Interactions between distal epiclastic and bio‐chemogenic sedimentation at the foothills of a mafic alkaline volcano: The case of the Oligocene Doupovské Hory Volcanic Complex (Czech Republic)

Late Oligocene (ca 25 Ma) volcano‐sedimentary successions exposed on the western periphery of the Doupovské Hory Volcanic Complex reveal a complex sedimentation history influenced in various ways by decay of the alkali basalt volcanic edifice. Weathering of the volcanic rocks supplied abundant reactants that promoted carbonate precipitation in the peripheral palaeolakes—as evidenced by strongly non‐radiogenic 87Sr/86Sr values (0.7038–0.7041). On the other hand, the sediments of the initial shallow lake became deformed by the bulldozing effect of a debris avalanche. The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano. At this stage, surface water influx from the surrounding granites conferred a more radiogenic character (87Sr/86Sr values 0.7046–0.7049) to the calcrete deposits. Fossil assemblages as well as limestone textures suggest significant seasonal water‐level fluctuations, possibly reflecting the alternating rainy and dry‐seasons of a prevalently humid Central‐European Late Oligocene climate. The seasonal drying out of the ponds resulted in significant 18O enrichments. Although the ca 0‰ δ13C values might suggest mixing of atmospheric and volcanic CO2 during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity. The lower δ13C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present. This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non‐radiogenic Sr isotopic ratios (87Sr/86Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates. Contrary to the notion of carbonatites being present when highly non‐radiogenic Sr isotopes are found, these results show that Sr isotopes in carbonates formed in alkali basalt‐sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.

The debris flow and avalanche deposits filled up the original depression, modified the basin morphology and shifted the peripheral lacustrine setting further away from the volcano.At this stage, surface water influx from the surrounding granites conferred a more radiogenic character ( 87 Sr/ 86 Sr values 0.7046-0.7049) to the calcrete deposits.
Fossil assemblages as well as limestone textures suggest significant seasonal waterlevel fluctuations, possibly reflecting the alternating rainy and dry-seasons of a prevalently humid Central-European Late Oligocene climate.The seasonal drying out of the ponds resulted in significant 18 O enrichments.Although the ca 0‰ δ 13 C values might suggest mixing of atmospheric and volcanic CO 2 during carbonate precipitation, no active volcanic conduits of relevant age are known in the close vicinity.The lower δ 13 C values are likely a result of mantle degassing through rift faults, a phenomenon observed in the magmatically extinct Ohře Rift until present.This paper demonstrates that limestones derived from weathered alkaline basalts are characterised by highly non-radiogenic Sr isotopic ratios ( 87 Sr/ 86 Sr ca 0.704), suggesting a magmatic origin for the Ca within these carbonates.Contrary to the notion of carbonatites being present when highly non-radiogenic Sr isotopes are found, these results show that Sr isotopes
In the western foothills of the Oligocene to Early Miocene Doupovské Hory alkaline shield volcano, limited outcrops of limestone are found in association with volcanic agglomerates.The scarcity of comparable volcanosedimentary systems in the literature, both for recent and ancient complexes, provided the motivation to deepen the understanding of this depositional association.Although the limited number of exposures and the localised nature of this study prevent a comprehensive reconstruction of the volcano's entire depositional history, involving multiple stages of edifice growth and decay, this research does shed light on the complex interplay between volcanic processes, carbonate sedimentation and palaeoenvironmental conditions.
This paper contributes to a better understanding of the interactions between epiclastic (volcanogenic) and biochemogenic sedimentation in alkaline volcanic systems, as well as the broader implications for interpreting Sr isotopes in alkaline basalt weathering-redeposition mechanisms.To achieve these objectives, a multidisciplinary study was conducted incorporating volcanology, sedimentary petrology, palaeobiology, geochemistry, geochronology and C-O-Sr isotopic systematics, including leaching experiments.

| GEOLOGICAL SETTING
The Doupovské Hory Volcanic Complex (DHVC) is one of the major remnants of Cenozoic alkaline magmatism in the Bohemian Massif in Central Europe (Figure 1A).The abundance of volcanic rocks in the Bohemian Massif follows the Variscan suture between the Saxothuringian and Teplá-Barrandian domains (Mlčoch & Konopásek, 2010).Two volcanic complexes and several volcanic fields erupted along this suture (Ackerman et al., 2015;Rapprich & Holub, 2008), but also numerous volcanic fields erupted further away from this zone (Rapprich et al., 2007;Figure 1B).Apart from a short period of magmatism around the K-Pg boundary, Cenozoic activity in the Bohemian Massif lasted from the Late Eocene until the Pleistocene (Ulrych et al., 2011).Volcanic activity in the DHVC itself started in the lowermost Oligocene (Fejfar & Kaiser, 2005), culminated in the early Oligocene (Holub et al., 2010) and lasted until the early Miocene (Sakala et al., 2010).Subsequently, the Miocene Ohře (Eger) Rift, representing the easternmost branch of the European Cenozoic Rift System (Dèzes et al., 2004), developed along the same Variscan suture zone (Rajchl et al., 2009).The entire volcanic edifice developed as a multiphase shield volcano dominated by lavas of alkaline basaltic composition.Subsidence of the Ohře Rift then contributed to the preservation of thick superficial volcanic sequences by limiting erosion.The preserved sequences are dominated by lava flows (Rapprich & Holub, 2008), associated with numerous deposits of lahars and debris avalanches suggesting several stages where the volcanic edifice decayed and was then rebuild (Rapprich & Dostalík, 2015;Sakala et al., 2010).Concurrently with volcanic activity, agglomerates (or debrites: lahars and debris avalanches deposits) sourced from slope-failure processes formed fans at the foothills of the volcanic complex.Here the focus is on the margin of one of such agglomerate fan in the western DHVC foothills (Figure 1C).It recorded interactions between volcanic and lacustrine palaeoenvironments.
On the western margin of the DHVC (Figure 1C), limestone occurrences associated with volcaniclastic deposits were first reported in the late 18 th century.Mining of 'hard' limestone ('quarry' site in Figure 1C) near the in carbonates formed in alkali basalt-sourced environments only reveal the source of the Sr (and Ca) ions, not necessarily the presence of carbonatite.

