Journal of Asian Earth Sciences 81 (2014) 28–37

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Journal of Asian Earth Sciences

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Diagenesis of clay minerals and K-bentonites in Late Permian/EarlyTriassic sediments of the Sichuan Basin (Chaotian section, Central China)

1367-9120/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2013.11.018

⇑ Corresponding author. Tel.: +33 380 396 331; fax: +33 380 396 387.E-mail addresses: jean-francois.deconinck@u-bourgogne.fr (J.F. Deconinck),

sylvie.crasquin@upmc.fr (S. Crasquin), francois.baudin@upmc.fr (F. Baudin).

J.F. Deconinck a,⇑, S. Crasquin b, L. Bruneau a, P. Pellenard a, F. Baudin c, Q. Feng d

a Université de Bourgogne, UMR 6282 CNRS Biogéosciences, 6 Bd Gabriel, 21000 Dijon, Franceb CR2P – UMR 7207, CNRS, MNHN, UPMC-Univ. Paris 06, T. 46-56, E.5, Case 104, 4 Place Jussieu, 75252 Paris Cedex 05, Francec UPMC-Univ. Paris 06, CNRS, UMR 7193 ISTeP, Case 117, 4 Place Jussieu, 75252 Paris Cedex 05, Franced State Key Laboratory of Geological Process and Mineral Resources, University of Geosciences, No. 388, Lumo Road, Wuhan 430074, People’s Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 March 2013Received in revised form 22 November 2013Accepted 25 November 2013Available online 9 December 2013

Keywords:K-bentonitesPermian-Triassic boundaryPalaeothermicitySichuan BasinSouth China

Detailed clay mineralogical analyses were carried out on Late Permian/Early Triassic carbonate sedimentsexposed on the Chaotian section (Sichuan Basin, Central China). The clay assemblages are dominantlycomposed of illite in platform carbonates and clay seams, and illite–smectite mixed-layers (I/S) in tufflayers (K-bentonites) intercalated in the carbonate succession. Detrital and authigenic volcanogenic clayminerals have been partially replaced through illitisation processes during burial, raising questions aboutdiagenetic effects. The precise determination of I/S occurring in K-bentonites shows that the sedimentsreached a temperature of about 180 �C, which is consistent with (1) previous estimates based on fluid-inclusion homogenisation temperature analysis, (2) the burial depth of the sedimentary series deducedfrom the post-Palaeozoic geological history of the Sichuan Basin and (3) the new data (Tmax) obtainedon organic matter indicating the transition between oil and gas windows. The Wangpo Bed, located closeto the Guadalupian–Lopingian Boundary, is interpreted either as a volcanic acidic tuff or as a clastic hori-zon. This controversial origin probably results from mixed volcanogenic and detrital influences. The Wan-gpo Bed is therefore interpreted as a reworked bentonite as revealed by the occurrence of I/S similar tothose found in tuff layers, together with preserved detrital kaolinite.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Numerous wells have been drilled and natural gas fields discov-ered in the Sichuan Basin, where oil and gas exploration has takenplace for over 45 years. The search for hydrocarbons is partly basedon good knowledge of the diagenetic and thermal history of sedi-mentary basins. In the case of the Sichuan Basin, hydrocarbon gen-eration originated in the diagenetic evolution of source rocks ofEdiacarian (Sinian) and Lower Palaeozoic ages. Studies focusingon hydrocarbon accumulations and thermal evolution histories ofsome parts of the Sichuan Basin have been performed, notablyusing vitrinite reflectance and apatite fission-track (Guoshenget al., 2008; Jianzhong et al., 2008). The Sichuan area evolved froma continental margin (Ediacarian to Middle Triassic) to a forelandbasin (Late Triassic), and then an intracontinental depression filledwith thick continental deposits (Jurassic and Early Cretaceous); up-lift and denudation occurred from the Late Cretaceous onwards(Jianzhong et al., 2008). In the Sichuan Basin, clay minerals havenot been used until now to estimate the thermal evolution of

Palaeozoic sediments, although illite/smectite mixed-layered min-erals, commonly found in bentonite beds, may provide accurateinformation about the thermal history of sedimentary rocks (e.g.Srodon et al., 2009, 2013).

The objective of this study is therefore to clarify the diageneticevolution of Permian sediments deposited in the Sichuan Basin byusing clay mineral assemblages and the degree of evolution oforganic matter. The Chaotian section is a suitable site, as manybentonite layers are intercalated in the Late Permian sedimentarysuccession, which has been thoroughly studied from sedimento-logical, geochemical and palaeontological points of view (e.g.Isozaki et al., 2004, 2008; Isozaki, 2009). The composition of claymineral assemblages was studied by X-ray diffraction and thedegree of evolution of organic matter was determined byRock–Eval pyrolysis.

