Impact ejecta

Two impact ejecta deposits related with the Spanish impact structures have so far been investigated in more detail: the Pelarda Fm. ejecta (Ernstson and Claudin, 1990, also see ), and the Puerto Mínguez impact ejecta (Claudin et al. 2001, Ernstson et al. 2002). Both may be traced back to an early observation of an “enigmatic deposit” at Puerto Mínguez (Moissenet et al., 1972). This at that time small outcrop of Paleozoic quartzite components was also addressed by Carls and Monninger (1974) by comparing it with the Pelarda Formation. The reported similarities between the “enigmatic” Puerto Mínguez deposit and the Pelarda Fm. initiated a new thorough investigation which related the Pelarda Fm. sediments with the Azuara impact structure (Ernstson and Claudin, 1990). A completely new insight into the deposit at the Puerto Mínguez has been provided by construction work for the new roadway of the CN 211 between Caminreal and Montalbán. From field work in the enlarged outcrops it is suggested that the deposits are related with the Rubielos de la Cérida impact structure. Here, we show a few images of typical exposures and deformed clasts and, over and above that, point to a full description of the Puerto Mínguez ejecta on

There are geologists pretending that the deposits are typical Tertiary fluvial sediments and that the deformations are the result of tectonics. Other geologists (e.g., Casas et al. 2000; Geodinamica Acta, 1-17) regard the deposits as to have resulted from syn-tectonic sedimentation. The reader may form her/his own view on these exceptional geological outcrops.

Puerto Mínguez outcrops

Image002Image004 Fig. 1. Puerto Mínguez impact ejecta. Part of about 5 km continuous deposits exposed by road construction. The rocks shown are an intense mixture (diamictite) of excavated and local material probably due to ballistic erosion and sedimentation (Oberbeck 1975).

Image006Fig. 2. Impact ejecta at the Puerto Mínguez. Intense mixture of prevailing Paleozoic (dark) and few Lower Tertiary (more reddish) material. The light components are Mesozoic limestone cobbles and boulders, probably originating from Lower Tertiary conglomerates. All (!) limestone cobbles and boulders show plastic deformation in the form of intense and deep striations and imprints. Mirror polish and rotated fractures are frequent. In the Paleozoic material, schist components are also striated. Outcrop height about 8 m.

Image008Fig. 3. Impact ejecta at the Puerto Mínguez. Different from the situation shown above, the large limestone components are not rounded but appear to be fragments of a Mesozoic megablock intermixed within the Paleozoic material. It is evident that this layering is not the result of fluvial deposition, as suggested by some geologists. – About 5 m in height.

Image010Fig. 4. Puerto Mínguez impact ejecta: Intense mixture of rounded, subrounded and angular Mesozoic and Paleozoic components in a sandy and marly matrix. The angular Paleozoic quartzite fragments may have made Moissenet et al. to write in their 1972 paper about the “enigmatic” deposit at the Puerto (which means pass height) Mínguez. Paleozoic rocks are not exposed up to a distance of about 15 km!

Macroscopic deformations

(more on

Image012Fig. 5. Puerto Mínguez impact ejecta: limestone component displaying extremely fine striations and mirror polish. Similar high-pressure striae and clay polish have been described by E.C.T. Chao (Science, 194, 615-618, 1976) for components of the Ries crater ejecta (Bunte breccia). Also note the distinct groove (arrow) formed before the polish.

Image014Fig. 6. Puerto Mínguez impact ejecta: Limestone component showing strong internal torsion. The distinct torsion relates to macroscopically untouched hinges (Fig. 7: rear side of the cobble) and rotated fractures cutting through the whole cobble without breaking it to pieces. These rotated fractures are very abundant in components of the Puerto Mínguez ejecta and are also observed in components of the Pelarda Fm. ejecta (see Ernstson & Claudín 1990). Very similar rotated fractures have been reported by E.C.T. Chao (Geol. Jb., A 43, 1977) for the Ries crater ejecta (Bunte breccia) where correspondingly deformed limestone concretions occur within soft Jurassic claystones. The torsionally fractured components of the Pelarda Fm. and Puerto Mínguez ejecta also occur in a soft matrix (more sandy in the case of the Pelarda Fm.). Therefore, rotated fractures are considered diagnostic of short-term impact deformation under high confining pressure. – Sample put at our disposal by Alvarez Marques Miguel Desiderio.

Image016Fig. 7. Reverse of sample in Fig. 6.

Image018Fig. 8. Puerto Mínguez impact ejecta: typical clast deformation, at first sight like a bread cut to slices. However, the clast is not broken to pieces at all. Very similar deformations are well known from the Ries crater impact ejecta to occur in carbonate concretions and in the famous so-called Ries belemnites both embedded within soft Jurassic claystones (also see

Image020Fig. 9. Limestone clast from the Puerto Mínguez impact ejecta showing regmaglyptic features otherwise known from the ablation of meteorites. It is suggested that the regmaglypts formed by partial melting of the limestone in the ejection process, when the expanding vapor plume passed through the ejecta curtain. The regmaglypts should of course not be confused with lapiés (karren) features. Similar thumbprints have been reported for clasts in the Belize ejecta from the Chicxulub impact structure (K.O. Pope & A.C. Ocampo, LPSC 31, abstract #1419: The complete story about the Puerto Mínguez regmaglypts can be read here: Ernstson, K. (2004): Regmaglypts on Clasts from Impact Ejecta. – Meteorite, 10/1, 41-42, and, in great detail, here:

Image022Figs. 10, 11. Miniature impact craters on limestone clasts from the Puerto Mínguez ejecta (the fields are 3.5 mm and 1.8 mm [Fig. 11] wide). Note the uplifted rims of the craters and the preserved projectile in the left-hand crater. It is assumed that the craters were formed by high-speed impacts of fine sand particles during the ejection process. Also see Glidden, M. et al. (2004): DISTAL IMPACT EJECTA, UPPERMOST EOCENE, TEXAS COASTAL PLAIN. LPSC 2004, abstract:

Image024Fig. 11. Another miniature impact crater on a limestone clast, Puerto Mínguez ejecta.

Ejecta between Mesquita de Jarque and Escucha

Image026…. Image028Fig. 12. Ejecta tunneling well-bedded Cretaceous sediments. Impact ejecta emplacement can be strongly erosive as shown in the image.

Ejecta near Blancas

Image030…. Image032Fig. 13. Patches of diamictic ejecta are exposed in the environs of Blancas. In the outcrop shown here, all (!) limestone pebbles, cobbles and boulders are striated all around frequently exhibiting penetration marks and polish. The similarity to the Puerto Mínguez ejecta is obvious.

Image034Fig. 14. Imprints and striations on a pebble from the ejecta deposit near Blancas.