Stud. Geophys. Geod., 53 (2009), 111−131 111 © 2009 Inst. Geophys. AS CR, Prague

COMPOSITION AND PROPERTIES OF THE IRON HYDROXIDES - CEMENTED LENSES IN ESTONIAN SANDSTONE OF MIDDLE DEVONIAN AGE

ALLA SHOGENOVA1, ANNE KLEESMENT1, ANN HIRT2, ENN PIRRUS3, TOIVO KALLASTE1, KAZBULAT SHOGENOV1 AND REIN VAHER1

1 Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn,

Estonia (alla@gi.ee)

2 Institute of Geophysics, ETH Zurich, Schafmattstrasse 30, CH-8093 Zurich, Switzerland (hirt@mag.ig.erdw.ethz.ch)

3 Department of Mining, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia (pirrus@starman.ee)

Received: August 17, 2007; Revised: January 2, 2008; Accepted: April 11, 2008

ABSTRACT

The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history of Martian iron hydroxide concretions.

A case study of iron-rich concretions from Estonian Middle Devonian sandstones exposed in ancient river valleys in southeastern Estonia was carried out based on the results of mineralogical, petrographical, geochemical, petrophysical and magnetic analyses. It was found that the high Fe2O3(total) content (25.0−39.5%), high magnetic susceptibility, bulk and grain density, very low porosity, corrosion and fracturing of the quartz grains of the platy iron concretions are in contrast with properties of the Devonian host sandstones. However the ferrous iron content (measured as FeO) of iron-rich concretions was as low as in the other Devonian rocks, suggesting an oxidizing environment and arid climate during the cementation by iron-hydroxides. The fracturing of quartz grains cemented by iron hydroxides could take place at near-surface conditions including vadose and phreatic zones in arid climate with high evaporation rates. Such climatic conditions have been reported for the Baltic region during Devonian, Upper Permian and Triassic times.

We have found that goethite is prevalent in the cement, replacing clay and carbonate minerals. We assume that this iron-rich cement is originated from the mobilization of iron in host sandstones by groundwater, associated with tectonic activity at the end of the Middle Devonian, evidenced by fracturing in Devonian outcrops and caves. Although this mobilization could occur under reducing conditions, precipitation of goethite and hematite for the cementation could take place in oxidizing environment along bedding planes close to the surface during short sedimentation breaks. Another possible time for the formation of iron concretions could be Permian, under the condition of both arid climate and tectonic activity.

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Ke y wo rd s : Middle Devonian, iron hydroxides, sandstone, hematite and goethite concretions, chemical composition, mineralogy, magnetic susceptibility, density, porosity

1. INTRODUCTION

Iron-hydroxides concretions in sandstones have been described in some regions of the world. Iron layers of different location, called “hilltop”, “foothill” and “groundfroor” iron layers, and thickness (corresponding to 20 m, up to 1 m and 5−10 cm) enclosed by “barren” sandstone of the Bahariya Formation of Cenomanian to Eocene age in Bahariya Oasis in Egypt are described by Mücke (1994).While “hilltop” iron layers capp the top of cone-shaped residual hills, “foothill” iron layers are found at the flanks as well as at different levels of the residual hills. “Groundfloor” iron layers locally cover the eroded base of the Bahariya depression. These three types of iron layers also differ in iron content, quartz grain size, goethite-hematite proportion, ilmenite alteration and graphite content, suggesting modifications of the conditions of formation and subsequent alteration. Hilltop iron layers exibit a brown to dark-brown color and composed mainly of goethite. Groundfloor and foothill iron layers are represented by hematite and are generally red-colored. Foothill iron layers were formed when iron-bearing solutions of meteoritic water-origin penetrate the porous sandstone and move downwards by gravity. When sandstone, underlain by an impervious horizon, is exposed to atmospheric oxygen, percolating waters encounter an oxidation barrier (Perelman, 1972), which penetrates into the sandstone in a tongue-like form (Mücke, 1994). Hilltop iron layers were classified along with the foothill layers, as stratabound oxidation products. Mücke (1994) links the formation of hilltop iron layers with tectonics. Fractures developed during formation of anticlines in the Baharaiya Oasis facilitate the movement of supergene solutions and increased the amount of solutions in the beds, resulting in leaching and a higher concentration of precipitated iron. Mücke (1994) postulated that the iron in the groundfloor iron layers could have originated from groundwater and was transported to the surface by capillary action. This led to the evaporation of water close to the surface and the precipitation of ferric hydroxides. Mücke concluded in his “ferruginization model” that iron was mobilized postdiagenetically from decomposing detrital oxides (ilmenite and magnetite) under reducing conditions and iron was precipitated along the oxidation front.

Other studies have reported the diagenetic formation of hematite and goethite concretions in bands of different thickness (from a few millimeters to tens of meters) in Jurassic eolian sandstones in southeastern Utah, USA (Chan et al., 2000, 2001; Eichhubl et al., 2004; Kwon et al., 2005; Ahmadov, 2006; Ahmadov et al, 2007; Busigny and Dauphas, 2007). These bands are associated with faults, and the iron oxides precipitated as cement to form concretions in the porous sandstones. They are thought to have formed when saline reduced brines were mixed with shallow oxygenated groundwater. The interest in the study of hematite and goethite concretions during the last years has increased due to the investigation of Martian Blueberries (Beitler et al, 2004, 2005; Busigny and Dauphas, 2007). The precipitation of hematite cement is also considered to be connected with burial history as reported by Khidir and Catuneanu (2003) from South-central Alberta, Canada.

