The Lefipán Formation is a Maastrichtian to Danian, straddling the Cretaceous-Paleogene boundary, geologic formation of the Cañadón Asfalto Basin in Chubut Province, Patagonia, Argentina. The up to 380 metres (1,250 ft) thick stratigraphic unit comprises mudstones, sandstones, siltstones and conglomerates, sourced from the North Patagonian Massif and deposited in a deltaic to shallow marine environment with a strong tidal influence. The basin that in those times was connected to the widening South Atlantic Ocean with a seaway connection to the Austral Basin and possibly with the Pacific Ocean. The formation has provided unique fossil flora assemblages dating to the Cretaceous and Paleogene ages, and are characteristic of the early Cenozoic after the extinction of the dinosaurs. The occurrence of the same taxa in the Maastrichtian and Danian successions suggests that the Cretaceous-Paleogene extinction event did not affect aquatic plant communities, which retained approximately similar structure and composition during the transition between the latest Maastrichtian and the earliest Paleocene. Insect predation on fossil leaves shows a considerably more rapid recovery from the extinction in event in Patagonia (about 4 Ma) than in the Western Interior of North America, estimated at 9 million years. Fossils of the mammal Cocatherium, the oldest known marsupial or any therian mammal in the Southern Hemisphere, and fish; Hypolophodon patagoniensis and shark teeth were found in the Danian section of the formation. The Danian part of the succession contains fossil flora of Lactoridaceae, presently a monotypic family restricted to the subtropical forests of the Juan Fernández Archipelago in the South Pacific Ocean, offshore Chile. The latest Cretaceous lower section of the formation contains fossils of a plesiosaur; Aristonectes parvidens. The genus Lefipania padillae and species Cocatherium lefipanum and Araucaria lefipanensis were named after the formation.
Description
The Lefipán Formation was first described by Lesta and Ferello in 1972. The formation was formerly considered a member of the underlying Paso del Sapo Formation. The Lefipán Formation is named after Lefipán, a prominent member of the Mapuche, who are the native inhabitants of the region where the formation crops out. The Lefipán Formation is found in the Cañadón Asfalto Basin, stretching from the Andean foothills to the Atlantic coast in Patagonia, stretching about 400 kilometres (250 mi). It reaches a maximum thickness of 380 metres (1,250 ft). The formation partly overlies and is partly laterally equivalent to the Paso del Sapo Formation and is unconformably overlain by the Barda Colorada Ignimbrite, part of the Middle Chubut River Volcanic Pyroclastic Complex. On both banks of the Chubut River where the Lefipán Formation crops out, it is overlain by the Huitrera Formation. In other parts of the Cañadón Asfalto Basin, the formation overlies the Cerro Barcino Formation and is unconformably overlain by the Colloncuran Collón Curá Formation or the basalts of El Mirador Formation. In the western part of the basin, the Lefipán Formation rests unconformably on the Early Jurassic Lonco Trapial Formation. At the fossil site of Cocatherium, the formation is 200 metres (660 ft) thick and comprises a Maastrichtian succession of massive mudstones with intercalating parallel and cross-bedded sandstone beds and coquinas that preserve a molluscan fauna. Bioturbation of Skolithos-Cruziana-type is present in the sandstones. Phosphatic levels occur in both the Maastrichtian and Danian parts of the formation, and the sandstones contain grains of biotite, zircon, kyanite, amphibole and pyroxene, all typical of an igneous and metamorphic provenance. Traces of volcanic glass are also present in the sandstones.
Basin history
The Cañadón Asfalto Basin started forming as a rift basin in the earliest Jurassic on top of Permian basement. During the Jurassic and Cretaceous, the basin went through an extensional tectonic regime, with transtensional movements. Several distinct tectonic reactivation cycles occurred, with block rotation due to transpressional forces, characterized in the stratigraphy by regional unconformities. The western side of the basin during the Late Cretaceous experienced a marine transgression of the Atlantic Ocean, depositing the fluvial and estuarine Paso del Sapo Formation and Lefipán Formation.
Depositional environment The lower part of the formation was mainly deposited in a shallow marine shoreface environment with strong tidal influence and beds rich in phosphatic concretions. The sandstones of the formation probably represent bars shoals on the coast of an epeiric sea. The environment evolved to a tide to wave-dominated deltaic system in the middle part of the sequence, with a maximum flooding surface representing the transgression and deepening of the basin at the start of the Paleocene. This was the first marine transgression of the Southern Atlantic in the Cañadón Asfalto Basin. The abundance of iron oxides in the sediments of the formation, as well as ferruginous cement and glauconite indicate the waters were rich in iron, coming from a continent with intense meteoric waters. This suggests the presence of a temperate to hot climate and high humidity in Maastrichtian to Paleocene Patagonia. The provenance area for the sediments was probably the North Patagonian Massif to the northeast of the Cañadón Asfalto Basin. The intense bioturbation in combination with phosphatic levels in the sediments points to a highly organic activity at time of deposition. The marine sediments of the Lefipán Formation have been correlated with the La Colonia Formation in the Cañadón Asfalto Basin, the Jagüel Formation of the Neuquén Basin to the northwest and the Salamanca Formation of the Golfo San Jorge Basin to the south. The strata of the Lefipán Formation show evidence of syntectonic deposition.
Paleontological significance The Lefipán Formation has provided fossil teeth of a newly described species of rays, Hypolophodon patagoniensis from the middle section of the formation, in the Paleocene strata, as well as Cocatherium lefipanum, the oldest known marsupial or any therian mammal in the Southern Hemisphere. The mammal, a member of Polydolopimorphia, probably represents a basal polydolopiform, closely related to Roberthoffstetteria. The presence of the mammal predates the Tiupampan South American land mammal age.
… excerpt ends here. Continue reading the full article.
