Chorrillo Formation
Geologic formation in Patagonia, Argentina
The Chorrillo Formation, also called the Chorillo Formation, is a Late Cretaceous (Maastrichtian age) geological 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. It constitutes a significant uppermost Cretaceous continental sedimentary sequence within the Magallanes Basin, located in Santa Cruz Province, Argentina, approximately 30 km (19 mi) 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.
01Geology
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.
02Paleoenvironment
Paleoenvironmental reconstructions indicate a temperate and humid subtropical climate with marked seasonality in rainfall. Mean annual temperatures were 12.1-14.5 °C with mean annual precipitation of 929-1321 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.
03Fossils
Color key
|
Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Arthropoda
Decapoda
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Astacidea indet. | Indeterminate | Locality 4 | MPM 21516, three gastroliths | Previously identified as crushing fish teeth |
Insects
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Coelolepida indet. | Indeterminate | La Anita farm | MPM-Pal 21835-3:W34/1; MPM-Pal 21835-23:W55/0; MPM-Pal 21835-22:R52/2; MPM-Pal 21835-20:Z47/3; MPM-Pal 21835-22: G55/0; MPM-Pal 2183-21: E42/3 | A glossatan lepidopteran | |
| Diamesinae indet. | "Morphotype 3" | La Anita farm | MPM-Pal 21835-9:E34/4; MPM-Pal 21835-19: V34/4 | A chironomid cematoceran. Similar to the living genus Paraheptagyia. | |
| Ephemeroptera indet. | Indeterminate | La Anita farm | MPM-Pal 21835-9:W37/1 | A mayfly | |
| Lepidoptera indet. | Indeterminate | La Anita farm | MPM-Pal 21835-10:X30/2; MPM-Pal 21835-7:S27/2 | ||
| Orthocladiinae indet. | "Morphotype 1" | La Anita farm | MPM-Pal 21835-24: M31/0; MPM-Pal 21835-3: P46/4 | A chironomid nematoceran | |
| "Morphotype 2" | La Anita farm | MPM-Pal 21835-25: V26/3 | |||
| "Morphotype 5" | La Anita farm | MPM-Pal 21835-9: Q58/3 | It resembles modern Cricotopus | ||
| Tanypodinae indet. | "Morphotype 4" | La Anita farm | MPM-Pal 21835-2:Y40/3 | A chironomid nematoceran |
Molluscs
Gastropods
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Allopeas | A. agnolini | Locality 4 | Shells | An achatinid gastropod | |
| Bocourtia | B. leonardodavincii | Locality 4 | Shells | A bulimulid gastropod | |
| Coelostemma | C. patagonica | Locality 4 | Shells | A holospirid gastropod | |
| Holospira | H. sp. | Locality 4 | Shells | An urocoptidae gastropod | |
| Leptinaria | Cf.L. sp. | Locality 4 | Shells | An achatinid gastropod | |
| Lilloiconcha | L. novasi | Locality 4 | Shells | A charopid gastropod | |
| Naesiotus | Cf.N. sp. | Locality 4 | Shells | A bulimulid gastropod | |
| Physa | P. sp. | Locality 4 | Shells | A physidae gastropod | |
| Physella | Cf.P. sp. | Locality 4 | Shells | A physidae gastropod | |
| Pleuroceridae indet. | Indeterminate | Locality 4 | Shells | Incertade sedis | |
| Pomacea | P. sp. | Locality 4 | Shells | An ampullariidae gastropod | |
| Potamolithus | P. fabiani | Locality 4 | Shells | A tateid gastropod | |
| Stenophysa | S. sp. | Locality 4 | Shells | A physidae gastropod |
Bivalves
Color key
|
Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Chlamys | "C." salamanca | Locality 5 | Shells | A pectinid bivalve | |
| Cubitostrea | C. ameghinoi | Locality 5 | Shells | An ostreid bivalve | |
| Gryphaeostrea | G. callophyla | Locality 5 | Shells | A gryphaeid bivalve |
Fishes
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Lamniformes indet. | Indeterminate | Locality 5 | Isolated upper lateral tooth | Similar to Cretalamna | |
| Vidalamiinae indet. | Indeterminate | Locality 2 | Isolated tooth | An amiid fish |
Amphibians
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Anura indet. | Indeterminate | Locality 4 | MPM 21518, isolated tibiofibula | ||
