Tectonic Environment of Western Tianshan during the Early Carboniferous:Sedimentary and stratigraphical evidence from the bottom of the Dahalajunshan Formation

Key words: 
• western Tianshan /
• Dahalajunshan Formation /
• Early Carboniferous /
• sedimentary facies /
• post-collision rift

Abstract: Volcanic-sedimentary rocks from the Dahalajunshan Formation of the Lower Carboniferous are widely exposed in the Yili area, western Tianshan. Large amounts of mineral deposits have been found in this association. However, there is still a considerable controversy on the tectonic setting where the Dahalajunshan Formation deposited. Through regional stratigraphic correlations and field observations, we discovered a regional angular unconformity between the Dahalajunshan Formation and its underlying strata in the study area. The lower Dahalajunshan Formation consists of conglomerate, coarse sandstone, sandstone, siltstone, mudstones, carbonate and andesite in an ascending order. In combination with sedimentological and stratigraphical analysis, we suggest that the terrestrial clastic rocks at the bottom of the Dahalajunshan Formation were formed in a fan-delta environment, which can be further divided into three subfacies:fan-delta plain, fan-delta front and pro-fan delta, and the carbonates were formed in carbonate platform. The stratigraphical sequences, in an ascending order, have creasing sediment sizes from coarse to fine, enlarging water depths from shallow to deep, but varying depositional environment from land to marine, and thus reflect an extensional setting. Therefore, we believe that the early Carboniferous Dahalajunshan Formation in western Tianshan could have formed in a post-collision rift environment.