K E Y W O R D S
Doupovské Hory Volcanic Complex, freshwater limestone, gastropod palaeoecology, kinetic deformations, lahar, Sr-C-O-isotope systematics village of Sedlečko was first mentioned by Schaller (1785).Glückselig (1842) appended that an outcrop of limestone in basaltic tuffs could be also found near Dubina ('roadcut' site in Figure 1C).The limestones from both localities were described as fine-grained, yellowish, and containing abundant leaf imprints.Glückselig (1842) also mentioned the limestone from Sedlečko.Exploitation in the quarry was complemented by short shafts and two adits drained by a steam pump, this ended in 1890 when the deposit was exhausted (Zerlik et al., 1974).The Sedlečko limestone quarry attracted the attention of palaeontologists since the mid-19th century.Reuss and von Meyer (1849) F I G U R E 1 Location map of the study area: (A) within the European Cenozoic Rift System (adapted after Dèzes et al., 2004); (B) among the Cenozoic alkaline volcanic systems of the Bohemian Massif (adopted from Holub et al., 2010).(C) Geological sketch map of the study area.
and Reuss (1854Reuss ( , 1863) ) were impressed by the richness of the imprints of dicotyledonous leaves in the limestone.However, the same authors noted a lack of fossil fauna.

| Field documentation, sample preparation
The entire DHVC is encircled by several agglomerate (lahar) fans (Rapprich & Dostalík, 2015;Sakala et al., 2010) developed during multi-phase growth and decay of the volcanic complex.The agglomerate fan in the western foothills studied here (Figure 1C) represents proximal facies exposed during reconstruction of the road-cut near Dubina, whereas the distal facies, where the lahar transitions into a lacustrine environment, are exposed in an abandoned limestone quarry near Sedlečko.An additional limestone occurrence near the Sedlečko quarry was temporarily exposed by excavation during construction of a house (Figure 1C).All three exposures are documented in detail here, with samples collected for further analytical work.
Thin sections from representative rock samples were prepared in the laboratories of the Czech Geological Survey (CGS), and studied under a petrographic microscope (Nikon Eclipse 80i).Fossils were mechanically separated before final cleaning with a vibro-tool.Carbonate fossils enclosed in carbonate rocks precluded the application of acid in fossil separation, except for marginal application of 8% acetic acid.Zoopalaeontological material described in this study is archived in the palaeontological collection of the CGS, while the palaeobotanical samples are stored in the repository of the Karlovy Vary Museum.Five representative samples of limestone were collected for geochemical and isotopic analyses conducted in the laboratories of the CGS in Prague.Two samples of basaltic lavas, one interbedding the debrite sequence and the other overlaying the debrites (Figure 1C), were collected for K-Ar geochronology.

| Bulk-rock chemistry
Whole-rock major-element concentrations (including volatiles: moisture, bound water, CO 2 , S and F) were determined in the CGS laboratories employing conventional wet chemistry involving titration, flame photometry and atomic adsorption spectrometry (Dempírová et al., 2010).The samples were first dissolved using HF + HNO 3 + H 2 SO 4 at 220°C (determination of SiO 2 ), and subsequently in a HCl + H 2 SO 4 mixture (remaining oxides).Trace-element contents were determined using an Agilent Technologies 7900 series ICP-MS.Prior to rare earth element (REE) determination, the samples were fused with LiBO 2 (after removal of organic compounds) and dissolved in dilute HCl.Additional trace elements (Ba, Cr, Ga, Hf, Nb, Ni, Pb, Rb, Sc, Sr, Ta, Th, U, V, Y, Zr) were determined after acid digestion with HF (total dissolution of sample with HF, treatment with HClO 4 and H 3 BO 3 mixture and redissolution with HNO 3 after drying).Data handling and plotting were performed using GCDkit software (Janoušek et al., 2006).

| Stable C-O isotopes
Stable C-O isotopic compositions were evaluated, after removal of organic carbon with H 2 O 2 , by implementing the method described by McCrea (1950).Samples of carbonate were decomposed by 100% H 3 PO 4 under vacuum at 25°C.The carbon and oxygen isotopic composition of the evolved CO 2 gas was measured using a dual inlet DeltaV Advantage IRMS (Delta V Isotope Ratio Mass Spectrometer), in the CGS laboratories.The results are reported in conventional δ (δ = R sample /R standard − 1, where R is the mole ratio of 13 C/ 12 C or 18 O/ 16 O) notation relative to V-SMOW (Vienna Standard Mean Ocean Water standard) for oxygen and V-PDB (Vienna Pee Dee Belemnite standard) for carbon.The analytical error was ±0.1‰ for both δ 13 C and δ 18 O values.The accuracy of the measurement was checked by analyses of the international standard (IAEA) NBS 18 (δ 13 C = −5.014‰,δ 18 O = −23.2‰)and two in-house standards: Carrara marble (δ 13 C = 2.29‰, δ 18 O = −1.32‰)and CS 2 (δ 13 C = 2.93‰, δ 18 O = −3.86‰).The long-term reproducibility for all standards is better than 0.05‰ for δ 13 C and 0.1 for δ 18 O.

| Radiogenic Sr isotopes
Five limestone samples were analysed for Sr isotopes.In order to better understand the carbonate source, four samples of recent hot-spring travertines from Karlovy Vary (5 km to the west of the studied area), as well as leachates and residuals of four representative basaltic lavas and a lahar deposit were prepared to complement the dataset.
Chemical separations for Sr isotope analyses were performed in a class ISO 7 ultra-clean laboratory at CGS. Doubly distilled acids were used for the Sr separation procedure and 18.2 MΩ•cm Milli-Q water was used throughout.Additional details of the sample preparation procedure are described in Erban Kochergina et al. (2021Kochergina et al. ( , 2022)).For each sample, about 100 mg of powdered carbonate rock was dissolved in 6 M HCl in Savillex® vials, water samples were evaporated to dryness and then dissolved in a H 2 O 2 and HNO 3 mixture (Erban Kochergina et al., 2021).
About 200 mg of each of the four samples of basaltic rocks representing the prevailing lavas of the DHVC (Rapprich & Holub, 2008) and one sample of lahar deposit from Sedlečko quarry were leached in ascorbic acid (for details see Kochergina et al., 2017;Nádaskay et al., 2019).The leachates were evaporated to dryness, dissolved in a H 2 O 2 -HNO 3 mixture, and re-dissolved in 2 M HNO 3 .Residual phases were removed by centrifugation.Then, the residuum was dissolved as a silicate rock sample in a HF-HNO 3 mixture (3:1, v/v).

