East Side of Valles Caldeira, Jemez Mountains

Friday 26th May 2006

Picture n°1: localisation map of Valles Caldeira, Jemez Mountains

During this day we observed mainly volcanic events products of Valles Caldeira, and also rift sediments with reworked volcanic products.

The first stop showed us this rift sediments : we can see on the picture an alternance between indurated conglomerates and thiner sandy beds with cross-stratification, which gives us the way of flow.

The clasts which are inside the conglomerate can also give us indications on the way of flow.

This clasts have a volcanic origin: we have intermediate lava clasts with feldpath and pyroxene minerals ; there are also pieces of quartz and metamorphic rocks.

The sorting in clasts shows a change in the deposit dynamics, in a shallow water environnement.

In these sediments we can see in several places ashes beds, so we can say that there was reworking of pyroclastic deposits in the sediments.

Cross stratification in syn-rift sediments

Cross stratification in sandy beds

Picture n°2: Cross-stratification in thin beds of sand

At our second stop we saw a very beautiful outcrop which shows at its basis a basaltic flow with many vesicles: they are oriented and allow us to determine the way of lava flow. This blue-grey colored rock lets us see altered olivine and pyroxene. The flow measures nearly 10 meters of thickness, its age varies between 5 and 6 My.

We find above a red altered bed which separate the basalt and a stratified, very light, pumice stone fallout: its very explosive features indicate that it must be a very viscous lava, certainly rhyolite, what confirms the presence of quartz. This ashes are dated at 1,5My.

When we come up to the summit, ashes beds become thiner and regular. On the top ashes are even lighter and thiner, which is normal: bigger stones fall down in first, and tinier stones fall down at the end.

This tuff is overlayed by a massive red flow with coarse columns.

Picture n°3: Ashes deposits interbedded with lava flows

We stayed the rest of the day inside the National Parc of Bandelier, where we saw this time a pyroclastic flow: we observed blocks of breccia and pumice stones within this blocks, which mesured untill 4 meters of high. Pieces of quartz indicate that it must be a rhyolite too.

There were 2 eruptions, whose products reach several hundreds feet of thickness. The first eruption happened 14My ago and the second 1,2My ago.

The caldeira of this volcano is huge: it measures more than 20 kilometers of diameter…

Picture n°4: Pumice stone in pyroclastic breccia