Formation of shallow-water glaucony in weakly oxygenated ...ganqing.faculty.unlv.edu/...PrecambrianResearch.pdfPrecambrian ocean: An example from the Mesoproterozoic Tieling Formation

Precambrian Research 294 (2017) 214–229

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Precambrian Research

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Formation of shallow-water glaucony in weakly oxygenatedPrecambrian ocean: An example from the Mesoproterozoic TielingFormation in North China

http://dx.doi.org/10.1016/j.precamres.2017.03.0260301-9268/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding authors at: State Key Laboratory of Biogeology and Environ-mental Geology, China University of Geosciences (Beijing), Beijing 100083, China.

E-mail addresses: [email protected] (D. Tang), [email protected] (X. Shi).

Dongjie Tang a,b,⇑, Xiaoying Shi a,c,⇑, Jianbai Ma c, Ganqing Jiang d, Xiqiang Zhou e, Qing Shi a,b

a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Beijing), Beijing 100083, Chinab Institute of Earth Sciences, China University of Geosciences (Beijing), Beijing 100083, Chinac School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, ChinadDepartment of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USAeKey Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

a r t i c l e i n f o

Article history:Received 20 December 2016Revised 18 March 2017Accepted 21 March 2017Available online 22 March 2017

Keywords:GlauconySeawater redox conditionsMesoproterozoicTieling FormationNorth China platform

a b s t r a c t

Authigenic glaucony precipitation in Phanerozoic oceans takes place mostly in middle shelf to upperslope environments with low depositional rate or sediment starvation. Precambrian glaucony, however,is more common in stratigraphic successions deposited from shallower-water environments with highand variable depositional rates. This phenomenon has long been noticed in literature, but the controllingfactors of shallow-water glaucony precipitation in Precambrian oceans have not been adequately inves-tigated. To better understand the glauconitization processes in Precambrian oceans, we have conductedan integrated study of the glaucony in stromatolitic carbonates of the Mesoproterozoic Tieling Formation(ca. 1437 Ma) in North China, using sedimentological, mineralogical, and geochemical data obtained fromfield observations, petrography, XRD, SEM, quantitative EDS and ICP-MS analyses. Macro- and micro-scopic observations show that the Tieling glaucony fills voids of varying sizes and shapes, and records dif-ferent maturation stages of glauconitization. Geochemical analyses show that the Tieling glaucony hashigh K2O (avg. > 8%) but low and variable total Fe2O3 (TFe2O3) contents (1.92–13.65 wt%). The TFe2O3

contents increase with maturation of glaucony. Titration results show that the Tieling glaucony containsboth Fe3+ and Fe2+ ions, but has Fe2+/Fe3+ ratios much higher than that of the Phanerozoic glaucony. REEresults of glaucony-hosting carbonates show weak negative and positive Ce anomalies with average Ce/Ce⁄ ratio close to 1.0, suggesting carbonate precipitation near the redoxcline of Fe-Mn oxides. All thesefeatures suggest that the Tieling glaucony was precipitated in seawater around the Fe-redoxcline, whereboth Fe2+ and Fe3+ were available throughout the glaucony maturation stages. High Fe2+/Fe3+ ratios in thedepositional environments led to Fe2+ occupation at octahedral sites of glaucony and negative charges onoctahedrons, which resulted in high K content (to balance the negative charges on octahedrons) and lowTFe2O3 (limited by dioctahedral structure). The formation of the Tieling glaucony and other similarPrecambrian glauconies is likely controlled by low oxygen concentration in seawater and a shallowredoxcline that controls the availability of Fe and K cations during initial precipitation and maturationof glaucony. The shift of authigenic glaucony precipitation from shallow water in the Precambrian to deepwater in the Phanerozoic may record the deepening of ocean chemocline in response to increased oceanoxygenation.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Glauconite [(K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2] refers to a phyl-losilicate mineral of dioctahedral mica group with 2:1 + interlayer

ion structures, while the term glaucony is used to represent a ser-ies of green clay minerals with a wide range of chemical/miner-alogical compositions including glauconitic mica, glauconiticsmectite, and ferric illite (Banerjee et al., 2015, 2016; Odin andLétolle, 1980). Glaucony typically occurs as 60–1000 lm green clayaggregates formed through marine authigenesis in sedimentaryrocks (e.g., Banerjee et al., 2015, 2016). Although detrital andreworked glaucony is sporadically present in both modern and

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ancient records (Amorosi, 1997), most autochthonous glauconiesform in depositional environments with low sedimentary rate orsediment starvation where persistent chemical exchange betweensediments and seawater promotes fixation of potassium (K) andiron (Fe) into mineralogical structures (e.g., Amorosi, 1997; Odinand Matter, 1981). Based on K-contents and morphological attri-butes, Odin and Matter (1981) divided glaucony into four evolutionstages: nascent (2–4% K2O), slightly evolved (4–6% K2O), evolved(6–8% K2O), and highly evolved (8–10% K2O). Most Phanerozoicevolved and highly-evolved glauconies are found in transgressivedeposits and/or condensed sections, and they are spatially dis-tributed in low-energy depositional environments from middlecontinental shelf to upper slope with water depths between 50and 500 m (Amorosi, 1997; Baldermann et al., 2017; Banerjeeet al., 2008, 2012a, 2012b; Bansal et al., 2017; Chattoraj et al.,2009; Chafetz and Reid, 2000; Giresse and Wiewióra, 2001;Harris and Whiting, 2000; Hesselbo and Huggett, 2001;Kitamura, 1998; Meunier and El Albani, 2007; Odin and Matter,1981).

A particular phenomenon noticed in literature is that most Pre-cambrian glaucony, in contrast to its Phanerozoic counterparts,tends to concentrate in shallow-water environments (e.g.,Banerjee et al., 2015, 2016; Chafetz and Reid, 2000; Mei et al.,2008; Zhou et al., 2009). Yet the mechanism behind this favorableglauconitization in shallow-water environments during the Pre-cambrian has not been adequately addressed. To better understandthe controlling factors of shallow-water glauconitization duringthe Precambrian, we have conducted a comprehensive study ofthe glaucony in stromatolitic carbonates of the MesoproterozoicTieling Formation (ca. 1437 Ma) in North China (Figs. 1 and 2),

Fig. 1. Simplified geological map of the study area (modified after Xie and Zhou, 2005). (Ageological map showing distribution of the Tieling Formation (in grey color) and locatio

using integrated sedimentological, mineralogical, and geochemicaldata obtained from field observations, petrography, XRD, SEM,quantitative EDS and ICP-MS analyses. With the integrated dataset,we discuss the potential seawater control on the source of Fe and Krequired for glauconitization in shallow-water environments.

2. Geological setting

2.1. Regional stratigraphy and age constraints

The Proterozoic succession of the North China platform wasdeposited in rift and post-rift basins associated with the tectonicevolution from the breakup of supercontinent Columbia to theassembly of supercontinent Rodinia. Paleoproterozoic to Neopro-terozoic strata in North China have a total thickness of �9 kmand are regionally well preserved with low metamorphic gradebelow prehnite–pumpellyite phase (Chu et al., 2007; Li et al.,2003). The Proterozoic succession consists of three groups(Fig. 2), in ascending order, the Changcheng Group (1660–1600 Ma, Pt1), the Jixian Group (1600–1400 Ma, Pt2), and the Qing-baikou Group (1000–800 Ma, Pt3). A hiatus of up to 400 Ma occursat the unconformity between the Jixian and Qingbaikou groups(Gao et al., 2009).

The Jixian Group comprises six formations including, in ascend-ing order, the Gaoyuzhuang, Yangzhuang, Wumishan, Hong-shuizhuang, Tieling, and the Xiamaling formations. This group ispredominated by carbonates with a total thickness of �6000 m,recording shallow-water deposits of an extensional epicontinentalsea. The Jixian Group is unconformably overlain by sandstones of

) Major tectonic subdivisions of China. (B) Location of the study area. (C) Simplifiedn of the studied section (star).

Fig. 2. Stratigraphical succession of the Jixian Group and occurrence of the Tieling glaucony in the central North China platform (modified after Zhou et al., 2009).

216 D. Tang et al. / Precambrian Research 294 (2017) 214–229

the Changlongshan Formation (Su et al., 2010), the basal unit of theQingbaikou Group.

In the last decade, a number of high-precision zircon U–Pb agesby ICP–MS, SHRIMP, and TIMS have been obtained from the

Proterozoic succession of the North China platform (Duan et al.,2014; Gao et al., 2007, 2008a, 2008b; Li et al., 2010, 2013, 2014;Lu and Li, 1991; Lu et al., 2008; Su et al., 2010; Zhang et al.,2013, 2015), providing a geochronologic framework for the


stratigraphic units (Fig. 2). Notably, the middle part of the TielingFormation was anchored at 1437 ± 21 Ma (Su et al., 2010) andthe lower part of the Xiamaling Formation was dated as1392 ± 1 Ma (Zhang et al., 2015) by SHRIMP and TIMS Zircon U-Pb ages, respectively. Based on the radiometric ages and the strati-graphic relationships between these units, the top of the TielingFormation can be faithfully assigned at �1.40 Ga and the base ofthis unit should be no older than �1.45 Ga (Fig. 2).

