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New approach to an old debate: The Pelarda Formation meteorite impact ejecta (Azuara structure, Iberian Chain, NE Spain)

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The Pelarda Formation (Fm.), located in the Iberian System in northeast Spain, is a sedimentary deposit with an extension of roughly 12 km x 2.5 km and an estimated thickness of no more than 400 m. The formation was first recognized as a peculiar unit in the early seventies and underwent interpretations like a fluvial or an alluvial fan deposit having a postulated age between Paleogene and Quaternary. Since the early nineties the Pelarda Formation has been considered an impact ejecta deposit originating from the large ca. 40 km-diameter Azuara impact structure and meanwhile being among the largest and most prominent terrestrial impact ejecta occurrences, which however is questioned by regional geologists still defending the fluvial and alluvial fan models. Roughly speaking, the Pelarda Fm. is a grossly unsorted, matrix-supported diamictite with grain sizes between silt fraction and meter-sized clasts and a big intercalated megablock. Strong clast deformations and abundant shock metamorphic effects like planar deformation features (PDF) are observed throughout the Pelarda F. deposit compatible with its impact ejecta origin. Aligned bigger clasts and smaller intercalated bands of sandstones, siltstones and clayey material indicate some local stratification obviously adjusted to flow processes within the impact ejecta curtain. This suggests that gravitational flows predominated in a transport by water in both liquid and gas states. Transport and deposition as a kind of pyroclastic surge are discussed. A sketch sequence describes the emplacement process of the Pelarda Fm. as part of the Azuara crater formation and the integration in the general frame of pre-impact geology and some post-impact layering.
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ARTICLE (August 2019)
New approach to an old debate: The Pelarda Formation meteorite impact
ejecta (Azuara structure, Iberian Chain, NE Spain)
Ferran Claudin1, Kord Ernstson2, Wolfgang Monninger3
_______________________________________________________________
Abstract. - The Pelarda Formation (Fm.), located in the Iberian System in
northeast Spain, is a sedimentary deposit with an extension of roughly 12 km x
2.5 km and an estimated thickness of no more than 400 m. The formation was
first recognized as a peculiar unit in the early seventies and underwent
interpretations like a fluvial or an alluvial fan deposit having a postulated age
between Paleogene and Quaternary. Since the early nineties the Pelarda
Formation has been considered an impact ejecta deposit originating from the
large ca. 40 km-diameter Azuara impact structure and meanwhile being among
the largest and most prominent terrestrial impact ejecta occurrences, which
however is questioned by regional geologists still defending the fluvial and
alluvial fan models. Roughly speaking, the Pelarda Fm. is a grossly unsorted,
matrix-supported diamictite with grain sizes between silt fraction and meter-
sized clasts and a big intercalated megablock. Strong clast deformations and
abundant shock metamorphic effects like planar deformation features (PDF) are
observed throughout the Pelarda F. deposit compatible with its impact ejecta
origin. Aligned bigger clasts and smaller intercalated bands of sandstones,
siltstones and clayey material indicate some local stratification obviously
adjusted to flow processes within the impact ejecta curtain. This suggests that
gravitational flows predominated in a transport by water in both liquid and gas
states. Transport and deposition as a kind of pyroclastic surge are discussed. A
sketch sequence describes the emplacement process of the Pelarda Fm. as
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part of the Azuara crater formation and the integration in the general frame of
pre-impact geology and some post-impact layering.
Key words: Pelarda Formation, Iberian System, Upper Eocene/Oligocene,
Azuara impact structure, proximal impact ejecta, pyroclastic flow
_______________________________________________________________
1 Associate Geological Museum Barcelona (Spain); fclaudin@xtec.cat
2 University of Würzburg (Germany), Faculty of Philosophy; kernstson@ernstson.de
3 Formerly DEMINEX oil company; retired geologist; wolfgang.monninger@t-online.de
Content
1 Introduction 3
2 Previous studies - overview 7
2.1 Lithology 7
2.2 Source for the Pelarda Fm. material 8
2.3 Age of the Pelarda Fm. deposit 9
3 Previous studies - geological setting, layering, and petrographic features 9
3.1 Geological setting 9
3.2 Layering 16
3.3 Petrographic features 16
Shock metamorphism 16
Mesoscopic deformations 19
Impact spallation 21
4 New investigations 22
4.1 Methodology 23
4.2 Investigated locations 23
4.2.1 Salcedillo S1 24
4.2.2 Salcedillo S 2 37
4.2.3 Road from Olalla to Fonfría (S 3 and S 4 in Fig. 6 and Fig. 23) 38
4.2.4 Contacts 42
4.3. Granulometric and textural analyses 48
4.4 Petrographic analyses of thin sections 49
Shock metamorphism (shock effects) 49
4.5 Paleocurrent analysis 52
4.6 Sedimentary environment 52
4.7 Origin of the clasts 54
4.8 Age 54
4.9 More Pelarda Fm. ejecta deposits: the San Roque deposit 55
5 Discussion 55
6 Conclusions 58
References 59
Appendix I: The Azuara impact cratering process and ejecta formation
in eight images - schematic 67
Appendix II: The Ermita de San Roque Pelarda Fm. ejecta deposit 71
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1 Introduction
The Pelarda Formation (Fig. 1) covers an area of about 12 x 2.5 km² (Fig. 2),
has a thickness of in part up to 400 m, and is exposed between 1,100 and
1,450 m altitude in the highest parts of a mountain chain within the Alpidic
Iberian system (Fig. 3). First described by Monninger (1973) and Carls &
Monninger (1974), the origin of the Formation experienced various and quite
different explanations like fluvial (Carls & Monninger 1974; Adrover et al. 1982;
J. Smit 2000 (written communication), alluvial fan (Lendínez et al. 1989, Pérez
1989; Aurell et al. 1993; Aurell 1994; Cortés et al. 2002; Díaz-Martínez et al.
2002 a, b, c; Díaz-Martínez 2005) and meteorite impact (Ernstson & Claudin
1990, Ernstson & Fiebag 1992, Rampino et al. 1997 a, b; Claudin et al. 2001;
Ernstson et al. 2002, 2003; Claudin & Ernstson 2003; Ernstson 2004)
deposition.
Fig. 1. Location map for the Azuara and Rubielos de la Cérida impact structures in Spain.
Modified from Ernstson et al. (2002). The arrow focuses on the occurrence of the Pelarda
Fm. in detail outlined in Fig. 2. CAM = Caminreal, CAL = Calamocha, CAR = Cariñena,
MUN = Muniesa.
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Fig. 2. Location map for the Pelarda Fm.
Fig. 3. View of the densely forested Pelarda Formation deposit forming the highest
mountains (Retuerta 1,512 m) of the Sierra de la Pelarda (or Sierra de Fonfría).
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Fig. 4. The Pelarda Fm. and the Azuara impact event within the stratigraphy of the
Iberian Chain. Data from Carls & Monninger (1974) and ITGE (1991). Modified from
Ernstson et al. (2002).
The meteorite impact model considers the Pelarda Fm. to be ejecta of the
about 40 km-diameter Azuara impact structure located at the margin between
the Iberian Chain and the Ebro Basin in northeast Spain (Fig. 1). In Fig. 1, the
Azuara structure is sketched as part of the proposed multiple-impact scenario
also comprising the Rubielos de la Cérida elongated impact basin (Ernstson
2001a, b, 2002, 2003; Hradil et al. 2001, Claudin et al. 2001). Stratigraphically,
the Pelarda Fm. and the impact event are positioned in the column of Fig. 4.
The Azuara structure first described as a probable impact crater by
Ernstson et al. (1985) has undergone a long and curious history. In the 1985
paper, Ernstson et al. presented clear and unambiguous shock effects in
Azuara rocks like planar deformation features (PDFs) in quartz, as well as other
typical impact-related features. Consequently, Azuara became listed as a
confirmed impact structure (Grieve & Shoemaker 1994, Hodge 2010, Norton
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2002) and was included in the Earth Impact Database of established impact
structures conducted by the Canadian Geological Survey (R.A.F. Grieve and
co-workers). Later, more evidence for the impact nature of Azuara in the form of
additional shock effects (e.g., shock melt, diaplectic glass, shatter cones),
abundant monomictic and polymictic breccias, extensive megabreccias,
dislocated megablocks, and geophysical anomalies was presented (Ernstson et
al. 1987, Fiebag 1988, Ernstson 1994, Ernstson & Fiebag 1992, Ernstson et al.
2001, Ernstson et al. 2002). In 1990, the Pelarda Fm. was for the first time
suggested to be Azuara proximal impact ejecta (Ernstson & Claudin 1990) thus
also supporting the extraterrestrial origin of the crater. The impact nature of the
Pelarda Fm. was mainly attributed to the abundant occurrence of shock-
metamorphic effects (multiple sets of planar deformation features, PDFs, and
multiple sets of planar fractures, PFs, in quartz) as well as on high-
pressure/short-term deformations (e.g., rotated fractures) of clasts regularly
found in the deposit (Ernstson & Claudin 1990).
Surprisingly, in 2003 when the management of the Canadian impact
database had changed from the Geological Survey to the university of New
Brunswick (J. Whitehead and J. Spray), Azuara was removed from that
database despite all established impact evidence the clear and unambiguous
shock-metamorphic effects included. For insiders, the reason for the removal of
the Azuara impact structure is obvious (Ernstson & Claudin 2013b). However,
this will not be discussed here, but playing a certain role in the discussion about
the Pelarda Fm., the fact of the removal is worth mentioning here.
As proximal impact ejecta, the Pelarda Fm. attracts considerable
attention, because ejecta deposits of such a size and extension are extremely
rare among terrestrial impact structures. The reason for the poor preservation of
this special kind of sediments is the in general rapid erosion of the ejecta
deposits even in the case of large craters. It has been suggested that
diamictites commonly interpreted as glacial (tillite) deposits are in fact preserved
impact ejecta of unidentified impact structures (Oberbeck et al. 1993, Rampino
et al. 1992, 1994, 2017), however no respective studies in proof of this model
have been published so far (also see Reimold et al. 1997).
Apart from the many distal occurrences (e.g., Addison et al. 2005),
Haines 2005, Glikson 2005, Hassler et al. 2011, Glikson et al. 2016, Glikson &
Pirajno 2018) numerous ejecta deposits from various established impact
structures have carefully been studied (e.g., Ries [Hüttner 1969, Hörz 1982],
Chesapeake Bay [Horton et al. 2005, Horton & Izett 2006)], Chicxulub [Pope et
al. 1999, 2005, Schulte & Kontny 2005, and others], Haughton [Osinski et al.
