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Another
break in our summer monsoon, and last weekend we were out again
back at our fossil outcrop in the Permian Fort Apache Limestone
east of Payson Az. First, to get you in the right mind set about
this formation, a bit of information. While the entire Mogollon
Rim area (south side of the huge Colorado Plateau) is all permian
both marine and terrestrial, There are several different transgressions
of the sea to visit here. The brick red Schnebly Hill formation
- which is equivalent to the Supai in the Grand Canyon to some
extent - is a huge fossilized Sahara style dune field, composed
of non fossiliferous sandstones with huge eolian crossbeds.
This is the very same rock which makes Arizona's town of Sedona
famous for its red colored buttes. BUT, a narrow band of marine
lies in the upper half of this red bed sequence! it is a grey
micrite like limestone ranging from about 10 feet thick in Sedona
to around 100 feet thick at the type section in Fort Apache on
the indian res. Now for the bad news. It is nearly completely
non fossiliferous. We have spent years exploring outcrops of
this limestone all over the state and come up empty handed.
Until now. A few years ago we found an outcrop along the Highline
Trail east of Payson which was rich with very tiny fossils.
A few selected specimens were placed in the pool acid and we
discovered that the limestone dissolved easily and left tons
of fine sand and dirt, and some very tiny fossils that were silicified.
FInally - our first site with fossils in the Fort Apache! But
why were all the fossils missing from everywhere else? We have
hoped to collect enough of the material from this site to begin
to answer that question. (we have, more on that later...).
Todays posting will initially start with one of the smallest
of fossil found, the ostracods. Recall from your basic first
grade biology class that ostracods are shrimp like crustaceans
enclosed in a bivalve shell. They feed by extending thier modified
legs through the opening in the shell and collect plankton. We
found at least three types picking through over the acid fines
from over 60 pounds of limestone. I wont attempt to identify
them, that is for an expert on these tiny crustaceans. First
lets look at the site, and give you a look at where we collected
our material.
From the parking lot at the trail head, you can see directly
ahead the 20 foot thick layer of Limestone. We are going to
an area just below its scree slope.
Here I am at the outcrop.
Looking closely at a fossiliferous boulder, you can see some
of the few larger fossils, productids. The layer is also packed
with some urchin material.
Looking for promising rocks for tiny fossils, I collected anything
that had fossils on the surface.
One slab of trace fossils too. It was the only one we found that
day.
Back home, in the paleo lab we sorted out the prospects for the
acid bath. We use 10% muriatic in pails out side and sieves
of various sizes to get rid of the loads of dirt mixed in. Also
I mention that the limestone is very "fetid". This
is a strong petroleum like smell that occurs when you hit the
rock with a hammer caused by enclosed organics.
Now for some microscope shots. Takes many hours to pick out
the specimens from the acid fines!
Ostracods, 5x with mm scale. this by far was the most common
type we found. They are fully silicified and have an oval or
rectangular shape, and a slit on one side for the arms to protrude.
This second type of ostracod had a very neat ornament on the
shell exterior. Very rare too, we only found half a dozen of
them in 60 pounds of rock.
15x close up of ostracode #1
Now for a few 30x shots. This is the max for my stereo microscope.
The second type is quite interesting at 30x.
and TWO stuck together~
Well, thanks for looking, we have more to write on this trip
and some of the other micro fossils we found. It is starting
to become clear to us that the Fort Apache limestone does indeed
have some very tiny fossils to be found. And next write up Ill
you why....
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