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Impact-related melting of sedimentary target rocks of the Rubielos de la Cérida structure in Spain

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The Rubielos de la Cérida impact structure forms a companion crater to the Late Eocene-Oligo-cene Azuara impact structure. Both are located more or less at the margin of the Iberian chains and the Ebro basin south of Zaragoza. Within the Rubielos structure, silicate melt rocks, carbonate-phosphate melts with small-scaled immiscibility features, very fine mixtures of silicate melt and carbonate forming clasts in suevite, as well as glassy particles of amorphous carbon were found. These melt rocks clearly reflect the chemical composition of various parts of the thick sedimentary pile in the target area and show the shock-induced high-temperature influence on these rocks.
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Impact-related melting of sedimentary target rocks of the Ru-
bielos de la Cérida structure in Spain
Ulrich Schüsslera, Klaudia Hradila, Kord Ernstsonb
aInstitut für Mineralogie der Universität Würzburg, Am Hubland, D-97074 Würzburg
e-mail: uli.schuessler@mail.uni-wuerzburg.de
bFakultät für Geowissenschaften der Universität Würzburg, Pleicherwall 1, D-97070 Würzburg
Abstract: The Rubielos de la Cérida impact structure forms a companion crater to the Late Eocene- Oligo-
cene Azuara impact structure. Both are located more or less at the margin of the Iberian chains and the Ebro
basin south of Zaragoza. Within the Rubielos structure, silicate melt rocks, carbonate-phosphate melts with
small-scaled immiscibility features, very fine mixtures of silicate melt and carbonate forming clasts in
suevite, as well as glassy particles of amorphous carbon were found. These melt rocks clearly reflect the
chemical composition of various parts of the thick sedimentary pile in the target area and show the shock-
induced high-temperature influence on these rocks.
1. Introduction
The impact of the disintegrated Shoemaker-Levi 9 comet with Jupiter in 1994 was not only
an exciting event for astronomers and for the impact-researcher community but has also reinforced
interest in multiple impacts and multiple impact structures in our planetary system [1, 2, 3, 4, 5].
Such structures are well known from the Moon, Mars, Venus, and Jupiter's satellites Ganymed and
Callisto [6, 7, 8]. On Earth, there are at least three paired craters, the West and East Clearwater
Lakes, the Ries and Steinheim Basins, and the Gusev and Kamensk structures [6]. Apart from as-
tronomical significance, impact cratering and ejection mechanisms of multiple impacts have been
considered in order to understand, among other things, a worldwide distribution of ejecta signature
possibly related with mass extinction as, e.g., at the K/T boundary [4].
Only some months ago, we provided first evidence that the previously established 35 - 40
km Azuara impact structure in Spain has a nearby companion crater of roughly equal size thus con-
stituting the presently largest doublet impact structure on Earth [9, 10, 11, 12]. This newly discov-
ered impact structure was named the Rubielos de la Cérida structure. Geological field work, petro-
graphical investigations and geochemical analyses have provided a host of data and findings from
there which have to be incorporated critically during the next time and then will be published in
detail. The present paper is limited to the study of silicate, phosphate and carbonate melts as well as
amorphous carbon from outcrops within the Rubielos de la Cérida structure which are important as
indicators for the impact-related high temperature influence on sedimentary rocks of the target area.
2. Geological Setting
2.1. The Azuara impact structure
The Azuara structure is located some 50 km south of Zaragoza at the margin between the
Alpidic fold belt of the Iberian Chains and the Tertiary Ebro Basin (Fig. 1). Its impact origin was
first suggested in 1985 [13] and later confirmed by detailed mapping, geophysical measurements,
petrographic and geochemical studies [14, 15, 16]. According to these investigations, the most im-
portant impact features are a large quantity of monomict and polymict breccias and breccia dikes,
1
extended megabreccias, a large number of dislocated megablocks, continuous deposits of impact
ejecta, and strong shock metamorphism, indicated by diaplectic glass, melt, and all kinds of planar
deformation features (PDFs).
The sedimentary target of the Azuara structure is assumed to have been thick (1000 - 2000
m or even more) and predominantly unconsolidated molasse sediments overlaying the Palaeozoic
and Mesozoic stable core of the Eastern Iberian Chain [15]. This sedimentary core has a thickness
of roughly 10 km [17], which shows that despite the large diameter, the Azuara structure had a
purely sedimentary target. The dominantly soft target and the advanced erosion may be the cause of
a weak morphological signature of the structure. Although attempts have been made, no radiometric
absolute age is so far available for the Azuara impact. From stratigraphic considerations, it is con-
cluded that the structure formed in Upper Eocene or Oligocene times thus overlapping Alpidic tec-
tonics in that region. As dated by gastropodes, post-impact sediments are Upper Rupelian or Chat-
tian marls [15].
Despite the clear shock metamorphim, the impact evidence is questioned by some regional
geologists [18], which claim an endogenetic origin for most of the impact-related observations.
Fig. 1. Location of the Azuara (A) and the
Rubielos de la Cérida (R) impact structures
south of Zaragoza in Eastern Spain.
2.2. The Rubielos de la Cérida impact
structure
The Rubielos de la Cérida structure,
about 50 km south-southwest of the
Azuara structure (Fig. 1), is defined by
an approximately circular uplift of
Mesozoic rocks (in the middle of Fig. 2),
surrounded by a semi-circular to ellipti-
cal depression of Quaternary and post-
impact Neogene deposits. The diameter
of the uplift is roughly 15 km and the
west-east diameter of the depression
amounts to some 40 km. The geological
sketch map (Fig. 2) and a simplified sec-
tion (Fig. 3) show that the existence of
the central uplift is structurally con-
trolled. In the western and northern part
of the structure its rim is made up by
Mesozoic and Paleozoic rocks belonging
to the Western Iberian Chain. Few hills
of Mesozoic rocks are emerging from the
Quaternary in the western part of the depression. In the south and east, the Rubielos de la Cérida
structure is less well defined.
Central uplift: The oldest layers exposed in the center of the uplift are of Muschelkalk age
corresponding to a stratigraphical uplift [19] of the order of one kilometer. The youngest layers are
Upper Cretaceous and occur only peripherally in the west (Figs. 2, 3). From morphometrical con-
siderations [19, 20]), one would expect a much larger stratigraphical uplift. The divergence could
2
possibly be related with the enormous thickness of the soft molasse top target layers (perhaps more
than 2 km; see above for the data for the Azuara structure).
