The Analysis Of Depositional Environment Evolution On The Bivalve Fauna Fossil Strata (the Source Bed) In The Latest Triassic Period

In Triassic period, Ordos Basin experiences the whole developing course, the Ordos lake shrivelled at Chang 1 epoch. The bivalve fauna fossil strata exposes at Niujiagou section, Zizhou. The Ordos lake depocenter at Chang 1 epoch is not far from Niujiagou section. The analysis on the bivalve fauna fossil strata helps scientists build a comprehensive understanding of Ordos basin depositional environment in the latest Triassic period. Besides, another bivalve funa fossil stratum is discovered at Chang 8 formation at Xuefengchuan section, Hancheng. The research of depositional environment is of great significance to palaeogeographical pattern.This paper focuses on the bivalve fauna fossil strata named Niujiagou section in the latest Triassic period. Besides, some research work at Chang 8 formation at Xuefengchuan section are finished. In order to accomplish research mission, lots of field works, analytical tests and fossils identification are done. Comprehensive using the knowledge of stratigraphy ? paleontology?sedimentology?geochemistry and so on and combing with the regional research achievements, the stratigraphic time? fossil species and sedimentary environment of Chang 1 formation at Zizhou are analyzed clearly. And we get several understandings as follows:It is the first time to identify bivalve fossil species at Niujiagou section, Zizhou. Shaanxiconcha are the dominant species, and several Unio constitute the bivalve fauna fossil strata. Besides, a little plant fossils and vestigiofossil are discovered. The bivalve fossils at Chang 8 formation in Hancheng are mostly the Shaanxiconcha longa. And there are little plant fossil fragments.Geochemical test results show that the depositional environment at Niujiagou section is periodic, bivalve fossils are moved to lakefront, and then deposite and lithifie in rock. Bivalve fossils at Hancheng are moved, deposite and lithifie in rock at fluvial controlled delta.Niujiagou section locates along the middle structural belt at Ordos Basin. The deep hydrothermal solution helps bivalve explosion, leads bivalve fauna to death and controls bivalve strata alteration.The bivalve fauna fossil strata at Zizhou section is a cheniers.