Shock effects (shock metamorphism) in rocks from the impact layer (ejecta, Bunte Breccia) at Lake Tüttensee

Quartz grains in mica gneiss from the Chiemgau impact Tüttensee ejecta layer – full of planar deformation features (PDFs).

Two sets of crossing kink bands in mica. Shock effect from the Chiemgau impact Tüttensee ejecta layer.

Numerous excavation pits have been performed in the surroundings of Lake Tüttensee that is proposed to have formed as a meteorite crater in the Chiemgau impact event. The excavation pits have revealed the existence of a more or less continuous ejecta blanket around the lake, and the peculiarities of the layer have already been described: (part 1) and (part 2). Now, samples (from crystalline and sedimentary Alpine cobbles) were selectively taken from this ejecta layer in order to systematically analyze them for shock effects, for the time being under the optical microscope only. The study of thin sections from 31 rock samples taken from 7 different excavations establishes a rich inventory of mineral deformations that with reasonable certainty or with great likeliness have originated from shock load. The photomicrographs (crossed polarizers) show planar deformation features (PDFs) in quartz from a mica quartzite (upper) and two sets of extremely closely spaced kink bands in biotite from a gneiss (lower). A more comprehensive report including 17 photomicrographs (in German with English abstract and figure captions) may be clicked HERE.


The Tüttensee “Bunte Breccia”

 Apart from the suevite the ejecta blanket of the Ries impact structure (Germany) is composed of the so-called Bunte (= multicolored) breccia (clast size < 25 m) and allochthonous megablocks (coherent clasts  > 25 m). The distinction has been made for mapping reasons but otherwise is arbitrary. The term “Bunte” refers to the many colors (black Jurassic claystones, white Malmian limestones, purple and greenish Keuper sandstones and claystones, various crystalline rocks) that on intensive mixture of the components cause the multicolored appearance of the breccia. New excavation pits at Lake Tüttensee (Chiemgau impact event) have encountered a “Bunte breccia” (the image shown here) very similar to its namesake of the Ries impact crater. The probable similar formation of the breccias (as impact ejecta) is especially evident when the Tüttensee breccia is compared with the small-grained clayey Ries breccia. A preliminary short report (in German with English abstract and figure captions) on the new excavation results may be clicked HERE.

On the Lake Tüttensee discussion:

Critics of the impact origin for the Lake Tüttensee crater (e.g., Doppler & Geiß 2005) argue that the strong deformations exhibited by the cobbles and boulders from the rim wall are the result of tectonic processes in the Alps and that the tectonically deformed clasts were transported and deposited in the region of the strewnfield. The impact advocates counter that such deformed cobbles and boulders would not have survived any transport over a distance of more than 50 m. Instead, they point to the typical features of a high-pressure/short-term process of deformation and to the fact that gravel pits outside the Lake Tüttensee area are void of these characteristically deformed rocks.

We have received a comment on this controversy by F. Claudin, geologist from Barcelona (Spain):

“We can compare the deformations observed in the Lake Tüttensee rim wall with the supposed deformations that must be present in other glacial moraines. In the Pyrenees, near Les Bordes de Llestui at the Clot and Malmarrui torrents, we can observe a glaciolacustrine-glacial complex. Neither in the clasts of the subglacial till nor in those of the supraglacial till, deformations as described for Lake Tüttensee can be seen. Only striae on the clasts are observed. The same observations can be made in the moraine deposits near Vilaller (Verge de Riupedrós, Sant Mamés, Sant Antoni), near the hospital of Vielha, or in the “Barranco de la runada” (near Senet). Glaciers may produce enough pressure to ensure striae deformations or fragmentation of clasts, but they are unable to produce short-term deformations under high confining pressure as seen in the photos of the Lake Tüttensee clasts. Typical aspects of the Pyrenees glacial deposits are shown down below the text.

The above-mentioned deposits have in more detail been described by

  • Bordonau, J (2000): Itinerario 3 (Vilaller, Hospital de Vielha, Valle de Llauset y Noguera Ribargozana), in Geopirineos, Monografias de Enseñanza de las Ciencias de la Tierra, Serie Itinerarios, nº 2.
  • Bordonau, J.; Pous, J., Queralt, P., Vilaplana, J.M. (1989) : Geometria y depósitos de las cubetas glaciolacustres del Pirineo. Estudios Geológicos, 45, 1-2: 71-79.
  • Bordonau, J., Vilaplana, J.M., Fontugne, M. (1993) : The glaciolacustrine complex of Llestui (Central South Pyrenees) : a key-locality for the chronology of the last glacial cycle in the Pyrenees. C.R. Acad. Sci. Paris, 316, série II: 807-813.
  • Vilaplana, J.M., Bordonau, J. (1989): Dynamique sédimentaire lacustre de marge glaciaire : le paléolac de Llestui (Noguera Ribagorçana, Versant Sud des Pyrénées), Bull. A.F.E.Q., 1989-4 : 219-224.”