Besides the soft argillaceous Keuper layers and few Cretaceous and Keuper sandstones, car-
bonate rocks, mainly limestones, strongly dominate. The most significant structural feature in the
central uplift is the enormous compressive signature. The strong deformations up to continuous
megabrecciation is evident nearly everywhere and can best be observed in cuts from road construc-
tions. Apart from the general megabrecciation in the uplift, all kinds of monomict and polymict
breccias and breccia dikes occur. The facies of these breccias is identical to those which have in
detail been described for the Azuara impact structure [13, 15, 21].
Fig. 2. Generalized geo-
logical map and drainage
pattern of the Rubielos de
la Cérida impact structure,
modified from [26, 54]. The
legend numbers 1-6 refer
to the respective strati-
graphical units in Fig. 3:
Arrows mark outcrops and
sample localities as re-
ferred to in the text: M,
megabreccia with melt
rocks near Barrachina; O,
inverted stratigraphy near
Olalla; P, Puerto Mínguez;
R, Rubielos de la Cérida
village; T, outcrops NW
Torre Los Negros; V,
quarry near Villafranca del
Campo.
Crater depression: Go-
ing outwards from the
uplift, the remarkable
deformations continue.
In the Mesozoic hills
emerging from the Qua-
ternary, few quarries
enable insight into dras-
tic and voluminous brec-
ciation of the limestones.
In the extended quarry
near Villafranca del Campo (UTM coordinates 639200; 4505200; see location V in Fig. 2), large
complexes are intensely crushed and ground down to display mortar texture. A compressive
strength of perhaps 150 - 200 MPa (= 1.5 - 2 kbar) for these massive and dense Muschelkalk lime-
stones assumed, they must have experienced pressures clearly exceeding these values not only lo-
cally but through and through.
Crater rims: At the rim of the (semi-circular) depression, the rocks continue to being heavily
deformed, however with varying intensity. Outcrops resulting from road construction show a
megabrecciation of the mostly carbonate rocks. Very often within the megabreccias, large blocks
occur to have rotated in situ, a feature also observed in the uplift. Frequently, well-bedded lime-
stones are sharply cut vertically by breccia dikes.
3
At the northern rim region near Olalla (location O in Fig. 2), an isolated complex of Paleo-
zoic and, to a minor degree, Mesozoic rocks is exposed to show extreme deformation and fractur-
ing. Paleozoic schists on top of Muschelkalk limestones overlaying Keuper marlstones and clay-
stones reflect inverted stratigraphy. At the extended Paleozoic - Muschelkalk interface so far ex-
posed, the limestones are continuously mirror-polished, show distinct and deeply engraved scour
marks and are heavily brecciated and ground down in a layer few centimeters thick immediately
below the mirror. An identical situation is well known from the Ries impact structure [22] where in
the Holheim quarry the same features can be observed to occur at the interface between soft exca-
vated Dogger/Liassic claystones and underlying Malmian limestones at the rim of the crater. Chao
et al. [23] have given a detailed description and argue in favor of high confining pressure which
must have acted on the limestone to overcome the compressive strength of estimated 180 MPa.
We emphasize that comparably heavy and voluminous deformations are restricted to the
Rubielos de la Cérida structure and are unknown from other regions in the Iberian chain overprinted
by Alpidic tectonics.
Impact ejecta: The wide-spread Pelarda Formation [17] as well as a small "enigmatic" de-
posit at the Puerto Mínguez [24] were formerly suggested to be impact ejecta of the Azuara struc-
ture [14]. Meanwhile, the outcrop at the Puerto Mínguez has enormously been enlarged by road
construction continuously exposing impact ejecta over more than five kilometers. The diamictic
material is composed of Palaeozoic, Mesozoic and Lower Tertiary sedimentary rocks. No crystal-
line rocks were excavated, which is in agreement with the enormous thickness (about 10 km, see
above) of the sedimentary target.
These Puerto Mínguez ejecta deposits (location P in Figs. 1, 2) as well as many others even
further to the south (e.g. near Mesquita de Jarque, 28 km east of the central uplift, location M in
Fig. 1) are now considered to probably not originate from the Azuara structure because of their
thickness with relation to distance [12]. Instead the Rubielos de la Cérida structure could well have
been the source of the ejecta. Moreover, even the Pelarda Formation ejecta are now suggested to
possibly be composed of material supplied not only from the Azuara structure but also from the
Rubielos de la Cérida structure. This is concluded from the enormous thickness (in part several 100
m) and the conspicuous position of the ejecta deposits exactly between both structures and oriented
tangentially to them (Figs. 1, 2).
Fig. 3. Generalized geological W-E section across the Rubielos de la Cérida structure.
4
3. Impact melts
Beside the pressure-related shock phenomena like diaplectic quartz, PDFs or spallation frac-
tures as observed in rocks of the Rubielos de la Cérida structure [9, 10], impact melts were found at
four locations of the structure.
3.1. The host rocks
The outcrops of interest occur in the north of the central uplift in the valley of the Pancrudo
brook roughly 10 km east of the town of Calamocha (location M in Fig. 2). This valley is part of the
drainage pattern closely associated with the morphology of the Rubielos de la Cérida structure [9].
Between the junction of the road to Cutanda/Olalla and the Barrachina village, the impact-related
rocks are exposed as a megabreccia over roughly five kilometres along the northeastern roadside. A
further outcrop is located opposite at the steep banks of the Pancrudo brook. On both sides of the
road between the junction and Barrachina, temporary outcrops exist and may be enlarged in con-
nection with quarrying for gravel exploitation. The megabreccia is 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 up to 50 m. Within the megabreccia,
two kinds of components may basically be distinguished for the present:
- a massive diamictic material which is not consolidated and has characteristics very similar
to those described for the Pelarda Fm. ejecta [14] and for the ejecta deposits of the Puerto
Mínguez [12]. This diamictic material incorporates, intermixes, and stratifies with multicol-
oured gypsum marls, claystones, limestones, and pure gypsum.
- strongly brecciated limestones showing mortar texture and a megatexture which resembles
of the gries breccia described for part of the Ries crater ejecta [25].
The diamictic 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. Lithologically, the components are 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 de-
tail been described for the Pelarda Fm. ejecta and the ejecta deposits of the Puerto Mínguez [12,
14]. Like in these deposits, we observe strongly plastically deformed components which point to
high confining pressures upon deformation [12, 14].
