The La Amarga Formation is a geologic formation with outcrops in the Argentine provinces of Río Negro, Neuquén, and Mendoza. It is the oldest Cretaceous terrestrial formation in the Neuquén Basin.
The type locality is La Amarga Arroyo and China Muerta Hill.Musacchio, 1970 The La Amarga Formation unconformably overlies the marine Agrio Formation of the Mendoza Group. It is in turn overlain by the Lohan Cura Formation, separated by another unconformity.
Composition There are three members within the La Amarga Formation. The oldest is the Puesto Antigual Member, which is approximately thick and consists mainly of sandstone deposited in the channels of a braided river system. Paleosols, or soil deposits, are well-developed. The Bañados de Caichigüe Member is the next highest, approximately thick. Alternating limestones, shales, and siltstones make up this member, indicating a lacustrine (lake) environment. Youngest and thickest is the Piedra Parada Member, approximately thick in some sections. This member consists of alternating sandstones and siltstones from an ancient alluvial plain, with some swamp and paleosol deposits.
Fossil content Most of the tetrapod fossils found in the La Amarga come from the Puesto Antigual Member.
Dinosaurs Dinosaurs Genus Species Location Stratigraphic position Material ImagesAmargasaurusWeishampel et al., 2004, "Dinosaur distribution (Early Cretaceous, South America).", pp. 563–570A. cazauiNeuquénPuesto Antigual"Braincase and associated postcranial skeleton.""Table 13.1," in Weishampel et al., 2004, p. 264AmargatitanisA. macniNeuquénPuesto Antigual"Two caudal vertebrae, an incomplete right ischium, a right femur, an incomplete right tibia, an incomplete right fibula, a right astragalus, and an incomplete right metatarsal I." LigabueinoL. andesiNeuquénPuesto Antigual"A cervical neural arch, a mid to posterior dorsal neural arch, a posterior dorsal centrum, the left femur, left ilium, articulated pubic shafts, and two pedal phalanges."Carrano et al., 2011 ZapalasaurusZ. bonaparteiNeuquénPiedra Parada Stegosauria indet.IndeterminateNeuquénPuesto Antigual"Supraorbital, cervical and caudal vertebrae, dermal armor".Galton & Carpenter, 2016Pereda Suberbiola et al., 2012
Other tetrapods Other tetrapods Genus Species Location Stratigraphic position Material ImagesAmargasuchusA. minorNeuquénPuesto AntigualA trematochampsid crocodylomorph, found in association with the holotype of Amargasaurus Pterosauria indet.Montanelli, 1987IndeterminateNeuquén An isolated femur VincelestesV. neuquenianusNeuquénPuesto AntigualA cladotherian mammal
See also List of dinosaur-bearing rock formations
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