0.57 4.42 0.81 2.20 0.20 1.67 0.25 166.67 0.40 13.52 3.40 2.26

2.48 0.78 2.17 0.19 0.71 0.10 0.32 0.05 0.25 0.05 119.85 1.04 79.25 7.08 7.34

5.82 1.27 3.78 0.80 4.19 0.78 2.50 0.35 2.40 0.38 158.47 0.84 8.52 3.12 1.33

0.89 0.22 0.81 0.13 0.75 0.15 0.43 0.06 0.40 0.06 23.16 0.80 7.00 2.79 1.71

6.49 1.47

All values are in parts per million. a Precambrian basement rock (Hong, 1992). b Triassic Yeongdeok Granite (Kim, 2002). c Mino terrane clastic rocks, SW Japan (Joo et al., 2007). d Mino terrane chert, SW Japan (Shimizu et al., 2001). e Chaeyaksan basaltic rocks (Kim et al., 1999).

31.55

3.28

1.51

1.09 16.05 3.31 1.50

9

Fig. 7. Chondrite-normalized REE patterns of five end members used for mass balance calculation. EM 1: Precambrian gneissic–granitic rock (Hong, 1992); EM 2: Triassic Yeongdeok Granite (Kim, 2002); EM 3: average clastic rocks of the Mino accretionary complex, SW Japan (Joo et al., 2007); EM 4: Triassic chert of the Mino accretionary complex, SW Japan (Shimizu et al., 2001); EM 5: Chaeyaksan basaltic rock (Kim et al., 1999).

importance (Lee and Kim, 2005). In shales the presence of chert and volcanic material in the source area can only be inferred from the chemistry of the shale. In sandstones, it is difficult to discriminate between the source-rock contributions of the Precambrian basement and the Triassic Yeongdeok Granite. Both types of source rocks produce sand-sized feldspar and quartz grains. However, it is possible to recognize their relative contributions in shales due to their different geochemical characteristics (Fig. 7). Basaltic source rocks weather easily, and, as a result, their role as a source rock may not be recognized in sandstones without the presence of volcanic rock fragments. Based on the chemistry of shales the contribution of basalt to Hayang Group sediments was considerable. In contrast, sandstone petrographic studies indicate that basaltic source rocks contributed significant detritus to the Dogyedong and Gisadong formations following continental-margin arc volcanism (Lee and Kim, 2005). Hence, it is interpreted that continental-margin arc volcanism occurred prior to or was coeval with deposition and formation of the Yeongyang Subbasin. 7. Weathering conditions A common method of quantifying the degree of source area weathering is to use the chemical index of alteration (CIA; Nesbitt and Young, 1982). CIA values for shales of the Hayang Group range from 64 to 76 with an average of 68 (Table 1). These values indicate a moderate degree of chemical weathering of the source area. Chemical data for Hayang Group shales are plotted in Al2O3 − CaO⁎ + Na2O − K2O (A–CN–K) compositional space in Fig. 10 (Nesbitt and Young, 1984, 1989). The A– CN–K plot reveals that shales of the Ullyeonsan, Gasongdong, Dogyedong, and Sinyangdong formations follow a weathering trend extending from the composition of granodiorite toward the A join. With the exception of one sample from the Sinyangdong Formation (1005-1) the weathering trends shown in Fig. 10 lie subparallel to the A–CN join. Samples plotting significantly off the weathering trends and toward the K-apex are interpreted to have been affected by potassium metasomatism (e.g., samples in the Donghwachi and Sinyandong formations) (Fedo et al., 1995). The degree of weathering is quite variable within each formation, showing scatter along the projected trends. Such scatter is

10

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

Table 4 Mixing results

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu sum REE Eu/Eu⁎ (La/Yb)n (La/Sm)n (Gd/Yb)n Th/Sc

Ullyeonsan

Donghwachi

Gasongdong

Dogyedong

Gisadong

Obs.

