Name: Liaoning Province
Location: China

Age: Early Cretaceous

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Geological Setting and Age


Geological Setting

In the late Mesozoic, East Asia was a landmass with large amounts of fresh water. Northeast-striking fault-bounded basins characterised northeast China where the Jehol Biota were living in, and around, freshwater lakes. Volcanic sediments from the nearby active margin, later to become the Japanese subduction zone, were frequently deposited into these basins. It was the rapid deposition of these sediments that allowed the exceptional level of preservation. The fossils are all found in the Jehol group, which is composed of two lacustrine formations, the underlying Yixian formation and the overlying Jiufotang formation. These both consist of a series of intercalated fine-grained lacustrine deposits and fine volcanic ash. These lake deposits were mainly deposited in between major eruptions, although there were low levels of volcanic activity throughout [3].

Age

There has been much controversy over the age of the deposits. Biostratigraphy and radiometric dating support either a Late Jurassic age or an Early Cretaceous age. It is of utmost importance that an age is established to understand the evolutionary significance of the fauna. The ages proposed range from latest Jurassic, Tithonian, to the Aptian of the Early Cretaceous. (See table 1 for timescale). These estimates cover a time range of 25 million years [3].

Period

 

Age/Ma

 

 

 

 

Early Cretaceous

Albian

 

98.9

Aptian

 

112.2

Barremian

 

121.0

Hauterivian

 

127.0

Valangian

 

132.0

Berriasian

 

137.0

 

 

Late Jurassic

Tithonian

 

144.2

Kimmeridgian

 

150.7

Oxfordian

 

154.1

 Table 1. Geological timescale of the Late Jurassic and Early Cretaceous. Redrawn after [6]

There are problems associated with biostratigraphic correlation in terrestrial/freshwater ecosystems. In the case of the Jehol Biota, the taxa used tend to have long stratigraphic ranges or are difficult to identify. It can also be difficult to correlate between lagerstätte sections and equivalent non-lagerstätte sections [3].

Some radiometric dates have been proposed in favour of a Late Jurassic age for the Yixian formation. However, these dates of 137 and 143 Ma depend upon the Jurassic-Cretaceous boundary being placed at 135 Ma. It is generally accepted that the boundary is, in fact, slightly older, at around 144 Ma. This places both these estimates within the Early Cretaceous [3]. More recent estimates suggest an age of around 125 Ma, which places the Biota within the Barremian stage [8].

Many of the taxa appear to be of Late Jurassic age, however, it is now thought that these are relict taxa, living in eastern Asia after they disappeared from other regions of the world [7].

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