Origin and evolution of unconformities in a continental rift basin: a case study from the Lishu Depression in Songliao Basin, China

A variety of unconformity types are commonly developed during the rifting of a continental basin. Identifying their origin and evolution is a prerequisite to understanding the complex tectono- sedimentary evolution of the basin and to evaluate the basin's oil and gas resources. Based on the 2D/3D seismic, geophysical logging, and subsurface lithological data, the unconformities within the rift layer of the Lishu Depression in the Songliao Basin are identified as disconformities, nonconformities, and four "types" of angular unconformities. Their evolution and origin are analyzed by associating the unconformities with the basin's tectonic evolution. The results show that the rifting of the Lishu Depression was not a continuous process. Instead, several regional tectonic episodes influenced the basin's evolution. Episodic compressional events and a phase of extension along the main faults determined the basin configuration and sedimentary fill characteristics of different sub-rifting stages. The tectonic history and related paleogeomorphology changes are the dominant factors in the formation of these different unconformity types. During the rifting process, the type and distribution of unconformity varied. In the initial rifting stage, the basin developed a nonconformity and four types of angular unconformities as the result of both regional uplift and fault block tilt. Rotation and tilting of the fault blocks during the intensive rifting resulted in two types of angular unconformities. Finally, in the recession rifting stage, a different form of angular unconformity and disconformity developed, mainly as a result of the regional uplift and subsequent subsidence.