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NOT JUST A RIMMED BOWL: GROUND PENETRATING RADAR (GPR) IMAGERY OF SMALL
CRATERS IN THE HOLOCENE CHIEMGAU (GERMANY) METEORITE IMPACT STREWN FIELD.
J. Poßekel1, K. Ernstson2 .1Geophysik Poßekel Mülheim, Germany, jens.possekel@cityweb.de 2University of
Würzburg, D-97074 Würzburg, Germany, kernstson@ernstson.de
Introduction: The cratering of earth media by
surface explosions is a complex process of combined
effects that are difficult to treat. Some 40 years ago,
the so-called Maxwell Z-model was a first analytical
approach to describe the formation of craters with
excavation flow and ejecta for a vertical impact (Fig.
1). The plausibility of the Z-model has not yet been
investigated, because the movement of the target
material cannot be directly observed in the laboratory
[2] and only the present final state is visible in nature,
which can be explored with single drillings or with
geophysical measurements. For simple bowl-shaped
craters, depth-to-diameter ratios, and possibly the
thickness of a breccia lens on the ground may be
determined. We report here on a program of high-
resolution GPR measurements over some craters of
different size in the soft Quaternary target of the
Chiemgau meteorite impact strewn field in southeast
Bavaria (Germany), which provides an unusual insight
into structures and movements during crater formation.
Fig. 1. Selected phases of crater formation in the Maxwell Z-
model [1].
The Chiemgau impact event: In a roughly
elliptically shaped strewn field (Fig. 2) more than 100
mostly rimmed craters with diameters between a few
meters and a few 100 meters occur.
Fig. 2. Location map for the GPR over craters (red circles)
within the roughly elliptically encircled Chiemgau impact
strewn field.
Apart from the craters and their distinct
morphology as revealed from precise Digital Terrain
Model analyses (1 m x 1 m grid, vertical resolution 0.2
m; [3]), the impact strewn field shows all and abundant
evidence of impact signature as is required within the
impact research community (impact melt rocks, impact
glasses, strong shock metamorphism, shatter cones,
geophysical anomalies, and meteoritic matter [4, 5, and
references therein]). The event happened in the Bronze
Age/Iron Age 900 - 600 B.C. as revealed from impact
catastrophe layers and their archeological inventory
[5].
Field work: So far, a total of 9 craters of the
Chiemgau strewn field have been investigated with
GPR (Fig. 2). A special program was dedicated to the
larger Lake Tüttensee crater, and a parallel campaign
was carried out by a research team from the Czech
Republic with special, very low-frequency equipments,
which has been reported on separately [6]. Our
measurements used different antenna systems with
200, 300 and 400 MHz.
Results: From the amount of data collected so far
we select typical radargrams for the #004 Emmerting
crater and the Aiching semi crater.
#004 Emmerting (Fig. 3) is the early and so far best
investigated small crater. With a diameter of 11 m it is
characterized by an impressive impact inventory with
extreme temperature and pressure effects (melt rocks,
shock effects PDF, diaplectic glass). Until today its
exact formation has not been clarified, since the
extreme temperature effects on the rocks, >1,500°C,
within a 20 m measuring halo cannot be attributed to
the impact of a projectile, but suggest a near-surface
heavy impact-related explosion [4].
Fig. 3. The 11 m-diameter #004 crater near Emmerting and
its 3D surface of the Digital Terrain Model DTM.
Fig. 4. Radargram across the #004 crater (25 MHz center
frequency with modulated 200 MHz; data from P. Kalenda
and R. Tengler) and interpretation.
2040.pdf11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251)
The radargram in Fig. 4 corresponds in a certain
way to the unexplained formation mechanism. Extreme
reflectivity down to a depth of 5 m with an outward
moving wall projection also to this depth, are the
special features. With the high GPR resolution,
abundant unconformities are also shown, revealing a
whole sequence of movement phases during
excavation. The simple approximate Z-model does not
do justice to this structure.
Aiching. The semi crater appears punched into the
embankment of the Inn river valley (Fig. 5, 7), and the
data of the DTM show its unmistakable contours of a
60 m- diameter crater with a weak ring wall (Fig. 7, 8).
Fig. 5. The Aiching semi crater. The arrow points to the
gravel excavation outcrop in Fig. 6.
!
Fig. 8. The DTM profiles for the Aiching semi crater. The
assumed reconstruction of the original full crater shows that
the GPR profile is located roughly midway between crater
center and rim.
While the erosion of the Inn river has exposed a
very coarse section of the crater in the past, a gravel
excavation, certainly unintended, has recently made an
exceptionally fantastic cut through the crater rim with
a ring wall (Fig. 6).
Fig. 9.!Radargram across the hidden half of the Aiching semi
crater in the Inn river valley. 300 MHz antenna. Note the
wavy deformations and unconformities.
The radargram in Fig. 9 reveals in beautiful
resolution the structure of the crater below its second
half eroded and leveled by the Inn river. Similar to the
radargram of the #004 crater (Fig. 4) a replication of
structures with wavy deformations downwards
implying layer unconformities are most suspicious. A
doublet mound of higher reflectivity in the very center
may have formed from crater refill with coarser
material from rim wall collapse. In this respect, today's
very flat ring wall of the Aiching crater (Fig. 8) could
have been the remains of an originally much higher
wall, which may well have been part of a Z-model
overturned flap (Fig. 1)
Conclusion: The results of a high-resolution GPR
presented here are not singularly selected two
examples of an exploration of the crater bedrock. Very
similar results of a complex subsurface with prominent
wave-like movements and multiple layer uncon-
formities are also found in the other craters in the
Chiemgau meteorite crater strewn field surveyed with
the GPR. These results should not be generalized or
applied to impacts on other targets, but they show to
what extent GPR can contribute to getting to the
bottom of impact processes, at least for impact craters
in the decameter range and in the range of some 100 m
diameter [6].
References: [1] ]Maxwell, D. E. (1977) in Impact
and Explosion Cratering, pp. 1003-1008. [2] Wada, K.
et al. (2004), LPSC XXXV, Abstract #1520. [3]
Ernstson, K. and Poßekel, J. (2020) This meeting. [4]
Ernstson, K. et al. (2010) J. Siberian Federal Univ.,
Engin. & Techn., 1, 72-103. [5] Rappenglück, M.A. et
al. (2017) Z. Anomalistik, 17, 235-260. [6] Poßekel, J.
and Ernstson, K. 50th LPSC, Abstract #1204.pdf.
2040.pdf11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251)
