Dear colleague.

Based on the sedimentary features (e.g. inverse to normal grading, floating pebble, bedding...), there are debris flows., I would also recommand you to look at the nature of the clast and for some more sedimentary features to check if these deposits could be pyroclatic flow. The background sedimentation (e.g. alluvial, fluvial, lacustrine...) will provide you key informations for the depositional environment (not feasible with your photos).

You can find some more information and references on http://volcanology.geol.ucsb.edu/facies.htm

Regards

I agree. Volcaniclastic deposits, debris-flows, Lahars. Apparentely, these coarser despoits are alternating with some finer and massive siliciclastics, eventually lacustrine (?). Some closer and detailed photos would be usefull.

Dear Mobin,

what I can see on these pictures you attached is a typical intercalation of distal ignimbrites with medial to distal volcanic fall-out with pumice components. If you have a look on the pictures I attached from southern Tenerife Island (Canary Islands) the outcrops show identical features with the exception that picture 3142 additionally shows lahar deposits in the middle of the outcrop wall. Your picture 3 probably shows an inverse graded ignimbritic layer at the base, overlayn by pumice-rich medial volcanic fall-out (compare to my picture 4836. Your picture 4 also shows medial volcanic fall-out (pumice), compare to my picture 3145. However, it seems that the pumice was reworked and transported over a certain distance, debris flow is not wrong, accordingly...

Best Elmar

Looking at the pictures, I think whites are pyroclastic deposits, perhaps ignimbrites. You should see a coarsening upward distribution clasts of pumice and fining in clastic, presence of pipes, or signs of the presence of hot gases or sinsedimentario hydrothermal (contact metamorphism in clasts or vacuoles) or fiammes if the ignimbrite is welded.

The detail photograph of the clasts, reworked them seem even deposits have an erosive surface below.

My opinion is that distal ignimbrite deposits are interspersed with lahars (darker brown and brown sediments) and / or volcanic processes channeled (without being a strict river system).

I hope my help will be useful.

I definitely agree with Ana! These are distal ignimbrites intercalated with reworked fall-out that can be interpreted as lahars.

I agree with Elmar and Ana. The coarse unit with mainly rounded pumice  clasts may be linked to  lahar flows

I agree with the previous comments providing a "one-step beyond interpretation" (a lahar is a mass or debris-flow, and an ignimbrites is a pyroclastic flow).  We are still missing information for the depositional environment (ephemeral lake?, alluvial/fluvial plain? subaerial catchment/flank?) even if observations suggest a distal volcanoclastic seeting that is already a good starting point.

Hi all, what I see in pictures 2 and 3 of Mobin are massive ash-lapilli tuffs that were deposited from pyroclastic density currents (PDC). The massive nature shows that the depositional conditions did not vary significantly over the course of time required to deposit this unit (looks like many hours to days to me). The current itself may have been fast but deposition took place without significant shearing at the base. The amazingly homogeneous grain size distribution of the deposit should not be taken as proxy of the composition of the travelling current and clearly does not indicate a non-stratified PDC. The deposit does not represent thickness of moving PDCs!  The brownish colour of the top of the underlying unit (figures 2 and 3) proves that the PDC emplacement was the first depositional process after a long hiatus exposing the underlying units to erosion and weathering. PDC deposits that are deposited below the temperatures required for thermal sintering are easily subject to erosion (rain, wind) and the rounded pumices are a good indicator for me for local reworking in a dense (= frequent particle interaction) medium. Abrasion of pumice clasts may also take place in PDCs (http://www.sciencedirect.com/science/article/pii/S1474706511002580) but they usually have a significantly lower concentration than suspensions in water.

Hi all, Tanks for your comments

There are many channel structures in this units specially in Lahar units.

Whether these sediments were deposjted in the a lake environment under the influence of river ?

Hi Mobin,

pictures 2 and 4 seem to show deposits of dilute pyroclastic density currents that had a tractional basal boundary layer. This is producing the thin laminae. (https://www.researchgate.net/publication/258159966_Sedimentology_and_geomorphology_of_the_deposits_from_the_August_2006_pyroclastic_density_currents_at_Tungurahua_volcano_Ecuador?ev=prf_pub)

Cross-bedding in PDC deposits can be a primary feature and does not require reworking by e.g. water. In images 1 and 3, the rounding of the clasts is intriguing as well as an apparent increased density of the clasts compared to the surrounding material. Picture 1 may be due to water but picture 3 is most likely the effect of local scouring of the fresh PDC deposits, most likely by an eddy or so, and the deposition of dense clasts that had been transported by rolling or saltation. I don't think that water, capable of transporting these pebbles, would only have eroded so few material. Also, as you see no layer boundary in the massive lapilli tuff to the left and to the right of the pebble-filled depression means to me that the erosional process took place while PDCs were still flowing and depositing.

Ulli

Article Sedimentology and geomorphology of the deposits from the Aug...

Hi all,

I agree with Ulrich. Additional comment: Picture 1 shows a small channel with sediments deposited by water. The componenets are subrounded, component based (components have contact) and show imbrication. However, this does not mean that these sediments were deposited in a river system. Heavy rain fall occur subsequently to volcanic eruptions and all kind of components can be reworked and transported over a certain distance in small channels like this one you show in picture 1.

Best Elmar

Beautiful images - difficult to comment without knowing anything of previous work - looks to me like shallow marine - tidal - bimodal cross bedded sandstones? possibly giving way to fluvial sequence (ie shallowing - up sequence.....

It may be volcaniclastics, forming a stratovolcano. Examine in detaio the petrogrpahy of clastics associated vith volcanic.