| Geochronology
Two samples of basaltic lavas were collected.These include lava of fine-grained aphanitic limburgite embedded within the agglomerate (lahar and debris avalanche deposits) sequence (DB02) and alkaline basalt lava covering the entire studied sequence (DR334).Whole-rock samples were analysed by the unspiked K-Ar method, following the procedure laid out by Matsumo and Kobayashi (1995) in the Geochronology Laboratory of the Institute for Nuclear Research, Debrecen, Hungary.Potassium content was measured on 50 mg sample aliquots, after dissolution by HF and HNO 3 , with a Sherwood-400-type flame spectra-photometer with accuracy better than ±1%.Separated mineral sample splits were then subjected to heating at 100°C for 24 h under vacuum, to remove atmospheric Ar contamination that was adsorbed on the surface of the mineral particles during sample preparation.Argon was extracted from the minerals by fusing the samples via high-frequency induction heating at 1,300°C.The released gases were cleaned in two steps in a low-blank vacuum system by cold St-707 and hot Ti-getters.The isotopic composition of the Ar was measured using an Argus VI© multi-collector noble gas mass spectrometer and corrected for atmospheric 40 Ar/ 36 Ar ratios.The accuracy and reproducibility of isotope ratio measurements were periodically controlled with the HDB-1 (Hess & Lippolt, 1994) and MDO-G (Gillot et al., 1992) international standards.Decay constants recommended by Steiger and Jäger (1977) were used for age calculations, with an overall error of ±1%.Error of the samples was calculated using the equation of Quidelleur et al. (2001).In the studied transition of agglomerate fan into marshy plains at the volcano foothills, the proximal facies of the investigated sedimentary system consists of volcanoclastic and sedimentary rocks displaying irregular boundaries (Figure 2A), with (sub)horizontal bedding seen only locally (Figure 2B).The bio-chemogenic sedimentary rocks (limestone, diatomite) in the Dubina section are present mostly in the form of deformed fragments within the debrite deposits (rip-up clasts; Tost et al., 2014).The laminated sediments adjacent to the agglomerate (debrite) accumulation are intensively folded (Figure 2C).The original horizontal position was revealed only at the base of the section, underlying the debrites.
The exposure is dominated by bedded clays of discrete colours ranging from light yellows, ochres, reddish brown to greenish grey (V1).The thickness of individual beds ranges from 1 to 10 cm (Figure 2B).Despite the actual clayey nature of these deposits, the textures of some beds suggest coarser grain-size (sandy), affected by post-depositional argillisation.These authigenic clays contain abundant clayey pseudomorphs after pyroxene.These bedded deposits are subhorizontally layered onto the outcrop periphery (undeformed sediments-s, Figure 2B), whereas in the central part, they are intensively deformed with detailed folding (deformed sediments-ds, Figure 2C).The highly deformed sediments surround chaotic debrites (V2).
The volcanogenic debrites (agglomerates, V2) occur mainly in the central part of the outcrop, but are present also as smaller 'windows' exposed within the deformed sediments (Figure 2A).The debrites (d in Figure 2A,D) are poorly sorted and matrix supported.The sub-rounded clasts consist of various types of volcanic rocks (alkaline basaltic rocks) up to 25 cm in size.Apart from the sub-rounded clasts, the debrites contain jig-saw fractured blocks or boulders (m in Figure 2A,D), which exceed the outcrop height (5 m) in size.These blocks are not coherent, but have a shattered structure with the jig-saw fit of the subgrains and matrix migrated into the block along subgrain boundaries (Figure 2E).
The laminated limestone facies (F1a) comprises millimetre-scale laminae or thin layers (3-20 cm; Figure 3A).Laminae are flat to undulating with small amplitude (Figure 3B) and very regular.Limestone disintegrates along these laminae into 1-2 mm thin plates.The rock resembles planar stromatolites, but lacks fibrous structures perpendicular to the lamination (indicative of stromatolite fabrics, cf.Martin-Bello et al., 2019).Light-coloured laminae (1.5-2.0 mm thick) alternate with dark laminae/interlaminae (0.1-0.5 mm), which locally pinch out.Individual light laminae are formed by microsparite without visible fossils and are very homogenous.Size gradation of crystals within light laminae may be present.Dark laminae are not well defined and they are formed by amorphous organic matter and iron oxides and a mixture of clay minerals (probably montmorillonite group; Figure 3C) generating a honey yellow to brown mass resembling amorphous silica.Detrital micas (up to 0.8 mm) appear rarely, and are oriented parallel to lamination.Dark organic matter and iron oxides (diagenetic products; interpreted here as remnants of microbial mats) are present locally.Admixed clastic material is very rare and this is represented by monocrystalline and polycrystalline quartz (up to 0.25 mm).Microsparite occurring near interlaminar joints recrystallised into coarser grained sparite.Interlaminar joints or dark laminae are commonly lined with these rhombohedral crystals, accumulating somewhere inside the dark laminae.
Massive limestone facies (F1b) are usually present as lenses (up to 12 cm thick), macroscopically grey and finely crystalline.They are massive, mostly microsparitic (only locally micritic; Figure 3D) and admixed with mica.Typical of this facies are oblique structural domains resembling stylolites or cleavage (Figure 3E,F), which indicate significant deformation.Small lenses (ca 1 mm long) of microsparite calcite crystals are structurally limited (Figure 3F).These lenses resemble laminae facies F1a.All massive F1b limestones lack bioturbation and they are palaeontologically sterile.
Diatomites are described for the first time at the DHVC.Their origin can be safely assumed to be similar to occurrences in other parts of the Ohře (Eger) Rift, for example the Eocene-Oligocene Kučlín and Bechlejovice palaeontological sites in the České Středohoří Volcanic Complex (Bellon et al., 1998;Kvaček, 2002;Mach & Dvořák, 2011), or the Lower Miocene Cypris Formation in the Sokolov Basin (Rojík, 2013).Diatomite facies in the Dubina section are exposed in their original position under the debrite accumulation, and as fragments inside the debrite body (as rip-up clasts).Diatomite generally predominates over limestone in the exposed profile.
Laminated diatomite facies (F2a, Figure 4A) are very light in colour, usually light grey to ochre, porous, thin layers (usually up to 7 cm), that do not contain a macro-palaeontological record.Massive opalised fragments (F2b, Figure 4B) are also classified as diatomites with their amorphous character representing a specific subfacies.Individual collected fragments of this rock type reach several decimetres in diameter.The colour varies from dark brown in the cores of individual boulders to a whitish crust (Figure 4B), which may reflect weathering.Distinguishable diatom valves are missing.Intensively wrinkled fine lamination is visible in some samples (Figure 4C,D,E).Silicified rhombohedral crystals are visible under the microscope (Figure 4F).The wrinkled lamination is remarkably similar to laminated diatomite (F2a, Figure 4A), including large crystals of sparite, but secondarily silicified.Ptygmatically folded cracks are also secondarily filled with chalcedony.