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Stud. Geophys. Geod., 53 (2009) 113

In Estonia lens-shaped interlayers of strongly cemented platy iron-oxide-rich sandstone are found in a loose yellowish sandstone of the Middle Devonian Burtnieki Regional Stage in the Ahja and Lutsu ancient river valleys in the south-eastern part of the country. Their occur in a limited area extending for approximately 10 km from East to West and 8 km from North to South. The iron-rich lenses are composed of 2–5 cm thick bands within 5–10 cm platy concretions having a diameter up to 1.3 m. The lenses may also contain interlayers of weakly cemented red-coloured sandstone between the strongly cemented plates. In contrast to the weakly cemented Burtnieki sandstones (containing 5% clayey cement), found in outcrops and boreholes, iron-rich sandstones are cement-supported. The cement, which is composed of hematite and goethite, forms 27−40% of the rock in the middle part of the lens, while the cement containing only goethite occurs in the upper and lower parts of the platy concretions. The sandstone consists mainly of subrounded to angular, often fractured quartz grains, with the grain size decreasing from East to West (extent 8 km). Strongly cemented iron-rich sandstone was found earlier in the form of weathered and partly rounded platy concretions and more rarely ball-shaped nodules in the glaciofluvial gravel deposits in the Quaternary cover of the same region. These concretions are often represented by coarse angular clasts with a many-storeyed frame texture. This frame and cores of some rounded nodules contains soft sand. Mineralogical composition of the sand is indentical to that of the sediments of the Burtneki Stage exposed abundantly in the Ahja River Valley. The cement in these concretions is made up of goethite. The nodules were interpreted as fragments of big concretions, occurring rarely in sandstones of the Burtneki Stage. The abundance of these concretions in the Quaternary cover of this region was explained by the zigzag outline of the Ahja River Valley North of Põlva and by extensive contact of moving glaciers with the exposed Devonian bedrock. Weakly cemented sandstones broke and crumbled rapidly, especially during the glaciofluvial process, while hard concretions were preserved and are enriched in the pebble fraction of gravel (Kleesment et al., 1993). The influence of diagenetic processes on iron-bearing minerals and genesis of the red-colouring in Devonian carbonate and siliciclastic rocks was discussed by Shogenova and Kleesment (2006) using borehole data. Primary colouring of Devonian rocks were red and removal of Fe from the sandstones of the Burtnieki Formation took place during diagenesis. The Devonian sediments of the Butneki Stage consist mainly of red-coloured material. The mineral composition of the red and diagenetically altered colourless sandstones is similar. Changing of red to colourless sandstones is connected with underground water activity (Kurshs, 1975; Kurshs et al., 1981).

The present study concentrates on the genesis of these iron hydroxide-rich sandstones, which have only been found in outcrops, and not in boreholes. Iron-rich concretions in Devonian rocks of the other regions of the world have been rarely found and described.

2. GEOLOGICAL SETTING

Devonian rocks found in southern and south-eastern Estonia (Fig. 1) are represented by alternating siliciclastic, mixed carbonate-siliciclastic and dolomitized carbonate rocks (Kleesment and Shogenova, 2005, 2006). The Burtnieki Stage with a total thickness of

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60 − 95 m occurs in south-eastern Estonia, and crops out in a 25–30 km wide belt (Fig. 1). It is mainly represented by light (white, yellowish, pinkish and greyish-brown) fine-grained (0.1−0.25 mm) medium to weakly cemented cross-bedded sandstones with interlayers of siltstone and clay. A regressive trend of the marine basin during the Burtnieki Stage dominated and near-shore facies and delta sediments were deposited in the region under consideration (Kleesment, 1997, Fig. 153B). The Burtnieki Stage (total thickness 60–94 m) consists of the Härma (13–30 m), Koorküla (20–32 m) and Abava Beds (15–32 m) (Table 1). Their composition changes from relatively coarse-grained sandstones at the base progressing to clayey silt layers at the top of the bed sections. Iron-

Fig. 1. Map of Estonia with distribution of Devonian stratigraphical units. Middle Devonian Pärnu (D2pr), Narva (D2nr) and Gauja-Amata Stages (D2gj-am) and Upper Devonian Plavinas Stage (D3pl) are shown in grey. Distribution of Burtneki Stage is shown by line pattern, studied boreholes are shown by black circles, sampled Kooskora and Lutsu outcrops by black squares, outcrops with fracture systems in sandstone by triangles (Kleesment and Pirrus, 2000; Pirrus et al., 2002; Kleesment et al, 2003).

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rich platy concretions occur in the lower Härma Beds, and these crop out in a well-defined area bounded to the East by the Ahja River and Lutsu Brook and to the west at the vicinity of Tõrva, Helme and Karksi (Kleesment, 1995). The strongly cemented Fe-rich platy-concretions are found only in a restricted area of Ahja and Lutsu (Figs. 1 and 2). Iron-rich sandstone concretions of similar character were found in Quaternary deposits in the same region (Kleesment et al., 1993).

The main mineral constituent in the Härma beds is quartz (70−90%) (Fig. 3), with 3−10% feldspars (mainly orthoclase), and 1−8% mica. Ilmenite is the most abundant heavy mineral (45−65%), followed by leucoxene which makes up 5−10% of the heavy minerals, and Fe-hydroxides, which have a variable content between 0.1−50%. The mud fraction (< 0.01 mm) consists of illite (50−70%) and kaolinite (30−50%) (data by Kleesment, 1995).

Table 1. Devonian Stratigraphy and lithology. Lithological column modified after Kleesment (2007).

Series Stage Regional

Stage Formation

Member/ Beds*

Age [Ma]

Lithology

Upp

er

Frasnian

Daugava Daugava 374.5 ± 2.6 lst Dubniki Dubniki dst

Plavinas Plavinas Izborsk dlm-mst, Snetnaya Gora 385.23 ± 2.6 dst, st-dst

Mid

dle

Givetian

Amata Amata slt, brc, snd

Gauja Gauja Lode

snd Sietini

Burtnieki Burtnieki Abava* slt, snd

Koorküla* snd Härma* snd

Aruküla Aruküla

Tarvastu* cl-slt, snd Kureküla* snd Viljandi* 391.80 ± 2.7 slt, snd

Eifelian

Narva Narva

Kernave cl-slt, snd

Leivu dlm-mst, slt

Vadja brc

Pärnu Pärnu Tamme snd

Tori 397.50 ± 2.7 snd

Low

er Emsian Rezēkne

Lemsi Rezēkne

407.00 ± 2.8

dlm-mst, dst snd

Pragian Kemeri Kemeri 411.20 ± 2.8 snd Lochkovian Tilžē Tilžē 416.00 ± 2.8 snd

Lithology: lst – limestone, dst – dolostone, dlm-mst – dolomitic marlstone, st-dst – silty dolostone, snd – sandstone, slt – siltstone, cl-slt – clayey siltstone, brc – breccia.

