Brief Devonian foray to the Iowa City River

[tombk]

This past Sunday, I had about 2 hours to spare, and wanted to look for fossils. I obviously couldn’t go far, so I decided to see whether I might have any luck fossil hunting along the Iowa River right in Iowa City. The exposure there is from the Coralville Member of the Cedar Valley Formation, which is mostly Devonian limestone and Dolomite. The faunal diversity in this strata is not great - some solitary and colonial corals, a smattering of brachiopods and bryozoans, and a few small crinoidal columnals. I had time to check out two locations very near downtown Iowa City, and both within two miles of my house.

 

The banks leading to the river in this area are relatively short (~10-15 feet), but can be steep and/or blocked by dense vegetation. The first location allowed access to the river via a small boat launching ramp. The river at that point is not scenic and the brief stretches of shoreline contain a surprising amount of large discarded slabs of concrete.

 

This must have been near somebody’s favorite fishing spot, as I saw a chair perched neatly on the stones of the shore.

 

I poked around for a bit and found quick collection of some species of the colonial coral Hexagonaria. I assembled the finds for this photo. They are not that well-preserved. I did find it interesting that some pieces are brownish and contained open corallites without intact septa. These were all thin sheets about 10 mm thick. Other pieces were thicker and larger, grayish stones whose corallites were filled in.

 

I didn’t find much else there other than a small patch of some species of the Bryozoan fenestella 

 

a beat-up brachiopod

 

and an even more beat-up solitary rugose coral.

 

With only 45 minutes left, I went about ¾ of a mile north, where there’s a rough, worn-out path to the river; a path likely made by people going down to fish. Here I had more luck, finding a place I’ll return to later when I have more time. I almost immediately found two very large rocks with plenty of brachiopods in them.

 

 

Unless you zoomed in very close to where my fingers touch the rock in the second picture, you are sure to have missed my favorite find. Here it is zoomed in more closely. Both valves of a fairly large Platyrachella iowensis.

 

I was in a hurry and at first was not going to try to get it out for fear of damaging it in my rush. But with just a little gently prying with the pick tip on my rock hammer, the specimen popped out quite nicely. Here are some pics of it after I got it home:

 

 

 

I then found a palm-sized chunk of solitary rugose coral. I’ve read that a common genus of this coral in this strata is Cystiphylloides, although I have no real basis to ID this as such.

 

I still had about 10 minutes to pry open a few pieces of one of the large, brachiopod-filled rocks and got what I think is a species of Vinlandostrophia,

 

as well as a few, small brachiopods that are not identifiable (to my unskilled eye).

 

 

[Fossildude19]

Great report, with some interesting finds!

Thanks for sharing here. 

[FossilDAWG]

Not bad for a quick impromptu hunt.  I especially like the Platyrachella!  Your "Vinlandostrophia" is not that genus, as Vinlandostrophia is an Ordovician genus that did not persist into the Devonian.

 

Don

[tombk]

11 minutes ago, FossilDAWG said:

Your "Vinlandostrophia" is not that genus, as Vinlandostrophia is an Ordovician genus that did not persist into the Devonian.

Thanks for the correction. I’m not sure what source I used for that. Frankly, IDing brachiopods can make my head spin!

[Al Dente]

36 minutes ago, tombk said:

They are not that well-preserved. I did find it interesting that some pieces are brownish and contained open corallites without intact septa. These were all thin sheets about 10 mm thick. Other pieces were thicker and larger, grayish stones whose corallites were filled in.

Some of these are partially silicified. The large one in the upper left isn’t silicified, it would look nice if you cut and polished it. I used to live in Iowa City back in the 1990s.

[tombk]

21 hours ago, Al Dente said:

Some of these are partially silicified. The large one in the upper left isn’t silicified, it would look nice if you cut and polished it. I used to live in Iowa City back in the 1990s.

Thanks @Al Dente! For my education, would the process of being silicified affect whether the individual coralites get filled in, or just affect the color of the fossil? 

I was thinking that the large coral would look nice polished. I don't have a rock saw or a polisher (other than some off-brand sandpaper disks I purchased for my rotary Dremel), though, so that'll have to wait. 

I moved to Iowa City in 2008. Where you here when the flood of 1993 created the Devonian Fossil Gorge? 

[Al Dente]

22 minutes ago, tombk said:

Where you here when the flood of 1993 created the Devonian Fossil Gorge? 

I was at the spillway the day water went over. If I remember correctly, the Corps of Engineers made the statement the day before that water won’t go over the spillway. For awhile the only way in and out of Iowa City was Highway 1 due to flooding of the roads.

 

The partial silicification of some of the corals will preserve some or all of the corallites but usually not the matrix filling the coral. Years of weathering will dissolve some of the calcite within the coral, making the septal of the corallites stick out. This has happened to some of your pieces.