Research on geology, geophysics, and petrology of impact structures (meteorite impact craters)
A FIELD GUIDE TO THE AZUARA IMPACT STRUCTURE ] Route ] STOP 1 ] STOP 2 ] [ STOP 3 ] STOP 4 ] STOP 5 ] STOP 6 ] STOP 7 ] Stages of Crater ] References ]


STOP 3: Pelarda Fm. ejecta (near Fonfría)


The Pelarda Formation covers an area of roughly 12 x 2.5 km2. It has a thickness of in part more than 200 m and is located on top of a mountain chain which belongs to the highest ones in that region (Carls & Monninger 1974). The deposits unconformably overlie the Lower Tertiary, which we will first study near Fonfría. The Lower Tertiary is composed of conglomerates alternate with multicolored marls. The limestone cobbles are strongly deformed displaying striae, imprints and polish. The striae strike is more or less homogeneously SW - NE.

On the whole, the Pelarda Formation shows a stratification into three parts (Ernstson & Claudin 1990). The contacts between the lower, middle, and upper zones are gradual and not very clear. The lower zone contains clasts of Paleozoic pelitic rocks (slates and schists) and quartzites. The pelitic rocks are more abundant than the quartzites. The clasts are angular-shaped or subrounded, smaller than the clasts of the middle and upper zones, and embedded in a brownish muddy matrix.

The middle zone contains pelitic, quartzite, quite a few buntsandstein, and sporadically limestone (Jurassic?) clasts. The ratio quartzite/pelitic clasts is increased compared with that of the lower zone. The clasts are more rounded and heterometric. In this zone (where we will stop again), Buntsandstein megaclasts as large as 9 m in length are intercalated. Exposures show a matrix-supported texture. The matrix itself is predominantly sandy and in some parts only muddy. Frequently, Lower Tertiary multicoloured marls and red Buntsandstein sandstones are intermixed into the matrix. In outcrops, the red sandstones may give a characteristic colour to the deposit.

The upper zone shows a matrix-supported texture as well, with rounded and heterometric clasts of dominantly quartzites and pelitic rocks in a silty matrix. The ratio of pelitic rocks to quartzites is low. On the whole, the quartzite boulders are bigger than the slate, schist, and Buntsandstein clasts. Quartzite clasts up to the size of 2 m can be seen in the field. Frequently, oriented components can be observed with a convex surface in the base and a flat surface in the upper part.

Locally developed and only small-sized bedding planes including oriented slab-shaped clasts allowed the measurement of strike and dip. A predominantly southwest and northeast dip is observed.

In all three zones of the Pelarda Formation, pebbles, cobbles and boulders with pronounced striae are observed. The surfaces show one or more sets and, in some cases, are irregular striated. Slates, schists, and quartzites are affected in like manner with the exception that the striated quartzites are obviously restricted to the lower zone. Measurements of the striae azimuth of more than 400 individual sets show that the striae directions are not randomly distributed within the Pelarda Fm. The rose diagram displays a broad but pronounced maximum for the southwest – northeast direction and a subordinate, roughly orthogonal accumulation (Ernstson & Claudin 1990).

More evidence of mechanically loaded stress is given by the presence of strongly fractured components (Fig.5). In general, a net of irregular fractures with complex bifurcations can be observed. A relation to planes of weakness such as bedding or foliation is missing. Simple shear or tensile stress can be excluded. Instead, a small-scale inhomogeneous stress field changing with place and/or time must be considered to have produced the conspicuous fracture pattern. A special feature are open fractures with rotated displacements (rotated fractures) which point to plastic deformation (also see STOP 6).

In thin sections, strong mechanical deformations are observed to occur even in mineral grains from Pelarda Fm. clasts (Ernstson & Claudin 1990). Apart from heavy fracturing, strong undulatory extinction, and deformation lamellae, quartz grains show PDFs and PFs (cleavage), and in biotites, kink banding can be observed. In a recent study, PDFs in quartz from the Pelarda Fm. have been investigated by Ann Thierault from the Canadian Geological survey. In the sections PDFK 5BB and PDFK 1A, she observed in 12 quartz grains a total of 24 sets of decorated PDFs with a maximum of 3 sets per grain. For 20 sets, the crystallographical orientation could be measured with a distinct 65 % accumulation at w {10-13}. As with the Santa Cruz de Nogueras PDFs (see STOP 2), the density of the PDFs is throughout high, their relative length in general 100 %, and the spacing less than 1 µm. According to Stöffler & Langenhorst (1994), the investigated non-porous quartzite samples experienced the shock stage 3 (moderately shocked).

Interpretation and relations

The Pelarda Fm. is interpreted as an impact ejecta continuous deposit, which formed by the mixing of materials originating from both the Azuara and Rubielos de la Cérida impact structures, according to a model of ballistic sedimentation (e.g., Oberbeck 1975). It is regarded as the remnant of an originally continuous ejecta blanket and has its exceptional thickness because of an accumulation exactly between both structures.

This impact-related interpretation is substantiated by the depositional characteristics of the rocks, the deformation mesoscopic features, the presence of Buntsandstein and other megablocks (see Fig.6) and Lower Tertiary marls intimately admixed with the clasts, and by the occurrence of shock metamorphism in the pebbles and cobbles from the deposit. This excludes the fluvial-deposit hypothesis (Carls & Monninger, 1974), a Quaternary deposition (ITGE 1990, Aurell 1994), and any kind of syn-tectonic sedimentary model for this Formation (e.g., Casas et al. 2000).

The presence of striated and plastically deformed components, found more or less homogeneously distributed throughout the large Pelarda Fm. volume, and the analysis of them allow us to say:

The Pelarda Fm. is a sediment which was transported from south-west or/and north-east and deposited under high confining pressure.

The plastically strongly deformed components have experienced their deformations in the excavation, ejection and deposition stage, probably especially upon landing by the effect of strong confining pressure. This confining pressure allowed the components to survive and not to break to pieces.

Most of the striae on the surfaces of the pebbles, cobbles and boulders must have been formed in the final stage of ejection and deposition. Otherwise one has to anticipate the striae directions being more randomly distributed throughout the large Pelarda Formation volume.

The origin of the striae is related with the action of the matrix components against the cobble and boulder surfaces under confining pressure. Based on surface features of the quartz grains seen by SEM, the matrix seems to have been hot during the emplacement (as is expected for impact ejecta (Melosh 1989).

Fig.5. Heavily deformed quartzite cobble.

Fig.6. Large quartzite block in the Pelarda Fm. ejecta. 

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last modified at: 2002-06-18