8 Petrographical and geochemical evidence

A program of petrographical and geochemical investigations (thin-section inspection, microprobe and x-ray analyses, etc) has been initiated. With respect to the host of material with quite different composition and texture, preliminary results are presented here.

Fig. 30. Sandstone cobble completely coated by silica glass. Note the smooth surface without any sinter traces. From crater 004.

Fig. 31. Detail from Fig. 14 in close-up. The colourless to greenish glass exhibits many minute vesicles. The field is 22 mm wide.

Evidence of unusually high temperatures is given by the occurrence of sandstone boulders and cobbles in and around craters completely coated by silica glass (Figs. 30, 31) [11]. The cobble in Fig. 32 is similarly coated by silica glass but, moreover, is homogeneously interspersed with feldspar melt glass (the dark strings, close-up in Fig. 33) embedded in quartz (white). In thin section, the feldspar glass is in most cases associated with feldspar crystals dispaying multiple sets of so-called planar deformation features (PDFs; Fig. 34) which are considered to be indicative of shock deformation (Engelhardt et al., 1969, French & Short 1968, and others).

Fig. 32. Sawed surface of a thermally shocked sandstone cobble completely coated by silica glass. Note the dark strings of partly recrystallized feldspar glass giving a gneiss-like aspect to the rock. From 11 m-diameter crater 004.

Fig. 33. Detail of Fig. 16: Feldspar glass embedded in quartz. Note the many spherical bubbles in the glass. The field is 3 mm wide

Fig. 34. Feldspar glass in sandstone cobble from Fig. 16. Photomicrograph, crossed polarizers and parallel light. The fields are 1.4 mm wide. Note the vesicular glass (black under xx nicols) and the multiple sets of planar features in the feldspar grains. Quartz grains are whitish to greyish.

Because of the smooth surface of the glass-coated cobbles lacking any traces of sinter processes from contact with neighboring rock material, an in situ origin of the glass, from human activities, can be clearly excluded. Instead, we have to assume that the cobbles were ejected and entered the super-heated impact explosion cloud where they became thermally shocked and partly melted. Moderate mechanical shock by impact shock waves is indicated by the occurrence of planar deformation features and multiple sets of planar fractures (cleavage) in quartz (see e.g., Stöffler 1972, Stöffler & Langenhorst 1994) from sandstone and quartzite cobbles and boulders (Fig. 35). Whereas PDFs in quartz are relatively rare in the samples so far examined, multiple sets of planar fractures (PFs) are abundant. Cleavage is normally absent in quartz and may only occasionally be observed in rocks from very strong regional metamorphism. In impact cratering, however, PFs belong to the regular shock inventory. Since the Mesozoic sedimentary rocks from the moraine material in the Chiemgau region underwent Alpidic tectonics only and were not subjected to any significant regional metamorphism, an origin of the PFs from shock is highly probable.

Fig. 35. Shocked quartz grain in sandstone cobble from crater 004: planar deformation features (NNE - SSW trending) and multiple sets of planar fractures (cleavage). Photomicrograph, crossed polarizers; the field is 560 µm wide.

Fig. 36. Highly porous carbonate clasts assumed to be crystallization products from a carbonate melt

More evidence of high temperatures is given by white, low density, highly porous carbonate clasts (Fig. 36). We suggest them to be the crystallization product of a carbonate melt from the melting of limestone cobbles. Very similar vesicular carbonate material to be also relics of carbonate melt is reported for the Azuara and Rubielos de la Cérida impact structures (Ernstson & Claudin 2002; also see Grieve & Spray 2003). Different from silicate rocks, carbonates may melt but cannot be chilled to form glass. Instead, upon cooling, the melt rapidly crystallizes to form again carbonate. Relics of calcite crystals in these clasts may show microtwinning which is assumed to be also a shock effect (Metzler et al. 1988, and references therein).

Bluish-grey, dark green and black glass-like material abundantly exhibits "splash" shapes (tear drops, dumbbells, plates, etc.) indicating rapid cooling and solidification during flight (Fig. 37). The same matter, that has to be studied in more detail, may also coat rocks.

Fig 37. "Splash" shapes indicating rapid cooling and solidification of glass particles during flight.

More strange material extended over the area of the scattering ellipse comprises metallic matter of very different size, shape and composition (Figs. 16 ,38, 39) (Beer et al. 2003). Corroded metal fragments up to the size of 8 cm are composed of dominantly Fe, in two cases of Fe and C, together with Rb, Se, Cu, Na, Mn, Al, Zn, Si, S, Cl, Lu, P, Tl. Re, Ru, Ni, Cr in amounts between about 1 % and 0.01 %. Metallic fragments (< 10 cm) without any oxidation traces (density 6.3 g/cm³, Mohs hardness 6-8) prove to be ferrosilicides, Fe_XSi_Y, containing inclusions of TiC (titanium carbide), alpha-iron and Al_XSi_Y. Among the Fe_XSi_Y phases, Fe₃Si corresponding to the mineral gupeiite, and Fe₅Si₃ corresponding to the mineral xifengite, have been identified (Beer et al. 2003). The Fe_XSi_Y material finely intersperses also the carbonate clasts assumed to have originated from a carbonate melt (Fig. 40). It is also found, down to the fraction of fine sand, in a halo of more than 100 km length around the crater scattering ellipse.