Geology of Post Oak Creek

[ThePhysicist]

I haven't seen much discussion on the geology of the well-known Post Oak Creek here in North Texas. I'd usually seen it referred to the Eagle Ford Group/Formation, but without much substantiation. I really hadn't given it much care, but as is a healthy practice in science, one should often reevaluate one's beliefs especially in light of new information. I believe there is some evidence to support the Eagle Ford assignment, granted I'm not a professional nor very knowledgable about geology and I welcome discussion and contrarian input.

 

The Eagle Ford (Kef) is exposed near Sherman, so it's a possibility. The Austin Group/Austin Chalk is also here (Kau):

 

^https://ngmdb.usgs.gov/Prodesc/proddesc_19387.htm

 

Here is the evidence that I believe favors that most of the Cretaceous fossils from POC are from the Eagle Ford:

 

1. Lithology

USGS describes the lithology of the Eagle Ford formation: "In Sherman Sheet (1967) (NE Texas) shale, selenitic, bituminous, [calcareous] concretions and septaria, thin platy beds of [sandstone] and sandy [limestone]. in middle and upper part, [medium] to [dark] gray; marine megafossils; thickness 300-400 ft." (https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=TXKef%3B0) (http://northtexasfossils.com/pdfs/shermansheet.pdf)

 

I've found several matrix pieces with shark teeth in it that match the preservation of the vast majority of sharks in POC, which are sandstones.

 

 

There are also septarian nodules like this one:

 

 

2. Presence of Cameleolopha bellaplicata

 

Anyone who has been to POC has seen these oysters everywhere. They are often embedded in the same sandstone. 

 

 

Note that this illustrated one from Hook & Cobban (2011) was collected near Sherman (probably POC since they refer to it). Here is a specimen I collected:

 

 

Hook & Cobban (2011) say that "Cameleolopha bellaplicata (Shumard 1860) is an easily recognized fossil oyster that occurs abundantly in sandy strata in Arizona, Colorado, New Mexico, Utah, and Texas, where it is restricted to the middle Turonian ammonite zones of Prionocyclus hyatti and P. macombi." 

 

These together are consistent with the description and age of the Eagle Ford formation. One possible strata for POC is also the lowest part of the Austin Group, the Atco. However, I've yet to find a description of the lithology which matches better than that above for EF. In addition, there is disparity in the fauna between the Atco and POC. There are many genera found in the Atco which I've yet to find after a few years of hunting Post Oak (e.g.  Heterodontus, Protolamna, Dallasiella, Pariasurus, Pseudocorax, Microcorax, etc.; see Hamm & Cicimurri (2011)). This may be because I haven't looked long enough or there could be some other bias. If you have any of the sharks mentioned above from POC, I'd love to see them. 

 

That's all I've got from a one-sitting internet exploration last night, so please do double check my rambling.

Edited June 6, 2022 by ThePhysicist

[JamieLynn]

@ThePhysicist thank you a million times for this in depth study! I also have just relied on "heresay" about POC and have been lazy and not done the research, so you deserve some serious kudos from all of us. You Rock (and Fossil) 

[DPS Ammonite]

You are right; the Cameleolopha bellaplicata oysters occur in the Arcadia Park Formation of the Eagle Ford Group. The upper part of the Arcadia Park Formation from Dallas north to Sherman contain flaggy sandstone layers that often contain oysters, shells and shark teeth. I suspect that the largest sandstone layers are related to the Bells Sandstone Member of the Arcadia Park Formation east of Sherman.

 

The lowest part of the Austin Group is not very productive near Sherman although I did find a fish jaw with teeth in west Plano in what sort of looked like the Atco Formation that is better developed south of Dallas.

 

I don’t remember seeing a single outcrop near Sherman where you could clearly see the Eagle Ford/Austin Group contact. The contact is covered with slope wash and plants. Just south of Center Street in Sherman, the Austin Group is at the top of the bank of Post Oak Creek. The creek bed is in the Arcadia Park Formation. Hike south in Post Oak Creek from Center Street and you will see lots of sandstone with teeth and shells in it; this is probably the source of the teeth in Post Oak Creek further south. In the oyster beds note the colonial corals and the ramose bryozoans that grow on the oysters.

South of Sherman and below the Austin Group rocks in McKinney, Frisco, Prosper and Plano you can find multiple flaggy layers with teeth and shells, but very few oysters.

 

A few miles west of Sherman along Choctaw Creek there is a large ledge of Cameleolopha bellaplicata oysters, but the shark teeth in the flaggy layers are nearly absent. I did find a layer of some freestanding lacy bryozoans a few feet below the oyster layer.

Edited June 5, 2022 by DPS Ammonite