2005]), the generation of ejecta deposits is still poorly understood (e.g., Housen
& Holsapple 2011, Osinski 2007, Osinski et al. 2011, 2013). While a ballistic
sedimentation and emplacement of impact ejecta on airless bodies is general
accepted (Oberbeck 1975), new models suggesting surface flow similar to
pyroclastic flow or as impact melt-rich ground-hugging flow (e.g., Newsom et al.
1986, Osinski et al. 2004, Artemieva 2006, Meyer et al. 2008, 2011, Artemieva
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et al. 2013, Stöffler et al. 2013, Siegert et al. 2017) in addition to ballistic
emplacement are also taken into consideration.
This new and increased interest in impact ejecta and ejection processes
initiated a new and comprehensive study of the Pelarda Fm. within the last
years including new separate companion deposits the results of which are
presented here.
A more comprehensive paper on the Pelarda Fm., in particular focusing
on new geologic mapping results and showing a host of outcrop and sample
photos and stratigraphical columns, has recently been published in Spanish
language (Claudin & Ernstson 2018).
2 Previous studies - overview
2.1 Lithology
In Fig. 5, a typical aspect of a Pelarda Fm. outcrop is shown, although the
facies may change considerably as is in detail described below.
Fig. 5. A: Typical aspect of the Pelarda Fm. facies that, however, may change
considerably throughout the deposit. B: Large quartzite clast from the Pelarda Fm.
Lithologically, the Pelarda Fm. has been described as
-- a boulder conglomerate composed of mostly well-rounded clasts
(predominantly Paleozoic sediments) in an unconsolidated gravelly yellowish-
brownish matrix (Carls & Monninger 1974),
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-- a diamictite with conglomeratic intercalations in a sandy-silty-clayey matrix
(Ernstson & Claudin 1990, 2002), and
-- conglomerates and diamictites composed of rounded to subrounded quartzite
clasts eroded from the local basement and immersed in a sandy-silty-clayey
matrix apparently without any orientation and configuration (Cortés et al. 2002;
Díaz-Martínez et al. 2002 a; Díaz-Martínez 2005).
In all cases, the practical lack of or at most very poor stratification and the
distinctly bad sorting including quartzite clasts up to the size of 1 m (Díaz-
Martínez 2005) or even much larger (Fig. 5) have been pointed out (Ernstson &
Claudin 1990, 2002).
2.2 Source for the Pelarda Fm. material
As for its origin, the Pelarda Fm. deposit has as listed above been attributed to
fluvial processes, alluvial processes and meteorite impact processes (impact
ejecta).
For the models of fluvial and alluvial deposition, the source of the
material has been located in the NW (Carls & Monninger 1994) and in the S-SE,
respectively (Cortés et al. 2002; Díaz-Martínez et al. 2002 a, b, c; Díaz-Martínez
2005), while the impact emplacement model considers a source in the NE
(Ernstson & Claudin 1990, 2002; Ernstson & Fiebag 1992). The S-SE direction
suggested by Díaz-Martínez et al. (2002 a, b, c) and Díaz-Martínez (2005) lacks
any support by specific investigations, e.g., by paleocurrent studies. Carls &
Monninger (1974) deduce the NW source location from the many Bámbola
quartzite clasts that contribute to the Pelarda Fm. and are assumed to originate
from the outcropping Paleozoic Bámbola layers near Codos village (see Fig. 2).
Cortés et al. (2002) believe that the material of the Pelarda Fm. is a proximal
alluvial deposit, which developed at the base of a hypothetical massif produced
by faulting during the Upper Miocene - Pleistocene (a faulting that is said to
have affected the Calatayud- Montalbán basin). Moreover, according to Cortés
et al. (2002) the Paleozoic rocks in the region under discussion never attainted
significant altitudes until the Miocene.
In the impact ejecta model of Ernstson & Claudin (1990), the Pelarda Fm.
material is derived from NE corresponding to the center of the Azuara structure.
The authors assume that the impact affected a target composed to a
considerable amount (up to 2,000 m thickness or more) of Tertiary molasse
sediments from the Alpidic Iberian System. Ernstson & Claudin (1990) analyzed
more than 400 sets of striations on the surfaces of clasts embedded in the
Pelarda Fm. resulting in a clear NE strike accumulation. The striae are
suggested to have been formed in the latest phase of ejecta emplacement and
thus to have preserved the main direction of the ejecta trajectory (also see
below).
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The early petrographic description by Carls & Monninger (1974) noted
the evident complete lack of limestone clasts in the deposits, which was
paleogeographically related with the source for the Pelarda Fm. material.
Meanwhile, we were able to verify the occurrence of Jurassic and/or Cretacious
limestone cobbles and boulders, although in fact represented scarcely, which
will in more detail be dicussed later. In the course of our field work we were also
able to establish the existence of coherent Buntsandstein megaclast (up to the
size of 9 m) embedded in the Pelarda Fm. material otherwise not mentioned in
the literature. Opponents of the impact model (Cortés et al. 2002; Díaz-Martínez
et al. 2002 a; Díaz-Martínez 2005) consider the megaclasts to be thin
sandstone layers belonging to the (from their point of view) alluvial fan
stratigraphy.
2.3 Age of the Pelarda Fm. deposit
The ages so far proposed for the deposition of the Pelarda Fm. material are the
Paleogene (Carls & Monninger 1974; Adrover et al. 1982; Ernstson & Claudin
1990; Ernstson & Fiebag 1992; Ernstson et al. 2002; Ernstson et al. 2003;
Claudin & Ernstson 2003), the Miocene (IGME 1981) and the Quaternary (ITGE
1989; ITGE 1991; Aurell et al. 1993; Aurell 1994; Smit 2000 (written comm.);
Cortés et al. 2002; Díaz-Martínez et al. 2002 a, b, c; Díaz-Martínez 2005).
Among these authors, Cortés et al. (2000), Díaz-Martínez et al. (2002 a, b, c)
and Díaz-Martínez (2005) suggest a Pliocene or Pleistocene age and consider
the Pelarda Fm. an alluvial fan deposit (of the “raña” type) that accumulated at
the base of a former Paleozoic relief meantime fallen to denudation and today
no longer existent. Nevertheless, at the same time, they strangely enough
consider the deposit as being still under investigation and admit a Paleogene
age as proposed by Adrover (1982).
3 Previous studies - geological setting, layering, and petrographic
features
3.1 Geological setting
As noted in their paper, Ernstson & Claudin (1990) appreciated the early sound
geologic field work on the Pelarda Fm. by Carls and Monninger (1974) and
conceded that new outcrops and especially petrographic work had enabled the
development of the new impact model. Both the early work spanning the
geologic frame and the hitherto existing petrographic data are summarized in
the following.
The Pelarda Fm. is located between the Azuara structure and the
Rubielos de la Cérida impact basin and extends more or less tangentially with
respect to both (Fig. 1, Fig. 2). According to the existing geological mapping
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(ITGE 1991; ITGE 1989; IGME 1977/81) and to own field data, Paleozoic,
Mesozoic, Eocene to Lower Oligocene, Lower to Middle Miocene, and
Quaternary rocks make the contact (Fig. 6), although in the official IGME
(1977/81) and ITGE (1989, 1991) maps the Pelarda Fm. does not exist and is
instead mapped as Miocene not further itemized (IGME), and Quaternary
(ITGE). More inconsistencies between our own field work and the existing
official maps will not be considered here unless they are immediately related
with the matter under discussion.
Fig. 6. Geological general map for the region of the Pelarda Fm. 1 = Paleozoic, 2 =
Triassic, (?) Jurassic, 3 = Cretaceous, 4 = Eocene-Lower Oligocene, 5 = Miocene, 6 =
Quaternary, P = Pelarda Fm. S1 - S4 = outcrop locations examined in the new field
campaigns. S1 is preferentiallly addressed in this paper.
Paleozoic contact zone
The kilometer-sized Paleozoic complexes are exposed near Olalla and Collados
(Fig. 6, 7), constituting there the "footwall" of the Pelarda Fm. According to
Monninger (1973) and ITGE (1991), the Embid, Jiloca, Ribota, Huérmeda,
Daroca, Valdemiedes, Murero and Almunia Formations contribute to the
stratigraphy, where lithologically slates, sandstones, dolomites, limestones,
quartzites and calcareous siltstones dominate. Especially the rocks of the
Valdemiedes and Murero Fms. are heavily fractured through and through
exhibiting grit brecciation and mortar texture. Here, Monninger (1974) mapped
extensive bands of "mylonites" (his designation; Fig. 8) striking 130°.
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Fig. 7. The contact between the Pelarda F. and the Paleozoic unit is rather diffuse,
because the Quaternary debris from weathering of the Pelarda layers are moving
downhill.
Fig. 8. Monomictic movement breccias ("mylonites", Monninger [1973]) - Paleozoic near
the contact to the Pelarda Fm. These "monomictic movement breccias" also refer to
strongest earthquakes and massive landslides of a strength and character that have not
been described for Alpine movements in the region (and also elsewhere).
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These "mylonites" (we prefer the term monomictic movement breccias (see the
discussion by Reiff [1978]) together with abundant breccia dikes and a suevitic
breccia (Fig. 9) adjacent to the Pelarda Fm. outcrops near Olalla (UTM
coordinates 0656513 E/4539035 N) are now interpreted as originating from the
proposed Azuara/Rubielos de la Cérida impact event and are in more detail
described and discussed in Ernstson et al. (2002) and Claudin & Ernstson
(2012).
Fig. 9. From the Paleozoic - Pelarda Fm. contact zone: Sawed and polished surface of a
polymictic suevite breccia exhibiting flow and breccia-within-breccia texture. Coin
diameter 18 mm.
Mesozoic contact zone
The Mesozoic of the contact is represented by Muschelkalk, Keuper and
(?)Jurassic rocks). Like the Paleozoic rocks, the Mesozoic rocks are intensively
fractured and heavily brecciated (Fig. 10). The Muschelkalk limestones and
dolomites partly overlain by the upper part of the Pelarda Fm. are abundantly
exhibiting mortar texture and hosting impact breccia dikes (Ernstson et al.
2002). Among these breccia dikes, accretionary lapilli have been established to
contribute to the dike material and in a few cases to make the breccia matrix in
form of lapillistone (Fig. 11). Quartz grains, limestone fragments, fragments of
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Paleozoic metamorphic rocks and accrecionary grains can form the nucleus of
the lapilli.
Fig. 10. Large volumes of pervasivly crushed Muschelkalk limestones near the contact to
the Pelarda Fm.
Fig. 11. From the Muschelkalk - Pelarda Fm. contact zone: Dike breccia composed of
Muschelkalk fragments (dark; coin diameter 16 mm) in accretionary lapilli matrix
(lapillistone; detail to the right). The lapillistone occurence will be addressed also later.
