Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
STOP 4: Breccia dikes (near Fuendetodos)
The breccia dikes are observed in new outcrops made by road construction between Fuendetodos and Jaulín (Fig.7). They cut Jurassic limestones and form a system of nearly vertical and, subordinately, nearly horizontal dikes. Frequently, the vertical and horizontal dikes are connected to form H shapes (also see below), which is well known from many other breccia-dike locations in the Azuara structure. The maximum width of the dikes is about 1.5 m, and they can be traced in the field over a distance of at least 10 m. The strike of the vertical dikes is roughly SW - NE.
The contacts between the dike breccias and the Jurassic host rock are sharp. We observe breccia apophyses penetrating into the host rock, thereby breaking the host rock to fragments. Frequently, laminated calcite crusts and calcite crystals are observed to follow the wall of the dikes.
Basically, the dikes are polymict. The components (maximum size about 1 m) are in general angular to subangular, and sometimes slightly subrounded.. They comprise single rock fragments, rock aggregates, and polymict breccias to form breccias-within-breccias. The most abundant components are Jurassic limestones, which are sometimes internally fractured and cut themselves by breccia dikelets. Less abundant is Triassic material, which may be assigned to Keuper. The polymict breccia components are similar to the host breccia.
Fig.7. Breccia dikes in Jurassic limestones. Fuendetodos.
The components are immersed in a medium hard, reddish and dominantly carbonate matrix. Reactions between the matrix and components have occurred as seen from precipitation seams and corrosion of their surfaces. Probable relics of former carbonate melt in the dikes are indicated by zones of whitish, highly vesicular, foamy carbonate material, where evidently degassing took place.
Interpretation and relations
The Fuendetodos breccia dikes belong to the very large „family" of breccia dikes in the Azuara structure. The large diversity and processes of formation were already addressed at STOP 2 (Santa Cruz de Nogueras). Here, we appreciate the much better outcrop conditions which allow to better understanding that and how the dikes formed by injection of brecciated material, probably in the excavation process. In Fig. 8, a simple model is shown, which explains how the typical H-type breccia system could develop in two phases.
On cursory inspection of the dikes, the laminated calcareous crust may remind of some karstification phenomena. Very probably, however, the crusts have originated from migrating carbonate-rich fluids after the impact, perhaps under hydrothermal conditions. An extended hydrothermal regime after the Azuara/Rubielos de la Cérida impact has been suggested by Siegert (1997).
In fact, no real karstification can be observed in the Fuendetodos outcrops. And we emphasize that especially the H-type dikes, the injection apophyses, the matrix reactions with the components, and the breccias-within-breccias are incompatible with any karstic interpretation. This holds true also for any tectonic model.
Another point of interest is that the contact between the two breccia generations is sharp-cut without mixing of the material. This requires a rapid change of rock strength from soft to rigid (in order to allow the second dike material to intrude into the host breccia).
Fig.8. A simple model of H-type dike formation for the Fuendetodos breccia dikes.
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