Mixing result

Obs.

Mixing result

Obs.

Mixing result

Obs.

Mixing result

Obs.

Mixing result

Sinyangdong Obs.

Mixing result

Median

73:3:0:0:24a

Median

34:17:13:20:16a

Median

37:34:0:0:29⁎

Median

47:12:0:0:42a

Median

41:11:19:9:20a

Median

32:17:13:19:19a

33.5 62.8 7.5 26.0

35.4 71.4 7.6 30.3

29.0 61.3 6.9 24.5

28.1 56.3 6.1 23.2

29.2 58.4 6.9 24.1

34.2 66.0 7.1 27.2

30.7 58.9 6.9 24.4

35.5 69.3 7.4 29.6

27.2 52.8 6.4 22.7

31.6 63.9 6.9 26.6

46.2 88.6 10.0 35.3

28.5 56.9 6.2 23.5

5.1 0.9 6.1 0.7 4.0 0.6 2.0 0.2 1.8 0.2 151.5 0.51 12.2 3.8 2.9 1.5

5.9 0.9 4.0 0.5 4.0 0.8 2.0 0.2 1.6 0.2 164.7 0.54 14.4 3.4 2.1 2.1

4.6 1.0 5.2 0.6 3.2 0.5 1.7 0.2 1.5 0.2 140.4 0.65 12.5 3.6 2.9 1.5

4.4 0.8 3.0 0.4 2.8 0.5 1.5 0.2 1.3 0.2 128.7 0.68 14.5 3.7 2.0 2.1

4.5 1.0 4.7 0.6 2.7 0.5 1.4 0.2 1.2 0.2 135.6 0.67 15.2 3.7 3.2 1.2

4.9 0.9 3.2 0.4 2.8 0.5 1.4 0.2 1.1 0.1 150.0 0.73 19.3 4.0 2.4 1.9

4.6 1.0 5.2 0.6 3.6 0.7 2.0 0.2 1.8 0.2 140.9 0.63 11.3 3.8 2.5 1.1

5.7 1.0 3.5 0.5 3.5 0.7 1.7 0.2 1.4 0.1 160.1 0.70 16.0 3.5 2.0 1.4

4.2 0.9 4.8 0.6 3.2 0.6 1.8 0.2 1.7 0.2 127.4 0.63 10.5 3.7 2.4 1.1

5.2 0.9 3.4 0.5 3.4 0.6 1.8 0.2 1.5 0.2 146.7 0.67 13.5 3.5 1.9 1.9

6.5 1.1 4.2 0.9 4.4 0.9 2.7 0.4 2.5 0.4 204.0 0.66 12.2 4.0 1.4 1.5

4.5 0.8 3.0 0.4 2.8 0.5 1.5 0.2 1.3 0.1 130.2 0.71 14.7 3.7 2.0 2.0

The numbers represent proportions of end members used for mixing modeling. a Precamb. basement: Yeongdeok Gr.: Mino clastics: chert: Chaeyaksan basalt.

typical of non-steady state weathering conditions, where active tectonism permits erosion of all zones within weathering profiles developed on source rocks (Nesbitt et al., 1997). This interpretation implies active uplift throughout Ullyeonsan, Gasongdong, Dogyedong and Sinyangdong deposition. However, shale samples from the

Donghwachi and Gisadong formations plot in a limited field with slight scatter. The datasets for these two formations are very similar to each other. These features suggest that the sediment sources for these two formations were similar in lithology to those of other stratigraphic units, but were more deeply weathered and experienced steady-state

Fig. 8. Results of mixing calculations for the REEs plotted with median Hayang Group shales. (A) Ullyeonsan Formation; (B) Donghwachi Formation; (C) Gasongdong Formation; (D) Dogyedong Formation; (E) Gisadong Formation; (F) Sinyangdong Formation. Mixing parameters and results are given in Tables 3 and 4.

