Udita Bansal,Santanu Banerjee,Dhiren K.Ruidas,Kanchan Pande

Department of Earth Sciences,Indian Institute of Technology Bombay,Powai,Mumbai,400076,India

Abstract This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India.Resting conformably over the Bagh Group,the Lameta Formation consists of～4—5 m thick arenaceous,argillaceous and calcareous green sandstones underlying the Deccan Traps.The sandstone is friable,medium-to coarse-grained,well-sorted and thoroughly cross stratified,and contains marine fossils.Detailed petrography,spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high K2O,MgO,Al2O3and moderate TFe2O3.Glauconite is formed by the replacement of K-feldspars,initially as stringers in the cleavages and fractures of feldspars.Incipient glauconite subsequently evolves fully,appearing as pellets.Fully-evolved glauconite pellets often leave tiny relics of K-feldspar.XRD exhibits characteristic peak of 10˚A from basal(001)reflection of glauconite,indicating the “evolved” character.The K2O content of glauconites in the Lameta Formation varies from 5.51%to 8.29%,corroborating the “evolved” to “highly-evolved” maturation stage.The TFe2O3content of glauconite varies from 12.56%to 18.90%.The PASS-normalized-REE patterns of glauconite exhibit a “hat-shape” confirming the authigenic origin of glauconites.The slightly-negative to slightly-positive Ce anomaly value and the moderate TFe2O3content of glauconite agree well with a suboxic,estuarine condition.The replacement of K-feldspar by the glauconite contributes towards the high K2O content.Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences.

Keywords Glauconite,Estuary,Mineral chemistry,Cerium anomaly,Redox condition,Lameta Formation,Narmada Basin

1. Introduction

Modern glauconite forms in the depth range of 50—500 m and is most abundant between 200 m and 300 m deep on the sea floor(Odin and Matter,1981).There is a tendency to apply the same bathymetric interpretation for ancient glauconites.However,authigenic glauconitemay form in a wide spectrum of shallow marine environments including wave-agitated estuaries and coastlines(Banerjee et al.,2015,2016a;Chafetz and Reid,2000).Very few studies,however,investigate the relationship between glauconite composition and palaeoenvironmental conditions(Banerjee et al.,2016b).K2O and TFe2O3content of glauconite varies with sedimentation rate,nature of substrate,and depositional environment(Amorosi,2013;Banerjee et al.,2015,2016a,2016b;Bansal,2017).Highlyevolved glauconites containing ＞7.5% K2O are commonly related to the mega-condensed section.Amorosi(2013)proposed three types of sequencestratigraphic condensed sections on the basis of glaucony abundance and maturity:(i)simple omission surfaces,containing ＜20%of“slightly-evolved” glaucony;(ii)condensedsections,including20%—50%of“evolved”glaucony;and(iii)mega-condensed sections,with＞50%of highly-evolved glaucony.However,glauconite forming within the K-feldspar substrate may also exhibit high K2O content(Banerjee et al.,2015;Bansal et al.,2017).The TFe2O3content of glauconite possibly depends on sufficient supply of Fe and redox condition of the depositional setting(Banerjee et al.,2016a;Meunier and El Albani,2007).Ce anomaly values of the glauconite provide valuable information regarding redox conditions of the depositional setting(Banerjee et al.,2015;Elder field and Pagett,1986;Wright et al.,1987).Composition of glauconite also depends on its origin and evolutionary paths,which are explained by theories including the “layer lattice” theory(Burst,1958a,1958b;Hower,1961), “verdissement” theory(Odin and Matter,1981),and“pseudomorphic replacement”theory(Banerjee et al.,2008,2015;Dasgupta et al.,1990).“Verdissement theory” involves an initial precipitation of K-and Fe-poor glauconitic smectite within porous substrate and its subsequent maturation by addition of K at constant Fe.An integrated study involving detailed petrography,mineralogy,mineral chemical analysis,and REE chemistry of glauconites is likely to provide a better perspective of factors affecting the composition of glauconite.

The present study explores the influence of depositional conditions on the composition of glauconite.This study investigates the geochemical characteristics of glauconites within the Maastrichtian Lameta Formation in central India.Resting conformably over the Bagh Group,the Lameta Formation comprises arenaceous,argillaceous,and calcareous green sandstones underlying the Deccan Traps.This study presents detailed petrography,X-Ray Diffraction(XRD),Mössbauer spectroscopy,mineral chemistry,and REE chemistry of glauconite.The major findings of the paper include evaluating the effect of depositional redox conditions and nature of substrate on glauconite composition.The composition of glauconite has been further related to the estuarine depositional environment.

