Sequence stratigraphy and structure of the south Xialiao basin, Bohai, offshore China.

摘要

To better understand tectono-stratigraphic development in ancient rift basins, this dissertation investigates the syn-rift sequence stratigraphy and associated structural features of the south Xialiao basin, Bohai, northeastern offshore China, based on (1) establishment of basinal, structural, and stratigraphic frameworks, (2) delineation of fourth-order seismic stratigraphic sequences, physical sedimentary characteristics, structural features, and (3) reconstruction of basin forming mechanisms based on an integrated analysis of 2-D and 3-D seismic reflection and borehole data.; During Paleogene time, lithospheric extension and asthenospheric upwelling deformed the crust of northeastern China, forming an intricate system of faulted basins, collectively known as the North China basin. Along the central axis of a major rift arm, the Xialiao basin, the right-lateral Tan-Lu fault contemporaneously sheared the crust. In south Xialiao basin area, this fault system displays transpressional and transtensional styles, forming structurally high and low areas, i.e., sources of sediments and basins, during the peak and fading phases of rifting, respectively. The syn-rift megasequence consists of the Kongdian, Shahejie, and Dongying sequences corresponding to evolving modes of rifting. Sedimentation kept pace with tectonic subsidence during the deposition of the Kongdian and Dongying sequences, but tectonic subsidence outpaced sedimentation during Shahejie deposition. The initial rift Kongdian sequence consists of alluvial conglomerate near the ramps of faulted margins, whereas lacustrine mudstone, evaporative mudflat strata, or fluvial sandstone may have dominated basin center. The rift climax Shahejie sequence is characterized by parallel and continuous seismic reflectors. Toward the basin center and margins, these reflectors represent mudstone deposited in lacustrine and marginal lacustrine environments, respectively, and the latter may include sublacustrine fan deposits or surfaces of subaerial exposure. The late rift Dongying sequence is characterized by prograding clinoform, consisting of mudstone to siltstone with an upward-coarsening character deposited in deltaic environments. This sequence includes several high-order sequences related to frequent lake-level fluctuations. In addition to mode of continental rifting, lake-level fluctuations related to climatic cyclicity may have shaped stratigraphy, as suggested by the presence of (1) variable spatial extent of lake through time, and (2) higher-order base-level fluctuation cycles.