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DIGITAL TERRAIN MODEL (DTM) TOPOGRAPHY OF SMALL CRATERS IN THE HOLOCENE
CHIEMGAU (GERMANY) METEORITE IMPACT STREWN FIELD. K. Ernstson1 and J. Poßekel2, 1Uni-
versity of Würzburg, D-97074 Würzburg, Germany, kernstson@ernstson.de 2Geophysik Poßekel Mülheim,
Germany, jens.possekel@cityweb.de
Introduction: In earth and planetary impact crater
studies various digital remote sensing tools have
increasingly supplied high-resolution data. On Earth
and here with a special focus on the German DGM 1,
DTM data have become an important tool within the
geosciences. Based on LiDAR data, topographic maps
in a regular grid down to a spacing of 1 m and with
highest altitude resolution down to 20 cm may be
produced for the bare ground, ignoring any objects like
plants and buildings and may even be processed in
thick forest (Fig. 1). In the strewn field of the
Chiemgau meteorite impact the resolution of the topo-
graphic crater shapes to a hitherto unknown precision,
has opened completely new perspective on the
formation of these structures, which is reported here.
The Chiemgau impact event: In a strewn field of
roughly 60 km x 30 km size far more than 100 mostly
rimmed craters with diameters between a few meters
and a few 100 meters occur in a moraine and gravel
plain landscape formed in the last Würm Alpine
glaciation. The impact strewn field shows all and
abundant evidence of impact signature as is required
within the impact research community [1, 2, and
references therein]). The event happened in the Bronze
Age/Iron Age between 900 and 600 BC as revealed
from impact catastrophe layers and their archeological
inventory [2].
Data processing: The DGM 1 terrain imagery
proceeds from a 1 m mesh and an elevation resolution
of 0.2 m, which may be even reduced by interpolation.
It produces topographic maps based on arbitrary
contour intervals and color scaling, 3D surfaces,
shadowed relief and vector maps. The computing of
topographic gradient (terrain slope) maps and various
data filtering procedures add to the manifold
possibilities, as do high-resolution crater profiles
providing not only very precise crater depths and
diameters but also very details of the overall crater
geometry.
Results: The current status of morphological
investigations with the DGM comprises roughly 50-60
craters with diameters between about 5 m and 250 m
(rim to rim) and different crater constellations (Fig. 2).
For space reasons, typical craters from the different
size groups are presented and discussed in this paper,
by omitting individual steps of data processing and
emphasizing that certain shapes repeat themselves
many times with an accuracy better than decimeters to
meters.
Fig. 2. Medium-sized craters: simple, doublet, multiple, cluster.
Medium-sized "multi-ring" craters. The enormous
resolution of the DTM points to a possibly impact-
specific peculiarity. As is marked in Fig. 3 and 4, the
in each case clearly visible rim wall is surrounded by a
roughly concentric ring depression a few decimeters
deep only, giving the structures a total size of more
than 30 m. Similar ring-like depressions are found also
for most other small craters, but because of general
rough terrain conditions they often lack the exemplary
geometry seen in Fig. 3. Even in Fig. 5 with the band
of stacked small crater profiles, the peripheral
depression extending into a wavy signature becomes
apparent. Although for the time being a reasonable
explanation is lacking, the mere existence of this
peculiar crater structure highlights once more the
enormous potential of the DTM terrain evaluation.
Fig. 3. The model crater #001 (Schatzgrube). Optical leveling on
the same profile confirms the DGM data.
Fig. 5. Stacking of 20 small craters from a limitedly selected
forest area (1 km2) and cross sections from DTM data. A wavy
enclosing is indicated.
2019.pdf11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251)
The model nature of the #001 crater in Fig. 3 is also
emphasized by a perfectly rotational topographic
symmetry found in a more or less identical manner
with most individual craters in the Chiemgau strewn
field.
Fig. 6. Perfect rotational symmetry: eight DTM radial
profiles with height deviations no more than 20 cm in the
crater bowl and over the rim wall, and no more than 50
cm over the 40 m complete sections.
The larger craters. More than a dozen craters with
diameters between roughly 50 m and 250 m have
aroused particular interest because they are
concentrated in a moraine landscape rich in lakes to the
north-west of Lake Chiemsee and have so far generally
been regarded as glacial (dead ice) depressions, partly
filled with water. As in the case of medium-sized
craters, a model structure is presented here (Fig. 7) that
illustrates essential features and practically excludes
ice-age formation.
Fig. 7. The almost perfectly circular ND crater (ND = natural
landmark) and eight mirrored radial profiles. Despite the
irregular moraine landscape, the profiles do not deviate by more
than 2.5 m over 200 m extension with one exception. A terraced
structure and slight walling occurs in many closely related
craters (Fig. 8).
Discussion and conclusions: The results presented
here are an extract of a meanwhile much larger data
and interpretation pool of the DTM application. The
essential points are:
The application of the DTM for crater
exploration even in dense forest and swamp areas has
led to an enormous increase in the number of
postulated impact craters. The previously valid rule
that impact nature is only proven by direct observation,
projectile remnants or shock effects is given the
additional aspect of morphology in the case of the
Chiemgau impact, and for the time being only here.
Fig. 8. A selection of larger circular craters within an area of
roughly 10 km x 5 km. In most cases, crater lakes, some of which
are up to 200 m in size, have sharply contoured riparian margins
that are more or less morphologically congruent on radial
profiles with deviations of no more than 1 m.
Fig. 9. Depth-to-diameter relationship for 18 larger craters in the
Chiemgau strewn field. The dispersed distribution shows a
generally low ratio and rather a clustering than a systematic
dependency.
Perfect circular symmetry including crater bowl,
ring wall and peripheral depressions, which enlarge the
structure to two to three times the size of the pure
crater, reasonably exclude any human installation and,
e.g., simple sinkhole formation. A central, point-like
force "from above" can explain this. The associated
mechanism of crater (explosion) formation is not yet
understood, although the wavy shape could be a
reaction to the mostly soft ground of loose, water-
saturated rock, with the possible effect of seismic
surface (Rayleigh) waves [3]. The soft ground may
also explain the very low depth-to-diameter ratio (Fig.
9) and a terraced morphology (Fig. 7) by reflux of rim
wall material. So far it is also unclear to what extent
the consistency of the Chiemgau impactor for the large
strewn field has influenced the crater formation. The
larger craters and their amazingly perfectly circular
shape fundamentally rule out the ice age genesis
assumed so far by geologists who would have to
explain how this sharply contoured, very flat picture-
book shape could have been preserved over more than
10,000 years (end of ice age) from dead-ice melting.
We conclude that the results have a certain statistical
significance due to the high number of crater shapes
precisely measured with the DTM. Well-known
cratering models for the group of simple craters with a
bowl and a ring wall that merges into a blanket of
decreasing ejecta thickness have to consider much
more complex shapes, at least in targets with a bedrock
of loose rocks. For critics of an impact genesis, this
poses the problem of explaining crater morphologies
for which human formation or simple sinkholes can be
absolutely ruled out.
References: [1] Ernstson, K. et al. (2010) J.
Siberian Federal Univ., Engin. & Techn., 1, 72-103.
[2] Rappenglück, M.A. et al. (2017) Z. Anomalistik,
17, 235-260. [3] Poßekel, J. and Ernstson, K. this
meeting.
2019.pdf11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251)
