Duho Formation

The Duho Formation is a Middle Miocene geological formation in Pohang, South Korea. This formation is deposited in a deep marine environment and is roughly dated to around based on various analyses. It is the uppermost unit of the Yeonil Group of Pohang Basin and mainly composed of yellow-brown to dark gray mudstones.

Geology

The deposition of the Pohang Basin began some time after, corresponding to the Early Miocene, based on zircon U-Pb dating. This deep-marine basin was probably produced by rapid subsidence around, following the abrupt ending of the strike-slip deformation and volcanic activity, and experienced tectonic inactivity for 5 to 7 million years until the crustal uplift in southeastern Korea around which caused the sedimentation to cease. The basin was likely a pelagic zone around based on biostratigraphy analysis.
The intrusive basalt of the Yeonil Group from the southern part of the basin is dated to the lower Middle Miocene around based on K-Ar dating, which implies the evolution of back-arc basin in the seas of southeastern Korea, followed by the tectonic inversion which changed the geodynamic setting and basaltic magma composition. Paleomagnetic dating of the Duho Formation yielded the age estimate ranging from. The age estimate of the Yeonil Group based on the same technique is around for the lower and upper part respectively with a reliable pole position of, while biostratigraphy analysis indicated that the age of the Duho Formation likely ranged from. It is probably not younger than based on the radiolarians recovered from the formation.

Paleoecology

Notable fossils from the Duho Formation include a variety of aquatic invertebrates such as ophiuroids, bivalves and mantis shrimps, numerous extant genera of plants, indeterminate remains of toothed whales with some diagnostic to the genus level, and diverse ichthyofauna ranging from small bony fish to large cartilaginous fish like the giant lamniform shark Otodus megalodon which would have been the apex predator of the Miocene seas of Korea. While the fauna consists of species ranging from the shallow coastal waters to the depths of over , the sediments were probably deposited in a pelagic deep-water setting as evidenced by the paleoecological and bathymetric features of the fossil assemblage.

Fossil content

Crustaceans

Insects

The probable ant fossil named as Aphaenogaster "koreana" in a 2018 conference abstracts is a nomen nudum.

Echinoderms

Molluscs

Mammals

Bony fish

Cartilaginous fish

The record of Otodus obliquus from this formation is most likely a misidentification of other otodontid or lamniform shark.