2. The Chaotian section (Sichuan Province)

The Late Permian/Early Triassic sediments now exposed in theSichuan Basin were deposited on the eastern side of thesupercontinent Pangea. In the Chaotian reference section, thesedimentary succession recorded major global environmentalchanges and events, leading to two major mass extinctions, at

J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37 29

the Guadalupian/Lopingian Boundary (GLB), and at the Permian/Triassic boundary (PTB) (Jin et al., 1994; Stanley and Yang, 1994;Yin et al., 2007; Isozaki, 2009; Clapham et al., 2009). The Chaotiansection exposes Late Permian and early Triassic carbonate sedi-ments along a gorge of the Jialingjiang River to the north of Chao-tian (north of Guangyuan, Fig. 1, Isozaki et al., 2004). This sectionmainly consists of shallow marine shelf carbonates deposited onthe Palaeo-Tethyan side of South China (Isozaki et al., 2008). Guad-alupian sediments (Maokou Fm.) consist of massive calcareousbeds with occasional flint layers, first informally designed ‘‘Unit1’’ (Lai et al., 2008) and recently named ‘‘Limestone unit’’, includingthree subunits (L1–L3) showing distinct lithofacies (Saitoh et al.,2013a,b) (Fig. 2). The carbonates consist mainly of packstonesand wackestones, with marine fossils including calcareous algae,foraminifers, rugose corals, bryozoans, crinoids, brachiopods andgastropods (Isozaki et al., 2008). These assemblages indicate shal-low-water carbonate-platform depositional environments. Theuppermost part of the Guadalupian is characterised by the occur-rence of dolomitic limestones, overlain by poorly exposed mollusc-and brachiopod-rich thinly bedded siliceous sediments, termed‘‘Unit 2’’ (Lai et al., 2008) of the Maokou Fm (Fig. 2). This unit, re-cently described in more detail and renamed ‘‘Mudstone Unit’’ bySaitoh et al. (2013a,b), corresponds to deep-slope mudstones yield-ing radiolarians, conodonts, ammonoids, gastropods and brachio-pods. The change in facies from Limestone to Mudstone Unitsclearly results from a significant deepening (Isozaki et al., 2008;Saitoh et al., 2013a,b) due to local, tectonic-related subsidence ofthe Sichuan Basin (Saitoh et al., 2013a,b). However, Lai et al.(2008) suggest a different interpretation. For these authors, Unit2 = Mudstone Unit was mainly deposited in a restricted dysaerobiclagoonal setting occasionally connected with open-marine envi-ronment. Our field work leads us to favour the first interpretation.The Mudstone Unit is capped by the Wangpo Bed (Fig. 2), a 2 m-thick deposit interpreted as a deeply weathered air-fall ash bed, re-ferred to as ‘‘G-L Boundary acidic tuff’’ by Isozaki et al. (2004) andtermed Xuanwei Fm in South China (He et al., 2007), where volca-niclastic deposits are frequently found at the GLB (Wignall et al.,2009; Bond et al., 2010). The volcanic origin of the Wangpo Bedwas recently challenged by He et al. (2010) who suggest that it cor-responds to a clastic deposit originating from the erosion of volca-nic rocks from the Emeishan Large Igneous Province (ELIP). Theyrely primarily on the abnormal abundance of quartz, incompatiblewith volcanic fall-out, the presence of a high proportion of roundedcrystals of zircon, and the absence of a significant negative Euanomaly, which is often a characteristic of acidic volcanic ashdeposits (Wray, 1999). The Wangpo Bed is covered by a black argil-laceous horizon considered as coaly mudstone with plant debris

SICH UAN

CHINA

N

500 km

BEIJING

Fig. 1. Location of the

(Isozaki et al., 2008). This ‘‘black level’’ (Fig. 2) is interpreted asbeing deposited in near-shore environments after a presumed ma-jor regressive event at the end of the Capitanian (Isozaki et al.,2008). Similar facies of the Heshan Fm are widespread in SouthChina (Shao et al., 1998). These coal seams intercalated in LatePermian carbonate successions were believed to have been depos-ited in swamps developed on or adjacent to tidal flats (Shao et al.,1998, 2003) but according to He et al. (2010) they could derivefrom the erosion of mafic lavas.

The Lopingian stratigraphic series (Fig. 3) comprises the Wujia-ping Fm, which is dominantly composed of bioclastic packstones,with foraminifera, calcareous algae, siliceous sponge spicules,ostracods and brachiopods (Lai et al., 2008). Numerous centimetricto decimetric, yellowish to buff-coloured tuff layers are interca-lated in this Fm with an upward-increasing frequency. The overly-ing Dalong Fm, is composed of siliceous rocks, organic matter-richdark shales and laminated limestones with ammonites, depositedin deep, lower slope and basin floor environments (Isozaki et al.,2007; Chen et al., 2012). Felsic tuff layers are also intercalated inthis Fm. The section ends with the Feixianguan Fm (uppermostLopingian/lowermost Induan), composed of thinly bedded lightgrey micritic limestones. The PTB is located in this Fm, betweenunits E and F (Fig. 3, Ji et al., 2007).

