ERNSTSON CLAUDIN ESTRUCTURAS DE IMPACTO – CRÁTERES METEORÍTICOS

Breccia dikes (dike breccias) are a prominent feature in impact structures, and they have told us a good many about the processes of impact cratering. Among the yet known roughly 200 terrestrial impact structures, Azuara and Rubielos de la Cérida are providing the probably by far best insight into these fascinating geological configurations concerning abundance, exposure accessibility, and diversity with regard to geometry, dimensions and material (also see breccia/dikes; http://www.impact-structures.com/spain; rubielos-breccia-dikes). Recent extensive construction works (storage reservoir, freeway, railroad) near Lechago, Calamocha (CAL; see Fig. 1, red circle) have yielded quite a few nice geological exposures once more documenting the almost everywhere existing impact signature thus underlining that construction work in the large impact region of the Azuara and Rubielos de la Cérida structures may be a hard undertaking – as was impressively shown when the new Autovía freeway line had to cut through the strongly destroyed Paleozoic of the Iberian Chain in the northern rim zone of the Azuara impact structure between Daroca and Cariñena (click here http://www.impact-structures.com/impact-spain/the-azuara-impact-structure/the-2005-autovia-mudejar-geological-exposures/ «The 2005 Autovía Mudéjar geological exposures»).

Below we show images of some newly exposed impact breccia dikes near Lechago exhibiting their characteristic properties and, for comparison, similar dikes and dike systems from the companion Azuara impact structure (location in Fig. 1).

Focusing here on breccia dikes in Paleozoic silicate rocks bears in mind the frequent claim of some Spanish geologists, especially from the Zaragoza university, that the impact breccia dikes are all karst phenomena. Their confusion of breccia dikes with karst features, however, meets some difficulties in the case of silicate siltstones. Also fault breccias can clearly be excluded since these dikes are filled with allochthonous material as is typical for impact-induced highly energetic injection processes.

Fig. 1. Location map.

Fig. 2. Thick impact breccia dike cutting through Paleozoic siltstones at the new storage reservoir near Lechago (in the red circle of Fig. 1)

Fig. 3. For comparison: impact breccia dike in Paleozoic siltstones; road to Santuario de la Virgen de Herrera (1 in Fig. 1). Azuara impact structure.

Fig. 4. For comparison: thick impact breccia dikes cutting roughly perpendicular through the bedding of the Paleozoic siltstones. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 1). Azuara impact structure.

Fig. 5. Large system of impact breccia dikes in Paleozoic siltstones at the new storage reservoir near Lechago. Note that the dominant set of dikes is cutting sharply across the bedding (see Fig. 6). In the top: Unfolded post-impact Tertiary sediments.

Fig. 6. Segment of the wall in Fig. 5.

Fig. 7. For comparison: System of impact breccia dikes cutting sharply through Paleozoic silicate rocks. Autovía Mudéjar in course of construction; near Cariñena (2 in Fig. 2). Azuara impact structure.

Fig. 1. Amazingly similar: Regmaglypts on the surface of the Tabor meteorite and on a limestone clast from the Puerto Mínguez impact ejecta.

Among the various deformation features exhibited by the carbonate clasts in the Puerto Mínguez ejecta deposit (striae, nail prints, grooves, rotated fractures, irregular fractures with complex bifurcations, mirror polish, etc), regmaglypted clast surfaces are a prominent feature (Fig. 1). For the first time described by K. Ernstson (2004), they establish clear evidences of an aerial transport of quite a few clasts of the ejecta.

 

Source: Cascadia Meteorite Laboratory, Portland State University

Fig. 2. A Gibeon meteorite exhibiting distinct regmaglypts.

Regmaglypts (or thumbprints) are reliefs commonly reported for the surface of some meteorites (Figs. 1, 2). The depressions originate from dynamic air pressure [continue …] and from selective erosion by material melting (ablation) off the surface of the meteorite on its passage through the atmosphere. The relief may show polygonal, spherical, rounded or elliptical shape, and a pattern like fingers over wet clay is frequent.

We suggest the ablation features observed in the ejecta have originated from a similar process that is carbonate melting and ablation off the surface on the passage of the ejecta clasts through the heated impact explosion cloud.

Of course, the depressions on the Puerto Mínguez clasts in a way remind of lapiés (karren) features on surfaces of exposed limestones. Lapiés consist of shallow, straight grooves or runnels incised into the limestone by solution. Because of the striae and polish regularly coating the regmaglypted surfaces, and because the regmaglyptic clasts are embedded as individuals in the ejecta matrix (Fig. 3), an in situ formation as lapiés can clearly be excluded.

Fig. 3. The regmaglypted individual embedded in the Puerto Mínguez impact ejecta is incompatible with an in situ dissolution («lapiés» formation).

Alternatively, the «lapiés» depressions existed already before the impact and survived rock fracturing, excavation, and emplacement of the ejecta. This can be excluded because many clasts are regmaglypted all around (Fig. 4), and the frequently observed sharp-edged ridges of the regmaglypts (Fig. 4) would not have survived the excavation and ejection process.

Fig. 4. Front and rear of a regmaglypted limestone clast from the Puerto Mínguez ejecta. The regmaglypts all around and the sharp ridges exclude a «lapiés» formation before excavation and ejection.

In a few cases, the ablation by obvious carbonate melting of the clasts has eaten deeply into the clast (Fig. 5) reminding of similar distinct ablation features of some meteorites (Fig. 6).

Fig. 5. A regmaglypted limestone clast from the Puerto Mínguez ejecta exhibits prominent ablation reaching deeply inside into the limestone clast. For comparison see the meteoritic ablation features in Fig. 6.

Fig. 6. The Derrick Peak, Antarctica, meteorite. Image courtesy of NASA.

An extended article on the Puerto Mínguez regmaglypted ejecta including many images can be read here.