Thursday, May 29, 2008


Two posts in one-day! I must be jacked up on coffee something fierce!

Anyway, I was going through my photos from the recent industry-sponsored field trip to Wyoming, and I realized I had taken some panoramas that needed some stitching, stat. So I put a pot of coffee on, started up Photoshop, and got to work (as an aside, using a computer is, for me, an experience I would best describe as "harrowing"). I persevered, however, and present the fruits of my labor, below.

Anyway, this panorama is of the Triassic Chugwater Formation, exposed at the Alcova Reservoir in Wyoming (which is a completely awesome place, I might add). I expect Jeannette, who was also on the field trip, might start posting some stuff too, so I'll wait and see what she puts up before going into to much detail on the trip (I know that both of us got some pretty slick pictures, too).

Anyway, the Chugwater Fm is largely composed of silts and fine sands, and has been seriously messed up by a fair bit of gypsum. Sed structures are very rare, sadly. On the plus side, it is really really really red. Separating the lower Chugwater from the upper Chugwater is the Alcova Ls, representing a phase of shallow-water, carbonate-rich lake sedimentation that contrasts with the heavily oxidized, pedogenically-altered alluvial and fluival intervals of the rest of the formation. Unfortunately, we didn't get to spend much time looking at the unit on the trip, as we were focused on hitting the underlying Pennsylvanian Tensleep Sandstone.

Anyway, if you find yourself with some time in Wyoming, I would heartily suggest checking out the Alcova Reservoir. Rent yourself a boat at the marina and go for a paddle; there are a lot of really nifty exposures in the area.


Julie said...

Hi Eric,

I was just out to Alcova last Sunday, and we hiked around the canyon there. Embedded in the sandstone layers were these strange iron-like balls (much heavier than surrounding rock, with a deep purple and rust color). They are not eroding as fast as the surrounding sandstone. I'm perplexed as to what they are. My theory (having a very limited education in geology) is they are some sort of debris from a meteoritic impact? Do you know what they are? The question has been bothering me for days now.

Eric said...

Hi Julie! Sorry for contributing for your suspense, but I'm only coming into town around once a week to stock up on supplies and get my internet fix.

Anyway, the little balls you found in the sandstone are not only iron-like, they are, in fact, iron.
Iron in fluids moving through the sandstone resulted in localized precipitation of what are called concretions. These little iron concretions formed in place, within the sandstone, after the sand had been deposited and buried.

Concretions have recieved a lot of attention, mostly in the field of geochemistry, where the fundamental question of why these things form where they form is still a big research area. Generally, it's believed that the iron-rich fluids encounter a localized micro-environment in the sandstone that results in extremely localized precipitation of iron-rich mineral phases in the area, often as a cement. The cement grows outward from a nucleus, and viola! You get concretions!

Julie said...

Thanks for the explanation, Eric. I appreciate it. I found another place with similar concretions.

Anonymous said...

Eric: On what do you base your decription of the Alcova limestone as a phase of shallow-water, carbonate-rich lake sedimentation. Is it possible that you have read my PhD dissertation? George F. C. University of Missouri

Eric said...

Howdy George! My interp of the Alcova as lacustrine comes from me boppin' around the Chug for a few days, and I thought it looked awfully lake-y. Of course, I'm working in the Green River Fm for my PhD, so those carbonate lake phases might be coloring my worldview, too.

I havn't read your work, but I guess I'll have to, now! I know Picard always interpreted the Alcova as a shallow marine tounge, so it's nice to know that someone out there has done the work and come up with an alternative interp.