Scot,

thanks for the information. The belemnites were found in a region known as Limestone Gap in the eastern Talkeetna Mts. You are probably familiar with the area. It is easily accessible by small plane from the Glenn Highway near Sheep Mt. Lodge. The calcareous mudstone with the belemnites overlies the Nelchina Limestone. Since it is rather weak and only about 10 m thick, the mudstone is usually weathered away. It is overlain by a sandstone. All are Lower Cretaceous, but have no official stratigraphic name. I have not observed any calcite concretions in the mudstone. Elsewhere in the area there is the Matanuska sequence exposed with some beautiful large ammonites. And I remember students reporting belemnites, but by far not as many as in the mudstone.

Janos: There is no evidence that these rocks experienced any metamorphism beyond diagenesis. However, I should literally dig deeper into the outcrop to get less weathered samples of the mudstone. It may indeed be stronger than what is obvious from the surface exposure.

Jenni: Permafrost, that is a very interesting point! I have to look into that.

Thanks for your interesting comments!

Greetings from Alaska,

Jochen

On Wed, Feb 24, 2016 at 7:27 AM, Krueger, Scot <[log in to unmask]> wrote:

Jochen,

 

Are these in the Matanuska-Kaguyak sequence?

 

When I was mapping in the Talkeetna Mountains in the mid 1990s we frequently encountered thin beds of belemnite hash which we referred to as death assemblage beds. They occurred in lenses up to a meter thick and could extend for more than 100 meters along the outcrop. Most of the ones we encountered were calcite cemented mudstone/siltstone that stood out in relief to the surrounding outcrops. I was reminded of the fossils one often finds in the heart of calcite cemented concretions and suspected the abundant coarse carbonate in the belemnites helped seed the deposition of calcite cement in the death assemblage beds during diagenesis. We also encountered many in situ ammonites as well. While I did not sample them, I seem to recall examples of both belemnites and ammonites with the types of shears you show.

 

It may be that our outcrops were of a fresher nature because we had the advantage of flying about in helicopters to study the ridge tops (MMS fieldwork sponsored by Arco). Some outcrops still had glacial scour marks visible. I recall a few times when we flew down to study river exposures and we found similar rocks with deep weathering where the calcite cement had been largely dissolved. Even though you could recognize stratigraphy, the outcrops were more like soil  and most fossils were found as clasts in the mud. I would not be surprised if your belemnite beds were tightly cemented and very brittle at the time of the shearing you observe, but have since been deeply weathered in their current location.

 

Scot Krueger

 

From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of Jochen Mezger
Sent: Tuesday, February 23, 2016 10:49 PM
To: [log in to unmask]
Subject: Re: Compressional deformation of belemnites

 

Thanks Mark for the original Badoux paper!

The calcareous mudstone which contains the belemnites forms a relative thin bed (ca. 10 m) overlying a thick limestone. The upper contact is a shallow dipping fault which thrust the same limestone layer over the mudstone. The mudstones probably acts as a weak layer accommodating fault motion. Unfortunately, the mudstone is so soft that it's hard to measure bedding. The belemnites are washed out by rain. Only very few are deformed and coherent. Hundreds and thousands of broken straight fragments "litter" the underlying limestone beds. It is impossible to find them in place, unless one is extremely lucky. Janos, I wish I could see them in place and get their orientation. I plan to go out there again this summer and take a closer look. My previous visits were with students and I had not much time other than to look for them on the ground. I think the calcareous content in the mudstone facilitated the precipitation of calcite in the cracks. What I cannot understand is that the mudstone is strong enough to develop enough differential stress. But then again, I have not taken a "fresh" sample of the mudstone, which I attempt to do this summer. I can't say much about the calcite cement yet, since I just made the thin section a few days ago (and they are still too thick). This is just the beginning.

Thanks Janos, Mark and Dirk for your comments/questions.

 

Shaocheng, I sent a picture with two prominent examples in my initial email. If you can't find it, I can send you some.

Jochen

 

On Tue, Feb 23, 2016 at 11:37 AM, Dirk Nieuwland <[log in to unmask]> wrote:

Dear All, 

 

As it happend I am just working on chalk. Not a reservoir chalk, but a formation with much lower porosity (5% to 10% max).

The idea is that water weakening has had a lot to do with the more than usual porosity reduction of the chalk.

Woud it be possible that slow compressional deformation in combination with a process similar to water weakening has enabled the deformation of the belemnites and keeping them more or less intact. An ongoing process of calcite dissolution and re-precipitation could result in such a deformation as seen in the belemnites. I think?

