Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, based on combined...

Journal of Volcanology and Geothermal Research 182 (2009) 97–112

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Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, basedon combined geophysical and geological surveys

J. Mrlina a,⁎, H. Kämpf b, C. Kroner b, J. Mingram b, M. Stebich c, A. Brauer b, W.H. Geissler d,J. Kallmeyer b,f, H. Matthes e, M. Seidl a

a Institute of Geophysics ASCR, Boční II/1401, 14131 Prague 4, Czech Republicb Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germanyc Senckenberg, Forschungsinstitute und Naturmuseum, Forschungsstation für Quartärpaläontologie, Am Jakobskirchhof 4, D-99423 Weimar, Germanyd Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, D-27568 Bremerhaven, Germanye Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg, D-14473 Potsdam, Germanyf University of Potsdam, Department of Geosciences, Karl-Liebknecht-Str. 27, Haus 27, 14476 Potsdam, Germany

⁎ Corresponding author.E-mail addresses: [email protected] (J. Mrlina), kaempf@

[email protected] (C. Kroner), ojemi@[email protected] (M. Stebich), [email protected]@awi.de (W.H. Geissler), jens.kallmeyer(J. Kallmeyer), [email protected] (H. Matthes), m

0377-0273/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.jvolgeores.2009.01.027

a b s t r a c t
a r t i c l e i n f o
Article history:Received 31 August 2008Accepted 30 January 2009Available online 8 February 2009

Keywords:Eger RiftQuaternary maar volcanismgeophysical surveySaalian lake sedimentsvolcanic hazard potential

Based on results of previous investigations of tephra-tuff volcaniclastic deposits and a geophysical survey inthe surroundings of the Železná hůrka Quaternary volcano, West Bohemia, we performed detailed geo-physical surveys using gravimetry, magnetometry and electrical conductivity techniques. Striking anomalieswere revealed in a morphological depression near Mýtina, West Bohemia, as a strong evidence of the assumedmaar-diatreme structure. The sharp isometric gravity lowof−2.30mGal, as well as the corresponding positivemagnetic anomaly of 200 nTwith a negative rim on its northern side indicate a steeply dipping geological bodyof low density and containing magnetic rocks/minerals. Magnetic survey also showed pronounced localanomalies outside the depression that can reflect relicts of the tephra rim of the maar.This geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly.Macroscopic on-site evaluation of the core, and more detailed sedimentological, petrochemical, palynologicaland microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m oflake sediments reflecting a succession of several warm and cold climatic periods. Results of palynologicalanalyses confirm the presence of a continuous palaeoclimate archive, with at least three successive warmerperiods of most probably interstadial character from the upper Quaternary Saalian complex. Therefore, therecovered sediment sequence holds strong potential for in-depth paleoclimate reconstruction and deep bio-sphere studies.At the bottom of the Mýtina-1 (MY-1) borehole (84–85.5 m), country rock debris was found, containing alsovolcanic bombs and lapilli.The discovered volcanic structure is considered to be thefirst knownQuaternarymaar-diatreme volcano on theterritory of the Bohemian Massif. Because of hidden active magmatic processes in combination with earth-quake swarm seismicity ca. 20–30 km north of the Mýtina maar, reconstruction of the palaeovolcanologicalevolution is important for evaluation of hazard potential of the NE and E part of the Cheb Basin.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Maar-diatreme volcanoes are the most common volcano typefollowing scoria cones. TheWesteifel, Germany, is the classical area formaar-diatreme volcanoes worldwide with more than 68 of such

gfz-potsdam.de (H. Kämpf),.de (J. Mingram),am.de (A. Brauer),@[email protected] (M. Seidl).

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structures of Quaternary age (Büchel and Pirrung, 1993). Maar-diatreme volcanoes develop when magma rises in dykes andintersects with water causing thermohydraulic explosions (Lorenzet al., 2003). Up to now several Tertiary maar-diatreme volcanoes areknown from the northern part of the BohemianMassif (Brus & Hurnik,1984; Lindner et al., 2006; Kopecky, 1968; Krutsky, 1994; Malaseket al., 1980; Rutsek,1973; Suhr & Goth,1996; 1999;Wiedemann,1961),which are related to volcanic activity of the Eger Rift. The Eger Rift(Kopecky, 1978; Mrlina, 1980) is part of European Cenozoic RiftSystem (ECRIS; Ziegler, 1992). Within ECRIS Quaternary maarvolcanoes are well known from the French Massif Central and thewestern Eifel (Fig. 1, inset map).

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Fig. 1. Location map of the western Eger Rift area (after Geissler et al., 2004 and Ibs-von Seht et al., 2008). Black full and dashed lines: fault zones; triangles: Quaternary volcanoes ofŽelezná hůrka (ZH) and Komorní hůrka (KH); red circle: location of Mytina Maar; white circles: seismicity (ML N0.5) from 1962 to 1999 (Neunhöfer, 2000); stars: locations of mainfocal zones; yellow ellipse: main area of active CO2 emissions sites; full and dashed lines: fault zones; Upper right: position of the study area (red square) within the Europeanvolcanic province (after Wilson & Downes, 1991). Abbreviations are as follows: MLFZ, Mariánské Lázně fault zone; PPZ, Pocatky–Plesna zone; TFZ, Tachov Fault zone; Cheb–Dom.Graben, Cheb–Domažlice Graben; MC, Massif Central; AM, Armorican Massif; BM, Bohemian Massif; MN, Moldanubian; RH, Rhenohercynian; ST, Saxothuringian.

98 J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97–112

The area under investigation (Figs. 1, 2) is located at the NW cornerof the Bohemian Massif. According to Fiala and Vejnar (1997, 2004)and Richter and Stettner (1993), the crystalline schists of the localVariscan basement comprise a succession of psammo-pelitic, carbo-natic, and volcanogenic rock sequences of an Upper Cambrian toOrdovician age. The crustal segment of the surroundings shows largeareas with Variscan intrusives dominated by granites (easternsurroundings of the area: Karlovy Vary and Zandov plutons, westernsurroundings: Mitterteich and Flossenbürg Granites).

During Late Eocene the Eger Rift started to evolve. According toKämpf et al. (2005 and citations therein), the following steps ofCenozoic alkaline volcanic activities were recognized in the westernpart of the Eger Rift:

1. Early Oligocene–Early Miocene (31–20Ma): volcanic activity of theRift in (i) the eastern and central part — the České Středohoří Mts.and the Doupovské HoryMts., (ii) the western Eger Graben and theadjacent Krušné Hory/Erzgebirge Mts. and the Smrčiny/Fichtelge-birge area (24–20/15Ma), (iii) thewesternmost continuation of therift as far to the Frankonian Lineament/northern Oberpfalz area(29–19 Ma).

2. Middle to Late Miocene (16.5–8.3 Ma): volcanic activity synchro-nous with the graben formation dated by its pre-Middle Miocene?(N11.7 Ma) up to Late Pliocene sedimentary fill.

3. Middle Pleistocene (0.7–0.3 Ma): two volcanoes (scoria cones) inthe Cheb Basin area, Komorní hůrka/Kammerbühl and Železnáhůrka/Eisenbühl.

The Cheb Basin, a small (c. 265 km2) intracontinental basin, islocated in the western Eger Rift. The basinwas formed during the Late

Tertiary as a result of the reactivation of three fault zones (N–Strending Regensburg–Leipzig–Rostock zone, the ENE–WSW trendingEger Graben faults and the NNW–SSE trending faults of the Cheb-Domažlice Graben) by continental rifting (Bankwitz et al., 2003 a,band citations therein).

