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

The Pelarda Fm. covers an area of approximately 12 x 2.5 km², 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 (Carls & Monninger, 1974). The Pelarda Fm. unconformably overlies the 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). In the environments of Fonfría, the limestone cobbles of these conglomerates are heavily deformed and show intense striations, deep imprints and polish (Fig. 1). The striations display more or less homogeneous SW – NE strike.

Fig. 1. 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.

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 color (Fig. 2). 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.

Fig. 2. Typical aspect of the lower unit of the Pelarda Fm.

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 photo 3, a Buntsandstein sandstone megaclast of about 9 m size is shown to be intercalated in the deposit.

Fig. 3. Buntsandstein megablock in the middle unit of the Pelarda Fm.

Fig. 4. Close-up; photo-cap diameter 51 mm. -

 Fig. 5. Aspect of the middle unit of the Pelarda Fm. showing matrix-supported texture.

In all outcrops of this zone so far studied the texture is matrix-supported (Fig 4). 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. 6. Middle unit; the reddish strings result from intermixing of Buntsandstein material.

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. 6).

Fig. 7. Quartzite megaclast in the upper zone of the Pelarda Fm.

In all three zones referred to, a stratification is completely lacking except for few conglomeratic intercalations in the middle zone which are more or less laminated but do not display tabular bodies (neither small nor large) nor do they form channeling structures. In these intercalations, the majority of clasts larger than 35 cm are oriented parallel to the lamination with their more convex part upwards (see, e.g., the clast in Fig. 6). These clasts enabled the measurement of dip which is either SW or NE.

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 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.

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. 8, 9, 10). 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. The SW – NE strike points to the centers of the Azuara and Rubielos de la Cérida impact structures (Ernstson & Claudin, 1990).

Fig. 8. Pelarda Fm.: large striated quartzite boulders.

Fig. 9. Multiple sets of striae on a quartzite cobble. The field is 2.5 cm wide.

Fig. 10. Irregular striae and imprints.

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.

Figs. 11 – 13. Strongly deformed quartzite clasts displaying rotated and other complex fractures.

Fig. 12.

Fig. 13.

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. 14, 15) 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.)

Fig. 14. 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

Fig. 15. 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.

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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