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The impact of cosmic bodies to produce large impact craters is related with the propagation of seismic waves which may release energies comparable to strongest earthquakes, and even beyond that. It is obvious that similar processes and well-known drastic deformations may result in the ground and at the earth’s surface.

In this context, W. Alvarez and coauthors (Alvarez W., Staley E., O’Connor D., Chan M.A., Synsedimentary deformation in the Jurassic of south-eastern Utah, a case of impact shaking? Geology, 1998, 26, 579-582) went into an interesting question when they related characteristic deformations in exposed older geological layers with a possible meteorite impact in a past geologic epoch thus pointing to the Upheaval Dome impact structure (Utah, USA) and distant rock liquefaction phenomena. Rock liquefaction is an established process during strong earthquakes which can lead to enormous modifications of the earth’s surface and catastrophic damage when the seismic shock affects water-saturated uncemented rocks. For the study of earlier earthquakes in the geologic past (paleoseismicity), observations in older layers may be important, and Alvarez had now pointed to the possibility that fossil sedimentary liquefaction features need not necessarily have originated from earthquakes but may be related with former large impact events. A critical debate, however, submitted that in the special case such a context of the relatively small 6 km-diameter Upheaval Dome impact structure and the geological outcrop some 260 km apart must be questioned.

Now for the first time, a compelling direct evidence for the relation of a large meteorite impact with distinct rock liquefaction features has been established. The recently printed article

Ernstson, K., Mayer W., Neumair, A., and Sudhaus, D. (2011): The sinkhole enigma in the alpine foreland, Southeast Germany: Evidence of impact-induced rock liquefaction processes. – Cent. Eur. J. Geosci., 3(4), 385-397.  DOI: 10.2478/s13533-011-0038-y 

describes the first geologic and geophysical investigations of the so-called “Thunderhole” (in German: Donnerloch) phenomenon in the region of the town of Kienberg north of Lake Chiemsee in Southeast Bavaria (Germany).

Fig. 1. A freshly caved-in and a somewhat older thunderhole near the town of Kienberg north of Lake Chiemsee. Strikingly different from customary and well known sink holes e.g. in karst areas, the thunderholes exhibit a highly energetic rock mass transport from the bottom up before collapse, which is typical for liquefaction processes. 

The authors conclude that the innumerable enigmatic sudden sinkhole cave-ins having happened in living memory originate from late and even today acting processes of an earlier shock-induced underground rock liquefaction known from strong earthquake shocks. The geologically prominent underground structures that have now been uncovered are considered the result of impact shocks in the course of the formation of the Chiemgau meteorite crater strewn field (Chiemgau impact), and a comparison is made with the famous widespread and strong rock liquefaction features in the large region affected by the catastrophic 1811/1812 New Madrid (Missouri) earthquake series.

More detailed information about the geophysical measurements (resistivity and induced polarization (IP) soundings) of the thunderhole structures was given in a presentation at the 2011 Fall Meeting of the American Geophysical Union (AGU) in San Francisco:

Kord Ernstson & Andreas Neumair: Geoelectric Complex Resistivity Measurements of Soil Liquefaction Features in Quaternary Sediments of the Alpine Foreland, Germany. – AGU Fall  Meeting, December 5-9, 2011, NS23A-1555.

The poster may be clicked here.

Fig. 2. From the geophysical investigation of the impact-induced thunderholes in the Chiemgau impact event. The geoelectrical measurements of induced polarization on a profile across an active depression as a precursor of a future thunderhole reveal perfectly clear the intrusion paths bottom up as a document of impact liquefaction.

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; https://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 https://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.

Impact-affected limestone clasts with regmaglypts have been reported also from the Chiemgau impact. An article may be clicked.

About 30 km north of the center of the Azuara impact structure (Spain) near the village of Jaulín (0°59.3′ W; 41°27.2′ N), a peculiar breccia is exposed. The breccia, not mapped geologically thus far, is intercalated between fossil-rich Jurassic limestones and brownish Miocene(?) gypsum marls. The breccia is unconformably overlying the Mesozoic rocks (Fig. 1) and may penetrate the  limestones (breccia dikes, Fig. 2) as well as corrode them (Fig. 3).

Fig.1. Unconformable contact between the Jaulín breccia and the Jurassic limestones.

Fig. 2. Penetration of the breccia into the Jurassic host rock in the form of a dike.

Fig.3. Corrosion features in the contact zone between breccia and host rock. Neither aeolian nor karstification processes are responsible of the corrosion, which probably is related with decarbonization.

On cursory inspection, the greenish rock looks like a massive bone breccia (Fig. 4). On closer examination, the “bones” prove to be limestone clasts having become hollow or more or less completely skeletal (Figs.5, 6).