Y.I. Lee / Sedimentary Geology 215 (2009) 1–12

11

was a significant process during deposition of the Donghwachi and Gisadong shales, and that recycling of fine-grained rocks may mask the weathering history of the source terrain. The result of this study suggests that active tectonism occurred throughout the deposition of the Hayang Group in the Yeongyang Subbasin. However, the locus of tectonic uplift varied over time. During the deposition of the Ullyeonsan, Gasongdong, and Dogyedong formations, active uplift occurred on the continent side; during the deposition of the Donghwachi, Gisadong, and Sinyangdong formations active uplift of the Jurassic accretionary complex occurred. 8. Conclusions

Fig. 9. (A) Stratigraphic variations in supply ratios of different source materials for Hayang Group shales. (B) Average proportions of source materials for Hayang Group shales: Precambrian basement (44%), arc basaltic rock (26%), Triassic Yeongdeok Granite (15%) and Mino accretionary complex (15%).

weathering conditions, where material removal rate matched the production of mineralogically uniform weathering products generated in the upper zone of soil development (Nesbitt et al., 1997). An alternative and more probable interpretation is that the CIA indices of the Donghwachi (64–74; av.= 69) and Gisadong (59–72; av. = 69) shales may reflect recycling, considering that much of these sediments were derived from the Jurassic Mino accretionary complex (terrane) in SW Japan (Chang et al., 1990; Kamata et al., 2000; Mitsugi et al., 2001; Lee and Kim, 2005). Joo et al. (2007) described the geochemistry of the Jurassic Mino terrane sediments, and showed that Mino terrane sands and muds exhibit a dispersed trend in A–CN–K space, from granodiorite source towards illite (CIA: 44–78) indicative of non-steady state weathering conditions in the source area. This interpretation suggests that recycling

The major and trace element compositions of shales in the Upper Cretaceous Hayang Group differ from typical arc volcanic rocks and PAAS. Most major elements except for SiO2 and K2O are slightly depleted with respect to PAAS. Chondrite-normalized REE patterns of Hayang Group shales are characterized by highly enriched LREE, negative europium anomalies, and varying depletion in HREE, which are in turn more fractionated than PAAS. Mixing calculations indicate that the average Hayang Group shale is composed of detrital components derived, in order of significance, from Precambrian granitic-gneissic rocks (43.7%), continental arc basaltic rocks (26.4%), and subequal amounts of accretionary complex clastic and cherty sediments (14.5%) and the Triassic Yeongdeok Granite (15.4%). As with the petrologic results derived from sandstones (Lee and Kim, 2005), fine-grained sediment derivation from the Jurassic Mino terrane, SW Japan was only recorded in the Donghwachi and Gisadong formations as well as in the Sinyangdong Formation. The average CIA value of the Hayang Group shales is 68, a value suggestive of moderate weathering in the source area. Active uplift occurred in the source area as indicated by non-steady state weathering regimes in most of the Hayang Group except for the Donghwachi and Gisadong formations. Although steady-state weathering conditions are indicated for the Donghwachi and Gisadong formations, the Jurassic accretionary complex was actively uplifted during deposition of these two formations but recycling of fine-grained sediments in the complex might have masked any evidence for its weathering history. The results of this study underscore the complementary aspects of data obtained from fine-grained and coarse-grained sediments in detecting the compositions of complex paleogeographic source areas. Acknowledgements This study was supported by a grant from the Korea Science and Engineering Foundation (R01-2008-000-20056-0). Dr. H.S. Lim, J.Y. Kim, S.H. Lee, M.K. Lee and Y.W. Lee provided field and/or laboratory

Fig. 10. A–CN–K (Al2O3 − CaO⁎ + Na2O − K2O) plots for Hayang Group shales (after Nesbitt and Young, 1984, 1989). Il: illite; Sm: smectite; Pl: plagioclase; Ksp: K-feldspar.

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