18 Department of Geology and Geophysics • University of Wisconsin-Madison

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Pyrenees Field Trip

Toni Simo

The Pyrenees—what a wonderful place to study the interaction of sedimentation and tectonics! In May 2001 a group of 15 graduate and undergraduate students went for a ten-day field trip to the Pyrenees (Fig. 1). The trip was the first (hopefully of many) combined sedimentology and structural geology groups, led by Profs. Alan Carroll, Toni Simo, and Basil Tikoff. The trip was partially funded by the Weeks Bequest, BP-Amoco, and individual contributions. The early start of the trip was chaotic since Lufthansa, the chosen airline, went on strike the day of travel. Field trip participants arrived in Barcelona over a day and a half interval, arriving at different times of the day, all having been diverted to other airlines. Upon arriving in Barcelona, we discovered that the janitors in the airport had gone on strike. In spite of the chaos, the field trip started with almost no delay, and we headed off (along strike) westward in the Pyrenees. Camping and cooking gear was loaded between jetlagged bodies (How about that cooking pot, Mike?) for the beginning of one of the two cross sections across the Pyrenees.

Tremp cross-section The field trip was dynamic—so much to see and enjoy. We started at Toni’s old stomping grounds in the Spanish foreland near the town of Tremp. The relationships of folding, faulting, and sedimentation are incredible. We saw piggyback basins, synfolding sedimentation, etc. in the beautifully exposed mountain-sized outcrops. Jaume Veges showed us the Oliana anticline and a lateral ramp in the Pyrenean fold and thrust belt. The recognition that the edge of the basin is also the edge of the thrusting made everyone realize that sedimentation and deformation are more inter-

related than one may think. During the traverses, Toni explained how the shallow water carbonates and deep-water turbidites were formed during the transition between Early Cretaceous rifting and Late Cretaceous-Tertiary contraction. Toni Teixell discussed the amazing structures along the Ribagorca canyon, where the syn-sedimentary Early Cretaceous normal faults are preserved in the thrust systems.

Andora We got to see the effects of low Andorran gasoline prices: the roads to France were jammed and we were stuck in a three-hour traffic jam near a large filling station. The road was along the axial zone of the Pyrenees, so we inched along while looking at schists and granites.

Lherz, French Cheese, and Dutch Geologists In an attempt to investigate the entire Pyrenean mountain belt, we also visited the North Pyrenean fault. This fault is the de-facto boundary between south-vergent deformation in Spain and north-vergent deformation in France. A variety of ultramafic

Fig. 1. A group photo on the Lherz ultramafic body (type locality of lherzolite). Front row, left to right: Renaud Visser, Toni Simo, Mike Smith, Scott Giorgis, Marwan Wartes. Standing, left to right: Colin Walling, Basil Tikoff, Hans Ave Lallement, Norlene Emerson, Pierre Bouilhol, Eric Horsman, Meredith Rhodes, Eric Carson,Liz Leslie, Jana Van Alstine, Jeff Pietras, Selena Mederos (front), Leonardo Piccoli, Kate McColgin, Sarah Titus, Alan Carroll.

2001 The Outcrop 19http://www.geology.wisc.edu

bodies are aligned along the North Pyrenean fault, including the Lherz massif which is the type locality of lherzolite. Two geologists from the University of Utrecht (Martyn Drury and Renaud Visser) and Hans Ave Lallement (Rice University) showed us around the massif (Fig. 1). Hans had done his PhD on the Lherz body in 1969 (it remains the standard reference), and it was the first time that he had returned to the area. We still covet Alan’s piece of peridotite breccia.

Jaca cross-section While the structure group stayed behind to look at one more ultramafic body in France (how could anyone not want to see mantle ultramylonites?), the sedimentologists returned to Spain. Although they apparently saw some fabulous syn-folding sedimentation, all the structure group heard about was staying in an eleventh-century monastery, eating and drinking in the cloister while contemplating snow-covered peaks. Finally, when the structural geologists rejoined the sedimentologists, we ate the worst food in all of Spain in Jaca (Wait, there is a trend here...), but we saw some fantastic geology. Toni Barnolas and Inma Gil-Peña acted as our guides to show us thrust deformation exposed in glacial cirques in the high Pyrenees. We saw spectacular examples of nappes, imbricate thrust sheets, box folds, and the back portions of the Pyrenean thrust belt (Figs. 2 and 3). All in all, it was a trip of combinations: long day hikes and road geology, Spanish and French cuisine (as well as our own camping specialties), and sedimentological and structural discussions. We share memories that will stay with us all our lives. Unfortunately, it seriously raised the standard for all subsequent field trips.

Fig. 2. In general, there was a very positive interaction between structural geology and sedimentology interests. However, every once in a while, people’s true colors came out. This picture shows the structural geologists examining one of the most fabulous exposures of a large-scale thrust fault placing Cretaceous (dark color, K) over Paleocene carbonates (light color, P), while the sedimentologists look across the cirque valley at the relations exposed on the other side.

Fig. 3. Opposite wall of figure 2 in the Aragues valley (near the town of Anso, southern Pyrenees) showing the folding and thrusting of Cretaceous strata (dark color, K) and the Paleocene carbonates (light color, P).