Nakdong Formation
Geologic formation in South Korea
From Wikipedia, the free encyclopedia
The Nakdong Formation (Also known as the Nagdong Formation) (Korean: 낙동층; Hanja: 洛東層; RR: Nakdong-cheung) is an Lower Cretaceous (Hauterivian to Aptian) geologic formation in South Korea. It has been dated from the Hauteravian to the Aptian, between 127 Ma and 118.0 ± 2.6 Ma.[2] It is also the lowermost formation-level stratigraphic unit of the Gyeongsang Supergroup. Plant fossils from the Nakdong Formation overlap with the floral assemblage of the older Myogok Formation and show a 'mixed type' of the Tetori-Type and the Ryoseki-Type flora present in this formation. Based on bivalve fossil records, it correlates with molluscan faunas of the Japanese Kitadani Formation and Sebayashi Formation.[3]
| Nakdong Formation | |
|---|---|
| Stratigraphic range: Hauterivian-Aptian ~ | |
| Type | Geological formation |
| Unit of | Shindong Group |
| Underlies | Hasandong Formation |
| Overlies | Unconformity |
| Thickness | ~2,100 meters |
| Lithology | |
| Primary | Conglomerate, Shale, sandstone |
| Location | |
| Coordinates | 35.6°N 128.2°E |
| Approximate paleocoordinates | 41.1°N 133.8°E |
| Region | Gyeongsang Province |
| Country |  South Korea |
| Type section | |
| Named for | Nakdong, Sangju |
| Named by | Tateiwa, 1929[1] |
 Nakdong Formation (South Korea) | |
Geology
The Nakdong Formation could be distinguished from the overlying Hasandong Formation by absence of red beds in the entire formation.
Based on facies shown in outcrops, the depositional setting of the formation has been changed from lower to upper:[4] (A): Alluvial plain with tributaries, (B): Low sinuous channel system with bars, and (C): Sinuous channel system with splays and floodplains. The channel system of the upper Nakdong Formation is continuously extended to the Hasandong Formation.
Meanwhile, pond deposits are restricted in the formation, represented by dark gray shaly mudstone intervened between fluvial sandy deposits, showing a concave lower contact and planar upper contact, up to 2 meters thick. Ferns and bivalve fossils are occassionaly from these sediments. Based on these characteristics, pond deposits of the Nakdong Formation represent small-sized channels or small ponds deposited over the floodplains. No evaporites or stromatolites have been recorded there.[5]
Fossil content
Color key
|
Notes Uncertain or tentative taxa are in small text; |
Molluscs
| Molluscs of the Nakdong Formation[3] | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Nagdongia | N. soni | Also known from the Kitadani Formation and overlying Hasandong Formation | ||||
| Pseudohyria | P. matsumotoi | Also known from the Kitadani Formation and the Sengoku Formation | ||||
| Nippononaia | N. ryosekiana | Also known from the Kitadani Formation and the Sebayashi Formation | ||||
| Trigonioides | T. bongkyuni | |||||
| T. wakinoensis | ||||||
| Sphaerium | S. coreanicum | |||||
| Viviparus | V. keishoensis | |||||
Vertebrates
| Vertebrates of the Nakdong Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
| Sauropoda | Indeterminate | Partial cervical vertebra and ribs[6] | ||||
