2 Scattering ellipse and crater dimensions of the Chiemgau strewnfield
On earth, six meteorite crater strewnfields are known. These are the Kaalijarvi field in Estonia, the Morasko field in Poland, the Sikhote Alin field in Russia, the Henbury field in Australia, Campo del Cielo in Argentina, and the Wabar field in Saudi Arabia (see Hodge 1994, Krinov 1963 a,b, and others; for details see below).
Compared with these known occurrences, the newly discovered crater strewnfield in the Chiemgau (Bavaria), a region in southeastern Germany (Fig. 1), is exceptional.
Fig. 1. The location of the Chiemgau strewnfield in Germany.
Up to now, 81 craters have been identified, measured and catalogued on the basis of topographic mapping, satellite imagery, systematic aerial photography and ground inspection [6] establishing the scattering ellipse shown in Fig. 2. The size of the ellipse is given by a major axis of ca 58 km and a minor axis of ca 27 km. The strewnfield covers an area of about 1,200 km² between 47.8° to 48.4° N and 12.3° to 13.0° E. The craters are situated at altitudes ranging from 362 m to 560 m asl.
Fig. 2. Distribution and size of craters in the Chiemgau strewnfield.
The preservation of the craters is quite different depending on their location on, e.g., farm land or in forests. On farm land, many of the craters recorded on older topographic maps have meanwhile been leveled out. Despite the levelling, they are frequently visible by satellite imagery or on aerial photographs (Fig. 3). On the other hand, many well-preserved craters are probably if not certainly hidden in forests that cover large areas of the scatter ellipse. These undetected craters as well as craters that have been destroyed and, therefore, are completely unrecognizable, may account for estimated roughly 40 – 50 % of the original number of craters.
Fig. 3. Craters having been leveled out by farming. Aerial photograph (Gerhard Benske) and satellite imagery (D-SAT).
The diameter of the documented craters ranges between 3 m and several 100 m (Fig. 4 - 9). Some of them are permanently filled with water. There is a large number of craters and depressions having diameters below 3 m. They have so far not been documented, and in many cases the origin of the smaller pits from meteorite impact may be questioned without closer inspection. The other end of the scale shows craters having diameters as large as 200 m or more. At present, a lake named Tüttensee and located near the well-known Chiemsee and the town of Traunstein, proves to be the largest crater (Fig. 9). The lake is surrounded by a more or less continuous ring wall and has a maximum diameter of about 400 m. Taking into account the size of the ringwall, a rough diameter of 500 m for the Tüttensee crater may apply. As Fig. 9 shows, the shape is far from being circular, but in parts, the shore matches exactly a circle. We suggest therefore that the Tüttensee was formed by the impact of a fragmented projectile similar to the irregularly shaped craters of the Kaalijarvi and Henbury meteorite crater fields. More evidence for the impact nature of lake Tüttensee is given below.
Fig. 4. This crater at Murshall (near Tyrlaching) is permanently filled with water. It is 16 m across and shows a clear wall.
Fig. 5. This 55 m crater is located at an ancient bank of the river Alz near the hamlet Dornitzen (near Marktl). It is conserved only at half, because of the influence of the nearby Alz and agriculture. Originally there had been a wall. But it was destroyed by plowing. In the centre of the crater pieces of the peculiar FexSiy-phases had been found. Aerial photo: Gerhard Benske.
Fig. 6. The crater close to the hamlet Bergham (near Tyrlaching) today has a diameter of about 150 m, a depth of 15 m and a small wall. Before 1960 the hollow hosted a lake. Later it was dewatered and filled up with gravel and soil. Originally the crater had a wall of at least 2 m high. The slope had been steep enough and the lake was sufficiently deep (more than 25 m), that children and adults sprang from the rim headlong into the water without any risk. Photo: Mathias Wurm, farmer of Bergham.
Fig. 7. 6 m-diameter Hohenwart crater exhibiting a distinct wall.
Fig. 8. The deep 15 m-diameter Einsiedeleiche crater.
Fig. 9. The hitherto biggest crater is the lake Tüttensee close to the hamlet Marwang (near Grabenstätt). The water surface measures about 370 m. But the original hollow probably had a diameter of about 500 m. Today the walls are 8 m high. At a depth of ca 17.5 m a layer of trees fallen down from the walls was discovered. Divers noticed that there are some gaps in this swimming ground. They took soundings through these blanks, but didn’t reach the ground at a depth of 70 m. Aerial photo: Gerhard Benske.
From Fig. 2 it is evident that the average diameter of the craters increases from the northern end of the strewnfield to its southern end. This is remarkably similar to other meteorite crater strewnfields (Morasko, Henbury, Kaalijarvi, Sikhote Alin) showing a comparable distribution (Fig.10). Such a distribution is generally assumed to be related with an atmospheric break-up of the impactor implying a rough grading of the fragments and of the diameters of the associated craters.
Fig. 10. Scattering ellipses for meteorite crater strewnfields. Modified from Krinov (1963) (Henbury, Kaalijarvi) and Hodge (1994) (Morasko, Sikhote Alin).
The depths of the craters range between 0.4 m (for the smallest 3 m-diameter craters) and about 70 m for the largest lake Tüttensee crater. In Fig. 11, the depths and diameters for 46 fully preserved craters are plotted exhibiting a general increase of the depths with increasing diameters. On average, a diameter-to-depth ratio of r = 6.7 applies.
Fig. 11. Diameters and depths for 46 fully preserved craters of the Chiemgau strewnfield. On average, a diameter-to-depth ratio of 6.7 has been determined (given by the straight line).
