Abstract:

This paper explores the depositional architectural characteristics of alluvial fan associated with contemporaneous reverse faults at compressional basin margin, for example of Triassic Karamay Formation at the northwestern margin of the Junggar Basin. Based on lithofacies study, hierarchical bounding surface analysis, outcrops and subsurface analogues as well as welltoseismic integration, the combination types of contemporaneous reverse faults at compressional basin margin are summarized. According to this study, the distribution of alluvial fan bodies at compressional basin margin was mainly controlled by the combination of contemporaneous reverse faults. Different combinations of contemporaneous reverse faults resulted in the scale and morphology variation between different fans. In the combination of multiple parallel faults and strikeslip faults, the movement of dextral strikeslip fault resulted in continuous erosion of the source provenance which eventually evolved into an accommodation area; under this condition the fan bodies presented a rapid retrograding thinbed overlying pattern of banding shape updipping towards the provenance. Meanwhile, the translation of strikeslip fault had an impact over the sediment transportation from provenance; thus the fan bodies could also display a lateral migration overlaying pattern. Where crossshaped contemporaneous reverse faults act strongly, a multiperiod thick vertical superimposed fan combination is formed.

Key words: contemporaneous reverse fault, alluvial fan, depositional architecture, outcrops, compressional basin, northwestern margin of the Junggar Basin