Compositional variability of glauconites within the ...

Sedimentary Geology 331 (2016) 12–29

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Compositional variability of glauconites within the Upper Cretaceous Karai Shale Formation, Cauvery Basin, India: Implications for evaluation of stratigraphic condensation Santanu Banerjee a,⁎, Udita Bansal a, Kanchan Pande a, S.S. Meena b a b

Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 4000765, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 9 August 2015 Received in revised form 21 October 2015 Accepted 23 October 2015 Available online 6 November 2015 Editor: Dr. B. Jones Keywords: Evolved and slightly evolved glauconite Condensed section Rim and core of glauconite pellet Mössbauer spectroscopy of glauconite Reverse glauconitization Vermiforms

a b s t r a c t A detailed mineral chemical investigation of glauconite within the condensed section deposits of the Cretaceous Karai Shale Formation, Cauvery Basin, India reflects a wide spectrum in chemical composition related to origin and evolution in different substrates, stratigraphic condensation, and post-depositional alteration. Fe- and Mgrich glauconite, comprising up to 60% of the sedimentary rocks, occurs as replaced forms of fecal pellets, as infillings within pores and chambers of bioclasts including those of foraminifera, ostracoda, bryozoa, and algae, and as altered forms of mica exhibiting vermiforms. Authigenic precipitation of K- and Fe-poor glauconite, followed by addition of Fe and K into the lattice and concomitant release of Al and Si explains the origin of glauconite pellets and infillings; the origin of glauconite vermiforms in partly degraded mica involves only the second stage of Fe and K addition. Glauconite pellets and vermiforms exhibit sharply defined alteration zones along peripheries to form rims, and in proximity to cracks or cleavages with reduced K2O and Fe2O3 (total) and enhanced Al2O3 and SiO2, related to late-stage meteoric water actions. Cores of glauconite pellets and unaltered zones of vermiforms reflect ‘evolved’ characteristics with N6% K2O, typical of a condensed section, while other glauconite varieties occurring at the same stratigraphic level exhibit ‘slightly evolved’ nature, not consonant with stratigraphic condensation. Increasing abundance of glauconite pellets from the bottom to the top of the transgressive systems tract, accompanied by slight increase in K2O within their cores, reflects the effect of stratigraphic condensation on the evolution of glauconite. High Fe2O3 (total) content of glauconite in the Karai Shale Formation may be related to upwelling, although the Fe may be contributed partly by the biotite substrate. Mössbauer spectroscopy of glauconites reveals significant total Fe substitution in both tetrahedral and octahedral sites. Detailed mineral chemical analysis enables us to distinguish stratigraphically significant glauconite within the Karai Shale Formation from the rest of the glauconite notwithstanding its wide compositional range. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Authigenic glauconite is a reliable indicator of low rate of sedimentation in marine sedimentary sequences and is characteristically associated with transgressive deposits (Odin and Matter, 1981; Amorosi, 1995, 1997; Harris and Whiting, 2000; Amorosi and Centineo, 2000; Wigley and Compton, 2007; Amorosi et al., 2012). Evolved glauconite with high K2O content (N 6%) has been reported within condensed sections (Amorosi, 1995, 1997; Huggett and Gale, 1997; Kelly and Webb, 1999; Harris and Whiting, 2000; Giresse and Wiewióra, 2001; Hesselbo and Huggett, 2001; Amorosi et al., 2012; Banerjee et al., 2012a). Abundance and K2O content of authigenic glauconite in combination distinguishes simple depositional breaks from significant stratigraphic condensations (Amorosi, 2012). Crystallo-chemical properties of glauconite, however, ⁎ Corresponding author. E-mail address: [email protected] (S. Banerjee).

http://dx.doi.org/10.1016/j.sedgeo.2015.10.012 0037-0738/© 2015 Elsevier B.V. All rights reserved.

vary significantly depending on physico-chemical properties of the substrate, freshwater influx in the depositional setting, redox conditions of pore water, background lithology and temporal variation in seawater composition, besides the rate of sedimentation (Courbe et al., 1981; Dasgupta et al., 1990; Chafetz and Reid, 2000; El Albani et al., 2005; Amorosi et al., 2007; Eder et al., 2007; Banerjee et al., 2008, 2012a,b). Post-depositional processes may alter glauconite composition as well (see Courbe et al., 1981; Dasgupta et al., 1990; Eder et al., 2007; Sánchez-Navas et al., 2008), and therefore, the sequence stratigraphic implication of glauconite composition depends on the retention ‘pristine’ signature. The origin and chemical evolution of glaucony involving a wide variety of substrates including fecal pellet, bioclasts, feldspar, quartz, and mica has been widely debated in the past (Burst, 1958a,b; Hower, 1961; Odin and Matter, 1981; Odom, 1984; Dasgupta et al., 1990; Clauer et al., 1992; Stille and Clauer, 1994; Meunier and El Albani, 2007). Furthermore, glauconite represents wide structural variability related to substitutions of Fe in tetrahedral and octahedral sites,

S. Banerjee et al. / Sedimentary Geology 331 (2016) 12–29

which is poorly understood. A better perspective regarding factors influencing chemical as well as structural variability of glauconite may be obtained by a detailed mineral chemical investigation highlighting its origin and evolution within a well-constrained stratigraphic framework. Unusually high sea level during the Cretaceous is recorded by the occurrence of widespread glauconite and phosphorite deposits in passive margin condensed section occurrences across the world (Jiménez-Millán et al., 1988; Delamette, 1989; Garzanti et al., 1989; Amorosi et al., 2012). Although the Cretaceous sediments are extensively studied in the Cauvery basin in southern India, occurrence of glauconite is overlooked and its stratigraphic implication is yet to be explored. This paper investigates the origin, evolution, compositional and structural variability of glauconite within the depositional and stratigraphic framework of the middle Cretaceous passive margin succession of the Karai Shale Formation in the Cauvery basin. The study attempts to evaluate the influence of substrate, mode of origin, stratigraphic condensation, and post-depositional alteration on glauconite composition. An attempt has been made to recognize stratigraphically significant glauconite within the Karai Shale Formation despite its wide compositional range. The foundation of the work lies in detailed mineral chemical investigation of glauconite samples by EPMA supplemented by XRD, X-ray mapping, X-ray microscopy, and Mössbauer spectroscopy. The paper also investigates the origin of zoned glauconite, which is poorly known. Mössbauer spectroscopy adds a new dimension to the understanding of glaucony in eliciting the crystallographic constraint on its intake of Fe. 2. Geological background The Cauvery basin encompasses a passive continental margin near the southeastern tip of India, formed as a result of the breakup of

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Gondwana during the Late Jurassic (Sundaram and Rao, 1979; Govindan et al., 1996, 2000) (Fig. 1). The basin has recorded up to 6000 m of sedimentary rocks and sediments ranging in age from preAlbian to Recent. The Middle to Upper Cretaceous Uttatur Group consists of the Dalmiapuram Formation, the Karai Shale Formation, and the Garudamangalam Sandstone Formation in ascending order and the transitions between them are gradational (Fig. 1; Tewari et al., 1996; Sundaram et al., 2001; Watkinson et al., 2007; Sarkar et al., 2014). The transition from the shallow shelf-originated Dalmiapuram Formation to the deeper shelf-originated Karai Shale Formation records a transgressive trend (Tewari et al., 1996; Govindan et al., 2000; Watkinson et al., 2007). Karai Shale Formation gradationally passes upward to the Garudamangalam Sandstone Formation in a regressive trend (Tewari et al., 1996). The present study focuses on the roughly ~330 m thick offshore Karai Shale Formation which is time equivalent of the 1000–1500 m thick offshore Bhuvanagiri–Sattapadi–Andimadam Formation, known for its organic-rich, hydrocarbon source rock of the oil- and gas-producing Cauvery basin (Govindan et al., 2000). The glauconitic Karai Shale Formation predominantly consists of shale and calc-arenite beds of variable thicknesses containing abundant phosphorite nodules. The shale is well studied by biostratigraphers because of its highly fossiliferous nature (Sastry et al., 1968; Kale and Phansalkar, 1992; Govindan et al., 1996; Tewari et al., 1996; Hart et al., 2000, 2001; Kale et al., 2000; Watkinson et al., 2007; Nagendra et al., 2011, 2013). Foraminifera and belemnites are abundant at the bottom, while ammonites are reported from the upper part (Hart et al., 2001; Sundaram et al., 2001; Das Gupta et al., 2007). The overlying Garudamangalam Formation represents a mixed carbonate-siliciclastic succession of shallow marine realm, deposition of which is bounded by an unconformity at its top (Figs. 1,2;

Fig. 1. Geological map showing Cretaceous outcrops of the onshore Cauvery basin with necessary stratigraphic elaborations to the right. Location of the study areas are indicated by the rectangles.

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Tewari et al., 1996; Ramkumar et al., 2004; Watkinson et al., 2007; Sarkar et al., 2014). 3. Samples and methods Detailed bed-by-bed analysis of exposures of the Karai Shale Formation was carried out to prepare a composite graphic log (Fig. 2) with precise sample positions, principally based on exposures east of the area around Karai and Maruvattur (Fig. 1). Petrographic investigations were carried out using a Leica DM 4500P polarizing microscope attached to a Leica DFC420 camera. Glauconite pellets were separated from rock samples for geochemical and mineralogical analysis following the procedures mentioned in Amorosi et al. (2007). The samples were sieved initially to prepare to prepare a 50–1000 μm fraction, which was soaked in water, treated with anhydrous Na2CO3 powder and H2O2 solution, and was kept on a hot plate for 15–20 minutes, followed by cooling, washing, and drying of the mixture. Glauconite concentrates

were obtained using a Franz magnetic separator (LB-1) with 15° slope and 15° lateral tilt and 1.5–1.8 A current. Glauconite pellets were finally hand-picked using a Zeiss Stemi 2000 stereo zoom microscope. The powdered samples were scanned from 4° to 30° with step size of 0.026° 2θ and with a scan speed of 96 sec/step, using nickel filter copper radiation in an Empyrean X-Ray Diffractometer with Pixel 3D detector at the Department of Earth Sciences, IIT Bombay. The samples were scanned each time after air-drying, treatment with ethylene glycol (100 °C for 1 hour), and finally, after heating at 490 °C for 2 hours under the same instrumental settings. Glauconite pellets were studied using a Carl Zeiss X Radia 520 Versa X-Ray microscope, at the Indian Institute of Technology Bombay. The mineral chemistry of the green pellets and substrates was investigated in 17 thin sections on 188 points using a Cameca SX 5 Electron Probe Micro Analyzer at Department of Earth Sciences, IIT Bombay, with accelerating voltage 15 kV, specimen current of 40 nA and beam diameter of 1 μm (peak: 10–20 sec and background counting: 5–10 sec). Minerals as well as synthetic phases were

