Marnes Rouges Inférieures Formation

The Marnes rouges inférieures Formation is located in the department of Aude and Ariège in Occitania region. The formation crops out mainly in the Rennes-les-Bains syncline, located in the Upper Aude Valley, whose rocks can be seen particularly near the villages of Campagne-sur-Aude, Espéraza, Fa, Granès, Rennes-le-Château, Brenac, Saint-Ferriol.

The Marnes rouges inférieures Formation conformably overlies the Grès d'Alet Formation (Alet Sandtones Formation) and conformably underlies the Calcaires de Vignevieille Formation (Vignevieille limestones Formation).

The Marnes rouges inférieures Formation is also present in Ariège department in the Benaix, Dreuilhe, Péreille, and Le Mas d'Azil anticlines, where it lies unconformably on several Upper Cretaceous formations Coniacian to late Campanian in age.

The Marnes rouges inférieures Formation reaches about 130 metres (430 ft) in thickness and is subdivided into four members including from bottom to top:

• The Marnes rouges de Campagne Member (Red Marls of Campagne) : this member is composed of marmorized, ochre to red sandy mudstones, with a few conglomerates and sandstones levels. These rocks correspond to floodplain silts. These layers reach about 20 metres (66 ft) in thickness according Fondevilla et al. or 25 to 30 m (82.0 to 98.4 ft) according to Bilotte.
• The Grès des Estous Member (Estous Sandtone) : about 10 metres (33 ft) in thickness (a little less according to Fondevilla et al.), this member is almost exclusively composed of sandstones layers sometimes separated by thin red mudstone levels. These sandstones correspond to braided river channels.
• The Marnes rouges de la Maurine Member (Red Marls of Maurine) : this member is predominantly composed of red mustones, with minor ochre brown, orange, and purple mudstones, and some conglomeratic and silty levels. The mudstones correspond to floodplains silts, while the conglomeratic intercalations represent ancient river channels.. The presence of numerous calcareous pedogenetic concretions and the intense marmorization of the mudstones indicate the existence of paleosols.. Bilotte attributes to this member a thickness of 20 to 30 m (65.6 to 98.4 ft). Fondevilla et al. assign it a much greater thickness, around 100 m (328.1 ft).
• The Poudingue Fleuri Member (Fleuri Conglomerate) : this member, 5 to 15 m (16.4 to 49.2 ft) thick, consists of a conglomerate with a sandstone-carbonate matrix and calcareous cement. This conglomerate is of fluvial origin and corresponds to former gravel banks with large pebbles. In places, the Poudingue Fleuri forms the top of the Marnes rouges inférieures Formation, directly supporting the Vignevieille Limestones Formation. In other sections, the Poudingue Fleuri is intercalated in the upper part of the Marnes rouges de la Maurine Member.

In 1985, Bilotte reported in the marnes rouges de Campagne Member the presence of a palynological association suggesting a late Campanian age, and consequently attributed a Maastrichtian age to the three overlying members..

In 2016, a magnetostratigraphic study revealed that the upper part of the Marnes rouges de Campagne, the Grès des Estous, and the first twenty meters of the Marnes rouges de la Maurine belong to the normal chron C32n corresponding to the beginning of the lower Maastrichtian. More specifically, the vertebrate localities in the basal part of the Marnes rouges de la Maurine are located at the top of subchron C32n.1n, corresponding to an age of 71.5 million years. The rest of the Marnes de la Maurine, as well as the Poudingue Fleuri, are located in the reverse chron C31r, corresponding to the upper part of the lower Maastrichtian. Thus, the age of the Poudingue Fleuri, at the top of the Marnes rouges inférieures Formation, is estimated to be between 70 and 69.5 million years old. Above, the upper Maastrichtian could be limited to a few meters in the Vignevieille limestones or is totally absent. Regarding the lower part of the Marnes rouges inférieures Formation, the Campanian-Maastrichtian boundary (set at 72.2 million years) would be located below the section sampled by Fondevila et al., probably in the middle or lower part of the Marnes rouges de Campagne Member, or even lower in the Grès d'Alet Formation. These results indicate that in the Upper Aude Valley, the Marnes rouges inférieures Formation dates almost exclusively from the lower Maastrichtian, with only the lower half of the Marnes rouges de Campagne Member possibly dating from the top of the Campanian.