2. Geological background

The Lameta Formation crops out at the western segment of the Narmada Basin in central India(Fig.1;Tandon,2000).Although most of the outcrops occur as disconnected patches along the basin,they are well developed around Phutlibaori area in western India(Fig.1;Tandon,2000).Ahmad and Akhtar(1990)and Kumar et al.(1997)considered that the Lameta Formation conformably overlies the Bagh Group(Table 1).It is a roughly 4—5-m-thick succession comprising arenaceous,argillaceous,and calcareous sandstone(Tripathi,2005).The Lameta Formation occurs beneath the basal Deccan Traps in the west through a 15-cmthick siliceous layer(Fig.2);while in the east(towards Jabalpur),it is contemporaneous with early Deccan volcanism(Salilet al.,1997;Tandon,2000).The Lameta Formation has gained attention for over a century because of its association with a thick pile of Deccan Traps and its diverse faunal assemblage.It has been assigned a Maastrichtian age on the basis of Igdabatis(Besse et al.,1986),palynomorph Aquilapollenites indicus(Dogra et al.,1988),and palaeobatid frogs(Buffetaut,1987;Jaeger et al.,1989;Sahni and Bajpai,1988).Recently,Prasad et al.(2016)reported isolated archosaur teeth of pre-Late to Late Maastrichtian age within the green sandstone beds of Phutlibaori area.The Late Maastrichtian(～66 Ma)age of the overlying Deccan Traps further corroborates the assigned age of the Lameta Formation(Courtillot et al.,1988;Tandon and Andrews,2001;Venkatesan et al.,1993).

Marine Cretaceous sediments in the western India were related to the formation of an epicontinental seaway which advanced from the west(Tandon,2000).The Late Cretaceous carbonate deposition of the Bagh Group indicates a marine transgression.Tripathi(2006)suggested that marine transgression took place in half-grabens parallel to the Precambrian tectonic framework.Whereas the marine influences were confined only to the west of the Narmada Basin(Fig.1),continental deposition possibly continued around in the Jabalpur in the east and Nagpur in the southeast(D'Emic et al.,2009;Kumar and Tandon,1977,1978,1979;Pascoe,1950;Tandon,2000).The sea regressed owing to the surface uplift related to the active rifting and plume tectonics during the late Maastrichtian(Tandon,2000).

Fig.1 Geological map showing outcrops of Lameta Formation in central India(after Tandon,2000).The map of India within inset shows the location of the Narmada Basin in central India(study area indicated).Maastrichtian coastline is after Tandon(2000).

Tandon(2000)worked extensively on the sedimentology of the Lameta Formation in the Phutlibaori area and interpreted the Maastrichtian coastline(Fig.1).The Lameta Formation of the Phutlibaori area consists of friable,medium-to coarse-grained and well-sorted glauconitic sandstone(Figs.2 and 3a).The lower part of the section is a large-scale trough cross-stratified glauconitic sandstone.Individual co-sets of crossstratified beds measure up to 1 m in height.The crossstratified beds show bimodal palaeocurrent direction along ENE—WSW direction in places.The upper part of the section exhibits small-scale cross-stratification,wave ripples and planar lamination(Figs.2 and 3b).Sandstone of the Lameta Formation around Phutlibaori comprises quartz(～50%),feldspar(～25%)and bioclasts(～25%).The sandstones are classified as bioclasts-rich arkose(Bansal,2017).Glauconite content of thesediment,originating from K-feldspars is～20%.Quartz grains are rounded to sub-rounded,moderately well sorted and exhibit mostly point contact and occasionally straight contact.The sandstone exhibits extensive replacement of framework grains by calcite.Mediumto coarse-grained,clear calcite frequently replaces feldspar and glauconite grains,and also occupies the pore spaces(Fig.4c).The studied section contains abundant bivalves(oyster),gastropods,shark teeth,and fossilized wood logs.

Table 1 Generalized stratigraphy of Phutlibaori area(after Tripathi,2006),Narmada Basin,central India.

Fig.2 Graphic log showing sample positions(P-1 to P-9)in the studied section of the Lameta Formation in Phutlibaori area,western Narmada Basin,central India.Sedimentary structures across the section are also provided.

The presence of shark teeth, oyster and wood logs in the Lameta Formation around Phutlibaori indicates a marginal marine depositional setting.Tandon(2000)considered an E—W-oriented estuarine channel origin of the Lameta Formation in the study area.The dominantly aggrading,well-sorted sandstones with largescale cross-stratifications,exhibiting bi-directional palaeocurrent as well as fossil assemblage indicate an estuarine depositional environment.Presence of abundant thick-shelled bivalves including oysters attests to the estuarine origin(Fig.3c).

