The Gaiman Formation (), in older literature also referred to as Patagonian Marine Formation (, Patagoniense), is a fossiliferous geologic formation of the Peninsula Valdés Basin in the eastern Chubut Province of northwestern Patagonia, eastern Argentina.

The thick formation overlies the Sarmiento Formation and is overlain by the Puerto Madryn Formation and comprises grey and white tuffaceous mudstones and sandstones, deposited in a shallow marine environment.

The Gaiman Formation has provided fossils of many extinct penguins, among which five species in the genus Palaeospheniscus, as well as whales and dolphins, most notably Aondelphis talen, Prosqualodon australis, Idiorophus patagonicus and Argyrocetus patagonicus, indeterminate seal and turtle fossils, shark and other fossils. The richness of the formation, and the other formations in the area, such as the underlying Sarmiento Formation, led to the establishment of the , with a Welsh name, reflecting the number of Welsh settlers in the region.

Description

The Gaiman Formation was first defined by Mendía & Bayarsky in 1981,Cione et al., 2010, p.434 and further described by Mendía in 1983, taking its name from Gaiman in Chubut Province, eastern Patagonia.Rodríguez, 2015, p.198 Gaiman in the local language of the native Tehuelche people means "rocky point". Significado de los nombres de los Municipios de Chubut The formation crops out in the lower course of the Chubut River,Noriega & Tambussi, 2008, p.272 and overlies the continental Trelew Member of the Sarmiento Formation and is overlain by the Late Miocene Puerto Madryn Formation. The formation is thick, and comprises marine tuffs, tuffaceous mudstones, sandstones and coquinas.

The Gaiman Formation is correlated with the Chenque Formation of the Golfo San Jorge Basin and the Monte León Formation of the Austral Basin to the south, the Saladar Member of the lower Gran Bajo del Gualicho Formation of the Colorado Basin to the north and the Vaca Mahuida Formation of the Neuquén Basin to the northwest.Reichler, 2018, p.185

Depositional environment The basal stratum of the Gaiman Formation is a thin transgressive lag with some gravels, bones and teeth from marine vertebrates. The unit displays a concentration of phosphatic concretions, ooids, bones and teeth.Cione et al., 2011, p.424 The marine sediments overlying this basal stratum are composed of white, tuffaceous, thoroughly bioturbated mudstones and fine sandstones with occasional mollusk molds and thin oyster horizons, deposited in a shallow shelf environment.Viglino et al., 2018, p.263

The marine transgression leading to the deposition of the Gaiman Formation is the first of two major South Atlantic transgressions of the Miocene, the second causing the deposition of the overlying Puerto Madryn Formation.Cuitiño, 2017, p.47

Facies analysis shows that the formation represents a transgressive-regressive stratigraphic cycle, with palaeoenvironments including coastal, storm-dominated shoreface, inner shelf embayment and open inner shelf.Cuitiño et al., 2019

The age of the Gaiman Formation is established on stratigraphic correlations to other absolutely-dated sections in Patagonia and biostratigraphic data and dates to the Early Miocene (Burdigalian, or Colhuehuapian in the SALMA classification).

Paleontological significance

The Gaiman Formation has provided several fossil penguins, of which nine are assigned to specific species and include five species of the genus Palaeospheniscus. Also, shark teeth, fossils of indeterminate seals and turtles, and several dolphins, baleen and toothed whales have been found in the formation. Argentine paleontologist Florentino Ameghino described an isolated tooth from the formation first as an ichthyodectiform fish, Portheus patagonicus in 1901, however this fossil was assigned to a delphinoid cetacean of the family Kentriodontidae by Cione and Cozzuol in 1990.Cione & Cozzuol, 1990, p.451 Fish recorded from the formation are hexanchiform hexanchids, lamniform isurids, odontaspidids, cetorhinids, carcharhiniform carcharhinids, heterodontiform heterodontids, squatiniform squatinids myliobatiform myliobatids, perciform oplegnathids and labrids, and tetraodontiform molids.Cione et al., 2010, p.443 Rays are abundant in the formation.Cione et al., 2010, p.429 The lack of somatic remains of most molluscs, bryozoans, polychaetes and cirripeds in the formation is attributed to the high-energy and corrosive environment at time of deposition, as well as diagenesis after the formation was deposited.Cione et al., 2010, p.450

The bird species Eretiscus tonnii solely occurs in the Gaiman Formation.Chávez Hoffmeister, 2008, p.82 Bite marks on several of the penguin bones are attributed to terrestrial mammals such as didelphid or hathliacyniid sparassodont marsupials, common in Patagonia during the Miocene.Cione et al., 2010, p.437 Other ichnofossils found on the bones are presumably caused by dental erosion by regular echinoids,Cione et al., 2010, p.439 and bite marks by sharks, as Galeocerdo aduncus.Cione et al., 2010, p.441

The paleontological richness of the formation, as well as the more fossiliferous underlying Sarmiento Formation, led to the establishment of the . Parque paleontológico Bryn Gwyn