| Calyptocephalella | C. gayi | Locality 2 | MPM-PV-22844, proximal right radioulna; MPM-PV-22845, left proximal end of tibiofibula; MPM-PV-22846, proximal half of urostyle. | A calyptocephalellid frog | |
| C. sp. | Magallanodon Site | maxillae MPM-PV-22841 and MPM-PV-22842; ilium MPM-PV-22843 | A species very small in size, distinct from C. gayi. | ||
| Calyptocephalellidae indet. | Indeterminate | Locality 4 | MPM 21519, distal end of right humerus with eroded distal margin | ||
| Pipoidea indet. | Indeterminate | Locality 2 | Distal end of right humerus | Resembles, yet is different from, Xenopus |
Plesiosaurs
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Elasmosauridae indet. | Indeterminate | Locality 5 | Incomplete cervical centrum and the lateral half of a very fragmentary dorsal vertebra | May belong to the Calafate Formation |
Lepidosaurs
Color key
|
Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Mosasaurs
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Mosasauridae indet. | Indeterminate | Locality 5 | Seven teeth | Its presence suggest marine influences |
Rhynchocephalians
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Notosphenos | N. finisterre | Monotreme site | Fragmentary jawbone with two complete teeth and one partial tooth | A sphenodontid rhynchocephalian |
Snakes
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Alethinophidia indet. | Indeterminate | Locality 4 | MPM 21522, a vertebra | ||
| Anilioidea indet. | Indeterminate | Locality 2 | MPM-PV-22847, a vertebra | ||
| Coniophiidae indet. | Indeterminate | Monotreme site | Two partial vertebrae | Shares some features with Coniophis | |
| Madtsoiidae indet. | Indeterminate | Monotreme site; Green Valley site | Two partial vertebrae | ||
| Rionegrophis | R. madtsoioides | Locality 2 | A partial vertebra | A madtsoiid snake |
Turtles
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Chelidae indet. | Indeterminate | Locality 2 | Shell fragments (two peripheral and one costal plates) | Largest plates suggest a maximum carapace length of 40 cm. | |
| Hydromedusinae indet. | Indeterminate | Localities 2, 4, 5 | Shell fragments (left peripheral 3 or 4, bridge peripherals, right hypoplastron, buttress, and plastral fragment) | Similar to Hydromedusa | |
| Meiolaniformes indet. | Indeterminate | Locality 4 | Distal end of a left humerus | Southernmost record for the clade in South America |
Archosaurs
Crocodylomorphs
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Kostensuchus | K. atrox | Estancia La Anita | Skull, jaws, and incomplete postcranial skeleton | A large-bodied peirosaurid |
Ornithischians
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Hadrosauridae indet. | Indeterminate | La Anita Farm | Two incomplete caudal centra | ||
| Isasicursor | I. santacrucensis | Locality 5 | Multiple specimens, including teeth, several vertebrae, shoulder blades, and limbs | An elasmarian ornithopod | |
| Ornithopoda indet. | Indeterminate | La Anita Farm | Proximal end of a femur | A small ornithopod | |
| Parankylosauria indet. | Indeterminate | La Anita Farm | Five fragmentary caudal vertebrae, incomplete ribs, two incomplete humeri, and nine osteoderms |
Sauropods
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Fusioolithus | F. sp. | Locality 2, Puma cave, or Locality 4 | Eggshells | Fusioolithidae ootaxon | |
| Lithostrotia indet. | Indeterminate | Río Leona | MACN 5205 (3 middle-posterior caudal centra) | ||
| Nullotitan | N. glaciaris | Estancia Alta Vista, Localities 1, 2, 4, and 5 | 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 | A lithostrotian titanosaur | |
| Titanosauria indet. | Indeterminate | Locality 2 and 4 | Seven isolated teeth |
Theropods
| Genus | Species | Locality | Material | Notes | |
|---|---|---|---|---|---|
| Enantiornithes indet. | Indeterminate | Locality 2, and 4 | Distal fragment of a pedal phalanx and two isolated ungual pedal phalanges | ||
| Kank | K. australis | Monotreme site | Fragmentary remains, including vertebrae, teeth, and pedal phalanges | An unenlagiid theropod | |