3. Methods

Detailed bed-by-bed analysis of exposures of the Lameta Formation in Phutlibaori area was carried out to prepare a graphic log with precise sample positions(Fig.2).Petrographic investigations were carried out using Leica DM 4500P polarizing microscope attached to a Leica DFC420 camera.The rock samples were soaked in water,treated with anhydrous Na2CO3powder and H2O2solution,and kept on a hot plate for 15—20 min,followed by cooling,washing and drying of the mixture to separate the glauconite pellets for geochemical and mineralogical analyses. Glauconite grains were concentrated using Franz magnetic separator(LB-1)with 15°slope and 15°lateral tilt and 1.5—1.8 A current at Department of Earth Sciences,Indian Institute of Technology Bombay.Glauconite pellets were finally hand-picked using Zeiss Stemi 2000 stereo zoom microscope.The powdered samples were scanned from 4°to 30°with a step size of 0.026°2θ,with a scan speed of 96 s/step,using nickel filter copper radiation in an Empyrean X-Ray Diffractometer with Pixel 3D detector at Department of Earth Sciences,Indian Institute of Technology Bombay.The samples were scanned each time under the same instrumental settings after airdrying,ethylene glycol treatment(100°C for 1 h)to evaluate the number of expandable layers and finally after heating at 490°C for 2 h to investigate if the hydroxides are extraneous to the glauconite structures.Mössbauer spectra of two powdered glauconite samples were measured to calculate the Fe2+/Fe3+ratio for structural analysis at Solid State Physics Division,Bhabha Atomic Research Centre Mumbai.Aγ-raysource of57Co in Rh matrix at room temperature was used and α-Fe absorber was used at room temperature to calibrate the Doppler velocity V and also as the standard for the isomer shift(IS).The major element geochemistry of the green pellets and substrates were investigated in 5 thin sections on 38 points using a Cameca SX 5 Electron Probe Micro Analyzer at Department of Earth Sciences,Indian Institute of Technology Bombay,with accelerating voltage 15 kV,specimen current of 40 nA and beam diameter of 1 μm(peak:10—20 s and background counting:5—10 s).Minerals as well as synthetically developed phases were used as standards.Duplicate analysis of individual points records analytical error of less than 1%.Concentrations of REE were determined using Thermo Fisher Scientific,Germany(Element XR)at Thermo Fisher Lab in Mumbai.About 25 mg of cleaned glauconite pellets were precisely weighed and dissolved in a mixture of～2.5 ml of HF—HNO3(2:1)in teflon bombs for REE analysis.The solution was kept in an ultrasonic bath for 45 min,evaporated and dried down.The residue was treated with concentrated 8N HNO3and evaporated.The same step was repeated thrice before diluting the sample with 25 ml of 1N HNO3.Ruthenium(Ru)was added as internal standard to all samples and standards to monitor the instrumental drift induced during analysis.USGS geochemical reference standards(SCo-1,SCo-2,SBC-1)were used to assess the accuracy of the analyses.

Fig.3 a—Field photograph showing cross-stratified green sandstone of the Lameta Formation;b—Wave ripples on sandstone bed;c—Close up of oysters(marked by blue arrows).The pens in a and b are 14 cm long;the coin in c is 2.5 mm in diameter.

4. Results

4.1. Mode of occurrence of glauconite

Glauconite mainly occurs as three different forms,i.e.,linear stringers within cleavages and fractures of feldspar,along peripheries of feldspars,and as pellets(Figs.4 and 5).The linear and interconnected stringers of glauconite extend upto 250 μm longand 50 μm wide.In places,glauconite completely replaces the K-feldspar grain,referred to as fully-evolved glauconite pellets.The incipiently-formed glauconite pellets often leave traces of K-feldspar.Fully-evolved glauconite pellets are mostlyovoid,elongated.The longaxis of the elongated pellets ranges from 250 μm to 300 μm.Occasionally,the pellets are perfectly rounded,varying in diameter from 100μmto150μm.The glauconite pellets are intact in nature and exhibit light green color under plane-polarized light.Under cross-polarized light,they exhibit high interference color ranging from third-order green to dark green color.The texture of glauconites appears pitted due to the random arrangement of micro-platelets with pinpoint extinction.Most glauconite pellets display deeply penetrating fractures originating at the periphery and taper toward the center.Feldspars as well as glauconite pellets are replaced extensively by calcite. Nearly-complete replacement of glauconite is often marked by relicts of glauconite completely surrounded by calcite cements(Fig.4c).Calcite cementation possibly formed in a shallow burial condition after the formation of glauconite.X-ray mapping clearly distinguishes glauconite forming within cleavages and peripheries of feldspar grains by high concentrations of K,Fe and Mg,and moderate concentrations of Al and Si(Fig.5).Textural evidences confirm that glauconite forms mainly by the replacement of K-feldspars.Incipient glauconite forms along cleavage and fractures of K-feldspars as small blebs and streaks within the feldspar grains(Fig.4a).Subsequently it grows as linear and interconnected stringers of glauconite till the K-feldspar is completely replaced.