| Sedlečko section
The more peripheral (more distant from the centre of the volcano) facies of the investigated sedimentary system were exposed in Sedlečko village (Figure 1C), where sparitic (F3) and brecciated travertine facies (F3a) were uncovered in excavations for home construction.Block samples of both lithofacies were gathered, but a sedimentary profile is lacking.In a nearby abandoned limestone quarry (Figure 1C), however, a transition from fine-grained volcanoclastic debrites (V3) to micritic limestone (lacustrine) and travertine (spring) (F3 and F4) was documented.In addition, laminated lithographic limestones have been reported in the literature (Reuss, 1854(Reuss, , 1863;;Reuss & von Meyer, 1849).Here, the term travertine is used in its broadest sense, comprising all non-marine carbonate precipitates in or near terrestrial springs, rivers, lakes and caves (sensu Fouke et al., 2000;Sanders & Friedman, 1967).This concept does not take into account the water temperature, contrary to the classification scheme of Glover and Robertson (2003) whose work defined travertine as hot-spring deposits from water temperatures >20°C.The moderate primary porosity and dendritic fabric of the studied samples are characteristic of travertine according to Flügel (2004).
Lithofacies F3 comprises sparite to microsparite limestone with conspicuous travertine textural features (Figure 5A through D).Macroscopically, it commonly developed as grey rocks with a dendritic fabric and nodular surfaces (Figure 5C).Locally, it displays horizontal lamination.The more prevalent nodular domains, without the laminar structures, resemble calcareous tufa.They are more porous than their laminated counterparts, and probably represent travertines with cauliflower-like structures (sensu Chafetz & Folk, 1984;Koutecký et al., 2019).Some samples have a dendritic fabric resulting from carbonate encrustation of plants (Figure 5B,D).Remains of fossils are represented only by rare and small fragments of plant leaves and stems or indeterminate reed fragments.A special subtype of lithofacies F3 is represented by brecciated travertine (F3a).This type of limestone occurs as blocks in slope deposits encountered in the excavation.The blocks seem to represent disintegrated variably sized lenses of travertine characterised by brecciated structure at micro-scale (Figure 5E), and to a limited extent, also at the macro-scale (in the limestone quarry, the microbreccias occur only rarely).Remnants of micritic shrubs and secondary silicification in pore spaces are present (Figure 5F).The F3 limestone contains secondary iron oxides and a clay admixture.This type of limestone is very rich in fossil remains, especially gastropods and reed fragments.
The micritic to microsparitic limestone lithofacies (F4) is developed as a grey homogenous, locally laminated deposit (Figure 5A).It may also be partly porous at a micro-scale.This micro-porosity is probably the product of secondary dissolution of carbonate material (mostly sparite).The laminae may represent (seasonal) fluctuations in the contribution of carbonate material.Fossil remains, especially leaf imprints, are rare.Lastly, a now-depleted calcareous lithofacies, lithographic limestone (F5) was also present in the study area.This, however, was fully exploited and is not preserved.From descriptions in the literature, in both localities it was finegrained, yellowish in colour and contained abundant leaf imprints.Its fine lamination resulted in platy fracture enabling the use of this limestone for lithographic purposes (Glückselig, 1842).
The fine-grained debrite lithofacies (V3) is exposed on the eastern margin of the Sedlečko limestone pit.It differs from the Dubina road-cut (V2 facies) in its significantly finer grain-size (Figure 6A) and absence of jig-saw-fragmented blocks.The deposits comprising the debrite facies are poorly sorted, matrix-supported, with a sandy to silty matrix significantly predominating over small and well-rounded pebbles.The pebbles do not exceed 7 cm in diameter (mostly around 3-5 cm, Figure 6B), and consist of various types of alkaline basaltic lavas.Poor sorting and moderate rounding can be also observed in the groundmass under microscopic investigation (Figure 6C).The agglomerate (probably two depositional units, with a poorly developed boundary) is crosscut by a system of post-depositional carbonate veins (Figure 6D).These veins are mostly subhorizontal, and their frequency increases from east (debrite source) to west (centre of the limestone pit). 4.2 | Elemental and C-O-Sr isotope geochemistry Five representative calcareous samples were investigated spectroscopically with regard to their major oxide, minor and trace element, stable C and O and radiogenic Sr isotope compositions.The sample set was obtained at the (i) Dubina road-cut (n = 2), (ii) Sedlečko limestone pit (n = 2), and (iii) Sedlečko excavation (n = 1).Analyses revealed a calcitic composition with relatively low MgO (0.2-1.1 wt%), FeO (0.1-1 wt%), and MnO contents (0.1-0.3 wt%; Table 1; Figure 7A).The samples contain notable amounts of SiO 2 (0.6-2.6 wt%), with a positive correlation between SiO 2 and TiO 2 (Figure 7B) suggestive of basaltic silt admixture.The only exception is one sample from Sedlečko limestone pit (Sed-I_01), where no TiO 2 was detected.
The chondrite-normalised (Boynton, 1984) REE patterns (Figure 7C) are characterised by smooth and relatively flat patterns with generally very low REE contents (ΣREE = 2.6-27 ppm).The lowest REE concentrations were detected in sample Sed-I_01, which is also characterised by uncoupled SiO 2 -TiO 2 .The analysed limestone samples display a negligible Eu anomaly, except for sample DR339 of the brecciated travertine (F3a) facies that probably represents a spring-outpouring site.Sample DR339 displays a slight positive Eu anomaly (Eu N /Eu N * = 1.35).When the extended set of trace elements is normalised to an average representative of DHVC basaltic lavas (after Rapprich & Holub, 2008), significant depletion in all elements, except for U, Sr and P is observed (Figure 7D).
The measured δ 13 C and δ 18 O values are listed in Table 2, and shown in Figure 8.With δ 13 C isotope values ranging from −1.0‰ to 2.2‰, the samples fall within the mode observed for 90% of travertine deposits (Pentecost, 2005).The lowest measured value occurs in sample Dub_02, followed by brecciated travertine sample DR339, with similarly positive values observed in samples representative of both localities (Table 2).The δ 18 O values of the samples are in the 19.9%-23.0‰range and display relatively 18 O-enriched values at the Dubina proximal site.If one considers the travertinedepositing waters to be mainly, but not entirely, of meteoric origin, and thus exhibiting a palaeo-δ 18 O value somehow similar to that of modern day stream waters in the Karlovy Vary (Carlsbad) region, which fluctuate on a yearly-basis between −11.0‰ and −8.4‰ (Buzek et al., 1991), then an equilibrium deposition temperature of between 10 and 23°C can be derived.This estimate applies the quadratic equation relating δc and δw with temperature, as first devised by O'Neil et al. (1968), and revised by Hays and Grossman (1991).Depletion in 18 O in the measured samples occurs in parallel with a 13 C enrichment.
Strontium isotopic data for the studied carbonates are presented in Table 3 and Figure 9, together with previously published data for basaltic lavas (a possible source of calcium), leachates and residuals of these basaltic samples and hot-spring travertines from Karlovy Vary.Sedimentary carbonates from Dubina and Sedlečko show rather homogeneous 87 Sr/ 86 Sr isotopic compositions.The limestone sample from the construction site in Sedlečko (DR339) and the limestones from Dubina (Dub_01 and Dub_02) show strikingly non-radiogenic values of 87 Sr/ 86 Sr (0.7038, 0.7041 and 0.7039 respectively), while the two samples from Sedlečko quarry (Sed-I_01 and Sed-I_02) have slightly elevated 87 Sr/ 86 Sr values (0.7046 and 0.7049).In contrast, travertines from the recent Karlovy Vary hot-spring (Vylita & Žák, 2009) are characterised by significantly more radiogenic 87 Sr/ 86 Sr values (0.7191-0.7199).
As the alkaline basalts of the DHVC are rich in CaO (9-15 wt%; Rapprich & Holub, 2008), weathering of these rocks may provide enough CaO for carbonate precipitation at the volcano foothills.The weathering and CaO release may be facilitated by fragmentation of the basaltic rocks, leading to increased reaction surfaces.To test this hypothesis, leaching experiments of four representative basaltic lavas and a lahar deposit were carried out.The leachate and residuum of lahar samples collected on the edge of the Sedlečko limestone pit display only a small difference, having 87 Sr/ 86 Sr values of 0.7048 and 0.7049 respectively.Higher variability can be observed in the case of basaltic lavas with leachates characterised by slightly more radiogenic values (0.7045-0.7049) than the residuum (0.7042-0.7047;Table 3).

