Shortly after the impact … A |
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| The exposure in image A (details in B, C) results from the construction of an irrigation channel and is located near Blesa village about 14 km from the center of the Azuara impact structure. | |
 BThe 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. |
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Categoría: Aspectos destacables
Ries impact structure (Germany); Azuara and Rubielos de la Cérida impact structures (Spain)
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 CAguilón; Jurassic limestones (Azuara structure). Note the bedding in the base speaking against a tectonic fault.
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 Dnear Santa Eulalia; Muschelkalk limestones (Rubielos de la Cérida structure). Note the block of bedded limestone floating in the highly brecciated material. |
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| In all three cases, a tectonic interpretation of the layering offers considerable difficulties. | |||
Rubielos de la Cérida impact structure (Spain)
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Azuara impact structure, Spain: shock metamorphism
Highly shocked polymictic dike breccia (near Santa Cruz de Nogueras, 30660971E, 4553223N). Typical shock effects in the breccia are |
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 A
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 BB: Diaplectic glass; photomicrograph of a sandstone fragment completely transformed to diaplectic quartz; plane polarized light and xx nicols. Note that there are a few holes in the thin section not to be confused with diaplectic quartz grains. The field is 600 µm wide.
C: Planar deformation features (PDFs) in quartz grains; sandstone fragment from the shocked breccia. Photomicrograph, plane polarized light; the field is 800 µm wide. Note the large number of grains showing PDFs, their high density, the small spacing and the multiple sets. Up to five sets of different PDF orientation per grain have been observed in the dike breccia.
D: Planar fractures (PFs; cleavage) in quartz. Photomicrograph, xx nicols; the field is 450 µm wide. Note that at least six sets of different orientation can be observed. Cleavage in quartz is very uncommon in tectonically deformed quartz. In rare cases, rhombohedral fracturing is observed to occur in rocks which underwent strong regional metamorphism. In rocks from impact structures, PFs in quartz belong to the regular shock inventory. E: Kink bands in biotite from the shocked polymictic breccia. Photomicrograph, crossed nicols; the field is 840 µm wide. – Although kink bands can form under static conditions of strong regional metamorphism, the high frequency of the kink bands shown here, their narrow width, and their high kink-angle asymmetry point to shock deformation. The shock-metamorphic effects shown here correspond to a broad range of shock pressures. The melt glass, however, shows that parts of the breccia must have experienced shock peak pressures exceeding 500 kbars (50 GPa).
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D
ERubielos de la Cérida impact structure, Spain:
A
B
C
Rubielos de la Cérida impact structure, Spain:
Part of a large (some 300 m size) quarry exposing limestones (Muschelkalk Fm.) drastically destroyed through and through (A).
Within the completely brecciated rocks (displaying gries brecciation and mortar texture), white blocks (up to cubic-meter size) of carbonate material (B) are intercalated.
The low-density, highly porous material shows a distinct vesicular texture (C – the field is 7 mm wide).
Interpretation: A compressive strength of perhaps 150 – 200 MPa (= 1.5 – 2 kbar) for these massive and dense Muschelkalk limestones assumed, they must have experienced pressures clearly exceeding these values not only locally but throughout the huge volume. Whereas a tectonic origin can be excluded without any doubt, deformations like that are expected to occur in the cratering process (excavation and/or modification stage) of large impact structures. The intercalated white vesicular material is considered to be the relics from decarbonization and/or carbonate melt produced by shock or strong frictional heating.