Accommodation Space Evolution and Third-Order Sequence Correlation in the Second-Order Sequence of a Foreland Basin

From the forebulge to the foredeep of a foreland basin,the stratigraphic framework appears asymmetrical and wedge-shaped. The stratigraphic sequence at the forebulge is thin or absent. Research on the Kuqa foreland basin in Northern Tarim basin and previous studies on lithospheric flexural simulations of foreland basins show that accommodation space from the forebulge to the foredeep develops inconsistently due to compressional tectonism,which results in the forebulge rising while the foredeep subsides. This inconsistency is dynamically associated with the formation and migration of the forebulge. In an active tectonic phase the forebulge rises and migrates gradually toward the thrust-fault belt,which results in the basin narrowing and deepening,and the development of accommodation space becoming more and more inconsistent. In an inactive tectonic phase the basin is wide and shallow and accommodation space develops consistently. Therefore,the reflection configuration of a second-order sequence is two-layered in seismic sections such as the Jurassic and the Kapushaliang Group,Cretaceous of the Kuqa basin: the lower layer is a group of wedge-shaped,regressive reflections shrinking gradually toward the thrust-fault belt; the upper layer is a group of strap-shaped,widespread reflections. Further,the correlation model showed that the third-order sequences in the lower part of a second-order sequence onlapped gradually toward the craton,which indicates the initial tectonism and the formation of the forebulge; that the third-order sequences in the middle part gradually decrease in size toward the thrust-fault belt,corresponding to intense tectonism; that the third-order sequences in the upper part are widely distributed,corresponding to an inactive tectonic phase such as the Lower Tertiary of the Kuqa basin.