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Home , Cementation (geology), Concretion, Iron oxide, Nodule (geology)

J. Syst. Sci. (2021) 130:49 Ó Indian Academy of Sciences

https://doi.org/10.1007/s12040-020-01542-6 (0123456789().,-volV)(0123456789().,-volV)

Understanding genesis of present in Dhandraul (Vindhyan) : Implications in formation of Martian spherules

1, 1 2 PRAKASH JHA * ,PRANAB DAS and DWIJESH RAY 1Department of Applied , Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, Jharkhand 826 004, India. 2Physical Research Laboratory, Ahmedabad 380 009, India. *Corresponding author. e-mail: [email protected]

MS received 16 February 2020; revised 18 September 2020; accepted 8 November 2020

The iron oxide concretions of Shankargarh (Allahabad), India belongs to Dhandraul Sandstone of Vindhyan Supergroup. Petrography of concretions shows abundant grains embedded within the iron oxide . XRD analysis of the shows diagnostic peaks for quartz, hematite, and . The A–CN–K and A–CNK–FM ternary diagrams drawn for concretion and host bulk composition clearly indicate the interaction of concretions rock with iron-bearing diagenetic Cuids. A negative Ce anomaly, lower Th/U ratio, and enrichment of responsive trace element (e.g., vana- dium) indicate concretion formation is redox-controlled. The concretions show Fe enrichment and Si depletion as compared to the host sandstone. The mass balance calculations indicate that the total Fe2O3 in the ferruginous sandstone system is 17.63 wt%. The iron mobilization and recycling in the sandstone pore spaces have formed concretions with Fe2O3 (25–35% by volume). The sandstone volume required to produce a 6 mm diameter iron oxide concretion is 1807.83 mm3. The Fe laminae and random colouration patterns in Dhandraul sandstone are consistent with the movement of iron-enriched Cuid through pores and spaces. These iron oxide concretions have similarities with the hematite spherules discovered in the Burn Formation, , . Keywords. Iron oxide concretions; ; Martian hematite spherules; ; Cuids interaction.

1. Introduction Seilacher 2001; Mozley and Davis 2005). They are formed through diagenetic processes and recognized The concretions are cemented masses, as post-depositional features (Chan et al. 2007). usually spherical or disk-shaped objects set in a host During diagenetic processes, mineral matter precipi- rock of a different composition (Mozley 2003;Chan tates in pore spaces of sedimentary rocksas a localized et al. 2005). Concretions are common and extensively mass of forming concretions. The iron oxide present in sedimentary rocks throughout the geologic concretions keep a record of Cow and record (Seilacher 2001; Mozley 2003;Chanet al. suggested to have formed due to Cuid– 2007). Concretions vary in their composition, espe- interactions resulting in mineral dissolution and cially the cementing materials range from , precipitation (Chan et al. 2006, 2007, 2012; Potter iron , sulBdes, and silicates (Mozley 1989; 2009). The diagenetic iron oxide concretions of 49 Page 2 of 20 J. Earth Syst. Sci. (2021) 130:49 sandstone are developed through low-temperature, identiBcation of iron oxide concretions bearing near-surface, or burial diagenesis (Chan et al. sedimentary strata on Earth and its relevance to 2005, 2007;Potteret al. 2011). The formation of iron infer the martian sedimentary diagenetic process oxide concretions involves interactions of Fe bearing indirectly. reducing Cuid and oxidized groundwater resulting in precipitation of metastable iron phases (Chan et al. 2. Geological setting 2000, 2005;Potteret al. 2011). The metastable iron phases gradually dehydrate to form stable hematite Iron oxide concretions occur in the Dhandraul Sand- (Chan et al. 2000, 2012). stone of Manikpur Formation, Kaimur Group, Vind- The exciting discovery of hematite spherules at hyan Supergroup (Bgure 1). Its formation history Meridiani Planum, Mars (informal name ‘Martian involves varying sedimentary environments including blueberries’) by (MER) Cuvial, aeolian, and nearshore (Chakraborty 1996). ‘Opportunity’ has renewed the terrestrial iron oxide The study area represents an area near Biharia and concretions research as similar terrestrial like sedi- Janma Village (25°3.5240N; 81°31.5170E), Shankar- mentary diagenetic processes might have operated on garh, Prayagraj District, Uttar Pradesh, India Mars during its early geologic history (Squyres et al. (Bgure 2). The Dhandraul sandstone beds are nearly 2004). However, the nature of Martian spherules and horizontal to sub-horizontal with low dips (dip[10°) the exact mechanism of the formation process (pre-, towards the south. The sandstone is Bne-to-medium syn, or post-depositional process) are still inconclu- grained and varies from compact, massive to friable sive. Recent identiBcation (by Mars Science Labora- types. Overall, the sandstone is of two varieties, i.e., tory ‘Curiosity’) of sedimentary rocks and the homogenous massive and well laminated. Cross-bed- characteristic primary structures (cross-bedding, Bne ding is a common primary feature in the sandstone laminated structures) at Gale crater on Mars is sig- outcrops and suggesting a paleocurrent movement. nificant and unambiguously support the past diage- The Dhandraul sandstone in Shankargarh appears to netic history of Mars (Grotzinger and Milliken 2012; form by Cuvial–aeolian processes. The thickness of the Kah et al. 2015; Frydenvang et al. 2017). Therefore, a quartz arenite unit in Dhandraul sandstone varies better understanding of iron oxide concretion from 30 to 150 m (Chakraborty 1996) overlain by formation may oAer an improved insight into both alluvium with a thickness ranging from centimeters terrestrial and by analogy, martian diagenetic and (cm) to few meters (m) (Bgure 3). post-depositional changes. The joints are common in the sandstone and Earlier studies on Shankargarh iron oxide found either near-vertical to vertical or parallel concretions suggest that the host sandstone is a to the bedding. The Dhandraul sandstone in typical quartz arenite type belonging to Mesopro- Shankargarh varies from white, light yellow, pale terozoic Dhandraul sandstone of Vindhyan Super- yellowish brown, buA and red. It shows reddish group (e.g., Geological Survey of India Beld reports tinges, red spots, reddish shade laminations, and by Ali and Ahmad 1978 and Pati et al. 2016). random red colouration patterns at various expo- Concretion characteristics like sphericity, internal sures (Bgures 4–6). The red spots in the sandstone band counting, along with its mineralogy are dis- are less than a millimeter in size. cussed by Pati et al. (2016). However, the reasons behind the random red colouration pattern, iron laminae, the role of advective and diffusive pro- cesses occurring during the formation of the con- cretion in Dhandraul sandstone are not discussed in detail. In this work, we have used Beld studies, petrography, mineralogy, whole-rock chemistry of iron oxide concretions, and host sandstone to understand the formation process of iron oxide concretions. Additionally, the Fe bearing pale- oCuid circulation, redox processes, and elemental mobility (enrichment and depletion of elements) are discussed in detail using geochemistry of Shankargarh iron oxide concretions and associated Figure 1. Stratigraphic sequence in and around the Shankar- sandstone. Finally, we justify the importance of the garh, Uttar Pradesh (Auden 1933). J. Earth Syst. Sci. (2021) 130:49 Page 3 of 20 49

