Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
STOP 6: Puerto Mínguez Ejecta
The Puerto Mínguez ejecta are exposed along the new stretch of the road CN 211 from Cosa to Vivel del Río, between kilometric milestones 111 and 117. Like the Pelarda Fm., the outcrops are located in the altitudes of the zone (roughly 1200 m).
Stratigraphically, the deposits - meanwhile continuously exposed over 5 km length - consist of largely uncemented Palaeozoic, Mesozoic, and Lower Tertiary materials which, in many places, may intimately be mixed. The cobble and block components are heterometric, show variable morphology (well-rounded, sub-rounded, and angular), and are supported by a sandy and marly matrix. Occasionally, megablocks with preserved stratification and original characteristics are intermixed within the deposit.
Paleozoic materials are generally more common near the village of Fuenferrada. Here, patches of terrain showing the original layering are found in the deposit, which as a whole presents a poor sorting and is massive or weakly layered. Towards Cosa, and near the Portalrubio junction, Paleozoic materials become more and more scarce, until they disappear altogether, while the conglomeratic parts of the deposit show a prominent layering and an increment in clast-supported textures. These conglomeratic patches consist of heterometric large (sometimes 1 m) limestone blocks with sub-angular to, more common, sub-rounded morphologies. Most of them are found dispersed in a sandy to clayey matrix, so textures are essentially matrix-supported. Conglomeratic patches appear as „islands" in this matrix, which is specially abundant around Fuenferrada. Near Cosa, they show up instead as more or less continuous layers or as lenticular bodies.
Side by side with these conglomerates, breccia zones can be found, consisting of polygenetic Paleozoic (shales, phyllites, and schists) boulders. They are heterometric (but the maximum size is smaller than that of limestone boulders), matrix-supported, and layering is completely lacking. Some of these breccia zones form isolated blocks, which have conserved their original layering and which are immersed in the conglomeratic, poorly layered deposit. In other parts of the outcrop, the contacts between breccias and conglomerates are sharp or even clearly erosive. Frequently, the engulfing of single limestone boulders into Palaeozoic-dominated zones can been observed.
One of the most intriguing observations which can be made at the Puerto Mínguez deposits is the generally strong deformation of the components, even as they are uncemented and embedded within a soft matrix. Most of the pebbles, cobbles, and blocks in the deposit (the limestone components practically without exception) are strongly deformed. They show striations all around (including nailhead striae), penetration marks, deep grooves, rotated fractures, irregular fractures with complex bifurcations, and polish. While some of these deformations occur in components independently of lithology, others are especially spectacular in limestone pebbles, cobbles and blocks.
Along the deposit, components of different lithology (slates, schists, limestones) display very dinstinct striae which, comparable with the Pelarda Fm. (Ernstson & Claudin 1990), may occur as multiple sets and, in some cases, show totally irregular orientations. The grooves, penetration marks and imprints are preferentially associated with limestone components. Often, distinct penetration marks remind of soft butter and a knife having spread over it, such pointing to elevated plasticity. Occasionally, penetration marks in contact result in a faceted sculpture of the cobbles and blocks.
The polish is restricted to carbonate components. Frequently, fractured components at first sight look like a bread cut to slices. However, the clasts are not broken to pieces at all. The rotated fractures, very frequent in the Puerto Mínguez components and for the first time defined for the Pelarda Fm. ejecta (Ernstson & Claudin, 1990), may result from pure torsion as well as from a superposition of torsion and tension. The intense internal torsion of many cobbles and blocks is documented by macroscopic hinges and rotated fractures affecting the whole component, however, without breaking it to pieces (Fig.9). In some carbonate clasts affected by irregular and rotated fractures, a reaction between the matrix and the fracture rims can be observed. In this case, the matrix (silty to muddy, with some allochthonous and autochtonous fragments included in it) literally „eats" the border rim, giving a wavy aspect to it.
Interpretation and relations
When the Puerto Mínguez rocks were for the first time described by Moissenet et al. (1972), the authors were speaking of an „enigmatic" deposit. Today, these deposits are no longer an enigma, because the macroscopic and especially the mesoscopic characteristics observed in the Puerto Mínguez deposits are compatible with an impact origin showing large similarities to other impact ejecta
On the other hand, the observations are not compatible with an origin from fluvial deposition, from quasi-static tectonic deformations, from the acting of glaciers, or from syn-tectonic sedimentation (e.g., Casas et al. 2000).
In fact, a fluvial or syn-tectonic sedimentary control of the setting with striated, faceted and polished components within a very soft matrix is excluded. We also point to typical deformations found in the Puerto Mínguez outcrops which exclude long-lasting tectonic stress. Especially the strong internal rotations with macroscopically untouched hinges and rotated fractures cutting through whole components without breaking them to pieces (Fig.9), are explained by short-time deformations under high confining pressure only. Within a conglomerate, tectonic stress would have sheared the cobbles; and within the soft matrix, power (stress) transmission for the very strong deformation is absent. Also, the frequent open, tensile fractures of cobbles in a soft matrix are not compatible with long-lasting tectonic crushing.
Summarizing, the Puerto Mínguez deposits are diamictites which have been deposited and deformed within a short span and under high confining pressure. Such a setting is well known from meteorite impact ejecta. Identical depositional features and defomations (striae, mirror polish, rotated fractures, bread-cut-to-slices features, all kinds of imprints, and others) have in detail been described for, e.g., the Ries crater ejecta (Bunte Breccia) (Chao 1977, Chao et al. 1978) and for the Belize ejecta of the Chicxulub impact structure (Rampino et al. 1996, 1997a, 1997b, Ocampo et al. 1996, 1997). The bread-cut-to-slices features, for example, are well known to occur in carbonate concretions and in belemnite rostra from soft Bunte Breccia material in the Ries crater. For the origin of the Belize brittle and plastic deformations, the authors suggest high-velocity flow, violent collisions and shock effects as well as partially melting during excavation, transport and ballistic emplacement with secondary cratering.
The origin of the heavy deformations of the Pelarda Formation components has previously been discussed by Ernstson and Claudin (1990) (also see STOP 3). They consider deformations during excavation and collisions of components in the expanding ejecta curtain. They propose, however, that the main deformation took place in the process of ballistic erosion and sedimentation (Oberbeck, 1975; Hörz et al, 1983), i.e. on landing of the ejecta and mixing with local material. A similar origin for the deformations of the Puerto Minguez materials is evident.
It is again pointed out that the matrix has obviously reacted with the carbonate components. This is ascribed to elevated temperatures in the matrix, to the system CO2-CaO-H2O from decarbonation processes, and possibly to carbonate melting. While these features are expected to occur in impact diamictite deposits, they are unexpected in non-impact diamictites, and, hence, enable a distinction. Compared with the basal breccia (see STOP 1), the largely uncemented ejecta point to limited contribution of carbonate fluids from decarbonation and carbonate melts.
We add that the visited Pelarda Fm. and Puerto Mínguez ejecta outcrops are only part of an emsemble comprising a large number of deposits with comparable macroscopic and mesoscopic features, and thus obviously belong to a great geologic scenario in a much larger region of the Iberian Chain.
Fig.10. Mesoscopic deformations of clasts from the Puerto Mínguez ejecta, and the Ries crater ejecta (bottom right).
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