2.2. Sedimentary facies

The Tieling Formation at the Jixian section can be further subdi-vided into two members. The lower member includes a set of man-ganiferous dolostones interfingered with thin Mn- and K-rich shalebeds, which have been interpreted as deposits from deep subtidalto intertidal environments (Fig. 3A; Mei et al., 2008). The uppermember consists of stromatolitic limestone with dolostone inter-beds in the lower and upper parts. This member was interpretedas having been deposited in shallow subtidal to intertidal environ-ments (Figs. 3B–F; Mei et al., 2008). Two stratigraphic discontinu-ities, indicated by palaeokarst and paleosol, are visible between the

Fig. 3. Photographs showing representative depositional facies of the Tieling Formation.the Tieling Formation. (B) Flat pebble–bearing dolostone from the lower part of MemColumnar stromatolites from Member II of the Tieling Formation. (E) Plane view of columextending direction (arrows). (F) Intraclasts in matrix between stromatolite columns.

two members and at top of the upper member, respectively (Meiet al., 2008).

The glaucony-bearing facies are present mainly in the uppermember of the Tieling Formation. Flat pebble-bearing dolostone(Fig. 3B) and hummocky–swaley cross stratification (Fig. 3C) arecommon in the lower part of this member, suggesting subtidalenvironments above storm wave base, but glaucony was not foundin this part. Stromatolitic reef (Fig. 3D) dominates most part of thismember. Glaucony is mainly distributed in the upper part of Mem-ber II (Fig. 2) and particularly concentrated at the contacts betweenstromatolite columns and matrix (Fig. 3E). In bed surface view,stromatolite columns show elongate shapes with consistent orien-tation (Fig. 3E) and intraclasts are present in matrices betweenstromatolite columns (Fig. 3F), indicating at least episodicallyhigh-energy, subtidal environments.

3. Materials and methods

Samples analyzed in this study were collected from the uppermember of the Tieling Formation at the Jixian section (N:40�05033.800, E: 117�23037.800), Tianjin City (Fig. 1). Well-preserved

(A) Manganiferous dolostone interfingered with dark green shale from Member I ofber II. (C) Hummocky–swaley cross-stratification from the lower Member II. (D)nar stromatolites showing elongate shapes of stromatolite columns with consistent


samples with glaucony were selected for microscope and SEMobservations and for mineral (XRD) and chemical (EDS and REE+ Y) analyses. Macroscopic features were observed in the fieldand on polished slabs. Microfabrics were observed on thin sectionswith a Stereo Discovery V20 microscope for large scope and a ZeissAxio Scope A1 microscope for high magnification. Ultrastructureswere studied using a Zeiss Supra 55 field emission scanning elec-tron microscope (FESEM) under 20 kV accelerating voltage with aworking distance of 15 mm, at the State Key Laboratory of Biogeol-ogy and Environmental Geology, China University of Geosciences(Beijing). Secondary electron imaging detector (SE2) was used tocharacterize topographic features, and an AsB detector was usedto characterize compositional difference (backscattered electronimage). Samples were coated with �10-nm-thick platinum forelectric conduction before analysis.

Quantitative element concentrations of micron-sized spotswere analyzed by an Oxford energy dispersive X-ray spectrometer(EDS) connected to the FESEM, operated at 20 kV with a workingdistance of 15 mm, specimen current of 200 nA, and beam diame-ter of �1 lm, at the State Key Laboratory of Biogeology and Envi-ronmental Geology, China University of Geosciences (Beijing).Minerals as well as synthetic phases (MINM25-53) were used asstandards. Duplicate analyses of individual points show analyticalerror less than 1%. Fe2+/Fe3+ ratio of glaucony was determined byXRF and titration technique (Kelly and Webb, 1999).

Glauconitic minerals were separated from the samples using acombination of magnetic, electrostatic and handpicking tech-niques, following the methods suggested by Odin et al. (1982).Separated glauconitic minerals were then milled to 200-meshpowders using an agate mortar for XRD analysis. The samples werescanned after air-drying and glycol saturated, respectively. Thepowder slides were scanned from 4� to 70� with step size of0.02� 2h and a scan speed of 1�/min, using nickel filter copper radi-ation in an SmartLab X-ray Diffractometer at China University ofGeosciences (Beijing).

In preparation for REE + Y analysis of the hosting carbonates,polished chips were drilled for powders, avoiding weathered sur-faces and recrystallized areas. To minimize the potential influenceof terrigenous components on REE + Y, carbonate samples weredissolved using 5% acetic acid following the method described inTang et al. (2016). To compare the digestion methods, we also dis-solve two samples using HF/HNO3 following the methodsdescribed in Guo et al. (2013a). The trace elements were measuredin a Bruker Aurora M90 ICP-MS at the Institute of Geochemistry,Chinese Academy of Sciences. The accuracy of all ICP-MS analysesis better than 5–10% (relative) for analyzed elements.

Samples for organic carbon analysis were treated with 10% HClto remove carbonate, rinsed with distilled water, and dried at 60 �Covernight. The organic carbon isotope composition is measuredusing an EA-ISOPRIME system at the State Key Laboratory of Geo-logical Processes and Mineral Resources, China University of Geo-sciences (Beijing). Analytical results are reported in standarddelta notation relative to the VPDB (Vienna Pee Dee Belemnite).Precision is better than 0.15‰, based on multiple analyses of labo-ratory standards (GBW04407 and GBW04408).

4. Results

4.1. Glaucony features in the Tieling Formation

4.1.1. Occurrences of glauconyGlaucony is largely concentrated in stromatolite reefs of the

upper member of the Tieling Formation. The reef is predominantlymade up of calcite, with some ferroan dolomites and minor to nonsiliciclastic components. Glaucony, occupying up to �2% in volume

of the carbonates, can be distinguished into three types accordingto their size, distribution and substrate properties.

Type 1 glaucony, the dominant type, occurs in belts betweenstromatolite columns and microsparitic matrix in troughs, wheremicro- to meso-pores are most abundant. Glaucony accounts formore than 30% in volume in these belts (Fig. 4A). Macroscopically,it appears as <4 mm (commonly �1 mm) wide green belts alongthe margins of stromatolite columns (Fig. 4A). Microscopically,glaucony shows as irregular particles filling voids of 0.3–3 mm insize. Most glaucony displays irregular edges indicative of auto-chthonous precipitation (Fig. 4B). The glaucony particles are largerthan background micritic carbonates, which may record the origi-nal mineral phases formed during deposition or have beenenlarged through dissolution and recrystallization of carbonateminerals during diagenesis. High magnification SEM image revealslamellar structure in glaucony particles (Fig. 4C). EDS analyses con-firm that these green particles are glaucony that have high K andlow Fe contents (Fig. 4D).

Type 2 glaucony occurs as irregular particles filling voids inmicrosparitic matrix between stromatolite columns and accountsfor �1% in volume in places of its presence (Fig. 4E and F). Theyare less abundant than Type 1 and have smaller particle sizes(commonly <10 lm, occasionally up to 200 lm, Fig. 4E and F).Commonly, this type of glaucony is too small to be observed undermicroscope, but SEM observation confirms its existence as void-filling components in calcite or ferroan dolomite matrix (Fig. 4E).Occasionally, it disperses in matrix as individual grains of 50–500 lm in size (Fig. 4F). High magnification SEM image also revealslamellar structure, similar to that in type 1 glaucony.

Type 3 glaucony, the least common type, occurs as void-fillingcolloidal cements within stromatolite columns and is less than0.1% in volume. This type of glaucony fills the smallest voids(<10 lm) in micritic stromatolite laminae (Fig. 4G and H).

All three types of glaucony display as irregular void-filling par-ticles and their abundance seems to be positively correlated withthe pore size of the substrates. Based on their distribution, particlesizes and their substrate properties, the three types of glaucony inthe Tieling stromatolitic carbonates may have initially precipitatedcontemporaneously under similar chemical conditions, but glau-cony evolution terminated asynchronously depending on the per-meability and porosity of substrates. Type 1 glaucony may havehad the longest evolution time and type 3 had the shortest evolu-tion time.

4.1.2. Substrates of glauconySemi-quantitative EDS analyses of glaucony-hosting carbonates

(Table S1) show that the main elemental compositions of themicrosparitic matrix are Ca (20.02 wt%), Mg (4.68 wt%), C(18.56 wt%), and O (51.11 wt%), with minor Si (3.06 wt%), Fe(1.05 wt%), K (0.54 wt%), Na (0.03 wt%), Al (0.73 wt%), Mn(0.17 wt%), and Ti (0.04 wt%). BSE image-combined EDS confirmthat the matrix is composed of calcite (�67% in volume), ferroandolomite (�30% in volume), and glaucony (�3% in volume), with-out other detectable siliciclastic components (Fig. 5A). The ele-ments Si, K, Na, Al and Ti in matrix are mainly sourced fromglaucony itself rather than other siliciclastic components in car-bonates. Fe and Mg are largely from ferrous dolomite and glaucony(Table S1).

