Hello Jean-Marie,

There is about 20 m breccia related litofacies under the massive breccia. Th grains of the breccia are lithics. The omlt has 1 cm median size. Majority of the lapilli are obsidian and there are few lithics and pumice. I added some other photos.

I didn't scrubbed it. I missed the gray matrix of the diffused breccia in the sketch. You catched it.

I think the pyroclastic flow encountered a topographic barrier there. The breccia settled immediately and a few grains with last bit of the energy of the flow send the grains into the omLT.

On the nearest part of the breccia to the contact, one may see the normal grading of the blocks. In the diffused breccia part grain size decreasing lateraly into the omLT. These are supporting the idea of the energy decreasing of the flow.

The ingestion model doesn't explains the lateral size decreasing into omLT.

The ingestion would also causes some other chaotic geometry but in this case the diffused breccia has nearly a linear shape at the top.

I resolved partly and edited the sketch.

In sketch2, your comments about surficial coloring has a role in some degree.

In sketch1, the concave contact line, has been changed to a straight line. To the black block (BB) in the sketch2, the contact line has coming straight. The part on the right side and below this BB, are not omLT and coloured surficially. But to the left it turns to the omLT and has a tongue (diffused to) the omLT

In this model the volume of the diffused part decreased. But the remained part still associating the previous ideas.

I have said that, there is a lateral size decreasing. But it starting from the BB, not starting from the blocks on the right side of the BB.

The chance that the omLT scratched some parts of the mlBr, as you proposed also can be a solution for the occurrence of the tongue, it is very logical. But there must be a relation, with the thick breccia deposits below and a tongue (with a size decreasing). One may also find the solution that, "the energy decreasing of a current while encountering a topographic barrier responsible for the result".

The critical point in here that whether the current erosive or not at that moment. If it was erosive, then it might scratched the ground. But the clues shows that the energy was decreasing, in this case "erosion related solution" for the tongue become less possible.

The current that have deposited the units, is a single current. The grains had been settling down according to the yield strength of the current continuously, while the (lighter) grains flowing at the upper parts of the current. I am not proposing that the mlBr settled after omLT. While the omLT settling, a few particles of the blocks were able to transported forward/upper a bit more by the current. And blocks, got a place according to their size into the omLT which decreasing gradually.

For summary the core of the question can be seen in the photo attached to this comment. Which line would be parallel to the ground on the deposition, the surface of the contact or the surface line of the tongue.

The relation between breccia deposition and topographic barriers may be seen in this paper on p.57 and erosiveness conditions p.3

Article Pyroclastic Density Currents and the Sedimentation of Ignimbrites

I recently had an opportunity to revisiting the outcrop.

Erosion effected the outcrop, which made the grains of mlBr gaining relief by the dissolution of the matrix.

I scratched the surface to see what is lying behind the surface.

Despite the erosional cleaning and my scratching the lighter matrix (omLT unit) still existing.

The New data show that the concave contact line that i had drawn at the very beginning of this discussion is valid. And data also implies that there is a diffusive part on the contact.

Note: The circle on the photos showing the point that i had scratched on my second trip. The point lies between the yellow hand knife (on the left) and hammer (on the right) on photos (path826 and path852).

The rectangles on the old photo shows the borders of the recent photos.