Ejecta of the Azuara impact structure (Pelarda Formation)

The Pelarda Formation ejecta (preliminary contribution preceding an extensive article to be published)

This article should now be replaced by the very comprehensive article on the Pelarda Fm. ejecta. Please click here!

General setting

The Pelarda Fm. covers an area of approximately 12 x 2.5 km² (Fig. 1), has in many cases a thickness of more than 200 m and, between 1,100 and 1,450 m altitude, forms the top of a mountain chain which belongs to the highest ones in the region (Fig. 2) (Carls & Monninger, 1974). The Pelarda Fm. unconformably overlies Muschelkalk and Paleozoic of the Olalla block (Figs. 2, 3) and  Fonfría Lower Tertiary (Paleogene) which is composed of alternately conglomerates and multicolored marls, and is overlain by the Olalla Tertiary materials (Adrover et al., 1982).

location map of the Pelarda formation ejecta deposit, Azuara impact structureFig. 1. Location map for the main deposit of the Pelarda formation in the rim zone of the Azuara impact structure.

photo of the Pelarda formation landscape, Azuara impact structureFig. 2. View of the densely forested Pelarda formation ejecta deposit (to the left) forming the highest mountains of the Sierra de la Pelarda (or Sierra de Fonfría).

Pelarda formation in contact to the downhill underlying PaleozoicFig. 3. The Pelarda formation ejecta overlying Muschelkalk and Paleozoic of the Olalla block. The contact between the Pelarda F. and the Paleozoic unit is rather diffuse, because the Quaternary debris from weathering of the Pelarda layers are moving downhill.

In the environments of Fonfría, the limestone cobbles of these conglomerates are heavily deformed and show intense striations, deep imprints and polish (Fig. 4). The striations display more or less homogeneous SW – NE strike, that is towards the center of the Azuara impact structure. It is suggested that the deformations formed in the Tertiary conglomerates when the Pelarda Fm. ejecta landed with high velocity and were emplaced under high pressure on top of them.

deformed, striated and imprinted cobbles from below the Pelarda Fm. depositFig. 4. Deformed cobbles from the Paleogene conglomerates exposed below the Pelarda Fm. The cobbles that were samples near Fonfría village exhibit distinct striations and mirror polish.


Structure and composition

In general, the Pelarda Fm. shows a stratification divided into three parts: a lower, middle and upper zone. The contacts between the zones are gradual and not very distinct.

The lower zone contains clasts of Paleozoic pelitic rocks (slates and schists) und quartzites. The pelitic rocks are in general more abundant than the quartzites. The clasts are angular to subrounded, their average size is less than that of the clasts from the middle and upper zones, and they are embedded in an argillaceous matrix of maroonish to reddish color (Fig. 5). The sediments of this zone are clearly different from the Miocene alluvial fans in the neighboring Olalla area which concerns the characteristics of the components and the texture as well as the sedimentary structures.

exposure of the lower unit of the Pelarda Fm. ejecta, Azuara impact structureFig. 5. Typical aspect of the lower unit of the Pelarda Fm. Near Salcedillo. The maroonish to reddish color of the matrix changes with respect to the in each case predominant contribution of Paleozoic and Buntsandstein material.

In the middle zone, the components are pelitic rocks, quartzites, few Buntsandstein clasts and sporadically (?Jurassic) limestone clasts. Compared with the lower zone, the ratio of quartzite to pelitic clasts is increased. Altogether, the clasts are more rounded and heterogeneously sized. In Figs. 6, 7 a Buntsandstein sandstone megaclast of about 9 m size is shown to be intercalated in the deposit.

exposure of the Pelarda Fm. ejecta, intercalated Buntsandstein megaclast,Azuara impact structureFig. 6. A  9 m long Buntsandstein megaclast (scraped) horizontally intercalated in the middle unit of the Pelarda formation.

