The Chorrillo Formation, also named as Chorillo Formation,[1] 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 (160 ft) thick and underlies the Calafate Formation and rests on top of the La Irene Formation.[2][3] It constitutes a significant uppermost Cretaceous continental sedimentary sequence within the Magallanes Basin, located in Santa Cruz Province, Argentina, approximately 30 km southwest of El Calafate. Initial investigations in the 1990s laid the groundwork, with detailed paleontological studies emerging since 2019, revealing a rich biotic assemblage.[4] 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.[3][4]
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.[4] 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.[5] 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.[3] 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.[5]
Paleoenvironment
Modern equivalent, Valdivian Rainforest
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.[6] 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.[5] 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.[7] 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.[8] These patterns align with Maastrichtian assemblages in southern Gondwanan basins (e.g., La Anita, Lefipán Formation), reflecting angiosperm diversification and Weddellian Province expansion.[3][8]
Fossils
Dinosaur remains are among the fossils that have been recovered from the formation.
Incomplete left maxilla, incomplete right maxilla, incomplete right ilium, proximal right radioulna, left proximal end of tibiofibula and proximal half of urostyle
A partial skeleton consisting of a third cervical vertebra, tail vertebrae, a neck rib, ribs, a left shoulder blade, the ends of a right thigh bone, a right shin, a right calf bone, and a right ankle bone
Axis, several dorsal and caudal vertebrae, cervical and dorsal ribs, gastralia, a left coracoid, fragmentary scapula, partial right pubis, and partial metatarsal
The largest megaraptoridtheropod of South America and possibly the largest member of this group
Partial left acetabulum and ischium; MPM-PV-23515, one fragment of thoracic vertebra, posterior half of one caudal vertebra and 38 indeterminate bones.
^ abcdefghMiquel, S. E. (2024). "New species and records of freshwater and land gastropods from the Cretaceous of Argentina". Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. 310 (2): 119–131. doi:10.1127/njgpa/2023/1173.
^ abcdefChimento, N. R.; Agnolín, F. L.; Moyano-Paz, D.; Manabe, M.; Tsuihiji, T.; Novas, F. E. (2025). "New fossil mammal remains from the Chorrillo Formation (Maastrichtian, Upper Cretaceous), Santa Cruz Province, Argentina". Journal of Vertebrate Paleontology e2531263. doi:10.1080/02724634.2025.2531263.
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