Re: Fastovsky vs Archibald

[frank bliss <frank@blissnet.com>]

On Jun 25, 2005, at 5:59 AM, David Marjanovic wrote:

> Drought in the Hell Creek?


I believe that the Hell Creek microsites I sample (unsorted mud pebbles 
mixed with medium sand) that have fossils are a result of pricipitation 
perhaps controlled by seasonality.  The mud pebbles are pretty 
obviously the result of a flood event ripping up mud polygons (dried 
out mud) and rolling them along until rounded prior to their final 
deposition.  I find these still somewhat friable mud pebbles in all 
shapes and sizes from cigar shaped (4 or 5 inches long) to spherical in 
aspect.  These microsite lenses are certainly accumulations of river 
sand mixed with the now rounded mud polygons washed from mud flats 
surrounding the river which also washed any potential fossil detritus 
sitting on the mud along with them.  In order to have dried out mud, 
you had to have a dry period lasting at least a month or so.  These 
microsite lenses are quite laterally discontinuous, often truncated by 
stream migration.  Some are quite fossiliferous and some are poorly so, 
but all have the lense shapped sand/pebble nature.  I can't speculate 
accurately on how much time the mud polygons were dried out over, just 
that they were dried out.  There had to be an overbank flood, 
evaporation of the standing water with the resultant deposition of the 
mud, complete drying out of the mud and then some time until the next 
flood washing the old stuff around.  This could certainly occur 
seasonally. Thus a dry season at a minimum and maybe even a drought or 
two mixed in over the 10 million years it took to accumulate the 
section.
Frank Bliss
MS Biostratigraphy
Weston, Wyoming