Pennsylvanian Shell Beds

[Missourian]

While scouting out road cuts on Google Street View, I came across an exposure of a favorite limestone formation that appeared to have some cross-bedding. Since these sometimes indicate a mollusk-rich oolite, I had to pay a visit. There was a series of 3 to 6-inch beds with feather-edge partings. Instead of an oolite filled with nautiloids, I found a delightful mix of micritic limestone interbedded with bands of shell debris. I had to bring a few chunks home:



The shells consisted of pelecypods (Aviculopecten and myalinids), small gastropods, a few brachiopods, and even bits of carbonized wood:



The fossils in the shaly parts between the beds, seen here on top of the block, include brachiopods (Composita, which are the darker round shells), pelecypods, bryozoans, crinoid debris, and trilobites (one pygidium). There were no gastropods that I could see. The crinoid debris and the pygidium seemed to be more abraded than the rest, which could mean they were transported from a greater distance:



A second piece. Looks like bacon to me:



In the upper left, you can see several small shells collected in the cup of a large pecten (and 'downstream' from it):



Oh yeah.... some scale:

Edited March 26, 2013 by Missourian

[Missourian]

The road cut in the Missouri River bluffs:

 

 

The base of the overhang is micrite with just a few shell bands. Upsection, the shell bands increase in number and thickness until the rock is composed entirely of fossil debris:

 

 

The gastropods stand out nicely on weathered surfaces:

 

 

Interestingly, at the same level on the other side of the road, the shell beds are entirely absent.

[Roz]

Very clearly defined banding.. I love pics like those

and I sure know what you mean by the other side can be fossil

free.... You lucked out on the fossils contained..

[Missourian]

On 11/8/2011 at 6:35 PM, Roz said:

and I sure know what you mean by the other side can be fossil

free.... You lucked out on the fossils contained..

 

Because the bands run the entire length of one side of the cut, but are absent in the other, I was intrigued by the possibility that the fossil debris had filled a tidal channel.

 

I've wondered if such a channel could be a meeting point of a local, shallow-water fauna (gastropods and pectens), plant debris from nearby mangroves (Cordaites), and animals further offshore (crinoids, bryozoans, and trilobites).

 

In the past, I'd spotted a lens-shaped channel fill at about the same level in the formation at another locality 30 miles away, but I hadn't yet seen it close up. I've attached a Google Street View of that channel fill below.

 

[Auspex]

It'd be fun to work out a temporal analog to the physical deposition. Is there any evidence of graded bedding?

[Missourian]

On 11/8/2011 at 7:21 PM, Auspex said:

It'd be fun to work out a temporal analog to the physical deposition. Is there any evidence of graded bedding?

 

The overall trend within the unit is to coarsen upward.

 

A quick description of the bedding, from top to bottom, is:

 

1. Micrite and shale (unit above, mostly covered or missing in the photos)

2. Fossil debris (most of the unit)

3. Fossil debris with micrite bands

4. Micrite with fossil bands

5. Micrite

6. Shale (unit below)

 

But within the beds in the lower part, it alternates between debris and micrite, with no fine-scale gradation. It's as if a bunch of shells were occasionally dumped onto an otherwise pristine sea floor.

 

An added complication is the shaly partings with the worn fossils between the beds. Somehow, stuff from further away, along with a little mud, was brought in and deposited on rarer occasions.

 

I'm sure I missed some details, as often happens when the site of interest is 20+ miles away.

 

I made a quick, preliminary schematic:

 

[Auspex]

Maybe seasonally transgressive; looks like a higher energy depositional environment than is to be expected from deeper water, no?

[Missourian]

Yeah, it's apparent the spot was shallow and periodically agitated.

However, trying to figure out where the fossil material came from is like trying to play 4-dimensional chess.

[MikeD]

Some pretty interesting deposits.

[Kehbe]

Thanks Missourian for the lesson! I have walked many roadcuts similar to the ones in your photos and I never knew which layer was which. You certainly give me a much better idea of what I am looking at and what to look for! Many of the interesting cuts around here that I would love to hunt are right on the highway and I am a bit leary of just pulling over on the shoulder. Have you ever been stopped or questioned by the police, highway patrol or state troopers for stopping on the highway? I see signs that say no stopping except for emergencies and I don't know if they would consider finding that elusive gastropod an emergency or not!

[Missourian]

On 11/14/2011 at 8:35 AM, Kehbe said:

Many of the interesting cuts around here that I would love to hunt are right on the highway and I am a bit leary of just pulling over on the shoulder. Have you ever been stopped or questioned by the police, highway patrol or state troopers for stopping on the highway? I see signs that say no stopping except for emergencies and I don't know if they would consider finding that elusive gastropod an emergency or not!

 

I've had highway patrol and police stop to see what I was doing, but they showed interest in my finds. But I wasn't on interstates at the time.

 

Regarding interstates or other no-stopping zones:

 

1. Find a nearby street or road (hopefully a frontage road) and scramble over. Google Maps is very handy for this.

2. Find the nearest exit/entrance ramp and hike.

3. Go on a Sunday.

 

In a couple of cases, I've stopped just long enough to satiate my curiosity. Most cuts end up not having much to find anyway.

[Missourian]

Here's a piece with a weathered surface:

 

 

Some of the gastropods are Hypselentoma, I think.

[Roz]

In that second layer to the right , is that a Baylea gastropod?

[Missourian]

In that second layer to the right , is that a Baylea gastropod?

Oops. You caught me in mid-edit.

I'm still not 100% sure.

[Roz]

Unusual looking...

[Missourian]

Another rock for the pile....

 

Three nice Hypselentoma gastropods among the shells:

 

[Indy]

Maybe seasonally transgressive; looks like a higher energy depositional environment

than is to be expected from deeper water, no?

Interesting...Several years ago while working with a Sedimentologist at my Pennsylvanian

research locality...I asked him how long it took to lay down some of the layers we were

studying (shallow marine shale). He replied that the exposure here in Missouri was at the

equator at the time and tropical storms. then as today, could easily lay down several

inches of sediment in days to just a few weeks. Some things were different back then

but physics remained the same

Edited November 23, 2011 by Indy

[Missourian]

On 11/23/2011 at 4:23 PM, Indy said:

Interesting...Several years ago while working with a Sedimentologist at my Pennsylvanian

research locality...I asked him how long it took to lay down some of the layers we were

studying (shallow marine shale). He replied that the exposure here in Missouri was at the

equator at the time and tropical storms. then as today, could easily lay down several

inches of sediment in days to just a few weeks. Some things were different back then

but physics remained the same

 

Yes. The nature of the bedding and fossils made me think that storms, too, could have brought the shell debris in from different locations.

[Indy]

Yes. The nature of the bedding and fossils made me think that storms, too, could have brought the shell debris in from different locations.

Thus the term "Storm Deposit"