Abstract

The southern Songliao Basin manifests itself as a wide-rift system that developed in northeast China from the Late Jurassic to Early Cretaceous. Individual basins in the system experienced marked rift subsidence, but the postrift subsidence was insignificant in most of the basins, contrasting strikingly with the tectonic subsidence history of the northern Songliao Basin. Two types of rift basins are defined according to whether the basins underwent prominent postrift subsidence. Type 1 basins are characterized by thick postrift accumulations. Type 2 basins, although experiencing minor postrift subsidence, represent most of the southern Songliao Basin and can be subdivided in accordance with bounding-fault geometry and areal extent: basins bounded by high-angle faults (type 2a), basins bounded by low-angle faults (type 2b), and basins with limited spatial area (type 2c). Many rift basins are expressed as narrow belts in map view and are composed internally of several segments linked through different types of transfer zones. Depositional processes and facies architecture of the basins are controlled primarily by dips and migration of active bounding faults. Synrift depocenters occur close to high-angle bounding faults, and deep-lake deposition commonly persists through much of the synrift subsidence. Lacustrine deposition can be enhanced by the backward (or toward the footwall) stepping of active bounding faults. Depocenters controlled by low-angle bounding faults, in contrast, tend to shift basinward through time, and deep-lake facies commonly develop in the middle stage of rifting. At the end of the Cretaceous, basin inversion was evident in type 1 basins, such as the Shiwu Basin on the north, but other basins appear to have mostly escaped the contractional deformation. Gravitational collapse of the previously thickened crust is considered the cause for the generation of the rift basins, and lateral flow of the ductile lower crust may explain the significantly induced postrift subsidence of type 2 basins. Effective plays occur in basins bounded by high-angle faults, and therefore, type 1 and type 2a basins are suggested to be the main targets of future oil exploration. Synrift source rock maturation might be partially attributed to the heating of Early Cretaceous magmatism because shallower burial alone could not elevate temperatures high enough for petroleum generation.

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