Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, based on combined geophysical and geological surveys
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represent warmer (Interglacial or Interstadial) conditions. The dominant tree component in these intervals is Pinus, accompanied by lower amounts of temperate deciduous trees as oak, elm, lime, and hornbeam. The low sampling resolution together with the absence of diagnostically important taxa as Pterocarya or Taxus makes correlation to the Holsteinian or Eemian interglacial unlikely. The preliminary palynological database, however, supports the radiometric age for the Mýtina Maar structure of 288 ± 17 ka ( Mrlina et al., 2007 ), relating the Mýtina sediment record to the Saalian Complex (MIS 10-6, Litt and
Turner, 1993, Turner, 1998, Vandenberghe, 2000 ). Presently known Saalian deposits of non-glaciated areas in the Czech Republic are mainly of fluvial and aeolian origin with some intercalated soil hori- zons, but their stratigraphic classi fication 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 three interstadials/interglacials in superposition from the palaeo-lake at Mýtina will be an important contribution to the discussion of climatic gradients in Central Europe and their connection with North Atlantic ocean currents during interstadial/interglacial periods (e.g. de Beaulieu et al., 2001; Müller and Kukla, 2004 ) and of vegetation refugia during cold 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 . The samples in Fig. 11 (TAS diagram) plot in the fields of basanite/tephrite. The critical point is the content of xenolithic olivine, which might cause the scatter of the diagram. The REE pattern of the volcanics from the MY-1 borehole agrees with the pattern of nephelinitic host rocks from the tuff-tephra deposit in the Mýtina trench ( Fig. 11
). Based on results presented in Figs. 11 and 12 we can conclude that the volcanics from the MY-1 borehole and from the tuff-tephra deposit in Mýtina are almost identical. From a petrochemical point of view both rock suites should have been generated from the same magma source. The crustal sample (clast) from MY-1 borehole ( Fig. 10 ) can be
classi fied as phyllite. The crustal xenolith sample XKZH10 from the tuff- tephra deposit in Mytina shows similar chemical composition ( Table 2
). 4.6. Geomicrobiology So far, most of the deep biosphere exploration has been carried out in the marine realm, only few studies investigated deep terrestrial subsurface environments. The majority of terrestrial studies was focussed 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 microbiologically previously. Due to their typically fine-grained, often organic-rich and laminated sediments they represent an excellent target for geomi- crobiological investigations of deep lacustrine sediments. It was shown previously that organic-rich sediment can provide the feedstock for an abundant microbial community which mainly resides in the adjacent organic-poorer sediments ( Krumholz et al., 1997, Parkes, et al., 2005 ). Maar sediments provide a similar setting with changes between organic-rich and -poor sediments: on the micro-scale there are differences between single laminae, on the macro scale between sediment sections. 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 according to Geissler, 2005, Geissler et al., 2007 ). Fig. 12. Chondrite-normalized REE-distribution patterns of the nephelinitic host rock samples (MÝ-1 = samples from borehole MY-1; samples Mý and XKZH = samples from Mýtina trench according to Geissler, 2005, Geissler et al., 2007 ). Fig. 13. Abundance of microbial cells in the sediment and the drill fluid, enumerated by fluorescence microscopy. 109 J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97 –112 The goal of this pilot study was to obtain a first overview about the microbial abundance and distribution in a maar sediments and to see whether there are any relationships between the geologic environ- ment and the microbial community. The abundance and distribution of microbial cells exhibit a typical trend, starting at values around 10 8 cells cm − 3 at the surface and gently decrease with depth to around 10 7 cells cm -3 at the bottom of the hole ( Fig. 13
). Cell abundances in the drill fluid are uniformly around 3 ± 1 x 10 6 cells cm − 3 , about an order of magnitude lower than in the centre of the core. Samples from 10.4 m exhibit a visible change in sediment composition of silty clay and the intercalated dm-sized organic-rich layer (for sediment type see Fig. 7
B). This compositional change is also re flected in cell abundance, showing higher values above the intercalation (3 x 10 8 cells cm
− 3 ) and lower values inside (1 x 10 7
− 3 ). A similar trend can be seen between 50 and 60 m, where two thin horizons of organic laminated sediment intercalate silty clays (for sediment type see Fig. 7 F). Around these two layers cell abundances are elevated (7 x 10 7 cells cm − 3 ) whereas above and below values are around 1.6 x 10 7 cells cm − 3 . 5. Conclusions and implications The initial hypothesis of a hitherto unknown volcano as a source of volcaniclastic sediments (tuff, tephra) investigated in an exploratory trench in Mýtina by Geissler et al. (2004) became realistic after the geophysical survey by Mrlina et al. (2007) revealed remarkable gravity and magnetic anomalies. The detailed follow-up gravity, magnetic and electromagnetic surveying supported the hypothesis of a maar-diatreme structure in the morphological depression near Mýtina, about 700 m from the Quaternary Železná hůrka volcano. The impressive isometric gravity low of −2.3 mGal ( Fig. 4
b) is comparable with anomalies observed in Saxony, Germany (e.g. Gabriel, 2003; 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 well as the upgraded 3D gravity and magnetic model ( Fig. 5 )
the style of maar-diatreme structures composition investigated in other regions and summarized by Lorenz (2007) and
Lorenz and Kurszlaukis (2007) . The composition consists of a diatreme fill of low density (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 scoria cone at the base of the maar crater or a later-stage intrusion into the maar filling (
Fig. 4 c).
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 likely indicate surface deposits of volcanic products as part of the tephra rim around the maar. The presence of sediments and volcaniclastics close to the surface was also proven by increased electrical conductivity ( Fig. 4
d). In order to provide a complete image of the character and extent of the rim and volcaniclastic deposits, further magnetic survey is needed in the larger surroundings of the discovered maar structure, as well as between the maar and the nearby Železná hůrka volcano to disclose the relation of the two structures. The above discussed results, concentrated in Fig. 14
, show that the applied geophysical techniques represent a very ef ficient tool for detection of unknown and hidden volcanic structures, like the Mýtina maar. Mapping of geophysical anomalies allowed the positioning of the exploratory drilling of the MY-1 borehole near the centre of the gravity anomaly. The recovered core consists of maar lake sediments down to 84 m depth ( Figs. 6 –8
Fig. 14. Maar features: 1 and 2 — approximate contour and centre of maar-diatreme volcano from gravity; 3 and 4 — deeper (volcanic breccia inside the diatreme) and shallow (erupted) magnetic rocks accumulations; 5 — relicts of tephra rim outside the crater; 6 — morphological edge of the crater; 7 — borehole. 110 J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97 –112 Yüklə 355,06 Kb. Dostları ilə paylaş:
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