Fig. 2. Composite graphic log showing three divisions of the Karai Shale Formation and sample positions in sequence stratigraphic context. The underlying Dalmipuram Formation and lower (light green) and middle segments (dark green) of the Karai Shale Formation comprises the TST. The MFS separate the TST from the overlying HST composed of the upper segment (black) of the Karai Shale Formation and the Garudamangalam Sandstone Formation. Glauconite concentration in sediments and K2O wt% of unaltered glauconite pellets are provided on curves to the right of the log. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


used as standards. Duplicate analysis of individual points shows analytical error of less than 1%. Mössbauer spectra of four powdered glauconite samples were measured at the Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, and cross-checked with wet chemical analysis (redox titration method using K2Cr2O7), at room temperature using a conventional constant acceleration spectrometer in transmission geometry. A γ-ray source of 57Co in Rh matrix at room temperature was used and an α-Fe absorber was used at room temperature to calibrate the Doppler velocity V and also as the standard for the isomer shift (IS). Fe2+ content of glauconite samples obtained by Mössbauer spectroscopic data was cross-checked with wet chemical analysis (redox titration method using K2Cr2O7). 4. Sedimentology of the studied section of the Karai Shale Formation The poor-quality exposures, their heavily weathered nature, and the extensive obliteration of internal structures in the study area prevent detailed facies analysis. The underlying shallow shelf-originated carbonate succession of the Dalmiapuram Formation gradationally passes over to the Karai Shale Formation as carbonate content decreases and finer siliciclastics increases upward. In the study area, the Karai Shale Formation is divided into three segments: lower, middle, and upper (Fig. 2). The lower segment of the Karai Shale Formation (av. thickness ~70 m) comprises glauconitic shales intercalated with sheet-like or tabular calc-arenite beds. Thickness of calc-arenite beds decreases upward from 80 cm near the base to 15 cm toward the top. The calc-arenite beds are characterized by sharp bases and gradational tops, with gutter casts at the base, planar to wavy lamination internally and wave ripples at the top (Fig. 3a,b). The beds exhibit normal grading at places. The shale contains abundant glauconite pellets, the concentration of which increases from ~30% at the bottom to ~ 45% at the top (Fig. 2). Calc-arenite beds also contain glauconites, mostly concentrated along certain laminae.

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Broken pieces of belemnites, bivalves, and worm tubes are moderately dispersed in the shale and calc-arenite. Phosphorite nodule may occur occasionally near the top part lower segment (Fig. 2). The middle segment of the Karai Shale Formation (ca. 65 m thick) consists of thick shales intercalated with thin calc-arenite beds. The thickness of calc-arenite beds varies from 4 to 11 cm, while shale bed thickness varies from 2 to 24 m. Glauconite content of the shale varies between 50% and 60%. The shale characteristically appears dark green on fresh surfaces, exhibiting crude planar lamination. Phosphorite nodules (av. diameter 5.2 cm) of fluorapatite composition occur frequently in the shales (Fig. 3c). The shale is highly fossiliferous, containing planktic foraminifera, intact belemnites, bivalves, ostracods, bryozoans, and algae. The upper segment of the Karai Shale Formation is composed of gray shales and calc-siltites alternating with thick calc-arenite beds. Bed thickness of calc-arenite increases from 10 cm at the bottom to 1.1 m at its top. The lower contacts of the beds are much sharper than their upper contacts and the internal structures of the calc-arenite beds are similar to those in the lower segment of the Karai Shale Formation. Thickness of shale decreases, and its color changes from dark green to light green from the bottom to the top of the upper segment. Glauconite content of the shale is very low (5–10%) and phosphorite nodules are completely absent in this segment. Megafossils including ammonites, belemnites, and bivalves occur in the lower part. 4.2. Interpretation of sedimentation sequence Upward transition from one segment to the other in the Karai Shale Formation remains gradational implying deposition in laterally adjacent environments. Sharp bases of calc-arenite beds with sole features and gradational tops suggest an allochthonous origin. Normal grading in the calc-arenite beds indicates deposition from steadily waning flows.

Fig. 3. Field photographs showing a) sharp-based calc-arenite beds overlying shale, b) close-up view of the Karai Shale Formation showing grading, c) phosphorite nodules occurring within the shale (marker pen length = 14 cm, coin diameter = 2.5 mm).

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The presence of wave ripples in the lower and upper segments of the Karai Shale Formation suggests deposition within the storm wave base. Allochthonous calc-arenite beds are mostly of storm origin. Resting gradationally over the Dalmiapuram Formation, the lower segment reflects a deepening upward relative sea level trend, indicated by the gradual decrease of calc-arenite bed thickness and increase in thickness of shale. The upper segment of the Karai Shale Formation records a shallowing upward trend indicated by the upward increase in the thickness of calc-arenite beds containing traction-generated structures, which gradationally passes upward to the Garudamangalam Formation of nearshore marine origin (Sarkar et al., 2014). Reduced thickness and frequency of calc-arenite beds, as well as paucity of wave ripples, suggest a relatively deeper marine origin of the middle segment of the Karai Shale Formation, close to the storm wave base. The middle segment also exhibits abundant diagenetic phosphorite nodules, maximum concentration of planktic and benthic foraminifera, as well as maximum concentration of autochthonous glauconite pellets. Its foraminiferal assemblage of middle to outer shelf origin corresponds to 100–150 m depth (cf. Tewari et al., 1996; Das Gupta et al., 2007; Nagendra et al., 2011). The middle segment, therefore, represents a condensed section deposit with a maximum flooding surface (MFS) at the top, which divides the Uttatur Group into a lower transgressive systems tract (TST) consisting of the Dalmiapuram Formation and the lower two segments of the Karai Shale Formation (Nagendra et al., 2011), and an upper highstand systems tract (HST) consisting of the upper segment of the Karai Shale Formation and the Garudamangalam Sandstone Formation. The unconformity at the top of the Garudamangalam Sandstone Formation has been interpreted in terms of a significant relative sea level fall (Raju et al., 1993; Tewari et al., 1996). Our interpretation is consistent with the overall stratigraphic framework of the shelforiginated Uttatur Group proposed by earlier workers (Tewari et al., 1996; Watkinson et al., 2007; Nagendra et al., 2011; Sarkar et al., 2014).

5. Occurrence and types of glauconite in the Karai Shale Formation Glauconite occurs in three principal modes within the Karai Shale Formation, as an altered form of fecal pellets, herein referred to as glauconite pellets, as infillings within pores, chambers, and borings in bioclasts, herein referred to as glauconite infillings and as highly foliated, replaced forms of mica with vermicular appearance, herein referred to as glauconite vermiforms (Fig. 4a–g). Proportions of pellets, infillings, and vermiforms remain more or less similar at around 2:2:1 in the lower and middle segments, while it is 1:1:1 in the upper segment of the Karai Shale Formation. Glauconite content of shale steadily increases from 30% at the base to 45% at the top of the lower segment of the Karai Shale Formation, which further increases up to 60% in the middle segment near the maximum flooding surface (Fig. 2). Glauconite content sharply decreases within the upper segment and it is altogether absent at the top part (Fig. 2). Glauconite pellet exhibits pleochroism from yellowish green to brownish green. They show third-order yellowish green to dark green interference colors and exhibit aggregate extinction, imparted by the packing of micro-platelets. The diameter of rounded pellet ranges from 200 to 450 μm. The long axis of the elliptical pellets ranges from 350 to 750 μm. The pellets have distinct physical boundaries with the surrounding calcareous shale matrix. Most glauconite pellets display cracks, which often originate at the periphery and taper toward the center (Fig. 4a). X-ray microcopy images reflect the complicated nature of the cracks, often penetrating the entire pellet (Fig. 5a,b). The cracks are often filled by carbonate cements in the calcarenite beds (Fig. 5a). Glauconite pellets commonly bear a 2–10 μm thick rim which appears lighter than the core in plane polarized light (Figs. 4a,6). EPMA backscattered images reflect sharp compositional contrast between cores and rims of glauconite pellet. Rim composition is frequently maintained inside the cores of pellets along zones of alteration in proximity to cracks (Fig. 6a–f). Evidently, the alteration

Fig. 4. Photomicrographs showing altered rim of a glauconite pellet (blue arrow) with crack (red arrow) at bottom and a glauconite vermiform at top (a), glauconite vermiform (b), glauconite infillings within foraminifera (c), ostracoda (d) (red arrow indicating outer calcareous test), bryozoa (e), and algae (f) (b is under crossed polars, the rest are under plane polarized light). Elemental mapping of the same glauconite pellet and vermiform shown in Fig. 4a is presented in Fig. 6. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 5. X-ray microscope image showing three-dimensional view of a pellet showing filled cracks marked by red arrow (a), X-Y section of pellet showing deeply penetrating nature of the crack marked by red arrow (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

postdates cracking which affected both the outer margin of pellets, forming the rims and the cores along an alteration zone close to the cracks. Glauconite infillings occur within the intra-particle pores or borings in bioclasts including chambers of foraminifera, carapace of ostracoda, zooecial aperture of bryozoan, and voids within algae (Figs. 4c–f). The long dimension of the bioclasts is comparable to pellets in size, ranging in size from 350 to 850 μm. The infillings within ostracoda resembles pellets in shape, but they bear a thin outer calcareous test (Fig. 4d). The average length of the infillings in ostracoda is 650 μm, whereas the average diameter of individual glauconite infilling in foraminiferal chamber is ~55 μm. The average diameter of individual algal and bryozoan infillings is even smaller, varying between 15 and 20 μm). X-ray mapping reveals formation of glauconite exclusively within the tiny pores and borings in bryozoan tests, while the calcitic tests remain completely unaltered (Fig. 7). Infillings within Ostracoda may often

exhibit cracks similar to those formed in pellets. Cracks are, however, absent in other glauconite infillings. Glauconite vermiform exhibits distinctive cleavages perpendicular to the long axis of the grains (Fig. 4b). The average length of vermicular glauconite grains is around 650 μm, while their width may be up to 150 μm. BSE images often exhibit sharp compositional changes across the glauconite vermiforms along cleavages (Fig. 8). Alteration is more prominent along the margins of vermiforms compared to the center, often forming wedge-shaped zones (Fig. 8). In rare examples, the vermiforms are mantled by a rim of altered zone similar to those in glauconite pellets. The distinction between allochothonous and autochthonous glauconite is critical for the stratigraphic interpretation of glauconite-bearing sequences (Amorosi, 1995). The authigenic nature of the glauconite pellets within the shale is indicated by the presence deeply penetrating fractures within them (cf. Deb and Fukuoka, 1998; Bandopadhyay,

Fig. 6. EPMA backscattered image showing altered rim marked by red arrows and alteration zone along fractures (green arrow); also note a vermiform to the bottom left (a). X-ray mappings showing elemental abundance of K(b), Fe (c), Al (d), Mg (e) and Si (f) within the same glauconite pellet and vermiform. Note higher Mg content in the vermiform compared to the pellet. Note also sharp change in concentrations of K, Fe and Al between the altered rim and the altered core close to arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. X-ray mappings showing abundances of Ca (a), K (b), and Fe (c) in bryozoa test containing glauconite infillings. Note occurrences of glauconite within the tiny pores of bioclasts (red arrow) and within boring (yellow arrow). Note that calcareous test is totally unaffected. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2007; Banerjee et al., 2012a). In contrast, broken glauconite pellets dominate calc-arenite beds, clearly indicating their reworking and transportation (Figs. 9a,b). Glauconite exhibits homogeneous distribution and poor sorting in shale corroborating its autochthonous nature, while the glauconite within the calc-arenite beds exhibits good sorting suggesting hydrodynamic actions (cf. Amorosi, 1997). Cracks have previously been interpreted as either expansion cracks related to displacive growth of glauconite within initially smaller substrate spaces (Odin and Matter, 1981; Odin and Morton, 1988), or as shrinkage cracks related to dewatering during the mineralogical evolution of glaucony (McRae, 1972; Odom, 1976). Deeply penetrating cracks characteristically evolved to highly evolved glauconites (Odin, 1988; Chamley, 1989; Jiménez-Millán et al., 1998; Bandopadhyay, 2007).