Presently, the area is characterized by remarkable geodynamicactivity and possibly magmatic unrest. It is known as one of the mostactive earthquake swarm regions in continental rifts worldwide withthousands of small and intermediate magnitude swarm earthquakes(MLb5) (Fischer and Horálek, 2003; Neunhöfer and Hemmann, 2006;Ibs-von Seht et al., 2008). Primarily, earthquake swarms are a pecu-liarity of volcanic regions andmid-ocean rifts; however at present, thewestern Eger Rift is classified as a non-volcanic region in CentralEurope. The earthquake activity seems to be connectedwith small, butactive surface movements related to tectonic stress evolution (Mrlinaand Seidl, 2008). The seismicity is furthermore correlated in space andtime with significant CO2 dominated degassing at the surface inmofettes and mineral springs (Bräuer et al., 2003; Kämpf et al., 2007).For the first time, a hidden magmatic activity in the eastern part of theCheb Basin has been traced by 3He/4He characteristics from repeatedgas sampling between 1993 and 2005 (Bräuer et al., 2005; 2008a, b).Various lithospheric studies indicate an updoming of the lithosphere–asthenosphere boundary (LAB) to approximately 80–90 km depth, avelocity anomaly at the base of the lithosphere between 50 and 65 kmdepth, an updoming of the Moho (up to 26 km from approx. 31 km inthe surroundings) or thickening of the lower crust (rift pillow?) at thebase of the crust (Mrlina, 1980, Hofmann et al., 2003; Hrubcová et al.,2005; Geissler et al., 2005; 2007; Heuer et al., 2006; 2007; Babuškaand Plomerová, 2008).

Fig. 2. Two views of topography (DEM) of the Mýtina surrounding region. Left: 3D view from south, view inclination 77°, sunlight fromwest. Morphological features show generaldrainage trends with prevailing direction NNW–SSE, perpendicular ENE–WSW, and N–S. Mýtina depression is in the centre, on top of a hilly massif (orange to ochre colour). Right:top view of gray scale DEM (sunlight from NW) with drainage system indicated by blue, red and green dashed lines. 1 — Železná hůrka volcano, 2 — CO2 emmission site, 3 —

exploratory trench in Mýtina (Geissler et al., 2004), 4 — morphological depression of the assumed maar.

99J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97–112

The working area is located at the southern periphery of theCheb Basin between the Mýtina village (north) on territory of theCzech Republic and the Neualbenreuth village (south) on the Germanside (Fig. 2). From NW to SE the crystalline basement consists ofphyllites, quartz phyllites and quartzite lenses bearing mica schists ofUpper Cambrian age belonging to the Cheb–Dyleň Crystalline Unit(Fiala and Vejnar, 1997; Tonica et al., 1998). The NNW–SSE trendingvalley between the depression of the assumed maar and the Kozlyvillage (Fig. 2) is marked by a fault zone/quartz vein structure,belonging to the Tachov fault of late Hercynian age. The Železná hůrkascoria cone is also located at the Tachov fault (ca. 700 m SE of theproposed maar).

2. Statement of problem

Our paper comprises the third step of our study of Quaternaryvolcanism at the southern rim of the Cheb Basin, western part of theEger Rift.

In the first step, Geissler et al. (2004) studied an app. 4 m thickprofile of volcaniclastic deposits in a temporary trench near Mýtina.The only known nearby volcano at that time was Železná hůrka/Eisenbühl (Reuss, 1852; Proft, 1894; Lochmann, 1961; Kämpf et al.,1993; Schwarzkopf, 1997) in about 1.5 km distance from the trench.Geissler et al. (2004) found geological indications for the possibleexistence of another source of volcanic material surrounding Mýtinathan just Železná hůrka — they assumed an unknown maar structure.A morphological depression of ca. 500 m in diameter and 50 m indepth was proposed as a potential maar structure (see Fig. 10 inGeissler et al., 2004).

In the second step, Mrlina et al. (2007) tested the hypothesis fromGeissler et al. (2004) by geophysical (gravity andmagnetic) profiling ofan N–S directed area of 200 m×1200 m, which crossed the mor-phological depression. The gravity map showed a striking isometricgravity low of about −2.3 mGal in the centre of the morphologicaldepression, whereas themagnetic map exhibited a positive anomaly upto 200 nT related to the same source. These distinct geophysicalanomalies were the first in-situ proof of the assumed maar. The age oftheMýtina tephrawas dated onphlogopite and olivine-nephelinite rockmatrix by 40Ar–39Ar laser dating. The average Ar–Ar age is 288±17 Ka(Mid Pleistocene). In the trench, the magmatic (juvenile) componentincreases fromabout 12% to 32% in the tuff layer at the base,whereas it isabout60% in the tephra layerat the topwithout systematic trend in time.In summary, Mrlina et al. (2007) concluded that a Quaternary maarstructure is most likely, but without drilling the final evidence was stilllacking.

In order to provide such evidence for a maar structure nearMýtina,we started a systematic aerial geophysical (gravimetric, magnetic and

geoelectric) mapping of the area approximately 1 km×1 kmwith theassumed maar structure in the centre. Finally, an exploratory drillingnear the centre of the gravity anomaly by wire line technique wasconducted in 2007 (borehole MY-1). The geophysical and drillingresults represent the principal content of this paper. Additional to ourcommon interest in the neovolcanic evolution of the western EgerRift, a strong motivation was directed to palaeoclimate reconstruc-tions, if maar sediments were present.

3. Geophysical investigations

In the last years the effectiveness of gravity and magnetic inves-tigations of maar-diatreme structures in Central Europe, namely in theEifel, Bavaria, Saxony, W and N Bohemia, was demonstrated amongothers by Mrlina et al. (1989); Büchel and Pirrung (1993); Gabriel(2003); Kroner et al. (2006); and Mrlina and Cajz (2006). In otherregions, for example the explosive craters in the Coastal Mts., Syria, asuccessful localization of volcanic channels was also based on gravityand magnetic measurements (Mrlina and Rychtar, 1990). Similargeophysical surveys were applied to study maar structures in NewZealand by Cassidyet al. (2007). Based onprevious experience, gravity,magnetics and geoelectrics were selected for detailed mapping of theassumed maar structure near Mýtina.

3.1. Geomorphology of the area

In order to demonstrate the position of the investigated struc-ture and the character of the geomorphology, we built a detailedDEM (Digital Elevation Model) based on SRTM90 data with approx.90 m×90 m sampling, supplemented by elevation data of allgeodetically observed points that we measured during geophysicalsurveys. Detailed editing was necessary to the data as the SRTMvalues were randomly offset by up to 20 m.

In Fig. 2 the unique shape of the morphological depression understudy can easily be recognized. This almost circular structure is locatedon top of the first hilly block to the south of the limit of the Cheb Basin.Further to the SE (Fig. 1) there is the massif of Dyleň (940 m a.s.l.) aspart of the Český Les morphological unit prolonged to the south alongthe Czech-German border.

The diameter of the depression between Mýtina and Neualben-reuth is about 500 m and the relative depth is 50 m (Figs. 2, 3). Thedepression opens to NNW into the valley of the Kozly brook. The valleymay have its origin due to a fracture/fault within the Tachov FaultZone (TFZ), Fig. 1. The TFZ was defined and discussed e.g. by Mahelet al. (1984), Tonica et al. (1998), and Bankwitz et al. (2003).

