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Fickle Basal Atco Member - Summer 2022 Hunt


LSCHNELLE

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I have been hunting the Basal Atco member of the Austin group in Central Texas for about 7 years. Others say that it is the uppermost part of the Eagle Ford. I don't doubt that it is some type of erosional zone involving the two. I have found hundreds of fossils in it. Most of them are beautiful reddish brown color.

 

Most of them have been damaged somewhat by paleo-erosion before they were left in their final state. Many Basal Atco fossils are very interesting to me, because I can't find them anywhere else. My first area to hunt back in 2015 was a lot of fun. An exposed bluff where a giant chunk of the overhanging bedrock limestone had fallen down into the creek.

 

I'm glad I wasn't under that whenever it fell. But, it left behind a nice exposure of what I came to think represented the Basal Atco. Boy was I wrong. Over 100 Ptychodus and other shark's teeth from the same area within a 1 ft thickness of conglomerate like material.

 

Since then I have been at multiple locations of the same Basal Atco with many different expressions of what that might look like from a soft shaley mudstone to a hard rock-like conglomerate to primarily white thin sandy limestone seams with only occasional fossils in the middle of the bedrock. Most of these Basal Atco exposures have no fossils WHATSOEVER! 

 

I have found only half again as many Ptychodus fossils in all the other Basal Atco sites combined. Maybe I'd have better luck hunting the gravel banks - which is not my forte.

 

I found a new location that I was not aware of early this summer. It was kind of fun hunting it because it was different than all the rest. I pulled out a partial ammonite and knew that I was in the Basal Atco formation and that there might be more fossils in there. It was obvious that other people had probably been hunting it some before I had been there.

 

I came a second time and walked the football field length of the seam. If you've ever hunted the Basal Atco, then you know you might find yourself under an overhang of a large bluff that could collapse on you at any moment. This one was not that bad!

 

I have generally avoided the areas with the most extreme overhangs. Those where you feel like you have entered a cave-like area (like Mesa Verde) where the rock acts like a huge roof over your head - that (in my overly cautious mind) could fall at any moment.

 

So my goal was to hopefully find a Ptychodus martini. I have developed long-term inflammation in my wrists and hands. So hammering with a big 3 lb hammer or geologic hammer to break open limestone rock is no longer a pleasant thing. In fact, it can be downright debilitating for the next few days.

 

But I did it this one time and was rewarded with a few Ptychodus teeth out of a 7 ft length x 1ft wide triangular seam of sandy limestone. The largest Ptychodus was 18 mm wide and it initially appeared to be what I was seeking. But, upon further evaluation it was instead a nice Ptychodus latissimus. 

 

The latter species is perhaps my favorite from the Atco. Under the light it has a little bit of a rainbow sheen which doesn't come out in the photos. I don't know that it has pyrite as part of the preservation. I've seen and found Ptychodus that were completely pyritized from South Dakota and this was not like them.

 

The first photo is initial shot of the Ptychodus latissimus in the rock. The second is of a similar limestone flaggy rock to the one I found these fossils in. The third is a shot of it in my display area partially still in the rock. The final picture is a shot of one found in North Dakota that is a Ptychodus marginalis that was completely pyritized. Mine is a little smaller. The last photo is a side view of the Ptychodus latissimus. It comes out of the rock but still has the root. You can see how low the crown is.

 

My wife and I have continued our monthly fossil forays (her Christmas gift to me) finding at least one nice Ptychodus fossil on each outing.

 

On the above featured hunt, she and one of my adult daughters were at home with a very unkind version of COVID19. So, my wife kindly invited me to "get some fresh air" and leave the house for a while. So I took her up on it and hunted several places on that hot day. Cheers! 

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Edited by LSCHNELLE
Added a sideview photo
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Awesome report, I've started to catch on to what you're saying about the hit or miss nature of the Atco, and I'm developing a true love/love-a-lot-less relationship with it :P

 

I've stumbled into one area that I'm sure to be basal atco so far, and of the several outcroppings in that area, only one has produced teeth of any kind - some very tall, sharply pointed Ptychodus mortoni and what looks like a Cretolamna. It captures such an interesting time in the Western Interior Seaway but the depositional environment and frequency of outcrops can be such a pain!

 

I have encountered P. whipplei in a place where they shouldn't be though (gravels from strata much too young), so I assume that somewhere close is an actually productive (though ever elusive) Atco outcrop that's taunting me. Of course, there is always the possibility that the mystery outcrop is upper Eagle Ford too, but that's nothing to complain about 

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“Not only is the universe stranger than we think, it is stranger than we can think” -Werner Heisenberg 

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8 hours ago, LSCHNELLE said:

I have developed long-term inflammation in my wrists and hands. So hammering with a big 3 lb hammer or geologic hammer to break open limestone rock is no longer a pleasant thing. In fact, it can be downright debilitating for the next few days.

 

But I did it this one time and was rewarded with a few Ptychodus teeth out of a 7 ft length x 1ft wide triangular seam of sandy limestone.


Sorry to hear about your inflammatory condition. Hopefully the finds were worth the next few days of pain. I think so, but then again it’s not my wrist that is hurting!

 

Nice Ptychodus teeth. Thanks for sharing pics of them and sharing your story. :) 

The good thing about science is that it's true whether or not you believe in it.  -Neil deGrasse Tyson

 

Everyone you will ever meet knows something you don't. -Bill Nye (The Science Guy)

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Great trip report and awesome find! The preservation is beautiful, and the story that goes with its discovery only adds to what an amazing specimen it is! :default_clap2:

'There's nothing like millions of years of really frustrating trial and error to give a species moral fibre and, in some cases, backbone' -- Terry Pratchett

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