Samples were collected throughout the section, focusing partic-ularly on the clay seams because the proportion of clay minerals incarbonates is extremely low. Most bentonite horizons were sam-pled. Three samples were taken in the least altered part of the2 m-thick Wangpo Bed, from the centre to the upper edge.

3. Methods

3.1. X-ray diffraction

Clay minerals were first routinely identified by X-ray diffraction(XRD) on oriented mounts of non-calcareous clay-sized fraction(<2 lm), following the analytical procedure of Moore and Reynolds(1997). After removing carbonate using 0.2 N HCl, defloculation ofclays was done by successive washing with distilled water. Parti-cles finer than 2 lm were concentrated by centrifugation. Diffrac-tograms were obtained using a Bruker D4 Endeavor diffractometerwith Cu Ka radiations, LynxEye detector and Ni filter, under 40 kVvoltage and 25 mA intensity. Three XRD runs were performed,respectively after air-drying (AD), ethylene–glycol solvation (EG),and heating at 490 �C during 2 h. The goniometer performed a scanfrom 2.5� to 28.5� 2h for each run. Identification and semi-quanti-tative estimates of clay minerals was made according to the posi-tion and the area of the (001) basal reflections on the three XRD

33°N

32°N

33°N

32°N

106°E 107°E105°E104°E

Ji a l ingj ia ng

River

Ch

anji ang R

iver

Chengdu

Chongding

Guangyuan

ChaotiansectionChaotiansection

Sichuan

GansuShaanxi

100 km

Chaotian section.

162161

160

sam

ple

nb.

162

1.5210 34567820

sam

ple

nb.

powder bulk rock

Kaolinite

black level

1607.16

5.059

4.463

4.172

3.5

7

3.52

3.248

2.561

2.3

36

1.892

1.6

66

12.10

3.83

4.3

62

1.489

Anatase

I/S

Kaolinite

7.16

12.33

14.14I/S

Kaolinite

Wangpo Bed

d-scale

14.517.14

5.01

Q KA

Q

C

161

4.465

3.841

3.5

7

3.52

5.01

4.1

716

3.345

3.241

3.032

2.5

61

2.338

1.8931.817

1.666

1.541

1.489

Quartz

Calcite

Anatase

Transition

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1

0

2

3

4

5

m

163164165

171

m

Capita

nia

n

siliceous mudstone

marly limestone

marl

ash-layer

limestone

dolomitic limestone

GU

AD

ALU

PIA

N Wangpo

Bed

black level

MA

OK

OU

FM

Wujia

pin

gF

mM

AO

KO

UF

M(

Mudst

one

unit)

159

Wangpo Bed 159

7.16

152

4.98

4.457

4.164

3.5

6

3.5

1

2.5612.334

1.892

1.6

66

1.489

LO

PIN

GIA

N

Wuchia

pig

ian

3 10 20 30 40 50

sample 165

clay fraction

Legend

1.997

3.349

4.2685.013

10.08

° 2θ (Cu-Kα)

3.85

1.4891.5411.6661

.8172.5

6

2.281

3.032

I/S

illite Q

14.90

Lim

esto

ne

Unit

(L3)

Fig. 2. Lithological succession from the uppermost Guadalupian Maokou Formation to the Guadalupian/Lopingian Boundary. XRD profiles (bulk rock) of samples from theWangpo Bed and the overlying ‘‘black level’’. I/S = illite/smectite mixed-layers, A = anatase, C = calcite, K = kaolinite, Q = quartz.

30 J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37

(Moore and Reynolds, 1997). To identify more precisely illite/smectite interstratifications according to Srodon’s procedure (Sro-don, 1980, 1984) the X-ray diffractograms were made on orientedpreparations with a scanning angle ranging from 2.5� to 60� 2hSome bulk rocks were also analyses by X-ray diffraction.

3.2. Scanning and Transmission Electron Microscopy (SEM and TEM)

SEM observations were performed using a Scanning ElectronMicroscope Philips XL30SM to verify the presence or absence ofauthigenic crystals of kaolinite. Four samples including the Wan-gpo Bed and the black level (Heshan Bed) located at the GLB wereobserved. TEM observations were performed using a TransmissionElectron Microscope Hitachi to examine the morphology of I/Sfrom the tuff layers and also to verify the presence or absence ofauthigenic crystals of kaolinite.