exposure of the Pelarda Fm. ejecta, intercalated Buntsandstein megaclast detail, Azuara impact structureFig. 7. Close-up of the large Buntsandstein megablock showing typical Buntsandstein sandstone facies in small pieces in the middle of the image. Other geologists confused this Buntsandstein intercalation with Tertiary sandy material (P. Carls, pers. comm.) or consider the megaclasts to be thin sandstone layers belonging to the (from their point of view) alluvial fan stratigraphic column (Cortés et al. 2002; Diaz-Martinez et al. 2002 a; Diaz-Martinez 2005). Cap diameter 51 mm.

exposure of the Pelarda Fm. ejecta, matrix-supported clasts, Azuara impact structure

Fig. 8. Aspect of the middle unit of the Pelarda Fm. showing matrix-supported texture.

In all outcrops of this zone so far studied the texture can be termed diamictic and the clasts are matrix-supported (Fig 8). Predominantly, the matrix is sandy and in some parts argillaceous. Frequently, multicolored Tertiary marls and red Buntsandstein sandy materials are intermixed in the matrix, and especially the red Buntsandstein sands give a typical color to the deposits when exposed (Fig. 5, Fig. 8).

The upper zone also shows a matrix-supported texture, and the matrix is basically muddy. The components are predominantly unsorted rounded to subrounded quartzite clasts and subordinately pelitic rocks. In general, the quartzite clasts are much larger than the slate, schist and Buntsandstein clasts. Occasionally, the size of quartzite boulders may exceed 2 m (Fig. 9).

quartzite megaclast, Pelarda formation impact ejecta, Azuara impact structureFig. 9. Quartzite megaclast in the upper zone of the Pelarda Fm. It is hardly to imagine how such a big block could have been transported to the uppermost part of the mountains as part of a fluvial block conglomerate as has been postulated by Carls & Monninger (1974).

In all three zones referred to, a stratification is completely lacking except for few conglomeratic and sandy intercalations (Figs. 10,11) that are sometimes laminated but do not display tabular bodies (neither small nor large) nor do they form channeling structures. In these intercalations, the majority of larger clasts are oriented parallel to the lamination with their more convex part upwards (Fig. 12). These clasts enabled the measurement of dip which is either SW or NE. This dip corresponds with the strike of striations of cobbles (see below) and is interpreted to reflect the direction of transport of the ejecta out of the Azuara structure.

exposure of the Pelarda Fm. ejecta, sandy blocky intercalations, Azuara impact structureFig. 10. Sandy blocky intercalations in the Pelarda Fm. ejecta lacking any stratification, however.

exposure of the Pelarda Fm. ejecta, sandy layer intercalated, Azuara impact structureFig.11. Bed of locally intercalated sandy material but no real stratification in the Pelarda Fm. ejecta diamictite.

exposure of the Pelarda Fm. ejecta, adjusted clasts, Azuara impact structureFig. 12. Larger clasts show an adjustment to a suspected flow texture of the matrix. In general the convex side of the boulders points upwards. This orientation enabled the measurement of dip that resulted in a preferred SW – NE direction.

As already mentioned, there is no clear contact between the three zones, and no structures can be observed indicating different periods of deposition (e.g., thin sandstone beds (Figs. 10, 11) from instantaneous breaks, zones of reworked material, and so on). Likewise, a clear grain size distribution cannot be observed, and, always, the texture is matrix-supported. The occurrence of Buntsandstein megablocks and limestone clasts suggests that the deposit had erosive capabilities. On the other hand, the orientation of the clasts in the middle zone, the distinct bad sorting and the general texture of the Pelarda Fm. deposit indicate conditions of a non-Newtonian flow.