6. Mineralogy of glauconite pellets The air-dried samples exhibit peaks of glauconite at (001) basal reflection at 10.5 Å, (020) reflection at 4.5 Å and (003) reflection at 3.3 Å (Fig. 10). On glycolation, the (001) reflection is slightly shifted from 10.4 to 9.5 Å, while (020) and (003) reflections are unmoved. Initially, the peaks are broad with asymmetrical sides, but appear more symmetrical and sharp on glycolation (Fig. 10). The peak shows negligible shift on heating at 400 °C. Glycolation and heating of all samples reveal minor kaolinite at 7.2 Å. The observations made in the Karai Shale Formation are consistent with the observation of Odin and Matter (1981) and Odom (1984) that on randomly oriented samples, the first-order basal reflection

Fig. 8. EPMA backscatter image showing wedge-shaped alteration zones in a glauconite vermiform (a). X-ray mappings of the same showing elemental concentrations of K (b), Fe (c), Al (d), Mg (e), and Si (f) within the same glauconite vermiform (red arrows indicating alteration zones). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Fig. 9. Photomicrographs showing glauconite in shale (a) and calc-arenite bed (b). Note poor sorting and intact nature of pellets within the shale and good sorting within the calc-arenite.

(001) may be located anywhere between 14 and 10 Å. The peaks are broad-based with asymmetrical sides indicating lack of discrete (001) reflections. Although partial hydration of the sample can cause the broadening of the basal reflection (Hassan and Baioumy, 2006), the absence of 112 and 112 reflections and the slight shift of the 10 Å peak after glycolation suggest minor inter-stratification in the glauconite phase. The glauconite can be described as the ‘disordered’ type containing around 10–15% of expandable layers, corresponding to 6–7% K2O (Thompson and Hower, 1975; Odom, 1984). Amorosi et al. (2007) found full width at half maximum value (FWHM) of the basal (001) reflection to be an accurate estimate of glauconite maturity. The FWHM value of the present samples (1.42) corresponds to ~6.5% K2O content and warrants the description of the ‘evolved’ type. 7. Major element composition of glauconite in Karai Shale Formation EPMA data of all varieties of glauconites provided in Table 1 exhibit wide variation in major element composition. K2O content of glauconite pellets varies from 5.05% to 7.49%, while the same for glauconite infillings and vermiforms, it varies from 3.33% to 7.30% and 4.43% to 7.34%, respectively. The Fe2O3 (total) content of glauconite pellets varies from 20.72% to 30.75%, while the same for glauconite infillings and vermiform varies from 16.56% to 30.13% and 18.35% to 28.87%, respectively. The MgO content of glauconite pellets, infillings, and vermiforms varies from 2.46% to 3.02%, from 2.56% to 3.11%, and from 2.59% to 5.88%, respectively. CaO content of all the varieties of glauconites is negligible, mostly b1%. SiO2 content of glauconite pellets, infillings, and vermiform varies from 47.85% to 54.49%, from 47.85% to 55.25%, and from 46.73% to 53.58%, respectively. Al2O3 content varies from 4.47% to 13.20% in glauconite pellets, while the same varies from 4.51% to 14.03% and 5.16% to 13.68% in glauconite infillings and vermicular glauconite, respectively. Fe2O3 (total) and MgO of these glauconite pellets and vermiforms are slightly higher than the average values provided in

literature (Odin and Matter, 1981; Strickler and Ferrell, 1990; Chafetz and Reid, 2000; Amorosi et al., 2007; Baioumy and Boulis, 2012a, 2012b; Banerjee et al., 2012a, b). SiO2 and Al2O3 values of glauconites are within the given range (McRae, 1972; Odin and Matter, 1981; Amorosi et al., 2007). All analyses have been normalized to 100 wt% on an anhydrous basis for different cross plots. The significant results of various cross plots are as follows. a) A cross plot of K2O versus Fe2O3 (total) (Fig. 11) exhibits moderate correlation (r2 = 0.7). Different types of glauconite form groupings despite moderate overlapping between them (Fig. 11). Cores of glauconite pellets and unaltered portions of glauconite vermiforms exhibit the highest K2O and Fe2O3 (total) values. These two unaltered varieties of glauconite occupy the upper end of the correlation line. In contrast, infillings, altered rims of glauconite pellets, and altered zones in glauconite vermiforms are depleted in K2O and Fe2O3 (total); all these varieties occupy the lower end of the correlation line in Fig. 11. Infillings within pores and chambers in different bioclasts tend to form a grouping, but those formed within borings of bioclasts exhibit higher Fe2O3 (total). Odin and Matter (1981) divided glauconite into four types on the basis of K2O content, as nascent (b 4% K2O), slightly evolved (4–6% K2O), evolved (6–8% K2O), and highly evolved (8–10% K2O). Glauconites occurring at the same stratigraphic level in the Karai Shale Formation reveal both evolved and slightly evolved character depending on the nature of the substrate. Glauconite pellets and unaltered zones of vermiforms belong to the evolved type with K2O content exceeding 6%. Rims and altered portions of glauconite pellets and glauconite vermiforms, as well as infillings belong to the slightly evolved type with K2O content less than 6%. Co-occurrence of both varieties of glauconites at the same stratigraphic level of the Karai Shale Formation suggests that glauconite composition is

Fig. 10. XRD diffractograms of glauconite pellets under different conditions—air-dried (a), glycolated (b), and heated (c).

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Table 1 Oxide weight percentage of three varieties of glauconites within the Karai Shale Formation. Substrate

Sample no.

Na2O

MgO

Al2O3

K2O

CaO

MnO

Fe2O3

SiO2

P2O5

Total

Pellet (core)

KS/1 KS/3

0.13 0.29 0.22 0.15 0.24 0.27 0.26 0.27 0.18 0.05 0.06 0.06 0.05 0.10 0.08 0.05 0.13 0.05 0.04 0.07 0.10 0.08 0.04 0.08 0.06 0.02 0.08 0.40 0.37 0.31 0.35 0.34 0.42 0.11 0.12 0.14 0.12 0.15 0.16 0.13 0.17 0.16 0.16 0.11 0.18 0.16 0.76 0.06 0.07 0.08 0.06 0.24 0.21 0.30 0.00 0.02 0.00 0.03 0.04 0.06 0.12 0.07 0.08 0.42 0.17 0.34 0.04 0.02 0.04 0.08 0.08 0.01 0.02 0.02 0.05 0.37

2.56 2.76 2.73 2.69 2.64 2.70 2.66 2.63 2.67 2.81 2.65 2.70 2.71 2.85 2.77 2.73 2.82 2.61 2.62 2.73 2.71 2.72 2.72 2.70 2.86 2.82 2.91 2.61 2.71 2.80 2.88 2.82 2.72 2.64 2.72 2.66 2.54 2.62 2.55 2.52 2.65 2.71 2.65 2.69 2.58 2.63 2.74 2.61 2.76 2.92 2.77 2.94 2.76 2.87 2.87 2.88 3.00 2.93 2.61 2.58 2.46 2.69 2.78 2.62 2.86 2.60 2.61 2.70 2.76 2.79 2.72 2.89 2.90 3.02 2.88 2.69

7.38 9.40 7.56 8.06 7.41 7.47 9.84 8.11 6.14 6.40 9.09 9.46 9.61 9.24 8.53 8.67 9.88 9.13 6.99 8.65 7.96 7.85 7.27 8.27 7.55 6.34 7.72 9.34 11.90 10.28 9.59 10.87 10.83 4.47 4.74 4.51 6.51 6.01 7.97 6.65 7.41 6.54 6.81 5.11 9.32 8.66 10.71 8.17 7.88 9.16 7.62 9.25 7.70 8.82 7.93 9.53 9.71 9.48 8.20 7.07 13.20 8.82 8.41 11.30 8.87 8.53 9.87 8.85 8.35 9.68 11.70 7.66 10.70 12.37 10.82 10.19

7.11 6.07 6.72 6.17 6.60 6.42 5.68 6.42 7.10 6.85 6.11 5.64 5.67 6.03 6.28 6.31 6.02 5.83 6.87 6.44 6.65 6.35 6.52 6.35 6.91 6.66 6.38 5.71 5.28 5.31 5.43 5.11 5.22 7.49 7.12 7.25 6.91 7.05 5.37 6.50 6.78 6.98 6.80 7.30 5.83 5.85 5.17 6.25 7.14 6.91 7.15 5.67 6.47 5.40 5.69 5.52 5.51 4.71 6.87 6.75 5.58 6.40 6.18 5.05 6.46 5.90 6.54 5.26 6.60 7.29 6.25 7.30 6.08 6.01 5.89 5.35

0.35 0.46 0.35 0.43 0.47 0.43 0.67 0.53 0.32 0.51 0.36 0.41 0.48 0.45 0.34 0.39 0.43 0.33 0.25 0.29 0.32 0.34 0.91 1.06 0.92 0.80 0.76 0.65 0.67 0.74 0.72 0.72 0.69 0.19 0.23 0.24 0.32 0.24 0.26 0.23 0.30 0.24 0.29 0.16 0.28 0.26 0.74 0.41 0.42 0.38 0.36 0.45 0.22 0.61 0.53 0.70 0.98 0.83 0.29 0.77 0.70 0.57 0.97 0.45 0.42 0.26 0.52 0.60 0.34 0.48 0.75 0.56 0.95 0.78 0.97 0.40

0.00 0.00 0.03 0.00 0.03 0.05 0.00 0.00 0.00 0.01 0.00 0.03 0.00 0.01 0.01 0.03 0.01 0.00 0.01 0.02 0.00 0.00 0.00 0.03 0.03 0.00 0.02 0.00 0.00 0.00 0.04 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.06 0.02 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.02 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.02

25.84 24.12 26.45 25.35 26.45 26.87 23.84 25.30 28.50 27.40 25.83 24.75 25.14 24.06 25.86 24.70 23.64 25.77 27.07 25.46 25.85 25.77 27.22 25.09 26.27 27.39 25.27 23.97 21.17 22.22 23.22 22.31 22.48 30.42 30.43 30.75 28.52 28.30 23.88 28.00 27.11 27.97 27.72 29.82 24.46 24.54 22.61 27.67 26.07 24.56 26.69 25.69 28.69 25.61 26.19 24.55 24.02 22.61 26.41 26.66 20.81 24.79 24.83 21.20 27.10 26.61 25.68 25.37 26.91 24.07 23.03 26.92 24.28 20.72 23.75 23.34

51.40 52.98 52.55 51.98 52.26 52.29 52.79 51.87 51.26 52.18 52.11 52.57 52.70 53.84 52.77 52.60 53.24 52.57 51.33 52.98 52.36 52.31 51.86 52.55 51.71 51.92 52.97 52.23 53.49 53.27 54.39 54.49 53.85 50.87 51.25 50.79 51.16 51.35 50.29 51.57 52.12 52.23 52.02 51.24 52.64 52.10 53.61 51.35 51.33 52.42 51.72 52.56 49.92 51.41 51.72 53.10 53.50 51.08 52.02 51.32 52.69 51.53 52.88 50.02 50.13 47.85 50.67 50.87 50.61 50.43 51.39 50.68 51.01 52.56 51.64 53.44