Figure 2. Geological map showing part of Vindhyan basin with location of concretion samples marked around Shankargarh, Uttar Pradesh (modiBed after Geological Survey of India, Misc. Publ. 30, 2012).

to discoidal shape (aspect ratio [ 1.07) (Bgure 4g and h). The size of the concretions ranges from a few mm to [ 25 cm (Bgures 4 and 5). The micro- concretions (size \5 mm) show a geochemically self-organized distribution pattern (Ortoleva et al. 1987, 1994; Mcbride et al. 2003; McLennan et al. 2005)(Bgure 5a and b). Chi-squared goodness-of- Bt test, P-value \0.001, conBrmed that the con- cretions exhibit a geochemically self-organized distribution pattern instead of random spacing. Additionally, iron oxide concretions show Cow structures and Cow tails suggesting the movement of iron-bearing Cuids (Bgure 6a–c). Also, the iron laminations are found lying parallel to the bed- dings (Bgure 6d–f). The Fe red layers and iron oxides in the form of red colouration and red iron spots are more common in surface and near-sur- face sandstone beds and gradually decreases with depth.

3. Analytical techniques

Mineralogy and textural studies were conducted Figure 3. Lithosection of the study area. using a Nikon optical microscope and overall, 21 thin sections of concretions and associated host rocks were examined. XRD analysis of the con- The dark red to brown coloured Shankargarh cretion bands, the core of the concretions, and the concretions vary from spherical (aspect ratio = 1), host rock sample was carried out at the Nano- near-spherical (aspect ratio varying from 1 to 1.07) technology Research Centre, SRM University 49 Page 4 of 20 J. Earth Syst. Sci. (2021) 130:49

Figure 4. Field photographs showing the distribution of iron oxide concretions present in Dhandraul sandstone, Shankargarh. (a–d) are showing a sectional view of the concretions. The concretions in (c and d) are rind concretions of micro variety. (d) is showing conjoined micro-concretions from Dhandraul sandstone. In (e), concretions are attached to the massive sandstone. (f)is representing erosional lag deposit of concretions. (g–i) are showing lab photographs of concretions collected from the Shankargarh, depicting the shape and interior of the concretions. (g and h) are showing spheroidal and discoidal concretions, respectively. (i) is showing a section view of the concretions. The section view showing layers or bands similar to liesegang rings surrounding a core.

Chennai. XRD samples (0.025 g approx.) were ICP-MS analysis was conducted using an analyzed using the XPERT-PRO diAractometer Inductively Coupled Plasma Quadrupole Mass system with Cu Ka1 radiation at 40 kV and 30 mA. Spectrometer (Q-ICPMS) facility, Department of The data analyzed were collected between 2h Earth Sciences, Indian Institute of Technology angles of 5° and 100° with 0.02° scan steps. The Kanpur, India (table 2). Concretion trace elements mineral phases were identiBed based on dominant were analyzed using a Q-ICPMS. International or intense X-ray diAraction peaks present in XRD standards (WGB-1, SBC-1, and AGV-2) were used pattern using the JCPDS, 1974, American miner- to verify the accuracy of the analyses. Average blank corrections were \1% for most of the ele- alogist Crystal Structure Database and RRUFF ments. The measured trace element concentration XRD database (Lafuente et al. 2016). The XRD was within 10% of the certiBed values (WGB-1, data treatment has been done using Panalytical SBC-1, and AGV-2). X’Pert High Score Plus software. XRF analysis was carried out at the XRF facility, NGRI, Hyderabad, India (table 1). This laboratory is well 4. Results equipped with a Philips PW 2400 spectrometer. International standard (GSR-4) was employed to 4.1 Concretions morphology and characteristics check the accuracy of the analyses. The 2r ana- The macro-concretions range in size from few mil- lytical uncertainty of the analyses is better than limeters to [25 cm (Bgure 4). The concretions are 1%. Further, four host sandstone samples (Q-10, of spherical, near-spherical to discoidal in shape 14, 16, and 18) are analyzed at XRF facility, (Bgure 4g and h). Generally, the spheroidal con- National Centre for Earth Science Studies, Thiru- cretions are present in massive sandstone, and vananthapuram, India. discoidal concretions are present in the laminated J. Earth Syst. Sci. (2021) 130:49 Page 5 of 20 49

Figure 5. (a) is showing micro-concretions in the laminated sandstone. The laminations in the sandstone are similar to Liesegang bands. (b) is showing micro-concretions present in the massive sandstone. They have a geochemically self-organized distribution pattern. (c) is showing spheroidal concretion within a massive sandstone bed. (d) is showing discoidal concretion along the bedding plane. Elongation of concretions in the plane of bedding suggests faster diffusion rates along the permeable sandstone laminae. Also, in the late stage of concretion growth, advective Cuid Cow processes dominated the diffusive iron cementation supply (McBride et al. 2003). sandstone (Bgure 5c and d). The external concre- (Bgure 4f). Macro-concretions long axes vary from tion surface has a rough and sandy nature and 6.4 to 18 cm, whereas the measured short axes vary varies from brown to reddish-brown colour. Many from 4.5 to 13 cm. Concretions may or may not of the macro-concretions show bumpy exterior include any nucleus. Micro-concretions are also surface, similar to an avocado fruit skin. The common in the Dhandraul sandstone with sizes interior of macro-concretions, noted in the Beld and ranging from less than a millimeter up to 5 mm. during lab studies, is layered (Bgure 4i). In micro- Micro-concretions are geochemically self-organized concretions (\5 mm) population in the Dhandraul (Bgure 5a and b). Micro-concretions are also pre- sandstone, rind type concretions (Potter et al. sent in association with Liesegang bands in the 2011) are common (Bgure 4c and d). Concretion sandstone (Bgure 5a). cross-sections show reddish-brown concentric Doublets (Bgure 7a and b), triplets (Bgure 7c), bands having iron-enriched cementation. The and multiple conjoined concretion (Bgure 7d) concentric concretion bands surround a lighter forms are common in the Dhandraul sandstone. brown to the grey-coloured iron-depleted core These are commonly found in the both macro and (Bgure 4i). The core of the concretions varies from micro-sized concretion population. The doublet circular to elliptical. The core is similar in miner- conjoined concretions look ellipsoidal in shape. The alogy to the host sandstone but with higher iron external surface is generally rough and sandy in oxides cementation. The concretion layers or bands nature and appears to be a fusion of two discrete are well-cemented, similar to Liesegang bands or concretions. In contrast, cross-section views sug- rings (Stern 1954; Stow 2009) diagenetic sedimen- gest a shared internal structure. The section view tary structures of authigenic (Bgure 4i). of the conjoined concretion shows layers sur- The modes of occurrences of iron oxide concre- rounding a core similar to the single concretions. tions include both in situ (Bgure 4a, b, d, and e) in The conjoined parts of the doublet are of similar the host arenite and as erosional lag deposits shape and sizes and also in section view; both parts 49 Page 6 of 20 J. Earth Syst. Sci. (2021) 130:49

Figure 6. (a–c) are showing iron Cow structure representing movement and recycling of Fe bearing paleoCuid. (d–f) shows iron lamination and red colouration pattern in Dhandraul sandstone, suggesting mobilization and precipitation of iron along and parallel to the bedding and pore spaces in sandstone during the post-depositional stage.

