Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
STOP 7: Megabreccia and impact melt rocks in the Rubielos de la Cérida structure (near Barrachina)
The outcrops to be visited are located in the valley of the Pancrudo brook east of the town of Calamocha, half-way betweeen the central uplift and the northern rim of the Rubielos de la Cérida structure. The rocks of interest are exposed along the roadside in the valley for several kilometers. A further outcrop is located at the steep banks of the Pancrudo brook. In addition, temporary quarries for gravel exploitation exist.
The outcrops in general show a megabreccia deposited in contact with bedded sediments, which are, however, strongly folded and faulted. The term "megabreccia" refers to both the size of the components and the thickness which may reach to 50 m. Within the megabreccia, which is different from the megabreccia in the Azuara structure (STOP 5), two kinds of components may basically be distinguished for the present:
A massive conglomeratic/brecciated material, which is not consolidated and has characteristics very similar to those described for the Pelarda Fm. ejecta (Ernstson & Claudín 1990; see STOP 3) and for the ejecta deposits of the Puerto Mínguez (see STOP 6). This conglomeratic material incorporates, intermixes, and stratifies with multicoloured gypsum marls, claystones, limestones, and pure gypsum.
Strongly brecciated limestones showing mortar texture and a megatexture which resembles of the gries described for part of the Ries crater ejecta (Hüttner, 1969).
The conglomeratic/brecciated material consists of heterometric components up to the size of 100 cm, which have a subrounded to subangular morphology. They are supported by a sandy to micro-conglomeratic matrix. The lithology of the components corresponds to quartzites (e.g., Bámbola qu., Armorican qu.) (60 - 70 %), limestones (25 - 30 %), and schists (< 5%). Lithologically, the matrix corresponds to a lithic to sub-lithic arenite of subrounded to rounded grains with a size between micro-conglomeratic and that of coarse sand.
Limestone and schist components regularly show intense striations as is observed and has in detail been described for the Pelarda Fm. ejecta and the ejecta deposits of the Puerto Mínguez. Like in these deposits, we observe strongly plastically deformed components which point to high confining pressures upon deformation (Ernstson & Claudín 1990).
Not rarely, black veins pass through the conglomeratic/brecciated unit. They are few centimeters wide, and the orientation is from vertical to sub-horizontal. Close inspection shows that within the trajectory of the veins all clasts are coated with a black film of so far unknown composition.
Together with a missing distinct stratification, the thickness of the conglomeratic/brecciated material undergoes rapid lateral changes and oscillates between about few meters and 50 m. Presently, this material is quarried for road-construction purposes.
The multicolored deposits of marls, marly limestones, gypsum marls and gypsum show a complex interlayering with the conglomeratic/brecciated material. The deposits may overlay the conglomerates, thereby tunneling them and forming apophyses within them. Frequently, bodies of the multicolored material are even floating within the conglomeratic material. In other places, the marly material may underly the conglomeratic/brecciated unit. In this case, we may observe a fluidal megatexture in the conglomeratic deposits obviously corresponding to a fluidal megatexture also in the underlying marly unit. In this zone, impact melt rocks may be embedded.
The heavily brecciated limestones are best exposed at the roadside to Barrachina. We observe large limestone blocks (occasionally exceeding the size of 2 m) embedded in a "matrix" of small monogenetic limestone clasts not exceeding the size of 3 cm. A "ghost" stratification may be preserved. The clasts of the "matrix" in general show a distinct fitting suggesting in situ fragmentation without displacements. At their base, the large limestone blocks may deform the "gost" layering. On formation of the exposure by road construction, many limestone blocks showed distinct mirror polish of their surfaces now removed.
Fig.12. Silicate melt (left - the whitish ribbon; see hammer for scale) and Ca-phosphate melt (right, coin diameter 23 mm) embedded in the megabreccia.