0.04 0.09 0.05 0.02 0.05 0.04 0.03 0.04 0.04 0.09 0.04 0.07 0.03 0.05 0.05 0.03 0.11 0.05 0.07 0.05 0.03 0.07 0.03 0.05 0.01 0.08 0.03 0.03 0.08 0.07 0.04 0.06 0.05 0.06 0.04 0.04 0.09 0.04 0.06 0.04 0.08 0.06 0.02 0.05 0.09 0.05 0.08 0.04 0.02 0.08 0.04 0.08 0.04 0.07 0.04 0.05 0.02 0.01 0.04 0.06 0.06 0.06 0.07 0.02 0.05 0.05 0.03 0.05 0.07 0.07 0.08 0.02 0.02 0.06 0.02 0.04

94.87 96.16 96.66 94.83 96.14 96.53 95.76 95.18 96.21 96.42 96.23 95.68 96.39 96.70 96.88 95.63 96.34 96.34 95.33 96.74 96.04 95.59 96.68 96.34 96.43 96.13 96.23 94.93 95.67 95.00 96.67 96.73 96.25 96.27 96.66 96.38 96.17 95.77 90.54 95.64 96.66 96.91 96.47 96.48 95.38 94.24 96.81 96.55 95.71 96.59 96.57 96.93 96.08 95.14 95.13 96.51 96.85 91.84 96.58 95.35 95.71 95.01 96.24 91.07 96.05 92.17 95.95 93.70 95.69 94.98 96.16 96.10 96.09 95.62 96.15 95.92

KS/5 KS/6

KS/10

KS/11

KS/16

KS/23 KS/24

KS/27

KS/29 KS/30 KS/31

KS/33

KS/34

KS/35


21

Table 1 (continued) Substrate

Sample no.

Pellet (core)

Pellet (rim)

KS/1 KS/5 KS/6 KS/10

KS/16 KS/24 KS/27 KS/30

KS/33

Infilling in Foraminifera

KS/35 KS/5 KS/10

Infilling in Ostracoda

KS/29

KS/30

Infilling within algae

KS/1 KS/3

Infilling in bryozoa

KS/23 KS/10

Infilling within borings of algae

KS/10

Infilling within borings of bryozoa

KS/16

Vermiform (unaltered)

KS/1 KS/3

KS/5

KS/6 KS/10 KS/11 KS/16 KS/32

Na2O

MgO

Al2O3

K2O

CaO

MnO

Fe2O3

SiO2

P2O5

Total

0.29 0.36 0.55 0.57 0.26 0.30 0.23 0.31 0.10 0.07 0.07 0.07 0.08 0.03 0.07 0.14 0.20 0.23 0.06 0.06 0.02 0.02 0.01 0.03 0.04 0.28 0.39 0.43 0.13 0.25 0.13 0.03 0.00 0.05 0.03 0.02 0.03 0.02 0.03 0.01 0.02 0.01 0.00 0.31 0.11 0.10 0.12 0.11 0.14 0.19 0.08 0.09 0.03 0.01 0.06 0.04 0.34 0.39 0.21 0.14 0.11 0.17 0.36 0.35 0.17 0.13 0.05 0.10 0.14 0.42 0.04 0.06 0.16 0.21 0.15

2.57 2.65 2.53 2.70 2.86 2.78 2.67 2.76 2.83 2.47 2.54 2.57 2.79 2.85 2.97 2.39 2.45 2.71 2.67 2.57 2.78 3.17 2.90 2.93 3.04 2.62 2.83 2.92 3.25 2.83 2.52 3.01 2.90 2.81 3.03 3.11 2.92 3.04 3.00 2.88 2.79 2.89 2.90 2.66 2.96 2.96 2.88 3.00 2.84 2.65 2.77 2.87 2.89 3.00 3.05 3.07 2.59 2.83 2.58 3.00 2.78 2.91 4.04 5.88 3.22 3.09 2.79 3.28 2.86 3.77 2.89 2.89 5.30 3.55 2.76

10.28 9.07 10.12 9.86 9.95 10.20 9.18 13.56 12.59 9.66 10.39 11.29 10.99 9.77 10.40 11.23 10.45 9.00 9.90 9.71 9.92 13.82 13.18 11.46 11.56 11.92 12.57 11.65 11.51 14.21 9.89 11.23 10.94 10.63 11.87 10.20 11.78 10.34 11.97 12.05 11.74 11.91 11.76 14.03 13.67 12.48 13.36 12.50 11.63 12.07 12.37 11.69 9.60 10.20 10.18 11.07 10.90 11.95 10.42 6.84 10.29 6.19 9.86 9.60 8.36 5.16 5.32 8.89 9.73 9.15 7.64 7.70 9.00 7.39 7.85

6.14 6.41 6.08 6.07 5.62 5.77 6.21 4.05 3.79 5.15 4.88 4.50 5.09 5.05 5.06 5.55 5.50 5.56 5.36 5.87 4.16 3.45 3.87 4.23 4.55 4.89 4.03 4.52 4.28 3.83 4.96 5.04 4.80 5.19 4.11 4.69 4.24 4.55 4.36 4.23 4.12 3.95 4.20 3.33 3.36 3.76 4.08 4.01 3.80 4.26 3.87 4.58 6.12 5.73 5.94 5.76 5.60 4.92 5.58 6.58 5.82 6.72 5.86 5.89 6.04 6.97 7.34 6.19 6.58 5.69 6.20 6.19 6.04 5.76 7.04

0.34 0.30 0.28 0.27 0.60 0.57 0.44 0.93 0.71 0.70 0.35 0.41 1.20 0.88 0.97 0.82 0.49 0.33 0.47 0.55 1.41 0.85 1.35 0.92 0.91 0.57 1.03 0.81 0.42 0.33 0.86 1.36 0.92 1.21 1.04 0.97 1.00 0.97 1.02 1.17 1.21 1.20 1.14 1.58 0.83 0.85 0.91 0.96 1.44 2.45 1.58 1.29 1.02 0.85 1.00 1.31 0.60 0.73 0.56 0.57 0.36 0.36 0.53 0.52 0.56 0.60 0.35 0.40 0.42 0.73 0.84 0.82 0.85 0.46 0.51

0.00 0.01 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.05 0.02 0.00 0.00 0.01 0.04 0.03 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.04 0.00 0.00 0.01 0.00 0.04 0.04 0.06 0.02 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.05 0.00 0.06 0.01 0.02 0.02 0.03 0.07 0.04 0.03 0.05 0.00

23.12 24.47 22.94 22.71 23.41 23.39 24.08 18.34 19.98 24.74 23.42 22.19 22.53 22.20 22.39 20.33 22.97 24.26 24.95 26.09 20.79 17.93 18.81 21.12 21.86 20.61 19.34 21.08 18.77 16.55 23.07 20.71 20.27 21.52 20.62 22.26 20.86 22.27 20.87 20.15 20.83 20.68 20.87 17.59 16.56 18.95 18.10 17.69 19.44 18.17 16.94 19.17 24.00 22.40 23.29 21.93 19.13 16.82 21.59 26.54 22.99 27.70 22.62 20.60 24.99 28.13 28.87 23.54 25.41 23.30 25.48 24.76 22.04 24.93 28.09

51.62 52.56 52.77 53.07 53.93 53.73 52.15 54.72 53.71 49.65 52.16 52.22 52.56 52.69 53.88 49.43 49.82 51.78 51.58 51.00 49.61 56.78 53.73 53.64 54.79 50.80 54.63 55.52 52.55 54.51 49.56 53.80 52.99 52.08 54.94 54.41 53.60 53.93 53.83 53.13 52.76 53.41 54.43 52.07 54.94 55.25 50.34 50.80 52.64 47.37 48.30 49.54 51.43 51.66 53.02 52.08 49.35 50.18 49.30 52.62 48.17 52.76 49.76 48.25 52.11 52.40 50.61 51.45 51.61 52.07 51.14 51.94 51.08 49.51 50.41

0.04 0.04 0.05 0.05 0.09 0.05 0.04 0.06 0.04 0.04 0.05 0.05 0.08 0.00 0.10 0.03 0.03 0.02 0.06 0.07 0.06 0.04 0.04 0.05 0.04 0.03 0.04 0.03 0.03 0.02 0.04 0.04 0.05 0.06 0.00 0.07 0.05 0.03 0.02 0.00 0.00 0.01 0.02 0.01 0.01 0.03 0.02 0.08 0.03 0.01 0.06 0.04 0.07 0.08 0.10 0.05 0.06 0.07 0.06 0.08 0.03 0.08 0.07 0.08 0.06 0.06 0.08 0.08 0.02 0.04 0.06 0.05 0.29 0.08 0.01

94.46 95.94 95.46 95.41 96.81 96.87 95.08 94.88 93.74 92.48 93.87 93.29 95.49 93.76 96.02 90.02 91.97 93.90 95.03 95.91 88.92 96.21 94.07 94.56 96.97 91.71 94.86 96.96 90.97 92.62 91.04 95.40 93.04 93.73 95.81 95.87 94.62 95.29 95.25 93.79 93.61 94.16 95.44 91.70 92.48 94.42 89.86 89.16 92.28 87.46 86.22 89.44 95.25 94.08 96.73 95.50 88.56 87.88 90.29 96.43 90.60 96.94 93.08 91.20 95.49 96.73 95.42 94.12 96.88 95.44 94.44 94.53 94.78 91.94 96.82

(continued on next page)

22

S. Banerjee et al. / Sedimentary Geology 331 (2016) 12–29 Table 1 (continued) Substrate

Sample no.

