7 Macroscopic deformations
Some of the smaller craters have been investigated in more detail exhibiting strong mechanical deformations at the floor and the walls and in the ejected material forming the ring (Figs. 17, 18) [5].
Fig. 17. Example of a heavily fractured sandstone cobble typically found in the craters of the strewnfield.
Fig. 18. Grit-brecciated quartzite clast (monomictic movement breccia) from crater 004.
Heavily fractured but coherent boulders (Figs. 19 - 23) prove in situ high-pressure/short-term deformation. A deformation by Alpidic tectonics or by glaciers can basically be excluded, because the boulders would not have survived any significant transport. For comparison, we show in Fig. 24 a similarly deformed boulder from the Pelarda Fm. ejecta of the Azuara/Rubielos de la Cérida impact structures in Spain (Ernstson & Claudin 2002). Rotated fractures and bread-cut-to slices features, special deformation types established by Ernstson & Claudin (1990, 2002), have also been reported earlier for the Ries impact structure (Nördlinger Ries) (Chao 1977; also see Rampino et al. 1996, 1997 a, b, and Claudin et al. 2001) [4].
Fig. 19. Heavily fractured quartzite block from the ringwall ejecta of the Tüttensee crater. Note the multiple sets of closely spaced fractures and the distinct displacements. The clast remains coherent and is not broken into pieces indicating high-pressure/short-term in situ deformation.
Fig. 20. From the Tüttensee ringwall ejecta.
Fig. 21. From the Tüttensee ringwall ejecta.
Fig. 22. From the Tüttensee ringwall ejecta. Note the coherence of clast and extended fragments (arrows).
Fig. 23. From the Tüttensee ringwall ejecta.
Fig. 24.Strongly fractured quartzite block from the ejecta of the Azuara/Rubielos de la Cérida impact structures. Note the remarkable coherence of the sample.
Likewise, the widely open fractures in the otherwise coherent cobble with smooth surface and without any shearing (Fig. 25) cannot possibly have originated from tectonics. Instead, these so-called spallation features are the typical result of dynamic shock deformation well known from shock experiments in fracture mechanics (Fig. 26) and also observed near large impact structures (Fig. 27) (Ernstson et al. 2001 a, b) [1].
Fig. 25. Spallation fractures in a sandstone cobble from crater 016 in the Chiemgau strewndfield.
Fig. 26. Spallation fractures in experimentally shocked ARMCO iron (by courtesy of M. Hiltl).
Fig. 27. Spallation fractures in naturally shocked quartzite cobble. Buntsandstein conglomerate near Rubielos de la Cérida impact basin.
Abundantly, strongly deformed components appear to have aerodynamically been deformed plastically (Fig. 28), similar to volcanic bombs. The formation process of the peculiar crackling or bread-crusting features exhibited by the sandstone cobble in Fig. 29 and by others is not clearly understood so far.
We emphazise that the examples shown in the Figures do not represent scarce finds but regularly occur in and around the strewnfield craters. In the wall surrounding the largest crater, lake Tüttensee, estimated 40 - 50 % of the so far examined larger cobbles and boulders exhibit strong deformations, whereas all gravel pits next to the crater are void of these characteristically deformed rocks.
Fig. 28. Banana-shaped sandstone clast (viewed side-on) similar to volcanic-bomb form. From crater 004.
Fig. 29. Sandstone cobble showing typical crackling or bread-crusting features. From the ringwall ejecta of the Tüttensee crater.
