4 Geology of the target

Apart from the most northern part of the strewnfield, where Miocene gravels, sands and marls are exposed in the hilly terrain, the target is predominantly composed of Pleistocene and Holocene moraine sediments. Pebbles, cobbles and boulders up to the size of 20 cm are intermixed with sands and clays. The components represent Alpine material in the form of sediments (mostly limestones and sandstones), magmatic rocks (mostly granitoids) and metamorphic rocks (mostly gneisses, amphibolites and schists). Occasionally, larger blocks of cemented conglomerates (Nagelfluh) are observed. Locally, loess and loamy soils may contribute to the uppermost target layers.

5 Crater structure and material

Only a few craters have so far been examined in more detail. A study of some hollows by digging vertical trenches through them reveals the typical bowl-shaped profile well known from meteorite craters of comparable size. The majority of the craters have clear walls, with a steep gradient inside towards the center and a flat one outside. On aerial photographs (partly taken in infrared light), the zone of ejecta around some craters becomes visible (Fig. 12). Often, the craters show a slightly assymetrical, mostly elliptical form.

Fig. 12. This photograph taken in infrared light shows clear the zone of ejecta around a 15 m crater at Perach (379 m above msl). Aerial photo: Bay.LfD.

The gravel in the center of the crater is sharp-edged broken and looks basically different compared with the usually well-rounded pebbles found in the landscape (Fig. 13). On the crater floor, immediately on top of the target gravel, an ash layer peppered with small charcoal fragments is regularly observed (Fig. 14).

Fig. 13. Typical material sampled from crater floors (to the left) strongly contrasting with target material from outside the craters.

Fig. 14. Ash layer from a crater floor. The test pit is about 20 cm wide.

Near to the main craters, a prominent kind of secondary cratering can be observed. After removing the upper soil layers down to about 0.5 m, sharp-edged rock fragments sized 0.2 - 0.3 m occur to be stuck in the otherwise untouched ground. They are surrounded by a striking ring-shaped discoloring of the host material up to the diameter of 1 m (Fig. 15).

Fig. 15. Uncovered secondary cratering. Projectile (to the left) and halo of discolored host rock.

Peculiar metallic material  (Fig. 16) is accumulated  around all the craters at a certain depth, 0.3-0.4 m below the top soil. Occasionally, it has penetrated the target gravels. In majority, it is found concentrated northeast of each crater, while with increasing crater size the distance increases, too.

Fig. 16. Fine fraction of peculiar metallic material concentrated near the large Bergham crater. Note the many perfect spheres contributing to the material.

Ferrosilicides (FeSi, Fe₃Si, Fe₅Si₃) are found distributed over a much larger area of about 3,000 km² besides the corridor set by the strewnfield. Following, from north to south, the major axis of the scattering ellipse, the material is traceable in the promontory of the Alps south of lake Chiemsee up to an altitude of 1,200 m.

6 Geophysical signature

Geophysical earth magnetic field measurements (Fehr et al. 2002) across smaller craters reveal faint anomalies that could not be related with definite causative bodies so far. Soil magnetic susceptibility measurements in the Burghausen area (Hoffmann et al. 2004) reveal substantially increased susceptibilties in the soil lacking typically industrial or geogenic signature. Preliminary pulse-electromagnetic soundings across a 20 m-diameter crater show a signature of larger metallic objects in the center of the structure.

Outside the craters, the abundant occurrence of strongly magnetic rocks of quite different lithologies among the target rocks is conspicuous. The high, dominantly remanent magnetization seems to be unusual compared with typically magnetic rocks from the Alps (e. g., amphibolites, serpentinites). We suggest that these rocks might have acquired their magnetization as a thermoremanent magnetization in contact with the super-heated impact explosion cloud (also see below).

Macroscopic deformations

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