Impact melt rocks
Silicate melt. - The silicate melt rocks in the Barrachina outcrops in general occur within the megabreccia as sharply defined whitish-yellowish bodies (Fig.12), frequently showing a phacoid structure. (Unfortunately, several melt bodies have yet been removed by quarrying in temporary outcrops.) The maximum size is of the order of few meters. In most cases, the melt rocks can be observed near the contact between the conglomeratic/brecciated component (Pelarda Fm. facies) and the clayey component (claystones, gypsum marls), embedded in the latter.
The samples so far studied petrographically and geochemically, contain more than 90 % silicate glass, consisting of a globular whitish phase and an interstitial dark phase (Fig.13), and only few mineral grains. From microprobe analysis, both phases are rather similar and match the whole-rock RFA data of SiO2 around 58 wt.%, variable Al2O3 up to 21%, MgO 5-6 %, CaO 1.5 %, variable Na2O+K2O around 2 %, LOI around 10 % (U.Schüssler, K.Hradil). This roughly fits the average composition of shales, which are assumed to be the pre-impact educt rocks.
Fig.13 (from the left to the right). Silicate melt (field is 14 mm wide). Calcite bodies (former carbonate melt) in glassy matrix of former Ca-phosphate melt (field is 24 mm wide). Ca-phosphate melt (field is 9mm wide). Amorphous carbon of bizarre shape (length is 2 mm).
Carbonate/phosphate melt (Fig.13). - A special kind of former melt consists of amoeba-like carbonate particles embedded within a glassy matrix. The carbonate bodies are coarse-grained in their centres, with decreasing grain size and perpendicular orientation towards the rims. The isotropic matrix is pervaded by tiny microcrystals. From microprobe investigations, the carbonate is pure calcite. The glassy matrix consists of nearly pure Ca-phosphate which locally may contain some additional Si. In part, this glass is recrystallized to form apatite, as veryfied by x-ray diffraction analysis. The diffraction peaks of this apatite, however, are much wider compared to those of a well crystallized one, indicating its very unperfect crystal structure (see Hradil et al. 2001). A similar melt rock has been reported for the suevite of the Ries crater (Graup 1999), where the identical carbonate particles are interpreted as quench products of a carbonate melt. Different from the Barrachina melt rocks, the matrix in the Ries samples is silicate glass as a result from immiscibility of carbonate melt and silicate melt. Likewise, our melt rock may display a small-scaled immiscibility of former carbonate melt and phosphate melt.
Beside these mixed melt rocks, we found fragments of pure former Ca-phosphate melt with glassy parts and unperfectly recrystallized apatite, but without carbonate (Fig.13).
Amorphous carbon (Fig.13). - In a rock unit of some 20 m size exposed as part of the megabreccia and so far interpreted as a micro-breccia, black glossy droplets occur, which look like quench products from a melt. Their size is below 2 mm. From microprobe analysis, their composition proves to be 98 % amorphous carbon, 1.5 % Ca, and 0.5 % S.
Sulfate melt? Shocked gypsum? - Frequently, the megabreccia outcrops expose whitish blocks of up to several meters size, intercalated in the rock units described above. Without closer inspection, this whitish material might be considered and mapped as caliche (calcrete). However, preliminary microprobe and x-ray diffraction analyses show that the material consists of CaSO4 with few contribution of silicate components. The material has a very low density (about 1.4 g/cm³, dry), and porosities as high as 45 % have been determined. Frequently, small rounded conglomeratic quartzite pebbles can be observed to float in the extremely fine-grained white matrix. In thin sections of the matrix, the CaSO4 crystals have sub-microscopic size, and flow texture can be observed to occur. Frequently, single heavily disintegrated feldspar crystals or feldspar aggregates are embedded in the matrix. They show strong mechanical twinning and multiple sets of planar deformation features (PDFs). These features are considered to result from strong shock metamorphism. More shock-metamorphic effects are displayed in the floating quartzite pebbles, which may show quite a few isotropic "quartz" grains (diaplectic SiO2 glass)
Interpretation and relations
From field studies, experimental research including nuclear tests and computer modelling, the basic features of impact cratering are meanwhile well understood. In general, three different stages (contact and compression, excavation, modification) are distinguished.
