For the gravimetric modelling, a regional gradient of
−1.56 mGal/
km in SSE to S direction, taken from
Hofmann et al. (2003)
was
removed from the Bouguer anomaly. Our 3D geophysical model
consists of 26 parallel pro
files extending E–W. As an example, we
present the Pro
file 16 in
Fig. 5
. The density model was parameterized
according to
Mrlina et al. (2007)
, but in more detail, based on the MY-
1 core observation and on preliminary density test in a laboratory (10
samples showed wet densities between 1.90 and 2.20 g/cm
3
). We
considered the following density values:
• maar lake sediments 1.90–2.20 g/cm
3
(10
− 3
kg/m
3
) (2
–84 m)
• outer maar debris/fracture zone 2.55 – 2.60 g/cm
3
(84
–? m)
• inner maar debris 1.95 g/cm
3
(84
–? m)
• colluvium 2.25 g/cm
3
(0
–2 m)
• phyllite/country rock 2.65 g/cm
3
surroundings.
The layers of organic sediments were less than 2 m thick, so no real
impact on gravity is expected from the depth of 75 m. The model
contains 15 bodies: 2 representing the sediment
filling of the maar,
11 the lower part of the chimney, 1 for the overlaying colluvium and
1 for country rock. The resulting gravimetric model is shown in
Fig. 5
(only 7 of the 15 bodies are present in the selected Pro
file 16).
The maximum density contrast to the surrounding rock is about
0.70 g/cm
3
. Density differences between outer maar debris/fracture
zone and country rock range from 0 to 0.10 g/cm
3
.
The layout of the magnetic model is identical to the gravimetric
one. The magnetic
field parameters (total intensity: 48,640 nT,
declination: 2.09°, inclination: 66°) were taken from the
Geological
Survey of Canada (2006)
, which provides these parameters in global
coverage. The rms for the gravimetric model is 0.14 mGal, and 20 nT
for the magnetic model, respectively. In total a good agreement
between the models and the observations exists.
The
final model is presented in
Fig. 5
. The contrasts in magnetic
susceptibility of the diatreme structure with respect to the country
rock range from 100 to 1100 × 10
− 6
SI. Magnetic susceptibility of the
sediment
filling is clearly lower with values of 300 to 400×10
− 6
SI.
The overlaying colluvium has an even smaller magnetic susceptibility
of 100 × 10
− 6
SI. The main contrasts exist between the diatreme itself
and the country rock.
From the gravimetric and magnetic modelling, a cone-shaped
structure with dip angles of about 80° can be inferred. In its central
part the thickness of the sediment deposits is about 100 m (84 m in
MY-1 borehole). Towards the border of the structure the sediment
thickness decreases to a few meters. The diameter of the structure at
the 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
Lorenz
and Kurszlaukis (2007)
. The modelling depth is constrained by the
lateral extension of the survey area. Therefore the models contain
only the region above the root zone. Due to the integrated modelling
and 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 that
evidently deviates from the observed ones. The variance in the density
value of the outer diatreme zone (body 7 and 9) in contrast to the
country rocks (body 2) can be explained by a halo, characterized by
slightly 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)
. The
variance in the susceptibility value of bodies 7 and 9 can be caused by
different amount of magmatic and country rock present in the two
bodies. For a modelling of more detailed features the survey grids need
to be re
fined.
4. Geological investigations
Geological investigations presented in this paper are based on the
core samples from the borehole MY-1. Beside macroscopic on-site
evaluation (
finding of maar sediments, volcanic bombs and lapilli, and
country rock breccia), further sedimentological, palynological, petro-
chemical and geomicrobiological analyses have been performed in
order to evaluate the potential of the core samples for a paleoclimate
study, 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 exploratory
drilling near the centre of the gravity anomaly in order to test/prove
the hypothesis of a maar structure. The expectation was to extract a
core with debris of country rock and volcanics, and possibly maar lake
sediments. Eventually, we continued as deep as 85.5 m. In the interval
0
–84 m the core is composed of Quaternary clay to silty clay
(unconsolidated maar lake sediments) with intersections of organic
sediments and gravel. In 84
–85.5 m there was phyllitic debris with
volcanic bombs and lapilli.
The borehole MY-1 has the following coordinates in WGS84/
UTM33N datum:
Lat 49°59'48.8
q, Long 12°26'15.4q
X = 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 and
opened under lab conditions at GFZ Potsdam in March/April 2008.
Besides core description and photographic documentation a
first set of
petrographic thin sections and pollen samples has been prepared,
whereas ongoing geophysical and geochemical investigations of the
core material will be published elsewhere.
Fig. 5. Geophysical model (bottom), gravity (centre) and magnetics (top). Selected
pro
file (W–E) passing close to MY-1 borehole, see
Fig. 4
. Pro
file 16 represents a central
plane of 26 planes of the 3D modelling set. Table of used density and magnetic
susceptibility values at bottom right.
103
J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112