Neat. Might the initial fracture surfaces have evolved into stylolites by pressure solution, keeping the fragments together?

John Waldron

On Feb 22, 2016, at 9:12 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/

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