Rubielos de la Cérida with its distinct central uplift belongs to the type of complex impact structures, and a sequence of the crater development as derived from previous and recent field studies is sketched out in Fig. 14 Stages of Crater.
The top picture (note the schematic drawing, not to scale) shows the target conditions (also for the Azuara structure) as assumed for the time of the impact (Upper Eocene/Oligocene).
In Fig. 14 B, a "snapshot" of the excavation stage is shown. The excavation cavity is in progress and is lined with melt produced in the contact and compression stage. Together with the excavation cavity, an ejecta curtain expands, depositing predominantly unconsolidated molasse material, in which stratigraphically older material may be intermixed. Fig. 14 C schematically describes the final shape of the Rubielos de la Cérida structure. The deep transient cavity has been refilled in the modification stage to flatten the structure, and a central uplift has formed. Excavated material is deposited both within the structure and outside to form an ejecta blanket around the structure. At the rim of the structure, the excavation may have led to inverted stratigraphy (the so-called "overturned flap" [Shoemaker 1960]). This may be verified near Olalla at the supposed northern rim of the structure. The impact melt originally lining the excavation cavity is deposited in the form of melt sheets within the final structure, which is further discussed below.
After the impact, the structure underwent erosion and sedimentation by Upper Tertiary and Quaternary sediments to obtain its present form and layering (Fig. 14 D).
From this Figure, it can be seen where we place the present Barrachina outcrops, and Fig.14 B shows that the megabreccia and the melt rocks must have originated from near the wall of the expanding excavation cavity. In principle, the Barrachina rocks belong to the excavated ejecta, which however were deposited within the structure. This is not surprising, since Pohl (1983) has shown that in very large impact structures this exactly may happen. We again point to the observation that the megabreccia is exposed in contact with deformed Lower Tertiary autochthonous and/or para-autochthonous rocks, and we suggest that we are today able to visit near Barrachina the former (transient) floor of the excavation crater.
Here, we also want to point to many remarkable similarities of the Barrachina megabreccia macroscopic texture to that of the ejecta of the Ries impact crater ("Bunte breccia"; Pohl et. al. 1977; Hüttner 1969; Hörz et al. 1983). This concerns especially the deformations of the soft clasts, showing bending, folding and flow textures, and the brecciation features of competent rock clasts, such as gries and mortar texture.
While the silicate and carbonate/phosphate melts can easily be derived from the target rocks (limestones, shales; phosphate-rich material from coprolite layers well known within sediments of the regional Lower Tertiary), the amorphous carbon droplets and the peculiar CaSO4 rocks need further investigations.
The amorphous carbon my be derived from highly shocked limestones under reducing conditions (Miura et al. 1998) or may originate from extremely shocked coal (Cretaceous Utrillas lignite in the target). Carbon melts at approximately 3,500 K. Possibly, the black veins within the conglomeratic/brecciated units are the result of also amorphous carbon driven by hot fluids and vapor.
From the observations (see above), we conclude that the blocks of the whitish CaSO4 material are not primary chemical sediments. Instead, we suggest that it was no doubt originally gypsum/anhydrite, which, however, was strongly shocked in the impact event to obtain its present composition and texture. We suggest a gypsum decomposition at strongly elevated temperatures followed by recrystallzation, or a formation of a gypsum/anhydrite melt which chilled and recrystallized to gypsum comparable with the carbonate-melt. A total disintegration of the original gypsum beds by strong friction (possibly by frictional melting) in the excavation process must also be taken into consideration. In any case, the intermixing of the highly shocked silicate components is assumed to have occurred during the excavation process.
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