Together with a missing distinct stratification, the thickness of the diamictic material undergoes
rapid lateral changes and oscillates between about few meters and 50 m. Presently, this material is
quarried for road-construction purposes.
The multicoloured deposits of marls, marly limestones, gypsum marls and gypsum, according to
ITGE [26] to be Oligocene and Miocene, show a complex interlayering with the diamictic material.
The deposits may overlay the diamictites thereby tunneling them and forming apophyses within
them. Frequently, bodies of the multicoloured material are even floating within the diamictic mate-
rial. In other places, the marly material may underly the diamictic unit. In this case, we may observe
a fluidal megatexture in the diamictic deposits obviously corresponding to a fluidal megatexture
also in the underlying marly unit. Very often, the impact melt rocks are embedded within this zone.
Both the multicoloured composition of the megabreccia and its strange texture have much in com-
mon with the Bunte breccia ejecta of the Ries impact crater [22, 25, 27]. Different from the Bar-
rachina breccia, however, no melt has yet been detected in the Bunte breccia.
3.2. Silicate melt
5
Within the outcrop wall at the northeastern roadside (UTM coordinates 6 55 100, 45 29
900), melt rocks occur as soft, porous, fine-grained, whitish blocks of variable size in a range of
decimetres up to 1 - 2 metres, intermixed in a polymict megabreccia (Fig. 4a). Two of these blocks
have been investigated in detail. As visible under the microscope, the rocks mainly consist of a
milky white glass which forms tiny spheroids and lens-shaped bodies with diametres around 0.5
mm, sometimes smaller (Fig. 4b). A second, subordinate glass phase is translucent greyish and oc-
curs interstitial within the white glass particles (dark parts in Fig. 4b). The glass is estimated to
make up more than 90 % of the rock. This is typically shown by a distinct amorphous glass “hump“
occurring in x-ray powder diffractograms (Fig. 5). Some relics of plagioclase and, in a minor
amount, of quartz and mica within the glass masses are indicated by respective reflection peaks.
Grains of quartz, twinned plagiocase, and few mica were also found in the thin sections to be dis-
tributed within the glass matrix. In rare cases, the quartz fragments show planar deformation fea-
tures (PDFs) and, more frequent, multiple sets of planar fractures (PFs). Feldspar grains show isot-
ropization in the form of multiple sets of isotropic twinning lamellae and isotropic spots (diaplectic
crystals, Fig. 4c), and they have sometimes become almost completely isotropic (diaplectic glass),
indicating shock peak pressures of the order of 30 GPa (300 kbar [28]).
a
b
b
a a b
a
d
d
c
Fig. 4a. Whitish blocks of silicate melt rock within an outcrop of a polymict breccia (see hammer for scale).
Fig. 4b. Unprepared surface of the silicate melt rocks under the microscope. The tiny spheroids and lens-
shaped bodies which make up the rock become visible (field is about 8 mm wide). Fig. 4c. Twinned feldspar
grain, included within the silicate melt rock, with one set of twinning lamellae being isotropic, typical for
diaplectic crystals (crossed polarizers; field is about 1 mm wide). Fig. 4d. Thin section of the silicate melt
rock shown in Fig. 4b (field is about 2 mm wide).
6
Fig. 5. X-ray powder diffractogram of the silicate melt rock shown in Fig. 4. Beside the sharp diffraction
peaks belonging to the feldspar phase ( f ), strongly broadened mica peaks ( m ) can be observed. The
broadening reflects the low crystallinity of this phase. A typical glass “hump” is visible in the 2-range be-
tween 20° and 30°, with peaks of feldspar and mica phases superimposed. (Measurement was carried out
using a Philips PW1710 diffractometer with secondary monochromator. Operating conditions for the x-ray
tube: 40 kV accelerating voltage, 30 mA beam current on a copper anode. Measuring conditions: range 4
2θ 100°; stepwidth 2θ = 0.02°; time/step 2 s. Phase analysis was carried out using MDI-JADE+ program
and ICDD database).
Four bulk samples of the melt rocks were analyzed by RFA Philips PW1480, and separated
particles of the white and the greyish glass were measured using a CAMECA SX50 electron micro-
probe with wavelength dispersive spectrometers at operating conditions of 15 kV accelerating volt-
age, 15 nA beam current and a defocussed beam size. The results are given in Table 1. Contents of
Mn, Cr, Sc, Co, Ni, Mo and S are below the detection limit of the respective instrument. The poor
totals of the microprobe analyses are most probably due to the presence of H2O which entered the
glass by corrosion of the rocks in the ground. If corrected by LOI-determination, the totals are close
to 100 % as shown for the bulk analyses.
No considerable difference could be observed between the composition of the milky white
glass spheroids and the interstitial greyish glass particles. The same holds true if microprobe analy-
ses and RFA bulk analyses are compared. Major oxides are SiO2 between 53 and 59 wt.% and
Al2O3 around 20 wt.%. The content of MgO in the glass particles (around 7 wt.%) is somewhat
higher than in the bulk analyses (4.8 – 6.1 wt.%). These differences and further variations in minor
oxides between the microprobe and the bulk analyses may be caused by secondary fillings of pores,
visible in the thin sections as brownish masses, or by influence of enclosed minerals like feldspars
or micas on the bulk analyses.