Three principal geomorphologic directions can be recognized inthe surrounding region (Fig. 2). Firstly, the NNW–SSE drainage system

Fig. 3. View into the maar depression from the eastern rim. The depression is covered by various types of forest and some wood cutting was necessary during the surveying work.Trees did not allow using the GPS instrumentation in most of the depression. (Photo by Jan Mrlina).

Table 1Technical parameters of geophysical surveys, rms — root mean square error (in case ofgravity rms of raw data).

Technique Instrument Measurement interval rms Totalstations

Gravity LCR D-188 and 10–40 m detail grid 0.008 mGal 440Scintrex CG-2 50–100 m

reconnaissance0.015 mGal

Magnetics GTL TM-4 20–50 m reconnaissance 1 nT 191(Cesium vapour) 0.5–5 m detail profiles +test lines

Geoelectrics Geonics EM31 –SH10 m on profiles 1 mS/rn 2189Positioning Trimble 5700 Accordingly 0.03 m (Z) n/a

Leica TCR 403 Accordingly 0.02 m (Z)Zeiss Ni025 Accordingly 0.08 m (Z)Garmin_76 S Accordingly 4.00 m (XY)


is expressed by a significant valley heading to NNW from aroundNeualbenreuth. This is likely the main expression of the TFZ. Secondly,the Eger Rift ENE–WSWdirections are documented by shorter valleys.In one of them the Železná hůrka scoria cone is located. The thirdsystem is more or less N–S oriented and corresponds to the trend ofthe Regensburg–Leipzig–Rostock zone (RLRZ) defined by Bankwitzet al. (2003).

The crossing of the three systems provides reason for an activetectonic process in this exposed area and is already documented bythe presence of the Quaternary Železná hůrka volcano. From thecharacter of drainage system in Fig. 2 we can deduce that the ENE–WSW system (Eger Rift) is affected by younger (rejuvenated) systemsNNW–SSE (MLF, TFZ) and N–S (RLRZ) that are also seismically active,especially about 30 km N of Mýtina (Fig. 1). The principal Nový Kostelfocal area of the wholeWest Bohemia/Vogtland region is indicated byhigh numbers of earthquakes in Fig. 1. The CO2 dominated degassing ismarked by 3He/4He ratios within the subcontinental mantle range(close to 6 Ra), which implies its magmatic origin (Bräuer et al., 2005,2008a).

3.2. Gravity, magnetic and electromagnetic surveys — methods

During the initial stage, the gravity survey was performed with aScintrex CG-2 gravimeter (Scintrex, Canada) (Mrlina et al., 2007), butthe principal part of the detailed mapping was carried out using theLaCoste&Romberg D-188 gravimeter (LCR, USA) with high resolutionof 1 µGal (10−8 m/s2). The overall accuracy of gravity measurementswas 8 µGal (0.008 mGal).

Magnetic survey (TMI — total magnetic intensity field) was con-ducted with a CS-vapour magnetometer TM-4 (GTL, Australia). Ondetailed profiles the option of continuous recording was applied inorder not to miss any local magnetic sources. The measurementresolution was better than 0.1 nT, final accuracy 1 nT.

The gravity and magnetic surveys (Inst. of Geophysics ASCR,Prague, Czech Republic) have been applied since the beginning of theinvestigation (Mrlina et al., 2007). In the later principal stage of aerialmapping, beside gravity and magnetics we also applied near surfaceelectrical conductivity measurements (Jena University, Germany)using an EM31-SH instrument (Geonics, Canada) with frequencyof 9.8 kHz, inter-coil spacing of 2 m and penetration depth of up to2–3 m. This equipment was successfully used, for example, in theinvestigation of a Tertiary maar in Saxony, Germany, by Kroner et al.(2006). The gravity survey covered the complete study area of 1 km2,

the magnetic survey approx. 0.7 km2 and the electrical conductivitysurvey about 0.5 km2, as shown in Fig. 4.

All geophysical measurements were based on accurate positioningof observation stations. We used Trimble 5700 DGPS (Global Position-ing System in differential mode) to determine precise coordinates of anumber of points around the morphological depression. These pointsserved later as reference for trigonometric measurements inside thedepression. For this purpose we used a digital Leica TCR 403 totalstation, as the access to detailed profiles or singular pointswas difficultdue to various types of forests covering the depression (Fig. 3). Thecoordinates were acquired with a horizontal accuracy of about 0.08 mand vertical accuracy of about 0.02 m. This was perfectly sufficientfor gravity data processing. In a few cases, the Zeiss Ni025 altimeterwas used for levelling. Technical parameters are given in Table 1.

Gravity points were distributed in an irregular grid respectingdifficult access, with distances between points down to 20 m in thecentral area, rarely even less in the very centre of the depression, seeFig. 4. Magnetic points were distributed in a similar irregular grid asthe gravity ones. The two sets of points, however, are not identical dueto different periods of surveys and different system of positioning.Thanks to random distribution of observation points in these grids,both gravity and magnetic maps cover sufficiently the assumed maararea without any major gaps. In areas without data in largersurroundings of the structure, further measurements are planned,especially in case of magnetics, in order to define the extent of tuff andtephra coverage.

The electromagnetic survey was oriented along profiles tied togravity points (geodetic control), see Fig. 4d, with some detailed

Fig. 4. Geophysical survey location maps. (a) air-photograph (Geodis, Praha) in WGS84/UTM33 grid coordinates, with gravity (kyan) and magnetic (red) points, location ofmodelling Profile 16 and position of MY-1 well (yellow circle). Geophysical maps: (b) gravity— contour interval 0.1 mGal; (c) magnetics— contour interval 25 nT, red polygonsmarkthe test detailed survey areas; (d) electrical conductivity — contour interval 1 mS/m, observation profiles in dark blue, principal anomalies A–F in white colour.


“infill” profiles positioned by a hand-held Garmin GPS with relativehorizontal accuracy of about 3–4 m.

Details on gravity and magnetic data processing were given byMrlina et al. (2007). There were no significant changes in this stage,except more accurate gravimeter's drift control based on highernumber of repeated measurements during the daily observationprogramme. On the gravity data the following corrections wereapplied: tidal correction, instrumental drift, normal gravity, free air,Bouguer plate and terrain corrections due to pronounced topography(a depression 50 m deep, with a diameter of almost 500 m). Terraincorrections (TC) were calculated to the same distance as in case of allprevious gravity mapping projects on the territory of formerCzechoslovakia— to 167 km from each point. Detailed DEM improvedby our own elevation data was used. Estimated TC accuracy is below0.05 mGal, however, this value represents the highest contribution tothe total accuracy of the final Bouguer gravity anomalies (better than0.10 mGal). For the Bouguer correction the standard density of2670 kg/m3 (2.67 g/cm3) was used.

Magnetic observations were corrected for temporal variations ofthe earth's total field using data from a continuously running protonmagnetometer (PM-2) at a base station, or using data from magneticobservatory in SW Bohemia that was tested already during previousstages of the survey. The area does not exhibit any industrial noise.Magnetic anomaly data were referred to the average value of theundisturbed surroundings.

3.3. Geophysical indications of the maar structure

The resulting geophysical data were gridded and presented ascontour maps in Fig. 4. In Fig. 4a the location of gravity and magneticpoints is shown on top of an aerial photograph, as a further stage ofFig. 3 in Mrlina et al. (2007).

We consider the negative gravity anomaly with an amplitude of−2.30 mGal to be the most significant feature (Fig. 4b). The sur-rounding area is characterized by a general gradient of 1.5 mGal/km(increasing gravity from SE to NW), while the gradient is sharper in


the NW part of the study area. The gravity field in the SE corner of themap is relatively flat.

