Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
STOP 1: Basal breccia (near Cucalón)
The basal breccia (front cover, and Fig.3) is a polymict and rather heterogeneous breccia which is up to 20 m thick and is found in many parts (inside and outside) of the Azuara structure (and also the Rubielos de la Cérida structure). It unconformably overlies Mesozoic and Lower Tertiary sediments and formed at the base (basal breccia!) of the post-impact fanglomeratic/conglomeratic Upper Tertiary. Both Paleozoic and Mesozoic sediments contribute to the breccia components. The clasts are in general sharp-edged fragments, but well-rounded components are also observed. Frequently, they show distinct reaction seams in contact with the matrix. Breccia generations (breccias-within-breccias) are abundant. Breccia specimens break into pieces across the clast boundaries due to the extremely hard matrix and an exceptional cementation.
The matrix regularly shows distinct flow texture and is composed of calcite and (from X-ray fluorescence analysis) some SiO2 (up to 10 %). X-ray diffraction analysis and thin-section inspection with strong magnification, however, could identify only minute traces of quartz and silicates (mica) (Mayer 1991). The major fraction of the SiO2 content is thus expected to be in an amorphous phase (glass?).
Many components underwent partial up to nearly complete decomposition, which is in general connected with highly vesicular zones and flow texture of the basal-breccia matrix. In these zones, Katschorek (1990) established relics of former carbonate melt.
Rarely, thin sections of the basal breccia show shock metamophism in the form of PDFs and PFs in quartz, kink bands in mica and diaplectic quartz crystals.
Interpretation and relations
The basal breccia is interpreted as an impact breccia, which formed by the mixing of shock-produced carbonate and (subordinately) silicate melt with strongly brecciated target rocks. This interpretation is substantiated by the sharp-edged clasts (especially the limestone fragments), which suggest an immediate embedding in a soft, fluidized matrix after the brecciation and mixing of the Paleozoic and Mesozoic components, and by the occurrence of shock-metamorphic effects.
The mixture was partly excavated and partly deposited in the excavation cavity, and, thus, remained also within the modified, final structure.
Consequently, the basal breccia may be seen in close relationship to impact formations of the suevite type (e.g., Pohl et al. 1977). Different from classic suevites (e.g., Ries crater), the impact melt was primarily carbonate melt of very low viscosity, which rapidly recrystallized to calcite and/or aragonite. (Carbonate melts do not quench to form carbonate glass.)
As discussed for the Haughton impact structure (Metzler et al. 1988), a shock decarbonation (Kotra et al. 1983, Lange & Ahrens 1986) of the Azuara target limestones and a subsequent H2O-CaO recombination must also be considered. Such a reaction may have been the cause of the unusual cementation of the basal breccia. A similar "hammer-breaking" cementation has been reported also for carbonate breccia dikes in the Sierra Madera impact structure (Wilshire et al. 1972).
The basal breccia gives evidence also for the validity of the KIEFFER-SIMONDS model on the difference between impact-melt occurrences in impact craters in crystalline, mixed and sedimentary targets. Kieffer & Simonds (1980) explain the obvious absence of impact melt sheets in the case of sedimentary targets as the result of shock-produced volatiles (from pore-water vaporization and limestone decarbonation) finely dispersing the shock-melted material. As the basal breccia contains up to 10% SiO2, which cannot be ascribed to quartz or silicates, the SiO2 is suggested to be glass finely dispersed within the basal breccia matrix.
Fig.3. Different aspects of the basal breccia. The fields are 12 cm (top left) and 22 cm (bottom) wide.
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