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Name:Popigai
Country:Russia
Location:Siberia
Diameter:100 km
Age:35.7 million
Coordinates:71°39' N 111°11' E
Link to Google Maps: here
KML file for Google Earth: here
Associated specimens:
Cataclasite
Diaplectic biotite-garnet gneiss
Mascelynite
Shocked biotite gneiss
Tagamite
Vitroclastic suevite

Source: http://www.mines.edu/academic/geology/faculty/klee/PopigaiImpact.pdf

OVERVIEW
About 36 million years ago in northern Siberia, a large asteroid impacted the Siberian platform to form a crater 100 km in diameter filled with melted and shocked material that included shock-generated impact diamonds.
71°38’N 111°11’E - crater center
North Central Siberia about 100 km from the coast of the Laptev Sea, an embayment of the Arctic Ocean.
Late Eocene - 35.7 (± 0.2) million years [by stepwise heating Ar-Ar].

PRE-IMPACT GEOLOGY
Impact occurred at the Precambrian-Phanerozoic contact on the northeast flank of the Anabar Shield.
Basement of Archean graphite-bearing gneisses.
A simple homoclinal layer of sedimentary rocks, about 1500 m thick, consisting of platform sandstones and carbonates of Proterozoic, Cambrian, and Permo-Triassic age.

 

IMPACTOR
Probably an ordinary chondrite, about 8 km diameter, 3.5 g/cm3, with a trajectory 210o -220o. Such a body, with a velocity of about 20 km/s, would deposit about 1.7 x 1023 J of energy [Masaitis].

Use of Melosh's cratering program back-calculates a stony asteroid impactor slightly less than 5.0 km in diameter.

 

 

CRATER
The crater is 100 km diameter, as measured by the extent of deformation of bedrock by thrusting.
The geophysical signatures of the structure are a magnetic low and a gravity low with a central gravity high.
The crater is filled with 2-2½ km of suevite [75%] and melt rock [tagamite] [25%].
The crater floor has a low topographic high in the center of a central basin, with an outer synform-antiform.

SHOCK METAMORPHISM
The impact created:
shatter cones in Archean gneisses
PDFs [planar deformation features]
lechatelierite and diaplectic glasses
coesite and stishovite [polymorphs, or high-pressure forms, of quartz].
Shock pressures instantaneously transformed graphite to diamond within 13.6 km of ground zero.

SIGNIFICANCE
Popigai is a world-class impact structure that is well preserved and well exposed, providing a good view of impact structures and rocks and permitting access to internal materials. Three other craters are larger, but they are either buried [Chicxulub], strongly deformed [Sudbury], or deformed and severely eroded [Vredefort]. The exposures and preservation at Popigai permit a detailed account of the impact mechanisms to be interpreted [see Impact Scenario below].

Popigai may represent one of two or three simultaneous impacts. The 35.7 (± 0.2) Ma date precedes the 35.2-35.5 Ma Chesapeake Bay impact and the same-age Toms Canyon impacts, but the error bars do not preclude synchronism.

Popigai precedes the 33.7 Ma Oligocene/Eocene boundary. The 35.7 ± 0.2 Ma date corresponds to the 35.7 ± 0.4
Ma Ir and PDF quartz layer at Missignano, Italy, the stratotype section for the Eocene/Oligocene boundary.


IMPACT SCENARIO
A probable sequence of events in the formation of the Popigai impact structure would include:

A transient cavity about 8-10 km deep was excavated through the 1.5-km-thick sedimentary rock cover and into the Archean graphite-garnet gneisses
Peak pressure was about 624 GPa
Impactor and surrounding target rocks were vaporized
a vapor plume and vapor-melt cloud were ejected
99% of the impactor was vaporized and 1% was incorporated in meltrocks
About 1750 km3 of molten rock formed, about half of it was ejected
Shock metamorphism created shatter cones, PDFs, coesite and stishovite, and diaplectic glasses
Shock pressures transformed graphite to diamond within 13.6 km of ground zero
[A fraction of a second has elapsed]

Bedrock was shattered into blocks that were forced downward, outward, and upward, at supersonic speeds, to form the allogenic breccias
Fusion of gneiss formed sheets of meltrock [precursor of the tagamite] that covered most of the cavity/crater floor and flowed radially out to form annular ridges and streams of meltrock into and over the allogenic breccias
Explosion cloud of fragmental ejecta, vapor, and melt [similar to a pyroclastic flow] formed suevite almost simultaneously with tagamite flows
Rebound of cavity floor into a central peak squeezed melt layers into upper layers of suevites and possibly to the surface as flows
Subsidence of central peak formed annular rings [antiform and synform pair], thus creating a peak-ring complex crater or a multi-ring crater
Ejected blocks fall at least 70 km beyond the crater; some diamonds were blown 150 km to the east
Centrifugal bottom-flow material of allogenic breccia and tagamite sheets overtaken by suevite clouds and material intimately mixed
[Probably minutes have elapsed]

Fallback of condensed volatilized material, mixed with fine fragmental ejecta, probably formed a surface blanket of material that was subsequently removed by erosion
Tagamites up to 600 m thick may have required thousands of years to cool
Crater was filled to a shallow depth by Neogene-Quaternary sediments