Palynofacies and sedimentary paleoenvironment reconstruction of Triassic Yanchang Formation in southern Ordos Basin
To reveal the organic matter sources of high-quality Triassic source rocks in southern Ordos Basin,palynofacies analysis and sedimentary paleoenvironment reconstruction were carried out on the core of the Triassic Yanchang Formation.The findings show as follows.(1)Three types of palynofacies can be identified from Member 9 to Member 6 of Yanchang Formation.Specifically,palynofacies Type Ⅰ is mainly composed of amorphous organic matter(AOM);palynofacies Type Ⅱ is characterized by co-develop-ment of AOM and plant debris;palynofacies Type Ⅲ is dominated by plant detritus.(2)Three types of palynofacies assemblage are classified.Assemblage A is mainly composed of palynofacies Type Ⅲ,with a small amount of palynofacies Type Ⅰ and H.It is formed in fluvial/lacustrine delta and lakefront facies,as being a near-source oxygen-rich product in high-energy environment.As-semblage B mainly contains palynofacies Type Ⅱ,with a small amount of palynofacies Type Ⅰ and Ⅲ.It is a shallow to semi-deep lacustrine subfacies product,reflecting the hypoxic to anoxic near-source or far-source environment.Assemblage C is dominated by palynofacies Type Ⅰ,which is only found in the 3rd submember of Member 7 of Yanchang Formation.It was formed in the semi-deep to deep lacustrine static water anoxic environment.(3)The element geochemical paleoenvironmental analysis shows that the study area had a continuous highly humid climate during the depositional period in the 3rd submember of Member 7 of Yanchang For-mation,the discontinuous humid climate might occur during the depositional periods in the 1st submember of Member 9 and Member 7,and the 3rd submember of Member 6 of Yanchang Formation,and semi-arid to semi-humid climate occurred in the remaining peri-ods.The lake level reached its highest during the depositional period in the 3rd submember of Member 7 of Yanchang Formation.The lake productivity began to increase in the depositional period of the first submember of Member 8 of Yanchang Formation,and reached its peak in the depositional period of the 3rd submember of Member 7 of Yanchang Formation.The simultaneous occurrence of highly humid climate,maximum lake depth,and maximum productivity indicates that the coupling of rapid subsidence in the In-dosinian period and global humid climate events in the Carnian period is the key factor for the algal bloom and large-scale accumula-tion of degraded AOM in the depositional period of the 3rd submember of Member 7 of Yanchang Formation in southern Ordos Ba-sin,leading to the development of high-quality source rocks.
Ordos BasinTriassic Yanchang Formationpalynofaciespaleoenvironmentpaleoclimatesource rock
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