7
wt.% white white white white white white mean
wt.% bulk-1 bulk-2 bulk-3 bulk-4 bulk-5
SiO259.95 59.72 59.38 57.19 59.95 59.18 59.23 SiO256.06 58.13 53.45 54.47 19.78
TiO20.24 0.24 0.21 0.20 0.23 0.20 0.22 TiO20.33 0.34 0.38 0.45 0.24
Al2O320.75 19.53 19.88 21.30 23.16 18.63 20.54 Al2O320.91 19.76 20.40 20.96 6.34
MgO 7.26 7.49 7.42 6.14 6.45 8.21 7.16 MgO 5.81 4.77 5.24 6.14 12.62
CaO 0.88 1.04 0.92 0.99 1.09 1.17 1.02 CaO 1.48 1.56 1.72 0.98 22.56
FeO 1.61 1.77 1.62 1.89 1.85 1.73 1.75 FeO 2.00 2.70 2.76 2.49 2.68
Na2O 1.92 1.87 1.82 1.63 1.56 1.66 1.74 Na2O 0.48 1.20 0.29 0.48 0.02
K2O 0.23 0.28 0.27 0.21 0.18 0.26 0.24 K2O 0.65 1.34 0.45 0.57 1.82
Total 92.84 91.94 91.52 89.55 94.47 91.04 91.89 LOI 10.30 9.24 14.02 11.70 32.91
Total 98.02 99.04 98.71 98.24 98.97
wt.% grey grey grey grey grey mean
ppm
SiO256.45 56.89 58.05 59.54 57.12 57.61 V 14 21 27 23
TiO20.27 0.21 0.26 0.22 0.25 0.24 Zn 36 46 68 81
Al2O320.81 19.88 19.66 15.99 22.74 19.82 Ga 35 38 30 33
MgO 6.77 6.34 7.18 6.90 5.93 6.62 Rb 16 38 5 7
CaO 1.14 1.17 1.23 1.24 1.14 1.18 Sr 492 363 327 364
FeO 1.68 2.18 1.63 1.51 1.79 1.76 Y 43 37 32 38
Na2O 1.42 1.19 1.49 0.79 1.31 1.24 Zr 493 475 491 522
K2O 0.21 0.28 0.24 0.23 0.19 0.23 Nb 56 50 47 53
Total 88.75 88.14 89.74 86.42 90.47 88.70 Ba 1250 171 48 1034
Pb 79 238 29 31
Th 68 59 64 59
Table 1. Electron microprobe analyses of white and grey glass particles separated from the silicate glass
(Fig. 4), and mean compositions. X-ray fluorescence bulk analyses of four samples of the silicate glass
(bulk-1 to bulk-4) and one melt-containing inclusion of the suevite (bulk-5)
3.3. Suevite
Silicate melt was also found in a polymict breccia quarried out as large blocks in a tempo-
rary outcrop at the northeastern roadside in the Pancrudo valley (UTM coordinates 6 55 400, 45 29
600). Apart from the melt, the components of the breccia are matrix-supported subrounded to angu-
lar limestone, sandstone and quartzite clasts with various grain sizes. The dominantly carbonate
matrix is peppered with fragmented, predominantly quartz and calcite grains, shows flow texture,
and the larger clasts are preferentially adjusted to the flow (Fig. 6). Few clasts are themselves brec-
ciated (breccia-within-breccia). In thin sections, shock metamorphism in the form of quartz grains
with multiple sets of PDFs and diaplectic quartz is observed.
The melt occurs in elliptic whitish clasts which range in their diameter between several mm
to 2-3 cm. The components of the clasts are extremely fine-grained. The clasts are soft and could be
easily carved by using a small spatula to get material for analysis. From x-ray powder diffraction
analysis, these clasts are no pure melt, but consist of a mixture of amorphous material (typical glass
hump) together with dolomite, calcite and minor amounts of quartz, muscovite and gypsum. This
mixture is also reflected by the chemical bulk composition, shown by analysis bulk-5 in Table 1.
High contents of MgO and CaO and the remarkable value for LOI are due to the dominating car-
bonates, mainly dolomite and subordinate calcite. The bulk-5 composition of the clast can be
closely approached, if a mixture of 50% pure dolomite, 10% pure calcite and 25% of a glass com-
position like the mean of the white glass in Table 1 is calculated. Some small amounts of SiO2,
8
Al2O3, MgO, FeO, CaO and K2O remaining from the calculation can be explained with the addi-
tional quartz, muscovite and gypsum and, especially concerning FeO, with unpure carbonates. A
dominant amount of carbonates and subordinate share of melt can also be estimated from the x-ray
diffractogram.
According to the current classification and nomenclature of impact rocks (IUGS Subcom-
mission on the Systematics of Metamorphic Rocks, Study Group for Impactites), this polymict im-
pact breccia composed of shocked and melt clasts is termed a suevite or suevite breccia.
Fig. 6a. Polished specimen of the suevite from the Pancrudo valley, with clasts of limestone, sand-
stone and quartzite in various grain sizes. The carbonate matrix shows flow texture, with the larger
clasts preferentially adjusted to the flow. Some larger melt inclusions are marked by red boxes
(length of the specimen is 14 cm). Fig. 6b. Quartz grain from the suevite showing planar deforma-
tion features (PDFs). Two different sets of PDFs are clearly to distinguishe, their directions shown
by red bars. Two further sets are harder to recognize, their directions are shown by the blue bars
(field is about 500µm wide)
9
3.4. Suevite-like breccia
Along the road between Barrachina and Torre Los Negros (location T in Fig. 2), exposed
limestones (Eocene, Oligocene?) are strongly deformed and show frequently allochthonous material
intercalated in the form of megablocks and dikes composed of multicoloured marls and shales and
Palaeozoic Pelarda Fm. facies. At UTM 6 57 900, 45 28 300, black shales have been injected into
the bedded limestones exposing a peculiar macroscopic breccia zone. On a smaller scale, frag-
mented black shales are intensivly mixed with a white material to form a finegrained breccia. In
thin section, flow texture can be observed within the dark brownish to black, extremely fine-grained
matrix. This matrix includes clasts of quartz and subordinate feldspar, and sometimes very fine-
grained aggregates of light minerals. From x-ray powder diffraction analysis, the rock consists of
quartz, kaolinite and illite, carbonate and an amorphous phase, the latter definitely documented by a
typical “glass hump” (Fig. 7). The amount of feldspar clasts is below the detection limit of the
method. During electron microprobe investigations, backscattered electron images show an ex-
tremely fine-grained mixture of medium greyish and darker greyish parts in a scale of few microns.
If analyzed, the medium greyish shares reproducibly give a mixed composition of illite and further
clay minerals, with about 53 wt.% SiO2, 25% Al2O3, 5% FeO, 3% K2O and 2.5% each, CaO and
MgO. The darker greyish parts are rich in SiO2 and contain variable, but more or less subordinate
shares of Al2O3. Most probably, this fine-grained mixture reflects strongly alterated relics of the
amorphous (glass) phase which in an uncorroded state could not be detected by microprobe analy-
sis. During corrosion, a glass may be transformed to clay minerals by means of an intermediate gel
stage with typically enhanced Si-contents [29]. No clear shock-metamorphic features have so far
been identified in mineral clasts of this glass-bearing breccia. Therefore, we chose the term suevite-
like breccia for this special rock.