| Kookne | K. yeutensis | Locality 2 | An incomplete right coracoid, lacking the sternal end and the proximal end damaged | An ornithuran bird similar to Anseriformes | |
| Maip | M. macrothorax | La Anita Farm | Axis, several dorsal and caudal vertebrae, cervical and dorsal ribs, gastralia, a left coracoid, fragmentary scapula, partial right pubis, and partial metatarsal | A large megaraptorid theropod | |
| Megaraptoridae indet. | Indeterminate | La Anita Farm | Isolated teeth | ||
| Noasauridae indet. | Indeterminate | Locality 4 | Isolated right pedal phalanx IV-2? | Similar to Velocisaurus and Vespersaurus | |
| Prismatoolithidae indet. | Indeterminate | Locality 4 | Eggshells | Theropod eggshells | |
| Yatenavis | Y. ieujensis | Estancia La Anita | Distal right humerus | An enantiornithean bird |
Mammals
| Genus | Species | Locality | Material | Notes | Images |
|---|---|---|---|---|---|
| Magallanodon | M. baikashkenke | Río de Las Chinas Valley and La Anita Farm | Tentative first upper molariform, anterior end of a left dentary bearing part of its gliriform incisor, and upper mesial incisor from the right side | A ferugliotheriid gondwanathere | |
| M. terrwerr | Locality 2 | MPM-PV-23088, isolated right lower molariform; MPM-PV-23089, isolated right lower molariform | A ferugliotheriid gondwanathere | ||
| Mammalia indet. | Indeterminate | Locality 4 | Two isolated caudal vertebrae | Characters shared with mammals include having a long and gracile tail | |
| Meridiolestida indet. | Indeterminate | Monotreme site | MPM-PV-21707, two fragments of a right dentary | Small taxon similar to Reigitherium | |
| Mesungulatoidea indet. | Indeterminate | La Anita Farm | MPM-PV-21708, fragment of left dentary with five alveoli corresponding with three teeth. | Larger than any known Mesozoic meridiolestidan | |
| Orretherium | O. tzei | Monotreme site | MPM-PV-23514, left upper second molariform. | A member of Meridiolestida belonging to the family Mesungulatidae | |
| Patagorhynchus | P. pascuali | Monotreme site | Lower right molar attached to a fragment of the dentary | A ornithorhynchid monotreme | |
| Patagomaia | P. chainko | La Anita Farm | Partial left acetabulum and ischium; MPM-PV-23515, one fragment of thoracic vertebra, posterior half of one caudal vertebra and 38 indeterminate bones. | A large therian or gondwanathere similar to Adalatherium |
04Flora
Color key
|
Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |
Algae
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Botryococcus | B. sp. | Botryococcaceae | |
| Cymatiosphaera | C. sp. | Zygnemataceae | |
| Leiosphaeridia | L. sp. | Zygnemataceae | |
| Ovoidites | O. parvus | Zygnemataceae | |
| O. spriggii | Zygnemataceae | ||
| Schizosporis | S. reticulatus | Zygnemataceae (aff. Spirogyra) | |
| Sphaeroplea | S. striatocristata | Sphaeropleaceae |
Liverworts
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Aequitriradites | A. spinulosus | Marchantiidae | |
| Stoverisporites | S. microverrucatus | Marchantiidae | |
| Taurocusporites | T. segmentatus | Marchantiidae | |
| Triporoletes | T. reticulatus | Marchantiidae | |
| T. sp. | Marchantiidae |
Hornworts
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Foraminisporis | F. wonthaggiensis | Anthocerotidae | |
| F. sp. | Anthocerotidae |
Lycopods
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Camarozonosporites | C. ambigens | Selaginellales | |
| Ceratosporites | C. equalis | Selaginellales | |
| Densoisporites | D. velatus | Selaginellales | |
| Foveosporites | F. sp. | Selaginellales | |
| Retitriletes | R. austroclavatidites | Lycopodiales | |
| R. sp. cf. R. douglasii | Lycopodiales |
Ferns
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Azolla | A. cf. andreisii | Salviniaceae | |
| A. sp. | Salviniaceae | ||
| Baculatisporites | B. comaumensis | Osmundaceae | |
| B. kachaikensis | Osmundaceae | ||
| B. turbioensis | Osmundaceae | ||