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Orientation of fractures in the Devonian sandstones outcropping in South Estonia was determined recently. Fracture systems with dominant NE-SW direction and 10 – 20 m distance between fractures were observed in caves as 5–15 m stretches and in outcrops as fissures reaching 40 m in horizontal direction (Kleesment and Pirrus, 2000, Fig. 1). Fracturing of the Burtnieki sandstones was studied in the outcrops of the Ahja River,

Fig. 2. a) Kooskora outcrop. 1 is a location of sample Kooskora 1 and 2 is location of sample Kooskora 2. b) Detail of a).

a)

b)

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where fractures of 15–20 m in horizontal and 1.5–3 m in vertical directions were determined. Orientation of the prevailing joint system in the Burtnieki Formation with the azimuth bearing of strikes 287° and 6° fit well with their orientation in the underlying Aruküla Formation, and was explained by a probable regional tectonic origin of the fractures in Devonian sandstones. An average fracture spacing of 15–20 m is prerequisite for groundwater circulation along the blocks of rocks (Pirrus et al., 2002). Tectonic and groundwater activity along fractures during the Devonian is also supported by the study of clastic dikes in Middle Devonian sandstones of the Gauja Formation in southeastern Estonia. Mineralogical and geochemical study of the dikes support their formation during short sedimentary breaks at the end of the Middle Devonian (Kleesment et al., 2003).

3. SAMPLES AND METHODS

Iron-rich sandstones have been found only in the Kooskora quarry (Ahja valley) and in outcrop at the Lutsu valley. Two samples from Kooskora quarry were studied (Kooskora 1 and Kooskora 2, Fig. 2). A third sample (Lutsu 5/Kauksi) from an outcrop at the Lutsu Brook, which no longer exists, was taken from the collection of the Institute of Geology, Tallin University of Technology (IG, TUT). The fourth sample (Lutsu 1) was found on the ground in the region of the Lutsu quarry where Burtneki sandstones outcrop; this sample is the most weathered one.

These four samples were compared to 93 Devonian siliciclastic and carbonate-siliciclastic samples from earlier and ongoing studies (Shogenova et al., 2004; Kleesment and Shogenova, 2005; Shogenova and Kleesment, 2006). The magnetic, mineralogical and chemical properties of these rocks were studied in detail.

The bulk chemical composition of the 97 samples (Shogenova and Kleesment, 2006) was studied by X-ray fluorescence analysis in the Allrussian Geological Institute, St. Petersburg, Russia, using the method described in Teedumäe et al. (2006). The insoluble residue (IR), MgO, CaO and iron forms were additionally determined by chemical analysis using gravimetric (IR) and titration (CaO, MgO, FeO and Fe2O3(total)) methods in the Institute of Geology, at Tallinn University of Technology (IG, TUT).

Insoluble Residue: An amount of 50 mL of HCl (1:4) was added to 0.5 g of rock powder and dried in a water bath. The remaining residue was mixed with 5 mL of concentrated HCl and 30−40 mL of hot water (90−100°C), filtrated, washed with hot water (90−100°C), and this filtrate is used for analysis of CaO and MgO concentrations The filtrated residue was heated to 1000°C for one hour and then weighed.

CaO: An amount of 200 mL of water (20°C), 0.1 − 0.2 g of sucrose and HCl (1:1) by drops were added to 25−50 mL of the filtrate until pH = 1.5−2.0. Then 2 mL of hydroxylamine (NH2OH⋅HCl, 5%) was added to the solution. Next 25 mL of KOH (25%), 10 drops of Na2S (5%) and 0.1−0.2 g of 1% mixture of fluorexon disodium salt (C30H24N2Na2O13) in KCl were added and the solution was titrated with 0.02 molar solution of trilon B (C10H14N2Na2O8⋅2H2O) until the green colour disappeared. The trilon B solution volume used for the CaO titration is determined using a standard solution of 1.22 g/L CaO. The titration result is recalculated into weight percent of CaO.

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CaO and MgO: An amount of 200 mL of water (20°C) and HCl (1:1) by drops were added to 25 mL of the filtrate until pH = 1.5−2.0. Then 2 mL of hydroxylamine (5%) was added. An amount of 15 mL of ammonia buffer solution with pH = 10 and 8−10 drops of Na2S (5%) were added to the solution and in 2 min 0.1−0.2 g of indicator chromogen black was added. The solution was mixed and titrated by 0.02 molar solution of trilon B until the dark red colour of the solution was changed to blue. The trilon B solution volume used for the CaO + MgO titration is determined and the trilon B solution volume used for CaO titration is substracted from this amount. The remainder is recalculated using a standard solution of MgO 0.8062 g/L. The result is recalculated into weight percent of MgO.

FeO: The concentration of ferrous iron (FeO) was determined photocolorometrically with α-α dypirisol (C12H8N2). Fe2+ forms with α-α dypirisol an orange-red coloured water soluble complex at pH 4. To reach pH 4, a buffer solution of sodium acetate trihydrate (CH3COONa⋅3 H2O) and 99% of acetic acid (CH3COOH) is added. It is stable with respect to oxygen and over long time periods. Even at low concentration the complex is a strong colouring agent (strongly depends on Beer׳s law for wide ranges of Fe2+ concentrations). The maximum photo absorbability is at a wavelength of 505 nm. Silicate rocks and minerals were decomposed by a mixture of HF and H2SO4. The solution was boiled for 5 min so that the carbonate rocks were decomposed by H2SO4. The value of pH 4 was controlled by a saturated H3BO3 solution. This method allows the FeO concentration to be determined in the range of 0.1 to 20% in samples with a mass between 0.1 to 0.5 g.