Fig. 7. X-ray powder diffractogram of the suevite-like breccia. Next to the sharp diffraction peaks of low
quartz ( q ) and calcite ( c ), more or less broadened peaks of mica phases (m, kaolinite, illite, montmorillo-
nite) can be observed. In comparison to the silicate melt rock, a less pronounced glass “hump” can be ob-
served due to the superimposed strong and sharp quartz and calcite peaks. The “hump” becomes clearly
visible in the enlarged part of the diffractogram with a logarithm scale of the intensity. (Operating and
measurement conditions similat to that described for Fig. 5 except for the time/step with 10 s).
10
3.5. Carbonate-phosphate melt
A very special kind of former melt was found within the megabreccia of the northeastern
roadside outcrops at UTM coordinates 6 51 800, 45 31 400.
The melt rocks are whitish, very soft and crumbly. Typically harder, irregular spheroids up
to 4 mm in size can be observed, which are embedded within an extremely fine-grained matrix.
Under the microscope, the spheroids turn out to be globular to amoeba-like calcite particles, coarse-
grained in their centres and with decreasing grain size and often perpendicular orientation towards
the rims; the contact towards the matrix is extremely fine-grained (Fig. 8). The isotropic glass ma-
trix in part is intensively pervaded by tiny, elongated, sometimes flaser-like microcrystals, often
orientated tangentially to the rim of the calcite particles (Fig. 8).
The whole rock composition is 52.7 wt.% CaO, 8.3 wt.% P2O5 and 1.5 wt.% BaO (RFA,
bulk in Table 2). From microprobe investigations, the carbonate of the particles is pure calcite. The
glassy matrix mainly consists of CaO and P2O5 (Table 2), with minor contents of F (1.0-2.5 wt.%),
S (1.1-2.1 wt.%, if calculated as SO3), Cl (0.5-0.8 wt.%) and NaO (0.3-0.6 wt.%). The poor totals
of the analyses point to high amounts of light components within the Ca-P-glass, presumably H2O
which may have entered the glass during corrosion. The existence of considerable amounts of C or
CO2, however, must also be taken into account. Locally, a strong enrichment of Ba and S was ob-
served within the Ca-P-matrix. Here, the CaO- and P2O5-contents are in the range of trace elements
or below the detection limit, whereas Al2O3 is present in minor concentrations of about 1 wt.%.
In part, the Ca-P-glass is recrystallized to form apatite, as veryfied by x-ray powder diffrac-
tion analysis. The diffraction peaks of this apatite, however, are broadened compared to those of a
well crystallized one (not shown here),
indicating its very low crystallinity
(Fig. 9). The existence of baryte has
also been proved by x-ray diffraction
analysis. This baryte may occur as a
very fine-grained phase within the Ba-
and S-enriched locations in the Ca-P-
matrix, detected by microprobe analy-
sis.
Fig. 8. Photomicrograph (crossed polariz-
ers) of amoeba-like calcite bodies within a
matrix of phosphate glass (dark) from the
Barrachina megabreccia. Note that the size
of the individual calcite crystals increases
towards the centers of the bodies. Also
note that the peripheral calcite obviously
has grown perpendicular to the rim be-
cause of the orientation. In part, especially
along the borders to the calcite bodies, the
phosphate glass has recrystallized to form
apatite (elongated, sometimes flaser-like
minerals tangentially orientated to the cal-
cite bodies). The field is 6 mm high.
11
Fig. 9. X-ray powder diffractogram of the carbonate-phosphate melt. Peaks of calcite ( c ) and baryte ( b )
and clearly broadened peaks of hydroxylapatite (h) are superimposed to a glass “hump”. The “hump” be-
comes visible in the enlarged part of the diffractogram with a logarithm scale of the intensity, but is less dis-
tinct due to a subordinate content of phosphate glass in the melt. (Operating and measurement conditions
similat to that described for Fig. 5 except for the time/step with 10 s).
wt.% 1 2 3 4 5 6 mean bulk
P2O522.13 21.26 24.47 27.52 32.61 32.42 26.74 8.25
Al2O30.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
CaO 35.83 35.97 37.46 42.93 48.76 51.62 42.10 52.65
BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.47
Na2O 0.35 0.32 0.46 0.57 0.53 0.55 0.46 0.23
SO31.67 1.15 1.77 2.12 1.47 1.37 1.59 0.92
F 1.57 1.56 1.02 2.26 2.24 2.39 1.84 n.d.
Cl 0.62 0.71 0.50 0.79 0.49 0.55 0.61 n.d.
LOI 34.31
Total 62.18 60.98 65.68 76.19 86.10 88.91 73.34 97.83
Table 2. Electron microprobe analyses of the glassy phosphate matrix (dark areas in Fig. 8), mean composi-
tion of this matrix, and x-ray fluorescence measured bulk composition of the carbonate-phosphate melt
12
3.6. Amorphous carbon
In the Barrachina megabreccia described above (location M in Fig. 2), blocks of a fine-
grained microbreccia are locally intercalated. The microbreccia consists of loosely cemented frag-
ments and particles mainly of carbonate, subordinate quartz. As a further component, black parti-
cles in a size between 0.5 and 2 mm are subordinate, but nevertheless widespread and typical in
these sediments. To separate these particles from the sediment, the carbonate was dissolved in HCl
and then the particles were removed from the remaining mineral fraction by hand-picking. Two
kinds of black components could clearly be distinguished. The first kind is ordinary charcoal which
under the microscope showed the typical charcoal-structure and because of the weakness could eas-
ily be destroyed e.g. by soft pressure with the preparation tool. This charcoal is not of further inter-
est for the present paper. The second kind of black particles is very hard, occurs in very irregular
forms and has a surface gleaming like glass. At least, the particles look like black, glassy micro-
slags (Fig. 10). Qualitative microprobe element scans showed that the particles are composed of
carbon and oxygen as the only major components. Additional elements are Ca in varying concentra-
tions up to 2.7 wt.% and S varying between 0.2 and 0.8 wt.%. X-ray powder diffraction analysis of
the particles resulted in diffractograms without any reflections, but showing a typical amorphous
glass “hump“.
Fig. 10. Microslag-like particles are common in blocks of a finegrained microbreccia within the Barrachina
megabreccia. They consist of carbon with subordinate amounts of Ca and S, but also contain remarkable
amounts of oxygen (black bar is 1 mm).