3.3. Rock Eval analyses

A Rock–Eval 6 Turbo apparatus (Vinci Technologies) was usedfor quantitative and qualitative study of the organic matter from8 selected samples. The method, described in details by Lafargue

et al. (1998) and Behar et al. (2001), consists in a two steps analysiswith programmed temperature: a pyrolysis, under inert atmo-sphere (N2), followed by an oxidation. Crushed samples are firstsubjected to a 3 min isotherm at 300 �C at which free hydrocarbonsare volatilised (peak S1). Then, a heating with a ramp of25 �C min�1 leads to the vaporisation of products from thermalcracking of organic matter up to 650 �C (peak S2). Pyrolysis efflu-ents are continuously detected by a flame ionisation detector(FID) and expressed in mg per g of sample. Organic CO and CO2

are measured online as well by an infrared cell (peak S3). At theend of the pyrolysis step, samples are automatically transferredinto an oxidation oven where they are subjected to a 1 min iso-therm at 300 �C then a ramp of 20 �C min�1 up to 850 �C. Total sig-nals of both organic and mineral CO and CO2 are expressed in mgper g of samples. All those parameters allow the calculation ofthe total organic carbon (TOC). The hydrogen index (HI), corre-sponding to the quantity of hydrocarbon compounds released dur-ing the pyrolysis relative to the TOC (S2/TOC) in mg of HC per g ofTOC as well as the oxygen index (OI) corresponding to the organicCO2 released during the pyrolysis relative to the TOC (S3/TOC) inmg of CO2 per g of TOC are also calculated. The HI and the OI arecorrelated to the H/C and O/C atomic ratios respectively, which

11CHA0122

11CHA0140

11CHA0502

11CHA02

11CHA04

11CHA05

11CHA08

11CHA09

11CHA011

11CHA019

11CHA028

11CHA032

11CHA035

11CHA044

11CHA047

11CHA054

11CHA014811CHA0149

UN

ITF

UN

ITE

UN

ITE

UN

ITD

UN

ITC

UN

ITB

UN

ITB

UN

ITA

Wujia

pin

gF

m

Triassic

Perm

ian

Dalo

ng

Fm

.

Dalo

ng

Fm

.

2m

Feix

ianguan

Fm

.

Wujia

pin

gF

m

siliceous mudstone

marly limestone

ash-layer

limestone

Legend

marls and clay seams

chert

11CHA020

11CHA024

11CHA043

11CHA054bis

11CHA058

11CHA061

11CHA094

11CHA089

Fig. 3. Lopingian to lowermost Triassic stratigraphic succession, including Wujiaping, Dalong and Feixianguan formations. The boundary between Dalong and Feixianguan islocated between units D and E.

J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37 31

allow determining the origin of the organic matter. Tmax is definedas the pyrolysis temperature at which the maximum amount ofhydrocarbon is yielded by kerogen (Espitalié et al., 1977). Tmax in-creases linearly with the maturation degree of the organic matter,thus giving a rapid estimate of the thermal maturity of sedimen-tary basins (Espitalié, 1986).

4. Results

4.1. Clays at the Guadalupian/Lopingian Boundary

Five samples from the uppermost part of the Maokou Fm werestudied. The proportion of clay minerals is extremely low in bothcarbonate and siliceous facies of this Fm and it is therefore difficultto extract a sufficient amount of clay minerals for X-ray analyses. Avery well-crystallised illite, associated with quartz, is the onlyidentifiable clay mineral. A typical X-ray diagram (sample 165) isshown in Fig. 2.

In the Wangpo Bed (samples 159 and 160), analysis of the bulkrock indicates the presence of clays, including ordered illite/smec-tite mixed-layers (I/S), kaolinite associated with feldspars (3.24 Å),and anatase identified by a well-defined peak at 3.52 Å (Fig. 2). Theabundance of quartz previously reported by He et al. (2007) is notfound here. Samples 159, 161 and 162 probably contain a mixtureof two expandable phases. The first phase shows a relatively sharppeak between 14.1 and 14.9 Å and may correspond to a chlorite re-cently weathered into a vermiculite/smectite mixed-layer. Such acase was documented on Silurian bentonites from Podolia,

(Ukraine, Srodon et al., 2013). The second phase, showing a broadpeak around 12.1–12.3 Å, corresponds to illite/smectite mixed-lay-ers. The analysis of the clay fraction (<2 lm, Fig. 4) indicates thepresence of ordered I/S, kaolinite, with discrete illite notably insample 160, and an expandable phase close to smectite in sample159.

The transition from the Wangpo Bed to the coaly black level isprogressive. Sample 161 was taken in this interval. The mineralog-ical composition (bulk) is quite similar to that of the Wangpo Bed,but quartz and calcite are present (Fig. 2). Due to the presence ofseveral expandable minerals, the D�2h technique (Srodon, 1984;Moore and Reynolds, 1997) cannot be used to estimate %S (percentof smectite layers), but based on low-angle peaks, samples 161,160 and 159 (Fig. 2) give respectively 37%S, 34%S, and 26%S, show-ing a clear trend from the edge towards the centre of the WangpoBed.

In the overlying organic matter-rich black level (sample 162),quartz and calcite are much more abundant, while the proportionof anatase decreases (Fig. 2). As in the underlying levels, the clayfraction is composed of I/S, kaolinite and possibly a weatheredchlorite. The sharpness of its main diffraction peak indicates thatthe kaolinite is very well crystallised.

