Reworked fossils, was: Paleogene Titanosaurs in South America? Paleogene ammonoids in the Persian Gulf? Not quite, but analogous...

["franklin e. bliss" <frank@blissnet.com>]

Fossils you find in clasts are (more or less) by definition in a  
material different than the surrounding matrix otherwise it would be  
difficult to interpret the clast as "reworked"  The Hell Creek mud  
pebbles I mentioned are obviously rounded (sometimes even cigar sized/ 
shaped by rolling down the smooth sandy Hell Creek River Bed and are  
an obviously different lithology.  I have little doubt on the  
penecontemporaneous association of these sediments though. The  
reworking was from adjacent sedimentary environments (mud flat to  
river channel) and not reworking across and unconformity of some  
kind.  The Hell Creek Formation in particular is full of reworked  
sediments from adjacent environments because of stream action moving  
mass quantities (barge loads!) of previously deposited sand around by  
the cut and fill process over and over again.  (I look for these  
pebble/sands to find microsites.)

While some of the HC sands that you find are almost loose like modern  
sediments, others are fairly well lithified and quite hard.  There is  
a continuous spectrum in between these two end members of  
consolidation as well.  Younger Fort Union (Tert) sediments are known  
to have an occasional dino bone contained within  ( EG:  Evidence of  
Reworked Cretaceous Fossils and Their Bearing on the Existence of  
Tertiary Dinosaurs by Jeffrey G. Eaton, James I. Kirkland and Kentaro  
Doi,  PALAIOS, Vol. 4, No. 3 (Jun., 1989),l).  I see no reason why  
blocks of lithified sand with fossils within couldn't move as well as  
just the bone.  I have seen whole bone and even wood chunks in very  
hard cemented sandstone that would move very nicely down a younger  
Fort Union river channel assuming that the channel cut into older Hell  
Creek sediments and dislodged the clast that contained the bone.  Some  
of the older sediments look (composition and texture) identical to the  
younger sediment but differ in their fossil content. (Having  
Cretaceous plant/animal remains in a Tertiary formation).

There is no reason that two formations separated by great time but  
representing similar re-occuring sedimentary environments couldn't  
have an obfuscating tendency of the upper formation to contain  
reworked fossils from the lower (older) formation.  Having an  
unfortunate location/sample selection could really throw off your  
analysis.  I think this is pretty rare though I could take you right  
to a spot where Hell Creek faunal fossils from a rich microsite up  
slope are fairly common in the Holocene loess blanket below. This  
phenomena would only be obvious to the trained eye, or when  
lithologies/associations between the clasts and the matrix are  
significantly different.

I have little question that sediments within the Hell Creek have been  
reworked from lower positions in the formation to more upper aspects  
by some of the fairly large rivers that were sweeping through the  
region at the time.  To what degree is the question, a big river might  
cut 20 or 30 feet down and move that sediment to a river bank and then  
repeat the process 20 times over the long length of a river and the  
time span of the formation.  This could lift a particular fossil quite  
a distance in the formation hypothetically.  This is a consideration  
to those trying to create zones of fossils is a hyper-active fluvial  
system like HC.

This could relate to seed ferns easily as the number of seeds would  
certainly suggest a greater probability of reworking.  Sample size,  
Sample size, Sample size.....I could attribute complete dinosaur bones  
to the Holocene taking only that one microsite into account (and not  
finding the real the source uphill).  The key is the mixed taxa  
present in the rocks or sediments.

Frank (Rooster) Bliss
MS Biostratigraphy
Weston, Wyoming
www.wyomingdinosaurs.com


On Apr 11, 2008, at 5:35 PM, David Marjanovic wrote:
> > You can find reworked seeds and leaves in clasts (chunks)
> > of older sediments.
>
> Yes, but no such things are mentioned in the paper, and they should  
> be pretty easy to recognize, shouldn't they?