BSE image and EDS analysis show that the light laminae in stro-matolites consist mainly of calcite (�96% in volume), with minorferroan dolomite (�3.5% in volume) and negligible glaucony(�0.5% in volume), while the dark laminae are made up of calcite(�83%), with more ferroan dolomite (�15%) and higher glauconycontent (�2%) (Fig. 4G and Table S1). In general, the light laminaehave lower Mg and Fe, but higher Ca concentrations than those in

Fig. 4. Three types of glaucony from the Mesoproterozoic Tieling Formation. (A) Polished slab showing type 1 glaucony (Gl) concentrated in the belts between stromatolitecolumns (SC) and carbonate matrices (CM). (B) Type 1 granular glaucony. (C) SEM image of type 1 glaucony showing lamellar structure. (D) EDS spectrum shows that themajor element compositions in glaucony are K, Mg, Al, Si, Fe and O. Pt is derived from coating. (E) BSE image shows that type 2 glaucony (Gl) occurs as void-fillings in thematrix between stromatolite columns where calcite (Cc) is the dominant mineral. Minor ferroan dolomite (FD) is also present. (F) Photomicrograph of type 2 granularglaucony (Gl) filling micropores in matrix. (G) BSE image showing that dark laminae (DL) in stromatolites are composed of calcite and significant amount of ferroan dolomite,while light laminae (LL) is dominated by calcite, with minor ferroan dolomite. (H) High- magnification SE image showing type 3 glaucony (Gl) that fills micropores in calcite(Cc) matrix of the dark laminae in stromatolites.


dark laminae (Table S1). Neither light nor dark laminae containdetectable siliciclastics except for the glaucony itself (Table S1).

Paragenetic analysis of diagenetic phases provides informationabout the cation source for glauconitization. Microsparitic calcites

in matrix and micritic to microsparitic calcites of stromatolite lam-inae are commonly anhedral to subhedral in morphology, whileferroan dolomites are larger euhedral to subhedral grains of10–200 lm (Figs. 4E and G, 5A and B). Occasionally, it can be

Fig. 5. Substrates of glaucony. (A) BSE image showing anhedral to subhedral microsparitic calcite (Cc), euhedral to subhedral ferroan dolomite (Dol) with growth rings, andglaucony aggregates (Gl). (B) BSE image showing ferroan dolomite (Dol) enclosing calcite (Cc). (C) EDS spectrum of calcite in (B). (D) EDS spectrum of ferroan dolomite in (B).(E) and (F), photomicrographs under plane polarized light (E) and cross polarized light (F) showing alternation of microsparitic light laminae (LL) and micritic dark laminae(DL) in stromatolites.


observed that non-ferroan calcite (Fig. 5B–D) is enclosed by ferroandolomite overgrowth. In addition, BSE image shows that some fer-roan dolomites have multi-stage growth rings (Fig. 5A). All thethree types of glaucony fill voids with irregular boundaries definedby microsparitic calcite and some euhedral ferroan dolomites, andsometimes anhedral calcites can be seen ‘‘suspended” in the glau-cony (Fig. 5A). Therefore, the paragenetic sequence of mineralphases within the Tieling stromatolite reef, from the oldest toyoungest, should be (1) non-ferroan micritic to microsparitic cal-cite, (2) void-filling glaucony, and (3) ferroan dolomite. Non-ferroan micritic calcite is most likely primary precipitate with littlediagenetic alternation, while void-filling glaucony may have beenprecipitated simultaneously with calcite from seawater and/orfrom pore fluids during early diagenesis. Ferroan dolomite wasformed during late diagenesis and could not have provided cationsfor the authigenesis of glaucony.

Glaucony-bearing light laminae in stromatolites contain her-ringbone calcite (cf. Sumner and Grotzinger, 1996a). Herringbonecalcite is a special type of carbonate cement composed of elongatecrystals in which the c-axis rotates from parallel to perpendicularto the long axis of the crystal, resulting in an extinction pattern

that sweeps along the long axis of crystal growth (Kah et al.,2009). Stromatolite laminae without glaucony do not contain her-ringbone calcite.

In summary, as all the substrates identified in the three types ofglaucony are essentially devoid of siliciclastics and ferroan dolo-mite was precipitated during late diagenesis, the elements K, Aland Si are most likely contributed by glaucony itself, and Fe inthe glaucony may not be supplied from siliciclastics in the sub-strates or from ferroan dolomite.

4.2. Mineralogy of glaucony

Only type 1 glaucony was chosen for XRD analysis because thecontents of types 2 and 3 glaucony are insufficient for XRD analy-sis. The air-dried samples exhibit intense basal reflections (001) at1.011 nm, (020) at 0.452 nm, (003) at 0.334 nm, (13-1) at0.258 nm and (060) at 0.150 nm d-spacing, and weak basal reflec-tions (002) at 0.501 nm, (11-2) at 0.365 nm, (112) at 0.304 nm,(201) at 0.240 nm and (005) at 0.199 nm d-spacing (Fig. 6). Theseare the characteristic peaks of glauconite. Minor quartz and calcitewere mixed in the sample (Fig. 6), but they were carefully avoided

Fig. 6. X-ray pattern of glaucony. (A) XRD result of air dried sample. (B) XRD result of glycol saturated sample.


during EDS analysis. There is no shift of these characteristic peakswhen treated with ethyl glycol (Fig. 6B). Neither characteristicpeak of chlorite at �1.4 nm d-spacing nor the peak of kaolinite at�0.7 nm d-spacing was identified.

4.3. Major element composition and structural formula of glaucony

Major element compositions determined by quantitative EDSanalyses in all the three types of glaucony are provided in Table 1(102 points). Their structural compositions are also presented inTable 1. When calculating the structural formula of glaucony, theratio Fe2+/TFe = 58% (determined by XRF and titration technique)obtained from type 1 glaucony was considered to representapproximately the values for all types of glaucony. All the elemen-tal determinations were made on an anion equivalent basis to thestructural formulae per O10(OH)2. The composition of Mesopro-terozoic Tieling glaucony was further compared with those of thePhanerozoic glauconies (Table 2). The results show that the Tielingglaucony has apparently higher K2O, Al2O3, MgO, SiO2 contents andhigher Fe2+/TFe ratio, but lower TFe2O3. Type 3 glaucony has thehighest Al2O3 and MgO contents but the lowest TFe2O3 and SiO2,while type 1 glaucony shows the opposite (Tables 1 and 2; Fig. 7).

Specifically, the K2O content in the Tieling glaucony is high andwithin a narrow range between 7.59 wt% and 9.58 wt%, indepen-dent of the glaucony types (Fig. 7A). However, TFe2O3 content islow and varies considerably in a range of 1.92 wt% to 13.65 wt%,with the lowest value in type 3 glaucony (avg. 3.02 wt%), mediumin type 2 (avg. 8.79 wt%), and the highest in type 1 glaucony (avg.10.42 wt%). The K2O content of the Tieling glaucony is comparableto that of highly evolved glauconies, but the TFe2O3 content islower than that in glauconitic mica and glauconitic smectite(Amorosi, 1997; Dasgupta et al., 1990; Odin and Matter, 1981).

There is no K2O–TFe2O3 correlation, but different TFe2O3 contentseasily identify the three types of glaucony (Fig. 7A). Similar K2O–TFe2O3 relationship has been reported for the glaucony from thesame section (Mei et al., 2008; Zhou et al., 2009) and from otherPrecambrian sedimentary successions (Fig. 7B; Banerjee et al.,2008, 2015; Dasgupta et al., 1990; Deb and Fukuoka, 1998; Dritset al., 2010; Guimaraes et al., 2000; Ivanovskaya et al., 2006;Sarkar et al., 2014), but it has not been observed in glaucony fromMesozoic–Cenozoic sedimentary successions.

Al2O3 content of the Tieling glaucony is higher than that of mostPhanerozoic glauconies (Table 2), but is comparable with that ofother Precambrian examples. Banerjee et al. (2015) reported simi-lar results for the Mesoproterozoic glaucony in India and used theterm ‘high-alumina glaucony’ proposed by Berg-Madsen (1983).The Tieling glaucony has the highest Al2O3 content in type 3(avg. 26.17 wt%), moderate in type 2 (avg. 16.59 wt%) and lowestin type 1 (avg. 14.89 wt%), showing a negative correlation withthe TFe2O3 content. A good correlation is also demonstrated bythe cross plot of Al3+(oct.) and Fe3+(oct.) (r2 = 0.95), with type 1and type 3 glaucony at the two ends of the correlation line(Fig. 7C). The relationship between Al3+(oct) and Fe3+(oct) indicatespredominant Al3+-Fe3+ substitution in the octahedral site (Banerjeeet al., 2008, 2012a, 2012b; Bornhold and Giresse, 1985; Dasguptaet al., 1990; Odin and Matter, 1981). The MgO content of the Tiel-ing glaucony is high (1.92–8.14 wt%), but it is consistent with thePrecambrian examples reported in a few studies (Table. 2;Banerjee et al., 2008, 2015; Sarkar et al., 2014). MgO contentincreases from type 3 (1.92–5.62 wt%) to type 1 (5.85–8.14 wt%),with medium values in type 2 glaucony (5.41–7.91 wt%).

SiO2 content has small variations among the three types ofglaucony. The average SiO2 content increases from type 3(47.84–58.78 wt%) to type 2 (52.57–58.85 wt%), and then to type

Table 1Quantitative EDS anlaysis results of oxide weight percentage and structural formula of glaucony within the Tieling Formation.