Fe2O3(total): An amount of 30 mL of HCl and HNO3 mixture (1:3) was added to 0.5 g of rock powder and dried in a water bath. The remaining residue was mixed with 5 mL of concentrated HNO3 and 30−40 mL of hot water (90−100°C), boiled, filtrated and

washed with hot water (90−100°C). An amount of 150 mL of water (room temperature), 1.5 mL of C7H6O6S⋅2H2O (20%) and NH4OH (1:1) (added by drops until violet colour

changed into yellow) were added to 50−100 mL of the solution and boiled together with 10 ml of one normal HCl. The solution was heated to 50−60°C and titrated by 0.02 molar solution of C10H14N2Na2O8⋅2H2O. The Fe2O3 content was determined by the formula Fe2O3(total) − 1.1113 FeO.

Mineralogical composition was studied by X-ray diffractometry and thin-section analysis in IG, TUT. The X-ray diffractometer was an HZG4 instrument (Fe-filtered Co Kα radiation). A powdered sample was mixed with alcohol and spread on a glass slide. For identification of minerals, an angular range of 20 − 45° 2θ was step-scanned (counting time 2 s, step of 0.02° 2θ). X-ray diffractograms of iron-rich sandstones were analyzed for the presence of goethite, hematite, magnetite, maghemite, lepidocrocite and quartz (Fig. 3).

Thin sections were studied under the polarized microscope with up to 200 times magnification and using scanning electron microscope JEOL JSM-840 A. An accelerating voltage of 20 kV and secondary electron imaging was used. Energy dispersive spectroscopic (EDS) analysis and X-ray line scan in the Link Analytical AN1000 analyser was performed. The mineralogical composition was evaluated qualitatively.

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Kooskora 1, an iron-rich sandstone, was analysed together with other Devonian siliciclastic and carbonate-siliciclastic samples in the paleo- and rock magnetic laboratory at ETH, Zürich (Switzerland). The IRM (Isothermal Remanent Magnetization) measurements were done on a 3-axis 2G Enterprises Superconducting Quantum Interference Device (SQUID) magnetometer with a sensitivity level of 1 × 10−11 Am2. The IRM was acquired in an ASC Scientific Pulse Magnetizer Model IM-10-30, where the sample was exposed to a pulsed DC field. First, a pulse of 1250 mT was applied to the sample and the magnetization was measured. Then, the sample was turned 180° and fields up to 1250 mT were given in the opposite direction. After each pulse, the remanent magnetization of the sample was measured on the cryogenic magnetometer. Thermal demagnetisation of a three-component IRM was investigated after an IRM with a field of 1.2 T was applied along the sample z-axis (hard component) followed by an IRM with a field of 0.5 T along the sample y-axis and 0.2 T component along the sample z-axis (soft component) (Lowrie, 1990). The low-field susceptibility was measured on an AGICO KLY-2 susceptibility bridge at room temperature (RT) and liquid nitrogen temperature (LNT) in order to estimate the contribution of the paramagnetic minerals to the total susceptibility. If only paramagnetic minerals contribute to the bulk susceptibility (κ), it should vary inversely with temperature, according to the Curie-Weiss Law. Assuming a paramagnetic Curie temperature of 0 K, the ratio of κ at LNT to κ at RT (κ77/κ293) would be 3.8 for a purely paramagnetic sample.

Magnetic susceptibility of rock samples was measured at the petrophysical laboratories of Helsinki University (Shogenova et al., 2006), Leoben University and St. Petersburg University (Shogenova et al., 2005, 2007) using cubes 24 × 24 mm or one inch-diameter cylinders. Porosity and density were measured using water saturation methods in these laboratories and in the Institute of Geology, Tallinn University of Technology (IGTUT). For density measurements samples were dried at a temperature of 100°C and the weight of dry samples (Pd) was determined. Then the samples were saturated with water under vacuum for 24 hours (in Helsinki University and Leoben University) and under normal pressure during one week (in St. Petersburg University and IGTUT) and after that weighed in air (Pw). From the obtained measurements the following parameters were calculated: dry density δd = Pd/V, where V represents sample volume calculated from sample size; wet density δw = Pw/V; effective porosity

ϕ = (Pw − Pd)/Vρ0, expressed in %, where ρ0 is water density (103 kg/m3), and effective density δe = Pd / [V (100 − ϕ)/100], which is close to grain density δg = Pd / Vg , where Vg represents grain volume.

The physical properties and thin-sections of the four iron-rich samples were studied and compared with other Devonian siliciclastic rocks and mixed carbonate-siliciclastic rocks that contain more than 50% insoluble residue, and that are taken from one sedimentary cycle beginning from the Kernave Formation of the Narva Stage (D2nr, Fig. 1, Table 1). Altogether magnetic susceptibility of 68 cemented rocks samples and density and porosity of 38 samples (which were not destroyed during water saturation) were determined.

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4. RESULTS

4 . 1 . P e t r o g r a p h y a n d M i n e r a l o g y

The host rock shows well-developed bedding surfaces, along which the iron-rich interlayers are well-defined by their contrasting colour and cementation (Fig. 2A,B). In contrast to the common weakly cemented Burtnieki sandstone, which has 5% of clayey cement (mainly illite and 20−40% of kaolinite), the strongly cemented iron-rich sandstone layers are matrix-supported. The former grain-supported fabric was replaced by a matrix-supported during cementation of sandstones by iron hydroxides. The cement forms 27−40% of the rock and is represented mainly by goethite. Hematite occurs only in the central part (about 5 cm) of the relatively thick lenses (Fig. 3A) and in platy concretions, which were subjected to oxidizing conditions while being close to the surface for a comparatively long time. Lepidocrocite was also recorded at these locations (Fig. 3C). Usually the strongly-cemented, iron-rich platy concretions are 2−3 cm thick and separated from each other by a pigmented layer of loosely cemented sand with thickness of some mm to 10 cm (Fig. 2B). The grain size of the detrital particles increases from West to East from very fine to fine-grained sandstone (0.07−0.15 mm) in the Kooskora quarry (Fig. 4A,B) to medium grained sandstone in Lutsu quarry (Fig. 4C) and Lutsu Brook (Fig. 4D). Their mineralogical composition is similar, however, over the study area (Fig. 3). The mineralogical composition of the detrital part of the sandstone is quartzarenite, which is common for the Burtnieki Stage. Grains are mainly quartz (85−90%), feldspar (< 10%) and muscovite (about 5%). Quartz grains are subrounded to

Fig. 3. Mineralogical composition by X-ray diffractometry. Kooskora 1a is taken from the middle more reddish part of the platy-concretion of the the Kooskora 1 sample. Kooskora 1b is taken from the darker part of the Kooskora 1 sample. Lutsu 1 is a sample found on the ground near Lutsu quarry. Lutsu 5 is a sample from Lutsu (Kauksi) outcrop.