Fig.4 Microphotographs under cross-polarized light showing a—“Incipient” glauconite within cleavages of K-feldspars(red arrows);b—Glauconite rinds along peripheries of feldspar grains(red arrows);c—Relict of feldspar substrate(yellow arrow)within a glauconite pellet(red arrow);d—Extensive calcite replacement(blue arrows)of a glauconite grain(red arrow).

Fig.5 a—EPMA backscatter image showing glauconite within the cleavage of K-feldspar grain(red arrow;the same follows in b—f)and along the periphery(yellow arrows;the same follows in b—f);b—f—X-ray mappings showing elemental concentrations of K(b),Fe(c),Al(d),Mg(e)and Si(f)within the same K-feldspar grain.Glauconite is marked by high concentrations of K,Fe and Mg,and moderate concentrations of Al and Si.Quartz grains are distinct by the red color in Si mapping.

4.2. Mineralogy and textural analysis of glauconite pellets

XRD peaks distinguish glauconite from other clay minerals and provide useful insights regarding the stage of glauconite maturation corresponding to the expandable layers.The air-dried samples of the glauconite pellets exhibit the prominent basal reflection(001)at 10.0poorly developed(020)reflection at 4.5and(003)reflection at 3.3(Fig.6).Theand(112)reflections are absent in these samples. The peaks are unmoved after glycol treatment and heating at 400°C indicating minimal inter-stratification between expandable and non-expandable layers(Thompson and Hower,1975).Peaks appear sharp and intense with narrow base in all three modes of sample scanning.A minor peak of illite at 5.0coexists in all modes of scanning(Fig.6).The prominent basal reflection at 10.0and the presence of(020)and(003)reflection at 4.5and 3.3are characteristics of glauconite(Odin and Matter,1981).Poorly-developed peak at 4.5indicates disordered nature of the lattice(Burst,1958b).Narrow and symmetric peaks further support the interpretation.The absence ofand(112)reflections indicate a “highly-evolved” to “evolved” nature of the glauconite containing approximately 10%expandable layers,corresponding to～7.5%K2O content(Odom,1984).Odin and Matter(1981)classified glauconite into four types:(i)2%—4%for the nascent,(ii)4%—6%for the slightly-evolved,(iii)6%—8%for the evolved,and(iv)＞8%for the highly-evolved.

Field emission gun-scanning electron microscopy(FEG-SEM)study of glauconite exhibits well-developed,slightly-sinuous and sub-parallel lamellae(Fig.7).The lamellar structure of the glauconite further corroborates the “highly-evolved” nature of glauconite(Odin and Matter,1981;Wigley and Compton,2007).

4.3. Structural analysis of glauconite using Mössbauer spectroscopy

Mössbauer spectra of glauconites in the Lameta Formation exhibit three doublets.Doublets A and C with isomer shift(δ)values of 0.37—0.42 mm/s corresponds to ferric ions,while doublet B(δ =1.06—1.13 mm/s)indicates ferrous ions(Table 2;Fig.8).

Fig.6 X-ray diffractograms of a glauconite pellet(sample no.P-4)under different conditions,i.e.,air-dried,glycolated and heated at 400 °C.Note the characteristic(001)basal reflection at 10.0 ˚A,(020)reflection at 4.5 ˚A and(003)reflection at 3.3 ˚A.The positions of the peaks are unmoved after glycol treatment and heating at 400°C.Peaks appear sharp and intense with narrow base in all three modes of sample scanning.

The ΔEQ has a lower value for doublet A(0.40—0.42 mm/s)than for doublet C(1.07—1.22 mm/s).Therefore,doublet A is assigned to ferric ions to cis M(2)position,and doublet C is assigned to ferric ions to trans M(1)position.Doublet B belongs to ferrous ions and has a higher ΔEQ value of 2.21—2.45 mm/s,corresponding to trans M(1)position in octahedral site(Hogg and Meads,1970;Kotlickiet al.,1981;McConchieet al.,1979;Rolf et al.,1977).The Fe2+/Fe3+ratio of glauconite was calculated by taking into consideration the χ2value( fitting parameter)and the relative area(under curve)of the component belonging to the cationic composition of the octahedral sheets.The Fe2+/Fe3+ratio of 0.15,obtained by Mössbauer spectroscopic study of glauconite,was used for calculation of octahedral and tetrahedral charge as well as the structural formula of glauconites.Mössbauer study confirms the exact allocation of total Fe in tetrahedral and octahedral sites and provides the general formula of glauconite(provided in next section).