The observed anomaly is of regular isometric shape, slightly elon-gated in NNW-SSE direction (ca. 280 m×240 m, based on inflectionpoints of gravity gradients), with a relatively flat centre and apronounced circular gradient of 1.3 mGal/100 m indicating a steepdip of the source body. This corresponds well with the usual shape ofmaar structures, which is an inverse cone. The apparently weakergradient (ca 0.7 mGal/100 m) to the S, SSE is caused just by theinterference of the regional gravity gradient and the gradient related tothe investigated structure.However, further to SSE there is an indicationof a very weak gravity low of about −0.2 mGal amplitude at aroundx=316,800, see Fig. 4b. It may be related to the northern surroundingsof the Železná hůrka volcano where we can expect accumulations oflowdensity volcaniclastic rocks. Also, bandingof gravity contours to theENE of the principal anomaly may indicate minor variations of rockdensity or layer thickness related to volcaniclastic deposits.

The principal positivemagnetic anomaly is also located inside themorphological depression. In general, it has an isometric shape aswell, but with much more internal variations compared to the gravityanomaly. The main maximum is located at the southern part of theanomaly and extends in ENE–WSW direction, with the maximumamplitude of about 200 nT relative to the normal magnetic total fieldin the adjacent region. However, there is another smaller, but pro-nounced local maximum that forms the northern part of the anomaly.Only averygentle increase ofmagneticfield intensity canbe recognizedin the centre of the anomaly.We have to point out that if we convert theobserved Total Magnetic Intensity field (TMI) into Reduced-To-Pole(RTP), the amplitudes will change in favour of the northern maximum.At the same time the negative ring (about −100 nT relative to normalmagnetic field) along the northern side of the anomaly (blue in Fig. 4c)will mostly disappear. It means that both southern and northernmaxima indicate the highest amount of magnetic minerals/rocks to bepresent in the subsurface. As the changes of amplitudes are not toosharp, we can estimate that the main source rock is located at a deeperposition. The maxima may correspond to near subsurface accumula-tions of magnetic rocks, possibly caused by a scoria cone at the base ofthe maar crater or a later intrusion into the maar filling.

Beside this principal anomaly, there are clear indications for veryhigh amplitude local anomalies up to, or over 1000 nT. They wereidentified by short magnetic test profiles in the SW, S, SE and NE areasoutside the morphological depression, see Fig. 4c. We assume theyreflect the volcanic fall material accumulations (tephra) at the flanksof the crater — the non-eroded relicts of the ejecta rim around thecrater. From these magnetic observations we also concluded that thevolcaniclastic material could extend (far) behind the edges of thedepression, as far as the exploratory trench NW of the Mytina village,where a 4 m section of tuff and tephra was found by Geissler et al.(2004). A more detailed image of the extent and structure of theerupted material can be obtained from further magnetic mapping.

Microgravity test profiles were measured in the SW corner of themap, just outside the depression, on the gentle slope to the south. Thereason for these profiles was the presence of volcanic bombs on thesurface of this site. Only small variations in gravity were observed withmaximum amplitude of up to 0.20 mGal. In contrast, magnetic testprofiles showedstriking sharp local anomalies of up to1000nT. Thismaybe evidence of one residual part of the ejecta rim around the crater. At alater stage also the electrical conductivity wasmeasured at this site andincreased conductivity values by up to 14mS/mwere found (anomaly Cin Fig. 4d). The highest amplitudes of magnetic anomalies wereobserved in the NE test area, up to 2000 nT, see Fig. 4c. This is mostprobably another place with significant relicts of the ejecta rim. In thisplace also the electrical conductivity shows slightly increased values 6–8 mS/m, with one local anomaly D of 14 mS/m (Fig. 4d).

Maar structures are set apart by higher electrical conductivityvalues as water can be better stored in higher porosity breccia and

young sediments than in the surrounding country rock. Thus, thesemeasurements can be deployed to scan for the outcrop of the struc-ture apart from morphological information. We observed such a pro-nounced conductivity anomaly E (up to 18 mS/m) elongated in theNNW–SSE direction in the NW edge of the assumed maar structure.We think that it is caused by a spring and a small creek directed toNNW through a small valley that forms the only open outlet of thedepression. As there is no obvious corresponding magnetic anomaly,an ejecta dominated source is unlikely. Future investigations willfocus on the possible existence of a paleo-valley filled by lahars andbase surges.

The largest, but relatively weaker (14 mS/m) conductivityanomaly A (Fig. 4d) is the triangular feature that is slightly shiftedsouthward from the centre of the depression. We expect this anomalyto be caused by highly water saturated unconsolidated subsurfacesediments. The local conductivity anomaly B on the northern edge ofthe structure correlates with a magnetic anomaly, supporting the ideaof a volcanic source there. All these conductivity anomalies aresurrounded by a low conductivity circle (2–5 mS/m, blue in Fig. 4d)following the steeper outer part of the depression slopes. These slopesare apparently formed by country rocks, mainly phyllite. The mea-surements performed along the outer side of the circular edge of thedepression show slightly increased values on the ENE and NE side,which is in agreement with high amplitude magnetic anomalies seenin the test profiles, as mentioned above. The anomaly F is likely ofman-made source. No further indications for obvious anthropogenicinfluences have been found so far.

We consider the results of the geophysical surveys as very signif-icant and clear evidence for the existence of a maar structure. Theprincipal gravity and magnetic anomalies are of comparable dimen-sions. Given the shape and sharpness of the gravity anomaly delin-eated by steep gradients, as well as the intensity of the magnetic totalfield anomaly, their source is expected to be a steeply dipping struc-ture, such as the assumed maar. The results of the magnetic surveyoutside the morphological depression indicate a high concentrationof volcanic material to NE, as well as SW, S and SE. The source ofanomalies can be the relicts of a maar ring (ejecta rim).

3.4. Gravity and magnetic 3D modelling

A preliminary gravity/density model of the investigated structurewas calculated byMrlina et al. (2007). It was composed of four differentgeological bodies: country rock (phyllite), near surface sedimentarycover, low-density fill in the upper part (high-porosity volcaniclasticmaterial and possiblymaar sediments), and increased-density fill in thelower part of a maar-diatreme inverted cone. Based on new geophysicaldata covering the whole maar structure, we performed 3D gravity and3D magnetic modelling. The information from the core analysis of theMY-1 well was used to upgrade the original gravity model.

For the forward gravity modelling, the program package IGMAS(Interactive Gravimetric and Magnetic Application System) by Götze& Lahmeyer (1988) is used. For modelling the model area is dividedinto a number of parallel vertical planes which itself consist of closedpolygon lines separating units of different density or magnetic sus-ceptibility. The model bodies are generated by triangulation betweenthe vertices of corresponding polygon lines between neighbouringplanes. This modelling procedure was similar to the interpretation ofthe Ebersbrunn Tertiary maar in Saxony, Germany, by Kroner et al.(2006). For the modelling of the total magnetic field anomaly onlyinduced magnetization was considered. The reason for neglectingremanentmagnetization is basedon the fact that themagnetic-relevantinterior of the structures cannot be expected to be ordered. The innerpart consists of a number of smaller bodies with randomly orientatedremanent magnetization. Topography was not included in the model-ling as the Bouguer gravity data onwhich the gravimetric modelling isbased are reduced for terrain effects.


