Structural controls on facies distribution in a small half-graben basin: Luanping basin, northeast China

摘要

Luanping basin is one of many small rift basins that developed in northeast China during mid-Cretaceous time. It is filled by alluvial, fan-deltaic and lacustrine strata of the Lower Cretaceous (post 130 Ma) Xiguayuan Formation. Distribution of facies and stacking patterns are controlled by position within the basin with respect to fault-bounded basin margins. Luanping basin is bounded by a normal fault consisting of two segments that are perpendicular to each other in map view; one or both of these faults probably accommodates a component of strike-slip. This geometry gave rise to three distinct depozones within the basin: (1) a region of maximum sediment thickness located near the intersection of these two basin-bounding fault segments; (2) a shallower part of the basin, located near the tip of the normal fault segment bounding the basin to the north; and (3) a low-gradient, north-dipping ramp. The facies found in each of these settings are different. Coarse sublacustrine sediment gravity flows interfinger with profundal black shale near the basin depocentre at the intersection of the two basin-bounding fault segments. Fan delta and shallow lacustrine sedimentation dominated in the shallower part of the basin near the northeastern tip of the master-bounding fault. Fine-grained shallow lacustrine sedimentation predominated along the low-gradient ramp. The facies in Luanping basin are different from those found in basins of similar size elsewhere in the northeastern China extensional tract. Specifically, profundal, organic-rich black shales are found in Luanping basin but are largely lacking in neighbouring basins. We suggest that this is due to higher rates of subsidence along more steeply dipping normal faults in Luanping basin, as opposed to the other basins.