Striations of cobbles and boulders

In all three zones of the Pelarda Fm., slate, schist and quartzite cobbles and boulders are observed to display distinct striations. The surfaces show one or more sets and are in some cases irregularly striated (Figs. 13, 14, 15). The quartzite cobbles and boulders are more distinctly striated in the lowermost zone. For more than 400 sets, the azimuth of the striae strike has been measured to show a non-random distribution. A rose diagram displays a maximum in the SW – NE direction and a subordinate, more or less perpendicular accumulation (Fig. 16). The SW – NE strike points to the centers of the Azuara and Rubielos de la Cérida impact structures (Ernstson & Claudin, 1990).

pelarda formation ejecta, large striated quartzite blocksFig. 13. Pelarda formation: large striated quartzite boulders. – Science may yield curiosities: When the impact ejecta origin for the Pelarda formation had been suggested and an article (Ernstson & Claudin 1990) was printed showing exactly this photo of the heavily striated quartzite boulders, these heavyweight objects resting near a drop-off had disappeared only shortly after and were never seen again.

pelarda formation ejecta, striated quartzite cobbleFig. 14. Multiple sets of striae on a quartzite cobble from the Pelarda formation ejecta. The field is 2.5 cm wide.

pelarda formation ejecta, striated schist cobbleFig. 15. Irregular striae and imprints on a schist cobble from the Pelarda formation ejecta.

rose diagram for strike directions of striations on cobbles from the Pelarda Fm. ejecta

Fig. 16. Rose diagram for the strike orientation of striae sets on cobbles and boulders from the Pelarda formation ejecta. The prominent SW – NE direction points to the center of the Azuara impact structure.


Strong fracturing and characteristic deformations of cobbles and boulders

In addition to the striae, more features are observed to indicate intense plastic deformations of the cobbles and boulders. We mention sets of irregular, frequently open, fractures with complex bifurcations and rotated displacements (rotated fractures) (Fig. 11, 12, 13). Because of an immediate disintegration to happen, any transport of such deformed clasts can be excluded, which actually proves an in situ deformation.

heavily deformed clast proves in situ damage, Pelarda formation, AzuaraFig. 17. Typical in situ fracturing of a quartzite cobble. A transport after the strong deformation can be excluded. This simple observation is hardly compatible with any hypotheses on conglomerate and alluvial fan deposition. Quaternary tectonics as proposed by some geologists/impact opponents as responsible of these deformations must be consigned to the realms of fantasy.

azuara impact, Pelarda formation, high-pressure/short-term deformationFig. 18. Heavily squeezed but coherent quartzite boulder from the Pelarda formation ejecta proves high-pressure/short-term deformation incompatible with other depositional processes suggested by impact opponents.

azuara impact, Pelarda formation, high-pressure/short-term deformationFig. 19.  Strongly deformed quartzite clast from the Pelarda formation displaying rotated and other complex fractures and  proving high-pressure/short-term deformation.

azuara impact, Pelarda formation, high-pressure squeezing of a quartzite boulderFig. 20. Heavily squeezed quartzite boulder from the Pelarda formation ejecta deposit – incompatible with conglomerate or alluvial fan deposition.

azuara impact, Pelarda formation, high-pressure squeezing of a quartzite boulderFig. 21. Like in Fig. 20: Heavily squeezed quartzite boulder from the Pelarda formation ejecta deposit – incompatible with conglomerate or alluvial fan deposition.

Shock spallation of quartzite boulders

Abundantly, quartzite boulders show a typical fracturing that can be ascribed to shock spallation. Spallation is a well-known process in fracture mechanics as well as in impact cratering and has been investigated theoretically and experimentally by many researchers. Unfortunately, it is less well known that spallation can also be observed in nature as an actually existing geologic phenomenon in and around impact structures. Spallation takes place when a compressive shock pulse impinges on a free surface or boundary of material with reduced impedance (= the product of density and sound velocity) where it is reflected as a rarefaction pulse. The reflected tensile stresses lead to detachment of a spall or series of spalls.Prominent spallation effects have been reported for shocked Buntsandstein conglomerates exposed around the Azuara/Rubielos de la Cérida impact structures. Details about these geologic spallation features have been described in Ernstson, K., Rampino, M.R., and Hiltl, M. (2001): Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts. Geology, 29, 11-14., and can be found here.

