Early Cretaceous Ammonite/nautiloid From Se France

[lukas1051]

Hi,

I'm currently undertaking some fieldwork in south-east France, looking at Jurassic and Cretaceous carbonates, and I found a fossil that has me stumped.

The fossil was found in a Berrisian (I think) rhythmite sequence within a limestone bed. The beds contained lots of other ammonites (of multiple species) and lots of belemnites too.

The fossil is coiled and has prominent ridges spaced about 3cm apart, but otherwise the surface is quite flat. It has a strange, almost elongate shape.

I'm not even sure whether this is an ammonite or nautiloid, let along a species for it. Any help would be greatly appreciated.

Thanks

[4circle]

It's a type of ammonite called a Lytoceratid. They're known from the formations you described - the darker layers of these interbedded units were formed in deeper water, and lytoceratids are almost exclusively known from deeper facies.

The prominent ridges you describe are typical lamellar flares which were formed during periods of reduced growth.

I've worked extensively in this region (especially around the Verdon and Alpes Maritimes) but never encountered one this good. I'd be interested to know a little more about where you found it?

[lukas1051]

It's a type of ammonite called a Lytoceratid. They're known from the formations you described - the darker layers of these interbedded units were formed in deeper water, and lytoceratids are almost exclusively known from deeper facies.

The prominent ridges you describe are typical lamellar flares which were formed during periods of reduced growth.

I've worked extensively in this region (especially around the Verdon and Alpes Maritimes) but never encountered one this good. I'd be interested to know a little more about where you found it?

Thanks for the help! It was found on a road near the town of St Julien du Verdon in the Provence region. I didn't get a picture of the exact outcrop, but this was very nearby (and part of the same formation). It's worth noting however that it was found within the more competent beds as opposed to the darker ones.

The reason I took a picture of this outcrop is actually because there is what seems to be a large submarine channel structure as well as slumping within these channels, but the fossil was found in some ordinary conformably bedded stuff to the left.

[Auspex]

...The prominent ridges you describe are typical lamellar flares which were formed during periods of reduced growth....

This is really interesting information!

[4circle]

4circle, on 10 Jun 2014 - 5:44 PM, said:

...The prominent ridges you describe are typical lamellar flares which were formed during periods of reduced growth....

This is really interesting information!

When times were tough, the ammonite - instead of secreting a further increment to the whorl and thus becoming larger - would simply stay put (with respect to the shell). This left the 'mantle' (the tissue which mineralised the shell) in contact with the aperture, so this simply became thicker and formed a flared band. These lamellar flares are typically at quite regular intervals on 'Lytoceras' (as indeed you can see on the photo above). Many ammonites (including lytoceratids) are often preserved only as internal moulds (which is how er see the sutures, so in all such cases, evidence of these external features is usually lost.

[4circle]

Thanks for the help! It was found on a road near the town of St Julien du Verdon in the Provence region. I didn't get a picture of the exact outcrop, but this was very nearby (and part of the same formation). It's worth noting however that it was found within the more competent beds as opposed to the darker ones.

The reason I took a picture of this outcrop is actually because there is what seems to be a large submarine channel structure as well as slumping within these channels, but the fossil was found in some ordinary conformably bedded stuff to the left.

I wouldn't worry too much about the difference between the two lithologies in the formations this high up the sequence (it's of more note further down in the Jurassic where the differences are more pronounced between the interbedded limestones and marls).

Thanks for the info!

And I woudl question yoru interpretation of submarine channel (certainly from the photo. Slumping yes (slumping was common on these carbonate platform fault blocks, whereas channels weren't). Your photo simply shows that overlying sequence thinning at the LH margin. You'll note it uprides the tilted beds below it at the far LHS - a channel wouldn't do this, as channels erode at the margins and deposit in the centre. Unless the RHS (out of shot) shows a perfect match up in the opposite, I wouldn't diagnose this as a channel!

[lukas1051]

I wouldn't worry too much about the difference between the two lithologies in the formations this high up the sequence (it's of more note further down in the Jurassic where the differences are more pronounced between the interbedded limestones and marls).

Thanks for the info!

And I woudl question yoru interpretation of submarine channel (certainly from the photo. Slumping yes (slumping was common on these carbonate platform fault blocks, whereas channels weren't). Your photo simply shows that overlying sequence thinning at the LH margin. You'll note it uprides the tilted beds below it at the far LHS - a channel wouldn't do this, as channels erode at the margins and deposit in the centre. Unless the RHS (out of shot) shows a perfect match up in the opposite, I wouldn't diagnose this as a channel!

Yeah I'm still not too sure about the channel thing, the only reason I thought so is because we seem to see the same thing going in the other direction about 100m down the road. But the area is really faulted anyway and there's some weird lateral ramp tectonics going on in addition to the slumping, so it's pretty challenging figuring out all these structures - hopefully over the course of the project I'll have a better idea about what's going on. Thanks for the help though! The ridges are actually really interesting.