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| Monte León Formation | |
|---|---|
| Name | Monte León Formation |
| Type | Geological formation |
| Period | Early Miocene |
| Primary lithology | Sandstone, siltstone, conglomerate |
| Other lithology | Mudstone, tuff |
| Region | Santa Cruz Province, Argentina |
| Namedfor | Monte León National Park |
Monte León Formation The Monte León Formation is an Early Miocene stratigraphic unit exposed in Santa Cruz Province, southern Argentina, known for marine and marginal-marine strata that preserve diverse fossil assemblages and record early Neogene paleoenvironmental change along the South Atlantic margin. It has been the focus of studies by researchers from institutions such as the Museo Argentino de Ciencias Naturales, the Universidad Nacional de La Plata, and international teams affiliated with the Smithsonian Institution and the Natural History Museum, London. The unit provides key data for correlations with contemporaneous South American formations like the Sarmiento Formation and the Gaiman Formation.
The Monte León Formation lies within the stratigraphic column of the Magallanes Basin sector of Patagonia and occurs stratigraphically above Paleogene units and below younger Miocene and Pliocene deposits correlated with the Monte Buey Formation and neotectonic sequences. Regional mapping links its boundaries to structural features such as the South Patagonian Andes foothills and fault systems associated with the Andean orogeny. Stratigraphic studies by teams from the Consejo Nacional de Investigaciones Científicas y Técnicas have subdivided the formation into member-scale intervals recognized by facies changes and marker horizons tied to tuff layers used for correlation with the South American Land Mammal Ages framework and marine biozones.
Biostratigraphic evidence places the Monte León Formation within the early Miocene (Burdigalian), with age control provided by marine microfossils, vertebrate assemblages, and radiometric dates from intercalated volcanic ashes analyzed using techniques practiced at laboratories in the University of California, Berkeley and the University of Buenos Aires. Correlations with the Colhuehuapian and Santacrucian biochronological units refine its temporal position relative to representative localities such as Cerro Bandera and sections near the Camarones River. Published zircon U-Pb and Ar-Ar ages from tuffaceous horizons supply absolute constraints that complement paleoceanographic marker events recognized in the Atlantic Ocean marine record.
Lithologically, the Monte León Formation comprises heterolithic successions of medium- to fine-grained sandstones, siltstone, mudstone, and pebble to cobble conglomerates with subordinate tuffaceous beds. Sedimentological descriptions highlight cross-bedded sandstones, bioturbated mudstones, and storm-influenced bedding coupled with shell beds similar to facies documented in the Golfo San Jorge Basin. Petrographic and heavy-mineral studies conducted by researchers affiliated with the Universidad Nacional del Comahue and the Instituto de Geología y Recursos Minerales provide provenance insights linking source terrains in the Patagonian Andes and cratonic sectors near the Río de la Plata Craton.
Fossil content in the Monte León Formation includes diverse marine vertebrates and invertebrates: well-preserved molluscs, echinoids, and crustaceans, alongside fish, pinniped and cetacean remains that have informed studies by paleontologists from the American Museum of Natural History and the Museo Paleontológico Egidio Feruglio. Notable discoveries include early otariid pinnipeds and archaic mysticete whales that contribute to understanding pinniped evolution discussed in works associated with Mary J. Reeder, and cetacean diversification explored by researchers linked to the Smithsonian Tropical Research Institute. Microfossils such as foraminifera and diatoms enable correlations with marine micropaleontology frameworks advanced at the Institut Français de Recherche pour l'Exploitation de la Mer and the University of Cambridge.
Interpretations infer a shallow marine to marginal-marine depositional setting influenced by episodic storm deposition, tidal reworking, and fluvial input, analogous to depositional models developed for the Bahía Blanca Basin and shelf sequences of the South Atlantic Ocean. Paleoclimatic reconstructions based on faunal assemblages and stable isotope proxies analyzed in laboratories at the McMaster University and the Universidad de Chile suggest temperate to cool-temperate sea-surface conditions during the Early Miocene, reflecting regional climatic trends tied to the evolving Antarctic Circumpolar Current and global Miocene climate signals recorded in the Deep Sea Drilling Project cores.
The Monte León Formation formed during a phase of Andean uplift and basin reconfiguration driven by the subduction of the Nazca Plate beneath the South American Plate, with tectonic influences modulating sediment supply and accommodation space in the Patagonian margin. Regional tectonostratigraphic frameworks developed by geoscientists at the Servicio Geológico Minero Argentino and the Instituto de Investigaciones Geológicas y Mineras place the unit within the complex interplay between foreland basin development, strike-slip faulting related to the Río de la Plata craton boundary, and flexural response to orogenic loading tied to the growth of the Andes.
Category:Geologic formations of Argentina Category:Miocene Series of South America Category:Patagonia