spallation fracture of a quartzite boulder, Pelarda Fm., Azuara impact structureFig. 22. Peculiar concave fracture of a quartzite boulder in the field of the Pelarda formation deposit is explained by dynamic shock spallation. Typically and for geometrical reasons, the concave fracture plane mirrors the original convex boulder surface (as shown dashed).

spallation fracture of a quartzite boulder, Pelarda Fm., Azuara impact structureFig. 23. Like in Fig. 22: A concave fracture plane mirrors the originally convex boulder surface – a typical shock spallation effect abundantly observed in the field of the Pelarda formation ejecta.


Shock metamorphism

In thin sections of quartzite cobbles and boulders (both Bámbola quartzite and Armorican quartzite), quartz grains regularly show strong mechanical deformations. We observe distinct fracturing, strong undulatory extinction, deformation lamellae, multiple sets of planar deformation features (PDFs; Figs. 24, 25) and cleavage (multiple sets of planar fractures, PFs). Kink bands in mica are frequent. The PDFs have been analyzed by three independent working groups which unanimously state that the PDFs have originated from shock metamorphism. (For more details, please click here.)

azuara impact, Pelarda formation, planar deformation features, PDFsFig. 24. Planar deformation features (PDFs) in quartz from the Pelarda formation ejecta. Photomicrograph, crossed nicols. The {10-13} and {10-12} crystallographical orientations of the sets suggest shock pressures exceeding 10 GPa (= 100 kbar). The field is 200 µm wide. Photomicrograph E. Guerrero

azuara impact, Pelarda formation, planar deformation features, PDFs under the SEMFig. 25. SEM image of two sets of crossing PDFs in quartz from the Pelarda Fm. ejecta. Note the spacing of the individual PDFs, which is distinctly less than 1 µm in many cases.

From these data, and in view of the location of the deposit and the source rocks of the clasts (in some cases more than 50 km apart), Ernstson & Claudin (1990) conclude that the Pelarda Fm. constitutes the remnants of an ejecta deposit originally extended around the Azuara and Rubielos de la Cérida impact structures. The unusual thickness is explained by the interaction of the material more or less synchronously excavated from both structures. The material incorporated and the stratigraphic position of the layers underlying and overlying the deposit correspond with an Upper Eocene to Oligocene age of the Pelarda Fm.

More about shock effects in the Azuara impact structure and a very bad story in science can be read here.


More distant ejecta of the Pelarda formation

In the course of mapping the environs of the Azuara and Rubielos de la Cérida impact structures we found more deposits exhibiting the very typical facies of the Pelarda formation, and from the general geologic setting and the geologic inventory we concluded they must represent also impact ejecta deposits. A characteristic example is given with the Ermita de San Roque deposit west of the Azuara Structure (Figs. 26, 27).

azuara impact, Pelarda formation, deposit at Ermita de San Roque Fig. 26. Google Earth location map for the Pelarda formation deposit near the Aldehuela de Liestos village at the Ermita de San Roque. The main deposit of the Pelarda Fm. extends between Fonfría and Olalla.

Google Earth, Azuara impact, Pelarda formation, deposit at Ermita de San Roque Fig. 27. The Pelarda formation deposit on top of the hill near Aldehuela de Liestos (Google Earth, oblique view). The deposit is found right in the middle of Mesozoic and Cenozoic sediments; Palezoic is not exposed within a radius of 10 km at least. On cursory inspection, no quartzite blocks of the size of the San Roque deposit have been observed in the plains around the hill.

A short report with quite a few photographs of the deposit and its rocks may be clicked here: San Roque report PDF.


Controversial debate on the origin of the Pelarda formation


Geologists refusing the impact origin consider the Pelarda Fm.