| Fossil associations
Three taxa of freshwater gastropods were distinguished in Sedlečko limestones, that is, Lymnaea sp., Stagnicola sp.(Figure 10A,B), and Radix sp.(Figure 10C,D,E) representing the Lymnaeidae family (pond snails).In addition, several individuals of Planorbis sp. from the Planorbidae family (ramshorn snails) were also found (Figure 10F,G).Several individuals could not be classified due to shell dissolution and preservation of the shell-core only (Figure 10H).In other cases, the gastropod shells had been shattered by compaction (Figure 10I,J).The relatively low diversity gastropod fauna is accompanied by a monospecific ostracod fauna represented by a single species Virgatocypris cf.virgata.The genus Virgatocypris is known to occur since the Late Cretaceous, but Virgatocypris virgata, in particular, is commonly recorded from the Upper Oligocene to Early Miocene lacustrine successions of Europe (Pokorný, 1986;Witt, 2001Witt, , 2002) ) and Turkey (Agbulut et al., 2020).
The fossil fauna is accompanied by numerous remnants of fossil flora, despite many of these remnants being too fragmented or incomplete for taxonomic classification.Volcaniclastic siltstones occur as thin layers atop individual flow units of debrites (debris flow and debris avalanche deposits).In Dubina, these contain abundant leaves of Platanus neptuni (Figure 11A).Leaves and stipules of this tree can also be found in Dubina limestones (Figure 11B), along with leaves of Populus sp.(Figure 11C), Acer cf.tricuspidatum, Daphnogene cinnamomifolia form lanceolata, Alnus sp., and fragments of other undetermined angiosperm leaves (possibly Dicotylophyllum sp.).Conifer remnants were found solely in diatomite, represented by strobili, probably of Calocedrus sp.Fine-grained facies associated with distal lahars at Sedlečko contain imprints of leaves of Engelhardia orsbergensis, Rumohra recentior and Eotrigonobalanus furcinervis (Figure 11D).The latter occurring also in the limestones along with Laurophyllum sp., Dombeyopsis lobata, Craigia bronii (Figure 11E) and Alnus sp.(Figure 11G).Genus Alnus is represented also by calcified wood A. tschemrylica (Figure 11F).Randomly or radially oriented calcite straws correspond to reed-bunches (Figure 11H,I).

| Geochronology
To better constrain the timing of the studied sequence, geochronological analyses were carried out on two lava samples, DB02 and DR334, interbedded and overlaying the sequence respectively (Figure 1C; Table 4).The aphanitic tephrite lava (sample DB02) is embedded in the debrite sequence near the documented road-cut in Dubina.This lava occurs ca 10 m up the slope above the studied sediments, deformed by debris avalanche indention.Two fractions prepared from this lava sample, representing both the bulk-rock and the light groundmass, provided almost identical age results of 25.04 ± 0.36 and 25.34 ± 0.36 Ma respectively (Chattian, late Oligocene; Table 4).The difference does not exceed analytical error.Taking rapid deposition of the volcanoclastic debrites and the span of analytical error into account, the obtained ages refer to the deposition of the agglomerate fan.In the area between the studied outcrops at Dubina and Sedlečko, other lava is exposed (Figure 1C).Unlike the tephrite DB02, the lava of the porphyritic trachybasalt DR334 fills a palaeovalley developed in the debrites, but orientated perpendicular to the general trajectory seen in the lahars.Age determination of this lava flow yields 21.64 ± 0.31 Ma (Aquitanian, early Miocene; Table 4).

| Age of the studied succession
Due to poor preservation, the zoopalaeontological material provides insufficient background for chronostratigraphic interpretations.Concerning the fossil flora, the co-occurrence of Eotrigonobalanus (Figure 11E) with Craigia (Figure 11F) and P. neptuni (Figure 11A,B) points to the early to late Oligocene floral assemblage Nerchau-Florsheim sensu Mai (Kvaček & Walther, 2001), and relates the flora described here with that of Suletice from the České Středohří Mountains (Kvaček & Walther, 1995, 2003).The geochronological data from the tephrite lava DB02 (25.04 ± 0.36 and 25.34 ± 0.36 Ma; Table 4) also place the studied succession in the Late Oligocene.The porphyritic trachybasalt DR334 lava erupted in the early Miocene (21.64 ± 0.31 Ma; Table 4), but was emplaced onto the eroded relief of an agglomerate fan.In addition, the DR334 lava erupted from local fractures on the rift scarp and transected the agglomerate accumulations almost perpendicularly (ca N-S) to the debris flows trajectory (from east; Figure 1C).It is therefore assumed that a significant time-gap exists between deposition of agglomerates and eruption of the DR334 lava.Taking the position of both lavas into account, the Late Oligocene age is accepted.T A B L E 1 (Continued)

