The Jangdong Formation (Jangdong Tuff in some older literatures; ) is an Upper Cretaceous geologic formation in South Korea. It is part of the Neungju Basin, one of the nonmarine sedimentary basins formed in South Korea during the Cretaceous period. Over 1,500 dinosaur tracks have been discovered from the Hwasun Seoyuri Tracksite, which is part of the formation and designated as a Natural Monument of the Republic of Korea.
Geology Among the Cretaceous sediments of the Neungju Basin, the Jangdong Formation is predominantly composed of silty mudstones and fine sandstones interbedded with pyroclastic materials, representing active volcanism in this area.
The Jangdong Formation shows lithological variety depending on the region. The lithology of the southern part of the formation is mainly composed of conglomerates, sandstones, silty mudstones, and pyroclastic breccia, and the facies association suggests an alluvial plain setting for this area. Whereas the lithology of the central and northern part of the formation is composed mainly of fine-grained sandstone to mudstone and dark gray siltstone to mudstone, with sedimentary features including mudcracks, ripples, evaporite casts and dinosaur tracks, and the facies association suggests a marginal playa lake setting of this area.
The age of the Jangdong Formation was initially suggested as latest Late Cretaceous (71-66 MA) based on K-Ar geochronology for volcanic clasts in the formation, but U-Pb ages from the Manwolsan Tuff which underlies the Jangdong Formation, and tuff bed in the middle of the Jangdong Formation shows 96 Ma and 94 Ma, indicating the age of the Jangdong Formation dates back to the boundary of Cenomanian-Turonian.
Fossil records Aside from diverse dinosaur and pterosaur tracks, and invertebrate traces associated with pterosaur tracks, a few carbonized wood and spinicaudata fossils have been found.
Dinosaur and pterosaur tracks Among 1,500 dinosaur footprints from 5 horizons at the Hwasun Seoyuri Tracksite, over 60 trackways made by theropods, sauropods, and ornithopods have been identified. Unlike other tracksites in Korea, nearly 88% of the tracks at this site are made by theropods. The lack of bird footprints from the site possibly shows regional dominance of pterosaurs in this area.
Genus Species Location Stratigraphic positionMaterialNotesImagescf. Magnoavipescf. M. isp.Hwasun Seoyuri Tracksite16-22 cm long. Has wide digit diversification (80–90°). Slipped footprints and a trackway showing acceleration have been reported in the L1 horizon.Theropod type Bindeterminate20-28 cm long. Has wide digit diversification (50–60°). Similar to Ornithomimipus and XiangxipusTheropod type Cindeterminate40-55 cm long. Has wide digit diversification (50°).OrnithopodichnusO. isp.L2 and L4 horizons12~80 cm long. Several trackways by small trackmakers in L2 show possible gregarious behavior.cf. Brontopoduscf. B. isp.L1, L4 and L5 horizonsPes-only trackway from L4 present, which represents manus tracks that were overprinted by the pes.PteraichnusP. isp.At least from three different horizons, all at least 20 meters above the L5 horizon.362 footprintsDifferent tracks attributed to various trackmakers, including dsungaripteroids, and small or juvenile azhdarchids or yet-to-be-reported group of pterosaurs.
See also List of dinosaur-bearing rock formations
References