More adventures in the Fort Apache Limestone

[Arizona Chris]

Greetings all,

 

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.  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.  Oh my.

 

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....

 

 

[gavialboy]

wow. great pictures thanks for posting