Correlation of Middle and Late Cambrian cyclic carbonates from the widely separated Zhangxia section, Qingxing section, Xishan section and Zhaogezhuang section has defined five major third-order depositional sequence...Correlation of Middle and Late Cambrian cyclic carbonates from the widely separated Zhangxia section, Qingxing section, Xishan section and Zhaogezhuang section has defined five major third-order depositional sequences. Correlation of biostratigraphy is used to establish the equivalency of stratigraphic sequences between individual sections. Even though the individ- ual sections are composed of different lithofacies and meter-scale cycles that reflect the varied environmental settings in the localities, the overall upward shallowing character of individ- ual sequences is evident. The sequences, named on their relation to the regional stages in range, are in ascending order: Zhangxia Sequence, Gushan Sequence, Guchang Sequence, Changshan Sequence and Fengshan Sequence, of which the upper four sequences were correlated with those defined by Osleger and Read in the United States, with the aid of biostratigraphic correlation. The Middle/Upper Cambrian boundary is tentatively suggested to be between the Guchang Sequence and the Changshan Sequence in North China. The optimization of the existing litho-and regional chronostratigraphic boundaries is discussed based on the sequence boundaries.展开更多
The Yanchang Formation is extensively developed in the Ordos Basin and its surrounding regions. As one of the best terrestrial Triassic sequences in China and the major oil-gas bearing formations in the Ordos Basin, i...The Yanchang Formation is extensively developed in the Ordos Basin and its surrounding regions. As one of the best terrestrial Triassic sequences in China and the major oil-gas bearing formations in the Ordos Basin, its age determination and stratigraphic assignment are important in geological survey and oil-gas exploration. It had been attributed to the Late Triassic and regarded as the typical representative of the Upper Triassic in northern China for a long time, although some scholars had already proposed that the lower part of this formation should be of the Middle Triassic age in the mid-late 20 th century. In this paper, we suggest that the lower and middle parts of the Yanchang Formation should be of the Ladinian and the bottom possibly belongs to the late Anisian of the Middle Triassic, mainly based on new fossils found in it and high resolution radiometric dating results. The main source rocks, namely the oil shales and mudstones of the Chang-7, are of the Ladinian Age. The upper part of the Yanchang Formation, namely the Chang-6 and the above parts, belongs to the Late Triassic. The uppermost of the Triassic is missed in most parts of the Ordos Basin. The Middle-Upper Triassic Series boundary lies in the Yanchang Formation, equivalent to the boundary between Chang-7 and Chang-6. The Ladinian is an important palaeoenvironmental turning point in the Ordos Basin. Palaeoenvironmental changes in the basin are coincidence with that of the Sichuan Basin and the main tectonic movement of the Qinling Mountains. It indicates that tectonic activities of the Qinling Mountains are related to the big palaeoenvironmental changes in both the Ordos and Sichuan Basins, which are caused by the same structural dynamic system during the Ladinian.展开更多
文摘Correlation of Middle and Late Cambrian cyclic carbonates from the widely separated Zhangxia section, Qingxing section, Xishan section and Zhaogezhuang section has defined five major third-order depositional sequences. Correlation of biostratigraphy is used to establish the equivalency of stratigraphic sequences between individual sections. Even though the individ- ual sections are composed of different lithofacies and meter-scale cycles that reflect the varied environmental settings in the localities, the overall upward shallowing character of individ- ual sequences is evident. The sequences, named on their relation to the regional stages in range, are in ascending order: Zhangxia Sequence, Gushan Sequence, Guchang Sequence, Changshan Sequence and Fengshan Sequence, of which the upper four sequences were correlated with those defined by Osleger and Read in the United States, with the aid of biostratigraphic correlation. The Middle/Upper Cambrian boundary is tentatively suggested to be between the Guchang Sequence and the Changshan Sequence in North China. The optimization of the existing litho-and regional chronostratigraphic boundaries is discussed based on the sequence boundaries.
基金supported by PetroChina Basic Research Programs (Grant Nos. 2011A0206, 2014A-0216)the National Basic Research Program of China (Grant No. 2014CB239001)
文摘The Yanchang Formation is extensively developed in the Ordos Basin and its surrounding regions. As one of the best terrestrial Triassic sequences in China and the major oil-gas bearing formations in the Ordos Basin, its age determination and stratigraphic assignment are important in geological survey and oil-gas exploration. It had been attributed to the Late Triassic and regarded as the typical representative of the Upper Triassic in northern China for a long time, although some scholars had already proposed that the lower part of this formation should be of the Middle Triassic age in the mid-late 20 th century. In this paper, we suggest that the lower and middle parts of the Yanchang Formation should be of the Ladinian and the bottom possibly belongs to the late Anisian of the Middle Triassic, mainly based on new fossils found in it and high resolution radiometric dating results. The main source rocks, namely the oil shales and mudstones of the Chang-7, are of the Ladinian Age. The upper part of the Yanchang Formation, namely the Chang-6 and the above parts, belongs to the Late Triassic. The uppermost of the Triassic is missed in most parts of the Ordos Basin. The Middle-Upper Triassic Series boundary lies in the Yanchang Formation, equivalent to the boundary between Chang-7 and Chang-6. The Ladinian is an important palaeoenvironmental turning point in the Ordos Basin. Palaeoenvironmental changes in the basin are coincidence with that of the Sichuan Basin and the main tectonic movement of the Qinling Mountains. It indicates that tectonic activities of the Qinling Mountains are related to the big palaeoenvironmental changes in both the Ordos and Sichuan Basins, which are caused by the same structural dynamic system during the Ladinian.