4.2. Clays of the Wuchiaping, Dalong and Feixianguan Fms

The felsic tuffs (pyroclastic horizons) and centimetric clayseams occurring in these Fms have yielded good-quality diffracto-grams (Fig. 5). Throughout the section, the clay fraction of detrital

Fig. 4. XRD patterns of glycol-solvated oriented preparations (Ca-saturated, <2 lm)from the Wangpo Bed and the overlying ‘‘black level’’.

54bis

61

58

24

149

89

94

° 2θ (Cu-Kα)3 10 20

sample

Illite

Illite

K

K

K

Quartz

Quartz

Illite

Illite

G

Wujiaping Fm.

Dalong Fm.

Feixianguan Fm.

Fig. 5. XRD patterns of glycol-solvated oriented preparations (Ca-saturated, <2 lm)of some clay seams from the Wujiaping, Dalong and Feixianguang Formations.G = goethite, K = kaolinite.

32 J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37

clay seams consists mainly of illite and quartz, with traces of illite–smectite mixed-layers, kaolinite and goethite. The clay fraction ofthe tuffs consists of almost pure illite–smectite mixed-layers,showing diverse XRD profiles (Fig. 6). The only admixtures de-tected are traces of goethite and anatase. The XRD results obtainedon I/S are summarised in Table 1, and some examples of XRD pat-terns are shown in Fig. 6, in stratigraphic order. Several types of I/S,including R1 and R3 (Reichweite parameters, Moore and Reynolds,1997), are identified with a range of %S from 6% to 22%. Sample 502shows a strong reflection at 5.4�2h The clay fraction of this sampleprobably contains a mixture of R0 with 65% S, and R3 (Srodon, pers.communication).

4.3. Characterisation of organic matter (Table 2)

The eight samples studied contain between 0.15 and 12.65%TOC, with values above 2% for most samples. The free hydrocarboncontent (S1) is very low (0.01–0.38 mg/g rock) and the pyrolysablehydrocarbon content (S2) ranges from 0.05 to 1.18 mg/g rock. Suchvalues are very low considering the organic carbon content of the

sample and suggest that most of the organic matter no longerhas petroleum potential. This is confirmed by the Tmax values rang-ing from 453 to 549 �C (Table 2). Note that Tmax cannot be deter-mined in sample 43 because it yields no or very littlehydrocarbon (<0.1 mg/g rock) during pyrolysis. These high Tmax

values indicate that the Chaotian section experienced high thermalmaturation.

The HI–Tmax diagram (Fig. 7) shows that all samples of theChaotian section have reached at least the end of the oil windowand are mainly in the gas window, with respect to oil generation.Due to this high thermal transformation, the origin of organic mat-ter is difficult to determine using the HI and OI parameters.

4.4. SEM and TEM

Pictures of selected samples are shown in Fig. 8. Particles (gen-erally less than 1 lm) of I/S R3 from tuff layers (sample 54) displayclear outlines, the typical morphology of mixed-layers originatingfrom the illitisation of smectites (Ferrage et al., 2011). In the Wan-gpo Bed, TEM observations confirm the occurrence of anhedral orsubhedral kaolinite, together with I/S. Although the Wangpo Bedsamples have been examined in detail by SEM, no euhedral crystalsof kaolinite could be highlighted (Fig. 8). In the ‘‘black level’’, I/S

502

28

122

3 10 20 30 40 50

7.31

A

9.30

17.09

26.72

44.8134.63G

5.40

9.0416.18

26.64

44.49

7.86

9.09

17.17

26.51

34.71

44.56

G

35

547.84

8.86

7.99 9.05

17.15

26.50

26.61

17.26

34.74

34.93

44.53

44.68

R1 - 22% S

Mixture R0 65% S

and R3

R3 - 16% S

R3 - 14% S

R3 - 12% S

G

G

G

A

A

sam

ple

num

ber

Wuchiaping Fm.

Dalong Fm.

Dalong Fm.

Dalong Fm.

Feixianguan Fm.

Fig. 6. XRD patterns of glycol-solvated oriented preparations (Ca-saturated, <2 lm)showing the variability of I/S from Lopingian K-bentonites (%S = percent of smectitelayers). A = anatase, G = goethite.

J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37 33

show more fleecy outlines, consistent with the higher %S of I/Soccurring in this sample.