Point Location Glaucony type Na2O MgO Al2O3 SiO2 Fe2O3 K2O Total K+ Na+ Fe3+ Fe2+ Mg2+ Al3+oct. Si4+ Al3+tet. R3+oct. Fe/Sum oct. M+/4Si

No. 001 Interface Type 1 0.04 6.07 17.27 55.45 10.12 9.23 98.18 0.78 0.01 0.21 0.29 0.60 1.04 3.68 0.32 1.25 0.22 0.86No. 002 Interface Type 1 0.00 7.59 14.89 56.67 11.25 9.01 99.41 0.76 0.00 0.23 0.33 0.75 0.89 3.73 0.27 1.12 0.24 0.81No. 003 Interface Type 1 0.00 6.70 12.48 57.80 12.66 8.33 97.97 0.71 0.00 0.27 0.37 0.67 0.85 3.87 0.13 1.11 0.28 0.74No. 004 Interface Type 1 0.07 6.96 16.25 56.63 10.73 8.56 99.20 0.72 0.01 0.22 0.31 0.68 0.97 3.72 0.28 1.19 0.23 0.78No. 005 Interface Type 1 0.14 6.29 13.02 56.32 12.48 8.43 96.68 0.73 0.02 0.27 0.37 0.64 0.87 3.83 0.17 1.14 0.28 0.78No. 006 Interface Type 1 0.10 7.78 13.58 53.96 10.42 8.72 94.56 0.77 0.01 0.23 0.32 0.80 0.85 3.74 0.26 1.08 0.24 0.84No. 007 Interface Type 1 0.00 6.64 13.08 54.48 10.09 8.39 92.68 0.75 0.00 0.22 0.31 0.70 0.91 3.83 0.17 1.14 0.24 0.79No. 008 Interface Type 1 0.00 7.26 13.21 55.81 11.67 7.93 95.88 0.69 0.00 0.25 0.35 0.74 0.86 3.80 0.20 1.11 0.26 0.73No. 009 Interface Type 1 0.11 7.13 13.00 53.59 11.63 8.87 94.33 0.79 0.01 0.26 0.36 0.74 0.82 3.75 0.25 1.08 0.27 0.86No. 010 Interface Type 1 0.27 7.69 12.75 57.74 11.80 8.76 99.01 0.74 0.03 0.25 0.34 0.76 0.82 3.82 0.18 1.07 0.26 0.81No. 011 Interface Type 1 0.09 6.63 12.47 54.74 12.80 8.94 95.67 0.79 0.01 0.28 0.39 0.68 0.81 3.79 0.21 1.09 0.30 0.85No. 012 Interface Type 1 0.11 7.28 13.61 54.44 10.70 7.92 94.06 0.70 0.01 0.23 0.33 0.75 0.89 3.77 0.23 1.12 0.24 0.76No. 013 Interface Type 1 0.00 6.80 14.90 54.54 10.72 8.31 95.27 0.73 0.00 0.23 0.32 0.69 0.94 3.74 0.26 1.17 0.24 0.78No. 014 Interface Type 1 0.00 6.99 15.30 54.63 9.69 8.38 94.99 0.73 0.00 0.21 0.29 0.71 0.97 3.74 0.26 1.18 0.22 0.78No. 015 Interface Type 1 0.07 6.87 16.02 56.84 9.32 8.82 97.94 0.74 0.01 0.19 0.27 0.68 1.01 3.76 0.24 1.20 0.20 0.80No. 016 Interface Type 1 0.00 7.08 14.89 55.73 10.33 8.45 96.48 0.73 0.00 0.22 0.31 0.71 0.94 3.76 0.24 1.16 0.23 0.77No. 017 Interface Type 1 0.00 7.11 12.07 53.88 12.72 8.58 94.36 0.77 0.00 0.28 0.39 0.74 0.78 3.78 0.22 1.06 0.30 0.81No. 018 Interface Type 1 0.19 7.34 17.02 57.62 10.11 8.19 100.47 0.67 0.02 0.20 0.29 0.70 1.00 3.71 0.29 1.20 0.21 0.75No. 019 Interface Type 1 0.00 7.19 12.57 55.43 13.65 9.07 97.91 0.79 0.00 0.29 0.41 0.73 0.77 3.76 0.24 1.06 0.31 0.84No. 020 Interface Type 1 0.00 7.25 12.27 54.16 12.50 8.40 94.58 0.75 0.00 0.27 0.38 0.75 0.79 3.78 0.22 1.07 0.29 0.79No. 021 Interface Type 1 0.00 6.56 15.88 54.49 9.43 8.50 94.86 0.74 0.00 0.20 0.28 0.67 1.01 3.73 0.27 1.21 0.21 0.80No. 022 Interface Type 1 0.52 6.77 14.37 53.75 10.43 8.15 93.99 0.72 0.07 0.23 0.32 0.70 0.92 3.74 0.26 1.14 0.24 0.85No. 023 Interface Type 1 0.00 6.36 13.83 54.59 10.78 8.69 94.25 0.77 0.00 0.23 0.33 0.66 0.92 3.79 0.21 1.16 0.25 0.81No. 024 Interface Type 1 0.00 6.45 13.27 54.19 12.26 8.77 94.94 0.78 0.00 0.27 0.37 0.67 0.86 3.77 0.23 1.12 0.28 0.83No. 025 Interface Type 1 0.14 7.19 15.25 55.90 9.38 8.44 96.30 0.72 0.02 0.20 0.28 0.72 0.97 3.76 0.24 1.17 0.21 0.79No. 026 Interface Type 1 0.07 6.28 15.89 54.69 9.48 8.42 94.83 0.73 0.01 0.20 0.28 0.64 1.02 3.74 0.26 1.22 0.21 0.80No. 027 Interface Type 1 0.18 6.16 14.12 53.41 9.83 8.43 92.13 0.76 0.02 0.22 0.31 0.65 0.96 3.78 0.22 1.18 0.23 0.83No. 028 Interface Type 1 0.12 5.85 15.21 53.85 8.28 8.35 91.66 0.75 0.02 0.18 0.26 0.61 1.05 3.79 0.21 1.23 0.19 0.81No. 029 Interface Type 1 0.00 6.19 15.17 53.45 10.11 8.45 93.37 0.75 0.00 0.22 0.31 0.64 0.98 3.73 0.27 1.20 0.23 0.81No. 030 Interface Type 1 0.00 6.61 13.82 52.39 10.33 7.75 90.90 0.71 0.00 0.23 0.32 0.71 0.92 3.76 0.24 1.16 0.24 0.75No. 031 Interface Type 1 0.00 8.14 14.28 56.73 11.81 8.70 99.66 0.73 0.00 0.24 0.34 0.80 0.84 3.73 0.27 1.09 0.26 0.78No. 032 Interface Type 1 0.04 6.40 13.65 53.21 11.46 8.49 93.25 0.76 0.01 0.25 0.35 0.67 0.89 3.75 0.25 1.14 0.27 0.82No. 033 Interface Type 1 0.33 6.80 17.40 56.44 8.38 8.87 98.22 0.74 0.04 0.17 0.24 0.67 1.06 3.71 0.29 1.23 0.18 0.85No. 034 Interface Type 1 0.06 6.81 16.21 58.79 9.79 8.99 100.65 0.74 0.01 0.20 0.28 0.65 1.01 3.78 0.22 1.21 0.21 0.79No. 035 Interface Type 1 0.08 6.52 14.60 53.63 10.53 8.73 94.09 0.78 0.01 0.23 0.32 0.68 0.93 3.73 0.27 1.16 0.24 0.84No. 036 Interface Type 1 0.14 6.04 17.58 54.31 7.66 8.83 94.56 0.77 0.02 0.16 0.23 0.61 1.11 3.70 0.30 1.28 0.17 0.85No. 037 Interface Type 1 0.10 6.50 13.25 52.61 11.36 8.26 92.08 0.75 0.01 0.25 0.36 0.69 0.87 3.76 0.24 1.13 0.27 0.82No. 038 Interface Type 1 0.00 6.43 17.03 55.63 8.77 8.25 96.11 0.71 0.00 0.18 0.26 0.64 1.07 3.73 0.27 1.26 0.19 0.76No. 039 Interface Type 1 0.00 7.14 17.36 57.05 8.09 8.60 98.24 0.72 0.00 0.17 0.23 0.70 1.07 3.73 0.27 1.23 0.17 0.77No. 040 Interface Type 1 0.00 6.11 14.99 53.67 9.25 8.41 92.43 0.75 0.00 0.20 0.28 0.64 1.01 3.77 0.23 1.21 0.22 0.80No. 041 Interface Type 1 0.35 6.19 16.61 54.19 7.87 9.29 94.50 0.81 0.05 0.17 0.24 0.63 1.06 3.72 0.28 1.23 0.18 0.92No. 042 Interface Type 1 0.36 6.65 14.31 55.68 12.47 9.58 99.05 0.82 0.05 0.26 0.37 0.66 0.85 3.73 0.27 1.12 0.28 0.93No. 043 Interface Type 1 0.00 6.41 17.27 54.99 8.97 8.69 96.33 0.74 0.00 0.19 0.26 0.64 1.06 3.69 0.31 1.25 0.20 0.81No. 044 Interface Type 1 0.28 6.64 16.44 56.66 10.38 9.22 99.62 0.77 0.04 0.21 0.30 0.65 0.98 3.71 0.29 1.20 0.23 0.87No. 045 Interface Type 1 0.13 6.88 15.23 55.50 10.70 8.28 96.72 0.71 0.02 0.23 0.32 0.69 0.95 3.74 0.26 1.17 0.24 0.78No. 046 interface Type 1 0.00 7.01 14.06 55.42 10.91 8.38 95.78 0.73 0.00 0.23 0.33 0.71 0.91 3.78 0.22 1.14 0.25 0.77No. 047 Interface Type 1 0.04 6.02 16.89 57.40 9.89 8.71 98.95 0.73 0.01 0.20 0.28 0.59 1.06 3.75 0.25 1.26 0.21 0.78No. 048 Interface Type 1 0.00 6.19 16.32 53.91 8.68 8.82 93.92 0.78 0.00 0.19 0.26 0.64 1.05 3.72 0.28 1.23 0.20 0.83No. 049 Interface Type 1 0.13 7.29 18.63 57.01 7.83 8.84 99.73 0.73 0.02 0.16 0.22 0.70 1.09 3.67 0.33 1.25 0.16 0.81No. 050 Matrix Type 2 0.02 7.91 15.65 56.64 10.56 8.48 99.26 0.71 0.00 0.22 0.30 0.77 0.92 3.71 0.29 1.14 0.23 0.77No. 051 Matrix Type 2 0.06 6.34 19.72 56.64 6.69 9.16 98.61 0.76 0.01 0.14 0.19 0.61 1.18 3.67 0.33 1.32 0.14 0.83No. 052 Matrix Type 2 0.21 6.41 16.10 52.92 8.82 8.81 93.27 0.78 0.03 0.19 0.27 0.67 1.01 3.69 0.31 1.21 0.20 0.88No. 053 Matrix Type 2 0.22 6.28 14.27 53.83 9.31 8.57 92.48 0.77 0.03 0.21 0.29 0.66 0.97 3.79 0.21 1.17 0.22 0.84No. 054 Matrix Type 2 0.00 6.86 17.50 55.84 7.84 8.24 96.28 0.70 0.00 0.16 0.23 0.68 1.09 3.72 0.28 1.25 0.17 0.75No. 055 Matrix Type 2 0.00 6.43 15.36 54.97 9.13 8.60 94.49 0.75 0.00 0.20 0.27 0.66 1.01 3.77 0.23 1.21 0.21 0.80