20 25 30 35 40 45°2θ (Co K α)

Kooskora1aKooskora1bLutsu1Lutsu5

Goethite

QuartzGoethite

Hematite

QuartzQuartz

Goethite

Hematite

Hematite + goethite

K-feldspar

Lepidocrocite

ABCD

A

B

C

D

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angular (Fig. 4), in contrast to rounded to subrounded quartz grains of the Burtnieki host sandstones (Kleesment, 1995). Quartz grains in the iron-rich sandstones are often corroded and fractured (Fig. 4C,D; Fig. 5). Occasionally the grains are fragmented and mica flakes are often dispersed in the goethite cement (Fig. 5B).

4 . 2 . C h e m i c a l C o m p o s i t i o n

The bulk chemical composition of the Kooskora iron-rich sandstone measured by XRF analysis in two samples (Kooskora 1 and Kooskora 2) consists mainly of 50.2−50.7% of SiO2, 35.6−39.5% of Fe2O3(total), 2.22−2.89% of Al2O3 and 1.02−1.07% of K2O (Table 2, Fig. 6). Chemical analysis by the gravimetric and titration methods showed that the rock contained 51.52−54.48% insoluble residue, 0.44−0.76% CaO and 0.4% MgO (Fig. 6C). All iron occurs in the form of Fe2O3 (Fig. 6D), whereby the measured FeO

content was only 0.17−0.30%.

Fig. 4. Thin section micrographs of studied iron-rich sandstone samples Kooskora 1 (A), Kooskora 2 (B), Lutsu 1 (C) and Lutsu 5 (D). Detrital part of the sandstone is composed of quartz (85−90%), feldspar (< 10%) and muscovite (about 5%). Quartz grains are subrounded to angular, in contrast to rounded to subrounded quartz grains of the Burtnieki host sandstones (Kleesment, 1995). Quartz grains in the iron-rich sandstones are often corroded and fractured.

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The bulk chemical composition of iron-rich sandstones in samples Lutsu 5 (taken from Lutsu brook outcrop) and Lutsu 1 (found on the ground near Lutsu quarry) are very close to each other. They consist mainly of 64.4−66.1% SiO2, 24.3−24.8% Fe2O3(total), 1.39−2.49% Al2O3 and 0.42−0.61% K2O measured by XRF analysis (Fig. 6A,B). Chemical composition measured by the gravimetric and titration methods yielded 66.4−68.0% insoluble residue, 0.33−0.55% CaO and 0.24−0.55% MgO (Fig. 6C). The FeO content was only 0.03−0.25% (Fig. 6D). The highest Fe2O3(total) iron content (39.5%) was determined in the Kooskora 1 sample, while in Lutsu samples the SiO2 content was higher. In the all studied samples FeO content was low and the lowest FeO content (0.03%) was in the most weathered Lutsu 1 sample (Fig. 6D). Magnetite and maghemite were not identified by XRD in any of the iron-rich sandstone samples.

The iron-rich samples show contrasting chemical compositions compared to the other Devonian siliciclastic and mixed carbonate-siliciclastic samples as seen in Fig. 6A−D. Only the FeO content in these samples is in the same range as the other rocks (Table 2 and 3, Shogenova and Kleesment, 2006). The Al2O3 content in Kooskora and Lutsu 5

samples (2.22−2.89 and 2.49%) is at the lower limits for Devonian rocks (2.5%) while in Lutsu 1 (the most weathered sample) it is lower (1.39%) than the minimum value (Tables 2 and 3, Fig. 6B). The K2O content in the Lutsu samples (0.42−0.61%) is also

lower than the minimum value (1.02%) while in Kooskora (1.02−1.07%) it is at the lower limit (Fig. 6C).

4 . 3 . M a g n e t i c P r o p e r t i e s , D e n s i t y a n d P o r o s i t y

The magnetic susceptibility of the all four iron-rich sandstone samples measured in the Petrophysical laboratory of Leoben University was in the range of 34−60.9 × 10−5 SI (Table 2, Fig. 7A), whereby the 3 samples collected from outcrops (Kooskora 1, Kooskora 2 and Lutsu 5) were in the range of 40.6−60.9 × 10−5 SI. Grain density of these samples was 2.99−3.27 × 103 kg/m3 and their porosity was in the range of 3.64−4.84%.

Fig. 5. SEM photographs of sample Kooskora 2. A) Fracturing and corrosion of quartz grains (Q). B) Fragmentation of quartz grain (Q) and displacement of mica (Mi) flakes in a Fe-hydroxide (Fe) matrix.

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Stud. Geophys. Geod., 53 (2009) 123

The weathered Lutsu 1 sample has a grain density of 2.84 × 103 kg/m3 and a porosity of 14.6%. Kooskora 1 has the highest magnetic susceptibility and grain density values with a total iron content of 39.5%. Kooskora 2 has a slightly lower total iron content of 35.6% with a lower magnetic susceptibility as well, while Lutsu samples have an even lower Fe2O3(total) content (24.3−24.7%) and magnetic susceptibility compared to samples from Kooskora. The susceptibilities of all four samples, however are significantly higher than have other Devonian siliciclastic rocks (Tables 2 and 3, Fig. 7A).

The magnetic susceptibility of the Kooskora iron-rich sandstone was nearly the same at 293 and 77 K (58.8 and 58.0 × 10−5 SI, respectively), which suggests that only ferromagnetic phases dominate the susceptibility. The magnetic susceptibility of the other Devonian rocks was in a range of 2−29 × 10−5 SI (Shogenova et al., 2004) and increased by a factor of 2.7 to 7.8 at 77 K (Fig. 7A,B), which suggests that both paramagnetic - temperature dependent and diamagnetic - temperature independent minerals are contributing to the bulk susceptibility.

