5/28/2008

Rock of the Week #8

EDITED *** WHY DIDN'T YOU GUYS TELL ME I HAD THE FOSSIL WRONG? THANK GOOD NESS FOR STUDENTS (NOW GLOATING STUDENTS). The Rock of the Week #8 contains a Receptaculid. It is maybe a green algae or maybe an echinoid but definitely not a bryzoan!!! Excuse me please. Receptaculids are most prevalent in early ordovician so early Paleozoic is still the correct answer.


For my birthday week, my very favouritest rock type. Note that I had to massively screw with bright/contrast/saturation to make these photos look like the rock really looks! Maybe I shouldn't take pictures of rock of the week after dark in my office while pretending to prep for tomorrow's early lectures. Hm.

Anyway:
1. Name the rounded reddish mineral.
2. Based on the colour (and any crystals if you can find them), what mineral comprises the very fine-grained matrix of the majority of this rock?
3. Rock name or metamorphic grade?




Solution to Rock of the Week #7:

This is a fossiliferous Ordovician (520-430Ma) Kanosh Formation limestone from Crystal Peak, Utah. It is almost completely composed of fossil material, so it would be called a biosparite or biomicrite (Folk classification) or a biograinstone (Dunham classification). Due to the large amount of broken shelly material, this rock is also called “hash”, in this case “brachiopod hash”. There is a wide variety of fossils in the rock, including the very large bryzoan and numerous brachiopod shells. Crinoid stem sections are present as well as lots of little very thin bits looking like fingernail clippings – I think these are part shed exoskeletons from trilobites. This assemblage is consistent with deposition on a shallow marine shelf. The prevalence and diversity of brachiopods, as well as the abundant trilobite fragments suggest lower Paleozoic, as brachiopod diversity and trilobite abundance never fully recovered after the end-Ordovician extinction. As such, this type of assemblage is sometimes called “Cambrian-type” assemblage.

1. (1 pt) What is large fossil?
: 1 point given for bryzoan.

2. (1 pt) Rock type?
: 1 point for either fossil hash, biosparite, biomicrite, or biograinstone.

3. (1 pt) Age:
: 1 point for b) early Paleozoic.

9 comments:

Trifarina said...

Christie's favorite rock = pseudotachylite, "could this be it?" ponders a soft rock sed head.

Karen said...

Dont'cha just love having birthday weeks?! I Enjoy your blog.

coconino said...

I've only seen one pseudotachylite, from the Whipple Mtns in eastern CA, but not in thin section under a scope. It didn't look like this. My guess is something very olivine-rich, but I'm not sure what the other mineral is.

Julie said...

For a little fun from a non-rock person, I would name this one "Worn Out Blue Jeans"! Happy Birthday!!!

Fault Rocks said...

Looks like the non-geologist is taking the lead on this one!

Bob said...

Looks like a blueschist. The red porphyroblasts are garnet, the bright green in the matrix might be epidote, the blue is from glaucophane, There's probably lawsonite in there as well. Or am I completely off

kes said...

Woah dang! you kept that quiet while you were down here... Happy b-day!

Kelly said...

I found your blog through Julie's site. I love it; you talk about rocks and succulents! Cool! My daughter has a rock museum at home. We decided leaving our rock collection in a box was a waste of time. Now we enjoy them year round. I didn't realize that there was that much material in a rock. I wonder if any of the rocks I've stumbled upon contain fossil material? Really interesting! I'm off to read more of your blog! Happy B-day!

Ron Schott said...

1. garnet
2. glaucophane
3. blueschist