In the Plantaurel massif in Ariège, the Marnes rouges inférieures Formation becomes younger due to the progressive and diachronic migration of continental facies towards the west in a context of marine regression. In this region, the Marnes rouges inférieures Formation grade laterally into the Marnes d'Auzas Formation, which has a lagoonal and continental origin and dates back to the late Maastrichtian.

The Bellevue quarry in Campagne-sur-Aude is the main vertebrate site of the Marnes rouges inférieures Formation and is one of the richest Upper Cretaceous sites in Europe. It is located at the base of the Marnes rouges de la Maurine Member just a few meters above the Grès des Estous Member, and corresponds to a former river meander where many disarticulated bones and entire corpses have accumulated. Vertebrate remains are concentrated in conglomerate and siltstone levels 4 to 6 m (13.1 to 19.7 ft) thick intercalated in the mudstones. The different fossiliferous levels (six recorded in 1999, then seven in 2002) are all located at the top of the conglomeratic levels. The variable state of preservation of the bones (from very eroded to well preserved), most often disarticulated, as well as the faunal composition, which varies from one level to another, testify to more or less significant transport and suggest considerable taphonomic biases in the composition of the fauna, which cannot be considered representative of an ecosystem.

The Bellevue quarry has yielded several thousand fossils including at least 1400 dinosaur bones belonging mainly to titanosaurian sauropod. This locality was long considered a monotaxic titanosaur bonebed represented by Ampelosaurus atacis. Since the 2010s, however, several studies have suggested the presence of a multispecific titanosaur assemblage at the Bellevue site. En 2012, Bernat Vila et al. distinguished three morphotypes based on femur morphology and tentatively assigned one of them to the genus Lirainosaurus. A generic attribution contested by Veronica Díez Díaz et al. in 2013. In 2021, Veronica Díez Díaz et al. also admitted the existence of a second titanosaur at Bellevue but found more similarities with the genus Garrigatitan from the Upper Campanian of southeastern France. In 2023 and 2024, Bernat Vila et al. presented at several scientific conferences the preliminary results of an in-depth study of the titanosaur remains of Bellevue confirming the presence of several titanosaur species (possibly up to four) at this site. The results of this study imply a future modification of the diagnosis of Ampelosaurus and the material attributed to it, and the description of at least one new species.

The Marnes Rouges Inférieures Formation contains many fossils of continental vertebrates. They are especially abundant at the base of the Marnes rouges de la Maurine Member but are also present in the Grès des Estous Member. These vertebrate remains are represented by teeth and bones as well as more or less complete eggs and shell fragments. Some invertebrates (freshwater gastropods and bivalves) and plant remains are also known. The macroflora, found particularly in Bellevue, is represented by palm and cycad leaves, as well as fragments of angiosperm wood from trees 10 to 15 m high.

• Buffetaut, Eric (1995). "Les dinosaures du Crétacé supérieur". In Buffetaut, E. (ed.). Dinosaures de France (in French). BRGM. pp. 101, 131. ISBN 2-7159-0766-4.

• Chanthasit, Phornphen (2010). The ornithopod dinosaur Rhabdodon from the Late Cretaceous of France: anatomy, systematics and paleobiology (PhD thesis). Université Claude Bernard.

• Laurent, Yves; Le Loeuff, J.; Bilotte, M.; Buffetaut, E.; Odin, G.S. (2001). "Campanian-Maastrichtian continental-marine connection in the Aquitaine-Pyrenees-Provence area (S France)". In Odin, G.S. (ed.). The Campanian-Maastrichtian Stage Boundary, Characterisation at Tercis les Bains (France) and Correlation with Europe and other Continents. Amsterdam: Elsevier. pp. 657, 674. ISBN 978-0-444-50647-4.

• Le Loeuff, Jean (2005). "Osteology of Ampelosaurus atacis (Titanosauria) from Southern France". In Tidwell, V.; Carpenter, K. (eds.). Thunder-Lizards: The Sauropodomorph Dinosaurs. Bloomington: Indiana University Press. pp. 115, 137. ISBN 978-0-253-34542-4.