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2005-03-22

Silverpit structure

There has been a bit of discussion lately on the interpretation of the "Silverpit" structure in the Southern North Sea.
A couple of years ago a paper was published in Nature about a circular set of faults surrounding a sub-surface depression in the top-Cretaceous reflector. The interpretation was that this was an impact structure. A summary of the paper can be found at the Geological Society website (if you're really interested, I can email you the paper). Well, there have been alternative interpretations published over the last few months, re-interpreting it as a sagging structure caused by withdrawl of Zechstein salt at depth. John Underhill (of Edinburgh University) has presented data orthogonal to the original section (link above), which does put a very different interpretation on the structure.
It still doesn't look right though - the "central peak" structure in the original paper is still a very anomalous structure, and Underhill's interpretation of thickness variations in the overburden to the salt is inconsistent - the Cretaceous thins in one direction and the Jurassic in another, combining to give the appearence of synsedimentary halokinesis. It still looks odd to me.
I am very surprised at a seismologist of Phil Allen's experience not looking out-of-plane though.
(In other comments since I first wrote this, Mr Allen claims he did, but disagrees with Underhill's interpretation.)

2 comments:

Anonymous said...

What do you mean, I didn't look out of the plane? It was a 3D data volume. I had an infinite number of planes to look at, both horizontal and vertical. (And I looked at a large number of these over a three-month period.) Can you explain or remove this unnecessary comment? Thanks

Phil Allen

Aidan Karley FGS, BSc said...

Hi Phil, long time/ no see.
Yes, in the cold light of day my original question does sound somewhat harsh. Obviously you would have been looking out of the plane, at various orientations, etc. But that still leaves the question of what your reasons are/ were for discounting the asymmetries that Underhill notes.
Obviously no-one (serious) would expect perfect "Meteor Crater" levels of symmetry from every proposed impact structure, but if I remember your original paper correctly, you did make quite a deal of the structure's symmetry. (BTW, I'm offshore at the moment and don't have access to my hard drive full of papers at the moment. Also, I don't have an email address for you, so I can't reply directly. But since you're the first person to comment on this in the 2 years since I wrote it, I guess the site is fairly low traffic.)
I was thinking about this very question just a week or so ago. While cruising the Jnl.Geol.Soc archives I had come across Journal of the Geological Society, Volume 156, Number 3, May 1999, pp. 487-499(13) by S.A.Stewart of Amerada (then), where he considers models for the formation of a number of circular structures in the Mesozoic of the Moray Firth. Impact structures are something he explicitly considers, but rejects (in favour of caldera/ maar structures). But examination of figures 3 and 4 will show how he addressed displaying the level of symmetry of the structures by putting 3 short intersecting sections together at 120deg azimuths.
When I saw that I thought "Aha, someone learned from the the Silverpit debate". But writing up this comment I just noticed that it was a 1999 article. Serves me right for clicking at random in the archives instead of systematically working my way through them.
Anyway, once I'm back onshore I'll snip and copy and paste and type a more detailed presentation of what I mean, referring to these various precedents. It would be interesting to have your input on why you chose the presentations you did. I've never had to write up a paper for the formal press, so I don't really know how tight they are on image scales, resolution, colour count etc for publication. It's enough of a PITA getting the image sizes right for presenting a safety alert to a [OilCompany] system that only accepts alerts of under a megabyte. That was last night's headbutting-the-brick-wall entertainment.

Hmmm, I see the JGS has a "Discussion" on Silverpit. I think I'd better read that at my next cigarette break. (I didn't have access to the JGS archive when I wrote the original article.) Oh well, blogs are meant to be "work in progress". And I don't have access to 'Nature' now (the wife objects to a quarterly mound of paper from the GeolSoc - so you can imagine what she'd say to a weekly mound!), which is why I've only just cottoned (back) on that S.A.Stewart was your Silverpit co-author. That really does make the question of the presentation of multiple planes of data in the Silverpit paper rather different. Nature has a noticably greater pressure on square inches then JGS, I guess.

BTW, have you noticed the discussions about the Java mud volcano recently? You know - the one that's an underground blowout that's broken through to surface and illuminates natural mud volcano processes.