The contact between the Pelarda Fm. and the Paleozoic/Mesozoic (see 4.2.4
Contacts; Fig. 52) is in part masked by an extensive deposit of an
unconsolidated breccia composed of angular, exclusively Paleozoic clasts. The
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portion of a shaly-sandy matrix of reddish-brownish color is poor. The breccia
has already been mapped and described ("rim breccia") by Monninger (1973)
who conceded that origin and age of the deposit were problematic. From the
comparison with dislocated (allochthonous) megablocks exposed in the Azuara
impact structure and in part exhibiting a very similar facies (Ernstson & Fiebag
1992), an origin from impact excavation, ejection and emplacement must be
considered. The immediate contact with the Pelarda Fm. and the basal breccia
(suevite breccia; see Fig. 5) obviously overlying the rim breccia substantiates
this interpretation. In the official geological maps (ITGE, IGME) the rim breccia
is attributed to the Miocene. The rim breccia is in more detail discussed in 4.2.4,
Lower Tertiary contact zone
Conglomerates, claystones, sandstones, siltstones and levels of
conglomerates, red clays, charophyte limestones and marls are represented in
the Upper Eocene - Lower Oligocene stratigraphy. Near Fonfría (Fig. 12), beds
of conglomerates of the Eocene-Oligocene are exposed immediately below the
base of the Pelarda Fm., and, without exception, the limestone pebbles and
cobbles exhibit peculiar surface deformations in the form of striations, polish
and remarkable imprints (Fig. 13). It is suggested that the plastic deformations
within the conglomerates have originated from the highly energetic
emplacement of the landing Pelarda Fm. ejecta.
Fig. 12. Lower Eocene-Oligocene conglomerates near the contact to the base of the
Pelarda Fm. ejecta.
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Fig. 13. Fom the Eocene/Oligocene - Pelarda Fm. contact zone: limestone cobbles from a
conglomeratic bed with imprints, heavy striations and polish all around.
In the zone of Salcedillo (Fig. 6) the conglomeratic levels are polygenetic, occur
as lenticular bodies with thicknesses not exceeding 1.5 m, and are intercalated
within sandy layers. No plastic deformations of the components like those in the
Fonfría conglomerates can be observed here.
Upper Tertiary contact zone
According to ITGE (1991) and own field work, the Lower to Middle Miocene is
built up of red clays, sands and conglomerates. In the environs of Olalla, the
Miocene conglomerates as mapped in the existing cartography (ITGE 1991)
prove in many cases to be in fact breccias. These breccias are polygenetic,
heterometric and matrix-supported. They form transversally oriented lenticular
16
bodies displaying variable but in general small lateral extension (less than 5 m).
Longitudinally, the breccia bodies are drop-to-tongue-shaped and elongated in
the direction of dip being some 30-35° towards SW. The age of the breccias is
left to assumptions.
Quaternary
The Quaternary is composed basically of material derived from weathering and
erosion of the Pelarda Fm. The contact to the underlying stratigraphical units is
always discordant implying more or less zero dip of the Quaternary.
3.2 Layering
According to the early general field work of Ernstson & Claudin (1990), the
Pelarda Fm. deposit shows a very rough stratification into three parts. The
contacts between the lower, middle and upper zones prove to be gradual and
not anywhere near traceable. On the whole, the zones differ by clast lithology,
clast size and shape, and matrix composition. A matrix-supported texture is
largely to be observed. Only locally developed bedding planes enabled the
measurement of strike and dip with a southwest and northeast dip preference
(also see below and Fig. 21).
3.3 Petrographic features
Shock metamorphism
The petrographic work on the Pelarda Fm. has in the past concentrated on
shock-metamorphic microscopic deformations and on mesoscopic deformations
of clasts. Pelarda Fm. shock effects in the form of multiple sets of planar
deformation features (PDFs; Figs. 14, 15), multiple sets of planar fractures
(PFs, Fig. 17), mosaicism and kink bands in quartz have been reported in
Ernstson & Claudin (1990), Ernstson & Fiebag (1992), Ernstson et al. (2002)
and Claudin & Ernstson 2003). The shock-metamorphic PDFs in Armorican and
Bámbola quartzites from the Pelarda Fm. were unambiguously confirmed in
analyses perfomed by Guerrero (2000) and Therriault (2000) (Fig. 16).
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Fig. 14. Decorated planar deformation features (PDFs) in quartz from the Pelarda Fm.
Photomicrograph, crossed nicols. The crystallographical orientations of the sets are
{10-13} and {10-12}. The field is 200 µm wide. Analysis and image: Guerrero (2000).
Fig. 15. SEM image of two sets of crossing PDFs in quartz; shocked Bámbola quartzite
clast from the Pelarda Fm.
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Fig. 16. Frequency diagram of crystallographic orientation of planar deformation features
(PDFs) in quartz from the Pelarda Fm. Data from Therriault (2000).
In Fig. 16, a frequency diagram of crystallographic orientation of PDFs from
Pelarda Fm. rocks is shown. The prevailing ω and, subordinately, π planes
suggest shock pressures exceeding 10 GPa (= 100 kbar) (Stöffler &
Langenhorst 1994, Grieve et al. 1996). Other parameters such as PDF density,
sharpness, spacing, and spreading over the grain were additonally analyzed
(Therriault 2000): Up to three sets of PDFs per grain were found in the Pelarda
Fm. quartz grains. 87.5 % of all sets exhibit a PDF spacing < 1 µm, 12.5 %
between 1 - 5 µm. The spreading over the grain is in most cases 100%, and the
PDF density always high. Practically all sets are decorated. All shocked grains
have reduced birefringence of 0,004 - 0,008. The prevailing {10-13}, ω, and {10-
12}, π, PDF orientations in the shocked samples from the Azuara structure are
unusual considering the sedimentary (porous) target in which {11-22}, ξ, and
{10-11}, r,z, directions commonly are more typical (Stöffler et al. 1994, Grieve et
al. 1996). The "crystalline" signature of the Azuara PDFs, however, may be
explained by the lithology of the dense shock-affected quartzite clasts. Cortés et
al. (2002), Díaz-Martínez et al. (2002) and Díaz-Martínez (2005) basically
denying any impact evidence don't take the shock effects as being existent.
19
Fig. 17. Multiple sets of planar fractures (PFs) in quartz. Bámbola quartzite cobble,
Perlarda Fm.
Mesoscopic deformations
Mesoscopic impact evidence in the Pelarda Fm. deposits is revealed by the
abundant occurrence of heavily deformed cobbles and boulders exhibiting
striations (Fig. 18, 19) and polish even on the surfaces of quartzite clasts,
rotated fractures and irregular fractures with complex bifurcations (Fig. 20).
Fig. 18. Pelarda formation: large striated quartzite boulders. Science may yield
curiosities: When the impact ejecta origin for the Pelarda formation had been suggested
and an article (Ernstson & Claudin 1990) was printed showing exactly this photo of the
heavily striated quartzite boulders, these heavyweight objects resting near a drop-off had
disappeared only shortly after and were never seen again.
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Fig. 19. Multiple sets of striae on quartzite cobbles from the Pelarda Fm. The field is 2.5
cm wide.
Fig. 20. Heavily deformed however coherent Armorican quartzite boulders from the
Pelarda Fm.
21
Being embedded in a soft, unconsolidated matrix, these deformations of
coherent cobbles and boulders are in proof of high confining pressure and
short-term deformation upon applied stress compatible with highly energetic
excavation, ejection and emplacement processes in the various stages of
impact cratering (Melosh 1989). As has been suggested earlier (Ernstson &
Claudín 1990), the Pelarda Fm. ejecta emplacement is reflected by a
preferential orientation of striae pointing to the center of the Azuara impact
structure (Fig. 21).
Fig. 21. Pelarda Formation: Equal area plot of the normals to locally developed bedding
planes and rose diagram of striae azimuth. Note the correlation of both distributions. The
arrow points to the center of the Azuara impact structure. Modified from Ernstson &
Claudin (1990).
Opponents of an impact scenario and advocates of an alluvial fan deposition
(Cortés et al. 2002; Díaz-Martínez et al. 2002 a, Díaz-Martínez 2005) explain
the peculiar in situ deformations by tectonic forces thus requiring an important
local tectonic phase in the Quaternary otherwise nowhere evident, however.
Moreover with regard to the soft unconsolidated matrix, the heavily broken
however coherent cobbles like those shown in Fig. 20 cannot possibly be
explained fracture-mechanically by tectonics.
Impact spallation
Abundantly, quartzite boulders show a typical fracturing that can be ascribed to
shock spallation. Spallation is a well-known process in fracture mechanics as well
22
as in impact cratering and has been investigated theoretically and experimentally
by many researchers. Unfortunately, it is less well known that spallation can also
be observed in nature as an actually existing geologic phenomenon in and around
impact structures. Spallation takes place when a compressive shock pulse
impinges on a free surface or boundary of material with reduced impedance (= the
product of density and sound velocity) where it is reflected as a rarefaction pulse.
The reflected tensile stresses lead to detachment of a spall or series of spalls.
Prominent spallation effects have been reported for shocked Buntsandstein
conglomerates exposed around the Azuara/Rubielos de la Cérida impact
structures. Details about these geologic spallation features have been described in
Ernstson et al. (2001) and Ernstson (2014). In impact research, spallation, which as
a shock effect can be observered down to microscopic scale in shocked quartz
grains, is largely ignored, and also in recent relevant literature (e.g. French and
Koeberl 2010) spallation as an important shock indicator is not even mentioned.
Fig. 22. The peculiar concave fracture of a quartzite boulder in the field of the Pelarda
formation deposit is explained by dynamic shock spallation. Typically and for
geometrical reasons, the concave fracture plane mirrors the original convex boulder
surface (as shown dashed in the left photo). This typical shock spallation effect is
abundantly observed in the field of the Pelarda formation ejecta.
4 New investigations
Now as before, the outcrop conditions in the large forest area corresponding
with the Pelarda Fm. deposit are poor. Although a ramified road network exists
enabling easy access, the roads are only sporadically carving the ground, and
Quaternary debris are broadly curtaining the original layering and stratigraphy.
Therefore in the past, the sidewalls of the road between Fonfría and Olalla more
or less traversing the Pelarda Fm. deposit (Fig. 6) gave the most instructive
insight. The new investigations are mainly based on a further opportunity to
study the Pelarda Fm. in more detail in sections along a cartway starting in the
village of Salcedillo and also traversing the deposit more or less perpendicularly
(Fig. 6).
