Continuing Olivier and Caroline’s epic cycle journey through South America (see the links at the end of this article for the earlier parts in this series).
We felt good in the temperate green of southern Chile’s Ring of Fire, but our journey was calling us. We left Pucón, nestled at the foot of Villarrica Volcano, for a new crossing of the Andes through the Mamuil Malal pass, 1,253m above sea level. The horribly steep dirt road zigzagged through an araucaria forest and climbed the dissected topography of the Chilean side of the Andes.
After a hard day’s cycling, we eventually reached the summit of the pass, and we looked forward to riding down the Argentinian flank of the Andes.
This delightful downhill ride never happened! We realized that the Argentinian Andean foothills consist of a wide plateau at around 1,000m. Here, there is no green forest, but yellow dry grass; no steep topography, but an endless elevated Patagonian pampa; and no nice climate, but a heavy burning sun. We had entered the domain of Neuquén.
Overview of Neuquén Geology
The Neuquén region is known among geoscientists mostly through the Neuquén Basin, which lies at the eastern foothills of the Andes, between 32°S and 40°S. The basin comprises Late Triassic to Early Cenozoic sequences covering an area of more than 120,000 km2, with up to 6,000m of well-preserved marine and continental deposits.
An initial Triassic rifting was followed by a Jurassic to Lower Cretaceous thermal subsidence phase, marked by the deposition of several high quality marine organic-rich shale formations, including the famous Lower Cretaceous Vaca Muerta Formation. From the Upper Cretaceous until the present day, the western margin of the basin was affected by east-west shortening as a result of the Andean orogeny, resulting in the deposition of thick continental deposits in a foreland setting. Since the Cretaceous, the Neuquén Basin has experienced successive igneous episodes, leading to the emplacement of voluminous intrusive complexes and large back-arc volcanic fields. The present configuration of the basin includes two main sections: the Neuquén Andes, which corresponds to the deformed western edge of the basin, and the Neuquén Embayment, which is made up of the eastern, non-deformed, flat part of the basin.
The textbook geological marvels of the Neuquén Basin can be discovered through a fabulous ~1,000 km-long road trip, starting from Neuquén city and travelling westwards to Zapala, before following the famous National Road 40 (RN40) towards the north along the Andean foothills. The road crosses vast, breathtaking landscapes, including the hostile desert of the southernmost Mendoza Province, and bends around the imposing Payunia volcanic field, following the Río Grande river. Finally, the itinerary leaves RN40 at Bardas Blancas to cross the Andes back to Chile, reaching the wild springs of Río Grande at the Vergara pass, at the foot of the Planchón-Peteroa volcano.
The Dinosaur Valley
Argentina hosts a tremendous paleontological inheritance. Abundant evidence of this treasure is found in the Neuquén Basin, which is sometimes known as the ‘Valley of the Dinosaurs’. Most dinosaur fossils are concentrated in the Upper Cretaceous continental deposits of the Neuquén Embayment. The most amazing pieces are exhibited in the least expected place on Earth: the charmless oil town of Plaza Huincul, isolated in the middle of the flat Neuquén Embayment.
When passing through Plaza Huincul, visitors should make sure they don’t miss the Carmen Funes Museum where, for less than €2, they can access a vast hall with incredible reconstructions of dinosaur skeletons. In particular, one can appreciate the impressive dimensions of the Argentinosaurus huinculensis, a herbivore of the sauropod family, considered one of the largest, if not the largest, dinosaurs discovered. Other stunning pieces exhibited at the museum are reconstructions of the Giganotosaurus carolinii, one of the largest carnivores ever found, and the most complete ichthyosaur fossil in South America. In addition, the museum features the only known embryos of sauropods, found in an extraordinary nesting site near Auca Mahuida, 150 km north-west of Neuquén City. The preservation of these embryos is such that they contain patches of delicate fossilized skin.
