Lithologies

The Cuche Formation is characterised by mostly cream and purple coloured shales, with a basal unit of micaceous siltstones with intercalated yellowish-grey shales and 30 metres (98 ft) thick quartzitic and feldspar-rich sandstone beds that colour red caused by meteoric waters. The sandstones have an iron-rich cementation.^[3] A middle unit of fine-grained sandstones with thin banks of siltstones follows the lower part and an upper sequence of shales with interbedded ferruginous red beds.^[2] The lower sequence contains runzelmark syn-sedimentary structures.^[3]

Stratigraphy and depositional environment

The Cuche Formation in some places discordantly and in other areas transitionally defined by colour changes,^[8][9] overlies the Floresta Formation in Boyacá and the Mogotes Formation in Santander,^[10] and is, by an angular unconformity up to 60 degrees,^[11] overlain by the Upper Jurassic Girón,^[12] and Early Cretaceous Tibasosa Formations.^[3] The angular unconformity between the Paleozoic and Mesozoic is exposed along the road between Duitama and Sogamoso, and is the location where the first flora fossils were found in 1978.^[11]

The age of the Cuche Formation has been estimated to be Late Devonian to Early Carboniferous,^[2][13] after an original designation as Permian-Carboniferous by Botero in 1950, further restricted to the Carboniferous by Julivert in 1968.^[14] The formation covers an area of approximately 36 square kilometres (14 sq mi) and ranges in thickness between 300 and 900 metres (980 and 2,950 ft).^[2] Stratigraphically, the Cuche Formation is time equivalent with the Diamante Formation of the Santander Massif to the north of the Altiplano Cundiboyacense.^[15] To the west of Paz de Río, the Cuche Formation is thrusted upon the Neogene Concentración Formation by the Soapaga Fault.^[16] In the northern part of the Floresta Massif, the contact between the metamorphic Otengá Stock and the Cuche Formation is formed by the Duga Fault.^[17]

Based on the preservation of fossils, the lithologies, syn-sedimentary structures and stratigraphic position, a depositional environment of shallow low energy waters has been proposed, possibly in a lagoonal setting at the edge of a regressional Paleo-Tethys Ocean.^[1][18] Other parts of the Cuche Formation were deposited in a continental environment, evidenced by the red beds and absence of marine fossils and abundant root imprints.^[19] Overall, the sequence represents a coastal deltaic environment with frequent marine incursions, a tidal deltaic setting.^[20][21]

Fishes

Remains of the cartilaginous fish Antarctilamna sp., the Placoderms Asterolepis sp. and two species of Bothriolepis,^[28] the spiny shark ?Cheiracanthoides sp., the Porolepiform Holoptychius sp., and the Rhizodontid ?Strepsodus sp. have been uncovered from the Cuche Formation.^[23] Several other fossils are less well recognizable at the genus level, among others Actinopterygii, Sarcopterygii,^[29] and Osteolepiformes.^[30] The fish specimens were found in sediments possibly representing localized transgressive marine incursions into brackish lagoonal settings,^[20][31] in all cases associated with the presence of bivalves, ostracods and brachiopods.^[32]

The fossil fish assemblage of the Cuche Formation presents a curious mixture of Euramerican "Old Red Sandstone" (Catskills, Greenland, Scotland and the Baltic states)^[20] species (Asterolepis and Holoptychius), and Gondwanan taxa (Antarctilamna), suggesting the interchange of species between the paleogeographical regions, possibly at closer distance than is presented in most paleogeographical models.^[33][34] This hypothesis is further strengthened by the discovery of typical Euramerican flora, as Archaeopteris.^[26]

Asterolepis has been known only from Euramerican fossils, except for a specimen found in Iran.^[20] The fish of the Cuche Formation are quite different from Bolivian Devonian fossils, with the exception of Antarctilamna. The similar sediments of the Colpacucho Formation in Bolivia have not provided the species discovered in the Cuche Formation, probably because of the cooler climate of the Devonian Bolivian seas more to the south than the paleogeographical position of northern Colombia (already around 51°S) at that time.^[34]

Fossils assigned to Florestacanthus cf. morenoi, Colombiaspis rinconensis and Colombialepis villarroeli were later described from the Cuche Formation.^[35][36][37]

Eurypterids

In 2019, fragments of the eurypterid Pterygotus were retrieved from the formation. The find represents the first sea scorpion from Colombia and the fourth from South America.^[38] The specimen (SGC-MGJRG.2018.I.5), assigned with uncertainty to P. bolivianus due to similarities with its holotype, represents the first eurypterid of Colombia and the fourth of South America. The fossil was dated as Frasnian (Late Devonian), showing that Pterygotus did not become extinct during the Middle Devonian as previously thought.^[39]