Table 2. Composition and properties of iron-hydroxide concretions.

Method Studied Parameter Iron-Rich Sandstone

Kooskora 1 Kooskora 2 Lutsu 1 Lutsu 5

Gravimetric Insoluble residue (%) 51.52 54.48 66.42 68.02

Titration

MgO (%) 0.4 0.4 0.55 0.24 CaO (%) 0.76 0.44 0.55 0.33 FeO (%) 0.3 0.17 0.03 0.25

Fe2O3(total) (%) 38.88 35.60 23.84 23.76

Calculations Fe2O3 (%) 38.55 35.41 23.81 23.48

XRF

SiO2 (%) 50.7 50.23 64.41 66.11 Al2O3 (%) 2.22 2.89 1.39 2.49 TiO2 (%) 0.205 0.192 0.07 0.082 MgO (%) < 0.05 0.56 0.37 0.4 CaO (%) 0.53 0.18 0.2 0.1

Fe2O3(total) (%) 39.50 34.78 24.76 24.32 MnO (%) 0.0827 0.256 0.411 0.029 K2O (%) 1.07 1.02 0.42 0.61 P2O5 (%) 0.245 0.122 0.101 0.352

Water saturation

Dry density (103 kg/m3) 3.15 2.98 2.43 2.86

Wet density (103 kg/m3) 3.15 3.01 2.57 2.9

Grain density (103 kg/m3) 3.15 3.09 2.84 2.99

Porosity (%) 0 3.64 14.62 4.27

Volume normalized

Magnetic susceptibility

(10−5 SI) 60.9 48.75 33.96 40.64

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124 Stud. Geophys. Geod., 53 (2009)

IRM acquisition was performed on selected siliciclastic rocks and one iron-rich sandstone (Kooskora 1) and subsequent thermal demagnetization was done only on the Devonian siliciclastic rocks. IRM acquisition and subsequent thermal demagnetization of the three-component IRM of the Devonian siliciclastic samples show that they can contain magnetite, and in the red-colored rocks they also have hematite. Goethite was not identified in the siliciclastic samples. IRM acquisition of the Kooskora sample showed

Table 3. Composition and properties of siliciclastic rocks from the boreholes. σ - standard deviation, N - number of samples.

Method Studied Parameter

Mixed Carbonate-Siliciclastic Rock

Siliciclastic Rock

Min Max Avg σ N Min Max Avg σ N

Gravimetric Insoluble residue

(%) 51.0 68.0 58.4 5.3 16 70.2 97.4 83.1 6.6 75

Titration

MgO (%) 4.4 8.6 7.3 1.4 8 0.3 4.6 1.8 1.2 54

CaO (%) 5.9 11.7 9.8 1.8 8 0.3 6.6 1.9 1.9 54

FeO (%) 0.01 0.27 0.14 0.07 16 0.02 0.4 0.14 0.09 75

Fe2O3(total) (%) 0.4 4.2 2.06 1.09 15 0.4 7.48 3.19 1.98 65

Calculations Fe2O3 (%) 0.39 4.12 1.92 1.05 15 0.33 7.36 3.05 1.96 65

XRF

SiO2 (%) 41.8 65.5 48.9 6.4 16 53.6 94.6 69.2 9.9 75

Al2O3 (%) 3.16 11.92 7.65 2.78 16 2.5 17.5 10.4 4.4 75

TiO2 (%) 0.14 0.58 0.42 0.14 16 0.14 1.05 0.65 0.6 75

MgO (%) 5.1 10.2 7.9 1.3 16 0.14 6.6 2.69 1.44 75

CaO (%) 5.9 16.1 11.0 2.3 16 0.05 10.8 2.61 2.69 75

Fe2O3(total) (%) 0.38 6.46 3.13 1.73 16 0.29 9.05 4.04 2.57 75

MnO (%) 0.07 0.21 0.13 0.04 16 0.003 0.11 0.045 0.025 75

K2O (%) 1.62 4.69 3.45 0.87 16 1.02 6.48 4.48 1.29 75

P2O5 (%) 0.03 0.68 0.24 0.19 16 0.01 0.4 0.13 0.09 75

Water saturation

Dry density (103 kg/m3)

1.71 2.50 2.27 0.21 13 1.5 2.36 2.03 0.2 50

Wet density (103 kg/m3)

1.81 2.57 2.39 0.24 8 2.03 2.44 2.04 0.12 26

Grain density (103 kg/m3)

1.9 2.73 2.59 0.28 8 2.42 2.73 2.61 0.09 26

Porosity (%) 6.66 17.53 12.16 3.71 8 10.6 37.14 23.8 5.9 26

Volume normalized

Magnetic susceptibility

(10−5 SI) 2.3 13.7 8.0 3.69 13 2.0 24.0 10.4 5.13 51

Composition and Properties of the Iron Hydroxides - Cemented Lenses in Sandstones

Stud. Geophys. Geod., 53 (2009) 125

a rapid increase in the IRM in low fields, and a further increase above 500 mT. The magnetization did not reach saturation in fields of 1200 mT. The increase in very low fields, suggests the presence of magnetite and/or maghemite, whereas the high field acquisition indicates the presence of hematite and/or goethite. The occurrence of the last two minerals is supported by XRD data (Fig. 3).

The rock porosity of the Kooskora 1 and Kooskora 2 samples was 0% and 3.6%, respectively, while for Lutsu 5 and Lutsu 1 samples, porosity of 4.27% and 14.6% was determined, respectively (Table 2). The bulk (wet) density of the Kooskora samples was 3.00−3.15 × 103 kg/m3 and that of Lutsu samples ranged 2.57−2.90 × 103 kg/m3 (Fig. 7D). The grain density of Kooskora samples was 3.09−3.15 × 103 kg/m3, while Lutsu samples had a lower grain density (2.84−2.99 × 103 kg/m3) (Table 2, Fig. 7C,D).