Table 1. Whole rock major element oxides (in wt%) of concretion and host sandstone from Biharia, Shankargarh, India.

Sample L-5 L-6 L-7 L-8 L-10 L-11 Q-10 Q-14 Q-15 Q-16 Q-17 Q-18

SiO2 65.352 65.879 64.517 65.402 66.33 65.516 93.81 93.47 94.219 93.13 95.578 96.46

Al2O3 0.098 0.36 0.192 0.157 0.261 0.169 3.31 3.65 3.054 3.82 2.067 2.12

Fe2O3 30.654 29.95 30.433 30.8 29.965 30.283 0.83 0.75 0.351 0.78 0.46 0.3 MnO 0.02 0.016 0.016 0.018 0.01 0.015 0.01 0.01 0.006 0.01 0.008 ND MgO 0.16 0.093 0.096 0.098 0.096 0.175 0.2 0.17 0 0.21 0 ND CaO 0.058 0.06 0.049 0.079 0.067 0.06 0.42 0.35 0.017 0.42 0.151 0.11

Na2O 0.04 0.02 0.01 0.03 0.02 0.01 0.25 0.21 0.021 0.26 0.101 0.02

K2O 0.11 0.21 0.01 0.1 0.01 0.02 0.07 0.06 0.01 0.07 0.01 0.02

TiO2 0.171 0.176 0.179 0.104 0.128 0.2 0.24 0.23 0.135 0.24 0.118 0.09

P2O5 0.045 0.046 0.042 0.05 0.042 0.045 0.04 0.04 0.03 0.04 0.028 0.03 LOI 1.99 1.98 2.5 1.94 1.6 2.3 0.62 0.66 0.8 0.62 0.7 0.53 Sum 98.7 98.79 98.04 98.78 98.53 98.79 99.8 99.6 98.64 99.6 99.22 99.68 CIA 25.24 49.36 62.26 34.26 61.19 53.45 72.58 75.09 97.56 75.091 82.07 89.28 ND: not detected.

Abbreviations. L: Concretion and Q: host sandstone. Also, the CIA (Chemical index of alteration) = –Al2O3/(Al2O3 + CaO* + Na2O+K2O)˝ 9100, CaO* is CaO associated with the silicate fraction of the rock (Nesbitt and Young 1982). show a sense of continuity between them have experienced simultaneous nucleation and (Bgure 7b). The concentric ellipse like rims or growth. If the parts of the conjoined concretions layers continues in both parts of conjoined con- may have nucleated and grown separately and cretion. The conjoined parts of the concretion may later on, they got fused together into a conjoined J. Earth Syst. Sci. (2021) 130:49 Page 7 of 20 49

Table 2. Whole rock trace and rare-earth element compositions (in ppm) of concretion and host sandstone from Biharia, Shankargarh, India.

Sample L-1 L-5 L-8 L-9 L-11 Q-10 Q-14 Q-15 Q-17 Trace elements (in ppm) Li 2.27 1.10 0.99 3.48 1.35 1.97 2.08 2.01 2.77 Be 0.52 3.22 1.46 1.11 1.56 0.29 0.30 0.25 0.32 Bi 0.08 0.23 0.09 0.10 0.12 0.14 0.09 0.09 0.11 V 63.15 83.54 63.25 64.65 51.00 5.00 3.50 3.69 5.23 Cr 44.51 52.49 92.29 74.10 55.50 2.15 10.88 4.36 2.87 Co 49.31 54.79 34.47 53.44 59.13 53.43 60.55 55.35 52.33 Ni 7.80 18.97 12.40 9.29 28.67 2.66 3.62 3.48 2.38 Cu 10.56 12.05 9.23 43.49 17.27 1.20 2.57 2.03 1.72 Zn 145.56 1133.50 861.05 393.50 1268.51 31.33 34.09 38.90 32.97 Ga 2.41 3.18 2.72 2.94 2.24 1.86 1.56 1.54 2.22 As 136.53 54.81 52.57 21.41 70.13 2.55 1.98 2.39 1.55 Se 6.86 3.19 1.49 8.25 4.53 0.35 0.19 À0.02 0.13 Rb 1.19 1.55 1.08 0.84 1.22 1.12 1.14 1.03 1.33 Sr 35.98 34.37 28.04 39.81 32.12 29.00 29.34 29.54 34.92 Cs 0.12 0.22 0.17 0.10 0.15 0.12 0.12 0.13 0.16 Ba 40.20 110.64 119.15 56.60 101.86 15.12 16.00 14.69 24.21 Pb 284.19 823.05 605.45 318.64 1291.92 51.11 52.72 35.32 54.16 Cd 0.20 0.53 0.41 0.22 0.49 0.12 0.12 0.07 0.16 Tl 0.02 0.04 0.02 0.02 0.03 0.03 0.03 0.02 0.03 Ge 7.09 14.42 13.83 12.65 14.73 1.41 1.51 1.33 1.35 Zr 77.32 124.03 74.46 103.56 216.14 112.11 121.69 71.10 135.89 Nb 2.61 2.70 1.61 1.96 3.54 1.03 1.07 0.88 1.54 Hf 2.30 3.38 1.84 3.09 6.09 3.31 3.70 2.04 4.22 Ta 0.37 0.33 0.24 0.25 0.47 0.16 0.19 0.11 0.21 Sb 22.29 12.24 9.35 5.30 21.64 0.77 0.43 0.49 0.39 Sc 3.38 2.36 1.84 1.96 3.12 0.57 0.49 1.05 0.55 Y 6.11 6.86 5.76 6.26 11.07 6.86 7.24 4.69 7.96 Rare-earth elements (in ppm) La 12.77 9.78 11.13 12.62 10.66 13.30 13.56 11.37 12.19 Ce 26.47 20.97 22.55 25.34 21.74 28.48 24.24 25.02 22.55 Pr 3.06 2.37 2.74 2.94 2.52 3.16 3.05 2.61 2.84 Nd 11.54 9.24 10.31 10.86 9.38 11.50 11.05 9.33 10.45 Sm 1.93 1.74 2.00 1.88 1.89 1.97 1.97 1.61 2.06 Eu 0.31 0.32 0.37 0.33 0.36 0.30 0.32 0.25 0.34 Gd 1.87 1.80 1.91 1.97 2.08 1.95 2.04 1.63 2.14 Tb 0.24 0.25 0.24 0.25 0.36 0.26 0.28 0.19 0.30 Dy 1.25 1.35 1.16 1.27 2.11 1.40 1.50 1.02 1.68 Ho 0.25 0.28 0.23 0.25 0.43 0.28 0.30 0.20 0.33 Er 0.81 0.82 0.67 0.77 1.33 0.88 0.92 0.61 1.06 Tm 0.11 0.12 0.09 0.11 0.20 0.13 0.14 0.09 0.15 Yb 0.77 0.83 0.64 0.78 1.41 0.91 0.97 0.62 1.09 Lu 0.12 0.13 0.10 0.12 0.22 0.14 0.15 0.09 0.17 Th 2.95 3.20 2.12 2.85 4.42 3.16 3.48 2.22 3.54 U 1.217057 5.105448 2.967369 2.719679 4.41085 1.069225 1.467408 0.945666 1.185251 Th/U 2.427572 0.627702 0.712969 1.046179 1.002984 2.95205 2.369216 2.34689 2.988625

form. In such conditions, the two conjoined parts 4.2 Petrographic study of the concretions rock should not be of similar size, and also in section views; there should be a discontinuity between two Petrographic studies of the Shankargarh iron oxide conjoined parts of the concretions. concretions show abundant detrital quartz grains 49 Page 8 of 20 J. Earth Syst. Sci. (2021) 130:49

