Research on geology, geophysics, and petrology of impact structures (meteorite impact craters)
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STOP 2: Shocked dike breccias (near Santa Cruz de Nogueras)


The dike is exposed in the Paleozoic part of the Azuara structure in a 5 - 10-m diameter, 1.5 m deep probable mining pit. The dike steeply dips, cuts dark Siegenian slates (Nogueras layers), has a maximum width of about 70 cm, and can be traced in the field over a distance of more than 10 m.

Components up to the size of several centimeters are in general slightly subrounded, larger fragments more angular. The maximum size of the components is 30 - 40 cm.

A homogeneous matrix is poorly defined and consists predominantly of strongly fractured host rock.

The components show a distinct diversity comprising single fragments, rock aggregates, and multi-colored breccias. We observe:

    Devonian slates frequently internally fractured and cut by small breccia dikes.

    Devonian Brachiopoda limestones also strongly deformed and cut by breccia dikelets.

    Devonian sandstones and quartzites, less deformed.

    Polymict multi-coloured breccias. The breccias are composed of carbonate and shale fragments which often show sandwich orientation within the matrix. Cataclastic flow texture can be observed. The colors are white, bluish-grey, red, brown, and black. The stratigraphic position of the components is unknown, but could be Paleozoic and/or Mesozoic/Cenozoic.

This dike breccia, and several others in the vicinity of Santa Cruz de Nogueras and Nogueras, are distinguished by the frequent occurrence of strong shock metamorphism, which has for the first time been described as early as 1985 (Ernstson et al.). We observe:

multiple sets of planar deformation features (PDFs) in quartz

multiple sets of planar fractures (PFs) in quartz

diaplectic ("quartz") glass

silicate melt; glass coating quartz grains

strong kink banding in mica

    multiple sets of PDFs (microtwinning) in calcite.

In a recent study, PDFs in a quartzite from this dike breccia were again investigated by Ann Thierault from the Canadian Geological Survey. In the section IDN 314, she observed in 13 quartz grains a total of 33 sets of decorated PDFs with a maximum of 5 sets per grain. For 28 sets, the crystallographical orientation could be measured with distinct accumulations at w {10-13}, p {10-12}, and x {11-22}. The density of the PDFs is throughout high, their relative length in general 100 %, and the spacing 1 µm or less. All shocked grains have reduced birefringence of 0.004 - 0.008. According to Stöffler & Langenhorst (1994), a shock stage 4 (strongly shocked) is indicated. This new analysis confirms the earlier measurements made by K.Ernstson and E. Guerrero.

Interpretation and relations

Breccia dikes (or dike breccias) are a prominent feature of impact structures. They have especially been reported for the Ries, Rochechouart, Sierra Madeira, Manicouagan, Haughton, Vredefort, and Sudbury structures. Most workers on this subject believe that they formed by hypervelocity cratering by the injection of material into the floor and walls of the growing and changing excavation cavity (excavation and modification stages). Thus, the study of breccia dikes is considered an important contribution to understanding cratering processes (see, e.g., Lambert 1981, Bischoff & Oskierski 1987).

At the Azuara structure, a large diversity and a large number of breccia dikes both on a macroscopic and a microscopic scale are observed. They occur around the structure in nearly all stratigraphic and lithological units (also see OUTCROP 4, on Friday). The dikes are up to 2 m wide and up to 300 m long. They are monomict and polymict, and they may occur in the form of dike generations (cossing dikes, dikes-within-dikes). A detailed description, classification and interpretation is given in Fiebag (1988) and Ernstson & Fiebag (1992).

The Santa Cruz de Nogueras dike belongs to the more complex breccia dikes, and it may tell us a story about the Azuara cratering process. The shock-metamorphic effects in the breccia components are witness of the contact and compression stage with the propagation of shock waves. Small breccia dikes formed in the shocked target rocks by injection of material in the excavation process which produced additional strong brecciation. Dike-bearing breccia components could intermix with other breccias and could have themselves been injected into the floor of the expanding excavation cavity. On collapse and rebound of the transient crater, a further mixing (e.g., with the multi-colored breccia) could have occurred producing what we are today observing in the mining pit east of Santa Cruz de Nogueras.

The prevailing {10-13} and {10-12} PDF orientations in the shocked samples from the Azuara structure are unusual considering the sedimentary (porous) target in which {11-22} and{10-11} directions commonly are more typical. The "crystalline" signature of the Azuara PDFs, however, may reflect the lithology of the dense Paleozoic quartzites.

Fig.4. Shocked polymict dike breccia (near Santa Cruz de Nogueras). Left: Polished cut section. Field is 11 cm wide. Right: Diaplectic glass; photomicrograph. The sandstone fragment is composed of diaplectic quartz grains embedded within partly recrystallized silicate melt. Plane polarized light (left) and crossed nicols (right). 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.

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last modified at: 2002-06-18