olelog

At the 2009 Portland GSA Annual Meeting (18-21 October 2009) Sankar Chatterjee will present a paper on the Shiva Crater (18 October 2009, 3:45-4:00 p.m.). The massive Shiva basin is a submerged depression west of India that is intensely mined for its oil and gas resources. Some complex craters are among the most productive hydrocarbon sites on the planet.

If Chatterjee is right this could be the largest, multi-ringed impact crater the world has ever seen, with a diameter of ~500 km. The diameter of the so far known largest impact crater, the Vredefort Crater in South Africa has a diameter of about 300 km. Furthermore the Shiva crater has an age that makes it a suspect for the killing of the dinosaurs ca. 65 million years ago.

According to Chatterjee it is the remnant of a giant meteorite impact that left high-resolution stratigraphic signals in the sedimentary and volcanic rocks such as shocked quartz, iridium anomaly, nickel-rich spinels, sanidine spherules, magnetic nanoparticles, high pressure fullerenes, megatsunami deposits, and melt lavas. If the author and his team are right, this is the largest crater known on our planet. The bolide may have been perhaps 40 kilometres in diameter - as compared to the bolide of between 8 and 10 kilometres that hit Yucatan Peninsula, and is commonly thought to have killed the dinosaurs.

The impact was so powerful that it led to several geodynamic anomalies: it fragmented, sheared, and deformed the lithosphere mantle across the western Indian margin and contributed to major plate reorganization in the Indian Ocean. It initiated rifting between India and Seychelles in the west and created the Laxmi Ridge; it shattered the Indian plate easterly along the Narmada-Son Rift extending 1500 km across, dividing the Indian shield into a southern peninsular block and a northern foreland block. Because of topographic barrier of the Western Ghat Mountain range, the impact-triggered tsunami was restricted along the Narmada-Son Rift at the KT boundary.

The team hopes to go India later this year to examine rocks drill from the center of the putative crater for clues that would prove the strange basin was formed by a gigantic impact.

The rest of us are waiting to hear more.

• http://www.geosociety.org/news/pr/09-54.htm
  
• http://www.physorg.com/news174827113.html
  
• http://thedragonstales.blogspot.com/2009/10/shiva-impact-rising-again.html
  
• http://www.eurekalert.org/pub_releases/2009-10/gsoa-gin101509.php
  
• http://gsa.confex.com/gsa/2009AM/finalprogram/abstract_160197.htm
  
• http://www.sawfnews.com/Health/60779.aspx
  

In Norwegian:

• http://www.forskning.no/artikler/2009/oktober/232020
  

PS of 21 October 2009:
The hypothesis met with sharp criticism. See
http://www.space.com/scienceastronomy/091018-dinosaur-crater.html

PS of 2 November 2009 - More about the Shiva Crater:
http://suvratk.blogspot.com/2009/11/end-cretaceous-how-many-impacts-how.html
and
http://books.google.co.in/books?id=3IORF1Ei3LIC&pg=PA35&lpg=PA35&dq=bombay+high+stratigraphy&source=bl&ots=ng1Gm3E1r-&sig=lYxnwzsvy96JmE821U2z40VDcKw&hl=en&ei=fLzmSvHfK5DOsQOf36TfBQ&sa=X&oi=book_result&ct=result&resnum=9&ved=0CB4Q6AEwCDgK#v=onepage&q=bombay%20high%20stratigraphy&f=false

I have just read a post that I would like to have written - but didn’t.

Nitish Priyadarshi at “Environment and Geology” did, and I am delighted to recommend his post, where he tells how economic deposits are associated with terrestrial impact structures.

Over to:

• http://nitishpriyadarshi.blogspot.com/2009/08/are-impact-craters-useful.html
  

Three meteorite impact craters have so far been recognised in Norway. The Gardnos structure was the first impact structure to be recognised. The Mjølnir crater (in the Barents Sea) was recognised just a couple of years after Gardnos. The Ritland crater outside Stavanger is apparently not yet really recognised, or at least not yet confirmed in The Earth Impact Database.