23
4.1 Methodology
The present new study of the Pelarda Fm. is based on the following
methodological proceeding:
-- re-examination of the hitherto existing field data
-- realization of stratigraphical columns as far as enabled by the outcrop
conditions, and possible correlations,
-- sampling and analysis of more than 150 clasts from the diamictic levels;
measurement of roundness and sphericity according to the visual criteria of
Powers (1953) and Krumbein & Sloss (1955),
-- petrographic characterization as well as analysis of 35 thin sections of the
collected rudites,
-- sampling and petrographic analysis including 25 thin sections of sandstones
intercalated between the diamictic leves as well as from isolated levels,
-- analyses of the sedimentary structures and identified facies,
-- analysis of paleocurrents as far as permitted by the outcrop conditions, and
-- sampling of fossils from accessible levels for dating purposes.
4.2 Investigated locations
In the following we consider four locations of differing importance. They are
designated and marked as S1, S2, S3 and S4 in the map of Fig. 6, and their
general stratigraphic position is shown in Fig. 23. The Salcedillo location no. S1,
because of its extension and the possibility to study the contacts between
different units, has supplied the crucial host of new data enabling the
construction of stratigraphic columns (Claudin & Ernstson 2018). S2, separated
from S1 by a long section of the cartway without exposures (Fig. 23), supplies
some insight into the upper part of the Pelarda Fm. S3, covering the lower and
middle part along the road from Fonfría to Olalla (Figs. 6, 23), serves for
comparison with the Salcedillo outcrops. S4 eventually also sheds some light on
the layering conditions in the upper part of the Pelarda Fm.
24
Fig. 23. The investigated locations in their general stratigraphic position. For details see
the following text.
4.2.1 Salcedillo S1
The outcrop S1 of the Salcedillo zone, easily accessible along the way from
Salcedillo village in a SW direction, has been divided into three parts, Salcedillo
Lower, Middle, and Upper S1. Middle S1 and Upper S1 are separated by a
segment lacking any significant outcrops (see Fig. 23).
Salcedillo Lower S 1
Lower S1 starts at UTM 30667751 E / 4536032 N (c. 1200m altitude) with a first
series of rudites (in the broadest sense) of Pelarda Fm. facies (Fig. 24). As for
the term "rudites" we in the following will avoid this very general, somewhat
diffuse and no longer often used name, and will instead speak of the more
current diamictite (adjective: diamictic) terminology (Flint 1960 a, b) much
better characterizing most volumes of the Pelarda Fm.
25
Fig. 24. Typical aspect of the Pelarda Fm. facies in the Salcedillo Lower S 1 part.
Heterometric and polygenetic clasts in a sandy matrix.
The contact with the underlying Upper Eocene - Oligocene is concordant
however erosive (Fig. 25). At a thickness of some 12 m the diamictites series
exhibits various sandstone levels and lenses (Fig. 26) intercalated and a
sandstone level at the top. The sequence shows a slight grading tendency with
grain fining upwards, although among the individual clasts significant variations
of grain sizes are observed.
Fig. 25. Contact between the Pelarda Fm. and the Tertiary. Salcedillo S 1 Lower.
26
Fig. 26. Intercalated sandstone lenses within in the Salcedillo S 1 Lower diamictites.
Altogether, the diamictites are (Fig. 24)
-- polygenetic including Bámbola and Armorican quartzites, shales, schists and
slates. The clast morphology varies from subangular to subrounded generally
implying a low to medium sphericity.
-- heterometric with clast sizes oscillating between 1 and 35-40 cm.
-- matrix supported. Although there are scarce zones with clast-supported
texture, the unit is dominated by a sandy-clayey matrix the sand having grain
size between fine and coarse and compositionally being similar to the
intercalated sandy levels.
Macroscopic superficial features of the clasts include subparallel open (tensile)
fractures, irregular fractures with complex bifurcations (Fig. 27), and striations.
Microscopically, planar features in quartz like planar deformation features,
PDFs, and planar fracture, PFs, are regularly observed (Fig. 28).
27
Fig. 27. Quartzite clasts with open parallel (left) and irregular, bifurcating fractures.
Fig. 28. Shocked quartz from the Pelarda Fm., Salcedillo S 1 Lower. Planar deformation
features (PDF) and strongly kinked quartz grain with PDF.
The color of the Salcedillo S 1 unit is between reddish and yellowish-orange
corresponding to the color of the underlying marly Upper Eocene-Oligocene
material thus suggesting a contribution of that material to the finest fraction of
the matrix of the Pelarda Fm.
In general, a certain stratification is defined by the orientation of
elongated clasts (SW general dip; Fig. 29), although a chaotic layering can also
be frequently observed.
28
Fig. 29. Elongated adjusted clasts Indicate some layering.
The sandstone leves, intercalated in the diamictic material are heterometric
lithoarenites (Folk 1968) composed of quartz, schist, quartzite and slate
particles with grain sizes between fine and coarse. At medium sphericity, the
grain morphology varies between angular (quartz and quartzite grains) and
subangular (metamorphic grains). No magnetic particles (magnet and binocular
applied) were found.
The sandstone level in the top also corresponds to a heterometric and
polygenetic (quartz, schist, quartzite, slate) lithoarenite (Folk 1968). Only
contrasting in sphericity (medium to low) and displaying some parting lineation,
the top level has the same facies as have the intercalated lithoarenites
described before.
Salcedillo Intermediate S1
Between Salcedillo S1 Lower and Upper, there is a powerful silty-sandy section
about 70 m thick. It is basically made up of shales with heterometric grains of
quartz, quartzite (Bámbola and Armorican), and metamorphic rocks (slate and
schist). The morphology of quartz grains is angular or very angular, while that of
the rest is subangular. The sandy levels correspond compositionally to
heterometric lithoarenites (Folk, 1968), with the same components of the larger
limolites. Their morphology is similar to that of the silty-sandy levels.
Between the set of shales and sands, at the base and close to Salcedillo
S 1 Inferior, a level of lenticular morphology, with a maximum thickness of 2 m
and with cross bedding of low angle (Fig. 30) occurs. It contains heterometric
clasts of Bámbola and Armorican quartzite and metamorphic rocks immersed in
29
a sandy matrix. The maximum size of the clasts does not exceed 40 cm. The
contact of this level with the underlying sandstones is clearly erosive.
Fig. 30. Aspect of the middle zone. The contact with sandy sections (in white), clearly
erosive, has been highlighted. The dip at this level is roughly 140/35 SE. Paleocurrents,
where they have been measured, indicate a contribution from N - NE.
In addition to this diamictitic layer, there are two levels - one nearby (Fig 31)
and another close to the contact with Salcedillo S 1 Upper - with a limestone
appearance and slightly reddish tones (Fig. 32).
Fig. 31. The first calcareous level interspersed between the sandy materials. It has a
lenticular morphology and its dip is about 140/35 SW.
30
Fig. 32. Detail of the "carbonate" level in Fig. 31. The yellowish zones correspond to
strongly altered slate clasts.
Salcedillo Upper S1
This some 26 m thick series is made up of an alternating sequence of diamictic
and sandy levels totaling nine and each grain-fining upwards. It is overlying the
intermediate lithoarenite unit (Fig. 33) and covered by a few meters thick
Quaternary layer (Fig. 34).
Fig. 33. The initial diamictite level of the Salcedillo S 1 Upper series is placed in erosive
contact on the fine-medium grain lithoarenites of the Middle S 1 part. Note the
accumulation of large clasts near the contact.
31
Fig. 34. The Quaternary, with horizontal or subhorizontal dip (< 5º), lies in discordant
contact with the Pelarda Fm. Its thickness never exceeds 8 m.
Each level is in general composed of a diamictic basal part (dia) and a sandy
upper part (ar; see Fig. 35). In the dia part, also sand bodies may be
intercalated.
32
Fig. 35. (upper and lower Figs.). Levels of sandstones (ar) and diamictites (dia) in
Salcedillo S 1 Upper. In the diamictic part there is a certain tendency to a parallel
arrangement to the stratification of the elongated clasts. In some points a certain
incipient imbrication can be observed. cu = Quaternary.
The diamictic basal parts are
-- polygenetic (Bámbola and Armorican quartzite clasts, shales, schists,
Permotriassic quartzarenites (Buntsandstein Fm.) and other quartzarenites,
slates). The clast morphology varies from subangular to subrounded implying a
low to intermediate sphericity.
-- heterometric with clast sizes between 1 and 60 cm.
-- matrix supported, although there are subordinate zones exhibiting clast-
supported texture. In the sandy-silty matrix the sand has grain sizes between
fine and coarse and regarding composition is similar to the sandy levels.
The color tends to reddish due to quartzarenitic Buntsandstein material
contributing to the deposit. In general a certain stratification is defined by the
parallel disposition of the elongated clasts (Fig. 35), although there are points
where it is chaotic. The general dip of the materials, where it can be measured,
oscillates between about 135/40 SW and 145/30 SW. The direction of the
paleocurrents indicates an origin of the N-NE.
The clasts may show rotated fractures (Fig. 36) (Ernstson & Claudin
1990), superficial striations (Fig. 37), irregular fractures with complex
bifurcations (Fig. 38), and occasionally an intense polish (Fig. 39). A
33
measurement of the strike of the striae sets gives a preferential SW - NE
direction identical to that published by Ernstson & Claudin (1990).
Fig. 36. Rotated fractures in a Bámbola quartzite.
Fig. 37. A phyllite clast with multiple sets of striae.
34
Fig. 38 The irregular fractures and the coherence of the clast show that the rupture must
have occurred in situ under high pressure, as a subsequent transport would have
disassembled the sample.
Fig. 39. Bámbola quartzite clast with intense polish.
Under the microscope, thin sections of Bámbola and Armorican quartzite clasts
exhibit impact metamorphic (shock) features (both PFs and PDFs).
The upper sandstone part (Fig. 35), where it has been sampled
corresponds compositionally to a heterometric lithoarenite (ranging from fine to
35
coarse sand), with grains of quartzite, schist, phyllite, slate and quartz.
Morphologically these grains range from angular (most quartz) to subangular
(metamorphic rocks), presenting a low sphericity. No magnetic particles were
found in any of the samples analyzed. The presence of parting lineation can be
mentioned as a remarkable sedimentary structure.
A notable aspect in this section is the presence of pseudotectonic
deformations, specifically tension faults that affect certain sections (sandy or
diamictic parts), but without affecting all the lower or superjacent parts (Fig. 40).
Fig. 40. Pseudotectonic tension faults affect the sandy part just in front of W. Monninger,
and the lower part of the next diamictic level, which however, "fossilizes" the fault by the
basal clasts.
