1 (2007) 141–148www.elsevier.com/locate/sedgeo

Sedimentary Geology 20

Origin and deposition of organic matter in continental chertof the Middle Permian Gufeng Formation in the

northeastern Yangtze platform

Masamichi Takebe a,⁎, Masao Kametaka b, Yukio Takayanagi c, Koichi Mimura d,Kenichiro Sugitani e, Koshi Yamamoto d

a Matsue National College of Technology, 4-14, Nishi-ikuma-cho, Matsue, Shimane, 690-8518, Japanb Dia Consultants Co. Ltd., Yoshino-cho 2-272-3, Kita-ku, Saitama 331-8638, Japan

c Aichi Prefectural Office, Sannomaru 3-1-2, Naka, Nagoya, 460-8501, Japand Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University,

Furo-cho, Chikusa, Nagoya, 464-8601, Japane Division of Environmental Engineering and Architecture, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8601, Japan

Received 14 September 2006; received in revised form 17 April 2007; accepted 17 May 2007

Abstract

The Middle Permian Gufeng Formation in the northeastern Yangtze platform, China, contains radiolarian chert that ischaracterized by black color, high content of Fe, Mo, Ni, Cu, and Zn. Based on the inorganic geochemical features, previous workconcluded that the cherts were deposited under sulfate-reducing conditions. In this study, organic carbon (Corg), N and S contents ofthe cherts and H/C and O/C ratios of kerogen were examined to explore the origin of the organic matter. The concentration of Corg

in the black chert is extremely high relative to other cherts in accretionary complexes and continental platforms. The average C/Nratio of organic matter is 35, indicating that the organic matter originates from land plants. The kerogen from the chert is classed astype IV based on the H/C and O/C ratios, suggesting that oxidized or reworked organic matter was supplied to the Gufeng basin.Therefore, the organic matter is considered to originate from terrestrial plants or reworked material from land. Due to thedepositional setting on a continental platform, nutrient supplied with organic matter from land probably played an important role inabundant biogenic productivity and sedimentation of the radiolarian chert on the continental platform.© 2007 Elsevier B.V. All rights reserved.

Keywords: Shelf-type bedded cherts; Middle Permian; C/N ratio of organic matter; Kerogen

1. Introduction

Bedded chert is one of the typical sedimentary rocksdeposited during the Paleozoic and has great potentialfor providing clues to reconstructing the Paleozoic

⁎ Corresponding author. Tel./fax: +81 852 36 5182.E-mail address: takebe@matsue-ct.jp (M. Takebe).

0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2007.05.011

marine environment. Many geochemical studies haveanalyzed cherts in accretionary complexes as a basis fordiscussing their depositional environments (e.g. Sugi-saki et al., 1982; Yamamoto, 1983, 1987; Aitchison andFlood, 1990; Murray et al., 1990, 1991; Girty et al.,1996; Takayanagi, 1998; Shimizu et al., 2001). Bycontrast, there are few studies of the chemistry of shelf-type bedded cherts that are deposited in platform basins,despite their frequent occurrence on paleoplatforms.

Fig. 1. Location of the Chaohu area.

142 M. Takebe et al. / Sedimentary Geology 201 (2007) 141–148

Kametaka et al. (2005) studied the mode of occurrenceand geochemistry of the Permian Gufeng chert in thenortheastern Yangtze platform of China. The GufengFormation is composed of black bedded chert with thickargillaceous intercalations. The Gufeng chert is charac-terized by high contents of normal paraffins, Fe, Mo, Ni,Cu, and Zn, which are concentrated in sediments undersulfate-reducing conditions (Ripley et al., 1990; Calvertand Pedersen, 1993). In addition, the chert containspyrite. Therefore, Kametaka et al. (2005) interpretedthat the cherts were deposited under sulfate-reducingconditions. Sulfate-reducing conditions are generallycaused by an abundant supply of organic material to thebasin.

