Category: Impact highlights

Rubielos de la Cérida impact structure (Spain): impact melt glass from the central uplift

 A  B
The glass shown in A, B (B: the field is 14 mm wide) is coating a sandstone exposed in the central uplift of the Rubielos de la Cérida impact structure. The glass has a greenish to whitisch color and is transparent to milky. In thin section (C, D (xx nicols) – the field is 6 mm wide), the sandstone shows heavily damaged, and intense cataclastic flow texture is observed to merge with the glass. Quartz grains are strongly fractured and show multiple sets of planar fractures (PFs) and planar deformation features (PDFs).
 C  D
Interpretation: Despite the occurrence of shock features in the sandstone, the glass probably did not form by shock melting. We suggest frictional melting by extreme dynamic metamorphism in the impact event (excavation or – more probably – modification stage when the uplift formed) and the glass to be pseudotachylite. Temperatures in excess of 2,000 °C were probably required for the homogenization of this glass (David Griscom, pers. com.).
The location of this spectacular exposure of the glass-bearing sandstone remains secret for the moment in order to prevent it from destruction by rock hunters.

Rubielos de la Cérida impact structure, Spain: at the crater floor


This peculiar fold is exposed in a region of an extended megabreccia near the village of Barrachina in the Rubielos de la Cérida impact structure. The fold is portrayed by a competent, however heavily brecciated Lower Tertiary limestone layer. The core of the fold is a pulp of nearly pulverized carbonate rock without any regular internal structure. Only a few limestone fragments are preserved.

Interpretation: The exposure is assumed to be located at or near the crater floor of the Rubielos de la Cérida impact structure (for more details see:

Fieldguide – Stages of Crater

Fieldguide – Stop 7

rubielos,

where giant rock masses moved in the excavation and modification stage of impact cratering to form the now exposed megabreccia. The fold is interpreted to be the result of a high-pressure injection of extremely brecciated material from below. A tectonic origin of this peculiar structure is hardly to understand. Local geologists (from the Zaragoza university and the Center of Astrobiology, Madrid) suggest collapse by dissolution of gypsum to have produced the megabrecciation – need we comment?

Azuara impact structure (Spain) – Ries impact structure (Germany): shortly after the impact

Shortly after the impact … A
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.
 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.

 C

 D

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.

Ries impact structure (Germany); Azuara and Rubielos de la Cérida impact structures (Spain): peculiar structural setting

 A B (close-up)
Peculiar structural setting in autochthonous Jurassic limestones at the eastern rim of the Ries impact structure (Wemding; formerly Schneider quarry). Photos: July, 2001.
Interpretation: The strange abrupt change from horizontal layering to steeply dipping and strongly deformed limestone beds has resulted from horizontal thrust under very high overburden pressure in the excavation and ejection process.Similar strange deformations can be observed at the rims of the Azuara and Rubielos de la Cérida impact structures (Spain):
C
Aguilón; Jurassic limestones (Azuara structure). Note the bedding in the base speaking against a tectonic fault.		 D
near Santa Eulalia; Muschelkalk limestones (Rubielos de la Cérida structure). Note the block of bedded limestone floating in the highly brecciated material.
In all three cases, a tectonic interpretation of the layering offers considerable difficulties.

Rubielos de la Cérida impact structure (Spain) – compressive signature – megabreccia

A B
Megabrecciation of Jurassic limestones in the southern central uplift near Bueña. Note the chaotic criss-cross layering (A) and some “ghost” layering having survived the intense brecciation (B).
Interpretation: A distinct megabrecciation is a typical structural feature in the central uplift of complex impact structures and well known from many craters.The giant compression occurs in the modification stage of impact cratering, when the transient cavity collapses and large rock volumes undergo a centripetal accelleration towards the center of the structure.In the Rubielos de la Cérida impact structure, the enormous compressive signature with strong deformations up to continuous megabrecciation is evident nearly everywhere and can best be observed in cuts from road constructions.

Azuara impact structure, Spain: shock metamorphism

A comprehensive article on the Azuara shock effects with emphasis on the F. Langenhorst and A. Deutsch quarrel may be clicked HERE.

Highly shocked polymictic dike breccia (near Santa Cruz de Nogueras, 30660971E, 4553223N). Typical shock effects in the breccia are
A

A: Melt glass with vesicles, schlieren and mineral fragments; photomicrograph, plane polarized light and xx nicols. The field is 9 mm wide.

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.CC: 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.
DD: 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

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).

 

  

 

 

 

 

Rubielos 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.