Chorrillo Formation

The Chorrillo Formation, also named as Chorillo Formation, is a Maastrichtian (Late Cretaceous Epoch, 72.1–66 million years ago) geologic formation in southern Patagonia, Argentina. The formation is more than 50 metres (160ft) thick and underlies the Calafate Formation and rests on top of the La Irene Formation. It constitutes a significant uppermost Cretaceous continental sedimentary sequence within the Magallanes Basin, located in Santa Cruz Province, Argentina, approximately 30km southwest of El Calafate. Initial investigations in the 1990s laid the groundwork, with detailed paleontological studies emerging since 2019, revealing a rich biotic assemblage. The formation correlates with the lower Dorotea Formation in Chile, sharing dinosaurian ecotypes, while both are clearly different from other northern Patagonian Maastrichtian units (e.g., Allen Formation, Los Alamitos Formation), which are dominated by saltasaurine sauropods and abelisaurids, suggesting potential temporal or environmental distinctions.

Geology

The Chorrillo Formation is embedded within the foreland Magallanes Basin, shaped by Late Cretaceous Andean orogeny driven by subduction along the Pacific margin. Tectonic subsidence, coupled with sediment supply from the rising Patagonian Andes, facilitated deposition. Lithologically, the unit comprises fine- to coarse-grained clastics: mudstones and siltstones dominate floodplain facies, often organic-rich and pedogenically modified; sandstones, arkosic to lithic, form channel fills with cross-bedding; and conglomerates appear as basal lags. Clay mineralogy is dominated by smectite, with minor illite and kaolinite, reflecting volcanic ash alteration. Sedimentologically, it records a meandering fluvial system on a distal floodplain, featuring fining-upward channel sequences, crevasse splays, and overbank deposits. High-frequency paleosol stacking indicates local hydrologic and topographic variability, while low-frequency patterns suggest avulsion-driven progradation. Regional ties extend to the Dorotea Formation in Chile, reflecting basin-wide tectonic influences.

Paleoenvironment

Paleoenvironmental reconstructions indicate a temperate-warm, seasonally humid climate with mean annual temperatures of 10-18°C and precipitation of 900–1500mm, inferred from paleosols, clay mineralogy, and biotic indicators. Depositional settings encompass low-energy fluvial floodplains with paludal and lacustrine elements, marked by poor drainage, hydromorphic paleosols (e.g., vertisols, histosols), and redoximorphic features such as mottling and iron nodules, suggesting fluctuating water tables and episodic anoxia. Aquatic communities feature Nymphaeaceae, Salviniaceae, Salviniaceae, and Zygnemataceae, indicative of freshwater ponds and swamps, while terrestrial elements include conifers, ferns, and angiosperms in riparian zones. Floral phytozones, derived from palynological data, comprise five assemblages across the section, exhibiting quantitative variations in taxa proportions without major turnover: lower zones are fern-dominated (e.g., Cyatheaceae), transitioning to angiosperm-rich upper zones (e.g., Proteaceae, Nothofagidites), attributed to subtle humidity fluctuations or habitat gradients relative to fluvial channels. These patterns align with Maastrichtian assemblages in southern Gondwanan basins (e.g., La Anita, Lefipán Formation), reflecting angiosperm diversification and Weddellian Province expansion.

Fossils

Arthropoda

Decapoda

Insects

Molluscs

Gastropods

Bivalves

Fishes

Amphibians

Plesiosaurs

Lepidosaurs

Mosasaurs

Rhynchocephalian

Snakes

Turtles

Archosaurs

Crocodylomorphs

Ornithischians

Sauropods

Theropods

Mammals

Flora

Algae

Liverworts

Hornworts

Lycopods

Ferns

Non-conifer Seed Plants

Conifers

Magnoliids

Monocots

Eudicots

See also

• Dorotea Formation
• Lists of dinosaur-bearing stratigraphic units
• List of fossiliferous stratigraphic units in Antarctica
• López de Bertodano Formation
• Snow Hill Island Formation
• Sobral Formation
• South Polar region of the Cretaceous

Bibliography

• Marenssi, S. A.; Casadío, S.; Santillana, S. N. (September 2003). [Stratigraphy and sedimentology of the Upper Cretaceous-Paleogene units outcropping on the southeastern margin of Lake Viedma, Santa Cruz Province, Argentina]. Revista de la Asociación Geológica Argentina (in Spanish). 58 (3): 403–416.