Fig. 4. At first glance, the breccia reminds of a massive bone breccia.

Fig. 5. Close-up of the carbonate breccia clasts.

Fig. 6. Close-up of hollow and skeletal clasts embedded in the matrix. Note that the decomposition with possible relics of carbonate melt is confined to the interior of the cobbles reminding of the construction of a wasp’s nest.

Beginning (Fig. 7) and complete fragmentation of the clasts is observed. Beside these decomposed clasts, fragments of the limestone host rock are intermixed in the breccia (Fig. 8). They frequently show distinct whitish rims (Fig. 9) which we interpret as the result of beginning decarbonization from enhanced temperatures. Occasionally, the fragmented clasts remain coherent giving evidence of confining pressure upon emplacement (Fig. 10). For the present, it is not clear whether the hollow and skeletal clasts originate also from the local limestones, but from the discussion below we have to assume they are allochthonous.

Fig. 7. Breccia sample exhibiting a decomposed, fragmented clast and distinct flow texture of the matrix.

Fig. 8. Evidence for the erosive strength of the breccia emplacement process: fragments of the limestone host rock have been carried away and embedded in the greenish matrix. Note the beginning “conglomeratization” (increasing roundness) of the clasts.

Fig. 9. Whitish rims of limestone fragments in contact with the breccia matrix: evidence of beginning decarbonization due to enhanced temperatures.

Fig. 10. Fragmented but coherent limestone clast as indicative of confining pressure during the emplacement.

The clasts are immersed in a greenish matrix (Fig. 8) partly exhibiting flow texture as indicated by lined-up small elongated clasts (Fig. 7).  In thin section (Fig. 11),  the matrix shows to be fine-grained carbonate streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Quite a few quartz grains exhibit planar deformation features (PDFs) as in proof of shock metamorphism (Fig. 12).

Fig. 11. Breccia matrix composed of dark carbonate material streaked with irregular bands of poorly rounded quartz grains in a slightly different matrix. Photomicrograph, plane light; the field is 10 mm wide.

Fig. 12. Several sets of decorated planar deformation features (PDFs) in a quartz grain from the breccia matrix. Photomicrograph, crossed polarizers; the field is 220 µm wide.

Breccia formation. – The contacts between breccia and underlying autochthonous rocks as well as the peculiar characteristics of the clasts absolutely exclude a karstification process. A “normal” sedimentation and any diagenetic processes are not consistent with the observations either. From the stratigraphic position at the base of the unfolded Upper Tertiary and from the evidence of enhanced temperatures and of shock metamorphism we conclude the breccia to be an impactite related with the formation of the Azuara structure in the giant multiple impact event that, beside the Azuara structure, formed also the large Rubielos de la Cérida impact basin and crater chain (see https://www.impact-structures.com/spain/). We explain the breccia to be impact ejecta excavated from a region in the Azuara structure where shock intensities were enough to decarbonize and melt limestone cobbles and boulders and to produce PDFs in quartz grains contributing to the breccia matrix.

Fig. 13. Breccia sample impressively exhibiting internally corroded limestone clasts. Also note the finely splintered components.

Hollow and skeletal limestone boulders, cobbles and pebbles are not uncommon in the impact region, and they have been found in e.g. the basal suevite breccia (see suevite)  and the impactite from Almonacid de la Cuba (see peculiarities). The process of the decomposition obviously confined to the interior of the clasts (see especially Fig. 13) is not completely understood thus far, but we suggest two possibilities that must not exclude one another. The clasts belonged to Lower Tertiary conglomerates in the target, and they experienced

• a shock concentration in the cobbles’ interior by shock-wave reverberation and focusing effects

and/or

• a shock heating of the cobbles and a rapid external cooling upon ejection, only allowing the interior to be decarbonized and melted.

On excavation and ejection, the shocked limestone clasts were mixed with the greenish matrix material possibly originating also from the Lower Tertiary. The emplacement was a process of ballistic erosion and sedimentation (Oberbeck 1975) under high pressure (penetration into the Jurassic host rock that was partially fragmented and incorporated into the breccia) and still enhanced temperatures (partial decarbonization of the host rock).

With regard to its stratigraphical position, the Jaulín impactite must be seen as a special variety of the Azuara/Rubielos de la Cérida basal breccia, and, due to the observed shock effects and the relics of probable carbonate melt,  as a special type of a suevite breccia (see IUGS classification, the_suevite_page).

Outcrop of Muschelkalk dolomite crosscut by a dark vein of impact melt rock in the vicinity of Monforte de Moyuela.

Black vein under the microscope: light matrix of carbonate minerals (Cc), black particles and gas vesicles (gv). Long side of the figure is about 1 mm.