| cf. Cladophlebis | C. sp. | Polypodiopsida | |
| Clavifera | C. sp. | Gleicheniaceae | |
| Cyatheacidites | C. annulatus | Dicksoniaceae (aff. Lophosoria) | |
| Cyathidites | C. australis | Cyatheaceae | |
| C. minor | Cyatheaceae | ||
| C. rafaeli | Cyatheaceae | ||
| Dictyophyllidites | D. sp. cf. D. concavus | Dipteridaceae / Matoniaceae | |
| Gleicheniidites | G. sp. | Gleicheniaceae | |
| Leptolepidites | L. verrucatus | Blechnaceae | |
| Ornamentifera | O. sp. | Gleicheniaceae | |
| Osmundacidites | O. diazii | Osmundaceae | |
| Peromonolites | P. vellosus | Blechnaceae |
Non-conifer seed plants
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Cycadopites | C. follicularis | Cycadales, Bennettitales, or Ginkgoales | |
| Ephedripites | E. ambiguous | Ephedrales | |
| Equisetosporites | E. notensis | Ephedrales | |
| E. sp. cf. E. dudarensis | Ephedrales | ||
| Vitreisporites | V. signatus | Pteridosperms |
Conifers
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Araucariacites | A. australis | Araucariaceae | |
| Callialasporites | C. turbatus | Araucariaceae | |
| Classopollis | C. sp. | Cheirolepidiaceae | |
| Cyclusphaera | C. psilata | Araucariaceae | |
| Dacrycarpites | D. australiensis | Podocarpaceae | |
| Dilwynites | D. tuberculatus | Araucariaceae | |
| Gamerroites | G. psilasaccus | Podocarpaceae | |
| Lygistepollenites | L. florinii | Podocarpaceae | |
| Microcachryidites | M. antarcticus | Podocarpaceae (aff. Microcachrys) | |
| Peripollenites | P. sp. | Cupressaceae | |
| Phyllocladidites | P. mawsonii | Podocarpaceae (aff. Lagarostrobos franklinii) | |
| Podocarpidites | P. elegans | Podocarpaceae | |
| P. elipticus | Podocarpaceae | ||
| P. exiguus | Podocarpaceae | ||
| P. herbstii | Podocarpaceae | ||
| P. major | Podocarpaceae | ||
| P. marwikii | Podocarpaceae | ||
| P. otagoensis | Podocarpaceae | ||
| P. sp. cf. P. langii | Podocarpaceae | ||
| P. sp. cf. P. microreticuloidata | Podocarpaceae | ||
| P. torquatus | Podocarpaceae | ||
| P. verrucosus | Podocarpaceae | ||
| Podocarpoxylon | P. dusenii | Podocarpaceae | |
| Trichotomosulcites | T. subgranulatus | Podocarpaceae |
Magnoliids
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Clavatimonocolpites | C. sp. | Magnoliids |
Monocots
| Genus | Species | Affinity / Family | Image |
|---|---|---|---|
| Arecipites | A. minutiscabratus | Arecaceae | |
| A. sp. cf. A. subverrucatus | Arecaceae | ||
| Aff.Eolirion | "Morphotype 5" | Palmae | |
| Longapertites | L. proxapertitoides | Arecaceae | |
| Liliacidites | L. lanceolatus | Liliales (aff. Lilium) | |
| L. variegatus | Liliales (aff. Lilium) | ||
| L. vermireticulatus | Liliales | ||
| L. sp. cf. L. kaitangataensis | Liliaceae/Amaryllidaceae | ||
| Monocotyledonean | "Morphotype 6" | Monocots | |
| Nuphar | N.? sp. 1 | Nymphaeaceae | |
| N.? sp. 2 | Nymphaeaceae | ||
| Palmoxylon | P. kospi | Arecoideae | |
| P. anciborae | Trachycarpeae (Aff. Chamaerops and Trachycarpus) | ||
| Pandaniidites | P. typicus | Araceae | |
| Proxapertites | P. operculatus | Araceae | |
| Spinozonocolpites | S. hialinus | Arecaceae |
Eudicots
Sources and credits
This article is adapted from the Wikipedia article “Chorrillo Formation”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
Images, from Wikimedia Commons:
- Perito Moreno ISS.jpg by NASA, Public domain
- Valdivian Rainforest (3184558287).jpg by Jason Hollinger, CC BY 2.0
- Crayfish.jpg by Unknown author, Public domain
- Potthastia.gaedii.-.lindsey.jpg by James K. Lindsey, CC BY-SA 3.0
- Midges.jpg by Jean and Fred, CC BY 2.0
- Tanypodinae - probably a melanic Procladius species - Flickr - gailhampshire.jpg by gailhampshire from Cradley, Malvern, U.K, CC BY 2.0
- Allopeas gracile 003.jpg by The Trustees of the Natural History Museum, London, CC0
- Scutalus tupacii (MNHN-IM-2000-24710).jpeg by Manuel CABALLER, CC BY 4.0
- Obese Funnel (Leptinaria unilamellata).jpg by Miguel Sho (mik3sho), CC BY 4.0
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