Avak is an 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...

 crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

 centered approximately 12 km (7.5 mi) southeast of Barrow, Alaska

Barrow, Alaska

Barrow is the largest city of the North Slope Borough in the U.S. state of Alaska. It is one of the northernmost cities in the world and is the northernmost city in the United States of America, with nearby Point Barrow being the nation's northernmost point. Barrow's population was 4,212 at the...

, United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

.

Avak is a subcircular structure about 8 km (5 mi) in diameter and 1 km (0.621372736649807 mi) deep. In the structure, metamorphic basement rocks and regionally flat lying sedimentary rocks are uplifted and intensely deformed. The Avak structure has no surface expression in the swampy, lake-dotted tundra

Tundra

In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...

 of the Arctic

Arctic

The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...

 coastal plain

Coastal plain

A coastal plain is an area of flat, low-lying land adjacent to a seacoast and separated from the interior by other features. One of the world's longest coastal plains is located in eastern South America. The southwestern coastal plain of North America is notable for its species diversity...

. The structure is covered by a thin veneer or permanently frozen Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

 and Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 rock. This means that the age of the impact took place anywhere between 95 million and 3 million years ago.

History

The Avak structure was first recognized from seismic and gravity surveys in the National Petroleum Reserve No. 4 (NPR4) by the U.S. Navy during the period from 1943 to 1953. The Navy drilled the Avak 1 well in 1951, and encountered deformed rocks from near the surface to a total depth of 1225 m (4,019 ft). This led to the suggestion that Avak might be an impact structure.

The Avak structure provides the structural trap for the natural gas in the adjacent South Barrow and East Barrow gas fields. These fields have accumulated about 37 bcf of natural gas in Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 sandstones.

In 1996, geologists from the U.S. Geological Survey found geologic evidence that indicated that the Avak structure was a meteorite impact structure. The presence of 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) was reported in 1995, so Avak is considered to a "confirmed" impact structure