| Palaeoclimate and depositional environment
The floral assemblage can be characterised as a warmtemperate to subtropical mixed mesophytic forest (MMF) to subtropical broad-leaved evergreen forest (BLEF; Teodoridis & Kvaček, 2015).Subtropical conditions would have been beneficial for chemical weathering of volcanic and volcanoclastic rocks.Insufficient preservation of the collected fossils precludes detailed determination and taxonomical categorisation, but it is sufficient for a palaeoecological estimation.The present species point to a habitat of shallow standing or slowly running freshwaters on a muddy substrate, which may include ponds or temporarily drying flood water bodies (Flügel, 2004).Some limestone samples have a dendritic fabric resulting from carbonate encrustation The recent species of Planorbis generally inhabit wetlands, pools or shallow and smaller ponds overgrown with macrovegetation, in standing or slowly flowing water.Palaeoecology of the fossil gastropod faunal assemblage from the Sedlečko locality was evaluated employing the ecological demands and distribution of closely related living species, as documented by Ložek (1964) and Kerney et al. (1983).The ostracod species V. virgata is usually part of the Candona-Cypridopsis assemblage, which is thought to have characterised a shallow lake or lake banks with water depths up to around 1 m (Malz & Moayedpour, 1973;Witt, 2002).Intermittent (possibly seasonal) sedimentation in a shallow lake or periodically flooded marsh would also correspond to the finely laminated character of the limestone.The appearance of laminar limestone resembles calcrete, or palustrine facies.Calcretes are defined as secondary accumulations of calcium carbonate (low-Mg and high-Mg calcite) in near-surface settings, which result from the cementation and/or replacement of host material by the precipitation of calcium carbonate from soil water or ground water (Wright, 1990).Definition of palustrine deposits is complicated-there are several ecological, hydrological and sedimentary parameters that should be considered.After definition by Cowardin et al. (1979) palustrine wetlands must fulfil one or more of the following requirements: (i) the land supports predominantly hydrophytes, at least periodically; (ii) the substrate is predominantly undrained hydric soil, and the substrate is non-soil and is saturated with a water cover or covered by shallow water at some time during the growing season.
In the Dubina section, there is no evidence of soil water or ground water, bioturbation, roots or ostracods, any mud cracks, etc. Plant remains are scarce and dominated by angiosperm trees.Leaves of P. neptuni can be found in both fine-grained volcaniclastic siltstones derived from debrites and limestones (Figure 10A,B).The laminar structure of limestone facies F1a is interpreted here as cryptomicrobial.Sedimentary rocks present at the Dubina section represents infill of a shallow water sedimentary basin or separate basins (lakes, ponds or puddles) originated in front of the volcanic complex.This basin (or basins) was indented by the debris avalanche.The unknown original position and interrelationship of carbonate and siliceous sediment fragments may represent one or more sedimentary basins, developed independently.
Carbonate sedimentation at Sedlečko was more complex.The brecciated travertine from the excavation (Figure 5E) corresponds to formation from a spring exiting a low mound close to the lake (Pentecost & Viles, 1994).This locality probably represents the margin of a very shallow lake, probably developed as paludal deposits (Pentecost & Viles, 1994), where the carbonate originated as cement around plant (e.g.reed) tussocks.Brecciation of the limestones probably formed by desiccation and/or is related to long-rooted grass colonisation of an exposed mud (Freytet & Verrecchia, 2002).They are formed with irregular periodicity and by reworking of desiccation breccias (Freytet & Verrecchia, 2002).The limestone lithofacies from the abandoned quarry at Sedlečko likely represent the main shallow lake sediments with carbonate development, and it is in the central part of this lake where the variably sized lenses of lithographic limestone (unfortunately not preserved) may have locally developed.
F I G U R E 8 Cross-plot of δ 13 C (‰) versus δ 18 O (‰).The linear correlation of the model is not significant (r 2 = 0.181) and the model is strongly biased by sample DR339 (standardised residual = −1.47).The 99% confidence range is also displayed.

| Stable carbon isotopes
To understand the source of reactants for carbonate precipitation, three independent isotopic systems were examined.The δ 13 C values obtained (−1.0‰ to 2.2‰) fit into the modal range (−1‰ to 10‰) of travertine-fixed carbonate ions originated from a variable combination of pre-existing limestone decarbonation and fluid-rock exchange reactions.Magmatically derived CO 2 was also important (Pentecost, 2005).This conferred an isotopically lighter signature to the resulting travertine, which exhibits a mixed source of dissolved inorganic C. The absence of pre-existing limestones in this area and the position of the studied sites at the volcano periphery, away from the degassing volcanic conduits, reduces the list of potential sources.Nowadays CO 2 outflows along the Ohře Rift faults are also characterised by mantle-like δ 13 C (ca −4‰; Mach et al., 2017), although no magmatic activity has occurred in the central segment of the rift since the Tortonian ca 10 Ma (Cajz et al., 2009).As the aqueous CO 2 phase containing a fraction of mantle-derived CO 2 approached atmospheric equilibrium, the resulting lake travertine carbonate precipitate would have become slightly enriched in 13 C.

| Stable oxygen isotopes
Highly positive δ 18 O (19.9-23.0‰;2) imply involvement of water highly affected by evaporation into building of carbonate ions.Such a scenario is in good agreement with palaeontological and sedimentological (e.g. Figure 3A) observations suggesting that ponds where limestone precipitated were ephemeral.Despite the fact that significant amounts of dolomite could be expected in limestones deposited in periodically evaporating ponds (Muir et al., 1980) and the surrounding basaltic lavas are rich in MgO (up to 12.5 wt%; Rapprich & Holub, 2008), only negligible amounts of dolomite were observed in the studied Oligocene limestone.This discrepancy can be explained by the character of the basaltic lavas weathering or the pH and alkalinity of the water in equilibrium with the precipitation environment.Formation of secondary iddingsite and smectite group minerals fixed large amounts of available MgO but almost no CaO, which remains mobile and soluble.Despite the elevated dissolved inorganic carbon levels, without a sustained alkaline generation and proton consumption mechanism also in place, calcite not dolomite would be favoured (Petrash et al., 2021).

| Radiogenic strontium isotopes
With the aim of confirming the hypothetical source of CaO from weathering of DHVC basaltic rocks and their volcaniclastics, several leaching experiments were carried out.During leaching of dry powders of fresh unaltered basaltic rocks at room temperature, the leachate yields a more radiogenic signature compared to bulk-rock and residuum (Table 3; Figure 9).In the case of fresh basaltic rocks, the low-temperature leaching preferably attacks the metastable phases (feldspathoids and glass) with a slightly more radiogenic signature.The opposite trend is observed in the case of the lahar deposit, where leachate provides a less radiogenic signature than the residuum (Figure 9).This trend possibly reflects the effect of longterm Oligocene weathering in subtropical conditions (Kvaček & Walther, 2001;Li et al., 2018), which occurred prior to fragmentation (triggering of the debris avalanche or debris flow) and continued also after its deposition.Such weathering leads to decomposition of mafic minerals first, namely olivine (Sr-poor) and clinopyroxene (Sr-rich).In a magmatic system evolving through open-system processes (e.g.crustal contamination), isotopic ratios of the magmatic liquid from which crystals grow may change through time.Consequently, minerals (indeed growth zones within crystals) reflect the isotopic ratios of their environment at crystallisation; early-grown minerals may be different (e.g. less radiogenic 87 Sr/ 86 Sr in a system with progressive incorporation of crustal material) to later-crystallised phases.In a system where two magmatic components mix, crystal cores may record the isotopic character of the mixing endmembers, whereas rims may record the isotopic ratio of the postmixing hybrid magma (Davidson et al., 2007).The isotopic character of the leachate, therefore, could be a complicated interplay of (1) the nature of open-system processes, leading to within-rock isotopic disequilibrium and (2) which mineral phases were preferentially dissolved.
The Sedlečko lahar (V3 lithofacies) is more radiogenic ( 87 Sr/ 86 Sr composition ca 0.7048; Table 3) than any of the analysed lava (Rapprich & Holub, 2008).Such a shift seems to be too pronounced even after long-term selective leaching of the less radiogenic Sr fraction.Moreover, it is expected that the lahar represents a range of basaltic rocks, not only those with more radiogenic 87 Sr/ 86 Sr. Present day 87 Sr/ 86 Sr composition may thus also be influenced by long-term groundwater circulations contributing small amounts of highly radiogenic Sr from country-rock granites (Dolejš et al., 2016).
Focussing on the Sr isotopic composition, it can be served limestones from Dubina and Sedlečko brecciated travertine have an even less radiogenic 87 Sr/ 86 Sr composition (0.7038-0.7041) than any of the lavas analysed by Rapprich and Holub (2008;Figure 9).That paper describes late Rupelian to Chattian (post 28 Ma) lavas, but Holub et al. (2010) analysed slightly older (29-30 Ma) intrusive alkaline rocks from the DHVC, suggesting compositionally similar lavas with lower 87 Sr/ 86 Sr values (as low as 0.7036; Figure 9) could have erupted during an earlier stage in the evolution of the DHVC.Weathering of these early Oligocene (Rupelian) low 87 Sr/ 86 Sr lavas seems to be an adequate candidate for the source of CaO precipitated in the Dubina limestones and Sedlečko brecciated travertine.The shift in 87 Sr/ 86 Sr towards more radiogenic values for the Sedlečko limestones might be explained by inclusion of a greater proportion of late Oligocene (higher 87 Sr/ 86 Sr) lavas into the source lahars.Such a scenario is contradicted by the low 87 Sr/ 86 Sr values for Sedlečko brecciated travertine, which should (according to its position) be a product of the same source.More likely, the brecciated travertine, as the outpouring site, carries the endmember isotopic character its source, the lacustrine limestones were contaminated by radiogenic Sr washed to the lake from surrounding country-rock granite (Dolejš et al., 2016;see Figures 9 and 12).
Strikingly low 87 Sr/ 86 Sr values of verifiably sedimentary carbonate is then linked to the origin of Sr (and CaO by proxy) from weathering of basaltic lavas and does not represent any evidence of carbonatite activity.The origin of CaO from weathered basaltic rock is also supported by minor amounts of smectite (Figure 3C) and fine basaltic silt (Figure 7B) admixtures in the limestones.

