Azuara impact structure (Spain) - Ries impact structure (Germany)

Shortly after the impact ... A

B

The channel cuts through highly fractured and brecciated Liassic limestone megablocks in sharp and steep contact with well-bedded Tertiary sands. Near the contacts, a few disintegrated limestone blocks are floating in the sands. The sand is composed of predominantly calcite and quartz grains and some altered glass fragments. In thin section (D, plane polarized light; the field is 1 cm wide), the quartz grains show to be mostly sharp-edged indicating fragmentation and short transport.

Many quartz grains display shock features like multiple sets of planar fractures (PFs) and multiple sets of planar deformation features (PDFs).

Interpretation: The peculiar contact between the sands and the overhanging and highly fractured rocks gives evidence of an obviously sudden and very short-term depositional process. The highly brecciated and partly overhanging flanks of the limestone megablocks would not have survived any substantial period of time, and faults can basically be excluded. Therefore, we suggest that the outcrop reflects the earliest phase of the post-impact sedimentation at the crater floor shortly after the impact.

In some respects, the sandy unit may be compared with the so-called "graded unit" which has been found as a 17 m core section in the research borehole Nördlingen 1973 in the Ries impact structure (Germany). The "graded unit" occurs within the crater between the suevite impact breccia and aquatic sediments, and it is assumed to be the result of a single-phase sedimentation. Alternative processes consider airfall of ejected impact material or a turbidity current-type transport mechanism in water or steam. Both are possible explanations also for the deposition of the sandy unit in the Blesa irrigation channel, which is currently investigated in more detail.