The Devonian cemented siliciclastic rocks usually have magnetic susceptibility between 2−24 × 10−5 SI and porosity in the range of 6.7−37.1%, bulk density in the range

Fig. 6. A) Fe2O3(total) content versus SiO2 content. Kooskora and Lutsu iron concretions measured by XRF analysis consist mainly of SiO2 and total iron. B) Fe2O3(total) content versus Al2O3 content. Al2O3 and K2O content in the concretions is low (Tables 2 and 3). C) MgO versus insoluble residue content. Iron concretions have anomalous to host rocks chemical composition with low content of MgO and CaO measured by the titration method. D) FeO content versus Fe2O3(total) content. Ferrous iron content measured as FeO by the titration method is in the same range as in the host rocks.

40 50 60 70 80 90 100

SiO2 (%)

0

5

10

15

20

25

30

35

40

45

Fe 2

O3t

otal

(%

)

Kooskora 1

Kooskora 2

Lutsu 1 Lutsu 5

A

0 2 4 6 8 10 12 14 16 18

Al2O3 (%)

0

5

10

15

20

25

30

35

40

45

Fe 2

O3t

otal

(%

)

Kooskora 1

Kooskora 2

Lutsu 5

Lutsu 1

B

50 60 70 80 90 100

Insoluble residue (%)

0

2

4

6

8

10

12

MgO

(%

)

Lutsu 1Lutsu 5

Kooskora 2

Kooskora 1

C

0 5 10 15 20 25 30 35 40 45

Fe2O3total (%)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

FeO

(%

)

D

Kooskora 1

Lutsu 5

Kooskora 2

Lutsu 1

A. Shogenova et al.

126 Stud. Geophys. Geod., 53 (2009)

of 2.10−2.48 × 103 kg/m3 and grain density 2.4−2.76 × 103 kg/m3 (Table 3, Fig. 7A,C,D), while the yellowish loosely cemented sandstones usually include less than 1% of total iron, their porosity is higher than 40% and density is lower than 2.1 × 103 kg/m3.

Fig. 7. A) Magnetic susceptibility versus Fe2O3(total) content with common correlation coefficient 0.96. Magnetic susceptibility and total iron content of samples from Kooskora outcrop are the highest. Two samples found in the Lutsu region, one from Lutsu brook outcrop (Lutsu 5) and (Lutsu 1) found on the ground near Lutsu quarry have very close values of Fe2O3(total) content and magnetic susceptibility. B) Magnetic susceptibility measured at room temperature versus measured at liquid nitrogen temperature. The magnetic susceptibility of the Kooskora iron-rich sandstone was nearly the same at 293 and 77 K, which suggests that only ferromagnetic phases dominate the susceptibility. The magnetic susceptibility of the other Devonian rocks increased by a factor of 2.7 to 7.8 at 77 K, which suggests that both paramagnetic - temperature dependent - and diamagnetic - temperature independent - minerals are contributing to the bulk susceptibility. C) Magnetic susceptibility versus grain density. Magnetic susceptibility of iron concretions (Kooskora 1, Kooskora 2, Lutsu 5 and Lutsu 1) has a high positive correlation with their grain density (Pearson correlation coefficient R = 0.96), while host rocks don’t show any correlation of these parameters. D) Bulk density versus porosity. Porosity and bulk density of iron concretions taken from outcrops (Kooskora 1, Kooskora 2 and Lutsu 5) are in contrast to the host rocks, while porosity and density of the weathered sample Lutsu 1 (found on the ground) are close to that in Devonian dolomite cemented siltstone and sandstone.

0 5 10 15 20 25 30 35 40 45

Fe2O3total (%)

0

10

20

30

40

50

60

Mag

netic

sus

cept

ibili

ty *

10-5

SI Kooskora 1

Kooskora 2Lutsu 5

Lutsu 1

A

10 20 30 40 50 60 70

Magnetic susceptibility *10-5 SILiquid nitrogen temperature

0

10

20

30

40

50

60

Mag

netic

sus

cept

ibili

ty *

10-5

SI

room

tem

pera

ture

Kooskora 1B

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2

Grain density *103 kg/m3

0

10

20

30

40

50

60

70

Mag

netic

sus

cept

ibili

ty *

10-5

SI

Lutsu 1

Lutsu 5

Kooskora 2

Kooskora 1C

0 5 10 15 20 25 30 35 40

Porosity (%)

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Bul

k de

nsity

*10

3 k

g/m

3

Kooskora 1

Kooskora 2

Lutsu 5

Lutsu 1

D

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Stud. Geophys. Geod., 53 (2009) 127

5. DISCUSSION AND CONCLUSIONS

Our previous studies have shown that the composition of Devonian siliciclastic and carbonate-siliciclastic rocks of Estonia is characterised by some common features (Kleesment and Shogenova, 2005; Shogenova and Kleesment, 2006), summarized as follows:

1. The MgO content has a negative correlation with the insoluble residue content with a correlation coefficient close to −1, explained by dolomite cementation of silicilastic rocks and replacement of clayey cement by dolomite.

2. The total iron content correlates with the Al2O3 and TiO2 contents, which reflect the clay content of the rocks. The correlation coefficient of Fe2O3(total) and Al2O3

is 0.89−0.93 for siliciclastic and mixed carbonate-siliciclastic rocks, which can be explained by association of iron minerals with clay minerals.

3. Devonian rocks have a rather low FeO content and excess Fe2O3 in comparison with other Palaeozoic rocks.

4. The iron-bearing mineralogy changed during diagenesis with Fe(III) minerals becoming dominant due to an oxygen-rich environment, low-water table and arid climate.

5. Iron oxides and hydroxides can be of primary, early diagenetic and or/late diagenetic origin.

Most probably Fe-rich layers have a diagenetic origin and were precipitated from Fe-rich groundwater close to the surface along a bedding plane marking a sedimentation break. They constitute plate-like concretions lying adjacent to each other on the same surface within an area of 8 × 10 km. This explanation is compatible with the groundfloor iron layers described in the Bahariya Oasis of Egypt (Mücke, 1994). As suggested by the variegate-coloured character of the Burtnieki Stage, with intricate alternating layers of whitish, yellowish, pinkish, violet and reddish sandstone, the sandstone layers of this unit have been strongly affected by groundwater circulation processes during which the iron minerals were removed and reprecipitated. Primary sandstone was probably reddish with Fe-hydroxide films on detrital grains. It was then bleached by groundwater under reducing conditions so that Fe-hydroxide films dissolved. Hematite or goethite was reprecipitated from Fe-rich fluids under oxidizing conditions, thus forming the plate-like concretions on bedding planes. Crystallization of Fe-hydroxides in the rock caused displacement of detrital grains so that the former grain-supported fabric was replaced by a matrix-supported fabric. The dilatation created additional stresses within the rock and caused the corrosion and breakage of detrital grains (Figs. 4 and 5). Similar features have been described for carbonate cementation (Buczynski and Chafetz, 1987).