| Mechanical interactions between lahars/debris avalanches and bio−/ chemogenic sediments
Limestones in the Sedlečko quarry do not display signs of intense post-depositional deformation.A well-defined linear trend for all samples in the TiO 2 versus SiO 2 binary diagram suggests that most of the silica in the limestones is carried by fine volcanoclastic silt.The only exception deviating from this trend is the sample Sed-I_01 from Sedlečko quarry, which lacks TiO 2 (Figure 7B).This most likely resulting from secondary silicification, as no source  of detrital was available in the area of the shield volcano.rocks in Dubina section are then affected by more complex post-depositional recrystallisation and deformation.
Besides weaker silicification, represented by smaller amounts of opal in F2a facies impregnating the porous structure produced by partial leaching of the limestone, the finely laminated sediments display intense deformation.Discrete folding at a millimetre-scale is clearly visible in both limestones (Figure 3D,E,F) and diatomites (Figure 4D,E).Additionally, the originally whitish diatomites in many places transition to dark massive rock (Figure 4B,E), suggesting compression and recrystallisation of initially highly porous (and low specific weight) material.All of these deformation features are restricted to close relationships with jig-saw fractured block-containing debrite (V2) facies (Figure 2A,D).Well-developed jig-saw fit fractures within the blocks and boulders (or even mega-blocks), partly filled with matrix, are indicative of debris avalanche deposits (Alloway et al., 2005;Bustos et al., 2022).The DHVC volcano extends ca 25 km across (studied sites are ca 11 km from the central part of the volcano), originally rising at least 600 m (more likely 800-900 m) above its basement.
Due to significant erosion since the Late Oligocene, it is challenging to reconstruct the volume of individual debris avalanches created during volcano evolution.Deformation, compression and recrystallisation observed in the Dubina sediments is closely and exclusively associated with the exposure of the debris avalanche deposit (V2 lithofacies).In addition, very detailed deformation suggests rapid and short events that most likely reflect indentation of the debris avalanche.Discrete (millimetre-scale) folding represented by stylolite-type structures in both laminated limestones and diatomites suggest at least partial lithification of the deposits prior to deformation, as liquification and roiling would appear in the case of non-lithified sediments (Marco & Agnon, 1995).The discrete and spatially limited character of the deformation suggest rapid and instant event, rather than a dragging process (creeping, spreading, or similar), which would produce more dispersed deformation with long-wave and low-amplitude folds (Katsman, 2013;Pietruszczak et al., 2002).Close space relationships between the deformed domains of the laminated sediments and the debris avalanche deposits suggest a genetic relationship between both features.
Deformation and bulldozing effects of sedimentary substrata and sediments at the volcano periphery indented by debris avalanche are known from the 10 ka collapse of the Jocotitlán volcano in Central México (Dufresne et al., 2010).Shock-metamorphosis associated with the impact of a debris avalanche onto the volcano basement in the French Massif Central created temperatures of around 800°C, possibly up to 1,500°C, differential stress up to 10 GPa (Bernard & Van Wyk Vries, 2017).the debris avalanche in the studied area possibly being of smaller volume, transformation of kinetic energy into heat is well-documented in deep compaction and recrystallisation of the diatomites.Later debris flows, reaching as far as the Sedlečko area, did not have the potential to deform sedimentary rocks and served solely as source of CaO.
Carbonate springs and ponds formed at the front of agglomerate fans on the foothills of large volcanoes seems to be a potentially interesting sedimentary environment not well described in existing literature.The studies of volcano edifice failures mostly focus on calc-alkaline volcanoes at convergent plate boundaries (Capra et al., 2002;Jicha et al., 2015;Keigler et al., 2011;Roberti et al., 2017;Tost et al., 2014Tost et al., , 2015;;Van Wyk de Vries et al., 2001); with different geochemistry and distinct weathering.Studies focussed on edifice failure processes at within-plate or continental-rift alkaline volcanoes are significantly scarcer (Delcamp et al., 2016;Kervyn et al., 2008), while reports on associated carbonate sedimentation are practically non-existent, although such deposits can be expected in these settings.
The entire development of the Dubina-Sedlečko volcano-sedimentary system, including its chemical evolution, is summarised in Figure 12.In the first stage, laminated lacustrine deposits at the volcano foothills were supplied from the weathering of basaltic lavas (Figure 12A).The laminated sedimentary sequence was deformed by indention caused by debris avalanches (Figure 12B).As a result, peripheral lacustrine sedimentation migrated further away from the volcano.A change in the local hydrological settings was documented to reflect deposition of Holocene avalanche debris on Parinacota, in central Andes (Jicha et al., 2015).As a consequence, the supply of CaO provided by the propagating distal debris flows (lahars) was also shifted further away from the volcano, but under increasing compositional influence exerted by country-rock granites (Figure 12C).

| CONCLUSIONS
• The studied localities at the foothills of the Doupovské Hory Mountains reveal a complex sedimentation history influenced in various ways by weathering and decay of the Oligocene alkali basaltic shield volcano.• Sedimentation at the foothills of the Oligocene DHVC was strongly influenced by volcanic edifice failure processes.Epiclastic mass flows (debris flows and debris avalanches) supplied the peripheral lakes to a lesser extent with fine detritus, but mainly in calcium liberated from fragmented and weathered alkaline basaltic rocks.The shallow lake sediments became deformed by the bulldozer effect of moving debris avalanches.
• Fossil assemblages suggest significant seasonal waterlevel fluctuations, possibly reflecting the alternating rainy and dry-seasons of a generally humid, Central-European Oligocene climate.Seasonal drying out of the ponds likely resulted in 18 O enrichment.• Limestones derived from weathered alkaline basalts are characterised by highly non-radiogenic Sr isotopic ratios ( 87 Sr/ 86 Sr ca 0.704), suggesting a magmatic origin for these carbonates.It is possible to demonstrate on this spectacular example that Sr isotopes do not reveal the origin of carbonate rock, but rather, the source of Ca ions.Hence, carbonates with non-radiogenic Sr isotope systematics do not necessarily mean the presence of carbonatite.

