Azuara impact structure - AbsoluteAstronomy.com

The 35–40 km-diameter Azuara impact structure
Impact structure
The term impact structure is closely related to the terms impact crater or meteorite impact crater, and is used in cases where erosion or burial have destroyed or masked the original topographic feature with which we normally associate the term crater...

is located in northeast Spain roughly 50 km south of Zaragoza

Zaragoza

Zaragoza , also called Saragossa in English, is the capital city of the Zaragoza Province and of the autonomous community of Aragon, Spain...

. The name is attributed to the small town of Azuara

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. Its postal code is 50140Cueva del Cabuchico is an ancient Iberian archaeological site near Azuara....

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

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

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

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 metamorphic effects unequivocally associated with explosions...

, and Azuara was established (Grieve & Shoemaker 1994, Hodge 1994, Norton 2002 as an authentic impact structure. From stratigraphic

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

considerations and paleontological

Paleontology

Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...

dating, its age is estimated to be Upper Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

or Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

(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 Tertiary

Tertiary

The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

sediments, however in the outer rim zone, the outcrop

Outcrop

An 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 deposits are covered by a mantle of soil and vegetation and cannot be...

conditions are excellent.

Impact features

The impact nature is documented by abundant polymictic and monomictic breccia

Breccia

Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....

s, breccia dikes, extensive megabreccias and impact ejecta

Ejecta

Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...

, dislocated megablocks, geophysical

Geophysics

Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...

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

Planar deformation features

Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals , consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.PDFs are only produced...

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

Quartzite

Quartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...

clasts contributing to the ejecta (Pelarda Formation).

Shock metamorphism in the Azuara impact structure: Planar deformation features (PDFs) in quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

. The histogram

Histogram

In statistics, a histogram is a graphical representation showing a visual impression of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson...

displays frequencies of crystallographically

Crystallography

Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

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

Nördlinger Ries

The Nördlinger Ries is a large circular depression in western Bavaria, Germany, located north of the Danube in the district of Donau-Ries. The city of Nördlingen is located about southwest of the centre of the depression....

, the Vredefort crater

Vredefort crater

Vredefort crater is the largest verified impact crater on Earth. It is located in the Free State Province of South Africa and named after the town of Vredefort, which is situated near its centre. The site is also known as the Vredefort dome or Vredefort impact structure...

, or the Sudbury Basin

Sudbury Basin

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

, 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

Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...

features, the impact ejecta (Pelarda Formation) are Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

alluvial fans and impact breccias and dike breccias are interpreted as karst

Karst topography

Karst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.Due to subterranean drainage, there...

features and soil formations.

The opposition against the impact origin for Azuara has been supported by a misleading analysis and paper (Langenhorst & Deutsch 1996) .

TEM investigations on deformed quartz disprove shock origin" (abstract). Lunar and Planetary Science XXVII.) denying the occurrence of shock metamorphism in Azuara rocks. Obviously based on this misleading paper and analysis, Azuara was removed from the Canadian Impact Data Base

Earth Impact Database

The Earth Impact Database is the authoritative source for information on confirmed impact structures or craters on Earth. It was initiated in 1955 by the Dominion Observatory, Ottawa, under the direction of Dr. Carlyle S. Beals...

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., and the Expert Database on Earth Impact Structures (EDEIS)

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

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...

s and impact melt rocks.

External links

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