Figure 7. (a) The external surface of the doublet appears to be a fusion of two discrete concretions. (b) Shows both the fused parts of the conjoined concretion that show a sense of formational continuity. (c and d) are showing multiple conjoined concretions from Dhandraul sandstone. (d) is showing the cluster of conjoined concretions and singlet concretions. The conjoined concretions are sharing internal morphology, whereas the singlet concretions appear to be fusing with distinct internal morphology obvious in the section view. The fusions of singlet concretion hinting towards Ostwald ripening phenomena in the micro-concretions populations.

(Bgure 8). The interstices or spaces between quartz bands can be appropriately termed as ferruginous mineral grains are Blled with iron oxide cementa- quartz arenite. tion. The modal analysis of concretions suggests iron oxide cementation percent varying from 28 to 35%, with the rest of the material being clasts 4.3 XRD percentage (Bgure 8a). Iron oxides cement appears to be formed through groundwater diagenetic The presence of quartz, hematite, and goethite in processes. The layers present within the cementa- the concretion band is conBrmed through X-ray tion indicates their precipitation one above the diAraction pattern studies of Shankargarh iron other in different precipitation events (Bgure 8c oxide concretions (Bgure 9a). The core of the con- and d). The precipitated iron oxide cementation cretion consists of quartz and goethite (Bgure 9b). layers are concentric (Bgure 8c) as well as elon- The host rock comprises quartz, hematite, goe- gated (Bgure 8d). The concentric nature of the iron thite, and orthoclase (Bgure 9c). In the XRD pat- oxide cementation layer suggests their precipita- tern of concretions, the quartz is recognised at d tion in the pore spaces of the , whereas spacing; 3.342; 4.261; 1.817 A, hematite at 2.697; the elongated layers of cementation suggest iron 2.515; 1.694 A, goethite at 4.24; 2.459; 2.70 A. In oxide minerals precipitation in the neck between the concretion’s core, the quartz is identiBed at d pore spaces. The colour shade of red and black in spacing 3.338; 4.261; 1.817 A, goethite at 4.241; 2.46; the iron oxide cementation indicates dehydrated  iron oxide mineral phases such as hematite, 1.54 A. In the host rock, the quartz is identiBed at d whereas the yellowish-orange colour shade in the spacing 3.339; 4.247; 1.816 A, orthoclase at 3.32; 4.2; iron oxide cementation indicates hydrated iron 1.81 A; goethite at 4.246; 2.451; 2.232 A. oxide minerals such as goethite (Bgure 8b–d). The cores of the concretions and the concretions bands have similar quartz grains distribution (Bgure 8e). 4.4 Major and trace element geochemistry But the concretion bands have higher ferruginous cementation as compared to concretions cores In major oxide chemistry, it has been observed that (Bgure 8a and e). The core of the concretion is the Fe2O3T average value is about 30.35 wt% in mostly quartz arenite, whereas the concretion the concretion and 0.58 wt% in the host sandstone. J. Earth Syst. Sci. (2021) 130:49 Page 9 of 20 49

Figure 8. Photomicrographs of the iron oxide concretions. (a–d) are showing microscopic view of concretions bands or layers under transmitted light. (a and b) clearly show detrital quartz grains having interstices or spaces Blled with iron oxides cementation. (c and d) are showing precipitated layers within the iron oxide cementation, suggesting multiple iron precipitation events. In (b–d), the red and black colour shades in the iron oxide cementation indicates dehydrated iron oxide phases such as hematite, whereas the yellowish-orange colour shades indicate hydrated iron oxide minerals such as goethite. (e) shows the core of the iron oxide concretion. It is clear from the Bgure that the core has less iron oxide cementation than concretion bands. (f) shows concretion nuclei consisting of -rich material.

The SiO2 average value is about 65.5 wt% in the plotting close to the Al2O3–K2O boundary towards concretion and 94.44 wt% in the host sandstone. K2O apex (Bgure 10a). This is due to the weath- Clearly, Fe2O3T shows an enrichment in concre- ering of K-feldspar in the advanced stage of tion with respect to host sandstone, while SiO2 . Very low values of CaO (average shows depletion in concretion with respect to host 0.24%) and Na2O (average 0.14%) in the bulk sandstone. As the iron-bearing Cuids move in the composition of the sandstone suggest weathering of sandstone, it precipitates the iron minerals in the Plagioclase feldspar in the initial stages of the pore spaces and dissolves silica from the sandstone. weathering. The high SiO2 of the sandstone indicates its mature The plotted samples of the concretions in the nature (Potter 1978). The average value of chem- A–CN–K diagram show scattering from the ical index of alteration (CIA) in the concretion rock weathering trend shown by the host sandstone is about 47.63% suggesting moderate weathering samples (Bgure 10a). This is due to the interaction and in the host, sandstone is about 81.95% sug- of concretion rock with the paleo-groundwater gesting high weathering of the rock –CIA, chemical diagenetic Cuid (Nesbitt and Young 1984, 1989). index of alteration =Al2O3/(Al2O3 + CaO* + In the Al2O3–(CaO*+Na2O+K2O)–FeO+MgO, Na2O+K2O) 9 100, CaO* is CaO associated with A–CNK–FM ternary diagram (Nesbitt and Young the silicate fraction of the rock (Nesbitt and Young 1989), plotted bulk composition of concretions is 1982)˝. In the Al2O3–(CaO* + Na2O)–K2O, showing deviation from the weathering trend of the A–CN–K ternary diagram (Nesbitt and Young host sandstone samples (Bgure 10b). This is due to 1984), the bulk composition of sandstone is the intense iron enrichment in the concretions rock 49 Page 10 of 20 J. Earth Syst. Sci. (2021) 130:49