Comparing the mode of occurrence and inorganicgeochemical features of other shelf-type bedded cherts,Kametaka et al. (2005) concluded that the thickargillaceous intercalations and enrichment of organicmatter and transition elements residing in sulfides aretypical features of bedded cherts deposited on conti-nental platforms. To obtain additional information aboutthe geochemical features of shelf-type bedded chert, thisstudy discusses the origin of the organic material in theGufeng chert. Because enrichment in organic matter issignificantly related to abundant biogenic productivityand an oxygen-depleted environment, the chemistry oforganic matter may provide information on the geo-chemical characteristics of the depositional basin.Although Kametaka et al. (2005) analyzed the normalparaffin composition of the Gufeng chert, informationabout the origin of the organic matter could not beobtained because the normal paraffin in the chert hadundergone severe maturation and thus showed a petro-leum-like composition.

This study examines the C/N ratio of organic matter,and H/C and O/C ratios of kerogen in the Gufeng chert.The C/N ratio of organic matter is usually well preservedin sediments that have undergone diagenesis (Meyers,1997). Algae typically have atomic C/N ratios between4 and 10, whereas vascular land plants have C/N ratiosof 20 and greater (Meyers, 1994). Therefore, the C/Nratio of sediment has been used widely as a proxy forthe origin of sedimentary organic matter (Meyers, 1994,1997; Sampei et al., 1997; Twichell et al., 2002; Gordonand Goñi, 2003). The H/C and O/C ratios of kero-gen can also provide information about the originof organic matter (Hunt, 1995). Although both H/C andO/C ratios of kerogen decrease with maturation, thematuration pathways of kerogen on the van Krevelendiagram (a plot of H/C and O/C atomic ratios) arerelated to the composition of source organic matter(Hunt, 1995).

2. Geological setting

The study area is located near Chaohu, Anhui Prov-ince, China (Fig. 1). The geology of the study area wasdescribed in Kametaka et al. (2005). In this area, Sinianto Middle Triassic formations are folded with wave-lengths of several kilometers and are unconformablycovered by post-Jurassic terrigenous coarse clastic rocks.The Middle Permian Gufeng Formation disconformablyoverlies the Lower Permian Qixia Formation and isconformably overlain by the Middle Permian YinpingFormation. The Gufeng Formation is divided litholog-ically into the lower and upper members, the PhosphateNodule-bearing Mudstone Member (PNMM) and theSiliceous Rock Member (SRM) respectively. PNMM ismainly composed of mudstone, which yields phosphatenodules and impressions of ammonoids and brachio-pods. SRM is mainly composed of alternating bedsof cherts, mudstones and siliceous mudstones, whichcommonly yield radiolarians and sponge spicules. In

Table 1Corg, N and S concentrations ofmudstone, chert, and porous chert (wt.%)

Sample no. Color Corg N S

Mudstone MS-02 Brown 0.585 0.242 0.110CH-01 Black 0.970 0.540 0.060CH-05 Black 1.86 0.090 0.130CH-10 Black with

brown part2.260 0.100 0.160

CH-11 Black 1.14 0.060 0.120CH-14bk Black 2.030 0.090 0.160CH-14bw Brown 2.030 0.090 0.180CH-15 Black with

brown part1.91 0.120 0.180

CH-21 Black withbrown part

1.58 0.070 0.120

Chert CH-24bk Black 0.910 0.080 0.080CH-24bw Brown 0.730 0.160 0.090CH-30 Black with

brown part1.80 0.100 0.200

CH-35 Brown withblack part

0.550 0.160 0.110

CH-37 Black 2.230 0.090 0.150CH-42 Black and brown 0.390 0.080 0.070CH-46 Black with

brown part0.330 0.050 0.020

CH-48 Gray 0.210 0.070 0.030CH-51 Blackish brown 0.340 0.070 0.020CH-52 Blackish brown 0.400 0.070 0.030NEC-01 Dark brown 0.630 0.180 0.050NEC-02 Brown 0.290 0.430 0.020

Porous chert NEC-03 Blackish brown 0.540 0.370 0.020NEC-04dbw Dark brown 0.500 0.090 0.030NEC-04bw Brown 0.830 0.130 0.060NEC-05 Brown 0.320 0.300 0.020

143M. Takebe et al. / Sedimentary Geology 201 (2007) 141–148

addition, SRM contains porous cherts with abundantrhombohedral cavities.