The full article can be read here:

http://www.uni-wuerzburg.de/mineralogie/schuessler/Monforte-vein.pdf

key words: Azuara impact structure, Rubielos de la Cérida impact structure, carbonate melt, breccia dike, manganese, quench crystallization.

Accretionary lapilli is a term originally solely related with volcanism. Accretionary lapilli are pellets that form by the accretion of fine ash around condensing water droplets or solid particles, particularly in steam-rich eruptive columns. Commonly, they exhibit a concentric internal structure, and, once formed, they can be transported and deposited by pyroclastic fall, surge, or flow processes (Allaby & Allaby, 1999; A Dictionary of Earth Sciences). Armoured lapilli is the term that is especially used in the case the ash has accumulated around a small rock fragment. The armoured-lapilli variety is frequently found in deposits of basaltic base surges.

Image 1: Accretionary lapillus (diameter 0.5 mm) from the basal
suevite breccia in the Azuara impact structure (Mayer 1990).
Photomicrograph, xx nicols.

Since similar processes are related with the turbulent explosion plume raising above the expanding excavation cavity in an impact cratering event, it is not surprising that accretionary lapilli have been found also in impact deposits. Graup (1981) describes accretionary lapilli to occur in the suevite fall-back breccia of the Ries impact structure. They are also reported for ejecta deposits of the K/T Chicxulub impact structure in Mexico (http://www.lpi.usra.edu/meetings/metsoc2000/pdf/5124.pdf,
http://www.lpi.usra.edu/meetings/largeimpacts2003/pdf/4113.pdf) and Belize (http://www.icdp-online.de/news/workshops/abstracts/EGS03/EAE03-J-06925.pdf).

Concentrations of lapilli formed lapillistone that occurs as discontinuous, reworked clasts within the megabreccia related with the Late Devonian Alamo impact (http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_42158.htm).

For the Azuara impact structure, accretionary lapilli was first reported by Mayer (1990). Image 1 shows a typical lapillus from the matrix of the basal suevite breccia near Muniesa. The interior is composed of very badly sorted material (calcite, quartz, ore). The rim zone is formed by concentric layers of finer material (mostly micritic calcite). Similar accretionary lapilli are observed also in the matrix of Azuara breccia dikes.

Accretionary lapilli also contribute to the matrix material of the basal suevite breccia and breccia dikes in the Rubielos de la Cérida elongated impact basin (as part of the Azuara/Rubielos de la Cérida impact crater chain, see https://www.impact-structures.com/2011/12/an-impact-crater-chain-in-northern-spain/ ).

 2 3

 4 5

Images 2 – 5: Accretionary lapilli from the basal suevite breccia in the Rubielos de la Cérida impact basin. Photomicrographs, xx nicols; the fields are 3.5, 5, 6.5 and 3 mm wide.

Images 2 – 5 show accretionary lapilli from the basal suevite breccia near Corbatón in the Rubielos de la Cérida impact basin. The lapilli are basically carbonate with some accessory silicate material (e.g., quartz fragments, Image 3) and regularly exhibit the typical “onion skin” internal structure. The angular core of the lapillus in Image 5 probably is a fragment of a former lapillus thus reflecting some kind of reworking in the lapilli formation and deposition process.

Near the village of Olalla in the northern rim zone of the Rubielos de la Cérida impact basin, a prominent breccia dike is exposed (Images 6, 7). In the early seventies, in the course of his geologic mapping activities, W. Monninger called this dike “Teufelsmauer” (Devils Wall) because of its peculiar structure and composition. At that time, impact processes and impact rocks were not well known among geologists.

 6 7

Images 6, 7: The Devils Wall breccia dike near Olalla.

 8 9

Images 8, 9: The matrix of the Devils Wall breccia dike composed of accretionary lapilli. The whitish crusts of some breccia fragments are suggested to result from high-temperature decarbonization of limestone. Polished sections, the fields are 13 and 15 mm wide.

In polished sections (Images 8, 9), the matrix of the carbonate dike breccia for the most part proves to be completely composed of accretionary lapilli (lapillistone). Limestone fragments from the disintegrated host rock of the dike abundantly make up the core of larger lapilli. In doing so, the fragments my be broken but still remain coherent within the lapillus, as can be seen in Image 10. This shows that the accretion process continued upon injection of the vapor plume material into the host rock.

Image 10: Fragments of the breccia dike host rock make up the core of larger accretionary lapilli.

Regional geologists from Spain (M. Aurell, E. Díaz-Martínez, A. L. Cortés, and others) vehemently refusing the impact origin of Azuara and Rubielos de la Cérida, suggest the basal suevite breccia and breccia dikes to be diagenetic, pedogenetic or karstification features. For these geologists, the accretionary lapilli is calcrete (caliche).