13
4. Discussion
4.1. Silicate melt
Due to the thick sedimentary cover in the target area of the impact, the melts should find
their educt rocks among these sediments. The blocks of silicate glass are assumed to originate from
shales, forming interlayers within the sedimentary pile of the target. This is corroborated by the
chemical composition of these melt rocks. The analyses shown in Table 1 compare quite well to a
composition of shales, especially concerning the high Al-contents and the low contents of Ca, Fe
and the alkaline elements. The discrimination log(SiO2/Al2O3) vs. log(Fe2O3/K2O) [30], not shown
here, also allows to classify the educt of the silicate melt rocks as shales, mainly basing on the low
SiO2/Al2O3-ratio which is typical for pelitic rocks [31]. Using the pure SiO2-Al2O3 system (compi-
lation in [32]), a maximum melting temperature of 1750°C can be deduced for the silicate glass
rocks which in any case is lowered due to the content of alkaline and earth alkaline elements.
Very similar shock-melted shales are described from the Haughton impact crater on Devon
Island in Canada to form a highly porous network of non-transparent silicate glass with some very
fine-grained shares of quartz, sheet silicates and calcite [33]. A minimum shock pressure for the
onset of melting in sandstones and shales is higher than 30 GPa ([33] for the melts of the Haughton
impact, using data of [34, 35]).
The silicate glass rocks are clearly not of volcanic origin. Apart from the occurrence of
strongly shocked clasts in the melt, if these melt rocks would represent a deformed ash layer, the
rocks should contain pyroclastic fragments and, with respect to an „intermediate“ SiO2-
concentration, mafic relic minerals or andesitic rock fragments. This is not the case. Furtheron, the
chemical composition should be similar to that of andesites or basaltic andesites. Those rocks, how-
ever, generally have distinct lower contents of Al2O3 and much higher contents of FeO, CaO and
(Na2O+K2O) than the investigated silicate melt rocks (a comparison was carried out with all analy-
ses of volcanic rocks given in [36]). At least, also the melting temperature estimated for the investi-
gated rocks does not really match the temperatures in an andesite volcanic system.
The only further geological possibility to produce glassy sediments besides impact events is
by frictional heating during extreme dynamic metamorphism in a thrust zone to form pseudo-
tachylites. This process is assumed to occur also in large impact events during the excavation and
modification stages of impact cratering. The Barrachina silicate melt is not assumed to have formed
by frictional melting of the target rocks. The coexistence of glass and highly shocked minerals
clearly speaks in favour of a shock-produced melt.
4.2. Suevite and suevite-like breccia
The melt clasts of the suevite are composed of a mixture of dolomite, calcite and glass
which is more or less similar to the silicate glass of the larger melt bodies. Carbonate which has
crystallized as calcite and dolomite may derive from a carbonate melt. A fine-grained carbonate-
rich suevite breccia with formerly melted carbonate material has been reported for the Chicxulub
impact structure [37, 38] and may serve for comparison.
From its macroscopic appearance, the suevite-like breccia is quite different from the suevite.
Like the suevite, however, it mainly consists of a very fine-grained mixture of carbonate and sili-
cate glass, the latter in part strongly corroded.
4.3. Carbonate-phosphate melt
The carbonate-phosphate rocks from Barrachina in the Rubielos de la Cérida structure do
not look like typical melt rocks, and at first glance, the only hints to a former melt are the glassy
14
relics within the phosphate matrix. The only comparable rock described until now has been reported
for the original Ries suevite which is the melt-bearing impact breccia of the Nördlinger Ries crater.
Within this suevite, irregular, amoeba-like carbonate particles are embedded within a matrix of sili-
cate glass. The carbonate particles show chilled margins in contact with the silicate matrix, and
along this rim, calcite crystals grew elongated perpendicular to the contact. This texture was inter-
preted as the result of a quench crystallization [39]. Taking into account also other features ob-
served in the suevite during a very detailed and extensive investigation, Graup [39] was able to
show that these parts of the suevite were formed in the course of immiscibility of impact-induced
carbonate and silicate melts.
In general, the carbonate-phosphate melt rocks from Barrachina resemble the suevite from
the Nördlinger Ries crater. Both rocks show these amoeba-like carbonate particles which result
from quench crystallization, with chilled margins and calcite orientated perpendicular to the margin.
In both rocks, also carbonate globules and carbonate particles with curvilinear rims towards the
matrix occur, and menisci of „matrix“ melt cut the carbonate particles in the suevite as well as in
the Barrachina samples. Like respective parts of the Ries suevite, the melt rocks from Barrachina
are assumed to result from an immiscibiliy of two kinds of impact-induced melts, different from the
Ries suevite, however, of a cabonate and a phosphate melt.
A thermal decomposition of carbonates is also described for the sedimentary target rocks of
the Haughton impact, with a beginning of this decomposition at a pressure of about 45 GPa [33,
40]. For the same impact, carbonate globules and irregulare blebs within silicate glass similar to
those from the ries impact are described and again interpreted as a result of carbonate-silicate liquid
immiscibility [41, 42].
4.4. Amorphous carbon
Carbon in elemental form, sometimes as diamonds, sometimes as C-rich shale fragments,
have repeatedly been found in relation to impact structures and in part have been investigated [43,
44]. One possible source of elemental carbon is carbonate, especially for target areas showing a
thick and carbonate-rich sedimentary cover. Hypervelocity impact experiments verified the produc-
tion of highly disordered graphite from dolomite or limestone targets [44]. For natural impacts, a
transformation of CO to CO2+C in the cooling atmospheric impact plume is assumed [45]. Miura et
al. [46, 47] showed that amorphous carbon occurring in impact structures is a result of multiple im-
pacts with reduction state and can be obtained from vaporized limestone target rocks, in natural
impacts as well as in artificial impact craters. Ca-contents within the carbon are interpreted to be
remains from the limestone target rocks .
Thick limestone sequences in the Rubielos de la Cérida target area thus may easily have
been a source for the carbon particles in the Barrachina megabreccia. This is underscored by the
Ca-contents up to 2.7 wt.% detected in these particles. As a further possibility and taking into ac-
count the glassy appearence and the irregular shapes of the particles, the amorphous carbon may be
quenched carbon melt from extremely shocked coal of the Cretaceous Utrillas lignite deposits in the
target. The melting temperature of carbon is roughly 3500°C which is exceeded at highest shock
levels. The role of remarkable contents of oxygen detected in the carbon particles is still unclear.