The Gardnos impact has been dated to 546 ± 5 million years old. Today it is about 5 km in diameter, and was created by a meteorite with an estimated diameter of 200-250 m and a velocity of around 20 km/sec.

A 400 m long core was drilled within the Gardnos structure in 1992, penetrating sediments and impactites (suevite and Gardnos breccia). The sedimentary post-impact deposits have been intensively studied and described in several papers. The Gardnos breccia was produced during impact by fracturing of target rocks, formed during the shock wave propagation. In the typical Gardnos Breccia angular white granitic or quartzitic clasts are embedded in a dark matrix. In central parts of the crater, a sheet of suevite breccia is found overlying the Gardnos Breccia. The creation of Suevite is a result of the enormous amount of energy released by the meteorite impact. The rock consists of rock melt, glass and rock fragments (see image below).

The complex crater is well exposed, easy accessible and conveniently located along the main road between Oslo and Bergen, just 170 km from Oslo. In addition, its great exposures offer a magnificent section through several different typical impact lithologies. The Gardnos structure is visited annually by about 20,000 tourists and geologists. It is possible to drive in to the centre of the crater. There are two trails through the surrounding natural region, with information signs describing the geology of the area. There is also a new information centre.

The Precambrian basement of the area consists mostly of granitic gneisses, along with quartzites and banded gneisses.

The Quaternary glaciers excavated the weaker crater-infill rocks and thereby refreshed some of the original crater shape.

An Excursion guide to Gardnos meteorite crater from Oslo University was at the time of writing available as pdf file. Click here for download. (Authors: Elin Kalleson, Tom Jahren and Henning Dypvik, Department of Geoscience, University of Oslo).

• http://www.gardnos.no/servlets/dispatcher?siteNodeId=584122&languageId=2
• http://www.unb.ca/passc/ImpactDatabase/images/gardnos.htm

In Norwegian:
• http://www.forskning.no/artikler/2009/mai/220027

Image of Suevite from the Ries crater in Germany:

When I visited Bosumtwi Lake in 2003 it was first and foremost to watch birds, I have to admit, although the geological aspects as usually kept lurking somewhere behind. It irritated me enormously that many people (still) thought that it is a volcanic crater - even supported in this belief by tourist guide books! Incredible!


The Bosumtwi impact crater is almost completely filled by Lake Bosumtwi - Ghana's largest natural lake. Maclaren postulated an impact origin of the Bosumtwi Crater already in 1931. Numerous studies have followed, and the impact hypothesis confirmed by all the typical impact indicators. The 1.07 Million years old crater has a rim-to-rim diameter of about 10.5 km.

Before I go on, a few words on tektites:

The term “Tektite” was coined by Suess in 1900 and is derived from a Greek word for “molten”, tektos. Tektites are natural glass objects up to a few centimeters in size, which most scientists argue were formed by the impact of large meteorites on Earth's surface. Tektites are typically black or olive-green, and their shape varies from rounded to irregular. Areas with concentrations of tektites on the Earth's surface are known as strewn fields.

One such strewn field is the so-called Ivory Coast strewn field (with Ivory Coast tektites) to the west of the Bosumtwi Crater. The first suggestions that the Bosumtwi crater is the source crater for the Ivory Coast tektites were made in the early 1960s. Ivory Coast tektites were first reported in 1934 from a geographically rather restricted area in the Ivory Coast (Cote d'Ivoire), West Africa. Microtektites and tektites were later reported from deep-sea sediments and other West African locations. Ivory Coast tektites and the Bosumtwi crater have the same age, and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks, which strongly support a connection between the crater and the tektites. This makes the Bosumtwi crater one of only three impact structures that have been identified as source craters of a tektite strewn field. (Ries Crater being another - connected to the moldavite strewn field that extends from about 200 to 450 km from the center of the Ries to the east north east).



The principal criteria for determining if a geological feature is an impact structure are listed at the Earth Impact Database. I shall here concentrate on two of them in connection with the Bosumtwi Impact Crater. Impact breccias and multiple planar deformation features (PDFs) in minerals (a.o. found in what is referred to as shocked quartz).