Fig.7 FEG-SEM image of a glauconite pellet showing well developed,slightly-sinuous and sub-parallel lamellae marked by red arrows.The lamellar structure of the glauconite confirms the“highly-evolved” nature of glauconite.

4.4. Major element composition of glauconite

Majorelement composition of glauconite is important to study the origin and evolution of glauconite.Electron probe micro-analysis(EPMA)data of glauconites are provided in Table 3.All elemental determinations were made on an anion equivalent basis to the structural formulae per O10(OH)2.All analyses have been normalized to 100 wt%on an anhydrous basis for different cross-plots.The K2O content varies from 5.51%to 8.29%in fully-evolved glauconite and from 7.04%to 7.90%for incipient variety in feldspar grains(Table 3).The K2O content of the glauconite suggests “evolved” to “highly-evolved” stage of maturation(Amorosi,1997;Odin and Matter,1981).The TFe2O3content of glauconite varies from 12.56%to 18.90%(Table 3).The MgO content of glauconite varies from 3.47%to 5.14%.The CaO content of all varieties of glauconites is negligible(av.0.50%).The SiO2content of glauconites varies from 48.98%to 55.73%,while the Al2O3content varies from 7.96%to 14.11%.

Fig.8 Mössbauer spectra of selected glauconite samples(P-4 and P-5)recorded at room temperature indicating relative abundance of Fe3+(red and blue doublets)and Fe2+(green doublet)cations.

Glauconite is defined as Fe-rich dioctahedral mica with tetrahedral Al(or Fe3+)usually greater than 0.2 atoms per formula unit and octahedral R3+greater than 1.2 atoms(Bailey,1980).The tetrahedral charge of glauconite in the Lameta Formation varies from 0.17 to 0.32 atoms per formula unit whereas the octahedral R3+varies from 1.32 to 1.54 atoms per formula unit(Table 4).The tetrahedral charges of a few glauconites,do not fully comply with the glauconite composition prescribed by the Association Internationale Pour I'Etude des Argiles(AIPEA).We therefore provide the glauconite composition in a cross-plot of 4M+/Si(M+=interlayered cations)vs.(Fe octahedral)/(sum of octahedral charge)(cf.Meunier and ElAlbani,2007)(Fig.9).All data points plot within the field of glauconite.The average formula of the glauconites is as follows:

Table 2Mössbauer spectral parameters of selected glauconite samples in the Lameta Formation.

Table 3 Oxide weight percentage of glauconite in the Lameta Formation.

Glauconites in the Lameta Formation contain high K,high Si,high Mg,high Al and moderate Fe.Most of the glauconite datum plots fall in the field of“glauconitic minerals”in a cross-plot of K2O and TFe2O3(Fig.10).A few data of glauconites,however,plot in the field of“compositional gap” of Odin and Matter(1981).An Al2O3versus TFe2O3cross-plot exhibits negative correlation(r2=0.9)(Fig.11a).The data indicate substitution of Al3+ions by Fe3+ions in the octahedral site during the course of glauconitization(Amorosi et al.,2007;Banerjee et al.,2008,2012a,2012b,2015,2016b;Bansal et al.,2017;Bornhold and Giresse,1985;Chang et al.,2008;Dasgupta et al.,1990;Eder et al.,2007;Odin and Matter,1981;Sánchez-Navas et al.,2008;Tang et al.,2017;Velde,1985).The relationship between K2O and Al2O3content of glauconite exhibits moderate negative correlation(r2=0.7)(Fig.11b).The plot suggests simultaneous addition of K in the interlayer with Al3+—Fe3+substitution in the glauconite structure.K2O exhibits moderate negative correlation with SiO2(r2=0.7),implying that the release of Si as K is added into glauconite structure(Fig.11c).K2O versus MgO cross-plot exhibits no correlation(Fig.11d).Thesecross-plots suggest that during the course of evolution,K and Fe are added while Al and Si are removed from the glauconite structure.The Mg content of glauconite is independent of stage of glauconite evolution.Fixation of K in the interlayer sites is synchronous with Al3+—Fe3+substitution in the octahedral site.

Table 4 Structural composition of glauconite in the Lameta Formation.

