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>
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