5. Discussion

5.1. Tuff layers

In the clay fraction of tuff layers, the major component is anillite–smectite mixed-layer (I/S), a characteristic mineral of mostPalaeozoic K-bentonites (Altaner et al., 1984; Teale and Spears,1986; Brusewitz, 1986; Kolata et al., 1996; Clayton et al., 1996;Huff et al., 1997; Spears, 2006; Srodon et al., 2009; Somelaret al., 2010). Many studies have concluded that I/S in K-bentonitesare a diagenetic product of smectite alteration (e.g. Lanson et al.,2009). Smectite usually derived from the submarine alteration ofvolcanic ash shortly after deposition in marine environments(Fisher and Schmincke, 1984). In Meso-Cenozoic sediments,smectite originating from the submarine weathering of volcanicglass shards is either preserved or slightly transformed into

I/S R0 (Deconinck and Chamley, 1995; Wray, 1995; Pellenardet al., 2003). In Palaeozoic series, which are often more deeply bur-ied, smectites may be exceptionally preserved (Calarge et al., 2003)but, as these minerals are sensitive to the thermal conditions andgeochemical environments that have characterised their post-depositional history, they are commonly replaced by I/S throughprogressive illitisation processes. These include either dissolutionand precipitation, or solid state transformation (Altaner andYlagan, 1997; Cuadros and Altaner, 1998; McCarty et al., 2009;Ferrage et al., 2011). Illitisation, which corresponds to the decreasein percent smectite, starts around 70–80 �C (Srodon et al., 2009).The rate of illitisation depends mainly on the relative effect oftemperature, time and chemical environment, notably K availabil-ity, water–rock ratio, and permeability (Ylagan et al., 2000).

The presence of I/S mixed-layers (R1 and notably R3) in thevolcanic horizons of the Chaotian section clearly indicates thatthey correspond to K-bentonites. Surprisingly, I/S show variablepercentages of smectite layers (%S) ranging from 6% to about 65%(Table 1), although K-bentonites are from the same section andwere submitted to the same burial history. The availability ofpotassium in each tuff layer is probably responsible for thesevariable %S. Knowing that the potassium necessary for illitisationcomes in part from the host rock, illitisation processes may belimited due to reduced diffusion of this element in thick levels ofK-bentonite (Altaner et al., 1984) leading to coexisting significantlydifferent I/S, including for example R0 and R1 I/S (McCarty et al.,2009). This is likely to be the case in the Chaotian section, as thehighest %S are encountered specifically in the 2 m-thick WangpoBed and in the thickest K-bentonite (15 cm, sample 502) whileother levels have a thickness ranging only from 1 to 7 cm.

5.2. Palaeotemperature estimation

The percentage of smectite layers present in I/S is commonlyused to estimate temperatures reached by sedimentary sequencesduring the burial, subsidence and tectonic history of a sedimentarybasin. The Phanerozoic subsidence history of the Sichuan Basincomprises four stages (Fig. 9): stable subsidence during the LowerPalaeozoic with continuous sedimentation from the Cambrian tothe Silurian, uplift and denudation during the Devonian and theCarboniferous, rapid subsidence from the Permian to the Late Cre-taceous and, finally, rapid uplift and denudation since the Late Cre-taceous (Meng et al., 2005; Shugen et al., 2008; Jianzhong et al.,2008). Permian to Late Triassic sediments are mainly marine, whilethick continental deposits characterise the Jurassic and the Creta-ceous. This burial history summarised in Fig. 9 shows that Permiansediments underwent burial to a depth of about 6000 m. In theSichuan Basin, hydrocarbon generation processes have been stud-ied, notably using fluid-inclusion homogenisation temperatureanalyses (Guosheng et al., 2008). Results show that oil originatedfrom Cambrian source rocks (Qiongzhusi Fm) gradually crackedto gas, due to high temperatures, reaching 200–220 �C.

The lowest reliable %S found in the K-bentonites studied rangefrom 6 to 16%, suggesting that the temperatures reached by thesediments are close to 180 �C (Šucha et al., 1993; Srodon et al.,2009). Assuming an average geothermal gradient of 30 �C/km, thisresult implies a burial depth for the Permian sediments of about6000 m, consistent with previous estimates based on other palae-othermometric methods. This burial depth and the temperaturereached by the sedimentary series together explain the illite-dom-inated clay fraction of detrital clay seams. It is obvious that the clayassemblages dominated by illite were homogenised under theinfluence of intense diagenesis.

The Tmax values may be used as a thermal maturation index(Espitalié, 1986), but cannot provide a direct palaeotemperaturedetermination, as organic matter evolution also depends on time,

Table 1Position of diffraction peaks of I/S, D�2H and %S in K-bentonites, the Wangpo Bed and the ‘‘black level’’. The grey lines correspond to samples with diffractograms shown in Figs. 5and 6.