For the gravimetric modelling, a regional gradient of−1.56 mGal/km in SSE to S direction, taken from Hofmann et al. (2003) wasremoved from the Bouguer anomaly. Our 3D geophysical modelconsists of 26 parallel profiles extending E–W. As an example, wepresent the Profile 16 in Fig. 5. The density model was parameterizedaccording to Mrlina et al. (2007), but in more detail, based on the MY-1 core observation and on preliminary density test in a laboratory (10samples showed wet densities between 1.90 and 2.20 g/cm3). Weconsidered the following density values:

• maar lake sediments 1.90–2.20 g/cm3 (10−3 kg/m3) (2–84 m)• outer maar debris/fracture zone 2.55 – 2.60 g/cm3 (84–? m)• inner maar debris 1.95 g/cm3 (84–? m)• colluvium 2.25 g/cm3 (0–2 m)• phyllite/country rock 2.65 g/cm3 surroundings.

The layers of organic sediments were less than 2m thick, so no realimpact on gravity is expected from the depth of 75 m. The modelcontains 15 bodies: 2 representing the sediment filling of the maar,11 the lower part of the chimney, 1 for the overlaying colluvium and1 for country rock. The resulting gravimetric model is shown in Fig. 5(only 7 of the 15 bodies are present in the selected Profile 16).The maximum density contrast to the surrounding rock is about0.70 g/cm3. Density differences between outer maar debris/fracturezone and country rock range from 0 to 0.10 g/cm3.

The layout of the magnetic model is identical to the gravimetricone. The magnetic field parameters (total intensity: 48,640 nT,declination: 2.09°, inclination: 66°) were taken from the GeologicalSurvey of Canada (2006), which provides these parameters in globalcoverage. The rms for the gravimetric model is 0.14 mGal, and 20 nTfor the magnetic model, respectively. In total a good agreementbetween the models and the observations exists.

The final model is presented in Fig. 5. The contrasts in magneticsusceptibility of the diatreme structure with respect to the country

Fig. 5. Geophysical model (bottom), gravity (centre) and magnetics (top). Selectedprofile (W–E) passing close to MY-1 borehole, see Fig. 4. Profile 16 represents a centralplane of 26 planes of the 3D modelling set. Table of used density and magneticsusceptibility values at bottom right.

rock range from 100 to 1100×10−6 SI. Magnetic susceptibility of thesediment filling is clearly lower with values of 300 to 400×10−6 SI.The overlaying colluvium has an even smaller magnetic susceptibilityof 100×10−6 SI. The main contrasts exist between the diatreme itselfand the country rock.

From the gravimetric and magnetic modelling, a cone-shapedstructure with dip angles of about 80° can be inferred. In its centralpart the thickness of the sediment deposits is about 100 m (84 m inMY-1 borehole). Towards the border of the structure the sedimentthickness decreases to a few meters. The diameter of the structure atthe top is in the range of 200 m. A good agreement exists to the prin-cipal shape of maar-diatreme volcanoes as e.g. discussed in Lorenzand Kurszlaukis (2007). The modelling depth is constrained by thelateral extension of the survey area. Therefore the models containonly the region above the root zone. Due to the integrated modellingand the laboratory-based density values clear constraints exist regard-ing ambiguities with respect to principal geometry and inner structure.E.g. a structure with smaller dip angles produces an anomaly thatevidently deviates from the observed ones. The variance in the densityvalue of the outer diatreme zone (body 7 and 9) in contrast to thecountry rocks (body 2) can be explained by a halo, characterized byslightly reduced density because of the joints and micro-fractures,formed by phreatomagmatic explosions in the root zone of the maar-diatreme volcano as described in Lorenz and Kurszlaukis (2007). Thevariance in the susceptibility value of bodies 7 and 9 can be caused bydifferent amount of magmatic and country rock present in the twobodies. For a modelling of more detailed features the survey grids needto be refined.

4. Geological investigations

Geological investigations presented in this paper are based on thecore samples from the borehole MY-1. Beside macroscopic on-siteevaluation (finding of maar sediments, volcanic bombs and lapilli, andcountry rock breccia), further sedimentological, palynological, petro-chemical and geomicrobiological analyses have been performed inorder to evaluate the potential of the core samples for a paleoclimatestudy, as well as for further volcanological investigation.

4.1. Exploratory drilling MY-1

The detection of striking geophysical anomalies (Mrlina et al.,2007, and this paper) was the basic motivation for an exploratorydrilling near the centre of the gravity anomaly in order to test/provethe hypothesis of a maar structure. The expectation was to extract acore with debris of country rock and volcanics, and possibly maar lakesediments. Eventually, we continued as deep as 85.5 m. In the interval0–84 m the core is composed of Quaternary clay to silty clay(unconsolidated maar lake sediments) with intersections of organicsediments and gravel. In 84–85.5 m there was phyllitic debris withvolcanic bombs and lapilli.

The borehole MY-1 has the following coordinates in WGS84/UTM33N datum:

Lat 49°59'48.8q, Long 12°26'15.4qX=316359, Y=5541430,Elevation=569.8 m Balt adj. msl.

4.2. Sedimentology, palynology, petrography, geomicrobiology — methods

All cores obtained from the well have been sealed in the field andopened under lab conditions at GFZ Potsdam in March/April 2008.Besides core description and photographic documentation a first set ofpetrographic thin sections and pollen samples has been prepared,whereas ongoing geophysical and geochemical investigations of thecore material will be published elsewhere.

Fig. 6. Lithological profile of the Mýtina 2007-1 (MY-1) well with indicated positions of preliminary pollen, thin section, and microbiological samples. Letters in brackets from thelegend refer to corresponding core photos from Fig. 7.


From all sediment lithotypes of the drill core a first set of large-scale (10 cm long) petrographic thin sections has been prepared, usingthe freeze-drying method after Werner (1966). Microscopic analyseshave been performed using a Carl Zeiss Axioplan microscope withpolarizing contrast, a Carl Zeiss Jenalumar for fluorescence withincident, broad-band blue light excitation, and a Leica DMC 480 digitalmicroscope camera for documentation.

Preparation of pollen samples followed methods described byBerglund & Ralska-Jasiewiczowa (1986). Lycopodium spores wereadded to each sample for calculation of pollen concentrations. Pollenpercentages were calculated on the basis of terrestrial pollen, exclud-ing aquatics and spores.

Samples from nephelinitic host rock and crustal xenoliths from thetuff-tephra deposit in the Mytina trench, as well as two lapilli and onephyllitic sample (pebble) fromMY-1 borehole (84–85.5 m depth), seeFig. 10, were analysed for their chemical composition by X-rayfluorescence (XRF) and inductively coupled plasma mass spectro-metry (ICP-MS). Samples were ground in an agate mill to homo-geneous rock powders of b62 µm grain size. Major element oxides andtrace elements (Ba, Cr, Ni, Rb, Sr, V, Zn and Zr) were analysed by X-rayfluorescence spectrometry on a Siemens SRS303AS spectrometer. Therare earth elements (REE)+Y, Cs, Hf, Pb, Th and U were analyzed byinductively coupled plasma-mass spectrometry (ICP-MS) on a Perkin-Elmer/SCIEX Elan 5000 ICP mass spectrometer according to the

method and with precision and accuracy outlined by Dulski (2001).Total water and CO2 were determined by IR-spectrometry (LECO CHelemental analyser) or thermal conductivity measurements (vario EL)after decomposition of the rock powder in a 1000 °C oxygen stream.FeO was analysed by potentiometric titration using a modification ofthe Wilson procedure (Wilson, 1955).