4.5. REE chemistry of glauconite and its implications

REE concentrations and patterns are useful in interpreting their relative inputs from seawater and/or substrate.Further,the Ceanomaly(Ce/Ce＊)values can provide useful insight in understanding the redox conditions.REE concentrations of six glauconite pellets were carried out(Table 5).Glauconites in the Lameta Formation show a moderate variation of REE(i.e.,a minimum of 148 ppm,a maximum of 528 ppm,and an average of 319 ppm).LREE concentrations are much higher(av.289 ppm)than HREE(av.30 ppm).

Chondrite-normalized patterns reveal weak LREE/HREE fractionation(8.17—12.48)and exhibit rare negative(Eu/Eu＊=0.17—0.18)or no Eu anomaly(Fig.12).The strong HREE depletion in chondritenormalized patterns suggests that∑REE is related to the presence of tiny relics of K-feldspar substrate in the glauconite.Post Archaean Sedimentary Shale(PASS)-normalized patternsare “hat-shaped”,exhibiting strong HREE depletion and MREE enrichment(Fig.13).The REE patterns of glauconite indicate the authigenic origin of glauconites as reported for other marine authigenic minerals(Banerjee et al.,2012a,2012b;Jarrar et al.,2000).All glauconites bear upper crustal signatures in a cross-plot of Yvs.La(Li et al.,2017;Zhao et al.,2013)(Fig.14).

Fig.9 Cross-plot between 4M+/Si(M=interlayered cations)vs.Fe/Sum of octahedral charge(after Meunier and El Albani,2007)(note that all data plot within the field of glauconite).MLM=Mixed layer minerals.

4.6. Palaeoredox of glauconite

Fig.10 Cross-plot between K2O and TFe2O3( fields of illitic minerals and glauconitic minerals are after Odin and Matter,1981).Note the good correlation between K2O and TFe2O3.

True Ce anomalies of the samples were calculated using the method of Bau and Dulski(1996).The Pr/Pr＊ratio was used to evaluate the effect of La abundance in four samples(P-1,P-3,P-5 and P-6),as the other two samples(P-2 and P-8)show anomalously high Pr values misleading the true Ce anomaly values(Table 5).The former group of samples plot in and around IIIb field in the cross-plot of Ce anomaly vs.Pr/Pr＊(Fig.15).The relationship between Ce anomaly and Pr/Pr＊indicates true negative Ce anomaly(cf.Bau and Dulski,1996,Fig.15).All true Ce anomaly values plot close to the oxic—anoxic boundary in a cross-plot between Ce anomaly and Nd concentration(Wright et al.,1987)(Fig.16).The logarithmic value of the true negative Ce anomaly varies from-0.07 to 0.04,representing suboxic depositional conditions(Elder field and Pagett,1986;Wright et al.,1987)(Fig.16).Further,the average of all glauconites provides negative Mn redox potential(Mn＊)values(-0.85)indicating suboxic conditions(Bellanca et al.,1996;Machhour et al.,1994),since Mn has a tendency to incorporate under more oxygenated conditions above the redox boundary.Nd concentrations of the glauconites ranging from 27.67 ppm to 95.84 ppm indicate moderate sedimentation rates(cf.Wright et al.,1987,Fig.16).

5. Discussion

Fig.11 Cross-plots of major element composition of glauconite in the samples.

Table 5 REE concentrations of selected glauconites in the Lameta Formation.

Fig.12 Chondrite-normalized REE patterns of selected glauconites in the Lameta Formation.Note the strong HREE depletion,indicating the source of REE is related to the presence of tiny relics of K-feldspar substrate.

Fig.13 PASS-normalized REE patterns of selected glauconites in the Lameta Formation.Note the REE patterns exhibit a “hat-shape”.

Authigenic glauconites have been reported from estuarine depositional conditions in a few studies(Banerjee et al.,2015;Chafetz and Reid,2000;Dasgupta et al.,1990).However,the influence of depositional conditions on glauconite composition has not been discussed before.The origin of glauconite is usually explained by three common theories,i.e.,verdissement,layer lattice and replacement.The“verdissement theory” (Odin and Matter,1981)involves precipitation of initial “glauconitic smectite”within the pores of bioclasts and faecal pellets,accompanied by continuous dissolution and recrystallization of the substrate.The incipient glauconitic smectite precipitates evolve to glauconitic mica as K2O content increases at a constant TFe2O3content(Fig.17).The “layer lattice theory” (Burst,1958a,1958b;Hower,1961)involves the transformation of degraded layer lattice silicates to glauconite with simultaneous increase of K2O and TFe2O3content(Fig.17).Textural evidences indicate pseudomorphic replacement of K-feldspars by glauconite in the Lameta Formation(see also Banerjee et al.,2015;Bansal et al.,2017;Dasgupta et al.,1990).The K2O content of the incipient variety is at least 7%.The compositional overlap between incipient and evolved varieties indicates that the K2O content is independent of the stage of glauconitization.The high K2O contents in both incipient and fully-evolved varieties reflect derivation of K+from the K-feldspar substrate.

