Geochemistry of Subsurface Late Quaternary Ironstones in Rajshahi and Bogra Districts, Bangladesh: Implications for Genetic and Depositional Conditions

Acta Geochim (2019) 38(3):404–413 https://doi.org/10.1007/s11631-018-00310-0

ORIGINAL ARTICLE

Geochemistry of subsurface Late Quaternary ironstones in Rajshahi and Bogra Districts, Bangladesh: implications for genetic and depositional conditions

1 1 2 Md. Sazzadur Rahman • Ismail Hossain • Pradip Kumar Biswas • 3 2 1 Md. Abdur Rahim • A. S. M. Mehedi Hasan • Md. Ibrahim Adham

Received: 4 June 2018 / Revised: 22 November 2018 / Accepted: 25 December 2018 / Published online: 3 January 2019 Ó Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract The present study deals with the geochemistry of mostly formed by bacterial build up in swamps and mar- Late Quaternary ironstones in the subsurface in Rajshahi shes, and was subsequently embedded in clayey mud. and Bogra districts, Bangladesh with the lithological study Within the coastal environments, the water table fluctuates of the boreholes sediments. Major lithofacies of the studied and goethite and siderite with mud and quartz became dry boreholes are clay, silty clay, sandy clay, fine to coarse and compacted to form ironstone. grained sand, gravels and sands with (fragmentary) ironstones. The ironstones contain major oxides, Fe2O3*(* Keywords Ironstone Geochemistry Goethite Siderite total Fe) (avg. 66.6 wt%), SiO2 (avg. 15.3 wt%), Al2O3 Bangladesh (avg. 4.0 wt%), MnO (avg. 7.7 wt%), and CaO (avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe2O3* along with Al2O3 and MnO indicate the 1 Introduction ironstone as goethite and siderite, which is also validated by XRD data. A comparatively higher percentage of SiO2 Iron, which is estimated to make up about 4.7% of the indicates the presence of relative amounts of clastic quartz earth’s crust, is present in significant amounts in almost all and manganese-rich silicate or clay in these rocks. These sedimentary rocks. More than 400 Fe-minerals are reported ironstones also have significant amounts of MnO (avg. 7.7 in different literatures; however, only iron ore minerals wt%) suggesting their depositional environments under with more than 15% of Fe are relevant (H.G. Dill, personal oxygenated condition. Chemical data of these ironstones communication). Aggregates of iron ore minerals, namely suggest that the source rock suffered deep chemical ironstone, are sedimentary rocks, either deposited directly weathering and iron was mostly carried in association with as ferruginous sediment or created by chemical replace- the clay fraction and organic matter. Iron concretion was ments that contains a substantial proportion of an iron compound. The iron minerals comprising ironstones can consist either of oxides (i.e., goethite, limonite, hematite, Electronic supplementary material The online version of this and magnetite), carbonates (i.e., siderite), silicate (i.e., article (https://doi.org/10.1007/s11631-018-00310-0) contains supplementary material, which is available to authorized users. chamosite) or some combination of these minerals (James 1996). The studied subsurface ironstones are mostly light & Ismail Hossain gray to brown in color, very hard and compact, and con- [email protected] coidal with spherical or elliptical nature. These types of ironstones are occasionally found during 1 Department of Geology and Mining, University of Rajshahi, Rajshahi, Bangladesh the drilling of a shallow or deep water pump setup at a depth of about 45–84 m below the study areas (Fig. 1). 2 Institutes of Mining, Mineralogy and Metallurgy (IMMM), BCSIR, Joypurhat, Bangladesh Dominant lithologies of these areas (from boreholes data) are clay, silty clay, sandy clay, fine to coarse grained sand 3 Department of Disaster Resilience and Engineering, Patuakhali Science and Technology University, and sands with gravel and/or ironstones. Generally, the Dumki, Patuakhali 8602, Bangladesh geochemical process for iron concentration is controlled by 123 Acta Geochim (2019) 38(3):404–413 405