The field observations show that the mechanism is syndepositional,
corresponding to a rapid sequence of erosion, sedimentation, faulting and flow
within a limited unit. This type of deformation cannot be explained by "normal"
geological forces, and in this particular case we refer to very similar
observations in the large Rubielos de la Cérida impact basin accompanying the
Azuara structure (Fig. 41) (http://www.impact-structures.com/2011/12/cutting-
into-an-impact-crater-rim-excavation-and-modification-signature-of-the-impact-
cratering-process/). Such a process (Fig. 41) is understood only by the complex
movements of impact excavation, modification and location of ejecta with
permanently varying strong stress and in a short period of time, probably
supported by the action of shock-produced water and volatiles, which
underlines the impact relationship of the Pelarda Formation. Fig. 42 shows a
very simple model of this so-called stop-and-go deformation.
36
Fig. 41. Stop-and-go deformations in the Pelarda Fm. compared to very similar
deformations in the Rubielos de la Cérida impact basin (Barrachina megabrecha, top
right, and southeast rim of the basin northeast of Teruel) that emphasize this typical
impact process.
Fig. 42. Simple model for the pseudotectonic process with multiple phases typical of
impact.
37
Salcedillo S 1, Quaternary
Wherever ithe Quaternary is present, it is easily identified, since its dip is
practical horizontal and overlaps the aforementioned materials discordantly
(Figs. 35, 35). The thickness of this cover, in the areas analysed, does not
exceed 15 m in any case. Its upper part presents somewhat darker colors, due
in most cases to the presence of organic matter.
4.2.2 Salcedillo S 2
Continuing along the path that goes from the village of Salcedillo to S 2 (at
coordinates 30665781 E / 4533873 N (1283 m altitude), the materials that
make up the upper part of the Pelarda Fm. are no longer exposed in outcrops.
The study of the lithology is enabled by the farmers' ploughing of the fields
(Figs. 43, 44). Evidently these materials correspond to the Pelarda Fm., since
the Miocene is formed, in this zone, exclusively by breccias of Paleozoic and
shale clasts (IGME, 1977; ITGE, 1989 and 1991; own field observations).
Fig. 43. Materials of the upper part of the Pelarda Formation (S 2). Buntsandstein,
Bámbola quartzite, Armorican quartzite and Eocene limestones and conglomerates can
be sampled in the field.
38
Fig. 44. Among the "waste materials", basically clasts that bother field ploughing, a clast
of Eocene-Oligocene conglomerates (next to the GPS locator) as well as clasts of
Bámbola quartzite, Armorican quartzite, Buntsandstein quartzite and Jurassic limestones
occur.
Continuing the same path and before reaching a fork, increasing prevalence of
limestone clasts changes the coloring of the fields from reddish-brown (typical
of the Pelarda Fm.) to whitish-brown. After the bifurcation and according to the
ITGE cartography of 1991 Miocene materials begin to appear.
4.2.3 Road from Olalla to Fonfría (S 3 and S 4 in Fig. 6 and Fig. 23)
The aspects of the S 3 and S 4 outcrops along the road between Fonfría and
Olalla do not change only with altitude (see Fig. 23) but also due to its bendy
course from roughly parallel to perpendicular of the strike/dip of the Pelarda Fm.
S 3 appears as a massive and diamictic aspect of the Pelarda Fm.
between Fonfría, the height of Pelarda and the first 2 km of descent towards
Olalla (Fig. 45).
39
Fig. 45. Massive and diamictite-like aspect of the Pelarda Fm. along S 3. A Buntsandstein
megaclast, about 9 m long, is interspersed between the materials. It should be
remembered that the reddish coloration of the matrix is due to the contributions of
Buntsandstein materials.
Following the road descent towards Olalla (S 4), the depositional characteristics
become more similar to those observed in Salcedillo (Figs. 46, 47). Moreover
advancing towards the roof of the Formation in section S 4, blocks of Bámbola
quartzite bigger than 1 m (see Fig. 5, Fig. 18) with grooves and striations on its
surface occur. As in Salcedillo, the clasts of the diamictic sections present
rotated fractures, striae on their surface, open parallel fractures and irregular
fractures with complex bifurcations. All these characteristics, already described
by Ernstson & Claudin (1990), are indicative of the action of intense confining
pressure at the time of deposition.
Fig. 46. Appearance of the Pelarda Fm. in the outcrops of zone 4. A sandy layer (ar) (with
diamictite intercalations) is interspersed within a diamictic section (dia). The dip of the
materials in this zone is 26º towards SW. qu = Quaternary.
40
Fig. 47. Another aspect of a diamictic part along S 4 with clasts of Armorican quartzite,
metamorphic rocks and Bámbola quartzite, immersed in a sandy-loamy matrix.
A remarkable depositional situation is met in the course of S 4, in front of the
Sanctuary of Pelarda, where an extended outcrop of Buntsandstein in the
middle of the materials of the Pelarda Fm. was mapped by Monninger (1973) as
an inverted megablock (Fig. 48, 49), indicated by inverted crossbedding.
Fig. 48. The Buntsandstein block in the center of the Pelarda Formation can be
recognised by its characteristic color and can therefore also be mapped in aerial
photography. According to Monninger (1973) the Buntsandstein block is inverted. Aerial
photography Apple Maps.
41
Fig. 49. Outcrop of the inverted Buntsandstein megablock that was probably transported
by the Pelarda Fm. The extension of this zone is more than 300 meters (Fig. 48).
Sampling in the field reveals Buntsandstein clasts with rotated fractures and
impact marks (Figs. 50 and 51).
Fig. 50. Buntsandstein clast with impact collision marks (not pressure dissolution; see
http://impacto.impact-structures.com/?page_id=1431) on its surface.
42
Fig. 51. Buntsandstein clast with rotated fractures but having remained coherent.
Altogether, moving from Fonfría to Ollala the complete sequence of the Pelarda
Fm. stratigraphy can be traversed.
4.2.4 Contacts
The rim breccia
The Pelarda Formation ends up in contact with a new formation that had
originally been mapped by Monninger (1973) and which he called the rim
breccia (rim related to the Pelarda Formation). He divided it into a P rim breccia
with Paleozoic components (mainly Cambrian) and an M rim breccia with
Mesozoic components. Its distribution was mapped by Monninger (1973) as a
strip extending in front of the mighty Olalla block (Claudin & Ernstson (2012) of
Cambrian and Mesozoic rocks. The P rim breccia (Fig. 52) is formed exclusively
by Paleozoic components of clayey-loamy composition with transition to
quartzite sandstones and quartzite that dominate towards the road (section S 4
of our Pelarda Fm. research). The components are completely angular, a
cement is missing, and the matrix contribution is very low (Fig. 53). Armorican
quartzite components may have rotational fractures (Figs. 54), although no
striations have been observed on their surface, possibly due to the granular
surface of the quartzite. The M rim breccia with a similar texture was not
included in our investigations, although the brecciated limestone blocks up to 1
m in size described by Monninger (1973) (Fig. 55) are also obvious. Monninger
could not give a satisfactory interpretation of the origin of this breccia.
In our opinion and considering the peculiar facies and the sedimentary
environment (Figs. 52-58) the rim breccia is also part of the Azuara impact
ejecta.
43
Fig. 52. The rim breccia (Monninger 1973) located above the upper end of the Pelarda
Fm., and in contact with the basal suevite breccia (Fig. 56-58).
Fig. 53 Detail of the P rim breccia. Photo taken at the road ahead of Olalla.
44
Fig. 54. Rotational fracture in a quartzite clast of the rim breccia under which the Pelarda
Fm. ends. There is no fracture in the rear part.
45
Fig. 55. A large block of heavily brecciated limestone exposed in the transition zone from
the rim breccia to the basal suevite breccia.
Fig. 56. Suevite breccia in contact with the rim breccia.
46
Fig. 57. Transition from the Paleozoic rim breccia to the basal suevitic breccia. Such a
transition from the rim breccia deposit to the basal breccia with components originating
from both the rim breccia P and M appears to be characteristic (Fig. 58).
Fig. 58. Basal suevitic breccias with dominant Mesozoic components (Keuper? on the
left, Muschelkalk? on the right) exposed between the rim breccia and the Olalla block
(Ernstson and Claudin 2012 URL).
General contacts (also see 3.1 Geologic setting)
Field data establish that the Pelarda Fm
a. overlies the Paleozoic and Mesozoic materials in discordant contact (Fig. 59).
This contact can be seen by following the section of the road which descends
from the Pelarda top towards Olalla.
47
Fig. 59. The Pelarda Fm. superimposes the materials of the Paleozoic (upper right of the
photograph) and those of the Mesozoic (Muschelkalk limestones intruded by the lapilli
dike (see Figs. 10, 11).
b. overlies Eocene-Oligocene materials at Fonfría (Fig. 12) and by erosive
contact in the zone of Salcedillo (see Fig. 25).
c. underlies Miocene and Quaternary materials in discordant contacts. The
contact with the Quaternary (Figs. 34, 35, 41 and 46) is easily visible in the
outcrops of Salcedillo and those located in part 4 of Fonfría-Olalla. As for the
contact with the materials of the Miocene, we refer to nº 63 in ITGE (1991) and
Tc33-1 A-Bb in IGME (1977) where the Pelarda Formation underlies the
materials dated as "Miocene". At coordinates 30655445E / 4536964 N (next to
the river towards Fonfría-Olalla, near Olalla), curiously some of these materials
dated as Miocene and in discordant contact with intensively folded Palaeozoic
(Fig. 60) are actually a breccia of Palaeozoic fragments (quartzites and shales),
heterometric and polygenetic. In the clasts rotated fractures and deformations
due to low crushing can be observed (Figs. 61).
48
Fig. 60. Contact (white line) between the Paleozoic and materials dated as Miocene by
IGME (1977) and ITGE (1991). The red line marks the limit between an upper microbreccia
a breccia below.
Fig. 61. Rotated fracture in a clast from the breccia in Fig. 67.
4.3. Granulometric and textural analyses
Studies on roundness and sphericity, according to the visual criteria of Powers
(1953) and Krumbein and Sloss (1955), have been carried out on more than
150 clasts collected in the outcrops visited. In the context of this paper the
results are of minor importance but may be addressed in Claudin & Ernstson
2018.
49
4.4 Petrographic analyses of thin sections
As mentioned in the chapter on the method used, a total of 35 thin sections of
clasts of the conglomerate sections were analysed, plus 25 corresponding to
the different lithoarenites of the sections carried out. The analyses covered
a. the lithology and microtexture of sandstones (see 4.3),
b. the presence or absence of impact (shock) metamorphism,
c. the search for fossils in the fine-grained "carbonate levels".
Since the analysis (a) was already dealt with in Section 4.3 and fossils could not
be clearly identified (c), the discussion is limited to (b) shock metamorphism.
