Impromptu Construction Site Hunt. Grayson Marl and Pawpaw Formation

[Mikrogeophagus]

Following a refreshing swim at the lake with a few friends over the weekend, I decided to take a spur-of-the-moment stop at a construction site I happened to be driving by. The sun was setting, cooling the area and finally making it hospitable despite the recent heat wave. I was not familiar with the exact geology of where I was, but with only an hour and a half of daylight left I decided to not waste too much time and immediately began scouring the dirt.

 

I quickly noticed that the ground was composed of two distinct formations. The higher layer was a grey limestone while the layer underneath was bright red and filled with a wide range of greens and yellows among other colors. It was only until I got home that I realized these were the Grayson Marl (/Mainstreet limestone?) and Pawpaw formation respectively. My first find was an unexpected, but welcome one. Coming from the grey rock, I noticed a rounded form that stood out against the jagged terrain surrounding it. After prying it out, I was surprised to find that it was indeed a large nautilus (A). Not too long after, I found a couple more that were in slightly worse condition. Continuing my search in the greyish area led me to later find a chunk of Mariella heteromorph ammonite (B) as well. I particularly like this specimen as it is interesting to compare it with my Mariella micromorphs from the Waco Pit and other locations. I also managed to snag a few pretty little brachiopods (C). I have a really difficult time differentiating between brachiopods so my ID for them is a bit iffy, but for now I'll go with Kingena wacoensis. 

 

The cool thing about this site is the fact that it sits on the border of two very distinct formations. As a result, one can find a wide variety of fossil life. The rest of the things I brought home from this trip were found in the red Pawpaw formation. For whatever reason, Texas loves to produce pyritized micromorphs. I was surprised to come across a multitude of ammonite genera with that classic bronze coloration I've come to be all too familiar with. I found a few un-pictured fragments of Mariella and other ammonite pieces. I was also able to pocket a few chunks of my favorite ammonite, Engonoceras (D). I didn't really capture its beauty well on camera, but something about the way light bounces off of the broad reflective shell is mesmerizing. I hope to one day come across a complete one. To round off the hunt, I found one last slightly enigmatic specimen (E) that came out of the Pawpaw. Its a big rock with lots of holes running in and out of it. The holes are covered with countless tiny divots. My guess is they are what's left of some crustacean burrows as they bear some resemblance to the poopy burrows I have seen elsewhere online. Some of the burrows have crystallization.

 

Here are the finds:

 

A). A large nautiloid and a smaller one coming from the Grayson Marl. I believe them both to be Cymatoceras. There is some extra matrix on the outside that I am debating on chiseling off. 

 

B). A fragment of Mariella heteromorph ammonite from the Grayson Marl.

 

C). Kingena wacoensis? from the Grayson Marl. They have some nice detail!

 

D).  Pyritized Engonoceras fragments from the Pawpaw Formation.

 

E). Crab burrows from the Pawpaw? 

 

All in all, I'm glad I made a quick stop at this particular site. It's a pretty interesting place with a wide variety of things to be found in a couple of formations I have only just begun to explore. I will likely make another trip there in the future!

 

Thanks for reading!

[Tidgy's Dad]

Nice finds. 

Love those brachiopods! 

[DPS Ammonite]

Kingena wacoensis is correct; it is very common in the Mainstreet and Grayson Formations. The Mainstreet if present, should be a ledge of limestone below the Grayson. The Grayson is a more clayey limestone, marl that easily erodes leaving behind great oysters, clams, echinoid, ammonites, nautiloids and brachiopods.

 

The shrimp? burrows are Opiomorpha trace fossils.

 

Roughly where were you in Texas?
 

 

Edited June 14, 2022 by DPS Ammonite

[Mikrogeophagus]

25 minutes ago, DPS Ammonite said:

Roughly where were you in Texas?

For this trip I was a bit North of Fort Worth. 

 

Ophiomorpha is a new word I'll be adding to my vocabulary!

[JohnJ]

46 minutes ago, DPS Ammonite said:

The Grayson is a more clayey limestone, marl that easily erodes leaving behind great oysters, clams, echinoid, ammonites, nautiloids and brachiopods.

 

True; these look more like Mainstreet and/or lower fossils.  

 

Nice finds.

[historianmichael]

Nice finds! I was up in the DFW area this weekend too. A nice swim should have been in my cards because it was hot! Looks like the 100+ degree weather in Texas is here to stay.  The Mainstreet Limestone overlays the Pawpaw Formation so your nautiloid and ammonite might be from there.

[Uncle Siphuncle]

It would be uncommon to find Grayson and Pawpaw together in absence of Mainstreet.

[Captcrunch227]

3 hours ago, Uncle Siphuncle said:

It would be uncommon to find Grayson and Pawpaw together in absence of Mainstreet.

Yeah this is what I came here to say.  Grayson and Mainstreet are very intertwined, especially north of Fort Worth. Most of the geological maps label the area as Mainstreet/Grayson undivided.