PROCESS-FACIES TYPES AND SEQUENCES OF DEVONIAN VOLCANIC-SEDIMENTARY SUCCESSIONS IN NORTHERN XINJIANG AND THEIR RELATION TO PLATE TECTONICS
Gong Yiming (China University of Geosciences, Wuhan)
The process-facies types of Devonian volcano-sedimentary successions in northern Xinjiang include explosive and effusive facies, pyroclastic and calcareous and terrigenous tempestite facies, and coarse-, medium-and fine-grained pyroclastic turbidite facies, volcanic eruption tsunamite facies, syndepositional and soft- deformed seismite facies, sandy and silt-muddy contourite facies. Their characteristics and diagnostic indicators have been presented. Sedimentary dynamic and facies dynamic studies indicate that from the Early Devonian to Late Devonian, the environment-facies sequence of the Zhifang section, east Junggar, is represented by the littoral→littoral and neritic→bathyal and abyssal transgressive one, rather than the facies sequence of marine facies (D_1)→transition facies (D_2)→ continental facies (D_3) as suggested by predecessors. The Process-facies sequence of the Zhifang section is dominated by the event-facies cycles. The proportion of the endogenic event-facies in the process-facies increases from 28.6%(D_1) to 46.5% (D_2) to 55.0% (D_3). The above features show that the interaction of the east Junggar slab and the south continental margin of the Siberian plate strengthened gradually from Early Devonian to Late Devonian. Extension might take place in the early Early Devonian. Convergence might be dominant from the late Early Devonian to Late Devonian. The Early Devonian to Late Devonian environment-facies sequence of the Mount Sarburti section, west Junggar, is bathyal, neritic and littoral facies (D_1)→continental facies(D_2-D_3~1)→littoral and neritic facies (D_3~2). The process-facies sequence of the Mount Surburti section shows complex volcano-sedimentary cycles. The proportion of the endogenic event-facies in the process-facies increases from 27.8% (D_1) to 50.1% (D_2) to 22.4%(D_3), which indicates that the interaction of the west Junggar slab and the south continental margin of the Siberian plate underwent the evolution from convergence to extension during the Devonian.