Shock metamorphism (shock effects)
As for the presence of features attributable to impact metamorphism, we first
refer to the analyses already carried out on samples of the formation from the
outcrops between Fonfria and Olalla. The results have been published in
Ernstson & Claudin (1990), Ernstson et al. (2002) and in http://estructuras-de-
impacto.impact-structures.com/?page_id=123. A selection of these earlier
observations is compiled in Fig. 62. This includes the comprehensive and
careful analysis of the PDFs conducted in the Canadian Geological Survey by
Dr. Ann Therriault (2000) (Fig. 63).
Fig. 62. Intersection of multiple sets of planar deformations (PDFs and PFs) in Pelarda
Formation quartzite from previous research (Ernstson & Claudin 1990, Ernstson et al.
2002). Photo upper left: Polarization microscope, crossed polarizers (the image is 200 µm
wide); right: scanning electron microscope, the PDF spacing is in part less than 1 µm.
Bottom left image: two intersecting sets of PDF (image width 220 µm); bottom right
image: multiple sets of planar fractures (PFs) (image width 440 µm).
50
Fig. 63. Frequency diagram of the crystallographic orientation of planar deformation
features (PDFs) in quartz of the Pelarda Formation. Data from Therriault (2000 ).
With regard to the results of the new analyses, they are in line with those
previously carried out. In other words, clear shock metamorphism has been
found in the case of of Salcedillo (Salcedillo S 1, intermediate section, and
Salcedillo S 2) clasts and in clasts from the S4 section. Planar features, both
PDFs and PFs, are present in most of the quartz of the various Pelarda Fm.
materials (Figs. 64, 65, 66). Identification criteria were:
-- first observation under the polarizing microscope to visualize planar structures
probably related to shock.
-- second universal stage measurements of their orientation with respect to
crystallographic planes (base c, ω, π and r).
Fig. 64. Different aspects of PDFs in shocked quartz of quartzites from different locations
within the Pelarda Formation. In many cases, there are several PDF systems crossing
each other. Right: also considered a shock effect in quartz of the Pelarda Formation:
strong kink banding with PDFs following the bands. Crossed polarizers; the images are
200 - 300 µm wide each.
51
The criteria set out in French (1998) were used for previewing under a
microscope. In the specific case of planar structures, among which are planar
fractures (PFs) and planar deformation features (PDFs), these criteria indicate:
1. PDFs can be distinguished from cleavage and Böhm lamellae (tectonic
deformation lamellae) by their width, spacing and crystallographic orientation.
Cleavage consists of relatively wide (>10 µm) and widely spaced ( 20 µm)
open fractures. The deformation lamellae width ranges from 10-20 µm and they
have a spacing > 10 µm. They also show optical disorientation with respect to
the grain they affect. On the contrary, PDF lamellae in quartz are highly
deformed or amorphous, more or less straight (they can be curved (Trepmann
& Spray, 2004, Ernstson http://www.impact-structures.com/2011/12/are-bent-
planar-deformation-features-pdfs-no-pdfs/), which are parallel to certain
crystallographic planes of the crystal they affect. Their width is small (2-3 µm
and less) and the spacing is between 2-10 µm or less (see Fig. 15).
2. PFs are sets of parallel fractures or cleavages with a thickness ranging
from 5-10 µm and a spacing of 15-20 µm or more. Occasionally, planar features
show typical PDF widths and are spaced between PDFs and PFs (Addison et
al., 2005), the crystallographic orientation being essential for differentiation.
3. The rest of microscopic deformations produced by shock (kink bands,
diaplectic glass, selective melting of minerals, ballen structures, toasted quartz),
are pervasive at a centimeter scale and are developed erratically and may
appear mineralogically and even grain selective.
4. While PDFs are considered indicative of impact alone, PFs are not, as
they can be produced by very strong tectonic deformation. However, the
development of a large number of planar fractures, widely distributed and with a
small spacing, is considered as indicative of shock. In addition, multiple PF
systems in a quartz grain are now considered shock and impact diagnostics
(French & Koeberl 2010 ).
Fig. 65. Left: Curved PDFs that follow the quartz grain deformation (undulatory
extinction, deformation sheets, kink bands). Right: Two PDF systems cross at a small
52
angle and appear to produce the image of curved PDFs. Quartz from Bámbola quartzite
clasts (PDFKS2 BA-B samples).
Fig. 66. PDFs in a quartz grain from a Bámbola quartzite clast (PDFKS2 BA-B). Unlike the
PDFs in Fig. 65, the PDFs appear very marked and narrow and extend along the whole
grain. The difference is reasonably explained by a deformation of the quartz grains (Fig.
65) after the shock PDF emplacement in the contact and compression stage, when
excavation and deposition of the ejected masses overprinted the quartz at high pressure.
4.5 Paleocurrent analysis
Sedimentary structures such as cross bedding, imbrication pattern of clasts,
channeling and primary current lamination have been used to measure
paleocurrents. The data obtained (Fig. 21) show a predominance of the NE-SW
directions (mean direction N 213) defining a transport of the Pelarda Fm.
deposits from NE.
4.6 Sedimentary environment
All the characteristics observed in the area of Salcedillo, as well as between the
road between Fonfria and Olalla and the roads that cross the Sierra de Pelarda,
establish a deposition by gravitational flows (Lowe, 1979, 1982; Colombo,
1989). The diamictic sections point to debris flows and matrix-supported clast
transport en masse by sufficient cohesion. This transport had prevented, in
most cases, clast collisions and hence preserved the abundant plastic
deformations (rotated fractures, parallel fractures, fractures with complex
bifurcations) and a parallel orientation of many clasts.
53
There are areas showing that this laminar (non-Newtonian) regime is replaced
by an evolution between laminar and turbulent behavior (Enos, 1977; Nemec
and Steel, 1984; Wilson, 1980; Colombo, 1989; Colombo and Martí, 1989),
mainly observed at the bases of the diamictic sections, which are clearly erosive
and incorporate fragments of downwelling materials including the described
megablocks along the sections S 3 and S 4. In addition, a certain tendency to
internal stratification points to high-density flows with enhanced particle
concentration.
The sandy sections interspersed between the diamictic sections indicate
a transport in multiple stages with a sequence of "casts" (flow units) that
overlapped.
An accumulation of carbonate fluids could have produced the calcareous
lens-shaped intercalations in S 1 Intermediate (Fig. 31, 32) immersed within this
section during transport. A karstification observed in them took place in a phase
subsequent to formation. From field observations and thin-section analysis an
ascription to hard-ground levels linked to lacustrine activity can be excluded.
Instead we propose an origin linked with melting and decarbonization and
carbonate recombination of limestone clasts in the impact event.
In the sedimentary environment of the Pelarda Fm. abundant impact
indications are unmissable:
a. macroscopic (strong plastic deformations in the clasts, shock spallation
structures) and microscopic (planar structures PFs and PDFs) features.
b. a conspicuous syndepositional failure ("stop-and-go")
c. megaclasts and intercalated dislocated megablocks (up to the size of several
100 m). Here it may me added that the general trend of the Pelarda Fm. is
coarsening upwards.
A summary of the observations suggests a genesis of the Pelarda Fm. deposit
by successive flow units belonging to the ejecta curtain of the Azuara impact
structure (Figs. A101-A108, Appendix I) and consisting of streams of
semifluidized, high-density materials moving basically in a laminar flow (Schultz
& Gault, 1979; Melosh, 1989; Mackaman-Lofland et al., 2014) despite some
observations of turbulence.
Taking into account the lapillistone breccias, that accompany the deposit
(Fig. 7), the transport agent could sometimes have be water vapour (produced
in large quantities during the impact) though the general massiveness of the
deposit suggests water as the predominant fluid, which was produced by the
condensation of the vapour or directly excavated from the target). The vapour-
liquid transitions, as well as the sudden loss of fluids within the transported
mass, must have conditioned its deposition.
In this respect, the measured NE-SW striae from a frictional contact of
the clasts with the matrix suggest a sudden arrest due to a loss of sustentation
54
in the fluid. The peculiar syndepositional faults ("stop-and-go") within some units
can be linked to the same phenomenon.
The observed diamictite-sandstone alternation seems to reflect a
tendency to pulsations within the Pelarda Fm. ejecta. Taking into account the
obvious differences, they resemble those observed in volcanic pyroclastic flows
and in deposits of some craters on Mars (Schultz & Gault, 1979; Melosh, 1989).
An analog line for the Pelarda Fm. would assume a flow morphology
similar to a pyroclastic flow as a large drop that moves over the pre-existing
surface and in which several parts can be differentiated, each of which gives
rise to a characteristic deposit. As in pyroclastic flows, and depending on
variations in flow velocity (sometimes influenced by topography) and on
particle/fluid ratio, a whole range of transitions can be observed giving rise to
different depositional facies, which in the case of massive deposits can range
from debris flows to hyperconcentrated flood flows (Colombo and Martí, 1989).
4.7 Origin of the clasts
The direction and sense of the paleo-currents and the azimuth of the striae
indicate an origin from N - NE. Correspondingly, the lithology of sandstone
sections and the clasts of the diamictic and agrees with that of the materials
(Armorican quartzite, Buntsandstein materials and others) of the S-SW part of
the Azuara structure (zone of Bádenas - El Colladito - Piedrahita - Monforte de
Moyuela - Rudilla - Anadón, Fig. 2), with the exception of the Bámbola quartzite
clasts. Today the nearest Bámbola quartzite outcrops near Codos.
From the coexistence of rounded, subrounded, angular and subangular
Paleozoic, Mesozoic and Eocen-Oligocene clasts, contributing to the Pelarda
Fm., an origin can easily be settled in the Azuara impact target region where
kilometer-thick molasse materials from the Pyrenees and the Iberian Chain
must have been affected by the impact excavation (see Appendix I).
4.8 Age
The lower and upper limits of the Pelarda Fm., as well as the contacts allow a
relative dating of the Azuara impact event and the deposition of the ejecta.
Hence, the official geological maps (ITGE, 1989 and 1991; IGME, 1977), date
the lower limit of the event to the Upper Eocene/Oligocene and the upper limit
to the Lower Miocene. In this sense, the paleontological data published in
Ernstson et al. (2002), and those of Peláez-Campomanes (1993), also confirm
this age.
An absolute K-Ar dating of an impact melt rock failed because of totally
unreliable ages.
55
4.9 More Pelarda Fm. ejecta deposits: the San Roque deposit
In recent years, with more extensive field work and mapping, we have
repeatedly encountered deposits and outcrops that in many cases showed the
particular characteristics of the Pelarda Fm. and in one case were so typical of
the Pelarda Fm. that we add a detailed description in an Appendix II.