Cores drilled in the Bosumtwi Lake have shown that the lake sediments are underlain by impact breccias, first a layer of polymict and suevitic impact breccia (suevite is also known as “fall back breccia”) and beneath that a thinner layer of monomict impact breccia. In a monomict all the small pieces of rocks in the breccia are of the same type, meaning they came from a single parent body. A polymict has more sources, indicating some sort of collision and melding of material from different areas of the same parent or different parents.

A study of shock metamorphism and shocked rocks of the Bosumtwi Impact Crater was published yesterday (12 December 2008) in the journal Science. I read it with great interest, but as it is rather technical I won’t go into much detail here. So far only three investigations have explored the relative vertical decay of recorded shock pressure in complex impact structures (i.e. craters with diameters greater than 2-4 km on Earth, see drawing below), two for the ~40 km diameter Puchezh-Katunki impact structure and another for the ~65 km diameter Kara crater. So also in this aspect the Bosumtwi Crater is special.

Cr isotope analyses of an Ivory Coast tektite indicates that the Bosumtwi impactor was an ordinary chondrite - a stony meteorite. Most meteorites that are recovered on Earth are chondrites.

Selected References:
Shock Metamorphism of Bosumtwi Impact Crater Rocks, Shock Attenuation, and Uplift Formation by Ferrière et al., Science of 12 December 2008.
An Ordinary Chondrite Impactor Composition for the Bosumtwi Impact Structure, Ghana, West Africa: Discussion of Siderophile Element Content and Os and Cr Isoptope Data by Koeberl and al., in Lunar and Planetary Science XXXV (2004)

• http://www.sciencemag.org/cgi/content/short/322/5908/1678
• http://www.unb.ca/passc/ImpactDatabase/images/bosumtwi.htm
• http://www.univie.ac.at/geochemistry/koeberl/bosumtwi/
• http://originoftektites.com/Chapter_2.php

Some time ago a geologist showed me an area that he believed to be an impact crater, and he is trying to gather enough evidence to prove it. Although he didn’t convince me, I wish him every success with his research. I have lately read about other cases of more or less doubtful impact stories. I wrote about one a few days ago. Another example is Bedout, that in 2004 was considered a Possible End-Permian Impact Crater Offshore of Northwestern Australia by Becker et al The Bedout impact hypothesis has, however, not been widely supported by impact cratering specialists (See a.o. abstract here. That is of course bad news for those who think that a Bedout impact is the culprit of the Great Dying, that is The Permian–Triassic extinction event that occurred about 250 million years ago, and was the Earth's most severe extinction event.

Let me today show you a picture of a more convincing looking candidate for an impact crater, and one that IS concluded in the Earth Impact database.



The Aorounga impact crater has a diameter of about 17 kilometres. The original crater was buried by sediments, which were then partially eroded to reveal the current ring-like appearance. The dark streaks are deposits of windblown sand that migrate along valleys cut by thousands of years of wind erosion. The dark band in the upper right of the (NASA) image is a portion of a proposed second crater. The crater is estimated to be less than 345 million years old. The formation age is indeed still disputed.



The impact origin is indicated by complex annular topography, the presence of shatter cones and breccias with quartz showing multiple sets of planar structures, indicative of shock metamorphism. Each ring rises approximately 100 meters above the surrounding plains.


The crater is accompanied by two nearby circular features, that may be related impact craters.

Koeberl et al. Aorounga and Gweni Fada impact structures, Chad: Remote sensing, petrography, and geochemistry of target rocks in Meteoritics & Planetary Science 40, Nr 9/10, 1455–1471 (2005)

• http://www.solarviews.com/cap/earth/aorounga.htm
• http://neo.jpl.nasa.gov/images/chad.html
• http://www.unb.ca/passc/ImpactDatabase/images/aorounga.htm
• http://www.lpi.usra.edu/publications/slidesets/craters/slide_22.html