 |
 |
Hi John, no stylolites. Some larger fracture "zones" feature tilted "fault blocks", almost looks like a fault breccia. Thinner fractures just have blocky calcite, and a few resemble fibrous veins with a center crack (antitaxial vein). Jochen On Mon, Feb 22, 2016 at 8:11 PM, John Waldron <[log in to unmask]> wrote: > 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] > <[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. Phone: +1 (907) 474-7809 http://www.uaf.edu/geology/faculty/