Southern Manzano Mountains: Estadio canyon

Tuesday 30th May 2006

We were on the East side of the Rio Grande rift. As in Picuris mountain, rocks are Precambrian. The aim of the day is to map the formations without the description of different rock units

b.

We have made up three groups. Two of them have mapped the crest and the last one was in the canyon.

Picture 1: a. localisation map (Larson and Sharp, 2003). b. Cross section. c. geological map of Estadio canyon.

The first formation was made of quartzite interbedded with thinner micaceous schist levels. Sedimentation in massive beds seems to be regular. We can only see few hummocky cross stratifications corresponding to storm deposits in shallow water but not enough to be sure of which side was up.

We have found two thin amphibolite levels of (1 and 5 m of thickness) with feldspar and amphibole probably corresponding to a basalt protolith. These levels do not cross cut the stratification, they are just interbedded.

A foliation plan N10-80W, nearly parallel too the stratification, was measured in micaceous levels. We can also find this direction in quartzite but less pronounced.

We can identify massive quartzite levels and amphibolite one in the landscape, forming highest summits. Finally, we have found some cross stratification and determined which way was up. Some pegmatite intrusions with pink alkaly feldspar, quartz and muscovite, cross cut the stratification. We have not observed any foliation in it.

The second formation that we have met seems to be similar to the first, however it is more deformed. Massive quartzite levels are more developed than in the first one and the sedimentation was not as calm as in the first case. Indeed, we have seen lots of hummocky cross stratifications with lenticular deposits in this part of the canyon. The difference between micaceous and quartzite levels are less marked. Some parts of this second formation seem to be more massive.

All along this way we have found some structural structures like tension gashes (picture 1), folds (picture 2) and little fault (picture 3).

Picture 1: Tension gashes full of quartz in a quartzite level. We can determine the sense of the shearing. To obtain this kind of structure, you have to open the fracture, so only one sense for the shearing is possible.

Picture 2. Big anisopachous folds in quatzite layers. This out crop shows that the temperature was hot enough to deform competent layer without important faulting.

Picture 3. Little thrust fault in quartzite levels

We have observed crenulation in micaceous layers which indicate that there were two different deformations or a change in the direction of strains during the same one.

The last formation that we have seen was a coarse grain porphyraceous granite called Priest pluton. It is in intrusion in sedimentary rocks and dated at 1,42 Ga. There is also a metamorphic aureole around this intrusion. Some metamorphic minerals like Sillimanite andalousite and kyanite can be found around the contact with the quartzite, in rare micaceaous layers. However they are rare because most of surrounding rocks are made of quartzite. According to Larson and Sharp (2003), the temperature increase from 540°C at a distance of 1 km from the granite, to 690°C at the contact.

Picture 4: Granite intrusion in quartzite layers

All these structural clues (foliation, folding, thrust fault and tension gashes) show us that these rocks were deformed by strong compressive strains. The temperature during the regional metamorphism was about 500°C and occurred at 1,65 Ma (Larson and Sharp, 2003). That is why we can find amphibolite rocks. However the most part of Priest granite is not deformed. Only some foliation planes can be found on the edge of the intrusion. This explains that some part of the deformation is interpreted as late or post-granite intrusion.

Reference: T.E. Larson and Z.D. Sharp. Stable isotope constaints on the Al2SiO5 ‘triple-point’ rocks from the Proterezoic Priest pluton contact aureole, New Mexico, USA. Journal of Metamorphic Geology, 2003, n°21, p.785-798.