The formation of glauconite within K-feldspar grains have been addressed before(Banerjee et al.,2008,2015;Bansal et al.,2017;Chattoraj et al.,2009;Dasgupta et al.,1990).The absence of broken pellets and the presence of deeply-penetrating fractures indicate the autochthonous origin of glauconite (cf.Bandopadhyay,2007;Banerjee et al.,2008,2016b;Deb and Fukuoka,1998).Autochthonous glauconite has been reported from estuarine/shallow marine depositional settings(Banerjeeet al.,2015;Chafetzand Reid,2000;Dasgupta et al.,1990).Although modern glauconites are reported abundantly from shelf region(200—300 m),Banerjeeet al.(2016a)recently reported that ancient glauconite formed in varying depositional environments ranging from deep marine(Baldermann et al.,2012)to shallow marine(Banerjee et al.,2012a;Bansal et al.,2017),lagoonal(Banerjee et al.,2012b),estuarine and tidal flats(Dasgupta et al.,1990;Huggett and Gale,1997)and inner to outer shelf(Banerjee et al.,2008,2016b;Chattoraj et al.,2009;Kelly and Webb,1999).The origin of glauconite in shallow marine depositional environments possibly involves different seawater composition and sediment accumulation rates than the present-day seas(Chafetz and Reid,2000).Shelf-originated glauconites form at sediment—water interface in a low energy environment where rate of sedimentation is considerably slow.However,Chafetz and Reid(2000)argued that glauconites within the Riley Formation and Wilberns Formation(upper Cambrian,southwestern United States)formed in very shallow-marine depositional conditions(from 1 m to 5 m)at normal sedimentation rates.Occurrence of glauconite within cross-stratified sandstone beds in the Lameta Formation indicates high-energy conditions;while Nd concentrations of the glauconites,ranging from 27.67 ppm to 95.84 ppm,indicate a moderate sedimentation rate(cf.Wright et al.,1987,Fig.16).

The compositional evolution of glauconite in the Lameta Formation is comparable to those reported in Precambrian varieties(Dasgupta et al.,1990;Ivanovskaya et al.,2006;Sarkar et al.,2014;see other examples in Fig.17).Precambrian glauconites form in shallow marine arkosic to sub-arkosic sandstones(Banerjee et al.,2016a).The Precambrian glauconites are distinguished by their chemical composition,containing high K2O,Al2O3,MgO,and low-to-moderate TFe2O3contents.Most Phanerozoic glauconites exhibit low-to-high K2O,moderate MgO,Al2O3,and high TFe2O3contents.The compositional evolution of these Phanerozoic glauconites relates their origin to either“verdissement” or“layer lattice” theories(Fig.17).Precambrian glauconites invariably consist of a high K2O content,related to the replacement of K-feldspar substrate(Banerjee et al.,2008,2016a).A high K2O content of glauconite,exceeding 7.5%,indicates mega-condensed section in the Phanerozoic(Amorosi,2013).However,the high K2O content of glauconite in the Lameta Formation corresponds to the K-feldspar substrate,and is unrelated to significant stratigraphic condensation.

Fig.14 Cross-plot of Y vs.La concentrations(after Zhao et al.,2013).Diagonal lines show the weight ratio of Y/La in modern seawater(4.7;Nozaki,2001)and upper continental crust(0.73;McLennan,2001).The light blue-shaded field encompasses the field of limited seawater uptake of REE.Note that glauconites bear upper crustal ratio signatures.

Fig.15 Cross-plot of(Ce/Ce＊)vs.(Pr/Pr＊)(after Bau and Dulski,1996).Note that glauconites within the Lameta Formation plot in and around the field IIIb implying true negative Ce anomaly.Field I:neither Ce nor La anomaly; field IIa:positive La anomaly,no Ce anomaly; field IIb:negative La anomaly,no Ce anomaly; field IIIa:positive Ce anomaly; field IIIb:negative Ce anomaly.

The mineral chemistry of glauconite is highly sensitive to local micro-environmental conditions(El Albani et al.,2005).The K2O content of the fullyevolved glauconite varies from 5.51%to 8.29%,while the same varies from 7.04%to 7.90%within the incipient variety(Table 3).This anomaly may be explained by the decrease in the K+ion activity by freshwater input in local micro-environments within glauconite.El Albani et al.(2005)recorded similar decrease in K2O contents of glauconite related to freshwater input in a lagoonal environment.

