The Azuara

impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface ...

is a structural feature of about diameter,Mikheeva, 2017 located in northeastern Spain, roughly south of

Zaragoza Zaragoza, also known in English as Saragossa,''Encyclopædia Britannica'"Zaragoza (conventional Saragossa)" is the capital city of the Zaragoza Province and of the autonomous community of Aragon, Spain. It lies by the Ebro river and its tributari ...

.Azuara
/ref> The name is attributed to the small town of

Azuara Azuara is a municipality located in the province of Zaragoza, Aragon, Spain. According to the 2010 census the municipality has a population of 220 inhabitants.Instituto Nacional de Estadística (Spain) Its postal code is 50140 Cueva del Cabuchic ...

located near the center of the structure. The first hint to a possible

impact Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Impact ...

origin was given by Wolfgang Hammann as early as 1980, and the first field evidence was provided by Johannes Fiebag in the early eighties. In 1985, Ernstson et al. published the occurrence of

shock metamorphism Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...

, and Azuara was established (Grieve & Shoemaker 1994, Hodge 1994, Norton 2002Grieve R. A. F. and Shoemaker E. M. (1994) The record of past impacts on Earth. In Hazards Due to Comets and Asteroids, edited by T. Gehrels. Space Science Series Tucson, Arizona, USA: Univ. Arizona Press. pp. 417-462Hodge P. (1994) Meteorite craters and impact structures of the Earth. Cambridge, UK: Cambridge University Press. 124 p."Norton, O.R. (2002) The Cambridge Encyclopedia of Meteorites" as an authentic impact structure. From

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...

considerations and

paleontological Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes t ...

dating, its age is estimated to be Upper

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

(roughly 30 - 40 Mill. years old). The Azuara structure exhibits a roughly circular morphology implying a partly pronounced outer ring. The inner part is covered by post-impact Upper

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

sediments, however in the outer rim zone, the

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...

conditions are excellent.

Impact features

The impact nature is documented by abundant polymictic and monomictic

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

s, breccia dikes, extensive megabreccias and impact

ejecta Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...

, dislocated megablocks,

geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

anomalies and shock metamorphism. Shock effects like melt glass, diaplectic glass,

planar deformation features Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orie ...

(PDFs) are found in breccias and breccia dikes, and PDFs are abundant also in

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

clasts contributing to the ejecta (Pelarda Formation). Shock metamorphism in the Azuara impact structure include planar deformation features in

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

. The

histogram A histogram is an approximate representation of the distribution of numerical data. The term was first introduced by Karl Pearson. To construct a histogram, the first step is to " bin" (or "bucket") the range of values—that is, divide the ent ...

displays frequencies of crystallographically controlled planes of microdeformation. The and occurrences are especially diagnostic and are generally considered as in proof of impact shock.

Controversy

Like many other impact structures in the world, such as

Nördlinger Ries The Nördlinger Ries is an impact crater and large circular depression in western Bavaria and eastern Baden-Württemberg. It is located north of the Danube in the district of Donau-Ries. The city of Nördlingen is located within the depression, a ...

, the

Vredefort impact structure The Vredefort impact structure is the largest verified impact structure on Earth. The crater, which has since been eroded away, was around across when it was formed. The remaining structure, comprising the deformed underlying bedrock, is loca ...

, or the

Sudbury Basin The Sudbury Basin (), also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geological structure in Ontario, Canada. It is the third-largest known impact crater or astrobleme on Earth, as well as one of the oldest. The cra ...

, the origin of the Azuara structure has been debated, and Spanish geologists still remain opposed to an impact origin. In their opinion the shock effects are

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

features, the impact ejecta (Pelarda Formation) are

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

alluvial fans An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

and impact breccias and dike breccias are interpreted as

karst Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...

features and soil formations. The opposition against the impact origin for Azuara has been supported by an analysis and paper (Langenhorst & Deutsch 1996) denying the occurrence of shock metamorphism in Azuara rocks. Based on this paper and analysis, Azuara was removed from the Canadian Impact Data Base when its management changed to the University of New Brunswick. Azuara is still listed as a confirmed impact structure in other data bases, e.g. Moilanen, J., 2009 and the Expert Database on Earth Impact Structures (EDEIS).EDEIS Expert Database on Earth Impact Structures

Companion impact structures

Since 1994 it has been hypothesized that Azuara is only part of a much larger multiple impact event (Azuara impact event), comprising also an elongated impact basin suggested to have resulted from the impact of a string of impactors. This Rubielos de la Cérida basin bears all evidence of impact features like morphological signature, polymictic and monomictic breccias, megabreccias, ejecta deposits, shock metamorphism,

suevite Suevite is a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as imp ...

s and impact melt rocks.Ernstson, K., Claudin, F., Schüssler, U., Anguita, F. and Ernstson, T. 2001. Impact melt rocks, shock metamorphism, and structural features in the Rubielos de la Cérida structure, Spain: evidence for a companion to the Azuara impact structure, in: Impact markers in the stratigraphic record, 6th ESF-IMPACT workshop Granada, abstract book: 23-2

/ref>

References

Bibliography

External links

EDEIS Expert Database on Earth Impact Structures
{{DEFAULTSORT:Azuara Impact Structure Geology of Spain Impact craters of Spain Possible impact craters on Earth Paleogene impact craters Paleogene Spain Province of Zaragoza