Fossil content The formation has provided the following fossils:Cerro del Castillo, Trelew at Fossilworks.orgBryn Gwyn at Fossilworks.orgPatagonian at Fossilworks.orgGaiman (marine vertebrates) at Fossilworks.orgPlaya Santa Isabel, south at Fossilworks.org

Mammals Cetaceans Taxa Species Locality Material Description Images Notes AglaocetusA. moreni Cerro del Castillo and Bahia Solano A skull An early baleen Whale Cione et al., 2011, p.428 AondelphisA. talen Bryn Gwyn Locality A partial skull An early river dolphin Viglino et al., 2018, p.264 ArgyrocetusA. patagonicus Bryn Gwyn Locality A skull An early toothed whale CrisocetusC. lydekkeriEstancia El Crisol A partial skull An early toothed whale DolgopolisD. kinchikafiforo Estancia El Crisol A partial skull An early toothed whale Viglino et al., 2020 Eurhinodelphinidae indet. IdiorophusI. patagonicus Bryn Gwyn Locality A complete skull An early toothed whale KentriodonK. sp. Bryn Gwyn Locality A specimen An early toothed whale MorenocetusM. parvus Cerro del Castillo, Trelew A partial cranium An early baleen whale PhoberodonP. arctirostris Playa Magagna and Cerro del Castillo, Trelew A partial skeleton with skull An early river dolphin PlesiocetusP. dyticus Cerro del Castillo A specimen An early rorqual ProsqualodonP. australis Playa Magagna and Cerro del Castillo A preserved skeleton An early toothed whale Gaetán, 2019 Squalodontidae indet.

Pinnipeds Taxa Species Locality Material Description Images Notes Phocidaeindet.

Birds Taxa Species Locality Material Description Images Notes Anseriformesindet. ApterodytesA. ictus Cerro del Castillo An early penguin ArthrodytesA. sp. CayaoaC. bruneti Cerro Castillo A specimen An early anatid water bird Tonni, 1979, p.12 EretiscusE. tonnii An early prenguin Cione et al., 2010, p.433 PalaeospheniscusP. bergi Cerro del Castillo, Trelew A set of limb elements (humerus, radius, tarsometatarsus) An early penguin P. bilocular Cerro del Castillo Two characteristic humeri (AMNH 3341 and 3346) An early penguin, formally named Chubutodyptes biloculata P. gracilis Gran Bajo de San Julian and Cerro del Castillo A specimen An early penguin P. patagonicus Cerro del Castillo An early penguin Acosta et al., 2008, p.568 P. wimani Cerro del Castillo An early penguin ParaptenodytesP. antarcticus A partial skeleton P. robustus Sphenisciformes indet.

Reptiles Taxa Species Locality Material Description Images Notes Dermochelyidae indet. HelianthochelysH. redondita. Coastal creek located 5 km east of La Redonda Chica House in La Redonda Chica Farm A near-complete carapace (secondary ossicles and nuchal bone) with a putative skull fragment (jugal) and postcranial elements. A sea turtle belonging to the family Dermochelyidae. Sterli, J.; Vlachos, E.; Cuitiño, J. I.; Cerda, I. A.; Buono, M. R. (2025). "Southernmost dermochelyid turtle from the Miocene". Ameghiniana.

Fish Taxa Species Locality Material Description Images Notes CarcharoidesC. totuserratus A mackerel shark Echinorhinus E. pozzii Galeocerdo G. aduncus Megascyliorhinus M. trelewensis Heterodontus H. sp. OdontaspisO. sp. PristiophorusP. sp. SqualusS. sp. Oplegnathidae Oplegnathidae indet TetraodontiformesTetraodontiformes indet

Mollusks Taxa Species Locality Material Description Images NotesCrassostreaC. ? hatcheri A bivalve BalanusB. sp. A bivalve Acosta et al., 2008, p.566 TurritellaT. sp. A gastropod

Ichnofossils Taxa Species Locality Material Description Images Notes Chondrites A Bioturbation Ophiomorpha Planolites Skolithos GnathichnusG. pentax Bite marks Entobia Bite marks

Flora

Taxa Species Locality Material Description Images NotesChenopodipollisC. chenopodiaceoides Isla Escondida A chenopodiaceaeCyatheaciditesC. annulatus A dicksoniaceaeEquisetosporitesE. claricristatusMargocolporitesM. vanwijhei A fabaceaeNothofagidites N. saraensisPeromonolites P. vellosus A blechnaceaeThalassotaeniaT. notophyllum A hydrocharitaceae

See also South American land mammal ages Uitpa Formation, contemporaneous fossiliferous formation of the Cocinetas Basin, Colombia Castillo Formation, contemporaneous fossiliferous formation of the Falcón Basin, Venezuela Biblián Formation, contemporaneous fossiliferous formation of Ecuador Bahía Inglesa Formation, contemporaneous fossiliferous formation of Chile

References

Bibliography Geology

Paleontology

Further reading