Friday Field Foto #78: Sampling volcanic ashes
As most of my readers already know, I recently returned from another trip to southern Chile to look at the Cretaceous sedimentary rocks exposed in the Patagonian Andes (my 6th year in a row). This year was different in that the primary purpose of the trip was to convene an SEPM field conference with my colleagues and show others in our field these great rocks.
But, since it is such a long way to go, we added a couple days before and after the conference to do a bit of field work. A student at my alma mater completed her final (?) field season down there this year. She is a structural geologist working on the evolution of the basin in relation to the fold-thrust belt.
In the last couple of years an old road was improved significantly, which created numerous new roadcut outcrops. These new roadcuts reveal some incredible exposures of previously unexposed deep-marine sedimentary rocks. What the roadcuts also reveal are some volcanic ashes! In this week’s photo the prominent white stratum (also marked by hammer at right side) is one of these ashes.
This ash layer, along with several others, were sampled and their ages will be determined sometime this year as part of this student’s dissertation. This is very exciting because this basin needs some more age control badly.
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Clastic Detritus 
Yay for some good road cuts and age constraints. (Is ELM advising the student?)
“Is ELM advising the student?” — no
What general age are the ashes? I used to peripherally do some work in volcanic ashes in the west, but only Quaternary ones. At the time, older ashes were thought to have some dating problems because of devitrification, but techniques have no doubt come along way since then.
SF … based on what we already know about the geology of the area, these ashes should be somewhere between 92 and 86 Ma … one of the best ways to obtain ages these days is to (hopefully) find several nice zircons and do some U-Pb dating.
This is one of numerous ashes that the students who are still working down here have found in the last couple of years. Assuming they get good ages out of them, they will help constrain the basin evolution nicely.
If the results are older or younger than what we are expecting … well, that would certainly be interesting!
Oh yes, zircons! Gotta love them.