Fig.16 Cross-plot of Ce anomaly vs.Nd concentrations.The anoxic—oxic boundary is placed at-0.10 following Wright et al.(1987).Note that the Ce anomaly values of glauconite plot near oxic—anoxic boundary indicating suboxic depositional condition while the Nd concentrations suggest moderate rate of sedimentation.

The MgO content of glauconite in the Lameta Formation is higher(av.4.18%)than those reported in other occurrences(see Banerjee et al.,2016b;Bansal et al.,2017).The high MgO content of the glauconites is possibly related to contemporaneous basaltic volcanism of Deccan Traps.The TFe2O3contents of these glauconites vary from 12.56%to 18.90%.The amount of TFe2O3content depends on the availability of Fe and redox conditions of the depositional environments(Banerjee et al.,2016a;El Albani et al.,2005).Seawater contains negligible Fe and therefore it is unlikely to contribute significant Fe to the glauconite.Fe and Mn significantly fractionates across the redox boundary.While Fe tends to be fixed under reducing conditions,Mn is incorporated more under oxidizing conditions(Bellanca et al.,1996).Mn redox potential(Mn＊)is calculated as log[(Mn sample/Mn shale)/(Fe sample/Fe shale)](cf.Bellanca et al.,1996;Machhour et al.,1994).The glauconites of the Lameta Formation exhibit negative Mn＊values(av.-0.85).The average negative Mn＊values support the formation of glauconite under suboxic conditions(Bellanca et al.,1996;Machhour et al.,1994).Cerium(Ce)is another potential indicator of palaeoredox condition.It fractionates under oxidizing conditions above the redox boundary.According to Wright et al.(1987),the negative values of Ce anomaly below-0.10 indicate oxic conditions.Ce anomaly values of glauconites in the Lameta Formation plot close to the oxic—anoxic boundary in a cross-plot between Ce anomaly and Nd concentration(Wright et al.,1987),indicating a suboxic depositional condition(Fig.16).The suboxic depositional condition restricted the mobility of Fe ions into glauconite structure and resulted in a moderate TFe2O3content(Banerjee et al.,2016a).

Fig.17 K2O vs.TFe2O3relationship of glauconites in the Lameta Formation(11)compared with the published Precambrian(1—10)and Phanerozoic(12—13)examples(the spread of data points are encircled):(1)Dasgupta et al.,1990;(2)Ivanovskaya et al.,2006;(3)Bandopadhyay,2007;(4)Mei et al.,2008;(5)Banerjee et al.,2008;(6)Sarkar et al.,2014;(7)Deb and Fukuoka,1998;(8)Guimaraes et al.,2000;(9)Banerjee et al.,2015;(10)Tang et al.,2017;(11)Lameta Formation(present study);(12—13)Fields representing most Phanerozoic glauconites.The compositional evolution of glauconites supporting “verdissement” (vertical dark-blue arrow),“layer lattice” (red inclined line)and “pseudomorphic replacement” (horizontal light-blue arrow)theories are marked(cf.Banerjee et al.,2016a).Also note that glauconites in the Lameta Formation follow the Precambrian evolution trend.Further note that these data overlap with the estuarine glauconites reported by Dasgupta et al.(1990).

6. Conclusions

The major conclusions of this study of glauconite are as follows:

1)Authigenic glauconite of the Lameta Formation formed in an estuarine depositional environment.

2)The glauconite in the Lameta Formation is characterized by high K2O,high Si2O,high MgO,high Al2O3and moderate Fe2O3.

3)The consistently-high K2O content and textural evidences indicate pseudomorphic replacement of K-feldspars by glauconite in the Lameta Formation.

4)The compositional evolution of glauconite in the Lameta Formation is comparable to those observed in many Precambrian varieties.

5)Ce anomaly values and Mn＊values indicate suboxic depositional conditions.The mobility of Fe ions is reduced during the glauconitization,resulting in moderate TFe2O3contents.

6)The “hat-shape” of the PASS-normalized-REE patterns of glauconite indicates the authigenic origin of glauconites.The REE content in glauconite derives from relicts of K-feldspar substrate.

Acknowledgements

Authors are indebted to their host institutes for infrastructure facilities.Santanu Banerjee is thankful to Ministry of Mines,Government of India for financial support through grant F No.14/77/2015-Met.IV.Authors thank S.C.Patel and Javed M.Shaikh for providing analytical support at the DSTIITB National facility for EPMA,Department of Earth Sciences,Indian Institute of Technology Bombay.Authors also thank S.S.Meena of Bhabha Atomic Research Centre for Mössbauer spectroscopic study of glauconite.