values show depletion in concretion with respect to host sandstone, and TiO2 values are comparable in concretions and host rock, indicating their immobile nature. For trace elements, the Li, Bi, Co, Ga, Rb, Sr, Cs, Cd, Tl, Zr, Nb, Hf, Ta, Sc, Y, and Th are at comparable level in concretions and host rock sediments, whereas the trace elements Be, V, Cr, Ni, Cu, Zn, As, Se, Ba, and Pb have higher abun- dance in concretions as compared to host rock (table 2). The Th/U ratios in the concretions are lower than the host rock. The Th/U ratios in the concretions are about 1.16, average value, and in the host rock about 2.66, average value. The lower Th/U ratios in the concretions in comparison to host sandstone is due to higher Uranium concen- tration. The higher concentration of Uranium in concretions is related to the incorporation of oxi- dized U6+ in the diagenetic Fe reducing Cuids. Another trace element, Vanadium is redox responsive and enriched in concretions, relative to host sandstone. The trace element distribution and Figure 9. (a) The XRD analysis of iron oxide concretion variations suggest varying redox conditions inCu- indicates the presence of quartz, hematite, and goethite as enced theP concretion formation. major mineral phases. In (b), the XRD analysis of the core of The REE valuesP of concretions range from 50 the iron oxide concretion indicates presence of quartz and to 61.5 ppm in whichP LREE varies from 46.22 to goethite as major mineral phases. In (c), the XRD analysis of 57.95 ppm and HREE from 3.13 to 6.05 ppm, the host rock of iron oxide concretion indicates presence of the P quartz, orthoclase, and goethite as major mineral phases. respectively. The REE values of hostP rock range from 54.65 to 64.67 ppm in whichP LREE varies during the interaction of diagenetic paleo-ground- from 51.83 to 60.66 ppm and HREE from 2.82 to water Cuid with sandstone. MnO is absent or is 4.78 ppm, respectively. Both the concretions and negligible in both sandstone and host rock host sandstone have similar REE abundances. The (table 1), indicating Mn2+ mobilization under (Post Archean Australian (PAAS: see, Tay- reducing conditions (Hylander et al. 2000). Al2O3 lor and McLennan 1985) normalized REE pattern

Figure 10. (a) is showing Al2O3–(CaO* + Na2O)–K2O, A–CN–K ternary diagram (Nesbitt and Young 1984). (b) is showing Al2O3–(CaO*+Na2O+K2O)–FeO+MgO, A–CNK–FM ternary diagram (Nesbitt and Young 1989). In both the ternary diagrams, concretion samples are deviating from the weathering trend shown by host sandstone samples. This is related to diagenetic Cuid interaction with concretion rock. J. Earth Syst. Sci. (2021) 130:49 Page 11 of 20 49 also show similar REE abundance in concretions Strong negative Eu anomaly is observed in both and host rock except for concretion sample L-11, concretions and host rock REE patterns, which which has elevated HREE (Bgure 11a and b). The is related to the weathering of feldspar in the PAAS normalized REE concretion pattern shows sandstone source rock. negative Ce anomaly, whereas the host rock REE pattern shows some variations; two samples 4.4.1 Element mobility showing high negative Ce anomaly and two sam- ples positive Ce anomaly. However, the negative Formation of concretion within the sandstone Ce anomaly is more prominent within the host involves the interaction of iron bearing diagenetic sandstone. The variation in Ce is related to Cuid with the concretion rock leading to enrich- changing oxidation–reduction conditions in the ment and depletion of various elements in the sandstone diagenetic environment (Wilde et al. concretion with respect to host sandstone. The 1996). The negative Ce anomaly in the sandstone mass gain or loss of various elements present in suggests Ce partitioning or fractionation due to concretion is being calculated using an equation 3+ 4+ 4+ oxidation of Ce to Ce and Ce precipitation proposed by Wilson et al. (2012). from solution as CeO2. Also, the negative Ce j i j i j i anomaly, within the concretions, suggests that the DC = C ¼ 100½Cs=CsŠ=½Cp=CpŠÀ1; ð1Þ diagenetic Fe Cuids involved in iron oxide minerals j precipitation were already depleted in Cerium. where Cs is the concentration of element or its oxide of interest (superscript j) in the concretion (sub- i scripts s) in weight percent; Cs is the concentration of immobile element or its oxide (superscript i) in the j concretion in weight percent; Cp is the concentration of elements of interest or its oxide in the sandstone i (subscript p) in weight percent; and Cp is the con- centration of immobile element or its oxide in the sandstone, in weight percent. Ti (TiO2) is considered as the immobile element for mobility calculations, although other elements such as Zr can also be considered (table 3). The percent enrichment of major oxides in concretions with respect to host rock are Fe2O3 (7496.99%) and depletion of the major oxides: SiO2 (–22.37%), Al2O3 (–91.38%), CaO (–40.64%), and Na2O (–66.3%). Similarly, the trace elements also show enrichment in the concretion with respect to host rock: Gd (1.53%) to As (3736.094%) and depletion in concretion with respect to host rock: Rb (–7.42%) to Hf (–50.45%).

4.4.2 Iron mass balance

Volumetrically iron oxides cementation in the form of hematite and/or goethite yields an aver- age value 30% (based on petrographic modal analysis point counting) and quartz about 70%. The average iron oxide concretions density is Figure 11. (a) is showing PAAS normalized (Taylor and 3 3 McLennan 1985) REE pattern of concretions present in about 3.26 g/cm (Fe2O3 density = 5.24 g/cm , 3 Dhandraul sandstone, Biharia (Shankargarh). It displays quartz density = 2.6 g/cm ; concretion density ¼ negative Ce anomaly, suggesting Ce fractionation. (b)is 30=100 Â 5:24þ 70=100 Â 2:6 ¼ 3:39g/cmÀ3Þ: showing PAAS normalized (Taylor and McLennan 1985) REE The mass balance equation for the concentration pattern of host Dhandraul sandstone. It shows Ce anomaly variation due to varying redox conditions in the sandstone of Fe2O3 in concretion bearing sandstone (JolliA diagenetic environment. et al. 2005): 49 Page 12 of 20 J. Earth Syst. Sci. (2021) 130:49

Cconcretion  mfconcretion þ Crock  mfrock

as the ¼ Cboth concretion and rock 2 32.2344 33.6587 percent – – where C is the concentration (wt%) of Fe2O3,mfis Enrichment g TiO the mass fraction of the concretion (mfconcretion) and rock (mfrock) and concretion density= 3.39 gm/cm3, sandstone density= 2.6 gm/cm3. 3

Trace For a 2 cm sandstone system comprising con- element cretions and host sandstone, total density = 5.99 gm/cm3, mass fraction of the concretion = con- cretion mass/total mass = 3.39/5.99, mass fraction of the sandstone = sandstone mass/total mass = 21.396 32.8378 32.3432 28.2432 11.1245 21.2336 percent – – – – – – 2.6/5.99. Enrichment Also based on XRF data, Fe2O3 in concretion = 30.8%, concretion sample, L-8 and, Fe2O3 in host sandstone = 0.46%, sandstone sample, Q-17. Trace

element Using weight% values and XRF oxide values in mass balance equation:

Cconcretion  mfconcretion þ Crock  mfrock 37.8304 Nd 11.92982 Lu 50.4451 Eu 25.74555 U 184.06 17.9121 Ho C percent ¼ both concretion and rock – – – Enrichment 3:39=5:99  30:8 þ 2:6=5:99  0:46

¼ Cboth concretion and rock ¼ 17:63%:

Trace Based on calculations, it is clear that the total element Fe2O3 in the Shankargarh Ferruginous sandstone system is 17.63%. This Fe2O3, through recycling and mobilization, repeatedly precipitates in the sandstone pores forming concretions having Fe2O3 23.3066 Ce 13.48717 Tm 8.89024 Ta 30.22431 Gd 1.530492 7.41621 Hf percent – – – value 25–35%. Enrichment The mass balance equation is also useful in ve sign indicates depletion. This calculation is based on the concretion sample, L-8 with respect to host

– understanding the volume of sandstone required to form the iron oxide concretion of diameter 6

Trace mm. element

3 3 rsst  ðÞFe2O3 sst¼ rconc  ðÞFe2O3 conc: The volume of sandstone required to form the 59.6434 As 3736.094 Zr 2.44663 Rb 25.2751 Ba 458.3433 Sc 275.6477 Dy

percent iron oxide concretion of 3 mm radius. – – – Enrichment 3 3 rsst ¼ rconc  ðÞFe2O3 conc=ðÞFe2O3 sst;

where rsst and rconc are the radii of bleached sand-

Trace stone and concretions, respectively. element (Fe2O3)sst is the weight percent iron oxide taken from the sandstone, and (Fe2O3)conc is the weight percent iron oxide gained in the concretion. 0 Be 423.9374 Se 1189.964 Nb 18.47892 Sm 10.34194 Th percent 7496.99 V 1272.19 Sr 102.6099 Ga 39.34232 Ge 1065.281 Pr 9.44515 Yb 22.3607 Li 66.2986 Cu 509.0949 Cd 187.8289 La 3.590736 Er 40.6393 Ni 490.0373 Pb 1168.301 Y 100 Co 91.382 Bi – – – – – Fe O 30:8%; concretion sample; L-8 Enrichment ðÞ2 3 concr ¼

Enrichment/depletion table for major and trace elements between the concretion and the host rock. Percent enrichment/depletion is calculated usin ðÞFe2O3 sst ¼ 0:46%; host sandstone sample; Q-17 3 3 3 ¼ 3 Â ðÞ30:8=0:46 5 2 O 2 O O 2 O 1034.615 Zn 2862.822 Tl 2 O 2

2 3 2 ¼ 1807:83 mm : Al Table 3. P K MgO MnO 155.2885 Cr 3547.866 Cs 21.19843 Sb 2626.065 Tb TiO Fe CaO immobile element oxide. +sign indicates enrichment, whereas the Na sandstone, Q-17. Major oxides SiO J. Earth Syst. Sci. (2021) 130:49 Page 13 of 20 49

Based on calculations, we estimate that for Liesegang bands present within the concretions forming concretions of diameter 6 mm, 1807.83 (Bgure 4i) suggest diffusion of iron, , and mm3 volume of sandstone is required. other cementing minerals such as clay from the Based on Berner (1969) and Wikinson (1993), concretion periphery inwards to the concretion the time required for concretion to form through center (Steefel 2008). The bumpy exterior surface diffusion in Million year, t = k9r2 where r is the similar to an avocado fruit skin, of the macro- radius of concretion in meter and k is growth concretions indicates coalescing of micro-concre- constant = 1.1 unit (Abdel-Wahab and Mcbride tions to form the outer part of macro-concretions 2001). Our studies suggest that it takes about 9900 (Potter et al. 2011). Ostwald ripening phenomenon years for forming a concretion of diameter 6 mm (Steefel and Van Cappellen 1990) may have been through diffusion. involved in coalescing micro-concretions into macro-concretions (Bgures 4b, d and 7d). The spheroidal nature of micro-concretions and their 5. Discussion association with Liesegang bands in sandstone suggest the formation of micro-concretions through The Dhandraul sandstone shows the presence of Fe diffusive processes (McBride et al. 2003; Steefel laminae (Bgure 6d–f) and iron-bearing Cuid Cow 2008; Potter et al. 2011). The absence of a nucleus structures in the form of red colouration patterns in many concretions suggests that its digestion in (Bgure 6a–c). The concretions are present parallel the chemical reactions occurred during concretion to and along the bedding planes (Bgure 5d) and Fe formation. laminae are presently associated with the micro- to Shankargarh macro-iron oxide concretions are of macro-sized concretions (Bgure 5a) in the Dhan- layered type (Bgure 4i) formed through concentric draul sandstone. These features suggest movement growth in which early iron oxide minerals precipi- of iron-bearing Cuids and precipitation of iron in tated in core zones with successive younger pre- the form of concretions. cipitated layers from core to periphery. Taking an During concretion-forming stages, the advection analogy from the concretion formation processes provide the mass reactants such as Fe model (Folk 1974; Jacka 1974; Longman 1980; and O, to the reaction site or front. Consequently, Mozley and Davis 2005), low saturation rate of concretions were formed through diffusive mass precipitated material leads to slow nucleation rate transfer processes occurring at the reaction sites and precipitations at the small number of sites. (Potter et al. 2011). It is quite logical in thinking This leads to the formation of layered or zoned that the larger size concretions in Dhandraul concretions. The higher saturation rate of precipi- sandstone have greater Fe supply at the reaction tated material leads to faster nucleation rate and fronts as compared to the smaller concretions. The precipitations at multiple sites. This leads to the spherical to near-spherical Shankargarh iron oxide formation of rind concretions. Shankargarh micro- concretions are found in the homogenous massive concretions population is of rind type (Bgure 4c). sandstone (Bgure 5c). In the homogenous sand- The macro-concretions have ferruginous clay stone, the growth conditions are isotropic. Thus, material, which possibly acted as the nucleus for there is uniform diffusion rate of the mass reactants concretions formation (Bgure 7b). Also, the uni- resulting in the formation of spherical shape con- formly spaced concretions (Bgure 5b) suggest that cretions in the host sandstone (Seilacher 2001; some chemical self-organization process commands McBride et al. 2003; Mozley and Davis 2005). The the nucleation site of the concretions (Abdel- discoidal Shankargarh iron oxide concretions are Wahab and McBride 2001). found in the laminated sandstone (Bgure 5d). It is Petrography and XRD studies conBrm the suggested that faster diffusion rates along (and presence of hematite and goethite, iron oxide parallel to) the permeable sandstone laminae and cementation phases in the concretions. The layers directional Cuid Cow lead to the formation of dis- present within the iron oxide cementation indicates coidal shape concretions (Mozley and Davis 2005). multiple iron oxide minerals precipitation events As the sediments go through compaction, porosity during concretions formation stages. The presence and permeability get reduced in the vertical of both goethite and hematite in cementation sug- direction. Thus, forming concretions get elongated gests dehydration processes transforming goethite in the direction of Cuid Cow along and parallel to into hematite. The initial precipitation in pore the bedding plane of sandstone (Seilacher 2001). spaces of sandstone occurs as amorphous hydrous 49 Page 14 of 20 J. Earth Syst. Sci. (2021) 130:49 ferric oxide phase such as . Ferrihydrite (ii) Later, this iron-rich sandstone unit was Cushed is an unstable HFO phase that converts into with reducing Cuids sourced from underlying stable HFO phases like goethite and hematite at rocks. This Cuid moved throughout the sand- diagenetic temperatures (*70°C) and intermedi- stone along weak zones such as fractures, ate pH (Cornell and Schwertmann 2003; Kukka- joints, bedding planes, and pore spaces. This dapu 2003; Potter et al. 2011). Major oxide Cuid mobilized and removed the disseminated chemistry indicates the dissolution of SiO2, detrital iron oxides from the sandstone (Hansley 1995; quartz, and precipitation of iron minerals in the Garden et al. 1997, 1998; Chan et al. pores spaces between grains by the iron-bearing 2000, 2005; Beitler et al. 2005). The iron Cuids moving in the sandstone. The deviation was reduced from ferric (Fe3+) to ferrous shown by the plotted concretion bulk composition (Fe2+) state and mobilized. The reducing from the weathering trend of host sandstone bulk Cuid contained Fe2+ in solution and caused a composition in the A–CN–K and A–CNK–FM bleaching eAect in the sandstone. The reduc- diagram indicates the interaction of iron-bearing ing Cuids are possibly petroleum (Chan et al. diagenetic Cuids with concretions rocks. 2000, 2005). The enrichment and depletion of various major (iii) The iron moved in reduced condition, as and trace elements in the concretion relative to the ferrous (Fe2+) ions, towards the reaction site. host rock indicate the transport of mass elements When the iron-rich reduced Cuids came in towards the reaction site where iron oxide concre- contact with the oxidized water below the tions are expected to form. The diagenetic Fe Cuid water table, diffusion of the reduced Cuid into moving through the sandstone, incorporated some the oxidized water at the reactive interface elements while leaving others depending upon the between the two types of water occurred. This redox conditions. These mass elements precipitated resulted in oxidation of Fe ions (Fe3+) and its at the reaction site along with iron minerals leading precipitation as iron oxide cement in available to enrichment of certain elements in the iron oxide pore spaces of the permeable sandstone (von concretions and depletion of other elements Gunten and Schneider 1991; Chan et al. (table 3). The absence of MnO in both sandstone 2005). The repetition of this process created and concretions indicates varying redox conditions the concretions. in the sandstone. The enrichment of redox responsive (iv) During the formation stages of iron oxide elements such as Uranium and Vanadium in the con- concretion, the amorphous hydrous ferric cretion relative to host sandstone, clearly indicates the oxides (HFO) precipitated in the sandstone involvement of redox processes in the formation of iron now at a slightly higher temperature oxide concretions. The negative Ce anomaly in the (\100°C) due to diagenesis (Parry et al. concretions and variations of Ce in the sandstone also 2004; Potter et al. 2011). On dehydration, supports the involvement of redox processes in the the amorphous HFO converts to goethite and formationofironoxideconcretions. then with time into hematite (Klein and Based on this study and comparison with Bricker 1977; Potter et al. 2011). Thus, the iron oxide concretions of (Chan younger, intermediate, and oldest phase of et al. 2000, 2004, 2005; Chan and Parry 2002; HFO growth are amorphous HFO producing Beitler et al. 2003; Parry et al. 2004; Potter et al. goethite and hematite, respectively. The geo- 2011), we envisage a multiple-step diagenetic pro- chemical reactions involved in the formation cess which can explain the Shankargarh iron oxide of diagenetic iron-oxide concretions (Chan concretions formation process. et al. 2000, 2012) are as follows: The hematite forming reaction: (i) During the early burial just after the sediment deposition, water moving beneath the sediment 2þ 1 surface interacts with the sandstone consisting 2Fe ðÞþaq =2 O2 ðÞþg 2H2OlðÞ þ of silicate minerals with Fe ions (Berner 1969; ¼ Fe2O3 ðÞþs 4H ðÞaq Chan et al. 2005). This promotes the break- down of the iron present in the sandstone. This or the goethite reaction: iron forms a grain-coating during early diage- 2þ 1 nesis. The Dhandraul sandstone has been 2Fe ðÞþaq =2 O2ðÞþg 3H2OlðÞ coloured red by the Fe3+ in hematite. ¼ 2FeOðÞ OH ðÞþs 4Hþ ðÞaq J. Earth Syst. Sci. (2021) 130:49 Page 15 of 20 49