5. Discussion
At the beginning and referring to the investigations presented here, we
unambiguously state:
The Pelarda Fm. is not a deposit of raña type, which is a glacis of clasts with
clay matrix and developed on Paleozoic soils (ITGE, 1989; ITGE, 1991; Aurell
et al., 1993; Aurell, 1994; Cortés et al., 2002; Díaz-Martínez et al, 2002 a,
2002b, 2002c; Díaz-Martínez, 2005), and it is also not a fluvial deposit as
originally suggested by Carls and Monninger (1974).
We discussed and justified this and summarize:
a. From the Salcedillo outcrops it is evident that the Pelarda Fm. materials are
overlying Upper Eocene-Oligocene, in obviously concordant (though erosive)
contact.
b. The deposition of the Pelarda Formation took place by gravity flows linked to
the expansion and deposition of the Azuara impact ejecta. Fluvial sedimentation
[Carls & Monninger, 1974; Adrover et al., 1982; Smit, 2000 (written
communication)] is incompatible with depositional characteristics observed and
described in this article. Here it is in particular worth noting that typical
deformations in the diamictic levels (rotated fractures, open parallel tensile
fractures and fractures with complex bifurcations) could not have survived
transportation under a fluid, mostly turbulent (Newtonian) regime.
We should also addres the comment of Smit (reputable sedimentologist
and impact researcher, written comm. 2000) "nicely rounded pebbles which
cannot originate in an ejecta deposit", which completely disregards that the
roundness of many Pelarda Fm. clasts has already been brought along from the
pre-impact molasse deposits in the Azuara target and that even in the
excavation and ejection process and under high pressure an "impact
conglomeritization" of bedded limestones (Ernstson & Claudin 2013a) could
have occurred.
A basic disaccord between our observation and descriptions of the
Pelarda Fm. characteristics and the depositional models of the impact
opponents (alluvila fan, Miocene relations) (ITGE (1991), Cortés et al. (2002),
Diaz Martínez, 2005)) is in more detail discussed in Claudin & Ernstson (2018).
56
Here we add the question, how to explain by fluvial processes or linked to an
alluvial fan and low transport efficiency
-- the presence of the Buntsandstein megablocks (Fig. 45, Fig.48),
-- the Bámbola quartzite clasts bigger than meter size (Fig. 5),
-- the clasts of Eocene-Oligocene materials (Fig. 43/44),
-- the surface and microscopic characteristics (shock effects) of the
clasts and
-- the peculiar syndepositional (stop-and-go) faults of Fig. 40?
c. The age of the Pelarda Fm. lies between the Upper Eocene-Oligocene and
the Lower Miocene. The attribution of a Quaternary age (ITGE, 1989; ITGE,
1991; Aurell et al, 1993; Aurell, 1994; Smit, 2000 (written comment), Cortés et
al., 2002; Díaz-Martínez et al., 2002 a, 2002b, 2002c; Díaz-Martínez, 2005)
comes into obvious, unanswerable conflict with the geologic contacts observed
and described here. Also, if the Pelarda Fm. were a deposit of a Quaternary
glacis, it should have developed at the feet of an abrupt relief, needing the
explanation, why the Pelarda materials are exposed in top altitudes of the
region. A priori, it is difficult to have a considerable uprising during the
Quaternary at the same time, however making way for erosion to eliminate
these reliefs. Also very strange is the assumption of tectonics so active during
the Quaternary (perhaps a late "fase Iberomanchega" deformation) and
particularly intense only in the Pelarda area.
d. Impact opponents (Cortes et al., 2002) argue that Ernstson and co-workers
propose complex explanations for the predominance of Palaeozoic clasts and
the practical absence of limestone in the Pelarda case, in contrast to the ejecta
of the Rubielos de la Cérida impact structure in Pto. Míngez, where carbonate
rocks predominate (Ernstson et al 2002). This argument fails to recognize
several factors. At the time of impact, the targets in the Azuara area and the
Rubielos de la Cérida area may have been significantly different in terms of
limestone and Paleozoic units. Ignored by the opponents is that in contrast to
the large Pelarda Fm. distribution, the ejecta outcrops (at Puerto Mínguez) with
alternating Mesozoic and Paleozoic units are relatively small, not allowing
generalization. The essential factors of the impact cratering process are also
overlooked, where shock propagation leads to extreme temperatures of many
1000°C following shock pressure, with the result that considerable volumes of
the target evaporate and melt prior to ejection. At the relatively low
temperatures at which limestones melt and/or decarbonate, this could have led
to very different "destruction" or elimination of the carbonate facies in different
targets. In the case of primarily different targets, the inverse stratigraphy that
occurs twice could also have been an important agent: in molasse
sedimentation with younger ages downwards and the following impact
57
excavation, in which the primary inverse layering of the molasse sediments is
inverted once again ("the overturned slab" [Shoemaker]). Exactly this could be
expressed in our observation that in the Salcedillo area to the top of the Pelarda
F. the Jurassic and Eocene/Oligocene components visibly increase significantly.
e. In Fig. A101-A108 (Appendix I), the illustrations exemplarily show the Azuara
impact cratering with the excavation, ejection and deposition of the Pelarda Fm.
The sequence of images is based on publications of several computer
simulations on the genesis of an impact crater. They show that the whole of the
ejecta are not deposited in a single pulse, but would be the result of various
thrusts of flow units.
These floods were deposited according to the ballistic erosion model of
Oberbeck (1975), as streams of semifluidized and high-density materials which
basically moved in a laminar flow (Schultz & Gault, 1979; Melosh, 1989). This
process allowed the plastically deformed clasts (with prominent rotated
fractures, parallel open tensile fractures, fractures with complex bifurcations) to
survive the transport.
On occasion, the diamictic levels show erosive capacity incorporating
materials from below. For comparison: Deep NASA drilling in the ejecta masses
of the Ries crater ("Bunte breccia") (Hörz 1982) show that during the landing of
the ejecta according to the drill core volumes up to 2/3 of the local material was
scraped and incorporated by 1/3 of ejecta material. In the Pelarda case
because of the dominantly water- and vapor-supported laminar, subordinately
turbulent behavior, this process was less effective. The contribution of volatiles
in the ejection process is suggested by the occurence of the lapillestone breccia
deposits and dikes adjacent to the Pelarda Fm. (Figs. 11) (also see Branney &
Brwn (2011), Brown et al. 2010).
The vapour-liquid transitions, as well as the sudden loss of fluids in the
heart of the transported mass must also have been an important agent. It was
the intermittend sudden arrest of the flows, together with the intense confining
pressure that produced the many striations and the in part strong polish of the
clasts. The dominance of the NE-SW direction of the striations together with the
measured paleocurrents is clear evidence of a relation to the Azuara impact.
A certain and not at all quantifiable erosive capacity at the base of the
Pelarda Fm. as a whole is attributed to the in part strong deformations of the
Eocene/Oligocene conglomerates at Fonfría, which were overrun here by the
ejecta (Fig. 12, 13).
f. Due to the pre-impact topography the thickness of the Pelarda Fm. may vary
from one zone to anothert. The maximum 200 m suggested by Carls &
Monninger (1974) may according to our obervations reach about twice as much
(400 m). The Quaternary, where observed never exceeds 20 meters.
58
g. The diamictitic, disordered aspect of the Pelarda Fm. during the ascent to the
height of Pelarda and the rather stratified aspect downhill relates to the curvy
course of the street running parallel and perpendicular to strike/dip. The related
study of the depositional character did not use the terminology of Miall (1978),
Moncrieff (1989) or Schönian (2003) but tried to refer to their descriptions only.
h. The source area of the Pelarda Fm. materials, according to the paleocurrents
and the azimuth of the striations, is located in the N-NE. The Bámbola quartzite
clasts embedded in the ejecta apparently speak against this provenance
(Adrover, 1982; Carls, 2005 (personal comm.), since the Bámbola quartzite is
now exposed at Codos in the NNW. As alreadfy emphasized, the composition
of the ejecta material with its clasts is strongly influenced by its origin from the
molasse basin targeted by the Azuara impact.
i. The distance of the Pelarda Fm. ejecta from the center of the Azuara crater,
less than 2 times its radius (Cortés and Casas, 1996; Cortés et al., 2002; Diaz-
Martínez et al., 2002 a and b; Diaz Martínez, 2005), is consistent with the
studies that exist on proximal ejecta. According to these (French 1998),
approximately half of the proximal ejecta is deposited (through various
mechanisms) within 2 times the distance of the crater radius (2Rc) or 1Rc from
the crater rim to form a continuous ejecta curtain that can present thicknesses
from tens to hundreds of meters. Moving further away i.e. for distances > 2 Rc,
the thickness of the ejecta is decreasing and its distribution becomes
discontinuous. Practically 90% of the ejecta is deposited inside a zone defined
by a circle of radius 5Rc.
Apart from the fact that this is a very simplistic model, which depends
very much on many additional factors, we mention the figures in this regard,
with an Azuara structure diameter of roughly 40 km, 2Rc = 40 km, 1Rc = 20 km
and 5Rc = 100 km. The center of the Pelarda Fm. between Fonfría and Olalla
has about 25 km distance to the center, Rp > 1.5 * 1Rc, in good agreement with
the model data.
After this little clarification, it remains a mystery why authors Cortés and
Casas, 1996; Cortés et al., 2002; Díaz-Martínez et al., 2002 a and b; Díaz-
Martínez, 2005 present a scheme of the Azuara structure with two circles and
radii of 1Rc and 2Rc respectively, claiming and insisting that the location of the
Pelarda Fm. excludes an origin as ejecta. We assume that they have never
read the corresponding literature on proximal ejecta.
6 Conclusions
After more than 20 years of discussion about the Pelarda Fm., after the early
works of Monninger (1973) and Carls & Monninger (1974), with numerous
publications of the impact proponents and impact opponents, as well as detailed
contributions of the three current authors on the Internet pages www.impact-
59
structures.com, here for the first time an extensive article in English language
including our recent investigations is presented, after an extended version
already appeared in Spanish language (Claudin & Ernstson (2018). To this
must be added the activities of some leading impact researchers of the so-
called "impact community", who still ignore the Azuara impact event, which,
apart from the strict rejection by Spanish regional geologists, has also
contributed to the fact that even in the most recent literature on Spanish
geology this striking geological event still does not take place.
In addition we conclude that with the observations, measurements,
analyses and descriptions presented here in detail, all claims of the impact
opponents can be refuted easily. We attribute the attitude of the opponents to a
lack of field work, the reliance on outdated literature, a lack of knowledge of
elementary and generally accepted impact criteria and, above all, to a general
rejection of a major impact event in a region, in which generations of geologists
have produced many thick theses, doctoral theses and extensive publications
regarding conventional regional textbook geology only. Such a constellation has
existed in principle for decades and still exists today all over the world, when
regional geologists are suddenly confronted with the completely new situation of
a postulated impact structure. In this Azuara case we wonder however that after
20 years of scientific evidence of impact, there is still a lack of scientific insight.
