Daohugou Formation
| Tiaojishan Formation Stratigraphic range: Bathonian-Oxfordian, 165–153 Ma |
|
|---|---|
| Type | Geological formation |
| Sub-units | Daohugou bed |
| Underlies | Tuchengzi Formation |
| Overlies | Haifanggou Formation |
| Thickness | 2420 meters |
| Lithology | |
| Primary | Pyroclastic |
| Location | |
| Region | Hebei, Inner Mongolia, & Liaoning |
| Country |
 China
|
The Tiaojishan Formation is a geological formation in Hebei and Liaoning, People's Republic of China, dating to the middle-late Jurassic period (Bathonian-Oxfordian stages). It is known for its fossil plants, and is made up mainly of pyroclastic rock interspersed with basic volcanic and sedimentary rocks. Previously, the Tiaojishan Formation was grouped together with the underlying Haifanggou Formation (also known as the Jiulongshan Formation) as a single "Lanqui Formation." Most researchers now agree that the Daohugou Bed, of formerly controversial dating, is a part of the Tiaojishan formation.
The geology of the Daohugou Bed is confusing because it is complex and does not conform; meaning that elements and layers of rock of different ages are mixed up together by folding and erosion and by volcanic activity. Liu et al. (2006) concluded that the rocks that bear the Daohugou Biota also include the Tiaojishan and Lanqi Formations. They demonstrated that the Jiulongshan Formation is older (Middle Jurassic), and that the Tuchengzi Formation is younger (Late Jurassic). However, many other researchers consider the Daohugou to be a part of the Jiulongshan Formation itself.
Fieldwork published in 2006 has also found that the beds are consistent over a large area; from western Liaoning into Ningcheng county of Inner Mongolia (Nei Mongol).
Using Argon–argon dating, Wang and colleagues in 2005 dated part of the Tiaojishan Formation to about 160 million years ago, the beginning of the Oxfordian stage, the first stage of the Upper Jurassic epoch. In 2006, a study by Liu and colleagues used U-Pb zircon dating to conclude that the Tiaojishan Formation correlates with the Daohugou Beds, and the complete chronological range of this shared biota dates to between 168 and 164/152 Ma ago. A subsequent study, published in 2008, refined the age range of the formation further, finding that the lower boundary of the Tiaojishan was formed 165 Ma ago, and the upper boundary somewhere between 156-153 Ma ago.
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