Early Cretaceous Maars, Depositional Environments and Their Relationship to the Fossil Preservation in Sihetun, Liaoning, Northeast China

摘要

Early Cretaceous localities in Sihetun area of Liaoning Province, northeast China have yielded extremely well preserved fossils. However, the controversies over the age and correlation of the deposits have not been settled in the past several decades. Field examination indicates that there probably existed many small and deep maar lakes in Sihetun rather than a large united lake. The morphological features of the maar imply strict physical (wave, current), biological (microorganism decomposition), chemical (oxidation) processes; they provide a favorable environment for ideal fossil preservation. Microstrati-graphic work shows that biogenic and clastic laminations were well developed in the sediments. The biogenic lamination comprises a couplet of organic-rich and organic-poor sediments. The clastic lamination mainly consists of clay and silt. Varved sediments support that the maar lakes were meromictic, and had a predominantly anoxic environment that favored the preservation of fossils and varves. If the varves were annually deposited, then the prosperous period of fossil animals probably lasted for only about 6,000 years. The co-existence of fossil layers with large charcoal occurrences and volcanic ash layers may suggest the catastrophic events resulted in the disappearance of birds and other vertebrates after short period prosperity.