Fig. 1 Location map showing borehole positions in Mohanpur, Rajshahi and Kahaloo, Bogra Districts, Bangladesh the dynamic tectonic evolution of earth (Mohanta 2007). 2 Geological setting The silicate mineral weathers to release Fe, which precipitates as goethite, chamosite, siderite and pyrite in the The Bengal Basin is an asymmetric basin. The sediments sedimentary geochemical environment depending upon the increase in thickness toward the southeast, with the thickest low temperature thermodynamics prevailing in the depo- being about 22 km thick (Curray 1991). The basin was sitional site (Mohanta 2007). The presence of ironstones in shaped by the dynamic interaction of three plates, namely the studied geological sections may occur through different the Indian, Eurasian and Burmese sub-plates (Fig. 2). geochemical processes or genetic inheritances. Uddin and Lundberg (2004) reported the tectonics of the It is a very remarkable phenomenon that the present basin, which is primarily related to the rifting due to the subsurface ironstones have complex genetic history. In this separation of the Indian Plate from Antarctica-Australia case, the Bengal Basin is an ideal museum in understand- during the Gondwana break-up, and then its movement, ing weathering, erosional and tectonic history. Raymo and initially northwestward and then northward. Most likely Ruddiman (1992) also established that chemical weather- during the Late Eocene, the initial collision with the Bur- ing in the Himalayas and the global climate have potential mese sub-plate resulted in the rising of an Eocene island connection. The study areas are within the southern slope arc, which created the Bengal Basin to the west and the of the Bogra Shelf within the Paleoproterozoic Irrawady Basin in the east (Fig. 2). After the separation of Stable Platform ﬂank of the Bengal Basin. Main aims of these basins during the Miocene, the Indian Plate sub- the study are to outline the lithology of the boreholes, ducted beneath the Burmese sub-plate with anti-clockwise identify the rock types, and assess the genetic history of rotation and ultimately the Bengal Basin commenced ironstones and depositional conditions. The geochemistry closing in the northeast and gradually turned into a remnant of the rocks with the overall lithologies of the boreholes basin (Uddin and Lundberg 2004). Bangladesh comprises a helps to reveal the mineralogical composition of the rocks, major part of the Bengal Basin. Several scientiﬁc articles and its depositional conditions, as well as the interacting have already been published regarding the regional geol- factors including provenance, weathering and tectonism. ogy of the Bengal Basin and its surrounding areas (Reim- The geochemical data also helps with understanding the ann 1993; Goodbred and Kuehl 2000; Hossain et al. 2018 ore, its utility and the recovery of valuable metals of the and others). Representative lithological succession of the rock. There are few studies on denuded sedimentary rocks study areas is recorded in Table 1. in the Bengal Basin from Himalaya (Najman et al. 2008; Bangladesh is divided into two major tectonic units:the Ferdousy 2011; Hossain et al. 2014), as there are not much Palaeoproterozoic stable platform in the northwest and the study on geochemistry in these areas. Due to the lack of Bengal Foredeep on the southeast, which are separated by studies on ironstone and iron-formation deposition in these the Hinge Zone. The northwest stable platform is divided areas, this research may help geologists reveal the new into three major elements: Bogra shelf, Rangpur saddle and geoenvironmental condition of these areas. Dinajpur slope. Geographically, the study area lies between

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Fig. 2 Dynamic nature of the Bengal Basin with interaction of three plates (Indian, Eurasian and Burmese sub-plate). Figure also showing two prominent basins of the Irrawady Basin (east) and the Bengal Basin (west). Hinge zone indicates the edge of the Indian Craton (modiﬁed after Steckler et al. 2016; Rangin 2017)

Table 1 Stratigraphic succession of Bogra slope and adjoining areas of shelf zone, Bengal Basin (slightly modiﬁed after Zaher and Rahman 1980; Islam 2001; Hossain et al. 2007) Age Group/Formation Lithology