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Table 1 (continued)

Point Location Glaucony type Na2O MgO Al2O3 SiO2 Fe2O3 K2O Total K+ Na+ Fe3+ Fe2+ Mg2+ Al3+oct. Si4+ Al3+tet. R3+oct. Fe/Sum oct. M+/4Si

No. 056 Matrix Type 2 0.00 5.96 16.00 53.46 8.43 8.66 92.51 0.77 0.00 0.19 0.26 0.62 1.06 3.74 0.26 1.24 0.20 0.83No. 057 Matrix Type 2 0.26 6.73 15.67 58.85 10.27 8.73 100.51 0.72 0.03 0.21 0.29 0.65 0.99 3.79 0.21 1.19 0.22 0.79No. 058 Matrix Type 2 0.05 6.61 15.29 55.37 10.38 8.13 95.83 0.70 0.01 0.22 0.31 0.67 0.97 3.75 0.25 1.19 0.23 0.76No. 059 Matrix Type 2 0.00 6.31 15.64 53.65 9.44 8.70 93.74 0.77 0.00 0.21 0.29 0.65 1.00 3.72 0.28 1.21 0.22 0.83No. 060 Matrix Type 2 0.04 5.96 15.20 53.77 8.46 8.14 91.57 0.73 0.01 0.19 0.26 0.63 1.05 3.79 0.21 1.23 0.20 0.78No. 061 Matrix Type 2 0.23 6.93 14.38 53.65 11.10 8.81 95.10 0.78 0.03 0.24 0.34 0.71 0.88 3.71 0.29 1.13 0.25 0.87No. 062 Matrix Type 2 0.19 6.89 15.73 52.96 8.78 8.37 92.92 0.75 0.03 0.19 0.27 0.72 0.99 3.70 0.30 1.19 0.20 0.83No. 063 Matrix Type 2 0.12 6.75 14.46 54.38 9.70 8.19 93.60 0.72 0.02 0.21 0.30 0.70 0.96 3.77 0.23 1.17 0.22 0.79No. 064 Matrix Type 2 0.14 6.35 18.14 55.07 7.34 8.80 95.84 0.75 0.02 0.15 0.22 0.63 1.12 3.69 0.31 1.28 0.16 0.84No. 065 Matrix Type 2 0.39 6.35 16.12 53.16 8.33 8.41 92.76 0.75 0.05 0.18 0.26 0.66 1.04 3.71 0.29 1.22 0.19 0.86No. 066 Matrix Type 2 0.00 6.29 16.58 53.63 7.74 8.23 92.47 0.73 0.00 0.17 0.24 0.65 1.09 3.73 0.27 1.26 0.18 0.78No. 067 Matrix Type 2 0.00 6.39 20.71 56.26 6.37 8.84 98.57 0.73 0.00 0.13 0.18 0.62 1.22 3.64 0.36 1.35 0.13 0.80No. 068 Matrix Type 2 0.31 6.50 17.67 54.48 7.51 8.40 94.87 0.73 0.04 0.16 0.22 0.66 1.10 3.69 0.31 1.26 0.17 0.83No. 069 Matrix Type 2 0.18 5.91 20.14 56.97 6.76 9.04 99.00 0.74 0.02 0.14 0.19 0.57 1.21 3.68 0.32 1.35 0.14 0.83No. 070 Matrix Type 2 0.00 5.94 17.28 52.92 8.46 8.41 93.01 0.75 0.00 0.18 0.26 0.62 1.09 3.68 0.32 1.28 0.19 0.81No. 071 Matrix Type 2 0.22 6.18 19.45 55.65 6.77 8.49 96.76 0.71 0.03 0.14 0.20 0.61 1.19 3.67 0.33 1.33 0.15 0.81No. 072 Matrix Type 2 0.29 6.10 18.28 55.56 7.26 9.06 96.55 0.77 0.04 0.15 0.21 0.61 1.13 3.70 0.30 1.29 0.16 0.87No. 073 Matrix Type 2 0.01 6.58 15.55 53.82 9.60 8.67 94.23 0.76 0.00 0.21 0.29 0.68 0.98 3.72 0.28 1.19 0.22 0.82No. 074 Matrix Type 2 0.00 6.07 20.35 56.99 5.80 9.02 98.23 0.74 0.00 0.12 0.16 0.59 1.24 3.69 0.31 1.36 0.12 0.81No. 075 Matrix Type 2 0.05 6.10 13.80 53.41 11.50 9.00 93.86 0.81 0.01 0.25 0.35 0.64 0.90 3.75 0.25 1.15 0.27 0.87No. 076 Matrix Type 2 0.00 6.14 15.87 53.86 8.69 7.59 92.15 0.68 0.00 0.19 0.27 0.64 1.06 3.76 0.24 1.25 0.20 0.72No. 077 Matrix Type 2 0.13 5.69 16.15 52.57 9.27 9.07 92.88 0.81 0.02 0.20 0.29 0.60 1.03 3.69 0.31 1.24 0.22 0.90No. 078 Matrix Type 2 0.27 6.11 14.07 55.24 10.86 8.47 95.02 0.74 0.04 0.23 0.33 0.63 0.94 3.80 0.20 1.17 0.25 0.82No. 079 Matrix Type 2 0.27 6.11 14.07 55.24 10.86 8.47 95.02 0.74 0.04 0.23 0.33 0.63 0.94 3.80 0.20 1.17 0.25 0.82No. 080 Matrix Type 2 0.22 6.59 14.87 55.78 10.25 8.54 96.25 0.74 0.03 0.22 0.30 0.66 0.96 3.77 0.23 1.18 0.23 0.81No. 081 Matrix Type 2 0.00 6.64 14.74 56.76 10.15 8.23 96.52 0.70 0.00 0.21 0.30 0.66 0.97 3.81 0.19 1.19 0.23 0.74No. 082 Matrix Type 2 0.00 5.41 21.35 54.72 5.62 8.89 95.99 0.75 0.00 0.12 0.16 0.53 1.29 3.63 0.37 1.41 0.12 0.83No. 083 Matrix Type 2 0.16 6.84 16.66 56.07 9.44 8.64 97.81 0.73 0.02 0.20 0.27 0.68 1.02 3.71 0.29 1.21 0.21 0.81No. 084 Matrix Type 2 0.29 6.68 14.77 55.92 10.93 8.71 97.30 0.75 0.04 0.23 0.32 0.67 0.93 3.76 0.24 1.16 0.24 0.84No. 085 Matrix Type 2 0.20 6.28 15.62 56.13 10.83 9.22 98.28 0.78 0.03 0.23 0.32 0.62 0.96 3.74 0.26 1.19 0.24 0.87No. 086 Matrix Type 2 0.28 6.05 18.85 55.38 6.66 8.52 95.74 0.73 0.04 0.14 0.19 0.60 1.18 3.69 0.31 1.32 0.15 0.82No. 087 Matrix Type 2 0.13 5.96 18.48 58.41 7.97 9.28 100.23 0.76 0.02 0.16 0.22 0.57 1.14 3.74 0.26 1.30 0.17 0.83No. 088 Column Type 3 0.30 3.57 23.06 52.79 4.35 8.86 92.93 0.77 0.04 0.09 0.13 0.36 1.44 3.59 0.41 1.54 0.10 0.90No. 089 Column Type 3 0.00 3.16 21.21 58.78 3.85 7.98 94.98 0.67 0.00 0.08 0.11 0.31 1.47 3.84 0.16 1.55 0.08 0.69No. 090 Column Type 3 0.00 3.46 24.57 53.74 3.40 8.70 93.87 0.74 0.00 0.07 0.10 0.34 1.52 3.59 0.41 1.59 0.07 0.83No. 091 Column Type 3 0.47 3.50 27.29 58.55 1.92 8.21 99.94 0.65 0.06 0.04 0.05 0.32 1.61 3.62 0.38 1.64 0.04 0.78No. 092 Column Type 3 0.00 3.30 27.94 53.82 1.96 9.16 96.18 0.76 0.00 0.04 0.06 0.32 1.63 3.49 0.51 1.67 0.04 0.87No. 093 Column Type 3 0.19 3.07 28.87 54.77 2.35 9.23 98.48 0.75 0.02 0.05 0.07 0.29 1.63 3.48 0.52 1.68 0.05 0.89No. 094 Column Type 3 0.40 3.11 23.38 47.84 3.66 8.85 87.24 0.82 0.06 0.08 0.12 0.34 1.49 3.48 0.52 1.58 0.09 1.01No. 095 Column Type 3 0.24 2.00 26.76 54.45 3.51 9.58 96.54 0.80 0.03 0.07 0.10 0.19 1.61 3.55 0.45 1.68 0.07 0.93No. 096 Column Type 3 0.19 3.14 24.41 54.32 2.71 8.26 93.03 0.71 0.02 0.06 0.08 0.31 1.56 3.63 0.37 1.62 0.06 0.80No. 097 Column Type 3 0.28 1.92 28.11 49.48 2.99 7.86 90.64 0.69 0.04 0.06 0.09 0.20 1.70 3.41 0.59 1.76 0.06 0.85No. 098 Column Type 3 0.11 3.13 26.81 52.04 1.99 9.12 93.20 0.78 0.01 0.04 0.06 0.31 1.61 3.49 0.51 1.66 0.04 0.91No. 099 Column Type 3 0.92 2.53 29.32 56.96 2.27 8.44 100.44 0.67 0.11 0.04 0.06 0.23 1.66 3.52 0.48 1.70 0.04 0.88No. 100 Column Type 3 0.19 2.44 29.87 56.85 2.70 8.31 100.36 0.65 0.02 0.05 0.07 0.22 1.69 3.51 0.49 1.74 0.05 0.77No. 101 Column Type 3 0.05 3.25 25.73 52.95 4.74 8.69 95.41 0.73 0.01 0.10 0.14 0.32 1.51 3.51 0.49 1.61 0.10 0.84No. 102 Column Type 3 0.33 5.62 25.26 54.34 2.97 8.26 96.78 0.68 0.04 0.06 0.08 0.54 1.44 3.52 0.48 1.50 0.06 0.82Min All types 0.00 1.92 12.07 47.84 1.92 7.59 87.24 0.65 0.00 0.04 0.05 0.19 0.77 3.41 0.13 1.06 0.04 0.69Max All types 0.92 8.14 29.87 58.85 13.65 9.58 100.65 0.82 0.11 0.29 0.41 0.80 1.70 3.87 0.59 1.76 0.31 1.01Average All types 0.13 6.08 17.18 54.97 8.72 8.61 95.69 0.74 0.02 0.19 0.26 0.61 1.07 3.71 0.29 1.26 0.20 0.82Average 1 Type 1 0.09 6.76 14.89 55.18 10.42 8.60 95.93 0.75 0.01 0.22 0.31 0.69 0.94 3.75 0.25 1.17 0.24 0.81Average 2 Type 2 0.13 6.36 16.59 55.02 8.79 8.62 95.51 0.74 0.02 0.19 0.26 0.64 1.05 3.73 0.27 1.23 0.20 0.82Average 3 Type 3 0.24 3.15 26.17 54.11 3.02 8.63 95.33 0.72 0.03 0.06 0.09 0.31 1.57 3.55 0.45 1.63 0.06 0.85