Samples of 2 cm3 each for geomicrobiological investigations weretaken immediately after the retrieval of the core of the MY-1 boreholefor enumeration of microbial cells by pushing cut-off syringes intothe centre of the core (Smith et al., 2000). Samples were fixed at thedrill site in a solution containing 120 mg/L NaCl, 250 mg/L Na2SO4,and 20 ml/L formalin. Also, 2 cm3 samples of the drill fluid weretaken in regular intervals and processed like sediment samples.Samples were stored at 4 °C until analysis. For enumeration, smallaliquots of the sample were filtered onto 0.2 µm pore size WhatmanAnodisc Filters and stained with SYBR Green I, according to Noble &Fuhrman (1998). No independent contamination control method,like dyes, microspheres, or perfluorocarbon (Phelps et al., 1989) wasemployed.

4.3. Sediment microfacies

Themain lithological profile (Fig. 6) exhibits a discontinuous recordof ca 82mof lacustrine sediments belowa 1.7m thick cover of colluvial

Fig. 7. Representative core photos of the main lithological types from M 2007-1 (MY-1). All depths given below are related to individual core barrels; scales with the photos arerelated to the corresponding core barrel. A— colluvial deposit with erratically distributed pebbles, B— silty clay, partly with faint lamination, with intercalations of dm-scale organic-rich, non-laminated horizons, C — mainly loose, medium- to coarse sand, D — silty clay, mostly with faint lamination, E — silty clay with some pebble-rich horizons, F, G — finelylaminated sediments with high organic content (F — transition from silty clay into laminated organic sediments), H — mainly pebbles and cobbles with some silty clay.


material (Fig. 7A). Visual inspection and amended preliminary smear-slide and thin section analyses revealed that most of the sediments aregreyish, minerogenic fine-grained lake deposits. Large parts of these

minerogenic sediments show faint lamination, but lack any signs ofclear gradation (Fig. 7D). Occasionally intercalated are horizons ofcoarser clasts (Fig. 7E), composed of typical phyllitic-quartzitic rocks of

Fig. 8.Mýtina sediment microfacies from thin sections. All photos, except A, are from laminated, organic-rich sediments of lithotype G (75.6–75.86m); photo A is from lithotype D. Allphotos with fluorescence taken with incident, broad-band blue light excitation. Arrows point to bottom. A — silty clay with faint lamination, caused by minerogenic grain sizevariations, crossed polarizers, B — alternation of minerogenic and organogenic laminae, partly crossed polarizers, C — lake sediments with high organic content, enrichments ofChrysophyte cysts, Botryococcus colonies, and Pinus pollen, parallel polarizers, D — same as C, but under fluorescence, E — laminated sediment with enrichments of Pinus pollen,fluorescence, F — laminated sediment with enrichment of Botryococcus colonies, fluorescence, G — lamination with a single diatom bloom, parallel polarizers, oil immersion, H —

massive enrichment of Pinus pollen, fluorescence.



the catchment, and, to a minor portion, of volcanic rock-clasts. Inaddition, loose, coarse sand has been recovered from 21–25.6 m drilldepth (Fig. 7C). The base of these sands can not be exactly definedbecause of a ca 20 m gap with no core recovery below ca 25.6 m drilldepth. Most likely this failure in core recovery is due to furtherextension of unconsolidated coarse material which could not beobtained by the drilling technique used. Some dm-scale, organic-richhorizons from the upper part of the profile could be related toreworking of catchment material as they lack any lamination, or mayrepresent sedimentary gaps with palaeo-soils (Fig. 7B). Most out-standing, however, are three horizons (at 54–54.65 m, 55.8–56.10 m,and 74.9–76.6 m drill depth) of darker, finely laminated sedimentswith increased organic content (Figs. 7F,G). Beneath the lowermostorganic horizon the amount of pebbles increases, endingwith a coarse,mainly phyllitic breccia in the lowermost 1.5 m of the profile (Fig. 7H).This sediment facies resembles the coarse basal sedimentary infill ofpreviously studiedmaar lakes of similar size (e.g. Eocene Eckfeld MaarLake; Mingram 1998; Bullwinkel 2003; Lago Grande di Monticchio;Brauer et al., 2007).

Thin sections from predominantly minerogenic sediments revealthe expected clastic input of the local catchment, mostly quartz, feld-spar, and mica. A faint lamination is caused by grain size variationsonly, without changes in sediment composition (Fig. 8A). The organiccomponents of these sediments are mostly allochthonous terrestrialplant remains, whereas remains of lake biota like diatoms or spongespicules are rare. Surprisingly, diatom frustules are rare also in thelaminated organic intervals (Fig. 8B–H) although a few diatom layershave been found (Fig. 8G). Instead, chrysophyte cysts and the colonial

Fig. 9. Mýtina Maar. Simplified pollen percentage diagram (selected taxa). ExaggerationInterglacial/ Interstadial (temperate), III = Glacial (cool-moist), IV = Glacial (cool-dry), V =(cool-moist), VIII = Glacial (cool-dry), IX = beginning terrestrialization. Analyst: M. Stebic

freshwater alga Botryococcus (Chlorophyta) are very abundant(Fig. 8C–F), clearly indicating open water conditions. Experimentaldata have shown a stronger resistance of chrysophyte cysts againstdissolution compared with diatoms (Miller et al., 1990). It has to beproven if this is the case also for the Mýtina sediments. An alternativeexplanation could be different ecological requirements since greenalgae can be dominant over diatoms under conditions of highertemperatures with moderate or low ratios of both N/P and Si/P(Tilman et al., 1986). Mass blooms of Botryococcus (Fig. 8F) are oftenrelated to nutrient-rich conditions, and may dominate the phyto-plankton under favourable conditions. Botryococcus is also favouredunder conditions of lake stratification and clear epilimnia with highpotential insolation (Reynolds, 2006). In any case, the frequentoccurrence of Botryococcus in the laminated, organic-rich horizonsindicates elevated temperatures and stable summer stratification ofthe lake. Numerous graded layers in the organic-rich horizons, rangingin thickness from sub-mm to cm scale (Fig. 8B), point to a high-energetic environment with steep slopes of the lake basin.

4.4. Palynology

A first set of 14 pollen samples has been analysed to underline theimportance of a continuous lake sediment section for palaeoenviron-mental and palaeoclimate reconstruction.

Palynological investigations reveal sufficient (app. 10,000 cm−3)pollen concentration from predominantly minerogenic sediments andextremely high concentration (with up to N1,000,000 cm−3) from thelaminated organic intervals. The overall pollen preservation is good.

(x10) is indicated by line. 9 zones (I–IX) have been distinguished: I = Glacial, II =Interglacial/ Interstadial (temperate), VI = no data (no core recovery), VII = Glacial

h.

Fig. 10. Photographs of typical hand specimen of rocks from borehole MY-1 (depth: 84–85.5 m) probable from uppermost part of debris-flow deposits at the base of the lakesediment), left: phyllitic sample (pebble), right: nephelinitic host rock (lapilli, volcanic bomb).