Vermiform (unaltered)

KS/33

Vermiform (altered)

KS/35 KS/1 KS/5

KS6

KS/11

KS/16 KS/24

KS/30 KS/33 KS/35

Na2O

MgO

Al2O3

K2O

CaO

MnO

Fe2O3

SiO2

P2O5

Total

0.07 0.05 0.03 0.32 0.31 0.27 0.45 0.33 0.17 0.11 0.14 0.08 0.07 0.08 0.09 0.06 0.08 0.07 0.08 0.09 0.06 0.08 0.15 0.09 0.09 0.46 0.10 0.09 0.03 0.04 0.29 0.45 0.30 0.34 0.35 0.07 0.08 0.05 0.02 0.37 0.10 0.14

2.59 2.73 2.80 3.14 2.90 3.09 3.07 5.66 4.85 2.78 3.81 2.70 2.68 2.60 2.78 2.64 2.57 2.68 2.60 2.78 2.64 2.57 4.41 4.16 2.85 3.18 4.12 2.76 2.85 2.83 5.06 3.66 2.89 4.79 4.59 5.21 5.69 4.21 2.64 2.78 4.49 3.81

9.29 8.47 7.99 9.01 9.17 10.12 9.32 11.52 11.54 10.29 12.34 10.55 10.22 10.36 10.86 10.39 11.05 10.22 10.36 10.86 10.39 11.05 10.24 9.55 9.53 9.79 9.45 10.34 9.75 10.21 10.41 10.40 9.98 8.36 8.39 10.61 10.88 11.11 13.68 13.28 12.34 12.34

6.99 7.28 6.89 5.84 5.74 5.76 6.11 5.68 5.62 5.82 5.38 4.39 5.06 4.86 4.50 4.51 4.03 5.06 4.86 4.50 4.51 4.03 4.79 4.95 5.39 5.43 5.37 5.18 5.28 5.10 4.76 5.56 5.49 4.88 5.10 4.70 4.43 3.94 4.86 3.85 5.15 5.38

0.56 0.46 0.43 0.37 0.40 0.27 0.80 0.53 0.49 0.36 0.38 1.06 0.91 0.95 0.68 0.85 0.66 0.91 0.95 0.68 0.85 0.66 0.78 0.42 0.88 0.84 0.70 0.57 0.71 0.67 0.70 0.30 0.36 0.38 0.43 0.84 0.75 0.86 0.66 0.45 0.59 0.38

0.00 0.02 0.00 0.00 0.00 0.00 0.01 0.02 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.01 0.06 0.04 0.00 0.01 0.03 0.02 0.00 0.06 0.03 0.00

26.08 27.07 28.58 24.87 24.33 24.30 25.56 20.95 19.78 22.99 20.39 21.95 23.33 21.31 20.68 21.14 18.33 23.33 21.31 20.68 21.14 18.33 22.69 19.40 23.15 23.29 23.71 23.84 25.11 24.08 19.14 21.46 26.41 22.79 22.82 21.79 19.74 20.56 20.59 16.89 18.35 20.39

50.48 50.48 49.12 49.04 46.73 50.83 50.91 47.91 48.61 48.17 47.85 50.63 52.40 50.48 51.89 50.62 49.63 52.40 50.48 51.89 50.62 49.63 52.14 48.29 48.68 53.51 52.77 50.95 52.83 51.80 51.04 50.99 48.52 49.45 49.50 53.58 51.66 52.37 52.70 50.72 47.70 47.85

0.06 0.08 0.07 0.05 0.04 0.04 0.00 0.03 0.05 0.03 0.05 0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.05 0.03 0.09 0.03 0.05 0.05 0.07 0.06 0.02 0.01 0.08 0.03 0.04 0.06 0.03 0.02 0.05 0.06 0.03 0.05

96.12 96.63 95.92 92.64 89.62 94.69 96.21 92.62 91.10 90.60 90.38 91.41 94.71 90.67 91.51 90.25 86.40 94.71 90.67 91.51 90.25 86.40 95.43 87.05 90.66 96.66 96.36 93.78 96.64 94.79 91.40 92.83 94.09 91.07 91.23 96.87 93.28 93.17 95.28 88.45 88.77 90.38

influenced by substrate. The moderate correlation between K2O and Fe2O3 (total) in the Karai Shale Formation glauconites is consistent with observation made elsewhere (Hower, 1961; Bornhold and Giresse, 1985; Eder et al., 2007; Sánchez-Navas et al., 2008; Banerjee et al., 2012b). However, lack of correlation in some cases is attributed to different modes of origin of glauconites (Odin and Matter, 1981; Dasgupta et al., 1990; Deb and Fukuoka, 1998; Jarrar et al., 2000; Meunier and El Albani, 2007; Banerjee et al., 2008). b) The transition from altered to unaltered portions in glauconite pellets and vermiforms is marked by a sharp decrease of av. ~ 2% K2O content and av. ~4% decrease in Fe2O3 (total) content in the latter (Fig. 12). Rims and altered zones in glauconite pellets and vermiforms contain higher Al2O3 and SiO2 compared to the cores. The variation in composition between cores and rims will be explored later. c) An Al2O3 versus Fe2O3 (total) cross plot (Fig. 13a) exhibits good negative correlation (r2 = 0.9) indicating substitution of Al3 + ions by Fe3 + ions during the course of glauconitization as suggested in a number of published investigations (Odin and Matter, 1981; Bornhold and Giresse, 1985; Velde, 1985; Dasgupta et al., 1990; Amorosi et al., 2007; Eder et al., 2007; Banerjee et al., 2008, 2012a,b; Chang et al., 2008; Sánchez-Navas et al., 2008). Different types of glauconite form grouping, although with moderate overlapping. The cores of glauconite pellets and unaltered portions of vermiforms plot around the lower end of the correlation line in Fig. 13a with high Fe2O3 (total) and low Al2O3. The rims and altered portions of glauconite pellets and vermiforms and glauconite infillings plot near

d)

e)

f)

g)

h)

i)

the middle and the upper end of the correlation line with low Fe2O3 (total) and high Al2O3. A K2O versus Al2O3 cross plot (Fig. 13b) displays moderate negative correlation (r2 = 0.6) suggesting simultaneous addition of K in interlayer sites and removal of Al from octahedral sites (Fig. 13b). The evolved glauconites tend to separate from the slightly evolved glauconites by having higher K2O and lower Al2O3. A K2O versus CaO cross plot exhibits poor correlation (Fig. 13c). CaO content is less within the cores of glauconite pellets and the unaltered glauconite vermiforms compared to other glauconite varieties. Poor correlation (r2 = 0.4) between CaO and K2O indicate that a low concentration of CaO content of glauconite is independent of its stage of evolution. A K2O versus SiO2 cross plot (Fig. 13d) exhibits moderate negative correlation (r2 = 0.6) implying decrease in Si content in the glauconite structure as K increases (Fig. 13d). The relationship between K2O and MgO exhibits poor correlation (r2 = 0.3) (Fig. 13e). Mg concentration is very high in glauconite vermiforms with less K2O content, which is inherited from the biotite substrate. Fe substitutes for Al in both octahedral as well as tetrahedral sites (Table 2). However, there is no correlation between K and tetrahedral charge (Fig. 13f), suggesting that K is independent of Al3+ or Fe3+ substitution in tetrahedral sites. Various cross plots indicate that during the course of evolution, K and Fe were added while Al and Si were removed from the glauconite


23

Fig. 11. The relationship between K2O and Fe2O3 (total) in Karai Shale Formation glauconite showing moderate correlation. Fields of illitic minerals and glauconitic minerals are after Odin and Matter (1981). The dashed line separates slightly evolved from evolved glauconite in the Karai Shale Formation. Note grouping of data for each variety, although with moderate overlap. Evolutionary trends in case of layer lattice theory and verdissement theory are presented, by red and blue arrows, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

structure. Ca and Mg content are independent of stage of glauconite evolution. Al–Fe substitution in the octahedral site was completed before fixation of K in the interlayer sites. 8. Mössbauer spectroscopic study and structural formula of glauconite Substitution of trivalent cations in the tetrahedral sheet and divalent cations in the octahedral sheet is common in glauconite. ‘Ordered glauconite’ exhibits a single doublet with broadened components (Annersten, 1975; Kotlicki et al., 1981; Daynyak and Drits, 1987). The Karai Shale Formation glauconite is of highly heterogeneous composition, commonly exhibiting multiple doublets. Mössbauer spectra of five glauconite samples presented in Fig. 14 indicate four doublets. Doublets A, B, and C with isomer shift (δ) value δ = 0.25–0.48 mm/s corresponds to ferric ions and doublet D (δ = 1.11 mm/s) indicates ferrous ions (Table 3). The doublet with the smaller quadrupole splitting (ΔEQ) is assigned to the less distorted cis M(2) position whereas the doublet with the larger quadrupole splitting is assigned to Fe3+ ions in the trans M(1) position (Hogg and Meads, 1970; Rolf et al., 1977; McConchie et al., 1979; Kotlicki et al., 1981). The ΔEQ has a lower

value for doublet A (ΔEQ = 0.46–0.49 mm/s) than for doublet B (1.18–1.25 mm/s). Therefore, doublet A is assigned to ferric ions to cis M(2) position and doublet B is assigned to ferric ions to trans M(1) position (Table 3). The largest intensity doublet C having lowest isomer shift value, δ = 0.25–0.28 mm/s and ΔEQ = 0.47–0.48 mm/s is assigned to Fe3+ in tetrahedral coordination (Taylor et al., 1968; Goodman, 1976; Cardile and Brown, 1988; Gates et al., 2002). Doublet D belongs to ferrous ions and has a high ΔEQ value equal to 2.684 mm/s, corresponding to trans M(1) position (octahedral site). In the present study, only one sample (KS/6) indicates the doublet for ferrous ion (Fig. 14). The Fe2+/Fe3+ ratio of glauconite was calculated by taking into consideration the χ2 value (fitting parameter) and the relative area (under curve) of the component belonging to the cationic composition of the octahedral sheets. The average Fe2+/Fe3+ ratio (0.02) obtained by Mössbauer spectroscopic study of glauconite was cross-checked with those obtained by the titration method and the same value was used for calculation of octahedral and tetrahedral charge as well as the structural formula of all glaucony. The isomer shift and quadrupole splitting values are comparable to other published data in Harding et al. (2014) and references therein. Mössbauer study confirms the exact allocation of total Fe in tetrahedral and octahedral sites (Tables 2, 3).

Fig. 12. Comparison of K2O content between cores (solid symbols) and rims (open symbols) of glauconite pellets in 8 samples marked by different colors (a). Comparison of Fe2O3 (total) content between cores (solid symbols) and rims (open symbols) of glauconite pellets in 8 samples (b). Note that rim is always depleted in K2O and Fe2O3 (total) compared to cores. Analysis at multiple points of a core reveals slight variation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

24


Fig. 13. Cross plots between Al2O3 and Fe2O3 (total) (a), K2O versus Al2O3 (b), K2O vs. CaO (c), K2O vs. SiO2 (d), K2O vs. MgO (e), K+ vs. tetrahedral charge (f).

Bailey (1980) defined glauconite as a 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. All glauconite data of the Karai Shale Formation have av. 0.34 atoms (av. 0.18 Al3 + and 0.16 Fe3 +) per formula unit in the tetrahedral site. The octahedral R3+, occupied by Fe3+ (octahedral) and Al3+, varies from 1.59 to 1.76 (av. 1.69) atoms per unit formula (Table 3). The tetrahedral charge of glauconite varies from 0.28 to 0.43, while the octahedral charge ranges between 0.15 and 0.26. These glauconites belong to the 1 M or 1 Md modifications (Kossovskaya and Drits, 1970). The Fe 2 +/Fe3 +

ratio (0.02) was used for all glauconite samples and stoichiometric formulas were calculated on the basis of 10 atoms of oxygen and two hydroxyl groups (22 layer charges). The structural composition of both varieties of glauconites and their octahedral and tetrahedral charges are presented in Table 2. The average formula of the Karai Shale Formation glauconite is ðK0:53 Na0:02 Ca0:05 Þ0:60 Fe3þ 1:12 Fe2þ 0:01 Mg0:29 Al0:57 Si3:66 Al0:18 Fe3þ 0:16 4 O10 ðOHÞ2 : n H2 O:

1:99


25

Table 2 Structural composition of glauconites in Karai Shale along with octahedral and tetrahedral charge. Structural formulae

Tetra. charge

Oct. charge

Al3+ (tetra.)

Fe 3+ (tetra.)