Holocene Alluvium Sand, silt, clay, sporadically present ironstones in some places Pleistocene Barind Clay Yellow–brown sticky clay, sandy clay Middle Pliocene to Late Miocene Dupi Tila Fm. Sandstone with subordinate pebble bed, grit bed and shale Early Miocene Jamalgonj Fm. Fine to medium grained sandstone, sandy and silty shale, siltstone, shale Oligocene Bogra Fm. Siltstone, carbonaceous shale and ﬁne grained sandstone Late Eocene Kopili Fm. Sandstone, locally glauconitic and highly fossiliferous, shale with the calcareous bands Middle to Late Eocene Sylhet Limestone Nummulitic limestone with sandstone interbeds Fm. Middle Eocene to Paleocene Tura Sandstone Fm. Gray and white sandstone, with subordinate greenish gray shale and coal Late Cretaceous Trapwash Coarse yellow brown sandstone, volcanic material, white clay Late Jurassic to Middle Rajmahal Trap Amygdaloidal basalt, Serpentinized shale and agglomerate Cretaceous Late Early Permian Paharpur Fm. Feldspathic sandstone, shale and coal beds Kuchma Fm. Sandstone and grit with subordinate shale interbedded with coal beds Paleoproterozoic Basement Rocks Diorite, Quartz-diorite, Granodiorite, Gneiss, Schist

a part of the Rangpur saddle and Bogra shelf. The study 3 Materials and methods areas cover the entire area of Keshorhat Pourashava, Mohanpur, in the Rajshahi district (latitude 24°35059.1100 Subsurface ironstone samples were collected from and longitude 88°390 45°300E) and some important samples Keshorhat, Mohanpur at 61–67 m depth and Kahaloo at are also collected from Kahaloo Upazila in the Bogra 46–55 m depth during drilling of testing wells and then district (latitude 24°50002.200N and longitude 89°18008.700 preserved in the polyethylene bags. Later these samples E) (Fig. 1). were dried up primarily by natural sunlight. From there, seven (7) samples were analyzed using the X-ray Fluo- rescence Spectrometer (XRF) method at the Institute of Mining, Mineralogy and Metallurgy, BCSIR, Joypurhat

123 Acta Geochim (2019) 38(3):404–413 407 following the procedures of Goto and Tatsumi Sand lithofacies: These lithofacies have different sub- (1994, 1996), and following instrumental precisions and lithofacies, mainly (1) very fine to fine sand, (2) medium standards discussed in Hossain et al. (2014). Sample sand and (3) coarse sand. These are mainly light gray in preparation techniques of the studied rocks for XRF anal- color, angular to sub-round grained, and composed of ysis were also followed according to Hossain et al. (2014). quartz, mica, dark and greenish minerals. Usually very fine In the analytical results, accepted highest uncertainties for to fine sand beds range from a few centimeters to 7 m, XRF major and minor elements are * 2% and trace ele- medium sand beds range from a few centimeters to 10 m, ments are \ 10%. The X-ray diffraction (XRD) analysis is and coarse sand beds range from a few centimeters to 14 m also supplemented for petrographic/mineralogical studies. (Fig. 3). Generally very fine to medium sands indicate a The XRD patterns were recorded using a GBC-Emma, deltaic or shore/beach environment with a low velocity of Australia diffractometer (Cu-Ka radiation generated at current. The coarse sand is deposited near the source area 35.5 kV and 28 mA), equipped with a 1° divergence slit, a of the river, within or near the active channel, where the secondary monochromator, a Solid-State Detector and a current energy essentially is found in a fluvial environment sample changer (sample diameter 16 mm). The samples (Ponteń and Plink-Bjo¨rklund 2007). were investigated from 10° to 80° 2h with a step size of Sandy Gravel lithofacies: This lithofacies is variegated 0.02° 2h and scan speed of 4.01°/min where measuring in color, and is commonly a mixture of sand to gravel size time of 4.987531E-03° per step. For specimen prepara- deposits. Thicknesses of these beds range from 2 to 5 m tion, the top loading technique was used. (Fig. 3). Sandy gravel indicates a continental environment near the source or few distances from the source of a river and very high velocity is required for the deposition of such 4 Lithology and lithofacies of study areas lithology with a fluctuation of energy (Rahman et al. 2004). Sand with ironstone lithofacies: This lithofacies consists Lithological analyses were carried out from the boreholes of a mixture of sand, gravel and gravely ironstone. These Mohanpur (MR), Rajshahi and Kahaloo (KB), Bogra, fragmental mixtures are mostly light gray to brown color, which were drilled about 70 and 60 m deep, respectively. gravel or boulder size, very hard, dense and compact, The representative lithological sections are presented in spherical or elliptical nature with concoidal and sharp Fig. 3a, b. These two boreholes were drilled by the edges, and composed of iron-rich minerals with siliceous Department of Public Health Engineering (DPHE) and the components. The samples of the studied ironstones were Institute of Water Modeling (IWM), and contain domi- collected from boreholes MR and KB, with depths of nantly clay, silty clay, sandy clay, fine to coarse grained 61–67 m and 46–55 m, respectively (Figs. 3, 4). sand, gravels and sands with ironstones. On the basis of These rarely present ironstone samples have enormous color, grain size, stickiness and visible minerals of these geological significance to these study areas as well as to the lithologies, four distinct sedimentary facies are recognized regional geology of Bengal Basin. Geochemical analyses in these boreholes. The individual lithofacies are described of these samples can provide some answers to solve the below: geological mystery of the genesis of this fragmentary Clay lithofacies: This facies shows light gray to yel- ironstone lithofacies. lowish brown clay with some organic matter. It is generally very sticky and soft. Sometimes very thin layers of silty or sandy clay lithofacies are also present, which are generally 5 Petrography and mineralogy of ironstones light gray to yellow color and few patches. It also contains a considerable amount of vegetal matter. Thicknesses of Generally studied ironstones are very fine-grained, the clay lithofacies range from a few centimeters to 13 m noticeably composed of matrix of quartz grains with clayey (Fig. 3). Clay lithofacies in the KB borehole show only a iron-bearing minerals, dominantly iron oxides and iron few centimeters of clay layers, whereas the MR borehole carbonates (e.g., goethite and siderite). The microscopic demarks two thick clay layers, ranging from trace to 13 m study of KB-1 shows burflower type matrices of light and and 15 to 18 m in thickness (Fig. 3). This is an indication yellow, orange or green color minerals (Fig. 5a, b), which of an abrupt decrease of hydrodynamic conditions or a low are mostly siderite, goethite with manganese-rich silicate energy standing water environment, away from the active bementite according to XRD data (supplementary data). channels (Ponteń and Plink-Bjo¨rklund 2007). Occasionally Reflected light photographs also show some developed present silty or sandy clay lithofacies indicate cool and metallic iron crystals in the rock (Fig. 5c). On the other calm environmental conditions, usually coastal, thought hand, the microscopic study of MR-7 authenticates the sometimes they may deposit in flood plain or lacustrine caviar type matrices of spotted light and yellow or green environments (Islam and Hossain 2006). color minerals (Fig. 5d, e), which has almost consistent 123 408 Acta Geochim (2019) 38(3):404–413