The most exciting finding is the presence of several alternating warmand cold periods in theMýtina record below the core gap (Fig. 9). Highvalues of non-arboreal pollen in the predominantly minerogenic

Table 2Whole rock chemistry of host rock samples (MY-1/=samples from borehole MY-1/2007;

Sample Nephelinite

MY-1/2-84 MY-1/3-84 My1 My1-B

Mass-%SiO2 43.30 40.90 40.4 39.8TiO2 2.74 3.05 2.96 2.93Al2O3 11.20 12.10 11.5 11.4Fe2O3(t⁎) 11.26⁎ 11.86⁎ 5.58 5.63FeO 5.26 5.11MnO 0.19 0.19 0.19 0.19MgO 10.96 11.38 13.64 13.7CaO 12.19 12.82 12.67 12.54Na2O 3.20 2.83 2.53 2.61K2O 1.89 2.06 1.55 1.53P2O5 0.75 0.73 0.67 0.72H2O 1.46 1.42 2.21 2.28CO2 0.13 0.12 0.11 0.1Total 99.22 99.38 99.38 98.49

Mg# 0.72 0.73

Trace elements (ppm)Rb 82 69 61 59Sr 766 776 759 748Cs 2.89 1.39 0.91 0.89Ba 896 1143 780 770Th 9.95 9.78 8.9 9U 2.53 2.64 2.4 2.4Pb 5.99 5.46 2.9 3.1Y 23.1 23.2 21 21Zr 274 273 225 220V 295 329 315 320Cr 490 469 726 735Ni 178 170 249Zn 96 100 70 72La 70.1 72.7 68.4 68.3Ce 134 138 131 128Pr 15.3 15.6 14.8 14.8Nd 57.8 59.7 56.2 55.7Sm 10.1 10.4 9.61 9.65Eu 2.93 2.94 2.92 2.83Gd 7.94 7.95 7.69 7.42Tb 1.03 1.07 0.97 0.97Dy 5.56 5.49 4.94 4.89Ho 0.97 0.96 0.81 0.83Er 2.3 2.35 2.06 2.05Tm 0.3 0.31 0.25 0.26Yb 1.8 1.78 1.51 1.51Lu 0.27 0.27 0.21 0.2Hf 6.03 6.15 5.26 5.35

sediment intervals point to open landscapes of steppic to woody-steppic character typical for late Quaternary stadial times of centralEurope. In contrast, the three organic-rich horizons obviously

My, XKZH=samples from Mýtina trench from Geissler et al., 2005, 2007).

Phyllite

My2 My2-B My-1/1-84 XKZH10 XKZH69 XKZH63

41.3 41.1 54 55.2 61 76.32.9 2.87 1.05 1.47 1.07 0.8411.6 11.6 25.3 24.7 20.2 10.15.68 5.39 6.58⁎ 7.47⁎ 6.16⁎ 6.51⁎5.05 5.270.19 0.19 0.05 0.08 0.05 0.07

12.6 12.68 1.46 1.8 1.4 1.1812.41 12.31 0.13 0.42 0.33 0.292.3 2.4 0.68 1.53 1.71 0.821.42 1.42 5.98 4.48 4.09 1.420.65 0.71 0.09 0.16 0.12 0.072.51 2.75 4.2 2.75 3.98 2.540.11 0.1 0.2 0.48 0.12 0.05

99.33 98.78 99.6 100.54 100.28 100.2

0.73 0.7

208 208 227 136 198 101738 724 125 129 123 702.09 2.03 11.2 10.2 10.2 5.8752 738 866 790 880 4929.5 9.6 20.4 20 20 182.4 2.5 3.55 3.2 3.3 3.63.7 4 26.7 36 20 723 22 33 37 32 30247 243 238 226 199 210316 306 154 109 58586 582 133 84 41225 54 32 2574 75 122 87 11370.3 68.2 58.1 55.6 59.9 49135 129 119 121 120 98.815.6 14.7 13.6 13.9 14 11.559 55.6 50.4 50 52.3 42.210.2 9.66 9.41 9.55 9.67 7.542.9 2.77 1.88 1.82 1.8 1.468.17 7.73 7.46 8.2 8.01 6.311.03 1 1.13 1.19 1.09 0.965.22 4.89 6.64 6.97 6.27 5.50.86 0.84 1.36 1.36 1.17 1.062.23 2.1 3.85 3.87 3.5 3.090.28 0.27 0.56 0.56 0.49 0.431.66 1.59 3.59 3.95 3.14 3.020.24 0.23 0.57 0.56 0.5 0.435.3 5.34 6.21 6.37 5.92 6.17

Fig. 12. Chondrite-normalized REE-distribution patterns of the nephelinitic host rocksamples (MÝ-1= samples from boreholeMY-1; samplesMý and XKZH= samples fromMýtina trench according to Geissler, 2005, Geissler et al., 2007).


represent warmer (Interglacial or Interstadial) conditions. Thedominant tree component in these intervals is Pinus, accompaniedby lower amounts of temperate deciduous trees as oak, elm, lime, andhornbeam. The low sampling resolution together with the absence ofdiagnostically important taxa as Pterocarya or Taxusmakes correlationto the Holsteinian or Eemian interglacial unlikely. The preliminarypalynological database, however, supports the radiometric age for theMýtinaMaar structure of 288±17 ka (Mrlina et al., 2007), relating theMýtina sediment record to the Saalian Complex (MIS 10-6, Litt andTurner, 1993, Turner, 1998, Vandenberghe, 2000). Presently knownSaalian deposits of non-glaciated areas in the Czech Republic aremainly of fluvial and aeolian origin with some intercalated soil hori-zons, but their stratigraphic classification often remains unclear (e.g.Macoun, 1981, Šibrava, 1986, Dolecki, 1999). In addition to the strati-graphic importance, the discovery of a succession of at least threeinterstadials/interglacials in superposition from the palaeo-lake atMýtina will be an important contribution to the discussion of climaticgradients in Central Europe and their connection with North Atlanticocean currents during interstadial/interglacial periods (e.g. de Beaulieuet al., 2001; Müller and Kukla, 2004) and of vegetation refugia duringcold stadials (e.g. Willis and van Andel, 2004).

4.5. Petrochemistry of volcanics and country rock samples

The photographs of typical hand specimen from borehole(volcanics and country rock) are presented in Fig. 10. The petrochem-ical results are listed in Table 2 and presented in Figs. 11, 12. Thesamples in Fig. 11 (TAS diagram) plot in the fields of basanite/tephrite.The critical point is the content of xenolithic olivine, which mightcause the scatter of the diagram. The REE pattern of the volcanics fromthe MY-1 borehole agrees with the pattern of nephelinitic host rocksfrom the tuff-tephra deposit in the Mýtina trench (Fig. 11). Based onresults presented in Figs. 11 and 12 we can conclude that the volcanicsfrom the MY-1 borehole and from the tuff-tephra deposit in Mýtinaare almost identical. From a petrochemical point of view both rocksuites should have been generated from the same magma source.

Fig. 11. TAS-diagram (Le Maitre, 2002) of nephelinitic host rock samples (MY-1 =samples from borehole MY-1; Mý and XKZH = samples from Mýtina trench accordingto Geissler, 2005, Geissler et al., 2007).

The crustal sample (clast) from MY-1 borehole (Fig. 10) can beclassified as phyllite. The crustal xenolith sample XKZH10 from the tuff-tephra deposit inMytina shows similar chemical composition (Table 2).

4.6. Geomicrobiology

So far, most of the deep biosphere exploration has been carried outin the marine realm, only few studies investigated deep terrestrialsubsurface environments. The majority of terrestrial studies wasfocussed on either hydrocarbon reservoirs (e.g. L'Haridon et al., 1995)or deep aquifers (e.g. Pedersen and Ekendahl, 1990). To our knowl-edge, maar structures have not been investigated microbiologicallypreviously. Due to their typically fine-grained, often organic-rich andlaminated sediments they represent an excellent target for geomi-crobiological investigations of deep lacustrine sediments.

It was shown previously that organic-rich sediment can providethe feedstock for an abundant microbial community which mainlyresides in the adjacent organic-poorer sediments (Krumholz et al.,1997, Parkes, et al., 2005). Maar sediments provide a similar settingwith changes between organic-rich and -poor sediments: on themicro-scale there are differences between single laminae, on themacro scale between sediment sections.

