Freethought & Rationalism Archive

Valentine Pontifex · 08-06-2002, 04:40 PM

Are there examples of desert varnish in the geologic record? Someone like Patrick could make good example out of it. It would demonstrate that the rock was exposed to heat and sun for hundreds of years. A few examples of this in the middle of the deposits from the so-called "Deluge" might be interesting examples to use against the YECs.

notto · 08-06-2002, 06:54 PM

<a href="http://www.glenn.morton.btinternet.co.uk/varnish.htm" target="_blank">http://www.glenn.morton.btinternet.co.uk/varnish.htm</a>

Dr.GH · 08-06-2002, 09:27 PM

There were attempts to use the thickness of desert varnish, and also the ratios of various isotopes, as the basis of dating methods. To date they have all been failures.

A good deal of Rock Art in the Colorado Desert is incised patterns cut through the dark varnish in to the light colored base rock. I presented one paper on this.

Gary S. Hurd, Melissa Pryor
1996 "The Close Analysis of a Desert Varnish Incised Boulder" 21th Annual Rock Art Conference,
San Diego Museum of Man.

ps418 · 08-07-2002, 12:57 PM

Quote:

Originally posted by LordValentine:
<strong>Are there examples of desert varnish in the geologic record? Someone like Patrick could make good example out of it. It would demonstrate that the rock was exposed to heat and sun for hundreds of years. A few examples of this in the middle of the deposits from the so-called "Deluge" might be interesting examples to use against the YECs.</strong>

I've got something much better up my sleeve for my next 'project.' I'm in the process of reading a bunch of papers and reviews of 'cosmogenic nuclide exposure dating' and its geologic applications. Using this method, it can be demonstrated that many minerals and/or 'geomorphic surfaces' have been at or near the earth's surface for at least millions of years, particularly in some parts of Australia and Antarctica, where chemical weathering rates are extremely slow.

Some of the coolest applications include: the dating of meteor craters, the dating of glacial tills and glacial striations, the quantification of erosional processes such as river incision and mountain denudation, the quantification of displacement rates along normal faults, and much more.

Patrick

Coragyps · 08-07-2002, 01:19 PM

Patrick - <a href="http://www.newswise.com/articles/2002/7/GRNDCNYN.UAZ.html" target="_blank">This press release</a> talks about cosmogenic nuclides in the context of dating lavas in the Grand Canyon. I didn't try very hard, but I didn't find any primary references online; the news release gives names/affiliations to go digging in the geology literature with, though.

ps418 · 08-07-2002, 01:50 PM

Quote:

Originally posted by Coragyps:
<strong>Patrick - <a href="http://www.newswise.com/articles/2002/7/GRNDCNYN.UAZ.html" target="_blank">This press release</a> talks about cosmogenic nuclides in the context of dating lavas in the Grand Canyon. I didn't try very hard, but I didn't find any primary references online; the news release gives names/affiliations to go digging in the geology literature with, though.</strong>

I dont think this research (on downcutting rates in Marble Canyon) has been published yet. At least the press release doesnt indicate that it has been. However, I have one of their (Webb and Cerling's) papers published in Geology last year, where they used cosmogenic 3He to date basalt flows, and showed that the dates derived from this method were concordant with those derived from thermoluminescence and new 40Ar/39Ar dates for the same flows. Just a coincidence, mind you!

ps418 · 08-07-2002, 03:36 PM

There is a free article available from the GSA disussing the use of rock varnish as a proxy for paleo-precipitation:

<a href="http://www.gsajournals.org/gsaonline/?request=get-document&issn=1052-5173&volume=011&issue=08&page=0004" target="_blank">Broecker, W., and Tanzhuo, L., 2001. Rock Varnish: Recorder of Desert Wetness? GSA Today 11(8), pp. 4-10.</a>

Quote:

ABSTRACT: Rock varnish is a thin coating (<200 �m) of a cocktail rich in Mn, Fe, and clay minerals that is ubiquitous in desert regions. It has become the center of a contentious controversy revolving around its use to date geomorphic surfaces and/or to evaluate past climate conditions. We observe pronounced temporal variations in Mn and Ba concentration that are similar over large regions and that likely relate to variations in paleo-wetness. The mode of formation of varnish remains uncertain, but anthropogenic Pb concentrated in outermost varnish layers indicates its continued formation, and experiments using cosmogenic Be suggest that, while precipitation is a primary control, dust, dew, and aerosols may also be important in delivering the ingredients of varnish. We suggest several steps that may lead to rejuvenation and future break-through in varnish studies.

08-06-2002, 04:40 PM	#1
Valentine Pontifex Veteran Member Join Date: Jul 2001 Location: Orion Arm of the Milky Way Galaxy Posts: 3,092	Desert Varnish Are there examples of desert varnish in the geologic record? Someone like Patrick could make good example out of it. It would demonstrate that the rock was exposed to heat and sun for hundreds of years. A few examples of this in the middle of the deposits from the so-called "Deluge" might be interesting examples to use against the YECs.

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08-06-2002, 06:54 PM	#2
notto Regular Member Join Date: Jul 2001 Location: Minneapolis, MN US Posts: 133	<a href="http://www.glenn.morton.btinternet.co.uk/varnish.htm" target="_blank">http://www.glenn.morton.btinternet.co.uk/varnish.htm</a>

08-06-2002, 09:27 PM	#3
Dr.GH Veteran Member Join Date: Jul 2001 Location: Dana Point, Ca, USA Posts: 2,115	There were attempts to use the thickness of desert varnish, and also the ratios of various isotopes, as the basis of dating methods. To date they have all been failures. A good deal of Rock Art in the Colorado Desert is incised patterns cut through the dark varnish in to the light colored base rock. I presented one paper on this. Gary S. Hurd, Melissa Pryor 1996 "The Close Analysis of a Desert Varnish Incised Boulder" 21th Annual Rock Art Conference, San Diego Museum of Man.

08-07-2002, 01:19 PM	#5
Coragyps Veteran Join Date: Aug 2001 Location: Snyder,Texas,USA Posts: 4,411	Patrick - <a href="http://www.newswise.com/articles/2002/7/GRNDCNYN.UAZ.html" target="_blank">This press release</a> talks about cosmogenic nuclides in the context of dating lavas in the Grand Canyon. I didn't try very hard, but I didn't find any primary references online; the news release gives names/affiliations to go digging in the geology literature with, though.

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