Sample XRD reflection (�2H) glycol-solvated oriented preparation D�2H Reichweite %S

Sup-reflect. 001/002 002/003

54 7.99 9.05 17.26 26.61 34.93 44.68 8.21 R3 1447 8.04 8.81 17.34 26.54 34.83 45 8.53 R3 744 7.9 8.93 17.17 26.52 34.69 Diff 8.24 R3 1335 7.84 8.86 17.15 26.5 34.74 44.53 8.29 R3 1232 7.9 8.78 17.15 26.5 34.69 44.61 8.37 R3 1028 7.86 9.09 17.17 26.51 34.71 44.56 8.08 R3 1619 7.8 8.91 17.09 26.4 34.69 44.43 8.18 R3 1511 7.85 8.7 17.17 26.45 34.79 44.58 8.47 R3 8

8 7.83 8.66 17.25 26.35 34.74 44.48 8.59 R3 64 7.77 8.82 17.1 26.42 34.6 44.51 8.28 R3 122 7.81 8.76 17.19 26.42 34.79 44.51 8.43 R3 9

502 5.4 9.04 16.18 26.64 32.33 44.49 7.14 Mixture149 7.82 9.03 17.25 26.6 34.84 44.83 8.22 R3 14140 7.35 9.03 17.11 26.63 Diff 44.65 8.08 R3 16122 7.31 9.3 17.09 26.72 34.63 44.81 7.79 R1 22162 Black-level 2.8 5.31 5.65 8.76 17.1 26.46 Diffuse 44.19 Mixture161 Transition 6.57 9.42 16.78 26.3 Diffuse 44.02 R1 37160 Wangpo bed 6.71 9.07 16.78 26.29 Diffuse 43.99 R1 34159 5.2 7.13 8.53 17.24 26.28 Diffuse 44.22 R1 26

Table 2Results of Rock Eval analysis. S1: free hydrocarbons, S2: pyrolysable hydrocarbons, S3: organic CO2, Tmax: temperature at the maximum of hydrocarbon yield, TOC: Total OrganicCarbon, Min C.: mineral carbon, HI: Hydrogen Index, OI: Oxygen Index.

Sample S1 (mg/g) S2 (mg/g) S3 (mg/g) Tmax (�C) TOC (%) Min. C (%) HI (mg HC/g TOC) OI (mg CO2/g TOC)

2 0.01 0.19 2.99 518 1.47 0.43 13 2035 0.04 0.62 1.92 453 2.99 1.31 21 649 0.03 0.55 3.29 518 4.64 0.31 12 71

20 0.03 0.91 4.06 521 5.44 0.60 17 7524 0.38 1.18 0.38 464 2.41 4.35 49 1643 0.01 0.05 0.40 – 0.15 1.75 33 267

148 0.02 0.29 3.58 526 6.40 0.35 5 56162 0.01 0.45 10.71 549 12.65 0.00 6 135

– Unsignificant.

400 420 440 460 480 500 520

150

300

450

600

750

0

Hydro

gen

Index

(mg

HC

/gT

OC

)

TypeII

TypeIII

Typ

eI

Tmax ( °C)

Immature Oil window Gas window

1.3

%V

itrin

itere

flect

ance

0.5

%

540 560

Fig. 7. Diagram HI/Tmax showing that the samples have reached the end of the oilwindow and the gas window.

34 J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37

and organic matter type. Nevertheless, a range of palaeotempera-tures may be estimated. The lowest mature sample of the Chaotiansection shows a Tmax of 455 �C, near the end of the oil window,whereas the most mature sample has a Tmax of 549 �C, near the

transition between wet and gas zones, for Type III organic matter.The transition between wet and gas zones is known to be around200 �C in most sedimentary basins, while the end of oil windowis near 150 �C. Several attempts have been made to compare dataon organic matter maturity and the degree of illitisation (e.g. Veldeand Espitalié, 1989). The most recent, that of Dellisanti et al.(2010), shows that the occurrence of I/S R3 are quite compatiblewith Tmax values higher than 465 �C. Consequently, the range ofpalaeotemperatures deduced from the evolution of organic matteris consistent with those estimated using the clay mineralogicalcharacteristics of K-bentonites.

5.3. Origin of the Wangpo Bed

The clay fraction of the Wangpo Bed shows similarities with theclay fraction of the tuff layers, notably the abundance of I/S and theoccurrence of anatase, but also a significant difference, the occur-rence of kaolinite. As in the tuff layers, I/S probably derive fromthe illitisation of volcanogenic smectite confirming a volcanic ori-gin of this bed (Isozaki et al., 2004).

Kaolinite, a common mineral in tonsteins from coal-bearing se-quences (e.g. Bohor and Triplehorn, 1993) has been more rarely de-scribed in ash layers deposited in marine environments (Schultz,1963; Pollastro, 1981; Cadrin et al., 1995). More recently, this min-eral was found in Oxfordian bentonites from the Subalpine Basin ofFrance (Pellenard et al., 2003) and in Turonian bentonites depos-ited in the deepest oxygen-depleted environments of the Paris Ba-sin (France), while coeval horizons deposited in shallower

0.1µ

0.5µ

0.1µK-

I/S

I/S

2µ

Fig. 8. TEM and SEM pictures of some selected samples.

T°C

50

75

100

150

125

175

200

225

Cambrian Ord. Sil. Devonian-Carboniferous Permian Triassic Jurassic Cretaceous CainozoicEdiacarian

Depth (m)

- 1000

- 2000

- 3000

- 4000

- 5000

- 6000

- 7000

stable subsidence

uplift and denudation

rapid subsidence

rapid uplift and

denudation

Fig. 9. Subsidence history of the Sichuan Basin deduced from the study of several wells (after Shugen et al., 2008 modified) showing that the burial depth of the Permiansediments reached about 6000 m and a temperature between 180 and 200 �C.