Fig. 3 Lithologs of a Kahaloo, Bogra and b Mohanpur, Rajshahi districts showing lithological types with sample locations

mineralogical composition with the KB-1 sample (supple- average of 15.27 wt%, Al2O3 from 2.11 (sample MR-6) to mentary data). Reﬂected light photographs show very few 4.54 wt% (sample MR-7) with an average of 4.00 wt%, developed metallic iron crystals in this ironstone (Fig. 5f). MnO from 3.82 (sample MR-7) to 11.68 wt% (sample MR- 6) with an average of 7.66 wt% and CaO from 1.89 (sample KB-1) to 4.91 wt% (sample KB-2) with an aver- 6 Geochemistry of ironstones age of 3.43 wt%. Other oxides are less than 1 wt% (Table 2). In this study, the rocks have very high Fe2O3*, Compositions of major and trace elements of the subsur- which indicates that the rock samples are ironstones. face Late Quaternary ironstones from the Rajshahi and Generally the iron-rich sedimentary rocks, which contain Bogra districts, Bangladesh are presented in Table 2. The 15 percent or more Fe of primary origin (depositional or chemical compositions of the rocks (ironstones) exhibit diagenetic), are commonly referred to as ironstone or iron- relatively wide variations, especially Fe2O3*, which is formation (James 1996; Al-Bassam and Tamar-Agha comparatively very high with a wide range (61.71–73.87 1998). It is a very signiﬁcant phenomenon that iron has wt%). The rocks also show wide variations of SiO2 from extraordinary capability to form different minerals in 6.46 (sample MR-6) to 24.46 wt% (sample MR-7), with an response to different depositional and diagenetic

