Brest, 11/12/2006 Compte-rendu scientifique de l'opération "Formation internationale sur les risques géologiques du rift continental du Rio Grande" Attribution BMO pour cette opération: 1 800 ⁄ Une subvention pour mobilité internationale a été obtenue pour cette opération pédagogique qui s‘est déroulée du 20 mai au 3 juin 2006, dans le cadre de la convention signée entre l‘Université de Bretagne Occidentale (UBO) et l‘Université de Purdue (Indiana, Etats-Unis). Les 8 étudiants français ayant bénéficié de cette mobilité internationale appartenaient au M aster Sciences de la M er et du Littoral, M ention Géosciences Océan, de l‘Institut Universitaire Européen de la M er. Cette activité faisait partie intégrante d‘une Unité d‘Enseignement (UE R8.2 « Stage International ») de leur parcours en M aster. Ils ont été évalués à la fois sur leurs prestations orales, leurs activités de terrain, et un rapport écrit publié sous forme d‘un site W eb (voir http://perso-sdt.univ-brest.fr/~jacdev/ens/purdue06.htm) dont nous fournissons ici les principaux éléments. Un complément du financement pour cette opération a été obtenu via la scolarité de l‘IUEM, mais il fut insuffisant pour rembourser les participants, qui ont eu finalement à leur charge une part importante de leur mission, dont le coût de transports avion et visas seuls s‘est élevé à 8716 ⁄. Nous tenons à remercier Brest Métropole Océane pour son soutien à cette opération, qui a été extrêmement fructueuse pédagogiquement, à la fois du point de vue scientifique (découverte sur le terrain d‘un objet géologique de première grandeur, un très grand rift continental actifs) et culturel (contact direct avec des enseignants et étudiants américains, pratique de la langue anglaise écrite et orale). Le responsable pédagogique Professeur UBO-IUEM Responsable du Master SML Pièces jointes : Rappel du cadre de l‘Echange International Purdue-UBO œ Page de garde du Projet 2006 Participants de l‘excursion RISQUES GEOLOGIQUES DANS UN GRAND RIFT ACTUEL : LE RIO GRANDE, USA Rapports d‘activités (7 sites et arrêts sélectionnés) Annexe : Planches des photos réalisées par les étudiants (site : http://www.students.bucknell.edu/rheuer/newmexico.htm) Time Schedule of the field Trip (May 22, 2006 to June 2, 2006) Monday May, 22: Arrival Albuquerque International Airport, Drive to Santa Fe - Stop at Albuquerque volcanic field & Sandia Crest for overview). Camp at Rancheros de Santa Fe Tuesday, May 23: Precambrian basement of New Mexico: Harding Pegmatite & traverse through Proterozoic stratigraphy. W ednesday, May 24: Picuris mapping project Thursday, May 25: Rift fill in Picuris and Taos area, Embudo fault zone, Taos for supper. Friday, May 26: Bandelier N.M. Santa Fe for afternoon/dinner Saturday, May 27: W hite Mesa, mapping in the Mesozoic Sunday, May 28: W hite Mesa, mapping Monday, May 29: Tent Rocks (1/2 day), Sandia Crest or Albuquerque volcanoes, Drive to Socorro, Stay at Motel 6 Tuesday, May 30: Manzano Mts, east rift margin (Precambrian/Paleozoic) W ednesday, May 31: Baca Canyon, west rift margin (volcaniclastics/sedimentary rift fill) Thursday, June 1: Baca Canyon, west rift margin Friday, June 2: Depart Albuquerque International Report 1. Picuris Mountains By Ronan DENNIEL and Nicolas AUBERT Tuesday 23 may 2006 : During the first day of this trip we went to the Picuris mountains. Theses mountains are located on the north of the Rio Grande Rift. The Picuris mountains are mostlly composed with metamorphic and igneous rocks. The first outcrop is an old mine called “Harding mine”. This outcrop is about ½ miles long. The pegmatite is 1,4 Byr. There is an intrusive contact between the p eg m atite and the am p hibolites which are 1 ,7 B y r The contact is shown on F ig ure 1 . There is some Zircon in these rocks: it could mean there is a lot of metamorphism and igneous rocks beside the pegmatite. Figure 2. Map of the pegmatite mine kindly provided by Aubri A. Jenson Figure 1 : contact between the pegmatite and Wednesday 24 and Thursday 25 may 2006: mapping in “Cerro de la Marquenas” in the Picuris mountains. W e started the mapping project in the Picuris mountains on this beautiful W ednesday morning. The formations we can observe are metamorphic Precambrian rocks. The first one is the Marquenas formation which is composed of quartzite and metaconglomerates. Some cross stratifications show us that this formation is overturned. Then, we enter in the younger formation of Piedre lumbre formation which is Silver gray , fin-grained, with graphitic garnet-biotite schist. the contact between the Marquenas formation and the Piedre Lumbre formation is not really easy to see on the field, but it seems to be a faulted contact which leads to this unconformity. This is most probably a reverse fault since older rocks of the Marquenas Formation are placed over younger rocks of the Hondo Group. While going down the hill (from south to north), we enter the older formations of Pilar formation (composed of dark gray to black phyllite with extremely fined grain). There are some beautiful garnets and other metamorphic minerals. In the rattle snake gorge (no rattle snake but some crows and Nico saw a bear) we walk Piedre Lumbre trough the Rinconada formation, where we see cooper deposits. The Rinconada formation is composed of PFimla.r Fm. interbedded quartzite and schist of diverse colors and composition. This Rinconada Fm. rinconada formation is 700 meters thick, it is the most thick formationof Ortega Fm. the precambrian. Then in the canyon we find older Precambrian Marquenas Fm. formation: the Ortega formation, which is composed of diverse massive to cross-bedded quartzite, andalousite, kyanite, and some garnet locally. This took about too days of hard mapping with about a hundred degrees Fahrenheit, we had good discussions about the faulted contact between the Marquenas formation and the Piedre Lumbre formation. But the final idea was a reverse fault dipping toward the south. We conclude that the Cerro de la Marquenas was composed by an anticline of Precambrian rocks with the Ortega formation in its heart. The unconformity between the Piedre Lumbre and the Marquenas formations was supposed to be a reverse fault even if we didn’t saw the contact. Copyright Ronan & Nico Report 2. East Side of Valles Caldeira, Jemez M ountains Friday 26th M ay 2006 Picture n°1: location map of Valles Caldeira, Jemez M ountains 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 W ay of flow Conglomerate Picture n°2: Cross-stratification in thin beds of sand Cross stratification in sandy beds 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. C e 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. W hen 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 W e 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 Report 3. W hite M esa mapping project Partie rédigée par Thomas bryant On May 27th and 28th, we went to map in the White Mesa area. White Mesa is located on a federal land (BLM - Bureau of Land Mangement) and is open to the public (most pueblos and reservations are not open to the public). It has a very hot climate with almost no vegetation (even cactuses seem to find it tough to survive there) and apart for rattlesnakes the only animals we saw were one antelope and a few cows (how on earth do they survive there???). White Mesa is near a gypsum quarry situated to the West/ North West of San Ysidro and about to the south west of the Valles Caldera (Map figure 2).