Chorrillo Formation

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 900–1500 mm, 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

Dinosaur remains are among the fossils that have been recovered from the formation.

Fossils recovered from the formation are:

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

References

Bibliography

References

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2. Marenssi et al., 2003, p.406
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4. Raigemborn, M. Sol. (2023-10-01). "Paleosols of the Maastrichtian dinosaur-bearing Chorrillo Formation (southern Patagonia, Argentina): Paleoenvironmental and paleoclimate implications". Cretaceous Research.
5. (2025-01-25). "Palynofloras from the Maastrichtian Chorrillo Formation (Patagonia, Argentina): Paleoenvironmental and paleoclimatic implications". Palaeogeography, Palaeoclimatology, Palaeoecology.
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12. Rozadilla, Sebastián. (2021). "Ornithischian remains from the Chorrillo Formation (Upper Cretaceous), southern Patagonia, Argentina, and their implications on ornithischian paleobiogeography in the Southern Hemisphere". Cretaceous Research.
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21. (February 3, 2024). "A large therian mammal from the Late Cretaceous of South America". Scientific Reports.
22. (2023). "Maastrichtian palynological assemblages from the Chorrillo Formation, Patagonia, Argentina". Review of Palaeobotany and Palynology.