Hi Frank,
Thanks for your note.
It's been great to get such a lot of positive feedback since the publication of
the article. I have enjoyed the debate and there has been a lot of useful
information sent to me now, which I really appreciate.
Sorry that you (and others) have had such difficulty seeing the text. I have
attached a copy of the final proof to this e.mail for information. If that fails
you should be able to download it from my University WWW page:
Home page: http://www.geos.ed.ac.uk/contacts/homes/jru/
In answer to your specific queries:
Yes, the lines are over the same feature. The line that was used in my article
is a representative SW-NE trending regional line that goes right through the
"crater". The line used by Simon and Phil runs NW-SE, that is parallel to the
regional structural trends. I am not sure why they chose that orientation to
illustrate the depression.
I agree that the differences between the lines are intriguing including the
absence of a "central peak". My understanding is that they interpreted on depth
migrated data (that is with an interpreted velocity profile already applied),
whilst mine is based on the raw time data for both 2-D seismic grid as well as
all the 3-D volumes.
The other obvious difference between the line that Simon and Phil used and the
one in my Nature article is, of course, theirs is cut-off well above the mobile
evaporites - leaving the link between their thickness and the "crater"
undocumented. I can show that the "crater is a roughly circular feature that
lies in the centre of a NW-NE trending narrow elliptical depression that
significantly, affects all stratigraphic levels down to and including the Top
Zechstein Group.
That halokinesis also occurred prior to the Cretaceous is well-documented by
truncation and thinning of earlier stratigraphic levels. I attribute the
truncation of the Triassic seen on the strike line of Stewart & Allen to an
earlier phase of salt movement when a pillow formed under what I now the "crater".
Importantly, thinning of the Upper Permian evaporites is entrely coincident not
only with the shape of the "Silverpit Crater", but also with at keast ten other
such depressions in the area. Unfortunately, Nature would only permit the use of
one figure to illustrate the article - understandably because of pressure of
space. However, any 2-D or 3-D line across the feature that is not cut-off
mid-way down, shows the same effect - irrespective of orientation
Finally, on the point of the style of faulting in the syncline, the nature of
deformation documented by the papers quoted in the article (such as by Mike
Branney and Steve Maione) demonstrate that extension can also form in just such
a setting.
Hope these answers help. Would be happy to hear more if others want to continue
the debate.
Best Wishes
John
Professor John Underhill
Chair of Stratigraphy & Fellow of the Royal Society of Edinburgh (FRSE)
Work Address & Contact Details:
Grant Institute of Earth Science,
School of Geosciences,
The University of Edinburgh,
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Edinburgh,
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Home page: http://www.geos.ed.ac.uk/contacts/homes/jru/
Quoting "Peel, Frank FJ" <[log in to unmask]>:
> John Underhill's note is interesting but I'm confused - I can't reconcile
> the seismic images of the feature shown by Stewart and Allen which can be
> found at
> http://www.geolsoc.org.uk/template.cfm?name=Silverpit
> with the 2D line that John Underhill shows at
> http://www.geolsoc.org.uk/template.cfm?name=Silverpit2
>
> On the Stewart and Allen picture, the feature is centered over an relative
> high at Triassic level - in the Underhill picture it's centered over a
> syncline. Are the two lines over the same feature?
>
> I've looked at the fault patterns related to many salt withdrawal basins and
> domes and in salt-cored fold belts in the Gulf of Mexico and West Africa,
> and I have not seen a circular fault pattern like the one shown by Stewart
> and Allen in any of the synclines. In general we see a crestal fault
> pattern, commonly radial unless it is in a compressional setting, usually
> centered on the salt dome or anticline (not the basin), and the intensity of
> faulting diminishes downdip - the synclines are usually very sparsely
> faulted. So on the basis of these pictures I'd favor the original S&A
> interpretation.
>
> John is correct - you can't access that Nature article without a
> subscription.
>
> There is a huge amount of good 3D seismic data of salt-withdrawal synclines
> so I'd start looking for analogs there - caldera collapse isn't a good
> analog, it's mechanically very different.
>
> Cheers
> Frank Peel BHP Billiton
>
> EOM
>
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