Compounds of carbon and oxygen do not occur in solid state. One idea is that the carbon may occur
as fullerenes which have been found for example in relation to the Sudbury impact structure and the
Permian-Triassic boundary [48, 49] and which are able to trap gases within their cages. More inves-
tigation is necessary, however, to clarify this.
15
5. Conclusions
As a consequence of the impact event at Rubielos de la Cérida, sediments of the target area
underwent high-temperature influence which led to partial melting of these rocks. Relics of these
melts were found in outcrops of the impact sediments and consist of silicate, but also carbonate-
phosphate melt rocks. These melts clearly reflect pre-impact lithostratigraphical units of the target.
Pelitic rocks, i.e. claystones and shales, as the deduced educt rocks of the silicate melt are
frequent in the stratigraphic sequence of the target. Only few kilometres to the north of the Bar-
rachina outcrops near Olalla, they contribute, e.g., to the Cambrian Valdemiedes and Huérmeda
Fms. [26, 50]. They also occur in the Buntsandstein Fm., exposed for example in the surroundings
of Visiedo southeast of the central uplift (erroneously mapped as Malmian there, see [26, 51]), in
the Upper Malmian Purbeck Facies, and in the Eocene/Oligocene yet presently exposed in the Ru-
bielos de la Cérida structure.
The carbonate of the carbonate-phosphate melt may easily be deduced from the Mesozoic,
especially the Jurassic and Upper Cretaceous limestones, which are very abundant in the strati-
graphic sequence of the central uplift of the Rubielos de la Cérida structure [26]. Limestones also
occur in the Eocene [26] and largely contribute to the conglomerates of the Lower Tertiary which
made up a large part of the pre-impact target area [15].
The phosphate of the carbonate-phosphate melt may be derived also from the Jurassic rocks
in the target. There are two „boundary oolithes“ [52] which mark the Lower/ Middle and Mid-
dle/Upper Jurassic boundaries in the whole Iberian Cordillera. From these boundaries, phosphoritic
components in the fossiliferous oolites are described [52]. Phosphate may also originate from cop-
rolites in the sedimentary sequence of the target. A prominent Oligocene coprolite layer has been
reported, e.g., to be exposed near the town of Calatayud some 80 km northwest of the Rubielos de
la Cérida structure [53].
Ba as locally observed in the carbonate-phosphate melt is common in the target area to oc-
cur as barite in fissures, dikes and as irregular masses in Palaezoic rocks [26].
The suevite is composed of clasts which, apart from the melt, represent a mixture of pre-
impact Paleozoic, Mesozoic and ?Cenozoic lithostratigraphical units. The melt clasts are composed
of glass similar to the silicate glass of the larger melt bodies and additional carbonate, both to be
deduced from the target rocks.
The source of the amorphous carbon can be established also in the sedimentary rocks of the
target. Beside the abundant carbonate rocks in the form of limestones, dolomites and marls in the
Palaeozoic, Mesozoic and Cenozoic rocks, the coal-bearing Cretaceous Utrillas layers must be con-
sidered. The lignite layers have been of considerable economic value in Spain and are deposited
roughly 20 km west of the Rubielos de la Cérida central uplift [26]. In the structure itself, the strati-
graphically adjacent layers of Albian to Senonian age are exposed in the central uplift [26], and
coal-bearing Utrillas layers could have been deposited immediately at the impact point.
This comparison of melt rocks and pre-impact lithology shows that the melt rocks in detail
reflect this lithology, on larger scale as well as even in these small-scaled clasts of the suevite,
where silicate melt and carbonate shares form an intensive and extremely fine-grained mixture of
previously distinct sedimentary layers. The Rubielos de la Cérida structure therefore can be seen as
a typical example for an impact into a very thick sedimentary cover.
Acknowledgements
16
Thanks to P. Späthe for the preparation of the thin sections and to K.-P. Kelber for all photographs
shown in this paper, but also to T. Ernstson who made some important discoveries in the field and
to F. Claudin for text contributions and helpful discussions. H. Müller-Sigmund, Institute of Miner-
alogy, Petrology und Geochemistry, University Freiburg, carried out the qualitative microprobe
element scans on the carbon particles. Thank you very much for this.
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19
... : Azuara impact structure located south of Zaragoza and north of the compagnion Rubielos de la Cérida impact structure [5,6,7] As shown by thin sections under the microscope, the vein consists of a light matrix of carbonate minerals, hosting a high amount of black spherical to amoeboidal particles which show gel-like layered structure in the reflective light. The rim towards the dolomite country rock is characterized by pure carbonate crystals having grown perpendicular to the vein´s wall. ...
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The Iberian System in NE Spain is characterized by a distinctive graben/basin system (Calatayud, Jiloca, Alfambra/Teruel), among others, which has received much attention and discussion in earlier and very recent geological literature. A completely different approach to the formation of this graben/basin system is provided by the impact crater chain of the Rubielos de la Cérida impact basin as part of the important Middle Tertiary Azuara impact event, which has been published for about 20 years. Although the Rubielos de la Cérida impact basin is characterized by all the geological, mineralogical and petrographical impact findings recognized in international impact research, it has completely been hushed up in the Spanish geological literature to this day. The article presented here uses the example of the Jiloca graben to show the absolute incompatibility of the previous geological concepts with the impact structures that can be observed in the Jiloca graben without much effort. Digital terrain modeling and aerial photography together with structural and stratigraphic alien geology define a new lateral Singra-Jiloca complex impact structure with central uplift and an inner ring, which is positioned exactly in the middle of the Jiloca graben. Unusual topographic structures at the rim and in the area of the inner ring are interpreted as strike-slip transpression and transtension. Geological literature that still sticks to the old ideas and develops new models and concepts for the graben/basin structures, but ignores the huge meteorite impact and does not even enter into a discussion, must at best cause incomprehension.
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Impact processes are among the most frequently occurring ones in the Solar System. The result of the collision on a solid surface is: crater. The book overview all aspects of crater formations on the solid bodies in the Solar System. It deals with the formation of the concepts, the development of methods which were used in extension of the stratigraphic principles to the Solar System bodies with solid surdace: planets, moons, smaller bodies. The book also analyses the processes of aging, weathering of the most spectacular planetary formations: the impact craters. Most of the book is written in Hungarian, however, several chapters have been translated to English.
