The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. It is known for its exquisitely preserved fossils, and is mainly composed of basalts interspersed with siliciclastic sediments.

Research history

The potential importance of the Yixian Formation was initially recognized during the time the Empire of Japan occupied China's Rehe ("Jehol") Province after the Defense of the Great Wall in 1933. Many Japanese scientists had noticed fossil remains of extinct fish and reptiles, possibly the champsosaurs. These initial fossil discoveries made by Japanese scientists vanished once World War II ended in 1945. By 1949, when administration of the area passed to the Chinese Communist Party and its leader Mao Zedong, the fossils of Yixian were studied only by Chinese scientists. It was not until the 1990s when remarkable fossils of birds and other non-avian dinosaurs were excavated. Since 1996, a number of dinosaur fossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-avian theropods with feathers.

Stratigraphy

thumb|left|A map of several geological formations in Liaoning, including the Yixian Formation (in orange)

The Yixian Formation is the middle formation of the Jehol Group. It is stratigraphically positioned between the older Dabeigou Formation and the younger Jiufotang Formation. These three formations together preserve broadly similar organisms and vary only in the composition of different groups. This assemblage is generally known as the Jehol biota and is widely known to preserve fossils in extreme detail, including those of small organisms. The Yixian Formation (and the Jehol Group more generally), is composed of lacustrine deposits interbedded with volcanic sediment, indicating the presence of a long-lived dynamic lake system punctuated by catastrophic volcanism. In older publications, these interbedded rock strata were given various names including the Jingangshan, Tuhulu, Jianchang, Lower Volcanic, and Volcanic Rock formations, but today, these are all recognized as being synonymous with the Yixian Formation.

Geological age

The precise geologic age of the Yixian Formation has been relatively uncertain since it was originally discovered. It has been previously considered to have been deposited during the Jurassic Period (specifically the Tithonian stage). If correct, this would mean a much earlier origin for angiosperms, birds, and eutherians than other evidence tends to suggest. It is also difficult to directly determine the age by comparing it to possible lateral equivalents because in addition to overlying the Dabeigou Formation, parts of the Yixian Formation also unconformably overlies the much older Zhangjiakou and Tuchengzi formations in some places. Modern estimates more confidently place the age of the Yixian Formation as being between the Barremian and Aptian stages, which is corroborated by biostratigraphic correlation of the Jehol biota with other biota of known ages. Specifically, the presence of fossilized pollen similar to other Early Cretaceous sediments as well as the genus Psittacosaurus, which is known from many other Early Cretaceous strata.

Refinement of this age has been controversial in the years since study began. The interbedded volcanic sediments allow for the use of radiometric dating of tuff, biotite, and zircons by various methods. Among these, potassium-argon dating, uranium-lead dating, and rubidium-strontium dating have been used to varying degrees of success. Most modern studies have settled on the use of argon-argon dating of basalts, which is generally believed to yield more precise age estimates.

Subdivisions

thumb|right|150px|Stratigraphic diagram of the various layers of the Yixian Formation

The Yixian Formation is broadly subdivided into several geological subunits. In ascending order (lowest to highest):

  • Jianshangou Unit: thick; composed of shale and tuff
  • Upper Lava Unit: more than thick; composed of basalt and andesite
  • Upper Yixian: thick; composed primarily of conglomerate with breccia, but also contains strata of shale and tuff, tuffaceous sandstone, and sandstone conglomerate (parts of the Upper Yixian are also sometimes called the Dawangzhangzi Unit)
  • Huanghuanshan Unit: thick; composed primarily of conglomerate with breccia with some sandstone conglomerate in the lower strata (sometimes called the Jingangshan Unit) Wang and colleagues made efforts to standardize the stratigraphy of the lower half of the Yixian Formation and their scheme further subdivided the Jianshangou Member into (in ascending order) the Dajianshanzi, Anjiagou, Hengdaozi, and Huangbanjigou beds. Evidence from the study of oxygen isotopes has shown that the average yearly temperature during this time period was 10 degrees celsius (50 degrees Fahrenheit), significantly colder than once thought. This indicates a temperate climate with unusually cold winters for the generally warm Mesozoic era, possibly due to northern China's high latitude during this time. A study by Wu et al. (2013) concluded that orbital forcing, which is the effect on climate caused by shifts in the tilt of the Earth's axis and by the shape of the Earth's orbit, contributed to the climate fluctuations of this formation.

Fossil content

thumb|right|Size comparison of the non-avian dinosaurs from the lower Lujiatun Member of the Yixian Formation

The Yixian Formation represents the second of three major faunal phases that characterize the Jehol Biota, mainly based on changes in invertebrate diversity. In the Yixian, ostracods (seed shrimp) had diversified considerably, despite a very low diversity in the earlier Dabeigou Formation. Other major invertebrate groups in the Yixian include clam shrimp and insects. Insects, as a group, experienced their largest diversification of the entire Mesozoic era in the Yixian. On the other hand, some invertebrate groups, such as bivalves and gastropods (snails and slugs), were numerous but low in diversity, being mainly represented by one or two dominant species (Arguniella in the case of the bivalves).