Stratigraphy and sedimentation in a rift basin modified by synchronous strike‐slip deformation: southern Xialiao basin, Bohai, offshore China

Abstract

ABSTRACTThe southern Xialiao basin, Bohai, offshore China, developed during Palaeogene time as the northern arm of the North China rift system. The right‐lateral Tan‐Lu fault developed contemporaneously along the central axis of the southern Xiaoliao basin. The syn‐rift stratigraphic megasequence of the southern Xialiao basin consists of Kongdian, Shahejie and Dongying sequences, corresponding to different phases of rifting. Sedimentation kept pace with tectonic subsidence during the deposition of the Kongdian and Dongying sequences, but was outpaced by tectonic subsidence during Shahejie deposition. The initial rift Kongdian sequence consists of alluvial conglomerate near the faulted basin margins, whereas deposits of lacustrine, dry‐pan or fluvial environments dominated basin centre. The rift climax Shahejie sequence is characterized by deep lacustrine mudstone and marginal lacustrine mudstone and siltstone towards the basin centre and margins, respectively, and the latter may include sublacustrine fan or surfaces of subaerial exposure. The late rift Dongying sequence consists of upward‐coarsening deltaic deposits, indicated by large southward‐prograding clinoform complexes, overlain by fluvial strata. Syn‐rift basin structures defined a basin‐and‐range configuration, consisting of half‐grabens bounded by normal and strike‐slip faults. However, an impinging, basement‐cored, rift uplift block forced transpression and surface uplift along the Tan‐Lu fault early in basin history, partitioning the Xiaoliao system basin into subbasins. As rifting waned and the basement block subsided, the Tan‐Lu system became transtensional, sedimentation overtook subsidence and the basin became a unified depositional basin. The Dongying sequence includes higher‐order sequences related to frequent lake‐level fluctuations. In addition to tectonism, lake‐level fluctuations likely were driven by climatic cyclicity, which shaped stratigraphic facies architecture by controlling areal extent of lakes and stacking pattern of higher‐order stratigraphic sequences.