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Fig. 4 Photographs showing ironstones of Kahaloo, Bogra (a, b: siderite-goethite-enriched ironstone, streak slightly light yellow or gray in color, present light spots indicating mixtures of quartz or mud), and Mohanpur, Rajshahi (d, e: goethite-siderite-enriched ironstone, streak brown to yellow brown or orange in color, present few light spots also indicating mixtures of quartz or mud)

environments. In general, the classification of the iron-rich ironstones have considerable amounts of bementite-types rocks is based in large part on the mineralogy of the iron, minerals or clastic quartz (SiO2). These ironstones also the character, composition, and occurrence of the iron have exceptionally higher MnO (3.8–11.68 wt%) with an minerals found in sedimentary rocks (James 1996). Usually average value of 7.66 wt%, which signify that these rocks mineral assemblages of ironstones vary widely, with abide considerable amount of manganese minerals. hematite and goethite being the main components of red Accordingly, to concentrate Mn into significant sedimen- deposits (Young 1989) and siderite being the most tary deposits, manganese needs to be oxidized to Mn(III) or important constituents of sedimentary ironstones (James Mn(IV) (Calvert and Pedersen 1996). These ions form 1996). Most ironstones contain three different types of iron oxyhydroxide minerals that are deposited in sediments minerals; oxides and oxyhydroxides, carbonates and sili- (Calvert and Pedersen 1996; Armstrong 2008). Conse- cates. Based on petrography and mineralogy, the studied quently, the presence of significant manganese content in rocks seem to have dominantly iron-carbonates, iron-ox- the sedimentary record should imitate the history of man- ides and considerable amount of iron-silicate (e.g., ganese oxidation (Johnson et al. 2016). These are the bementite). In these ironstones, especially siderite or goe- common behaviour of major oxides in ironstones: usually thite have higher contents of Fe2O3* (60–75 wt%) and Fe2O3* and CaO show strong linear negative trends and Al2O3 (3–6 wt%) (James 1996; Al-Bassam and Tamar- Al2O3 displays positive trends with increasing SiO2 Agha 1998). In these regards, comparing these studies with (Fig. 6). Besides, MnO and Al2O3 have strong positive and the geochemical data of seven (7) samples of the study negative correlation, respectively, with increasing Fe2O3*, areas, the constituent minerals of these rocks are mostly while CaO shows an insignificant positive trend (Fig. 6). siderite and goethite, with few iron-manganese silicates. These trends demarcate significantly increasing iron and Although goethite usually contains about 5–10 wt% and manganese minerals, as well as a few calcium minerals siderite shows \ 2 wt% SiO2 (James 1996), the studied during hydraulic fractionation (Hossain et al. 2014). It is geochemical data show about 6.46–24.46 wt% (avg. very remarkable that all ironstones show relatively high Cr

15.72 wt%) SiO2, which is higher than the normal goethite contents (375–974 ppm). It is noted that dominantly pre- and siderite (James 1996), which means the studied sent siderite and goethite with few clay minerals may serve

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Fig. 5 Petrographic microphotographs showing ironstones of Kahaloo, Bogra (a plane polarized photograph showing burflower type matrices of light and yellow, orange or green color minerals, b cross nicol photograph also showing similar textural pattern with red/orange or deep green birefringences, c reflected light showing some developed metallic crystals of iron minerals) and Mohanpur, Rajshahi (d plane polarized photograph showing caviar type matrices of spotted light and yellow, or green color minerals, e cross nicol photograph also showing similar textural pattern with light green birefringences, f reflected light showing very few developed metallic crystals of iron minerals), Bangladesh

Table 2 Major and trace Oxides wt% KB 1 KB 2 KB 3 MR 4 MR 5 MR 6 MR 7 SD SE element compositions of the studied ironstones of Rajshahi SiO2 22.63 11.12 13.85 13.44 14.93 6.46 24.46 6.31 2.39 and Bogra Districts, Bangladesh TiO 0.20 0.18 0.19 0.17 0.22 0.09 0.23 0.05 0.02 with the corresponding 2 uncertainties Al2O3 4.35 3.95 4.12 3.95 4.96 2.12 4.54 0.90 0.34 Fe2O3 63.70 66.62 66.84 69.09 64.33 73.87 61.71 4.01 1.52 MgO 0.96 0.10 1.11 1.07 1.17 0.86 0.96 0.10 0.04 MnO 4.26 10.73 7.62 7.01 8.46 11.68 3.82 2.97 1.12 CaO 1.89 4.91 4.29 3.35 3.98 3.50 2.08 1.11 0.42