  • a Quaternary mud flow deposit of the “raña” type (Lendínez et al, 1989; Pérez, 1989; Ferreiro et al, 1991; Aurell et al, 1993; Cortés & Martínez, 1999, Cortés et al. 2002).
  • a fluvial conglomerate of uncertain age (Smit, 2000, written communication)

With regard to the observations and material so far presented, the interpretation of the Pelarda Fm. as a “raña” type deposit or a fluvial conglomerate implies basic problems:

  • The almost completely missing stratification (except for a few laminated conglomeratic patches in the middle part), the orientation of the large clasts (> 35 cm size) parallel to the lamination with their more convex part upwards, and the matrix-supported texture suggest a transport by plastic (Binghamian) flow rather than a fluvial transport (see Lowe, 1979; Colombo y Marzo, 1987).The observations also discard models of fluvial meander sedimentation (Miall, 1977 y 1981; Bridge, 1975 y 1978; Allen, 1963, 1964, 1965 y 1970; McGowen y Garner, 1970) and models of braided stream sedimentation (Miall, 1977, 1978; Ramos y Sopeña, 1983;Tunbridge, 1981; Friend, 1978; Castelltort y Marzo, 1986). Sheet flood deposits as described by Friend (1983) don’t occur either.
  • The Pelarda Fm. deposit is located at the highest altitude of the region. Therefore, a fluvial or mud flow deposition requires exceptional Quaternary tectonics explicitly in this region or a sedimentation against gravity. Likewise, the suggested “raña” deposit would have developed at the base of a relief but not at a topographic high.
  • The observed deformations (macroscopic and microscopic (PDFs!)) can’t be explained by “normal tectonics”, and an origin from syn-tectonic sedimentation is actually excluded.
  • The comparison with Ebro delta conglomerates (near Tortosa, UTM coordinates 31295825E; 4515362N) where striae are clearly produced by tectonics, excludes such a process for the Pelarda Fm. Moreover, given a Quaternary age of the Pelarda Fm and with regard to its location at a topographic high, we miss the overlying thick sediments required for the confining pressure to produce the observed striations and plastic deformations. We explicitly also exclude faults (from Quaternary tectonics!) to have caused the deformations, because faults have never been observed in this zone. Also, the striae differ notably, especially by their strike consistency throughout the huge volume of the deposit. A formation of the striae by the action of glaciers can also be basically excluded. Otherwise, we had to establish very special conditions during the Quaternary in exactly this local zone.
  • Moreover and as already mentioned, the principal orientation of the striae points to the centers of both the Azuara and Rubielos de la Cérida impact structure.
  • About 30 % of the clasts display plastic deformations (see Ernstson & Claudin, 1990) in such a manner that even a very short fluvial transport would not have been possible without a complete disintegration of the clasts. Therefore, the field data let us again take up the idea of a transport by non-Newtonian flow (contrasting with a normal fluvial deposition). On the other hand, such plastic deformations have never been reported for debris flows and mud flows (McGowen y Groat, 1971; Rust, 1979; Sáez, 1985; Boothroyd y Nummedal, 1978; Miall, 1981; Heward, 1987; Bluck, 1987; Cabrera, Colombo y Robles, 1985; Gloppen y Steel, 1981).
  • Moreover, it is very difficult to explain the rotated fractures in these clasts by normal tectonic deformation (in comparison with synorogenic molasse deposits – e.g., breccias from Cairat/Montserrat, conglomerates from Sant Llorenç de Munt, conglomerates from Sant Llorenç de Morunys, conglomerates from Ortoneda, all located in Catalonia; conglomerates from the western North-Alpine molasse basin, Switzerland, Germany).
  • In the Pelarda Fm. deposit, a slight vertical sorting is observed to display three zones (or sections): a lower, a middle and an upper zone showing gradual transitions without any distinct interruption (Ernstson y Claudin, 1990). This suggests a transport as a coherent mass in a single episode (perhaps in several phases separated by minutes …).


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