The cherts are less than 10 cm in thickness and arecolored black, grayish black to blackish brown. Somecherts have brown upper and lower margins severalmillimeters or more in thickness.

3. Samples and analytical methods

The analyzed samples are the same as for those ofKametaka et al. (2005). One mudstone from PNMM,seventeen cherts, and six porous cherts from SRM wereanalyzed. For analysis of the cherts and porous cherts,the central part of each bed was used except for CH-14,CH-24, and NEC-04. As a result, only the central blackpart of the chert was analyzed in the black cherts withbrown margins, except for CH-14, CH-24, and NEC-04.The black and brown parts of CH-14, CH-24 and NEC-04 were analyzed separately. To discuss the analyticaldata, the term black chert is used for the analyzed chertwith black or grayish black color, whereas the termbrown chert is used for chert in which the blackishbrown part or brown part is dominant.

Concentrations of organic C and total N were ana-lyzed by a NC analyzer (SUMIGRAPH NC-H-900) atShimane Institute for Industrial Technology. Beforeanalysis, the samples were washed with 0.1 mol/l HCl.The S concentration was determined by a CHNS ana-lyzer (Fisons EA 1108) at Nagoya University.

Kerogen was isolated from ten black cherts. A sam-ple of 30 to 40 g was rinsed with acetone for 8 h toremove soluble organic matter and then acidified with6 M HCl for 8 h to remove carbonate. The samplewas digested with 50% HF at about 90 °C for 8 h. Thedigested residue was centrifuged and rinsed with de-ionized water. The residue was rinsed again with 6 MHCl at about 90 °C for 8 h. The dark, viscous resi-due (kerogen) was sampled by filtration with a 100-μmmembrane filter and rinsed repeatedly with deionizedwater. The isolated kerogen was analyzed for C, H, N, Oand S by CHNOS analyzer (Fisons EA 1108) at NagoyaUniversity. Analytical errors of each analysis wereestimated to be less than 10%.

4. Results

The C, N and S concentrations of the analyzed sam-ples are shown in Table 1. The organic carbon (Corg)concentrations of cherts from Gufeng formation rangefrom 0.2% to 2.3%, and the average concentration is1.1% (Fig. 2A and C). The Corg concentrations of blackcherts are higher than those of the brown cherts. Porous

cherts show Corg concentrations of 0.2% to 0.8%,which are comparable to those of the brown cherts. Themudstone has a Corg concentration of 0.58%. All chertsshow nitrogen concentration (Ntotal) less than 0.2%,except for CH-01 (0.54%). Except for CH-01, theaverage Ntotal concentration of cherts is 0.06 %. Ntotal

concentrations of the porous cherts show 0.09% to0.4%, and that of the mudstone is 0.24%. Sulfur con-centrations of cherts range from 0.02% to 0.2%, andthe average concentration is 0.11% (Fig. 2C). Sulfurcontents of cherts are positively correlated with Corg

contents. Sulfur concentrations of porous cherts are0.02% to 0.06%, and that of mudstone is 0.11%.

Fig. 2A shows a correlation diagram of Corg/Ntotal ratio(molar ratio) andCorg concentration. The analyzed samplesdivide into two groups. One group is composed of blackcherts with high Corg concentration and Corg/Ntotal ratio.The other group consists of two black cherts, brown cherts,porous cherts, and a mudstone, which show low Corg

concentration and Corg/Ntotal ratio. A correlation diagramof Ntotal/Ti and Corg/Ti ratios (weight percent) is given asFig. 2B. The Ntotal/Ti and Corg/Ti ratios of black cherts,

Fig. 2. Correlation diagrams of elements contained in Gufeng cherts: (A) plot of C/N ratio (atomic ratio) vs. C (wt.%); (B) plot of Corg/Ti vs. Ntotal/Tiratios (atomic ratios); (C) plot of S vs. Corg (wt.%); (D) S/Ti vs. Fe/Ti ratios (wt.% ratios). Data of Ti concentrations are from Kametaka et al. (2005).NASC data are from Gromet et al. (1984).

