Hey, I got a day off from field school because we did not have to use our
"rain day"...

>      What is the nature of layering in the limestone revealed in
> the walls of the Grand Canyon ?

 It depends upon which formation you are refering to (Temple Butte  
Formation, Muav Limestone, Redwall Limestone, parts of the Kaibab  
Formation, etc.).  The limestone units in the Grand Canyon have a variety  
of origins.

 I recommend:

Beus, S.S. and Morales, M., (eds.) 1990.  Grand Canyon Geology.  Oxford  
University Press: New York, p.1-518.  ISBN 0-19-505015-0

 For details of the units that are present.

> Particularly, what is it that separates layers ?

 Sedimentary layering in carbonate rocks (e.g., limestones) is  
usually produced by variations in composition and grainsize.  For example,  
there may be more limestone and less clay or silt mixed in with the  
limestone (calcium carbonate), or the carbonate fragments in the limestone  
may change in composition (e.g., from mainly fossil shell fragments to  
mainly ooids) or grainsize.  This produces differences in weathering and  
development of horizontal fractures, which is expressed as layers.

 It is easiest to describe by referring to a specific example.  You   
might see this on the side of a cliff:

                  || edge of the cliff with beds sticking out
                  ||
                  VV          All lithologies are limestones

-------------------\         grainstone (mostly sand-sized carbonate
 |   |   |   |   |  |   <--- fragments with little carbonate mud) composed
-------------------/         of crinoid fragments
----------------|       <--- wackestone (few sand-sized carbonate
-------------------\         fragments, mostly carbonate mud)
 |   |   |   |   |  |   <--- more grainstone
-------------------/    

 The wackestone, because it contains more carbonate mud, is  
weathering back into the cliff a bit more than the grainstone, and makes a  
horizontal layer that defines the bedding between the grainstone beds.   
The only real difference between the rock types is sorting -- the  
grainstone has less carbonate mud.  Producing variation between a  
grainstone and a wackestone is as simple as changing the strength of the  
currents depositing the sediment on the ocean bottom.  If the bottom has  
strong currents, the carbonate mud will stay in suspension and you will  
get a grainstone.  If the current subsides a bit, you will get wackestone.   
In many limestone formations, alternations between grainstones and  
wackestones are produced by storm activity, with each grainstone bed  
representing the passage of a major storm (e.g., a hurricane), and each  
wackestone representing quieter conditions between.  This interpretation  
is supported by the presence of erosive bases of the grainstones, the type  
of cross-bedding present within the grainstones, and the occurrence of  
similar variations in modern environments.


 
> In an argument with a Creationist, the claim
> was made that the layering in the Canyon limestone resembles the
> layering produced in an experiment at the University of Colorado
> in which multiple layers were formed simultaneously from an
> advancing mix of sediment. (Sorry, I don't have anything more
> specific.)

 This sounds like the presentation I have seen in a creationist  
video.  Yes, sorting of sediment occurs on slip faces (i.e. dipping  
surfaces of loose sediment close to angle of repose) between sediments of  
slightly different density/size (the experiments were done with   
siliciclastic sediments -- i.e. sand and silt).  This is commonly  
observed.  But the bedding normally observed is nothing close to angle of  
repose (although laminations in cross-bedding within them may be), and  
this process can not explain the many other features found within  
limestones and many other sedimentary rocks.  For example, how could this  
process explain the presence of entire coral reefs within limestones?  Or  
the presence of burrows (trace fossils) within the sediments? 

> Apparently, the claim is that hydraulic sorting can
> produce layering like that revealed in the limestone but that
> steady carbonate precipitation from sea-water should produce no
> layering at all.

 That is the point.  If it were steady, it would not be  
geologically realistic.  There are few, if any, environments that could be  
considered to have "steady" deposition.  It *always* varies to some degree  
(even in the deep sea, depositional rates are fairly constant, but  
differences in surface productivity cause slight changes in the  
composition of the sediment raining down).

 To solve the (apparent) mystery of layering in carbonate rocks  
(e.g., limestones), one only has to observe modern environments like the  
Bahamas -- storms roll through there on a regular basis and radically  
modify the sediments on the bottom, then the conditions settle back to  
"normal".  Geologists and oceanographers have extensively studied these  
sedimentary environments.  They have observed the conditions before,  
during, and after storms and seen the production of individual beds.  They  
have seen bars and dunes migrating on the ocean shelf.  If these modern  
examples were preserved in the rock record, they would look identical to  
what is observed in ancient geology, and would look quite different from  
the table-top experiments which produce only millimetre-scale laminations  
unless the conditions are changed during deposition (which mimics the real  
world conditions anyway).

--

	-Andrew
	macrae@geo.ucalgary.ca
	home page: "http://geo.ucalgary.ca/~macrae/current_projects.html"
	Check out the U. of Calgary resident Peregrine falcon:
	http://ksi.cpsc.ucalgary.ca/falcon
END****************************************************************************