Fig. 13. Abundance of microbial cells in the sediment and the drill fluid, enumerated byfluorescence microscopy.


The goal of this pilot study was to obtain a first overview about themicrobial abundance and distribution in a maar sediments and to seewhether there are any relationships between the geologic environ-ment and the microbial community.

The abundance and distribution of microbial cells exhibit a typicaltrend, starting at values around 108 cells cm−3 at the surface andgently decrease with depth to around 107 cells cm-3 at the bottom ofthe hole (Fig. 13). Cell abundances in the drill fluid are uniformlyaround 3±1x106 cells cm−3, about an order of magnitude lower thanin the centre of the core. Samples from 10.4 m exhibit a visible changein sediment composition of silty clay and the intercalated dm-sizedorganic-rich layer (for sediment type see Fig. 7B). This compositionalchange is also reflected in cell abundance, showing higher valuesabove the intercalation (3x108 cells cm−3) and lower values inside(1x107 cells cm−3).

A similar trend can be seen between 50 and 60 m, where two thinhorizons of organic laminated sediment intercalate silty clays (forsediment type see Fig. 7F). Around these two layers cell abundancesare elevated (7x107 cells cm−3) whereas above and below values arearound 1.6x107 cells cm−3.

5. Conclusions and implications

The initial hypothesis of a hitherto unknown volcano as a source ofvolcaniclastic sediments (tuff, tephra) investigated in an exploratorytrench in Mýtina by Geissler et al. (2004) became realistic after thegeophysical survey by Mrlina et al. (2007) revealed remarkablegravity and magnetic anomalies. The detailed follow-up gravity,magnetic and electromagnetic surveying supported the hypothesisof a maar-diatreme structure in the morphological depression nearMýtina, about 700 m from the Quaternary Železná hůrka volcano. Theimpressive isometric gravity low of−2.3 mGal (Fig. 4b) is comparablewith anomalies observed in Saxony, Germany (e.g. Gabriel, 2003;

Fig. 14. Maar features: 1 and 2 — approximate contour and centre of maar-diatreme volca(erupted) magnetic rocks accumulations; 5 — relicts of tephra rim outside the crater; 6 — m

Kroner et al., 2006; Lindner et al., 2006), but also e.g. in New Zealand(Cassidy et al., 2007). The former tentative (Mrlina et al., 2007) as wellas the upgraded 3D gravity and magnetic model (Fig. 5) fit well withthe style of maar-diatreme structures composition investigated inother regions and summarized by Lorenz (2007) and Lorenz andKurszlaukis (2007). The composition consists of a diatreme fill of lowdensity (country rock breccia with volcanic products such as bombs,lapilli and ash) covered by younger sediments of the maar lake (clay,sand, gravel), and is also characterized by low density (Figs. 6, 7, 10).

The magnetic map shows a positive anomaly in the depression,indicating the presence of volcanic rocks, possibly caused by a scoriacone at the base of the maar crater or a later-stage intrusion into themaar filling (Fig. 4c).

The shape of the anomaly indicates such volcanic rocks at depth,not straight at the surface, which is in agreement with the non-magnetic maar lake sedimentary cover of the diatreme.

As the survey was extended beyond the limits of the depression,we also observed very sharp magnetic anomalies that most likelyindicate surface deposits of volcanic products as part of the tephra rimaround the maar. The presence of sediments and volcaniclastics closeto the surface was also proven by increased electrical conductivity(Fig. 4d). In order to provide a complete image of the character andextent of the rim and volcaniclastic deposits, further magnetic surveyis needed in the larger surroundings of the discovered maar structure,as well as between the maar and the nearby Železná hůrka volcano todisclose the relation of the two structures.

The above discussed results, concentrated in Fig. 14, show that theapplied geophysical techniques represent a very efficient tool fordetection of unknown and hidden volcanic structures, like the Mýtinamaar. Mapping of geophysical anomalies allowed the positioning of theexploratory drilling of the MY-1 borehole near the centre of the gravityanomaly. The recovered core consists of maar lake sediments down to84 m depth (Figs. 6–8). The recovery of a long lacustrine sediment

no from gravity; 3 and 4 — deeper (volcanic breccia inside the diatreme) and shalloworphological edge of the crater; 7 — borehole.


record including a succession of at least threewarm climatic intervals insuper-position (Fig. 9) is unique for the area and provides a greatpotential for (i) solving pending problems of the stratigraphy of theSaalian complex, and, (ii) disentangling Saalian climatic variability inthis part of Europe with high-resolution multiproxy approaches.

Although the cell count data have to be treated with some caution,because no contamination control was employed during drilling, thegeneral decrease of cell abundance with depth as well as variations,which follow small-scale lithological changes (Figs. 6, 13) can be seenas good indicators for the existence of an active microbial communityin these sediments. Future drilling operations with appropriatecontamination control and sampling strategies would allow for amore detailed investigation of this subsurface ecosystem. Below thelake sediments, at 84–85.5 m depth, a country rock breccia (mainlyphyllite with quartz) was recovered, with some lapilli and volcanicbombs of identical petrological composition as the bombs from theMýtina exploratory trench. Hence, the Mýtina trench tuff–tephradeposits as far as 1500 m from the Železná hůrka volcano mostprobably originated from the Mýtina maar eruption (Fig. 10–12).

Indications for a presently active magma intrusion from lithosphericmantle into the crust beneath the eastern part of the Cheb Basin(marked by yellow ellipse in Fig. 1) have been published by Bräuer et al.(2005, 2008a, b). Their findings are result of repeated annual samplingsof free gas phase in mineral water springs and mofettes. Between 1993and 2005 an increase of mantle-derived helium has been observed(Bräuer et al., 2008a, b). The 3He/4He ratios are clearly higher than inthe Eifel area and theMassif Central (Fig.1, inset map), the two classicalEuropean regions for Quaternary maar eruptions. The area of hiddenmagmatic and earthquake swarm activity (with a magnitude ofpossibly up to 5) at the NE and E part of the Cheb Basin is locatedonly ca. 20–30 km north of the Mýtina maar. Because of the indicationsto presently active magmatic process, a potential future hazard(phreatomagmatic eruption) for that area cannot be excluded (Fig. 1,yellow ellipse). These findings are unique for Central and WesternEurope (Lorenz, 2007) and shed new light on the reconstruction ofpalaeovolcanic evolution of the Mýtina maar. This comprehensive pilotstudy argues for detailed future operations considering palaeovolcanicevolution, palaeoclimate, deep biosphere, and hazard potential.

Acknowledgements

This research is supported by the German Science Foundation (KA902/11, KR 1906/10) and GFZ, and partly by the Grant Agency of theAcademyof Sciences of the Czech Republic (IAA300460602). Gratitudeis to R. Naumann and P. Dulski (both at the GFZ) for their analyticalsupport (XRF, ICP-MS). We thank Ina Neugebauer, Lucas Kämpf (bothTU Dresden), Maria Wahle (EMA University of Greifswald) and NicoleWeiske (FU Berlin) for assisting of sampling, core description andtechnical work, as well as Václav Polák (IG ASCR Prague) and MarekSpěšný (Charles University, Prague) for participation in geophysicaland geodetic surveys. We appreciate detailed comments of M.Pirrungand one anonymous reviewer that helped us in improving themanuscript. We also appreciate the handling editor Joan Marti forthe decision in favour of the complex character of the publication.

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