Late Carboniferous Conodonts From Southeastern Liaoning And Re-study Of The Benxi Formation

The Carboniferous strata are well exposed in southeastern Liaoning, China, and rich in fossils. Among them, the Benxi Formation is very characteristic,with marine or alternation of marine and non-marine deposits. This formation has been studied for over a century, in which a lot of fossils were found, including mainly the fusulinid, brachiopoda, corals, as well as the plants. However, the fossil conodonts in this formation was not studied thoroughly during the past years, which brought about some inadequacy in the international stratigraphical correlation accurately, and shortage of the information on the conodonts fauna and its related paleogeography in this area during the Carboniferous.Through the field investigation and laboratory work on the biostratigraphy in the areas of the southeastern Liaoning, focusing on the Benxi of eastern Liaoning, and the Jinzhou and Fuzhouwan of southern Liaoning, with the beginning work in the upper Benxi Formation in its denominated geological section in Niumaoling of Benxi City, the several achievements have been gained by the author, as follows.1. The author firstly systematically studied the Late Carboniferous conodonts and strata from the Beishan section of Qianshihuiyao in Jinzhou and the Dingtun section of Fuzhouwan (Fuzhou Bay) area nearby Dalian City. Some important conodonts fossils of the Bashkirian to early-middle Moscovian were found by the author from the Beishan section of Jinzhou, such as Declinognathodus noduliferus,Idiognathoides corrugatus,Id. sinuatus,Idiognathodus sinuosus,for the first time. And, Idiognathoides corrugatus-- Declinognathodus marginodosus Assemblage Zone is to be proposed for the lower part of Beishan section, representing the earliest limestone deposits of Upper Carboniferous in the southern Liaoning.2. Found the index fossils of the middle and middle-upper Moscovian conodonts Idiognathodus podolskensis and Neognathodus roundyi in the upper of Niumaoling section and Beishan section for the first time, with being proposed Idiognathodus delicatus--I. podolskensis Assemblage Zone; and to be proposed Idiognathodus delicatus-- Neognathodus inaequalis for the upper part of Beishan section; and Idiognathodus delicatus- Neognathodus roundyi Assemblage Zone for the Beishan section in the west side of the fault. Based on the discovery of some index Moscovian fossils, e.g. Idiognathodus podolskensis, I. delicatus, Neognathodus roundyi, N. inaequalis, the existence of the middle-upper Moscovian strata can be confirmed in this region, which represents the middle Late Carboniferous deposits in the southeastern Liaoning.3. Firstly discovered the conodonts fauna characterized by Streptognathodus simulator in the Dingtun section in Fuzhouwan. Based on the finding of S. elongatus, S. simulator , S. wabaunsensis, the conodonts-bearing strata in this section can approximately be correlated with the Gzhelian Stage of the International Stratigraphic Chart. The fact implies that the late Late Carboniferous deposits existed in this area which would be equivalent of the main part of Taiyuan Formation. Meanwhile, since S. isolatus, indicating the earliest Permian appeared just above the C-P boundary, has not been found so far, it appears that the top strata in the Dingtun section have not achieved Permian.4. The conodonts Streptognathodus subexcelsus was found firstly in the upper part of Beishan section, with no findings of Idiognathodus sagittalis and Swadelina indicating the late Pennsylvanian in age, it appears to be unable to determine whether the existence of the Kasimovian deposits in this area.5. The above-mentioned new discoveries and conodonts research not only lay the foundations for the Upper Carboniferous conodonts stratigraphical sequences in southeastern Liaoning region,with comparison to related strata of China and abroad,but also provide a more detailed evidence for confirming the geological age of the Benxi Formation, even for the Taiyuan Formation.6. Based on the stratigraphical correlation and fossil comparisons of the southeastern Liaoning region to the Taiyuan area, the author has discussed on the Late Carboniferous geography briefly, and considered that the transgression might start from the Jinzhou and Fuzhouwan area of the southern Liaoning where was one of the edge of the North China Plate during the Carboniferous time, and then expanded to the Benxi area of the eastern Liaoning, and after that extended to the Taiyuan area where was the center of the North China Plateform. A sketched map made by the author, is presented in this paper for showing the palaeogeographic reconstruction in this region of the Late Carboniferous.In sum, the above research work is significant for the further biostratigraphical study of the Carboniferous in North China (especially the southeastern Liaoning), particularly for promoting the accuracy in stratigraphic correlation internationally. Meanwhile it would be also beneficial for widening the prospect of finding sedimentary mineral resources related to Carboniferous deposits in the southeastern Liaoning areas.