extremely high-count sample

[BobWill]

I am new to micros but I have had at least a cursory look at bulk samples from a variety of sites and ages, mostly Texas and Pennsylvanian. The material from this one Pennsylvanian site in Oklahoma seems so far out of the typical range for quantity of fossils I am wondering what others think. Most of what I have looked at will show me a fossil for every 50 to 100 rocks and I consider that normal. This site has matrix that, when cleaned and screened to remove the finest shale particles (60 mesh) has hardly anything but fossils! The biggest problem with that is deciding what to keep and what to discard, however, if the trade-off is diversity over quantity I might prefer to see less fossils.

 

I found mostly broken pieces of bryozoans but quite a few crinoid parts and not mostly stem sections like I usually see. There are some brachiopods and a few corals but the paucity of mollusks seems odd. In the first batch I took home there were no bivalves, no cephalopods and only one gastropod! Odder still I did find a trilobite genial spine and the tip of a conulariid, things that are usually far less common. There were also a few conodonts or maybe scolecodonts and some ostrocods. I went for another gallon baggie-full and finally got a few more gastropods, very few bivalves and one orthoconic nautiloid. Besides the large quantity of fossils I am curious about what conditions might contribute to that lack of diversity.

 

I can not be certain about the formation but it appears to be in the Deese group. Geologists I have asked say the area is a geological mess so anyone who could volunteer to clear up the confusion is welcome to have a look! It is west of I-35 and south of Ardmore. Below are photos of the matrix after I screened out the finest shale particles. Scale is millimeters.

 

 

 

 

[DPS Ammonite]

I see several questions here.

 

Why is the density of small fossils so high? 
 

Answer: the fossils were sorted by size because of deposition conditions such as current speed.

 

Why is there not a variety of major fossil types?

 

Answer: most Carboniferous rocks have a low diversity of fossils. The diversity of Pennsylvanian fossils at Jacksboro is an unusual exception. You found a more ordinary low diversity deposit. I think that you found the most common Pennsylvanian types that are preserved: crinoid parts, brachiopods, bryozoans and corals.
 

I see that most of the fossils preserved are calcite. The ones that are more aragonitic (think mother of pearl lined) such as bivalves, gastropods and cephalopods tend to be less stable over time and more likely to disappear from the record. If you grind up the fossils the less stable ones are even more likely to disappear.

 

Most fossil deposits have only the most likely to be preserved organisms present. The vast majority are never preserved.

 

 

 

 

[Missourian]

Re: size, bioturbation could be a factor