ACKNO WLE DGE MENTS
This manuscript contributes to the Strategic Research Plan of the Czech Geological Survey (DKRVO/ČGS 2018-2022), projects 310950 and 310450.We are indebted to Bohuslava Čejková (ČGS) for analyses of C-O stable isotopes, to Zlatko Kvaček ( †) for assistance with identification of palaeobotanic findings, and to John M. Hora and Greta Mackenzie for language checking.The manuscript benefited from constructive comments by Károly Németh and an anonymous reviewer.

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Volcaniclastic and sedimentary rocks exposed in the Dubina road-cut.(A) Overall view of the major part of the Dubina road-cut.(B) Subhorizontal planar bedding of weakly to undeformed sedimentary sequence (s) comprising mainly colourful volcanigenic clays.(C) Close-up of intensively deformed stratified sediments (ds).(D) Contact between jig-saw fractured blockscontaining debrite (debris avalanche deposit) and deformed sediments.(E) Close-up of jig-saw fit texture of subgrains within a jig-saw fractured block (m).d = debrite, ds = deformed sediments, m = jig-saw fractured block, s = sediments.

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I G U R E 3 Carbonate sediments from Dubina road-cut.(A) Laminated limestone in polished section, mm-scale laminas are laterally constant (each mark on the scale represents 1 mm).(B) Detail of laminae in thin section (the same sample as in photograph A), very regular laminae are flat to undulating with small amplitude, dark interlaminae that are locally reduced.(C) Backscattered electron image showing detail of light and dark laminae, Cal = calcite, Sme = smectite.(D) Massive micritic limestone with 'ghost' stylolite-type structures, thin section.(E) Oblique structural domains resembling stylolites or cleavage, thin section.(F) Detail of stylolite-type structure with structurally limited microsparite domains, thin section.

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I G U R E 4 Diatomites from Dubina road-cut.(A) Laminated diatomite with regular lamination (F2a).(B) Opalised fragment with massive (amorphous) character, the colour varies from a dark brown in the cores of individual boulders to a whitish crust.(C) Intensively wrinkled fine lamination, macroscopic view.(D) Intensively wrinkled fine lamination in thin section, same sample as in photograph C. (E) Sample of amorphous opalised diatomite fragment with visible remains of wrinkled lamination.(F) Thin section of massive diatomite with silicified calcite or dolomite rhombohedral crystals inside microcrystalline to cryptocrystalline matrix.Each mark on the scale represents 1 mm.

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Limestones from Sedlečko outcrops.(A-D) Sedlečko abandoned limestone quarry.(E and F) Temporary excavations for family house.(A) Thin laminated micritic limestone.(B) Encrusted bioclast (reed or twig fragment) with dendritic fabric.(C) Typical shrub-like structure probably supported by bacterially induced precipitation or by growth of bryophyte.(D) Internal structure of travertine in thin section, internal concentric growth lines usually formed around the bioclasts.(E) Brecciated travertine with higher content of iron oxides.(F) Secondary silicification (Sil) partly affecting the calcite sparite crystals and micritic matrix with clay admixture.Each mark on the scale represents 1 mm.F I G U R E 6 Volcanoclastic agglomerate (debrite) exposed at Sedlečko (V3 facies).(A) Poorly sorted matrix-supported fine-grained debrite at Sedlečko.(B) Close-up of small volcanic pebbles enclosed in the sandy matrix.(C) Microphotograph of the poorly sorted debrite groundmass with moderately to poorly rounded sand-sized clasts of various basaltic rocks.(D) System of postdepositional carbonate veins cross-cutting the debrite accumulation.

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C-O stable isotopes of studied freshwater limestone.ofplants 5B,D), and would have formed close to a spring, or at the shore of a carbonate-oversaturated shallow lake (sensuPentecost & Viles, 1994).The molluscs and terrestrial gastropods are usually very restricted to certain habitats.The species composition of the gastropod community, that is, a combination of their ecological requirements, is a very accurate ecological indicator of the environment at a specific locality.Palaeoenvironmental reconstruction presented here compare to that of palaeontological localities at Sandelhausen (Southern Germany, Early/Middle Miocene;Moser et al., 2009), and Lapsarna (Western Lesvos, Greece, Early Miocene;Vasileiadou et al., 2017).The recent representatives of the Lymnaeidae family, genera Stagnicola, Lymnaea and Radix inhabit standing waters, such as lakes and ponds, swamps and marshes.The genus Radix indicate peaty waters on muddy or stony ground near the bank of lakes, slowly flowing rivers and smaller and shallower water bodies.Some species can withstand drier conditions or short droughts.

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Fossils collected from limestones at Sedlečko excavation.(A) Stagnicola sp.-preserved core/filling of dissolved shell uncovered by natural weathering.(B) Stagnicola sp.-flattened shell-core on the crash-surface.(C, D) Radix sp.-preserved core/filling of dissolved shell uncovered by natural weathering.(E)?Radix sp.imprint of the shell.(F) Planorbis sp.-preserved core/filling of dissolved shell on the crash-surface.(G) Planorbis sp.-cross section of core of partly dissolved incomplete shell.(H) Part of core of dissolved shell of undetermined gastropod on the crash-surface.(I, J) Parts of flattened and shattered gastropod shells.

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I G U R E 1 2 Schematic model of carbonate sedimentation and deformation at the western periphery of the Doupovské Hory volcano.(A) Lacustrine carbonate deposition in a shallow Late Oligocene palaeolake (Dubina) with reactants supplied from weathering of early DHVC lavas.(B) Deposition of debris avalanche leading to deformation of the Dubina lacustrine deposits.(C) Shift of the lacustrine sedimentation further west of the volcano, with mantle-derived CO 2 -rich efflorescence producing a travertine mound (Sedlečko excavation).More radiogenic 87 Sr/ 86 Sr signature in the lacustrine limestones (Sedlečko quarry) suggests possible contribution from country-rock granites to chemistry of limestone precipitated in the Sedlečko palaeolake.
Chemical composition of studied freshwater limestones.
T A B L E 1 Major oxides in wt.%, trace elements in ppm. Note: Radiogenic Sr isotope systematics of studied freshwater limestones compared with potential sources.
T A B L E 3 b Data from Holub et al. (2010).