144 M. Takebe et al. / Sedimentary Geology 201 (2007) 141–148

except for CH-01, correlate positively (R=0.88), and thecorrelation line has a slope of 35.

Table 2 shows the chemical composition of kerogenfrom black cherts. Fig. 3 shows a diagram of H/C and O/C ratios (atomic ratios) of kerogen in black cherts (vanKrevelen diagram). The kerogen in black cherts ischaracterized by low H/C and high O/C ratios.

5. Discussion

5.1. Depositional environment

SRM of the Gufeng Formation is characterized by theoccurrence of porous cherts (Kametaka et al., 2005).

The dissolution of authigenic dolomites after lithifica-tion is considered to have produced the rhombohedralcavities of the porous chert (Kametaka et al., 2005). Thedissolution of dolomite infers the possibility that thechemical composition of chert is affected by weathering.The black cherts have brown layers close to their upperand lower bedding plane margins (Kametaka et al.,2002, 2005). The boundary between the black andbrown zones is gradational, irregular, and indistinct.

Because black chert rich in Corg and microscopicobservations show that the matrix of black chert consistof black materials, the black color is probably ascribedto enrichment of organic matter. Kametaka et al. (2005)suggested the possibility that the brown coloration

Fig. 3. A diagram of H/C and O/C ratios (atomic ratios) of kerogen inblack cherts (van Krevelen diagram). Fields of type I, II, III and IVarefrom Hunt (1995).

Table 2Chemical composition of kerogen (wt.%)

Sample no. C H O

CH-01 68.4 2.19 17.6CH-05 68.4 2.24 19.0CH-10 67.8 2.34 19.2CH-11 68.4 2.16 18.2CH-14bk 67.7 2.23 19.3CH-15 65.9 2.37 21.4CH-21 68.8 2.24 20.1CH-24bk 69.0 2.21 19.7CH-30 67.9 2.09 20.3CH-37 68.7 2.22 20.7

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results from loss of organic matter by oxic weathering,because brown cherts and porous cherts shows low Corg

concentrations relative to black cherts and the boundarybetween the black and brown color is associated withbedding planes, veins and cracks, which are easily in-fluenced by weathering. To avoid the possibility thatthe depositional environment is discussed based onthe geochemical feature modified by weathering, wefocused on the chemistry of organic matter in the blackcherts.

Most of the black cherts have Corg concentrationshigher than 1.5%. The high Corg content is remarkable interms of geochemistry of cherts. Permian and Triassiccherts from the accretionary complex in southeast Japangenerally show Corg concentrations of less than 0.2%(Kakuwa, 1996). Even cherts close to the Permian–Triassic boundary in the accretionary complex have Corg

concentrations of about 1%, though the cherts areknown to be enriched in organic matter. The Corg con-centration of the Rex chert in the Phosphoria Forma-tion, which has similar lithofacies and geochemistry tothose of the Gufeng Formation, is generally less than0.5% (Hein et al., 2004).

Because the Corg concentration of chert is affected bythe dilution effect of SiO2, the extent of organic mattersupplied to the basin is hard to estimate using only Corg

concentration. The Corg/Ti ratio indicates the composi-tional ratio of organic matter and silicate detritus in thesediment, because Ti in sediment originates mainly fromsilicate detritus (Sugisaki, 1984; Murray and Leinen,1993; Takebe and Yamamoto, 2003). The Corg/Ti ratioof the Gufeng black chert is 91±45 (1σ). The Corg/Tiratios of Permian and Triassic cherts except for thoseoccurring along the P–T boundary in the Japaneseaccretionary complex are less than 2 (Kakuwa, 1996).The Rex chert has Corg/Ti ratio of less than 6. The highCorg/Ti ratio indicates that organic matter is abun-dantly supplied to the Gufeng basin, and the deposi-

tional environment was appropriate for the preservationof organic matter.