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The Ries crater represents one of the best investigated, large, complex terrestrial impact structures with a well preserved 'ejecta blanket'. The Ries crater is, therefore, used as an important reference point for the discussion of impact cratering mechanics. A synopsis is presented of field and laboratory data relevant to the mechanics of the crater-forming process. The structure and composition of the target is considered and a description is presented of the surface formations of the crater, taking into account present morphology and surface geology, classification and composition of impact formations, structure and stratigraphy of outer impact formations, and shock metamorphism and thermal history of various impact formations. The subsurface structure of the crater is examined, giving attention to drillholes, geoelectric measurements, and gravity measurements. A cratering model is also presented.
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Beads from graves of the Samad Culture. Sultanate of Oman, and from an ancient craftsmen quarter of the old kingdom of Ruhuna. excavated in Sri Lanka, were investigated using electron microprobe analysis and X-ray powder diffraction. Both experimental methods were optimized towards a non-destructive analysis of archaeo­ logical finds. Based on their analysis, the beads from Oman can be divided into those made from natural rocks or minerals, metal, glass, Egyptian Blue and synthetic enstatite. Preferred natural rock types are serpentinite, chloritite and massive chlorite amphibolite which occur in the Samail Ophiolite of Oman and indicate a local production of these beads. Garnet beads are almandine-pyrope-rich and are interpreted as imports from the Sri Lanka/India area. Metal beads are made from pure Ag. ± pure Au or from Ag-Au-Cu alloys. Reddish-brown glass beads from Oman are Na-rich and coloured by Cu present in the glass matrix. Opaque red glass beads from Sri Lanka are commonly K-rich and coloured by tiny cuprite droplets which recrystallized from the melt and which are intensively disseminated within the glass matrix. Blue-while-blue and brown-white-brown sandwich beads from Oman and Sri Lanka are stylistically similar, but differ in composition of the white glass. Parts of the glass beads from Oman is partially or completely altered to form smectite. A cogged wheel bead from Oman was cut from steatite and then hardened by transformation of the steatite lo synthetic enstatite during firing at about 1000'C. Large amounts of microbeads from a Samad grave also consist of synthetic enstatite and most probably were produced from Mg-rich clay by firing. Comparable beads have been recovered from excavations in the Indus area, especially Harappa. but also in the Arabian Emirates.
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APPROXIMATELY ten per cent of the impact structures on the Earth and Venus are doublets1,2-pairs of craters formed by the near-simultaneous impact of asteroids of comparable size. It has been suggested that these doublet craters form from asteroid fragments dispersed by aerodynamic forces during atmospheric entry1,3, or from asteroids that were tidally disrupted by gravitational forces shortly before impact4-6. But to form a doublet, the progenitors of the craters must have been well separated before final impact1, which poses problems for both mechanisms. Here we argue that a hitherto undetected population of well separated binary asteroids can explain the occurrence of doublet craters. By modelling asteroids as weak, gravitationally bound aggregates ('rubble piles'), we show that the tidal forces experienced during close encounters with the Earth can generate binary asteroids, in a process similar to that which fragmented the comet Shoemaker-Levy 9 (ref. 7) as it passed by Jupiter. Although the resulting binary asteroids may eventually separate or coalesce before colliding with a planet, repeated close encounters with the Earth maintain a steady-state population that is sufficiently large to explain the observed number of doublet craters.
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Ejecta surrounding the 26-km-diameter Ries Crater, Germany, may be helpful to the interpretation of the Cretaceous Tertiary (C T) Boundary Event. The Ries ejecta can be classified into three major facies: (1) moldavite tektites, (2) Bunte Breccia, and (3) suevite, each of which represents a temporally and spatially distinct ejection regime. The petrographic and geochemical characteristics of each facies are also distinct, reflecting an orderly stratigraphic succession of the Ries target. Moldavites represent early high-speed ejecta originating at or close to the projectile-target interface; Bunte Breccia reflects the major excavation and ejection phase and comprises >90% of all ejecta beyond the rim crest; suevite is deposited last and is derived from the most deep-seated target strata. Details of the tektite transportation mechanism) are still poorly understood, but it is virtually certain that drag forces in a rapidly ascending cloud of vaporized target and projectile materials must be invoked. In contrast, individual components ofthe Bunte Breccia are ejected in direct ballistic trajectories and at sufficient velocities to generate a secondary cratering action upon landing, which in turn leads to a highly turbulent, ground-hugging debris surge. Finally, transportation and deposition of suevite seems to require a turbulent, radially expanding gas phase composed of volatiles (H2O; CO2) liberated from the target rocks. It is estimated that this vapor cloud persisted for a minimum of 5 min and that the ambient atmosphere was severely perturbed for at least this long. Using various scaling laws that relate the bolide's kinetic energy to crater geometry or volume and assuming 25 km/s as impact velocity, a projectile diameter of 1 to 2 km results for a stony object; corresponding ratios of ejecta mass (Me ) and projectile mass (Mp) range from 6 ×lO1,) to 4 × I04 (max); Me/Mp of = 102 seems to be a reasonable estimate. These calculations contrast with the bolide mass as estimated by geochemical means. Geochemical studies reveal that projectile dissemination is heterogeneous and that maximum extraterrestrial contamination modeled as a C( chondrite is 4 × 10 3 wt %; moldavites-at most-contain 4 × 10 s wt % C chondrite. Thus, projectile masses based on cratering theory conflict by orders of magnitude with measured concentrations of meteoritic indicator elements. This discrepancy seems to imply that most of the bolide mass is lost to the atmosphere. Observations from the Ries and other terrestrial craters indicate that tektites and.
The Pelarda Formation is an isolated continuous deposit which is up to 200 m thick and extends over an area of about 12 X 2.5 km2. It was originally described as a Tertiary fluvial boulder conglomerate. From the occurrence of striated and plastically deformed boulders and pebbles which partly show moderate shock effects, we conclude that the Pelarda Formation is the remnant of an originally extended ejecta blanket around the large Azuara impact structure. This interpretation is substantiated by statistical analyses of the striae azimuth and the normals to locally developed bedding planes. The admixture of lo- cal substrate with the Pelarda Formation indicates secondary cratering as a consequence of ballistic transport and ballistic sedimentation of the primary ejecta. — Models of palaeogeography and morphogenesis based on the fluvial-deposit interpretation must be re-examined.