Note: R3+ represent trivalent cations in octahedrons of glaucony; M+ represent monovalent cations in interlayer sites of glaucony.

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Table 2Comparison of chemical compositions between the Mesoproterozoic Tieling and Phanerozoic glauconies.

Glaucony MgO (wt%) Al2O3 (wt%) SiO2 (wt%) Fe2O3 (wt%) K2O (wt%) Fe2+/TFe (%) References

Phanerozoic 2.4–4.6 5.0–8.0 47.5–50.0 19.0–27.0 3.0–9.0 5–12 Odin and Matter, 1981; Banerjee et al., 2016Tieling (range) 1.92–8.14 12.07–29.87 47.84–58.85 1.92–13.65 7.59–9.58 58 This paperTieling (average) 6.08 17.18 54.97 8.72 8.61 58 This paper

Table 3REE + Y concentrations and Ce anomalies of the glaucony-bearing carbonates from the Tieling Formation.

Sample Acid for rockdissolution

La(lg/g)

Ce(lg/g)

Pr(lg/g)

Nd(lg/g)

Sm(lg/g)

Eu(lg/g)

Gd(lg/g)

Tb(lg/g)

Dy(lg/g)

Y(lg/g)

Ho(lg/g)

Er(lg/g)

Tm(lg/g)

Yb(lg/g)

Lu(lg/g)

Ce/Ce*

Jxt-2-1a Acetic acid 7.747 13.468 1.629 6.558 1.296 0.270 1.394 0.212 1.274 14.364 0.280 0.832 0.119 0.716 0.100 1.03Jxt-2-1b Acetic acid 7.074 12.103 1.423 5.616 1.128 0.224 1.302 0.195 1.121 13.031 0.262 0.754 0.091 0.638 0.096 1.04Jxt-2-1c Acetic acid 10.748 25.018 2.646 9.894 2.160 0.464 2.395 0.339 1.990 19.551 0.457 1.224 0.156 0.946 0.131 1.09Jxt-2-2a Acetic acid 9.618 16.945 1.960 7.513 1.464 0.296 1.635 0.244 1.515 14.997 0.314 0.921 0.126 0.796 0.120 1.03Jxt-2-2b Acetic acid 7.488 12.570 1.594 5.962 1.230 0.233 1.261 0.206 1.195 13.902 0.269 0.814 0.128 0.715 0.102 0.91Jxt-2-2c Acetic acid 9.069 16.167 2.098 8.218 1.671 0.376 1.907 0.300 1.795 18.902 0.412 1.264 0.164 0.929 0.137 0.93Jxt-2-4a Acetic acid 4.354 8.679 1.168 4.580 1.167 0.208 1.240 0.174 1.098 10.409 0.243 0.749 0.088 0.581 0.078 0.90Jxt-2-4b Acetic acid 5.286 9.251 1.085 4.385 0.916 0.182 1.004 0.144 0.930 8.844 0.202 0.603 0.084 0.465 0.075 1.07Jxt-2-1a HF/HNO3 8.435 13.309 1.623 6.171 1.249 0.228 1.263 0.206 1.290 17.600 0.298 0.857 0.124 0.827 0.115 0.96Jxt-2-1b HF/HNO3 6.868 10.378 1.287 4.846 0.935 0.180 0.979 0.159 0.951 15.700 0.231 0.699 0.092 0.620 0.089 0.94

Fig. 7. Comparison of chemical compositions of the three types of glaucony in the Tieling Formation. (A) Cross plot of K2O vs. TFe2O3, showing constantly high K2O butvariable TFe2O3 contents. Type 1 glaucony has the highest TFe2O3 and type 3 glaucony has the lowest TFe2O3 content. (B) Evolutionary trends of the Tieling glaucony andother Precambrian glaucony. Arrows indicate evolutionary trends of glaucony formed through different mechanisms. Blue arrow indicates ‘layer lattice theory’; red arrowindicates ‘verdissement theory’; and the black arrow at top left indicates evolutionary trend of glaucony in the Tieling Formation. The K2O–TFe2O3 relationship in mostPhanerozoic glauconites follows the first two evolutionary trends (not presented here) (modified from Banerjee et al., 2016). (C) Cross plot of octahedral Al3+ and Fe3+. Notethat the overall increase in Fe3+ from type 3 to type 2 and then to type 1 glaucony at the expense of Al3+. (D) Cross plot of Fe/Sum of octahedral charges vs M+/4Si(M = Interlayered cations) (modified from Meunier and El Albani, 2007). Note that type 1 and 2 glaucony data fall into the field of glauconite and Fe-illite, while type 3glaucony data are outside these fields. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


1 (52.39–58.79 wt%) glaucony. High SiO2 content (47.84–58.85 wt%) in the Tieling glaucony is also comparable with that of the Pre-cambrian glauconitic pellets (Banerjee et al., 2008; Guimaraeset al., 2000; Ivanovskaya et al., 2006). In contrast, SiO2 content ofthe Phanerozoic evolved glaucony pellets ranges from 45.4% to

52.6% (Odin and Matter, 1981). Na2O, CaO, MnO and P2O5 concen-trations of the Tieling glaucony samples are negligible.

In the octahedral site, Al3+ is the major cation, varying between0.77 and 1.70, and Mg2+ is higher than most of the reported values(Banerjee et al., 2015, 2016; Jarrar et al., 2000). The Fe3+ is low,

Fig. 8. REE + Y patterns of carbonate-hosted glaucony from the Tieling Formation, showing slightly anticlockwise-skewed patterns and lack of obvious Cerium (Ce) anomaly.(A) REE + Y patterns of acetic acid digested samples. (B) REE + Y patterns of HNO3/HF digested samples.

