When St. Louis met St. Patrick

After a very long 20 hours of travel, a group of students and profs from Washington University in St. Louis, found ourselves in the village of Lucainena de las Torres, in the Almeria region of SE Spain.  We had traveled to meet another university class from TCD, to explore the geology of the region and forge academic relationships between our institutions. The TCD group welcomed us the first night with open arms, and we spent the rest of the week in the field learning from the very different backgrounds that each group brought.

Almeria is part of the Betic-Rif Arc, a series of mountain ranges that curve along the southern portion of Spain, across the straits of Gibraltar, and along the northern coast of Africa.  These mountain ranges formed by convergence of the African plate with the Iberian plate during the late Mesozoic into the Tertiary and consist of a series of three main thrust sheets. The region remains tectonically active, and is now dominated by NE-SW trending strike-slip motion along two main fault systems and by extension in the Alboran Sea that separates the Betic Cordillera in Spain from the Rif in Africa.  Extensive uplift of metamorphic basement rocks in the older thrust sheets, along with later Neogene marine and terrestrial sedimentation, volcanic activity, and strike-slip faulting, makes Almeria an exceptional place for new and seasoned geologists alike to lose themselves in a bonanza of interesting geologic phenomena.

Our first day in the field was the first time many of the WashU students had seen metamorphic rocks in the field – metamorphic rocks are not abundant in the flat lying carbonates of the American mid-Continent! With the help of the professors and students from TCD we began to appreciate the complex deformation histories revealed by different deformation fabrics, and their relations to one another, from s-folded quartz veins running oblique to the dominant planar foliation, to sinistral shear expressed in foliation-parallel quartz veins with tails, and to later stage brittle fractures infilled with hematite-bearing quartz.

Our second day brought us to one of the oddest geological sights I have ever seen.  As we walked along a curving road cut with the knowledge that we were supposed to see some sort of fault placing basement rock next to Neogene sediment, we merely transitioned from pink schist to purple schist until our professors urged us onward and around the corner. Suddenly we found ourselves looking at light-colored schist on top of red, powdery sediment fault gouge with a sliver of something reminiscent of a Van Gogh painting squished between the two.

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The Van Gogh rock was full of large, sinistrally-sheared clasts of basement rock in a dark, bluish-gray, fine grained matrix.  It was a cataclasite, or a fault-rock formed from shear motion facilitated by small-scale fragmentation, grinding, and sliding of the faulted rock unit.  We were looking at part of the Palaomares Fault Zone, and though we knew from our reading that it was a strike-slip fault, here it looked like a thrust.  We left with the reaffirmed notion that geologic structures in the field are often not the idealized ones we see in our textbooks.

On our third day in the field we spent the first part of our morning sketching a prograding carbonate reef and then moved closer to get a closer view.  Beneath the reef, we found ourselves standing in a stream bed full of detrital garnet.  Garnet from a carbonate reef?

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Naturally, we had to find out where these garnet grains were coming from, so we walked along the stream bed until we came to an outcrop of dacite, a felsic igneous rock, that was full of garnet crystals!  Garnet in an igneous rock?!  While apparently it can happen, the garnet in this dacite did not actually crystallize from the melt; it was instead incorporated as the magma that eventually formed the dacite moved to the surface and partially melted some metamorphic country rock.  The garnet crystals did not melt, but were transported and erupted at the surface largely unchanged.  As we walked further we realized we were in an eroded caldera, a former dacite dome, fringed on all sides by reef. We stuffed our faces with lunch and stuffed our pockets with garnets.

Our last two days, gave us more spectacular views of reefs, deltas, evaporites, lagoon deposits, and the Gordo Megabed – a thick, extremely high-energy debris flow deposit sitting on top of a series of turbidites.

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Pretty cool.

The time passed far too quickly, with all days filled with equally exciting geology. As the trip ended, American and Irish students together celebrated St. Patrick’s day in Spain. We all left with new friends, and I count myself lucky to have spent a week in such an incredible geologic region with some incredible geologists and people.  ¡Viva España! ¡Viva TCD! ¡Viva WashU!

By Zoe Lefebvre, undergraduate student, Washington University in St. Louis, USA.

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