J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37 35

environments are composed of I/S R0 (Deconinck et al., 2005). Asimilar observation is reported for the well-known Ordovician Kin-nekulle K-bentonite (Kiipli et al., 2007; Somelar et al., 2009). In thishorizon, the clay fraction is composed of I/S, except when it wasdeposited in deep marine sedimentary environments. In all cases,kaolinite formation in the tuff layers is probably controlled by rel-atively low pH values, due to the decomposition of organic matterthroughout the water column and at the sea-water/sediment inter-face. This origin would suggest a deposition of the Wangpo Bed inrelatively deep marine environments, although it is capped by anorganic matter-rich horizon containing plant debris, obviouslydeposited in shallow environments. There are two possibilitiesfor the origin of the kaolinite. This mineral can either be authigenicand formed by in situ weathering of volcanic glass in deep oxygen-depleted environments or of detrital origin from emerged areas. Inthe first case, euhedral crystals of kaolinite would occur in the

volcanic horizon (Pollastro, 1981; Deconinck et al., 2005) while,in the second, anhedral or subhedral particles should be observed.The SEM and TEM observations did not reveal the presence ofeuhedral crystals of kaolinite, which would demonstrate its authi-genic origin (Fig. 8). This fact suggests a detrital rather than anauthigenic origin for kaolinite particles in the Wangpo Bed. Conse-quently, the Wangpo bed should be considered as a reworked ben-tonite (secondary deposit) formed by the accumulation of volcanicash, transported from ash-blanketed local land areas into marineenvironments. Such epiclastic deposits are characterised by theirplurimetric thickness, as it is the case for the Wangpo Bed, and pre-cisely by the mixture of volcanogenic and detrital particles (Jeanset al., 1982; Clayton et al., 1996; d’Atri et al., 1999). Consequently,the Wangpo Bed seems to be of mixed volcanic and detrital origin,which may explain the two divergent interpretations of this partic-ular deposit, interpreted as an acidic tuff by Isozaki et al. (2004)

36 J.F. Deconinck et al. / Journal of Asian Earth Sciences 81 (2014) 28–37

and as a clastic deposit by He et al. (2010) using local occurrence ofquartz and rounded zircon. The geological significance, of this par-ticular level, with a mixed origin does not affect the fact that it re-sults from major volcanic eruptions probably related to theEmeishan Large Igneous Province. The accumulation of volcanicparticles was probably made in part by direct fall-out of ash intothe marine environment, and secondly by the reworking of volca-nic material first deposited on continental areas along with detritalparticles. Like the horizon of bentonite, this bed is a good strati-graphic marker allowing long-distance correlations.

6. Conclusions

(1) Clay mineral assemblages of the Chaotian section have suf-fered significant diagenetic changes, well-expressed in tufflayers weathered to K-bentonites. Consequently, the originalconstitution of detrital clay assemblages changed consider-ably through illitisation processes. However, detrital kaolin-ite is preserved in the Wangpo Bed.

(2) The precise determination of I/S suggests a thermal evolu-tion reaching approximately 180 �C, consistent both withTmax values measured in organic matter-rich layers, andthe burial history of the series. The study of I/S in K-bento-nites provides an additional technique to estimate palaeo-temperatures, and may help to determine hydrocarbongeneration processes in the Sichuan Basin. In the future,their precise determination in the Permian felsic tuff shouldbe extended to other sections and boreholes of the SichuanBasin.

(3) The study of clay minerals of the Wangpo Bed brings newinsight into its controversial origin. The composition of theclay fraction reveals that the Wangpo Bed is probably com-posed of volcanic particles directly deposited in marine envi-ronment, mixed with reworked volcanic material firstdeposited on continental setting and terrigenous supply.The clay fraction contains I/S deriving from the illitisationof volcanogenic smectite, mixed with detrital kaolinite orig-inating from the erosion of emerged areas. This horizon istherefore considered as a reworked bentonite, resulting fromthe accumulation of detrital and volcanic material in quietdeep marine environments. This particular bed whichresults from huge eruptions probably related to the Emei-shan Large Igneous Province constitutes an excellent strati-graphic marker located close to the Guadalupian/LopingianBoundary.

Acknowledgements

The authors would like to thank A. Bonotte and J. Lherminier(INRA-Dijon) for technical assistance on SEM and TEM and Alexan-dre Lethiers (CR2P, UPMC) for the drawings. The manuscriptgreatly benefitted from the corrections and suggestions of review-ers, Y. Isozaki and J. Srodon. The authors thank Carmela Chateau-Smith (Université de Bourgogne) for English proof-reading. Thiswork was supported by the Chinese 111 program (B08030) andthe Ministry of Education of China (20110145130001).

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