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Heteromorph

In June of 2018, Kieth Minor alerted me to a new apartment complex that was being developed in the middle Atco Formation of North Texas. They were cutting a huge cubic area of rock out of a hillside, piling up multiple large mounds of Atco which seemed to be begging for someone to carefully search out their freshly exposed contents. On the 15th of that month we got to the site, Kieth asked permission of the site foreman for us to carefully take a look around during the crew's work hours, and we made our way into the pit. We were on a mission to save as many ammonites as we could.

 

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FIG 1: First impressions.

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Upon entering the pit, I was immediately struck with a familiar scent; the scent of long dead fish. Or more accurately, the smell of the (hydrocarbon rich?) dark gray marl that seems to always contain a much higher concentration of vertebrate material than average white chalk. It is the same scent that pervades the basal Atco sections in the Martin Marietta quarry, especially if there hasn't been rain since their last round of quarrying. This was a promising sign on the vertebrate front. 

 

We continued in until we got to the western wall of the pit, clearly exposing some lovely geology, but more specifically a thin, dark gray horizon that was roughly what I was suspecting would be there. Looking around further we found another dark gray layer, about as thick, not quite a meter below the first we spotted. A few hammer whacks proved that these two were indeed vertebrate bearing, with fish bits scattered across almost every laminated sheet of "blue" rock I split open, along with lots of squashed inoceramids.

 

Going down section a bit further we came across an unusual horizon. Walking though a corridor that was excavated for a concrete wall that was yet to come, we found an approximately third of a meter thick layer of orange/red clay with white waxy clumps. This layer stands out very prominently, situated between the normal gray chalk directly above and below it. It could be easily dug out with my pick, and was already crumbling out along the sides when we got there. There are no visible fossils in it.

 

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FIGS 2-4: General section shots. Can you spot the odd clay layer yet? Moreover, can you spot the fault? 

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FIG 4: Keith measuring sections. The clay layer can be seen on both sides. 

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FIGS 5-7: Clay layer and fault closeups.

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FIG 8: Clay close-up.

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Heteromorph

Not quite sure what to think about that, Keith went on to thoroughly search the piles and measure sections while I spent most of the day splitting the dark laminated marl. Along with the usual unidentifiable fish bits and tiny verts, I came across my first Enchodus jaw, which turned out to be the best paleo find of the day.

 

Despite our valiant efforts, the only identifiable fossils found were inoceramids and the fish material. No signs of ammonites were found, which makes unfortunate sense since every middle Atco site that I have thus far hunted shows a similar dearth of ammonites. Not sure why, but Keith says the middle Atco is known to generally be the ammonite doldrums.

 

Keith made sure to take some samples from the odd clay layer, which he thought might be a bentonite bed that could be used to stratigraphically correlate this site with others and give us a better idea of where we were.

 

After this first hunt he asked if I could go back and gather some more samples from the clay layer, which I did. A few weeks after that, the concrete walls were in place, then the piles were hauled away, and now it is just part of our ever growing urban sprawl with no sign of the history the underlies it. It is probably full of people at this point, but such is the ephemeral nature of sites in North Texas.

 

 

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FIG 9: Keith surveying the piles, with no luck except pretty inoceramids.

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FIG 10: A section of the exposure showing one of the dark gray layers.

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FIG 11: My area to rip up the dark laminated marl in search of fishy success. 

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FIG 12: My Enchodus sp. jaw.

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FIG 13: Clay samples.

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Last year I donated the samples and some of the fish bit plates to a museum (that I already forgot the name of) for an interested expert there to see if it really was a bentonite bed. Last I heard, the expert did not think that I was a bentonite bed, but just an oddly placed clay layer. Still interesting to me.

 

 

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FIG 14: Finally, Keith's outcrop log of the site.

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Tidgy's Dad

Very interesting adventure you've just taken me along on. 

Thank you.:)

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Uncle Siphuncle

I too savor the petroliferous effluvium of the Eagle Ford.  Watch for Coilopoceras, Scaphites and Prionocyclus in that section.  I've taken all of the above at different sites in or near the contact zone from Central to North TX.  In fact, Keith put me on a good Atco construction site in downtown Austin a few years back. He hooked my wife and me up with the project foreman, who was curious about the Ptychodus and other things he was bumping into on the job.  

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2 hours ago, Uncle Siphuncle said:

I too savor the petroliferous effluvium of the Eagle Ford.  Watch for Coilopoceras, Scaphites and Prionocyclus in that section.  I've taken all of the above at different sites in or near the contact zone from Central to North TX.  In fact, Keith put me on a good Atco construction site in downtown Austin a few years back. He hooked my wife and me up with the project foreman, who was curious about the Ptychodus and other things he was bumping into on the job.  

Isn’t it lovely?

 

I am afraid we were a bit above the section of which you speak, but I have found Scaphites in the Atco far above the contact zone. I remember reading your Meandering’s post about that hunt, and the only information I have to add to it is that the Scaphites that you found appear to be Scaphites semicostatus, or an allied species. I have found that species in the lower and upper Atco myself.

 

That site must have been exhilarating to hunt! Keith is a good friend to have.

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3 hours ago, Tidgy's Dad said:

Very interesting adventure you've just taken me along on. 

Thank you.:)

Thank you. Telling stories is a fun way to relive them. :)

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The Amateur Paleontologist

Great report, very nicely illustrated :) Of course the best part is the Enchodus jaw!

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Thanks for this very interesting and infomative report!

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Uncle Siphuncle
7 hours ago, Heteromorph said:

Isn’t it lovely?

 

I am afraid we were a bit above the section of which you speak, but I have found Scaphites in the Atco far above the contact zone. I remember reading your Meandering’s post about that hunt, and the only information I have to add to it is that the Scaphites that you found appear to be Scaphites semicostatus, or an allied species. I have found that species in the lower and upper Atco myself.

 

That site must have been exhilarating to hunt! Keith is a good friend to have.

That makes sense as I was hugging the contact +/- 20 strat feet, and I kept being drawn to the Kef, probably by petroliferous effluvium intoxication!

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Fossildude19

Great report!

Thanks for posting it. :) 

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Heteromorph
On 4/12/2020 at 3:57 AM, The Amateur Paleontologist said:

Great report, very nicely illustrated :) Of course the best part is the Enchodus jaw!

 

On 4/12/2020 at 6:28 AM, Ludwigia said:

Thanks for this very interesting and infomative report!

 

On 4/12/2020 at 9:51 AM, Fossildude19 said:

Great report!

Thanks for posting it. :) 

I am very glad that you all enjoyed it! :)

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