The Steinheim basin impact structure

The 15 Ma Steinheim basin, very probably a companion to the Ries impact structure, is a typical complex impact structure with a distinct 1,000 m-diameter central uplift as shown by a topographical profile, in a satellite imagery and on the geological map (see images below). Commonly, a diameter of about 3.7 km is quoted, but there are indications that the structure is much larger (see Gravity survey).

Satellite imagery of the Steinheim basin. Central uplift encircled by a red dashed line (small image). Diameter of the circle 1 km.

Geological map of the Steinheim basin.

The sedimentary target consisted of Jurassic limestones, marlstones, sandstones and claystones which contribute to the megabreccia of the central uplift and to a polymict breccia in the basin. Moderate microscopic shock metamorphism has been described to occur in Dogger sandstones. A well known macroscopic shock effect are the nice shatter cones, which were described in the literature as early as 1905. A distinct closely spaced subparallel fracturing of limestones has also been ascribed to shock (Reiff 1979).

Shatter cone in Malmian limestone; 6 cm maximum size. Upon clicking here, more shatter cones from the Steinheim impact crater can been seen

Closely spaced subparallel fracturing in Malmian limestone. 10 cm horizontal maximum size.

Polymict basin breccia in the Steinheim impact structure. 10 cm sample size.

Post-impact Tertiary and Quaternary sediments are widespread in the basin, and, therefore, outcrops exposing impact features are very rare. In the abandoned Burgstall quarry at the basin rim, excavated rocks are exposed to form a breccia composed of large blocks partly displaying mortar texture and gries brecciation.

Burgstall abandoned quarry.

Limestone breccia, Burgstall.

The Galgenberg (“gallows hill“) is an allochthonous uplifted block of Malmian limestones in the eastern basin.

Part of the Galgenberg (“gallows hill“) uplifted megablock.

Subparallel open fractures in a chert nodule, Malmian limestone in the Galgenberg megablock. The fracture strike points to the center of the structure. Coin diameter 22 mm. For comparison see the shock-induced subparallel fracturing in Buntsandstein quartzite cobbles, Rubielos de la Cérida impact structure (Spain)..

Strongly deformed and fractured Malmian limestones at the NW crater rim, Hirschtal