I have attached a few pictures below, wondering if people might offer their interpretation about some rhyolite tuffs in central Washington. I'm interpreting the first two photos from section of surge deposits, and the last two photos from a section of block and ash flow deposits. The tuff section in the last two photos contains large 5-7 m vitric blocks, presumably transported from near the source? Any feedback, or start of dialogue much appreciated!
1) pyroclastic flow deposits are relatively thick, poorly sorted, commonly containing abundant fine-grained ash in the matrix (4 phi), and with crude or no internal bedding
2) pyroclastic surge deposits are relatively thin, better sorted than flow deposits, with or without abundant matrix fines, and well bedded to cross bedded.
Surge deposits may occur beneath or on top of pyroclastic flow deposits, or by themselve
Poor sorting, subtle grading or its absence, poor or no bedding, characterise pyroclastic flow deposits.
Pyroclastic surge deposits are copmpositionally much like pyroclastic flow deposits, the main difference being that surges are commonly richer in crystals and lithics than flows and are better sorted.
Surge deposits have a more restricted grain size than pyroclastic flow deposits, lacking both the very fine and the coarse fractions.
Hope you keep these points in mind before interpreting!
At a glance, I agree with you. In the first two photos. It´s possible see thin material with little clastic to vitreous fragments,in overral well sorted and exbiting cross bedding in the second photo. The other photos seem to be a pyroclastic flow with vitreous blocks and probably bombs interlayered with ash flows. Some lobes indicating the sense of flowing. Nice outcrops!
Dear Mrs. Patridge,
If you are a novice in pyroclastic deposits but have some knowledge in sedimentology apply what you have learnt for arenaceous and fluvial deposits to what you suspect of being a surge deposit. Pyroclastic flow deposits are more difficult to interpret due to the complex mode of transport and the welding process which may be occur in these deposits also known as ignimbrites and nuees ardentes. For a more detailed interpretation I refer to the textbook: Fisher,R.V., Schmincke,H.-U. 1984. Pyroclastic Rocks. Springer-Verlag, 196 pp which is still worth to be consulted in a case like that.
You may also download the paper "DILL, H.G., TECHMER, A., BOTZ, R., DOHRMANN, R. and KAUFHOLD, S. (2012) Hypogene and supergene alteration of the zeolite-bearing pyroclastic deposits at Tell Rimah, Jordan, and rift-related processes along the Dead-Sea-Transform Fault System during the Quaternary.- Journal of Volcanology and Geothermal Research, 239-240: 49-68.
It may provide you with some kind of a guideline how to deal with such pyroclastic deposits (of economic interest) in the field, from the bird’s eye view and the laboratory.
As far as the interpretation of your images are concerned I concur with Mr. Lages, even if I am a bit reluctant to hand down remote-controlled diagnosis of this kind.
I wish you much success
H.G.Dill
Hope this link may be useful.
http://www.geo.umass.edu/courses/volcanology/Pyroclastic%202.pdf
http://www.sciencedirect.com/science/article/pii/0377027379900088
- Badges
- Science topic
- Similar topics
- Computer Science
- Research Topics