Based on the high content of normal paraffin andpresence of pyrites, Kametaka et al. (2005) consideredthat the Gufeng chert was deposited under sulfate-re-ducing conditions. Fig. 2D is the correlation diagramof Fe/Ti and S/Ti ratios (weight percent). Similar to theC/Ti ratio, the S/Ti ratio is high in the black chertsand low in the brown cherts, porous cherts and themudstone, suggesting that as well as organic matter,sulfur was probably lost during oxic weathering. InFig. 2D, most black cherts plot between two lines thatpass through the point of North American ShaleComposites (NASC) and have slopes correspondingto S/Fe ratios of FeS2 and FeS, respectively. The linesshow the mixing trend between detrital component(NASC) and iron sulfide (FeS2 or FeS). Fig. 2D suggeststhat Fe and S in black cherts reside in sulfide, whichsupports the interpretation of Kametaka et al. (2005).Brown cherts and some porous cherts plot below thelines of FeS2 and FeS in Fig. 2D. Although Kametakaet al. (2005) ascribed the high Fe contents of the Gufengchert to high sulfide content, Fig. 2D suggests that Fe inbrown and some porous cherts does not reside in sulfide.Considering that the geochemistry of brown and porouscherts was probably affected by weathering, iron sulfide

146 M. Takebe et al. / Sedimentary Geology 201 (2007) 141–148

can also be oxidized during weathering. As a result ofthe oxidization of the iron sulfide originally contained,Fe in brown and porous cherts is probably retained asiron oxide and S was probably lost.

5.2. Origin of organic matter

Algae usually have atomic C/N ratios between 4 and10, whereas vascular land plants have C/N ratios of 20and greater (Meyers, 1994). Ntotal concentration ana-lyzed in this study is the sum of organic and inorganicnitrogen concentrations. The contribution of inorganicnitrogen depresses the Corg/Ntotal ratio of a sediment(Müller, 1997; Meyers, 1997; Sampei et al., 1997).Fig. 2A shows that samples with low Corg concentrationhave low Corg/Ntotal ratios, which is probably related tothe relatively high content of inorganic nitrogen. Hence,any discussion of the origin of the organic matter insediments should focus on the Corg/Ntotal ratios of chertsshowing high Corg concentration. The samples with Corg

concentration higher than 1.5% have Corg/Ntotal ratioshigher than 25 (Fig. 2A).

To estimate the average Corg/Norg ratio of sediments,Sampei et al. (1997) used the slope of the regressionline on a diagram of Corg versus Ntotal concentrations.To remove the dilution effect of SiO2, we used thecorrelation diagram of Corg/Ti versus Ntotal/Ti ratios toestimate the average Corg/Norg ratio of the Gufeng cherts(Fig. 2B). The regression line for black cherts, except forCH-01, has a slope of 35 in Fig. 2B, suggesting that theaverage Corg/Norg ratio of the black chert is 35. If dia-genetic alteration is negligible, the Corg/Norg ratio in-dicates a large contribution of terrestrial organic matterto the bulk organic matter content in the Gufeng chert.

The diagenetic environment in the Gufeng basin isconsidered to be sulfate-reducing. Von Mooy et al.(2002) reported that nitrogen-rich organic matter undersuboxic condition is preferentially degraded and theCorg/Norg ratio increases. Hence, the high Corg/Norg ratioin the Gufeng chert could reflect either the originalcomposition of terrestrial organic matter or preferentialloss of N in organic matter originating from phyto-plankton during suboxic diagenesis.

We also examined H/C and O/C ratios of kerogensin black cherts (Table 2). The lower mudstone ofPNMM yields glauconite (Kametaka et al., 2005),which forms oxidized shallow marine (Tucker, 1991;Prothero and Schwab, 1996), suggesting that anoxicfluid have not flowed during diagenesis or weathering.Therefore, the kerogen from the Gufeng cherts, whichprobably have lower permeability than mudstone ofPNMM, is considered not to migrate from the continent

during diagenesis or weathering on a large scale, ratherto originate from in situ organic matter deposited in theGufeng basin. Although both H/C and O/C ratios ofkerogen generally decrease during maturation, the mod-ification pathways of these ratios are related to eachkerogen type.