Whether this article will change the opinion of the persons addressed
remains questionable, but it should familiarize the more open-minded
international impact research community with one of the greatest terrestrial
impact ejecta occurrences and, if necessary, initiate a discussion.
Acknowledgements. We thank P. Carls, who has worked for decades since the
sixtieth in the region of the Paleozoic of the Eastern Iberian Chain, for many
discussions about the Azuara impact and the Pelarda Fm., although we were
unable until today to convince him of the reality of the big impact. We thank
Pepe for years and years of really great hospitality as principal of the among
geologists famous "Legido" hotel, bar and restaurant in Daroca.
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Appendix I: The Azuara impact cratering process and ejecta
formation in eight images - schematic
Fig. A101. Block diagram of the Azuara zone corresponding to the Upper Eocene-
Oligocene period based on data from different sources. The Azuara zone would be
located within an endorheic basin where materials from the erosion of the Iberian
mountain range and the Pyrenees were deposited. This deposition was linked to sets of
alluvial fans. The blue colour corresponds to areas of lagoons and lakes formed by the
accumulation of water from these two large mountain masses.
Fig. A102. Cross section of the Azuara impact zone. At the time of impact, the target
rocks were made up of Paleozoic, Mesozoic, Cenozoic materials and extensive molasse
deposits accumulated at the foot of the reliefs of the Iberian mountain range in
emergence.
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Fig. A103. Shortly after the impact (contact and compression stage): A shock wave
compresses the materials of the target in a first phase and then - after release -
fragments and ejects them (excavation stage). A curtain of ejecta develops, including
megablocks, and comprises all molasse materials removed. Due to the pre-impact
geologic setting (Fig. A102) Paleozoic and Mesozoic materials predominate over
Cenozoic materials ejected to the southwest. On the NE side, Mesozoic and Cenozoic
materials predominate. In the fractures developed in the walls and floor of the crater,
materials generated during the impact (impact melts and breccias) are injected giving
rise to the formation of breccia dikes.
Fig. A104. The ejecta curtain is advancing, with increasing primary crater. In the central
part an ejecta plume grows progressively, accompanied by a conical zone of ejecta
between plume and ejecta curtain.
69
Fig. A105. Progressive growth of the crater to become the transient crater, advance of
the ejecta curtain, and increase of ejecta plume and conical zone.
Fig. A106. Modification stage: An inner ring begins to form, further expansion of plume
and conical zone. The ejecta of the latter zone begin to leave the crater giving rise to the
intracrater gap (Fig. A107).
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Fig. A107. The materials of the conical zone continue to advance from the intracrater gap.
In the case of the SW flank they will give rise to the Pelarda Fm.
Fig. A108. Materials from the central plume and conical zone begin to deposit to form a
breccia of Pelarda Fm. facies within the crater and layers of suevite, which e.g., can still
be observed in Cucalón and near Olalla, and which cover all ejecta previously deposited
in the Azuara zone.
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Appendix II: The Ermita de San Roque Pelarda Fm. ejecta
deposit
Near the Aldehuela de Liestos village, 50 km west of Olalla (Fig.1) on top of a
distinct hill emerging roughly 100 m from the plain, a peculiar deposit is
observed to overlay Eocene (Oligocene) sediments (according to the geological
map; Figs. 2, 3). The 5 m thick deposit bears all aspects of the Pelarda Fm. as
shown in the images below. The deposit is composed of uncemented, very
badly sorted, mostly (more than 95 %) Paleozoic material with well-rounded,
subrounded and angular clasts. The main mass is Armorican quartzite, and a
few Cretaceous/Jurassic limestone boulders are intermixed. Bámbola quartzite
like in our classic Pelarda Fm. deposit is not observed.
Typical deformations of the clasts occur in the form of spallation
fractures; Hertzian fracture cones, dinstinct small impact imprints and squeezed
cobbles.
The deposit is found right in the middle of Mesozoic and Cenozoic
sediments; Palezoic is not exposed within a radius of 10 km at least. On cursory
inspection, no quartzite blocks of the size of the San Roque deposit have been
observed in the plains around the hill.
We conclude that the San Roque deposit with all aspects of the Pelarda
Fm. is suggested to be also ejecta of the Azuara impact event that survived
erosion. Alternative explanations for the deposition are meeting serious
difficulties.
The considerable distance of about 40 km to the Azuara/Rubileos de la
Cérida rims is not a problem at all comparing the figures with the Ries crater
ejecta that in the form of limestone blocks (so-called Reuter blocks) can be
found up to 60 km distance. In one case one has found a several tons big
strongly shattered limestone block in a gravel quarry about 150 km (!) distant
from the Ries crater. This block is also interpreted as ejecta although also an
origin from Miocene volcanism has been discussed despite the complete lack of
any volcanic concomitant material. As for these distal ejecta we must not forget
that the Ries is much smaller than Azuara/Rubielos. Possibly in both cases the
large ejecta distances may be explained by the spall plate mechanism (Melosh
1989).
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Fig. A201. Location map for the deposit at Aldehuela de Liestos (Ermita de San Roque)
west of the villages of Azuara, Fonfría and Olalla.
Fig. A202. Geological map of the Ermita San Roque (arrow) area.
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Fig. A203. The Ermita de San Roque hill and the geological situation of the Pelarda Fm.
deposit. Google Earth oblique view.
In the following we show typical images taken on the Ermita de San Roque hill:
Fig. A204. Pelarda Fm. deposits over autochthonous Eocene limestone beds.
74
Fig. A205. Closer view of the contact.
Fig. A206. View downhill from the outcropping Eocene limestone beds (lower left corner)
to the southwest. The large blocks are Armorican quartzite. Note that downhill the blocks
rarefy.
75
Fig. A207. Aspect of the outcropping Pelarda Fm. The hammer is lying on one of the rare
Jurassic/Cretaceous limestone blocks. The outcrop wall is about 3 - 4 m.
Fig. A208. Aspect of the Pelarda Formation. in the field.
76
Fig. A209. Aspect of the Pelarda Fm. in the field. The larger blocks have obviously been
removed by the farmer.
Fig. A210. Mapping the Eocene - Pelarda Fm. boundary in the field.
77
Fig. A211. Assemblage of Pelarda Fm. clasts. To the right of hammer:
Jurassic/Cretaceous limestone boulders. The lowermost quartzite clast shows distinct
spallation fractures.
Fig. A212. Large Armorican quartzite block exhibiting irregular fracture and spallation
features.
78
Fig. A213. More spallation fractures.
79
Fig. A214. Strongly squeezed however coherent quartzite cobble.
Fig. A215. Large quartzite block with distinct concussion marks. Close-up in Fig. 16.
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Fig. A216. Close-up of concussion mark with Hertzian fracture cone - arrow head in the
center.
Fig. A217. Concussion (spallation) crater in a quartzite block.
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Fig. A218. Ermita San Roque built of impact ejecta (except for door and window curbs).
This Ermita nicely meets the Ries and Rochechourt impact structures where Impact
ejecta have intensively been used as building stones.
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A 25 70-cm-thick, laterally correlative layer near the contact between the Paleoproterozoic sedimentary Gunflint Iron Formation and overlying Rove Formation and between the Biwabik Iron Formation and overlying Virginia Formation, western Lake Superior region, contains shocked quartz and feldspar grains found within accretionary lapilli, accreted grain clusters, and spherule masses, demonstrating that the layer contains hypervelocity impact ejecta. Zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca. 1878 Ma and 1836 Ma. The Sudbury impact event, which occurred 650 875 km to the east at 1850 ± 1 Ma, is therefore the likely ejecta source, making these the oldest ejecta linked to a specific impact. Shock features, particularly planar deformation features, are remarkably well preserved in localized zones within the ejecta, whereas in other zones, mineral replacement, primarily carbonate, has significantly altered or destroyed ejecta features.
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Diamictites, especially those deposited before the break-up of Gondwana in the Late Carboniferous and Permian, hirve recently been suggested to represent ejecta deposits from large comet or meteorite impact events. This is in contrast to the commonly held interpretation that these rocks represent glaciomarine sedimentary deposits. To test this controversial hypothesis, we carried out a detailed petrographical study of over 75,000 mineral and rock clasts from a large number of Dwyka Group diamictite samples from localities covering an extensive part of the Karoo basin in southern Africa. No definitive evidence of impact-diagnostic shock metamorphic deformation in mineral or lithic clasts from any of these samples was detected. We conclude, therefore, that to date no unequivocal evidence for an impact origin of these diamictites in the South African stratigraphic record has been documented. What is more, the general hypothesis that some diamictites in the stratigraphie record could represent impact ejecta is not supported by first-order observations of bona fide shock (impact) related phenomena in such rocks.
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Aspects of base surge transport are considered along with questions regarding the applicability of base surge transport to lunar sedimentation, the ballistic transport of crater and basin ejecta, Copernicus crater ballistics, and the effects of ejecta impact on preexisting lunar ground. An ejecta emplacement model is discussed and attention is given to the structure of the surface of continuous deposits of craters and basins, the thickness of crater and basin deposits, and the characteristics of impact melts.
Impact cratering and distal ejecta: the Australian record
  • R A F Grieve
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  • P W Haines
Vol. 14). Springer. https://doi.org/10.1007/978-3-319-74545-9_2 • Grieve, R. A. F. & Shoemaker, E. M. (1994): The record of past impacts on Earth. In: T. Gehrels, Hazards Due to Comets and Asteroids (eds.): Space Science Series. Univ. Arizona Press, Tucson, Arizona, USA 1994, p. 417-462. • Haines, P.W., 2005. Impact cratering and distal ejecta: the Australian record. Aust. J. Earth Sci. 52, 481-507. • Hassler, S.W., Bruce M. Simonson, B.M., Dawn Y. Sumner, D.Y. &
Alluvial fan sequence and megasequence models: with examples from Westphalian D Stephanian B coalfields, northern Spain
  • L Bodin
  • P Hodge
Bodin, L. (2011) Paraburdoo spherule layer (Hamersley Basin, Western Australia): Distal ejecta from a fourth large impact near the Archean-Proterozoic boundary • Heward, A.P. (1978): Alluvial fan sequence and megasequence models: with examples from Westphalian D Stephanian B coalfields, northern Spain. In: Miall, A.D. ed., Fluvial Sedimentology. Canadian Society of Petroleum Geologists, Memoir 5: 669-702. • Hodge, P. (2010): Meteorite Craters and Impact Structures of the Earth.