Total Oct. R3+

(K 0.62 Na 0.01 Ca 0.04)0.66 (Fe3+1.31 Fe2+0.02 Mg 0.30 Al 0.35)1.97 (Si 3.69 Al 0.18 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.54 Na 0.01 Ca 0.03)0.58 (Fe3+1.21 Fe2+0.01 Mg 0.28 Al 0.52)2.02 (Si 3.64 Al 0.23 Fe3+0.14)4 O10 (OH)2 . n H2O (K 0.50 Na 0.01 Ca 0.03)0.54 (Fe3+1.16 Fe2+0.01 Mg 0.28 Al 0.57)2.02 (Si 3.66 Al 0.21 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.50 Na 0.01 Ca 0.04)0.54 (Fe3+1.16 Fe2+0.01 Mg 0.28 Al 0.56)2.02 (Si 3.65 Al 0.22 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.53 Na 0.01 Ca 0.03)0.58 (Fe3+1.12 Fe2+0.01 Mg 0.29 Al 0.57)2.00 (Si 3.71 Al 0.18 Fe3+0.11)4 O10 (OH)2 . n H2O (K 0.56 Na 0.01 Ca 0.03)0.59 (Fe3+1.21 Fe2+0.01 Mg 0.29 Al 0.49)2.01 (Si 3.67 Al 0.21 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.56 Na 0.01 Ca 0.03)0.60 (Fe3+1.17 Fe2+0.01 Mg 0.28 Al 0.52)1.99 (Si 3.69 Al 0.19 Fe3+0.12)4 O10 (OH)2 . n H2O (K 0.53 Na 0.02 Ca 0.03)0.58 (Fe3+1.10 Fe2+0.01 Mg 0.29 Al 0.60)2.00 (Si 3.68 Al 0.20 Fe3+0.12)4 O10 (OH)2 . n H2O (K 0.52 Na 0.01 Ca 0.02)0.55 (Fe3+1.20 Fe2+0.01 Mg 0.27 Al 0.53)2.02 (Si 3.65 Al 0.22 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.63 Na 0.01 Ca 0.02)0.65 (Fe3+1.31 Fe2+0.02 Mg 0.28 Al 0.38)1.99 (Si 3.66 Al 0.21 Fe3+0.13)4 O10 (OH)2 . n H2O (K 0.57 Na 0.01 Ca 0.02)0.60 (Fe3+1.20 Fe2+0.01 Mg 0.28 Al 0.51)2.00 (Si 3.68 Al 0.20 Fe3+0.12)4 O10 (OH)2 . n H2O (K 0.60 Na 0.01 Ca 0.02)0.63 (Fe3+1.23 Fe2+0.02 Mg 0.28 Al 0.46)1.99 (Si 3.68 Al 0.20 Fe3+0.12)4 O10 (OH)2 . n H2O (K 0.57 Na 0.01 Ca 0.03)0.61 (Fe3+1.24 Fe2+0.02 Mg 0.29 Al 0.46)1.99 (Si 3.69 Al 0.20 Fe3+0.12)4 O10 (OH)2 . n H2O (K 0.58 Na 0.00 Ca 0.07)0.66 (Fe3+1.24 Fe2+0.02 Mg 0.28 Al 0.43)1.98 (Si 3.65 Al 0.17 Fe3+0.18)4 O10 (OH)2 . n H2O (K 0.57 Na 0.01 Ca 0.08)0.66 (Fe3+1.14 Fe2+0.01 Mg 0.28 Al 0.52)1.96 (Si 3.68 Al 0.16 Fe3+0.17)4 O10 (OH)2 . n H2O (K 0.62 Na 0.01 Ca 0.07)0.70 (Fe3+1.20 Fe2+0.02 Mg 0.30 Al 0.46)1.97 (Si 3.64 Al 0.17 Fe3+0.18)4 O10 (OH)2 . n H2O (K 0.69 Na 0.02 Ca 0.01)0.72 (Fe3+1.43 Fe2+0.02 Mg 0.28 Al 0.24)1.97 (Si 3.66 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.65 Na 0.02 Ca 0.02)0.68 (Fe3+1.42 Fe2+0.02 Mg 0.29 Al 0.26)1.98 (Si 3.66 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.66 Na 0.02 Ca 0.02)0.70 (Fe3+1.44 Fe2+0.02 Mg 0.28 Al 0.24)1.98 (Si 3.65 Al 0.14 Fe3+0.21)4 O10 (OH)2 . n H2O (K 0.63 Na 0.02 Ca 0.02)0.67 (Fe3+1.30 Fe2+0.02 Mg 0.27 Al 0.40)1.98 (Si 3.64 Al 0.15 Fe3+0.21)4 O10 (OH)2 . n H2O (K 0.64 Na 0.02 Ca 0.02)0.68 (Fe3+1.31 Fe2+0.02 Mg 0.28 Al 0.37)1.98 (Si 3.67 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.50 Na 0.02 Ca 0.02)0.55 (Fe3+1.14 Fe2+0.01 Mg 0.28 Al 0.57)2.01 (Si 3.71 Al 0.12 Fe3+0.17)4 O10 (OH)2 . n H2O (K 0.59 Na 0.02 Ca 0.02)0.63 (Fe3+1.29 Fe2+0.02 Mg 0.27 Al 0.42)1.99 (Si 3.67 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.61 Na 0.02 Ca 0.02)0.65 (Fe3+1.22 Fe2+0.02 Mg 0.28 Al 0.47)1.98 (Si 3.66 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.63 Na 0.02 Ca 0.02)0.67 (Fe3+1.27 Fe2+0.02 Mg 0.28 Al 0.41)1.98 (Si 3.67 Al 0.13 Fe3+0.19)4 O10 (OH)2 . n H2O (K 0.61 Na 0.02 Ca 0.02)0.65 (Fe3+1.26 Fe2+0.01 Mg 0.28 Al 0.43)1.98 (Si 3.67 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.66 Na 0.02 Ca 0.01)0.69 (Fe3+1.39 Fe2+0.02 Mg 0.29 Al 0.29)1.98 (Si 3.66 Al 0.14 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.52 Na 0.02 Ca 0.02)0.57 (Fe3+1.09 Fe2+0.01 Mg 0.27 Al 0.64)2.01 (Si 3.68 Al 0.13 Fe3+0.19)4 O10 (OH)2. n H2O (K 0.53 Na 0.07 Ca 0.03)0.64 (Fe3+1.21 Fe2+0.02 Mg 0.27 Al 0.48)1.98 (Si 3.66 Al 0.20 Fe3+0.14)4 O10 (OH)2. n H2O (K 0.45 Na 0.10 Ca 0.05)0.61 (Fe3+1.02 Fe2+0.01 Mg 0.28 Al 0.67)1.99 (Si 3.67 Al 0.19 Fe3+0.13)4 O10 (OH)2. n H2O

0.31 0.36 0.34 0.35 0.29 0.33 0.31 0.32 0.35 0.34 0.32 0.32 0.31 0.35 0.32 0.36 0.34 0.34 0.35 0.36 0.33 0.29 0.33 0.34 0.33 0.33 0.34 0.32 0.34 0.33

0.18 0.16 0.16 0.16 0.17 0.17 0.17 0.17 0.15 0.16 0.16 0.17 0.17 0.17 0.17 0.18 0.17 0.17 0.17 0.16 0.16 0.16 0.15 0.16 0.17 0.16 0.17 0.15 0.16 0.16

0.18 0.23 0.21 0.22 0.18 0.21 0.19 0.20 0.22 0.21 0.20 0.20 0.20 0.17 0.16 0.17 0.14 0.14 0.14 0.15 0.14 0.12 0.14 0.14 0.13 0.14 0.14 0.13 0.20 0.19

0.13 0.14 0.13 0.13 0.11 0.13 0.12 0.12 0.13 0.13 0.12 0.12 0.12 0.18 0.17 0.18 0.20 0.20 0.21 0.21 0.20 0.17 0.20 0.20 0.19 0.20 0.20 0.19 0.14 0.13

0.18 0.16 0.16 0.16 0.17 0.17 0.17 0.17 0.15 0.16 0.16 0.17 0.17 0.17 0.17 0.18 0.17 0.17 0.17 0.16 0.16 0.16 0.15 0.16 0.17 0.16 0.17 0.15 0.16 0.16

(K 0.45 Na 0.00 Ca 0.07)0.52 (Fe3+1.02 Fe2+0.01 Mg 0.30 Al 0.67)2.00 (Si 3.71 Al 0.14 Fe3+0.15)4 O10 (OH)2 . n H2O (K 0.44 Na 0.01 Ca 0.07)0.52 (Fe3+0.99 Fe2+0.01 Mg 0.30 Al 0.70)2.01 (Si 3.70 Al 0.14 Fe3+0.15)4 O10 (OH)2 . n H2O (K 0.51 Na 0.03 Ca 0.04)0.58 (Fe3+1.01 Fe2+0.01 Mg 0.26 Al 0.73)2.02 (Si 3.61 Al 0.16 Fe3+0.23)4 O10 (OH)2 . n H2O (K 0.50 Na 0.03 Ca 0.03)0.56 (Fe3+1.09 Fe2+0.01 Mg 0.29 Al 0.62)2.01 (Si 3.68 Al 0.13 Fe3+0.19)4 O10 (OH)2. n H2O

0.29 0.30 0.39 0.32

0.18 0.18 0.15 0.17

0.14 0.14 0.16 0.13

0.15 0.15 0.23 0.19

0.18 0.18 0.15 0.17

(K 0.46 Na 0.02 Ca 0.07)0.55 (Fe3+1.10 Fe2+0.01 Mg 0.27 Al 0.63)2.01 (Si 3.62 Al 0.22 Fe3+0.16)4 O10 (OH)2. n H2O (K 0.44 Na 0.00 Ca 0.10)0.54 (Fe3+0.94 Fe2+0.01 Mg 0.31 Al 0.73)1.99 (Si 3.70 Al 0.18 Fe3+0.12)4 O10 (OH)2. n H2O (K 0.43 Na 0.00 Ca 0.07)0.50 (Fe3+0.95 Fe2+0.01 Mg 0.30 Al 0.74)2.00 (Si 3.72 Al 0.16 Fe3+0.11)4 O10 (OH)2 . n H2O (K 0.47 Na 0.01 Ca 0.09)0.57 (Fe3+0.99 Fe2+0.01 Mg 0.30 Al 0.69)1.99 (Si 3.67 Al 0.19 Fe3+0.14)4 O10 (OH)2 . n H2O

0.38 0.30 0.28 0.33

0.16 0.18 0.18 0.17

0.22 0.18 0.16 0.19

0.16 0.12 0.11 0.14

1.73 1.67 1.69 1.68

(K 0.34 Na 0.02 Ca 0.11)0.47 (Fe3+0.87 Fe2+0.01 Mg 0.30Al 0.82)2.01 (Si 3.71 Al 0.14 Fe3+0.15)4 O10 (OH)2. n H2O

0.29

0.17

0.14

0.15

0.17

(K 0.53 Na 0.05 Ca 0.05)0.63 (Fe3+0.87 Fe2+0.01 Mg 0.29 Al 0.82)1.98 (Si 3.67 Al 0.14 Fe3+0.19)4 O10 (OH)2. n H2O (K 0.59 Na 0.03 Ca 0.05)0.67 (Fe3+1.00 Fe2+0.01 Mg 0.31 Al 0.68)2.00 (Si 3.60 Al 0.16 Fe3+0.23)4 O10 (OH)2. n H2O (K 0.52 Na 0.03 Ca 0.04)0.60 (Fe3+0.96 Fe2+0.01 Mg 0.28 Al 0.75)2.01 (Si 3.63 Al 0.15 Fe3+0.22)4 O10 (OH)2. n H2O