Chan et al. (2000, 2001) and Beitler et al. (2004, 2005) explained the formation of hematite and goethite concretions in the Jurassic sandstones of Utah by a multi-stage process. Firstly, the reducing and/or low pH fluid transported iron along fractures and through permeable materials, which led to the precipitation of hematite within the formation where the fluid transporting the reduced iron met oxidizing conditions. Near the surface, meteoritic waters and weathering processes distributed the disseminated iron films that imparted a pink to orange-red colour to the sandstone early in the depositional

A. Shogenova et al.

128 Stud. Geophys. Geod., 53 (2009)

or burial history. The disseminated iron oxides were then mobilized and removed by reducing fluids, leaving the sandstone white. When these fluids later mixed with oxidizing groundwater, the concentrated hematite precipitated as cement to form concretions and tabular deposits in porous sandstones. Tabular concretions are deposited either along fractures or form layers of hematite-cemented sandstone (Chan et al., 2000, 2001; Beitler et al., 2004).

Many case studies associate the iron-rich formations of diagenetic origin in sandstones with fault zones, where the host rock is divided by a network of slip surfaces and joints providing pathways for the migration of fluids (Chan et al., 2000; Eichhubl et al., 2004; Kwon et al., 2005; Ahmadov, 2006; Ahmadov et al., 2007). In south-eastern Estonia Devonian rocks were deformed and open fractures were formed at the end of the Middle Devonian. The fracturing of Middle Devonian sandstones in South-eastern Estonia is probably connected with tectonic activity at the end of the Gauja Age (Table 1, Kleesment and Pirrus, 2000; Kleesment et al., 2003, Fig. 1). Therefore, we may suggest that the mobilization of iron from the red-colored Devonian sandstones is connected with these tectonic processes. The first post-Devonian deformation is tentatively assigned a post-Early Carboniferous to pre-Late Permian age. Faults of the same age have been well documented in the southern part of the Baltic sedimentary basin by deep drilling as well as by seismic surveys (Puura et al., 2003; Monkevičus and Sliaupa, 2007). Important movements causing stresses in the rock complexes are assumed to have taken place in the Northwestern part of Europe in the Late Devonian and Middle Permian (Edwards et al., 1997).

Grain breakage (Figs. 4−5) is observed in sandstones with early diagenetic ferrous (Mücke, 1994) and carbonate cement (Braithwaite, 1989) in nearsurface environments that include the vadose and phreatic zones in arid conditions with high evaporation rates (Buczynski and Chafetz, 1987; Metcalfe at al., 1994). Such conditions existed in the studied region during the deposition of Givetian sediments (Ziegler, 1988). Arid climate conditions also occurred during Upper Permian and Triassic, while in the Mesozoic and Cenozoic colder climate predominated in the Baltic region (Paškevičus, 1997).

In conclusion the present study revealed both marked differences and similarities between iron-rich sandstones to the Devonian siliciclastic host rocks, as listed below.

1. The iron-rich sandstones have 4−140 times higher total iron content, which is in the form of Fe(III) occurring as goethite and less commonly as hematite. Mineralogical and magnetomineralogical data for Devonian host rocks suggest that the main ferromagnetic minerals are magnetite, hematite and only in rare cases goethite.

2. Mobilization of iron from the host sandstones by groundwater fluids was possible under reducing conditions associated with tectonic activity at the end of the Middle Devonian. Precipitation of goethite and hematite and the cementation of sandstones was possible along bedding surfaces in an oxidizing environment that was associated with short sedimentation breaks.

3. The FeO content of the studied iron-rich sandstones is as low as of the other Devonian siliciclastic rocks, which suggests an oxidizing environment and arid climate during formation of iron-hydroxide cementation.

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Stud. Geophys. Geod., 53 (2009) 129

4. The very low content of Al2O3, K2O, K-feldspar and clay minerals also provides evidence for diagenetic alteration and replacement by iron hydroxide cement.

5. Iron-rich plates have high magnetic susceptibility, bulk and grain density and very low porosity as compared to the Devonian host rocks. The weathering and redeposition of the iron-rich rocks in the Quaternary led to an increase in their porosity (up to 14.5%) and to the formation of lepidocrocite, as seen in the Lutsu 1 sample. The magnetic properties of this weathered ironstone, however, are as high as of the other iron sandstones.

6. The total iron content of the iron-rich sandstones increases with decreasing grain size. In contrast to the host rocks with rounded to subrounded grains, iron-rich sandstones include angular to subrounded grains, which are often fractured and corroded. The fracturing could be formed close to the paleosurface, i.e. in the vadose and phreatic zones, in arid conditions with high evaporation rates.

To find proof for the likely later formation time of the iron concretions paleomagnetic studies would be helpful. Oriented samples from the Kooskora outcrop are already subject of such study.

Acknowledgements: This research was funded by the governmental target funding project

No.SF03320888s02 and No.SF0320080s07 from Ministry of Science and Education of Estonia). We are grateful to our colleagues from Institute of Geology at Tallinn University of Technology (TUT), T. Linkova for wet chemical analysis, Ü. Kestlane for thin-sections preparation and to G. Baranov for microphotographs of thin-sections. We thank V. Mikli from the Centre for Materials Research at TUT who operated the SEM microscope. We are grateful to colleagues from the petrophysical laboratories of Helsinki University, Leoben University and St. Petersburg University for possibilities to work in their laboratories. We are also grateful to reviewers (M. Hounslow and one anonymous) and to the SGG editor M. Dekkers for useful comments, which helped to improve the article very much.

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