The low H/C and high O/C ratios of the kerogen inthe black chert suggest that the organic matter in thechert are classed as type IV, which corresponds toreworked and highly oxidized organic material (Hunt,1995). The oxidation of organic matter in the Gufengchert did not occur during diagenesis because theconditions were sulfate-reducing at that time (Kametakaet al., 2005). Although Petsch et al. (2000) reportedthat the O/C ratio of kerogen potentially increases withorganic carbon and pyrite loss during oxic weathering,the O/C ratio of the black chert shows little variation anddoes not correlate either with Corg/Ti or Corg/Ntotal ratios,which were probably altered by weathering. In addition,the black chert is enriched in S and contains pyritessuggesting that black chert is not affected by oxicweathering. Therefore, the high O/C ratio of the blackchert was not caused by oxic diagenesis or weathering.Hence, the organic matter in the Gufeng chert is con-sidered to originate mainly from terrestrial or reworkedorganic matter. This interpretation of source is supportedby the average Corg/Norg ratio of 35.

The high Corg/Ti ratios suggest that terrigenous orreworked material supplied to Gufeng basin is rich inorganic matter relative to silicate. On the Yangtze plat-form, there are Qixia Limestone and Mokou Limestone,which underlies the Gufeng Formation and is theshallower water facies equivalent of the Gufeng For-mation, respectively (Jiang et al., 1994; Kametaka et al.,2005). Therefore, the high Corg/Ti ratios may reflect thatterrigenous soil around the Gufeng basin is calcareous.

Even though both the organic matter and the sili-cate detritus are terrigenous, Corg/Ti ratios of the blackcherts show some variation. In addition to the terrige-nous or reworked Corg, abundant radiolarian tests inthe cherts infer that marine planktons also supply theorganic matter. Because Gufeng chert intercalated withtuff, silicates in chert also supplied the basin through aneolian pathway which differs from that of terrigenous orof reworked detrital materials. Therefore, variation ofCorg/Ti ratio may reflect the contribution of terrigenousor reworked detrital materials, marine productivity, andeolian dust.

Based on sedimentological and lithological study,Kametaka et al. (2005) reported a transgression duringthe deposition of the lower and middle members of theGufeng Formation. The transgression probably forced

147M. Takebe et al. / Sedimentary Geology 201 (2007) 141–148

the supply of terrestrial and recycled organic matterfrom soils to the basin. High biogenic productivity isinferred from abundant phosphate nodules and radio-larian tests in the Gufeng Formation (Kametaka et al.,2005). The supply of terrestrial organic matter wasprobably associated with nutrient supply from land tothe basin. The high biogenic productivity in the basintherefore resulted from the contributions of terrestriallyderived nutrients in addition to the upwelling systemestablished along the equatorial margin of the Yangtzeplatform (Kametaka et al., 2005).

6. Summary

For this study, we determined Corg, Ntotal and Sconcentrations in the Gufeng chert, and performed CHOanalysis for kerogen in the black chert. The black chertis enriched with Corg, Ntotal and S, suggesting that theGufeng chert was deposited under sulfate-reducingconditions. The average Corg/Norg ratio is 35, and theH/C and O/C ratios of kerogen are classed as type IV.These geochemical features of the organic matter sug-gest that terrestrial or reworked organic material wassupplied to the Gufeng basin from land. The nutrientassociated with the organic matter supplied from landprobably played an important role in high biogenicproductivity, and may be the reason for the deposition ofradiolarian chert in the Gufeng basin on the continentalplatform.

Acknowledgements

We especially wish to express our gratitude to Ms.P. J. Murrow, Matsue National College of Technologyfor her patience in revising the English version of thepaper. We also thank Dr. T. Shiomura for CN analysis atShimane Institute for Industrial Technology. We are alsograteful to Dr. Zhu and Dr. C. Wu of Nanjing Universityand Dr. H. Nagai of Nagoya University for their helpwith fieldwork.

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