0.33 0.40 0.37

0.17 0.18 0.16

0.14 0.16 0.15

0.19 0.23 0.22

0.17 0.18 0.16

(K 0.55 Na 0.00 Ca 0.08)0.63 (Fe3+1.06 Fe2+0.01 Mg 0.30 Al 0.61)2.00 (Si 3.62 Al 0.18 Fe3+0.20)4 O10 (OH)2 . n H2O (K 0.52 Na 0.00 Ca 0.06)0.58 (Fe3+1.00 Fe2+0.01 Mg 0.32 Al 0.68)2.01 (Si 3.65 Al 0.17 Fe3+0.18)4 O10 (OH)2. n H2O (K 0.52 Na 0.01 Ca 0.07)0.60 (Fe3+1.01 Fe2+0.01 Mg 0.31 Al 0.66)2.00 (Si 3.65 Al 0.17 Fe3+0.18)4 O10 (OH)2. n H2O (K 0.51 Na 0.01 Ca 0.10)0.61 (Fe3+0.94 Fe2+0.01 Mg 0.32Al 0.73)2.00 (Si 3.62 Al 0.18 Fe3+0.19)4 O10 (OH)2. n H2O

0.38 0.35 0.35 0.38

0.18 0.19 0.19 0.19

0.18 0.17 0.17 0.18

0.20 0.18 0.18 0.19

0.18 0.19 0.19 0.19

Bored bryozoan infilling KS/10 (K 0.41 Na 0.03 Ca 0.20)0.63 (Fe3+0.80 Fe2+0.01 Mg 0.30 Al 0.86)1.97 (Si 3.57 Al 0.21 Fe3+0.22)4 O10 (OH)2. n H2O (K 0.37 Na 0.01 Ca 0.13)0.51 (Fe3+0.77 Fe2+0.01 Mg 0.31 Al 0.93)2.01 (Si 3.64 Al 0.17 Fe3+0.19)4 O10 (OH)2. n H2O (K 0.43 Na 0.01 Ca 0.10)0.54 (Fe3+0.86 Fe2+0.01 Mg 0.31 Al 0.83)2.02 (Si 3.63 Al 0.18 Fe3+0.19)4 O10 (OH)2. n H2O

0.43 0.36 0.37

0.18 0.18 0.18

0.21 0.17 0.18

0.22 0.19 0.19

0.18 0.18 0.18

0.29 0.35 0.41 0.32 0.35 0.36 0.41 0.34 0.30

0.20 0.18 0.18 0.19 0.24 0.26 0.17 0.18 0.18

0.17 0.20 0.25 0.20 0.22 0.22 0.26 0.16 0.15

0.12 0.14 0.15 0.12 0.13 0.13 0.15 0.18 0.16

0.20 0.18 0.18 0.19 0.24 0.26 0.17 0.18 0.18

Substrate/ Sample no. Pellet core KS/5 KS/6

KS/10

KS/16

KS/23

Pellet rim KS/10 KS/16

Foraminiferal infilling KS/5 KS/10

Algal infilling KS/23 Bryozoan infilling KS/16

Bored algal infilling KS/10

Vermiform unaltered KS/5 KS/6

KS/10

(K 0.63 Na 0.02 Ca 0.05)0.69 (Fe3+1.36 Fe2+0.02 Mg 0.33 Al 0.26)1.96 (Si 3.71 Al 0.17 Fe3+0.12)4 O10 (OH)2. n H2O (K 0.68 Na 0.01 Ca 0.03)0.71 (Fe3+1.41 Fe2+0.02 Mg 0.30 Al 0.25)1.98 (Si 3.65 Al 0.20 Fe3+0.14)4 O10 (OH)2. n H2O (K 0.51 Na 0.01 Ca 0.07)0.59 (Fe3+1.12 Fe2+0.01 Mg 0.31 Al 0.58)2.02 (Si 3.59 Al 0.25 Fe3+0.15)4 O10 (OH)2. n H2O (K 0.48 Na 0.06 Ca 0.06)0.60 (Fe3+1.07 Fe2+0.01 Mg 0.33 Al 0.59)2.00 (Si 3.68 Al 0.20 Fe3+0.12)4 O10 (OH)2. n H2O (K 0.51 Na 0.06 Ca 0.06)0.62 (Fe3+1.08 Fe2+0.01 Mg 0.39 Al 0.54)2.03 (Si 3.65 Al 0.22 Fe3+0.13)4 O10 (OH)2. n H2O (K 0.47 Na 0.01 Ca 0.05)0.54 (Fe3+1.08 Fe2+0.01 Mg 0.42 Al 0.55)2.07 (Si 3.64 Al 0.22 Fe3+0.13)4 O10 (OH)2. n H2O (K 0.58 Na 0.02 Ca 0.03)0.63 (Fe3+1.16 Fe2+0.01 Mg 0.30 Al 0.54)2.02 (Si 3.59 Al 0.26 Fe3+0.15)4 O10 (OH)2 . n H2O (K 0.57 Na 0.01 Ca 0.06)0.64 (Fe3+0.18 Fe2+0.02 Mg 0.31 Al 0.48)1.99 (Si 3.66 Al 0.16 Fe3+0.18)4 O10 (OH)2. n H2O (K 0.56 Na 0.01 Ca 0.06)0.63 (Fe3+1.16 Fe2+0.01 Mg 0.31 Al 0.50)1.97 (Si 3.70 Al 0.15 Fe3+0.16)4 O10 (OH)2. n H2O

(continued on next page)

26


Table 2 (continued) Substrate/ Sample no. Vermiform altered KS/5

Structural formulae

Tetra. charge

Oct. charge

Al3+ (tetra.)

Fe 3+ (tetra.)

Total Oct. R3+

(K 0.40 Na 0.01 Ca 0.08)0.50 (Fe3+01.03 Fe2+0.01 Mg 0.29 Al 0.69)2.02 (Si 3.65 Al 0.21 Fe3+0.15)4 O10 (OH)2. n H2O (K 0.45 Na 0.01 Ca 0.07)0.53 (Fe3+1.08 Fe2+0.01 Mg 0.28 Al 0.64)2.01 (Si 3.66 Al 0.20 Fe3+0.14)4 O10 (OH)2. n H2O (K 0.45 Na 0.01 Ca 0.07)0.54 (Fe3+1.02 Fe2+0.01 Mg 0.28 Al 0.69)2.01 (Si 3.67 Al 0.19 Fe3+0.14)4 O10 (OH)2. n H2O (K 0.41 Na 0.01 Ca 0.05)0.47 (Fe3+0.98 Fe2+0.01 Mg 0.30 Al 0.74)2.03 (Si 3.70 Al 0.17 Fe3+0.12)4 O10 (OH)2. n H2O (K 0.42 Na 0.01 Ca 0.07)0.49 (Fe3+1.01 Fe2+0.01 Mg 0.29 Al 0.71)2.02 (Si 3.68 Al 0.19 Fe3+0.13)4 O10 (OH)2. n H2O (K 0.39 Na 0.01 Ca 0.05)0.45 (Fe3+0.91 Fe2+0.01 Mg 0.29 Al 0.81)2.02 (Si 3.72 Al 0.16 Fe3+0.11)4 O10 (OH)2. n H2O

0.35 0.34 0.33 0.30 0.32 0.28

0.17 0.16 0.16 0.17 0.16 0.17

0.21 0.20 0.19 0.17 0.19 0.16

0.15 0.14 0.14 0.12 0.13 0.11

0.17 0.16 0.16 0.17 0.16 0.17

Tetra: Tetrahedral, Oct.: Octahedral.

9. Discussion The chemistry of glauconite varies widely and its Fe2O3 (total) content is often used to distinguish illitic minerals (b 10% Fe2O3) from ‘glauconitic minerals’ (N 15% Fe2O3), with a compositional gap from 10% to 15% Fe2O3 (total) as suggested initially by Odin and Matter (1981). Again, its K2O content distinguishes between poorly evolved glauconitic smectite (at least 3% K2O) from highly evolved glauconitic mica (N 8% K2O). All glauconite data of the Karai Shale Formation plot within the field of ‘glauconitic minerals’ proposed by Odin and Matter (1981). Salient aspects of the Karai Shale Formation glauconites are discussed as follows. 9.1. Origin and chemical evolution of glauconite in different substrates The origin of authigenic glauconite is explained by two popular hypotheses, the ‘verdissement theory’ proposed by Odin and Matter (1981) and the ‘layer lattice theory’ proposed by Burst (1958a, 1958b) and Hower (1961). The ‘verdissement theory’ accounts for the authigenic precipitation of glauconitic smectite within the pores of bioclasts and fecal pellets followed by progressive dissolution and recrystallization of

Fig. 14. Mössbauer spectra of five glauconite samples recorded at room temperature indicating the relative abundance of Fe3+ (doublet A-red) and Fe2+ (doublet D-cyan) cations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the substrate. Initial glauconitic smectite precipitates are evolved by the addition of K at a more or less fixed Fe2O3 (total). The ‘layer lattice theory’ involves transformation of degraded layer lattice structures by synchronous fixation of K and Fe. Other suggestions regarding the origin of glauconite include a) transformation of mechanically infiltrated detrital kaolinites to glauconite inside the intra-particle pore systems (Ehlmann et al., 1963), b) authigenic crystal growth beginning with either illite– smectite (Bjerkli and Östmo-Saeter, 1973) or Fe-rich smectite precipitation (Odom, 1984) inside empty tests. Clauer et al. (1992) proposed a two-stage glauconitization process, the early stage of which is influenced by chemical exchange from surrounding detrital clay minerals (until b4% K2O), which is followed by predominant seawater influence on glauconite composition. Evolutionary trends of glauconites for both theories are presented in Fig. 11. Moderate correlation (r2 = 0.7) between K2O content and Fe2O3 (total) in the Karai Shale Formation glauconites supports the layer lattice theory. However, the least evolved glauconite infillings contain N17.9% Fe2O3 (total) and N3.6% K2O, while the least evolved glauconite pellets and vermiforms contain 5.1% K2O and 21.6% Fe2O3 (total), which is not fully consistent with degraded layer lattice structures (Table 1). Furthermore, the pores in foraminifera, bryozoan, and algae are too tiny for infiltration of clay, although seawater could have percolated through them. Clay infiltration, however, may not be ruled out in case of relatively larger pores in ostracoda and borings on other calcareous tests. Therefore, the glauconitization in the Karai Shale Formation may be broadly envisaged in two stages, initial authigenic precipitation of K- and Fe-poor glauconite and its subsequent evolution. Glauconite infillings and pellets were evolved later by addition of Fe in the octahedral sites and fixation of K in the interlayer sites of the lattice; Al and Si were released concomitantly from the glauconite structure. A similar evolutionary trend of glauconite infillings and pellets was also noted by Banerjee et al. (2012b) for the Oligocene glauconite in onshore Kutch. The origin of glauconite vermiforms involves only the second stage, addition of Fe and K into the partly degraded mica was accompanied by the release of Al and Si. The unaltered nature of bioclasts containing the glauconite infillings suggests alkaline conditions during the glaucontization. The nature and composition of the substrate possibly influenced the evolution of the Karai Shale Formation glauconites. Mineral chemical data of glauconite obtained separately for cores and rims of pellets, infillings within chambers and pores of bioclasts and altered and unaltered zones of vermiform exhibit distinct grouping in most of the cross plots, although with moderate overlapping (Figs. 11, 13). K2O content of cores of glauconite pellets and unaltered glauconite vermiforms frequently exceed 6%, which corresponds to the evolved variety, characteristically associated with a condensed section. Other varieties fail to reflect the typical signature of condensed sections glauconite, even within the middle segment of the Karai Shale Formation. The variation in K2O content in different glauconite infillings is possibly related to pore size, nature of pores, presence of detrital clay, and decomposable organic content within pores. K- and Fe-poor glauconite precipitated initially within the tiny pores in bryozoans, algae, and foraminifera failed to evolve as sea water percolation was very restricted. Relatively large pores in borings within algal and bryozoan tests allowed greater


