Carboniferous Tectonic Framework And Sedimentary Filling Evolution In The Junggar Basin And Adjacent Area, NW China

The Junggar Basin and adjacent area belongs to Paleo-Asian Ocean tectonic domain, and records the dynamic processes of the Central Asian Orogenic Belt(CAOB) from subduction-accretion-collision to later intercontinental modifications. Carboniferous is the key period from subduction to closure in the tectonic evolution of Paleo-Asian Ocean, during which the complex ocean-continent transition processes resulted in the Carboniferous hydrocarbon reservoirs of the Junggar Basin. However, the research on the Carboniferous proto-basin remains relatively weak, hindering our evaluation of the Carboniferous source-reservoir condition and favorable zone. Thus, a study of the Carboniferous tectonic framework and filling evolution in the Junggar area not only enriches our understanding of tectonic evolution of the CAOB, but also has important implications on pushing the progress for the hydrocarbon exploration of the deep volcanic-enriched basin. Based on a combination of the borehole, outcrop, seismic and gravity and magnetic anomaly data, this paper made an detailed analysis of the Carboniferous stratigraphic framework, geological structure, tectonic setting and sedimentary filling sequence in the Junggar Basin and adjacent area. On this basis, it is suggested that the Carboniferous tectonic framework of the Junggar Basin and adjacent area and its tectono-sedimentary evolutionary characteristics within the multiple periods of tectonic backgrounds.Geochronological results for the borehole volcanic rocks suggest that the Junggar Basin and adjacent area had five periods of volcanic activities, including two periods in the Lower Carboniferous(359-347 Ma,347-331 Ma and 331-324Ma) and three periods in the Upper Carboniferous(323-307 Ma and 307-300Ma). Regional unconformities divided the Carboniferous into two tectonostratigraphic sequences: Lower Carboniferous and Upper Carboniferous. Lower Carboniferous are characterized by compressional structures and involve massive calc-alkaline basalts, andesites, dacites and rhyolites, whereas Upper Carboniferous are mainly controlled by extensional faults and dominated by intermediate-mafic volcanic rocks, with bimodal volcanic rocks in parts. On the basis of tectonic subdivisions, tectonic setting of the volcanic rocks and structural characteristics, the author determined four Carboniferous arc-basin belts in the Junggar Basin and adjacent area from north to south: the Saur-Fuhai-Dulate, Heshituoluogai-Wulungu-Yemaquan, Darbut-Luliang-Karamaili, and Zhongguai-Mosuowan-Baijiahai-Qitai, and identified multiple types of basins, such as fore-arc basin, retro-arc basin, intra-arc rift basin, foreland basin and passive continental margin basin. Based on established tectonic framework, the author divided the Carboniferous tectono-sedimentary evolutionary processes of the Junggar Basin into three phases by the anatomy of the sedimentary filling sequences of the typical Carboniferous basin, involving the early to middle Early Carboniferous subduction-related compressional phase, the late Early Carboniferous to middle Late Carboniferous subduction-related extensional phase and the late Late Carboniferous intracontinental fault-sag phase. The study results demonstrates that the Junggar Basin is likely underlain by juvenile continental crust rather than unified Precambrian basement, and also implies that the Junggar Basin and adjacent area, even the entire CAOB, were built by successively northward collision/amalgamation of multiple linear arc-basin systems characterized by southward accretion.