Fig. 9. A depositional model suggested for the Tieling Glaucony. Fe redoxcline likely positioned in shallow subtidal zone during deposition of the Tieling Formation. This zoneroughly coincides with the Ce redoxcline where carbonates have the potential of recording Ce anomaly variations. Mixing of photosynthetic and chemoautotrophic organicmatter within this zone may also result in variable d13Corg values. Sufficient Fe2+ and Fe3+ supply in this zone favored glauconization. Substrate porosity and permeability mayhave been the major controls for glauconization and glaucony evolution. Type 1 glaucony distributed in the most porous zone has the longest time for evolution, whereas type3 glaucony precipitated in the least porous zone has the shortest time to evolve.



varying between 0.04 and 0.29 atoms per formula unit, whereasFe2+ is relatively high, varying between 0.05 and 0.41. The octahe-dral R3+ varies from 1.06 to 1.76 atoms, with an average of1.26 atoms per formula unit. The average Si4+ and Al3+ contentsin the tetrahedral sites are 3.71 and 0.29 atoms per formula unit,respectively, suggesting low Al3+-Si4+ substitution at the tetrahe-dral sites in the Tieling glaucony (Table 1).

Meunier and El Albani (2007) proposed a cross plot of Fe/Sum ofoctahedral charge vs M+/4Si (M+ = interlayered cations) to differen-tiate the compositional fields of glauconite, Fe-illite and Fe-Alsmectite. Most of the analyzed Tieling glaucony falls in the fieldof glauconite and Fe-illite (Fig. 7D).

4.4. REE+Y composition

Acetic acid digested carbonate samples (Table 3; Fig. 8) showREE concentrations of 24.41–58.57 ppm. PAAS (Post-Archean Aus-tralian Shale)-normalized REEs (REESN) display seawater-like butanticlockwise-skewed distribution patterns, with light-to-heavyREE ratios (calculated as PrSN/YbSN; Table 3) close to 0.7, highY/Ho ratio (avg. 47), non or weak negative Eu anomaly [calculatedas Eu/Eu⁄ = EuSN/(0.66SmSN + 0.33TbSN)], and slightly positive Gdanomaly [calculated as Gd/Gd⁄ = GdSN/(0.33SmSN + 0.67TbSN)].The Ce/Ce⁄ ratios [Ce/Ce⁄ = CeSN/(PrSN2 /NdSN)] range from 0.91 to1.09, similar to those reported from the Mesoproterozoic Wumis-han carbonates of ca. 1.50–1.45 Ga (Tang et al., 2016), but theyare much higher than those observed in modern (e.g., 0.65–0.80,Webb and Kamber, 2000) and Paleozoic (e.g., Nothdurft et al.,2004) non-skeletal and microbial carbonates. The two samplesdigested using HNO3/HF (Fig. 8B) show similar Ce and Eu anoma-lies, Pr(SN)/Yb(SN) and Y/Ho ratios, confirming that the Tieling car-bonates contain negligible siliciclastics and the influence ofsiliciclastics on REE is minimum.

4.5. Organic carbon isotopes

Three samples of the Tieling carbonates (1407JxT-3, 1407JxT-6and 1407JxT-7) have d13Corg values of �28.7‰, �28.3‰, and�28.4‰, respectively. These values are within the d13Corg rangeof �25‰ to �34‰ reported from the Mesoproterozoic carbonatesof the North China platform (Guo et al., 2013b; Luo et al., 2014).

5. Discussions

5.1. Redox conditions for the Tieling carbonate deposition

During the period between the Great Oxidation Event (GOE;Holland, 2002) and the Neoproterozoic Oxygenation Event (NOE;Och and Shields-Zhou, 2012; Shields-Zhou and Och, 2011), theatmospheric oxygen level was previously estimated as >1% PresentAtmosphere Level (PAL), which resulted in stratified oceans withoxygenated surface water but anoxic or euxinic deep-water (e.g.Bekker et al., 2004; Canfield, 1998; Farquhar et al., 2000;Holland, 2006; Pavlov and Kasting, 2002). Recent studies, however,indicate that the Earth’s oxygenation history was more complexthan previously thought (e.g., Crowe et al., 2013; Frei et al., 2009;Kump, 2008; Lyons et al., 2014; Planavsky et al., 2012, 2014;Scott et al., 2014), and the pO2 may have declined significantly(to <0.1% PAL) shortly after the GOE and low-oxygen conditionsmay have lasted for more than a billion years up to the late Neo-proterozoic (Bekker and Holland, 2012; Lyons et al., 2014;Planavsky et al., 2014; Sahoo et al., 2016; Tang et al., 2016; butsee Zhang et al., 2016 for a different view).

Since the Mesoproterozoic Tieling glaucony was formed duringearly diagenetic stage when active exchanges existed between

porewater and seawater, the seawater redox conditions may havehad significant impacts on glauconization, especially on the incor-poration of Fe into the evolving glaucony. To date, the redox stateof shallow waters from which the Tieling glaucony was precipi-tated has not been investigated directly. To assess the backgroundredox conditions during glauconitization, we used REE + Y signa-tures, particularly Ce anomalies and organic carbon isotopesrecorded in the carbonates as proxies.

Ce anomalies record both secular seawater redox changes andlocal redox conditions where carbonates precipitate (e.g., Kamberand Webb, 2001; Ling et al., 2013; Planavsky et al., 2010; Tanget al., 2016). In redox-stratified ocean environments (basins),reductive dissolution of settling Fe-Mn (hydro)oxides below Fe-Mn redox boundary results in Ce(III) enrichment and therefore,higher Ce/Ce⁄ values below the chemocline (Byrne andSholkovitz, 1996; German et al., 1991; Slack et al., 2007) and lowerCe/Ce⁄ values in shallow-water environments at or above thechemocline. The Ce/Ce⁄ ratios of 0.91 to 1.09 from the Tieling car-bonates (Table 3) are much higher than those from the modernshallow-water carbonates (0.65–0.80; Webb and Kamber, 2000)but are similar to those from the older Mesoproterozoic Gaoyuz-huang and Wumishan Formation (Tang et al., 2016), implyinganoxic oceanographic conditions that may lack remarkable oxicFe-Mn oxide sink. The fluctuations between 0.91 (slightly negativeCe anomaly) and 1.09 (slightly positive Ce anomaly) suggest thatthe Tieling carbonates may have been deposited around the Feredoxcline, with the negative Ce anomalies formed above thechemocline and positive Ce anomalies recording deposition belowthe chemocline (Fig. 9).

The d13Corg values obtained from the Mesoproterozoic carbon-ates of the North China platform range from �25‰ to �34‰(Guo et al., 2013b; Luo et al., 2014). High d13Corg values were inter-preted as recording isotope signature of primary photosyntheticorganic matter, while lower d13Corg values (<–30‰) may have beenresulted from chemoautotrophic/heterotrophic biomass contribu-tion to total organic carbon below the chemocline (Guo et al.,2013b; Hayes et al., 1999; Luo et al., 2014; Jiang et al., 2010,2012). The average d13Corg value of –28.5‰ from the glaucony-bearing carbonates of the Tieling Formation falls in the range ofthe Mesoproterozoic carbonates in the same area (Guo et al.,2013b; Luo et al., 2014), which is consistent with partial chemoautotrophic/heterotrophic biomass contribution to organic matter insuboxic environments close to the chemocline (Guo et al., 2013b;Luo et al., 2014).

Facies analyses suggested that the carbonates of the Tieling For-mation were mainly deposited in shallow waters above fair-weather wave base (Mei et al., 2008). This implies that duringthe Tieling carbonate deposition, the redoxcline may have beenvery shallow (above the fair-weather wave base). Because oxygenin shallow seawater is well mixed and is in equilibrium with atmo-spheric oxygen, it also implies low atmospheric oxygen level dur-ing the Tieling carbonate deposition. This is consistent with theestimate of low atmospheric oxygen level (�0.1% PAL) during thetime based on chromium isotope study (Planavsky et al., 2014).

5.2. Genesis and Environmental significance of the Tieling glaucony

Previously, the high K but low Fe contents of Precambrianglaucony have been interpreted as a diagenetic feature(e.g., Chen, 1994). Several lines of evidence argue against substan-tial diagenetic alteration of the Tieling glaucony: (1) the hostingcarbonates of the Tieling glaucony, including stromatolites, havewell-preserved micro-textures that do not favor a strongdiagenetic alteration. This is consistent with the low maximumburial temperature (as low as �90 �C) obtained from its immediateoverlying Xiamaling Formation (Zhang et al., 2015); (2) the hosting


carbonates of the Tieling glaucony have low siliciclastic contentsand may not have sufficient K to elevate and homogenize the Kcontents in all three types of glaucony during diagenesis; (3) thehigh K contents in the Tieling glaucony are similar to those mea-sured from other time-equivalent Precambrian units globally(e.g., Banerjee et al., 2015, 2016; Mei et al., 2008; Zhou et al.,2009), which have different burial depths and metamorphicgrades; (4) the high K and low Fe contents of the Tieling glauconyfit well with the secular change in glaucony K concentration gener-ated from the global record (e.g., Banerjee et al., 2015, 2016); and(5) diagenetic recrystallization does not necessarily lead to high Kcontent in glaucony. In the Xiamaling Formation (1.40–1.35 Ga; itsimmediate overlying unit) of the same region, we found that thetransformation of glaucony to berthierine (and chamosite in laterdiagenesis) is clearly accompanied by the loss of K but additionof Fe (Tang et al., submitted for publication). Therefore, we inter-pret that the high K and low Fe contents of the Tieling glauconyare primary features that were most likely controlled by specificchemical conditions of ocean seawater. No feldspar grains or theirpseudomorphs have been observed in the glaucony-bearing sam-ples, suggesting that detrital feldspar dissolution may not be theK source for glauconitization. Instead, the K-bentonite layers foundin the vicinity of the Tieling glaucony (Gao et al., 2007, 2008b; Suet al., 2008, 2010) suggest that volcanic ashes may have suppliedabundant K for glauconitization in seawater.