27

Table 3 Mössbauer spectral parameters of five glauconite samples of the Karai Shale. Sample

Iron sites (doublets)

Isomer shift (δ) mm/s ±0.03

Quadrupole splitting (ΔEQ) mm/s ±0.05

Line width (Г) mm/s ±0.05

Relative area (RA) %

Fitting quality (χ2)

KS/5

A (Fe3+) M2(cis) B (Fe3+) M1(Trans) C (Fe3+) tet A (Fe3+) M2(cis) B (Fe3+) M1(Trans) C (Fe3+) tet D (Fe2+)M1 A (Fe3+) M2(cis) B (Fe3+) M1(Trans) C (Fe3+) tet A (Fe3+) M2(cis) B (Fe3+) M1(Trans) C (Fe3+) M2 tet A (Fe3+) M2(cis) B (Fe3+) M1(Trans) C (Fe3+) tet

0.443 0.409 0.247 0.455 0.416 0.262 1.113 0.473 0.405 0.278 0.478 0.419 0.284 0.465 0.42 0.266

0.476 1.207 0.485 0.467 1.189 0.476 2.684 0.472 1.192 0.470 0.478 1.248 0.483 0.493 1.252 0.484

0.405 0.297 0.413 0.396 0.325 0.383 0.298 0.335 0.328 0.410 0.362 0.292 0.438 0.398 0.271 0.427

42.3 16.3 41.4 42.2 18.1 37.6 2.1 27 21.3 51.7 28.8 12.2 58.9 37.0 12.5 50.5

1.0319

KS/6

KS/10

KS/16

KS/23

maturation compared to other infillings. Fecal pellet is considered as the most ideal substrate for glauconitization as it is enriched with required ions for glauconite formation. Furthermore, decomposition of organic content of the pellet creates a semi-confined, sub-oxic microenvironment which facilitates mobility of Fe (Odin and Matter, 1981; Meunier and El Albani, 2007). Additionally, relatively high porosity and permeability of the pellet allows access to seawater for chemical alterations necessary for evolution of glauconite. The glauconite vermiform possibly involved the fewer chemical reactions in converting a partly degraded biotite into an evolved glauconite by the addition of K and Fe into the lattice and release of Mg. Mg content exceeds 3% in most ‘slightly evolved’ glauconite vermiforms but is always less than 3% in the ‘evolved’ type (Fig. 13e), supporting the view of Stille and Clauer (1994) that chemical exchange with the substrate continues in glauconite as long as K2O content remains less than 6.5%. 9.2. Post-depositional alteration of glauconite EPMA backscatter images clearly reveal distinct change in chemical composition between cores and rims in glauconite pellets, as rims exhibit a sharp decrease in K2O and Fe2O3 (total) contents compared to the adjacent cores. Thin zones of alterations, close to the cracks, exhibit similar depletion of K2O and Fe2O3 (total) (Fig. 6). Similarly, glauconite vermiforms in exhibit frequent wedge-shaped zones of alteration along which K2O and Fe2O3 (total) values decrease sharply compared to the adjacent unaltered zones (Fig. 8). SiO2 and Al2O3 content in the altered zones of both glauconite pellets and vermiforms are higher than in the unaltered zones (Figs. 6,8). The origin of zoned glauconite has, in general, been explained by either degradation in contact with meteoric water in the soil profile (Courbe et al., 1981) or as an intermediate stage of evolution from glauconitic smectite to glauconitic mica (Velde and Odin, 1975; Odom, 1976; Velde, 1985). However, the sharp compositional contrast between cores and rims in glauconite pellets (Fig. 6) rules out the second possibility. The typical fibro-radiating structure of glauconitic smectite rim, reported by Odom (1976), is not observed in the Karai Shale Formation glauconite. Furthermore, the chemical alteration is not restricted within the rims but is also maintained within the cores through fractures (Fig. 6). Eder et al. (2007) and Sánchez-Navas et al. (2008) reported zoned glauconite with brown rims related to late precipitation of Fe-oxyhydroxides in oxidizing conditions. The rims in glauconite pellets of the Karai Shale Formation are lighter than the adjacent cores, which possibly reflects degradation of glauconite in the presence of freshwater, a process similar to the ‘reverse glauconitization’ (Courbe et al., 1981). K2O and Fe2O3 (total) were selectively leached from the glauconite by the ‘reverse glauconitization’ process by the action

1.23151

1.30436

0.990664

1.0509

of meteoric water at a very late stage. Altered zones in glauconite vermiforms can be similarly explained by ‘reverse glauconitization’ process. Dasgupta et al. (1990) recorded higher Fe2O3 (total) in cores than rims at a constant K2O for zoned glauconites in the Precambrian Pakhal and Sullavai Group of India, which they attributed to degradation of glauconite in the presence of K-rich pore water. 9.3. Sequence stratigraphic implications of glauconite Detailed mineral chemical investigation amply recognizes widely variable chemical composition of glauconite within the Karai Shale Formation. Composition of glauconite infillings as well as of altered zones in glauconite pellets and vermiforms cannot be used for stratigraphic interpretations; the former represents incomplete evolution of initial glauconite, while the latter reflects post-depositional alteration of glauconite composition. The relationship between K2O content of glauconite and abundance of pellets reveals intensity of stratigraphic condensation (Amorosi, 2012). The abundance of glauconite pellets increases steadily from the lower to middle segment of the Karai Shale Formation reaching up to 60% close to the maximum flooding surface (Fig. 4). Cores of glauconite pellets and unaltered zones in vermiform record a steady increase in K2O content from 5.1% at the base of the lower segment to 7.5% at the top of the middle segment, which decreases to 6.1% at the upper segment (Fig. 4). This gradual increase in K2O content as well as increasing abundance of pellet in the middle segment reflects the effect of stratigraphic condensation on glauconite evolution. The combination of the highest K2O content and greatest abundance of glauconite pellets coincides with the maximum flooding surface. The data are consistent with condensed section-related autochthonous glauconite as proposed by Amorosi (2012). 9.4. Causes of Fe enrichment in glauconite The Fe2O3 (total) content of unaltered glauconite pellets and their cores is higher than most reported glauconites. However, glauconite containing N 25% Fe2O3 (total) is reported from both modern and ancient deposits (Bornhold and Giresse, 1985; Kelly and Webb, 1999; Lee et al., 2002; Ketzer et al., 2003; Wigley and Compton, 2007; Sánchez-Navas et al., 2008; Boukhalfa et al., 2015). Because of negligible Fe content, seawater is unlikely to contribute significant Fe to the glauconite. Continental weathering-related supply of K, Fe, and Si is possibly responsible for glauconite formation in shallow marine sediments (Odin and Matter, 1981; Peters and Gaines, 2012). Continent-derived supply would have formed relatively higher Fe content in glauconite in the upper segment of the Karai Shale Formation compared to the middle segment. Furthermore, the inferred outer shelf origin of the middle

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segment of the Karai Shale Formation rules out the possibility of supply of Fe from the land. In modern oceans, Fe is available in upwelling areas on the outer shelf or in a slope environment where glauconite often occurs in condensed horizons (Cook and Marshall, 1981). High Fe content in the Karai Shale Formation may thus be related to upwelling. Furthermore, the co-occurrence of organic-rich, phosphatic and glauconitic sediments from offshore ward to landward as observed in the Karai Shale Formation and its equivalent sediments is consistent with those in well-developed modern upwelling zones (Burnett, 1980; Bremner, 1981; Glenn and Arthur, 1988; Glenn et al., 1994; Parrish et al., 2001). However, Fe may also have been contributed partly by the biotite. 10. Conclusions The major conclusions of the study of glauconite in the Karai Shale Formation are as follows. 1) Glauconite pellets, infillings, and vermiforms reflect wide compositional variability. Evolved glauconite containing N 6% K2O forms within glauconite pellets and vermiforms, while most glauconite infillings belong to slightly evolved type. The co-occurrence of evolved and slightly evolved glauconites with variable chemical composition at the same stratigraphic level is related to their origin and evolution in a wide variety of substrates. 2) Glauconite exhibits good correlation between K2O and Fe2O3 supporting the ‘layer lattice theory’ of its origin. However, the least evolved glauconite contains higher K2O and Fe2O3 than the expected values. Therefore, the formation of glauconite infillings and pellets is envisaged in two stages, initial authigenic precipitation of K- and Fe-poor glauconite and its subsequent evolution by addition of Fe and K into the lattice and concomitant release of Al and Si. Origin of glauconite vermiforms involves only the second stage from partly degraded biotites. 3) The rims and altered zones in glauconite pellets and vermiforms are characterized by lower K2O, Fe2O3, and higher Al2O3 and SiO2 compared to the unaltered zones, which is related to late-stage ‘reverse glauconitization’ by meteoric water actions. The K2O content within unaltered portions of glauconite pellets and vermiforms is consistent with the condensed section, but the remaining varieties provide erroneous interpretations regarding stratigraphic condensation. 4) Increasing abundance of glauconite pellets from the bottom to the top of the transgressive systems tract accompanied by slight increase in K2O content, reflects the role of stratigraphic condensation in glauconite evolution. 5) Mössbauer spectroscopy of glauconites reveals significant total Fe substitution in both tetrahedral and octahedral sites. 6) The high Fe2O3 (total) of glauconite in glauconite cores and vermiforms in the Karai Shale Formation is possibly related to upwelling, although there may be some contribution from the biotite substrate. Acknowledgements Authors are indebted to their host institutes for infrastructure facilities. SB is thankful to the Department of Science and Technology, Government of India, for financial support through grant IR/S4/ESF-16/ 2009(2). The authors thank S.C. Patel and Javed M. Shaikh for providing analytical support at the DST-IITB National facility for EPMA, Department of Earth Sciences, Indian Institute of Technology Bombay. Authors are thankful to the anonymous reviewers for their constructive criticisms and useful suggestions on an earlier version of the manuscript. References Amorosi, A., 1995. Glaucony and sequence stratigraphy: a conceptual framework of distribution in siliciclastic sequences. Journal of Sedimentary Research 65, 419–425. Amorosi, A., 1997. Detecting compositional, spatial, and temporal attributes of glaucony: a tool for provenance research. Sedimentary Geology 109, 135–153.

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