Geological setting
The NC was initially stabilized in the Middle Proterozoic and Neoproterozoic and was covered by Paleozoic shallow marine sediments (Qian et al., 1986). The earliest platform-type sedimentary rocks belonging to the Mesoproterozoic Changcheng Group are preserved with an almost horizontal stratigraphy in the Mt. Taihang area and rest unconformably on the Archean crystalline basement of the central NC. Horizontal Paleozoic strata in the eastern NC commonly covered flat-topped (Karst-like) mountains of the western Shandong Province. These observations suggest that the NC was stable in the Proterozoic and Paleozoic (Li, 1998; Lin, 1985). The Cenozoic extensional basin, Bohai Bay Basin, is surrounded by the Wutai, Taihang, Luliang, Zhongtiao, Songshan, Taishan and Yanshan mountains. They are block-faulting mountains, not typical orogenic mountains except intraplate orogenic Yanshan mountain. Mt. Yanshan displays overthrusts and anticlines related to the final close of the Mongolian-Okhostk Ocean before the Late Jurassic (Chen, 1998). These mountains probably once formed an Early Mesozoic plateau, the Eastern North China Plateau. The mountains and Bohai Bay Basin together constitute the basins-mountains system which resulted from the extensional event associated with collapse of the plateau.
In the eastern NC, the Cenozoic Bohai Bay Basin was developed on the site of a Late Mesozoic basin (Hou et al., 2001). The outline of the basin envelopes a rhomboidal central area with slender arms that extend to both the northeast and southwest (Figure 2) and, at the eastern edge of the basin, the well-known, north-northeast trending Tanlu strike-slip fault zone occurs (Xu, 1993; Xu et al., 1993; Ross et al., 1996). The basin consists of a series of uplifts and depressions containing numerous secondary uplifts and depressions. Six major depressions are recognized: the Jizhong, Huanghua, Liaohe, Bozhong, Jiyang and Linqing depressions (Ye et al., 1985; Liu, 1987; Lu et al., 1997). The offshore portion of the basin includes the Bozhong, southern Liaohe, eastern Huanghua and northern Jiyang depressions (Gong et al., 1987; Hou et al., 2003). Tertiary strata rest unconformably on a variety of older pre-rift strata and are covered conformably or unconformably by Quaternary sediments. This succession is typically 4-7 km thick in the main depression, with a maximum thickness of more than 10 km at the depocenter of the Bozhong depression (Central Bohai Sea) (Ye et al., 1985; Liu, 1987; Hu et al., 2001). Lithologies are dominated by sandstone, mudstone and interbedded volcanic sedimentary rocks. The Cenozoic succession has been divided into six formations: the Kongdian (Ek), Shahejie (Es), Dongying (Ed), Guantao (Ng), Minghuazhen (Nm), and Pingyuan (Qp) Formations, which are further sub-divided into several members. The basin experienced two phases of rifting in Cenozoic times (Ye et al., 1985; Hou et al., 2001).
Depressions within the basin mostly take the form of half-grabens with the geometry of Cenozoic successions controlled by listric master faults (Ye et al., 1985; Lu et al., 1997). The Bohai Bay Basin is divided into three parts: the western, central and eastern parts (Figure 2). The dominant structures of the Bohai Bay Basin are WNW and E-W trending uplifts and depressions in the central basin, and NNE trending uplifts and depressions in the eastern and western parts. The Tanlu Fault Zone developed in the east of the basin, while Huanghua-Dongming Fault developed in the basin centre (Hou et al., 2001; Hu et al., 2001) (Figure 2). Syn-rift deposits rest on pre-rift basement and are covered by post-rift deposits. The Bohai Bay Basin experienced a typical rifting sequence from tectonic subsidence to thermal subsidence; however, the relationship between the Mesozoic and Cenozoic basins of the Bohai Bay Basin and their implications for the destruction of the NC remain ambiguous.