The Tieling glaucony appears as irregular void fills (Fig. 4) thatmay have precipitated from K- and Fe-rich porewater. The threetypes of glaucony distributed in different parts of the Tieling stro-matolite reef may record different evolution stages of glauconitiza-tion (Fig. 9). Since the carbonate substrate with variable porosityand permeability may control the supply of elements from seawa-ter, type 3 glaucony with smallest grain size may have had theshortest evolution time and represents the nascent stage of glau-conitization. In contrast, type 1 glaucony with largest grain sizemay have had the longest evolution time and represents the mostevolved glaucony. The constant and high K2O content of three glau-cony types (Fig. 7A) suggests that the nascent glaucony had high Kcontent; while the progressive increase of Fe content from type 3to type 1 glaucony requires persistent Fe supply during the matu-ration of the Tieling glaucony. Type 1 glaucony is mostly concen-trated in coarser-grained carbonate matrix between stromatolitecolumns, suggesting that larger pore space favored ion exchangesduring the maturation of the Tieling glaucony.

Three hypotheses have been proposed to explain glauconitiza-tion, namely the ‘layer lattice theory’ (Burst, 1958a, 1958b;Hower, 1961), the ‘verdissement’ theory (Odin and Matter, 1981),and the ‘pseudomorphic replacement’ theory (Dasgupta et al.,1990). The ‘layer lattice theory’ ascribes the formation and evolu-tion of glaucony to simultaneous incorporation of Fe and K intothe lattice of precursor clay minerals such as high alumina smec-tite, illite or degraded mica. The ‘verdissement’ theory invokes ini-tial precipitation of glauconitic smectite within micropores of thesubstrates, followed by maturation through incorporation of K intomineralogical structure at constant total Fe2O3 (TFe2O3). The ‘pseu-domorphic replacement’ theory emphasizes the supply of K andsilica (Si) into porewater through dissolution of K-feldspars andquartz in substrates, which facilitates glaucony authigenesis. Pre-cambrian glaucony commonly shows constantly high K2O butlow and variable TFe2O3 contents, which is only consistent withthe ‘pseudomorphic replacement’ theory when elemental compo-sitions are considered (e.g., Banerjee et al., 2015, 2016). However,there is no evidence, either texturally or compositionally, ofK-feldspar or quartz dissolution or replacement in all three typesof glaucony in the Tieling Formation. In addition, there are negligi-ble detrital components in the hosting carbonates. Thus, the ‘pseu-domorphic replacement theory’ is difficult to explain the increase

of Fe during the evolution of the Tieling glaucony, which requirespersistent Fe supply from dissolution of siliciclastic componentsin substrates.

We interpret that the extra Fe during glaucony evolution wasmainly derived from ferruginous seawater. As the substrate isdevoid of siliciclastics, neither the Fe in calcite that is commonlybelow detection limit (Table S1) nor the Fe from ferroan dolomitethat postdates glaucony precipitation (Fig. 5) could provide enoughFe for progressive glauconitization. Initial K and Fe concentrationsthat promoted nascent glaucony precipitation may have beenrelated to contemporary volcanic eruptions as evidenced by widelydistributed K-bentonite beds in the basin (Gao et al., 2007, 2008b;Su et al., 2008, 2010), but the Fe supply required for glaucony evo-lution from type 3 to type 1 (Fig. 7A) may have been from Fe2+ inseawater and porewater (Fig. 9). Since both Fe2+ and Fe3+ are simul-taneously required for glauconitization, glaucony preferably pre-cipitates around the Fe redoxcline where Fe2+ is partiallyoxidized to Fe3+. In K- and Fe-rich environments, fully reducingconditions (e.g., ferruginous condition) favor berthierine forma-tion, while oxic conditions may promote goethite precipitation(Velde, 1992). The fluctuating redox condition was perhaps moreideal for glaucony precipitation, which is also supported by theCe anomaly and d13Corg values of the glaucony-hosting carbonates.The abundance of herringbone calcites in stromatolite laminae thatcontain glaucony also suggests ferruginous shallow seawater con-ditions because carbonate precipitation inhibitors, such as Fe2+ andMn2+, are essential for the precipitation of herringbone calcites(Sumner and Grotzinger, 1996a, 1996b).

The maximum of Fe content in glaucony could be limited by theredox conditions and by glaucony textures. In the glaucony tex-ture, cations of an octahedron are coordinated with six oxygensor hydroxyl units, while each tetrahedron silicon atom is sur-rounded by four oxygen atoms. In reducing environment, abundantFe2+ (but rare Fe3+) would occupy octahedral sites in glaucony (inTieling glaucony: Fe2+/TFe = 58%), resulting in negative charges ofoctahedrons that may be balanced by incorporating K+ into theinterlayer sites. However, if the interlayer sites are saturated withK+, no more Fe2+ can be incorporated into octahedral sites; onlyFe3+ can be incorporated into the octahedral sites by replacingAl3+ during times of redoxcline fluctuations (to more oxic condi-tions). Thus, we argue that low oxygen concentration in the Meso-proterozoic shallow seawaters was likely the essential cause forthe low TFe2O3 but high K contents in the glaucony.

The constantly high K content from type 3 to type 1 Tielingglaucony (Fig. 7A) indicates that the K content was not controlledby the amount of glauconization time. K+ cations occupy the inter-layer sites of glaucony and their fundament function is to balancethe negative charges in octahedrons and tetrahedrons. Both Fe2+

and Mg2+ in octahedral sites, and Al3+ and Fe3+ in tetrahedral siteswould cause negative charges on the octahedrons and tetrahe-drons. Since the Tieling glaucony has high Si content (Table 2),replacement of Si4+ by Al3+ or Fe3+ in tetrahedral sites will notresult in high K content. Thus, the most possible cause for high Kcontent is that a large number of Fe2+ and Mg2+ occupied octahe-dral sites during initial glaucony precipitation. Considering thatthe glaucony with relatively low Mg content also has high K con-tent (Table 1), Fe2+ in octahedral sites must have been the majorcations in octahedrons, which is consistent with the high Fe2+/Fe3+ ratios of the Tieling glaucony.

In summary, the Tieling glaucony was formed throughauthigenic precipitation of initially high-K but low-Fe glauconiticsmectite in micropores within the substrates, followed by progres-sive incorporation of Fe (but not K) into the glaucony texture. BothK and Fe required for glauconization were sourced from seawater,but the high K and increasing Fe contents were constrained by lowoxygenated seawaters with high Fe2+/Fe3+ ratios and dioctahedral


texture of glaucony. Because the glaucony-hosting carbonate is vir-tually devoid of detrital components, the Tieling glaucony can beused as a proxy for fluctuating seawater redox conditions in thedepositional environment.

6. Conclusion

An integrated study of the Mesoproterozoic Tieling glauconyreveals information that may help understanding the glauconitiza-tion processes in low-oxygen conditions of Precambrian seawater:

(1) The Tieling glaucony was mainly deposited in shallowmarine environments above fair-weather wave base, and ischaracterized by high K, Al and Mg but low and variable Fecontents. Maturation of the glaucony is facilitated by pro-gressive incorporation of Fe into the glaucony texture andsimultaneous release of Al.

(2) Ce anomaly and d13Corg values suggest authigenic precipita-tion of the glaucony around the Fe-redoxcline where bothFe2+ and Fe3+ were available. The occurrence of authigenicglaucony in shallow-water carbonates above fair-weatherwave base suggests a shallow chemocline in Mesoprotero-zoic seawater. The deepening of authigenic glaucony precip-itation from shallow subtidal environments in Precambrianoceans to middle shelf–slope environments in Phanerozoicoceans is likely linked to the secular change of ocean chemo-cline in response to increased ocean oxygenation.

(3) Low oxygen concentration and high Fe2+/Fe3+ ratios inshallow-water environments were possibly responsible forthe low TFe2O3 contents in Mesoproterozoic glaucony, sinceFe3+ rather than Fe2+ is required for glauconization. Thecause of high K content in the Tieling glaucony may berelated to the high proportional occupation of Fe2+ in octahe-dron sites, which requires additional K+ cations to balancethe excess negative charges of individual octahedrons.

Acknowledgments

The study was supported by the National Natural ScienceFoundation of China (Nos. 41672336 and 41402024), and by ChinaUniversity of Geosciences (Beijing) (No. 2652014063).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.precamres.2017.03.026. These data include Google maps of the most importantareas described in this article.

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Formation of shallow-water glaucony in weakly oxygenated ...ganqing.faculty.unlv.edu/...PrecambrianResearch.pdfPrecambrian ocean: An example from the Mesoproterozoic Tieling Formation