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Abstract: The authors introduced two kinds of newly found soft‐sediment deformation‐syn‐sedimentary extension structure and syn‐sedimentary compression structure, and discuss their origins and constraints on basin tectonic evolution. One representative of the syn‐sedimentary extension structure is syn‐sedimentary boudinage structure, while the typical example of the syn‐sedimentary compression structure is compression sand pillows or compression wrinkles. The former shows NW‐SE‐trending contemporaneous extension events related to earthquakes in the rift basin near a famous Fe‐Nb‐REE deposit in northern China during the Early Paleozoic (or Mesoproterozoic as proposed by some researches), while the latter indicates NE‐SW‐trending contemporaneous compression activities related to earthquakes in the Middle Triassic in the Nanpanjiang remnant basin covering south Guizhou, northwestern Guangxi and eastern Yunnan in southwestern China. The syn‐sedimentary boudinage structure was found in an earthquake slump block in the lower part of the Early Paleozoic Sailinhudong Group, 20 km to the southeast of Bayan Obo, Inner Mongolia, north of China. The slump block is composed of two kinds of very thin layers—pale‐gray micrite (microcrystalline limestone) of 1–2 cm thick interbedded with gray muddy micrite layers with the similar thickness. Almost every thin muddy micrite layer was cut into imbricate blocks or boudins by abundant tiny contemporaneous faults, while the interbedded micrite remain in continuity. Boudins form as a response to layer‐parallel extension (and/or layer‐perpendicular flattening) of stiff layers enveloped top and bottom by mechanically soft layers. In this case, the imbricate blocks cut by the tiny contemporaneous faults are the result of abrupt horizontal extension of the crust in the SE‐NW direction accompanied with earthquakes. Thus, the rock block is, in fact, a kind of seismites. The syn‐sedimentary boudins indicate that there was at least a strong earthquake belt on the southeast side of the basin during the early stage of the Sailinhudong Group. This may be a good constraint on the tectonic evolution of the Bayan Obo area during the Early Paleozoic time. The syn‐sedimentary compression structure was found in the Middle Triassic flysch in the Nanpanjiang Basin. The typical structures are compression sand pillows and compression wrinkles. Both of them were found on the bottoms of sand units and the top surface of the underlying mud units. In other words, the structures were found only in the interfaces between the graded sand layer and the underlying mud layer of the flysch. A deformation experiment with dough was conducted, showing that the tectonic deformation must have been instantaneous one accompanied by earthquakes. The compression sand pillows or wrinkles showed uniform directions along the bottoms of the sand layer in the flysch, revealing contemporaneous horizontal compression during the time between deposition and diagenesis of the related beds. The Nanpanjiang Basin was affected, in general, with SSW‐NNE compression during the Middle Triassic, according to the syn‐sedimentary compression structure. The two kinds of syn‐sedimentary tectonic deformation also indicate that the related basins belong to a rift basin and a remnant basin, respectively, in the model of Wilson Cycle.

Based on studies of sequence stratigraphy, event stratigraphy, biostratigraphy and lithostratigraphy, it is concluded that the Sailinhudong Group is a part of the Bayan Obo Group. Some trilobite fragments are first found in thin sections of the rock from the lower part of the Sailinhudong Group and some Ordovician acritarchs and chitinozoans are also found in this group. A formationa unit of carbonate seismites is first recognized in the upper part and a huge micrite mound is first identified at the top. Dolomite, the host rock of the super giant Bayan Obo Fe–Nb–REE deposits, is neither an igneous carbonatite nor a common bedded sedimentary carbonate, but a huge micrite mound. It has the same macroscopic characters as the micrite mounds at the top of the Sailinhudong Group, which suggests that they should be of the same horizon. According to the fossils, the Sailinhudong and Bayan Obo Groups should be of the Early Palaeozoic rather than the Middle Proterozoic. The new discovery and new idea will throw light on the explanation of the genesis of the supergiant Bayan Obo Fe–Nb–REE deposits.