Na2O 0.32 0.08 0.17 0.20 0.32 0.05 0.36 0.12 0.05

K2O 1.16 0.71 0.82 0.99 1.14 0.36 1.23 0.31 0.12

P2O5 0.26 0.49 0.73 0.54 0.24 0.81 0.31 0.23 0.09 Elements (ppm) Cr 799 375 651 681 551 391 974 – – Zr 85 39 63 46 67 na 122 – – Ni 97 171 300 na na 237 129 – – Co Na 124 na na na na Na – – Rb 54 na na na 77 na Na – – Sr Na na 64 46 48 na Na – – Y Na 56 52 42 Na 107 39 – – S 430 206 273 247 302 255 369 – – Cl 154 485 288 335 291 422 143 – – *na not analyzed

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Fig. 6 Harker major variations (in wt%) diagrams for the investigated ironstones with error (uncertainty) bars (symbol: diamond: samples from Kahaloo, Bogra, and square: samples from Mohanpur, Rajshahi) as host minerals for Cr. The Cr/Ni ratio in the ironstones is paleoclimatic deposits, which are common in several semi- high and variable. The relatively higher Cr/Ni ratio in the arid regions of the world (Lal et al. 2010). These types of studied ironstones is due to the association of Cr with clay ironstones also developed under temperate humid climatic (Tobia et al. 2014). Rubidium and strontium contents are conditions, typical of the post-glacial period (Dill and comparatively medium or slightly depleted with UCC and Techmer 2009). The comparatively light yellow color PAAS (Hossain et al. 2014, Table 2). Most of the samples ironstones in Bogra (Fig. 4a, b) may be siderite-enriched, have a considerable amount of S (206–430 ppm), which which may be developed mostly syndiagenetically under indicate higher sulphur oxidation that increased iron-sulfur reducing conditions. However, relatively higher reddish/ minerals (Jørgensen and Nelson 2004) in ironstones. High brown ironstones in Rajshahi (Fig. 4c, d) area indicates Cl (143–485 ppm) suggests more marine input in sedi- goethite-enriched, which may be developed syndiageneti- ments. Generally, higher Cl contents prove that the studied cally under oxidizing conditions (Dill and Techmer 2009). rocks have undergone evaporation at a greater rate than the From the geochemical data of the ironstones, the overall marine incursion (Worden 1996). chemical composition of the rocks shows comparatively

wide variations; Fe2O3* is quite high (avg. 66.6 wt%), indicating the constituent minerals of these rocks are 7 Discussions and conclusions siderite and goethite. However, the average percentage of

SiO2 is comparatively higher (15.72 wt%) than the com- Major lithofacies of the studied boreholes are clay, fine to mon siderite and goethite, indicating the presence of rela- coarse grained sands, sandy gravel, and sand with iron- tive amounts of manganese-rich silicate bementite or stones. Clay to fine sand lithofacies suggest their deposi- clastic quartz (SiO2). It is noted that manganese-rich sili- tional conditions as mainly low energy conditions, gently cate formation is usually restricted to marine environments sloping intertidal areas or shallow marine neritic environ- wherein supply of iron (Fe) is high, conditions are suboxic, ment. Medium sands are deposited in deltaic or shore/ and, most important, sediment influx is very low (Roy beach environments, whereas coarse sand suggests some- 1968; Curtis et al. 1975). where near the source area of the river within or near the The high Cr contents usually found in ironstones active channel, where the current energy is essentially high developed from basic and intermediate igneous rocks and found in a fluvial environment. Sandy gravel indicates (Aubert and Pinta 1977). Generally, Co and Ni occur the enormous fluctuation of energy within a continental together with Fe in parent rocks, as Fe is more easily environment. Sand with ironstone lithofacies may be oxidized to Fe3? and readily precipitates. Rubidium and

