Although competent rocks are widespread in the Rubielos de la Cérida basin and are especially well exposed in the central uplift chain, only very few shatter cones of moderate quality have been found so far. This is believed to be due to limited outcrop conditions (freshly exposed rocks are rare) and to the lack of a detailed mapping. Also, there may have been influencing factors which prevented shatter cone formation. We point to the assumed very thick unconsolidated molasse sediments of the target and a possible buffering effect. Moreover, we refer to a peculiar behavior of the limestones in the central uplift chain, that is breaking into small fragments and splinters upon hitting them gently with a hammer. The process that evidently froze stress within the limestones is unknown so far, but may have been incompatible with shatter-coning.
In this context it is interesting to note that in the 25 km-diameter Ries impact structure (Nördlinger Ries crater) shatter cones have so far been found in crystalline target rocks (granitoids, gneisses, kersantites) only but never in sedimentary rocks. The Ries target consisted of roughly 600 m Triassic and Jurassic rocks over crystalline basement among them about 200 m thick dense Malmian limestones and marlstones in principle best conditioned for shatter cone formation. During impact large volumes of these rocks that must have been exposed to the necessary shock levels, however shatter cones did obviously not develop. We note that the Ries belongs to the best investigated and mapped impact structures.
Below, we show some moderately well developed shattercones from the Rubielos de la Cérida impact basin.
Fig. 1. Shatter cones in Cambrian siltstones; near Olalla at the northern basin rim. Note the conical fracture surface and the typical horse-tail fracture markings. The joint surface in the base displays faint plumose fracture markings not to be confused with shatter cones. Sample from collection P. Bockstaller.
Fig. 2. More shatter cones in Cambrian siltstones. 10 cm scale bar.
Fig. 3. Shatter cones of moderate quality in Malmian limestones; central uplift chain near Bueña.