The reactants involved in the formation of quartz. The evidence of groundwater movement 2+ hematite and goethite are Fe (aq), O2 (aq), is present in both Burns Formation (Squyres and H+ (aq). et al. 2004; Grotzinger et al. 2005; McLennan et al. Hematite may form from dehydration of 2005; Andrews et al. 2007) and Dhandraul goethite: sandstone. Shankargarh iron oxide concretions vary from FeOðÞ OH ðÞ¼s Fe2O3ðÞþs H2OlðÞ: spheroidal to discoidal shape. The Martian hema- The dehydration reaction of ferrihydrite can tite spherules are near-spherical to spheroidal also result in hematite. (McLennan 2005; Squyres and Knoll 2005; Calvin et al. 2008). The Martian hematite spherules have 2Fe2O3 Á 0:5H2OsðÞ¼ 2Fe2O3ðÞþs H2OlðÞ: a bumpy external texture with an exterior surface similar to avocado fruit skin (Calvin et al. 2008; In the sediments, the iron present is in the Potter et al. 2011). The bumpy external surface Fe3+ oxidation state, which is immobile. So, suggests coalescing of micro-concretions to form the Fe3+ oxidation state must be reduced to macro-concretions via Ostwald ripening processes Fe2+ for transport. On reactions with hydro- (Potter et al. 2011). Shankargarh iron oxide con- carbons, organic acids, methane, or cretions also have a bumpy external texture with sulBde, the Fe3+ in hematite is reduced to an exterior surface similar to avocado fruit skin. Fe2+ (Garden et al. 1997; Chan et al. 2000). Conjoined forms (doublets or triplets) concretions (v) The microbial and bacterial processes in the are present in the Burns Formation, Meridiani sandstone may have increased the iron dissolu- Planum, Mars (Calvin et al. 2008). Similar con- tion and precipitation rate (Coleman 1993; joined concretions are also common in the Dhan- McBride et al. 1994; Ehrlich 1996; Langmuir draul sandstone of Mesoproterozoic Vindhyan 1997; Chan et al. 2000). The iron-oxidizing Supergroup. Both in-situ and erosional lag type such as Thiobacillus ferrooxidans oxi- concretions are common in Burns Formation dizes, and Gallionella ferruginea oxidizes iron in (Calvin et al. 2008) and Dhandraul sandstone. The acidic medium (Ehrlich 1996; Chan et al. 2000). Martian hematite spherules have a solid interior (vi) The reduced saline groundwater comes from structure (Calvin et al. 2008), whereas the underlying reservoirs present beneath the Shankargarh iron oxide concretions have layered Dhandraul sandstone reservoir. Thus ground- internal structures. water, on interaction with hydrocarbons like The Martian hematite spherules range from methane and organic acids present in the \1 mm (micro-spherules) to macro-spherules of underlying sedimentary reservoir beds, gets 1–6 mm in diameter (Calvin et al. 2008). The reduced. This reduced groundwater moves Shankargarh iron oxide concretion varies from \1 upward into the ferruginous sandstone mm to as large as 25 cm. The Martian hematite reservoir. spherules have in-situ geochemically self-orga- nized distribution pattern (McLennan et al. 2005; Calvin et al. 2008; Potter et al. 2011). The 6. Shankargarh iron oxide concretions: Shankargarh iron oxide concretions also show self- A Martian analogy organized geochemical spacing. The smaller size of the Martian hematite spherules suggests restricted The host rock of the Burns Formation in Meridiani water hydrogeochemical conditions in the Burns Planum of Mars is a Bne-grained homogenous Formation (Calvin et al. 2008). The Shankargarh eolian (Squyres and Knol 2005; concretion’s larger size and size variation indicate Grotzinger et al. 2005, 2011; Calvin et al. 2008; ample water was present in the Dhandraul sand- Grotzinger and Milliken 2012). The host rock of the stone reservoir. The hematite content of the Shankargarh iron oxide concretion is also a Bne to Martian hematite spherules is about 71 wt% medium-grained eolian sedimentary rock. The sand (Schneider et al. 2007), whereas the iron oxide grains of the Burns Formation comprise , mineral concentration of the Shankargarh con- calcium, and magnesium sulfates (Klingelhoefer cretions varies from 20 to 35 wt%. The number et al. 2004; Squyres et al. 2004; Christensen et al. density of hematite spherules is significantly 2005; Morris et al. 2006). In contrast, the grains of higher in the Burns Formation as compared to the the Shankargarh concretions host rocks are mainly Shankargarh iron oxide concretions. This suggests 49 ae1 f20 of 16 Page

