Monday, November 2, 2009

End Cretaceous: How Many Impacts? How Many Iridium Anomalies?

Looking over the recent GSA geoblogosphere page I was alerted to a presentation by Shankar Chatterjee and colleagues on the Shiva crater, a hypothetical impact crater that Chatterjee claims is of end Cretaceous age and is likely responsible along with the Deccan volcanism for the mass extinction. He suggests that the Chicxulub impact event in Mexico also timed to the end Cretaceous did not alone cause the mass extinction.

Here is the abstract. The talk apparently got a skeptical reception. Chatterjee envisions the following scenario. The meteor hit the Indian continental plate and maybe initiated rifting between Seychelles and India.

Source: Chatterjee- Proc 30th Int. Geol. Congr. Vol 26, pp - 31-54

The crater is now broken up along the rift. Part of the structure lies along the Indian west coast and part of it along the Seychelles margin.

The argument is at present based mostly on the geometry and structure of the basins and rifted margins matching what a meteorite hit will produce if it hit the crust at a  shallow angle from the southwest.

But according to what I have read so far there is little direct local evidence of this being an impact crater. There is no core recovered from the supposed impact site and so no information available on whether the features that are seen at the Chicxulub crater in Mexico i.e. impact melt rock, high pressure quartz phases, chaotic breccia,  tsunami deposits and so on are present at this site too.  Also Chatterjee has not given as far as I know a convincing account of why various models of continental rifting, flood volcanism and denudational isostacy are inadequate explanations of the continental margin geology of west India and the Seychelles.

Yet, end Cretaceous always fascinates.

Chatterjee think this impact occurred either simultaneously with the Chicxulub impact or it post-dates the impact in Mexico. I found this view point very interesting for the response it drew from Gerta Keller who is a strong proponent of the theory that the Chicxulub impact was not the primary cause of the K-T extinction. She is pushing for the Deccan volcanics being the real culprit.

Keller has been quoted rejecting Chatterjee's theory, saying there is no evidence for such an impact in Indian deposits. But she leaves a big question unanswered, one that is prompted by her own work and theory.

How do you explain the iridium anomaly associated with many K-T boundary sediments?

Keller contends that the Chicxulub impact took place about 300,000 years before the K-T boundary (read the debate here). The iridium anomaly at the K-T boundary is widely acknowledged - including Keller - to be of meteorite impact origin. In a previous paper she has proposed the possibility of multiple impacts to account for this iridium anomaly, even naming the Shiva crater as a potential candidate for a second impact site.

Now she seems to be against the Shiva impact theory and cites a lack of local evidence in Indian continental deposits for the Shiva impact event, but see....let me add an intriguing possibility... the Deccan volcanics complicate the stratigraphy and everything depends on the exact timing of the impact.  If the impact did not coincide with deposition of sediments and instead took place during eruptions, then all that evidence could be lost..obliterated within the lava flows. At end Cretaceous in the Indian continent these events, volcanism and sedimentation phases, alternated over periods of tens to a few hundred thousand years, and how can one be sure if the impact didn't coincide with a period of non-deposition and volcanism?

Given Keller's strong stance that there was no single smoking gun for the K-T mass extinction its a little bizarre that she is now dissing Chatterjee's hypothesis so readily and apparently rooting for the Deccan volcanics as the smoking gun!

So one impact definitely but how many more? ...still unanswered..

The most sympathetic view of Chatterjee's hypothesis was from The Economist who suggested that if the Shiva impact post-dates the Chicxulub impact, there should be two stratigraphically separated iridium enriched layers. The article pointed out to the Anjar deposits in west India as a site which might contain these two iridium anomalies.

I followed up on the work on the Anjar sediments. These are lacustrine (lake deposits)  inter-trappean sediments i.e. they are sandwiched between Deccan volcanic layers. Magnetostratigraphy indicates the sediments to be deposited in the latest Maastrichtian (the uppermost division of the Cretaceous) within the magnetic zone 29R (R- reversed magnetic field), which contains the K-T boundary at many locales. There are three iridium enriched layers each occurring below a clay rich bed.

Sounds like an ideal candidate in support of multiple impacts.

...Except there seems to be no evidence that these iridium anomalies are due to meteorite impacts or that they encompass the K-T boundary. Geochemical analysis shows that the section lacks the negative carbon isotope excursion that is characteristic of the K-T event found in terrestrial deposits elsewhere (see here). There is evidence in the form of high temperature low pressure cristobalite ( a variety of quartz) in the clay beds associated with the iridium (see here) that these could represent leaching and enrichment from a alkali volcanic tuff. Impact deposits should contain high pressure varieties of quartz.

Early alkali Deccan volcanics along the western margin of the rifted Indian continent are known to be enriched in iridium (see here) and leaching and secondary enrichment could form concentrated iridium layers.

Finally the sediment above the iridium enriched layers contain dinosaur egg shells and late Cretaceous ostracods (see here). Along with sedimentological features this indicates that the iridium layers at Anjar  predate the K-T extinction significantly and were formed in the early part of magnetic cron 29R.

It does not appear that these particular iridium layers are a result of the Shiva impact if it did occur. These deposits are well within the impact fallout zone for the Shiva impact and should have contained direct impact evidence like shocked quartz.

So....looks like many iridium layers but not all of them associated with the K-T boundary or with meteor impacts. 

The Chicxulub impact and the Deccan volcanism overlapped. The stratigraphy of end Cretaceous is going to be messy and it won't be possible to sort out events that took place with a few tens of thousands of years of each other and tie them to either an impact or volcanism. If there was a second impact its record could easily be amalgamated with that of volcanism and very hard to resolve within Indian continental deposits as an independent event .

Direct drilling and study of the recovered core from the Bombay High basin basement which is supposed to be the impact site should decide the issue

...if and when that data comes around.

My head is reeling with these details and nuances. I'll leave readers to sort through the evidence.

No comments:

Post a Comment