123 412 Acta Geochim (2019) 38(3):404–413 strontium are depleted, which indicate these elements are goethite with clayey mud and quartz became as dry and due to complete leaching of their host minerals during compacted as ironstone. Later, these ironstones were sub- lateritization (Al-Bassam and Tamar-Agha 1998). These jected to weathering and transported to form the present ironstones also confirm significant amounts of MnO (avg. ironstones. 7.66 wt%), suggesting their depositional environments under oxygenated condition. Moreover, the presence of Acknowledgements The authors woud like to thank the Director, significant manganese content in the sedimentary record Institute of Mining, Mineralogy and Metallurgy (IMMM), Joypurhat, Bangladesh for his cordial support to us carrying out our geochemical should imitate the history of manganese oxidation (Johnson and mineralogical analyses using XRF and XRD in the IMMM lab- et al. 2016). However, there are a couple exceptions to this oratory. The authors are grateful to Prof. S. Kabir for his effective general relationship where manganese enrichment in the proofreading of the manuscript and special thanks to M.A. Rahman sediments signifies manganese oxidation above the redox for his kind support. We are also grateful to F.H. Tobia and an anonymous reviewer for their constructive reviews and comments. boundary (Krauskopf 1979; Bellanca et al. 1996). When there are high levels of soluble Mn2? (generally low Eh conditions trigger the production of reduced, soluble forms), this divalent ion can substitute for Ca2? in authi- References genic carbonate phases. Due to the high-potential redox chemistry of Mn, the geologic record of manganese Al-Bassam KS, Tamar-Agha MY (1998) Genesis of the Hussainiyat deposits should reflect the ancient oxygen availability and ironstone deposit, Western Desert, Iraq. Miner Depos 33:266–282 the paleo-environmental chemistry (Bellanca et al. 1996; Armstrong FA (2008) Why did nature choose manganese to make Maynard 2010). oxygen? Philos Trans R Soc B 363:1263–1270. https://doi.org/ The genesis of iron-bearing sediments has been one of 10.1098/rstb.2007.2223 the most debated topics in sedimentary petrology (Pettijohn Aubert H, Pinta M (1977) Trace elements in soils (developments of soil science 7). Elsevier, Amsterdam 1984). The deposits were derived from a variety of parent Bellanca A, Claps M, Erba E, Masetti D, Neri R, Premoli-Silva I, rocks i.e., metamorphic rock, basic and intermediate Venezia F (1996) Orbitally induced limestone/marlstone igneous rocks and pre-existing sediments of the surround- rhythms in the Albian-Cenomanian Cismon section (Venetian ing areas (Ferdousy 2011; Hossain et al. 2014). 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Iron minerals (Fe-carbonate, Fe-oxides, Mn-sili- and origin of a concretionary siderite sheet (clay-ironstone band) cates) mostly originated from bacterial build up in in the Westphalian of Yorkshire. Miner Mag 40:385–393 Dill HG, Techmer A (2009) The geogene and anthropogenetic impact (mangrove)-swamps and marshes and were subsequently on the formation of per descensum vivianite–goethite–siderite embedded in clayey mud. Iron is present in a number of mineralization in Mesozoic and Cenozoic siliciclastic sediments mineral species (siderite and goethite) in sediments and in SE Germany. Sediment Geol 217:95–111 these minerals also display different reactivities (Poulton Ferdousy MF (2011) Paleoenvironments of Bengal Basin during Holocene as interpreted from geophysical and biogeochemical et al. 2004). The primarily formed carbonate in muddy studies: [Dissertation] M.Phil. thesis, Department of Geology 2? 2? 2? sediments is rich in Fe and Mn , and poor in Mg and and Mining, University of Rajshahi, Bangladesh Ca?, which is established rapidly in reducing conditions Goodbred SL Jr, Kuehl SA (2000) The significance of large sediment (Curtis et al. 1975). The Late Quaternary coastal stratig- supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of raphy of Bangladesh was characterized by alternating strata the Ganges-Brahmaputra delta. Sediment Geol 133:227–248 of organic and clastic minerogenic origin. The sedimentary Goto A, Tatsumi Y (1994) Quantitative analysis of rock samples by sequence of different areas in the Bengal basin reveals a an X-ray fluorescence spectrometer (I). 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