True and Fake Volcanoes
The road north of Zapala approaches imposing volcanoes which protrude out of the vast, low-lying Neuquén Basin. The most prominent is the Plio-Pleistocene Tromen volcano, the second highest peak in Patagonia, situated at the gates of the town of Chos Malal. This 4,114m-high sentinel, located 100 km east of the main Andean range, is visible from most of the Neuquén Embayment up to several hundred kilometers distance. RN40 offers various stunning viewpoints over the slopes of this very picturesque volcano. In addition, the visitor should not miss its northwestern flank, which offers a unique scenery of shallow lakes (Laguna del Tromen), flamingos, high Patagonian plateaus and gauchos, dominated by a 2 km-high volcano that recently erupted voluminous lava flows.
In addition to its beauty, Tromen can offer something of particular geological interest: it crowns the hanging-wall of a large-scale thrust fault that is well exposed around the eastern foot of the mountain. In fact, what looks like an enormous 3,000m-thick accumulation of volcanic products is a tectonic structure consisting of Mesozoic sediments uplifted 3,000m above sea level and covered by thin volcanic products. In addition, Tromen built up during tectonic compression, providing one of the first proofs that volcanism does not need tectonic extension to occur.
Further north, RN40 traverses the Miocene-Pleistocene Payunia volcanic field (see summary above). It is one of the largest volcanic fields on the planet (5,000 km2), dominated by the flat Payún Matrú (3,680m) and pointed Payún (3,715m) volcanoes, and containing about 800 individual volcanic cones. Recently, it was discovered that Payún Matrú emitted the longest known individual Quaternary lava flow on earth – 181 km (Pasquarè et al., 2008).
Crossing the Payunia volcanic field is an unforgettable, almost mystical experience. It offers a true feeling of isolation in the middle of the elevated Patagonian pampas and invites the visitor to meditate on the beauty of such pristine nature. Because of its uniqueness, the Payunia volcanic field is strictly protected and is a candidate to be a UNESCO world heritage site.
North of Tromen stands the ‘Roof of Patagonia’, a massive, rounded snow-capped mountain 4,709m high and visible from much of the Neuquén Basin. All maps of the region mark this landmark as Domuyo Volcano; the problem is, Domuyo is not a volcano! Despite its prominent, sub-conical shape, it is a tectonic structure brought to high altitude by the Andean orogeny. The confusion probably results from the presence of hot geothermal fields at the western foot of the mountain. Whatever its nature, the shining white summit of Domuyo is attractive to every mountain lover.
The Neuquén Basin is a significant economic player in Argentina, as it is the most important hydrocarbon-producing province in South America. Most of the fields are found in the Neuquén Embayment, where the petroleum systems are buried, but in the western margin of the basin the main formations of these systems are brought to the surface in a well-developed and well-exposed fold-and-thrust belt. The resulting outstanding outcrops have been the targets of voluminous academic research, and have global significance having been intrumental in defining new concepts in paleontology, sequence stratigraphy, reservoir geology and structural geology.
The numerous road-cuts along the RN40 spectacularly expose several organic-rich shale formations, the most famous of which is the Lower Cretaceous Vaca Muerta Formation. This world-class source rock potentially hosts gigantic reserves of shale oil and shale gas, and the outcrops along the RN40 are thus of primary importance for those studying it. Beautiful views of reservoir sandstone and Upper Jurassic gypsum are also found along the route. The fantastic, wide outcrops allow the visitor to understand large-scale textbook tectonic structures such as anticlines, synclines and thrust faults.
The Gauchos’ Spirit
Definitive geology and stunning scenery make the Neuquén Basin an incredible place for geologists to visit, but the experience would be incomplete without coming into contact with what makes the spirit and soul of the land. Everywhere along the route and in the desert there are stone or wooden huts standing in small green oases: the abodes of gauchos, or Argentinian shepherds.
Despite living in remote areas and in harsh climatic conditions, gauchos are incredibly welcoming, curious and generous people. How many times during our journey have we spent hours chatting with them, sharing homemade bread and ‘mate’, a typical local tea? We will never forget our fabulous encounters with, among others, Faustino, a gaucho in the Río Grande Valley and the leader of a folkloric music band with his family; or a group of gauchos travelling down from the Vergara pass for the autumn transhumance, who spontaneously shared with us an amazing meal of grilled meat (asado) in the heart of the mountain.
Discussions with gauchos seem to slow time down, and bring the visitor into a state of harmony with the extraordinary geological environment of the Neuquén Andean mountains.