 

Cheers,   Dirk

 

 

Dirk Nieuwland
4e Binnenvestgracht 13
2311NT Leiden

 

On 23 Feb 2016, at 17:25, Urai, Janos <[log in to unmask]> wrote:

 

Dear Jochen,

these are really spectacular little Rheometers. The mudstone was apparently strong enough to develop a differential stress sufficient to break the belemnites. And your inference on the calcite cementing is interesting too: can we deduce something abiut the strength of this glue?

Any chance to collect information on the orientation of the deformed belemnites?

kind regards,

Janos

Prof. Dr. Janos L. Urai
Structural Geology, Tectonics and Geomechanics
RWTH Aachen University, Lochnerstrasse 4-20
D-52056 Aachen, Germany
T: +49 241 809 5723  M: +49 151 140 42552
e-mail: [log in to unmask]
www.ged.rwth-aachen.de





On 23 Feb 2016, at 1916:, Mark Brandon <[log in to unmask]> wrote:

Jochen,
Thanks for your interesting observation. Judging from your photograph, I would guess that the belemnites have failed by shortening parallel to their long dimension.

Of course, our thinking about deformed belemnites is strongly influenced by the nice examples documented by Badoux (1963) at the Leytron quarries in the Helvetic Alps (and highlighted in Ramsey and Huber, 1983). The belemnites there are entirely confined to bedding planes. Badoux showed that the belemnites were extended (i.e. broken and separated) in the direction of the lineation, and undeformed in the orthogonal direction in the bedding plane. Thus, one would conclude a plane strain deformation, with principal directions Y and X in the bedding plane, and Z perpendicular to bedding, and principal stretches of Sz < 1, Sy = 1, and Sx > 1.

Of course, there is no reason to expect that your Talkeetna study area would have the same strain history as Leytron. Do you have other observations to help judge the geometry of the deformation associated with your belemnites.

Best,
Mark Brandon

If you want to see Badoux's paper, the citation and web link are:

Badoux, H., 1963, Les bélemnites tronçonnées de Leytron (Valais). Université de Lausanne (UNIL) (Lausanne),
Bulletin des laboratoires de géologie, minéralogie, géophysique et du Musée géologique, 7 p.

http://retro.seals.ch/cntmng?pid=bsv-002:1962-1964:68::773





On Feb 22, 2016, at 11:12 PM, Jochen Mezger <[log in to unmask]> wrote:

Hi,

in the Talkeetna Mts in central Alaska I found several deformed belemnite fragments up to 5 cm in length that  suggested compressive stress or pure shear. Some appear to be bent, others have developed shear fractures that are reminiscent of triaxial deformation experiments in geologic engineering (see attached photo). The belemnites are from the Lower Cretaceous, probably family Cylindroteuthididae.

Two things surprise me. First, the deformed belemnites are still coherent, not just simply broken. This suggests that recrystallisation or precipitation of calcite healed the fractures.
Second, the sediment containing the belemnites is a rather soft calcareous mudstone (the belemnites are washed out by rain). I would expect any strain affecting the mudstone to partition around the rigid belemnite  rostrum.

I should point out that the overwhelming majority of belemnites found are undeformed (except broken) fragments. The mudstone unit is about 10 m thick and overlies massif limestone banks. In the immediate area the units are not folded, but both shallow thrust faults and steep strike-slip faults are prominent. My impression is that the belemnites are deformed by slow continuous thrust faulting, allowing calcite to precipitate and keep the fragments glued together, as evident from thin sections. Unfortunately, but not surprisingly, there's no evidence for internal plastic deformation - that would have been spectacular.

I have searched the literature for similar examples. There are several papers on extensional deformation of belemnites (most know stretched belemnites from the Swiss Alps, used as an exercise to construct strain ellipse in Ramsay & Huber), but I only came upon one showing similar compressional fabrics like the Alaskan. These were described in 1969 from Jurassic mudstones found in the Ries Meteorite Crater in Bavaria, and attributed to shock deformation resulting from the impact of the meteorite. Clearly, this is not the case. Certainly not in Alaska.

I would welcome comments from others who may have found similar bent and sheared belemnites. I am sure that I am not the only one. If someone has photos to share they can contact me directly.

Thanks a lot!!!

Jochen

--
Jochen E. Mezger

Term Instructor of Geology / Field Camp Director
Department of Geosciences
University of Alaska Fairbanks
Fairbanks, AK 99775-5780
U.S.A.

Phone: +1 (907) 474-7809

http://www.uaf.edu/geology/faculty/


<Def_Belemnites.jpg>

 

 




--

Jochen E. Mezger


Term Instructor of Geology / Field Camp Director

Department of Geosciences

University of Alaska Fairbanks

Fairbanks, AK 99775-5780

U.S.A.




--
Jochen E. Mezger

Term Instructor of Geology / Field Camp Director
Department of Geosciences
University of Alaska Fairbanks
Fairbanks, AK 99775-5780
U.S.A.

Phone: +1 (907) 474-7809