The Tremp basin, Llerida province, Catalunya, Spain

The Pyrenees foothills on both the French and the Spanish side have yielded quite some vertebrate fossils in recent years. These fossils occur mainly in the upper Cretaceous (Maastrichtien) sediments.

On the Spanish side, the Tremp basin constitutes one of the most important sites. The Tremp basin includes roughly the area between the villages of Aren and Santa Maria de Meia/Coll de Nargo, a distance of about 30 km from East to West. From North to South, the sediments are exposed over an area of about 50 km, from Senterada to Balaguer. The basin is split into two parts, separated by the Sierra de Montsec d'Ares and the Sierra de Montsec de Rubies. Both Sierras expose older Jurassic and early Cretaceous sediments. In many parts of the basin the Maastrichitien (Cretaceous) is covered by Danien (Tertiary) sediments. The geology of the area is rather complex as the sediments have been subject to significant uplifting and folding during the past 70 million years as the Iberic peninsula clashes with the Eurasian continent (the origin of the Pyrenees, and the cause for the oceanic shoreline conditions in the late Cretaceous). Therefore the older sediments can on many places be found on the top of the hills/mountains, while the younger (Tertiary) sediments cover the valleys (e.g. in the Vall d'Ager). The geology is unique as the Tremp formation contains one of the worlds best exposures of the K/T boundary and it constitutes a continental/shore type of environment, so perfect conditions to study the extinction of dinosaurs and other animals. The whole Tremp formation covers about 700 metres of sediment that represent about 10 million years. The K/T boundary is not easily discernible in the terrain.

Many dinosaur fossils and other vertebrate fossils have been found in these sediments, but they are limited to certain locations, most formations do not contain any fossils. For example during a three day search I checked an exposure of a large part of the Tremp formation, but discovered only a single 30-cm thick bonebed with dinosaur remains, a mere 0.05% of the whole 700 m formation. Literature suggests another layer with bones somewhat above the K/T boundary (obviously without dinosaurs), which I could not find.  Dinosaur footprints alledgedly abound (e.g. around Santa Maria de Meia, and Figuerola), but are not signposted. I encountered what looked like a theropod footprint on a stone slab along the road (not mentioned in the literature). Dinosaur nesting localities occur on various locations between Tremp and Coll de Nargo, with some eggs exposed in the municipal museum in Isona.

Unfortunately the whole area seems to be off-limits to amateur collectors (contrary to a lot of information on the Internet, where e.g. some campings in the area advertise fossil collecting). There are no roadsigns whatsoever that ban fossil collecting, but I was stopped by two friendly policemen who explained that the whole area is a regional park (PEIN - Pla d'espais d'interès natural), where fossil collecting is strictly forbidden because of the high geological importance of the area (and they confiscated what was probably a nice dinosaur jawbone, sigh…, see below). Be warned.

Still the area is extremely interesting from a geological perspective. Below some photos. I hope they send in some professors to pick these bones up, or grant me a permit. If not, all this material will be lost to Science soon because of natural erosion.