Table 4. The similarities and difference between the Hematite spherules present at the Burns Formation, Mars and its studied terrestrial analogues including the present study.

Shankargarh Iron oxides concretions and Host rock (present study) Utah1 Mars2 Lake Brown3 Western Australia4 Hawaii5 Concretions characteristics Spheroidal geometrical shape Present Present Present Present Present Present Concretions interior structure Layered and Layered, rind and Solid Solid Solid Solid rind solid Doublets or triplets Present Present Present Not known Not known Present Macroconcretions and/or Both type Both type present Both type Only macro Only macro Only micro microconcretions present present concretions concretions concretions [ 5mm Host rock characteristics Fine grained, homogenous, Aeolian Yes Yes Yes No No No sedimentary host rock Host rock composition Quartz arenite Quartz arenite Sulfate, basaltic Evaporites Lateritic Sci. Syst. Earth J. Concretions mineralogy Hematite Present Present Present Present Present Present Goethite Present Present As precursor Present Present Absent Jarosite Absent Absent Present Present Absent Absent Clastic grains Present, Quartz Present, Quartz Not known Present, Present, Quartz Basic igneous Quartz rock

Clay Present Present Not known Present Absent Absent (2021) 130:49 1Potter et al. (2011); 2Calvin et al. (2008); 3Bowen et al. (2008); 4Jun et al. (2016); 5Morris et al. (2005). J. Earth Syst. Sci. (2021) 130:49 Page 17 of 20 49 a comparatively higher amount of iron was sites aAect element dissolution and composition of prevalent in the Burns Formation as compared to moving diagenetic Fe Cuids. Major and trace ele- Dhandraul sandstone. ments geochemistry further conBrm iron oxide The Shankargarh iron oxide concretions and the concretions formed from Fe diagenetic Cuids. The Martian hematite spherules of Burn Formation Shankargarh iron oxide concretions may consider have similar morphology, size (Shankargarh micro- as potential terrestrial analogue for studying concretions), distribution, conjoined forms (dou- Martian hematite spherules. The similarities blets), and iron-bearing mineral phases (table 4). include morphology, size (Shankargarh micro-con- Thus, the diagenetic Cuid movement processes and cretions), distribution, conjoined forms (doublets), multiple iron precipitation events might have pre- and iron-bearing mineral phases. However, the vailed in Burns Formation, Meridiani Planum, mode of formation is different between Shankar- Mars during the formation stages of Martian garh iron oxide concretions and Martian hematite hematite spherules. The Martian hematite spher- spherules. ules probably precipitated as a hydrous iron oxide mineral, which later dehydrated into hematite (Glotch et al. 2004). The diagenetic Shankargarh Acknowledgements iron oxide concretions are also initially precipitated as amorphous hydrous iron oxide minerals, which Mr Prakash Jha acknowledges IIT (ISM) Dhanbad later on dehydrated into goethite and hematite for the Institute fellowship. The authors acknowl- phases. The genesis of Martian hematite spherules edge the XRF analysis facility at NGRI Hyderabad is related to the solubility of ferruginous sulfate in (India), ICPMS analysis facility at IIT Kanpur the Burns Formation host rock, possibly through (India), and XRD analysis facility at SRM diagenesis (Mclennan et al. 2005). The Shankar- University, Chennai, India. garh iron oxide concretion formation occurs via precipitation through the mixing of reduced Fe bearing brine and oxygenated groundwater. Author statement

Mr Prakash Jha has done the Beldwork, petrogra- 7. Conclusions phy, and analyzed the photographs and data gen- erated from it. He has also analyzed the Shankargarh iron oxide concretions are of diage- geochemical data generated from various scientiBc netic origin as evident from Beld, petrographic and labs and organizations. Mr Prakash Jha, Dr Pra- geochemical studies. The formation mechanism nab Das and Dr Dwijesh Ray have cooperatively involves mixing Fe bearing reduced Cuid and oxy- written this research paper. genated groundwater resulting in precipitation of iron oxide cementation in the pore spaces of the sediment. Advection and diffusion of Fe bearing References paleo-Cuids through pore spaces of the sediment played a pivotal role in the formation of iron oxide Abdel-Wahab A and McBride E F 2001 Origin of giant calcite- concretions. The iron-bearing mineral phases in the cemented concretions, temple member, Qasr El Sagha Shankargarh iron oxide concretions include hema- formation (Eocene), Faiyum depression, Egypt; J. Sedim. tite and goethite. The texture of iron oxide Res. 71(1)70–81. cementation indicates its formation through mul- Andrews-Hanna J C, Phillips R J and Zuber M T 2007 tiple iron precipitation events. Red colourations Meridiani Planum and the global hydrology of Mars; and random red patterns in the Dhandraul sand- Nature 446(7132) 163–166. Auden J B 1933 Vindhyan in the Son Valley, stone demonstrate iron-bearing Cuid Cow move- Mirzapur district; Memoir Geol. Soc. India 62 141–250. ment through the sandstone. The negative Ce Beitler B, Chan M A and Parry W T 2003 Bleaching of anomaly, lower Th/U ratio or high U concentra- Navajo Sandstone on Colorado Plateau Laramide tion, and enrichment of redox responsive trace highs: Evidence of exhumed hydrocarbon supergiants; elements such as Vanadium indicate the involve- Geology 31(12) 1041–1044. Beitler B, Parry W T and Chan M A 2005 Fingerprints of Cuid ment of varying redox processes in the formation of Cow: Chemical diagenetic history of the Jurassic Navajo the Shankargarh concretions. Redox conditions Sandstone, southern Utah, USA; J. Sedim. Res. 75(4) present in the sandstone and concretion-forming 547–561. 49 Page 18 of 20 J. Earth Syst. Sci. (2021) 130:49

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Corresponding editor: SAIBAL GUPTA