Cenozoic extension of the offshore portion of the North China Basin

Over the past two decades, a number of models for the Cenozoic tectonic evolution of the North China Basin have been proposed and reviewed (Ye et al., 1985; Liu, 1987; Hong, 1989; Allen et al., 1997; Hu et al., 2001; He and Wang, 2003; Zhao and Zheng, 2005; Wang et al., 2006). Here we do not wish provide a review of tectonic models for the North China Basin however, generally, we find these models are based predominantly on the regional geology of the onshore portion of the basin, using very few data from the offshore portion of the basin, the only paper to discuss the thermal history of offshore portion of the basin being Hu et al. (2001). To date, the tectonic evolution of Mesozoic basin is unresolved. The crustal thickness of the offshore portion of the North China Basin (Bohai Sea) is thinnest in the North China Block, suggesting that the offshore portion of the basin played an important role in the evolution of the entire basin. The high sensitive seismic reflection sections and wells collected in recent years have inspired us to research the extension of the offshore portion of the North China Basin.

We chose four representive interpreted seismic sections that cross the orientation of the master faults in Bohai Sea, the offshore portion of the basin. The construction of these balanced-cross sections was analyzed after correction of the compaction curves and thermal subsidence based on the well control and thermal history. The depth (c. 12.6 km) of detachment for the basement faulting was calculated from the earthquake source depth. The method of the construction employed for the cross-sections was introduced by Gibbs (1983). Extension estimates indicate the intensity of extension in the Mesozoic and Cenozoic. Only one section gives an extension estimate of 13.7% for the late Mesozoic time, which is similar to the major extension in the late Eocene and Oligocene (Es1-3 and Ed). This comparison may suggest that the Late Jurassic and Early Cretaceous also witnessed major extension in the evolution of the basin.

The extension estimates of the offshore portion in the Paleocene and early Eocene (Es4-Ek) were calculated to between 3.6% and 8.9%, the mean being 5.8%. The character of sediments and volcanics deposited during this period also suggest that it represented an initial rifting stage (Figure 2a). The Paleocene Kongdian Formation (Ek) and early Eocene Shahejie-4 Member (Es4) are restricted to a few isolated and narrow northwest trending grabens in the east and south of the Bohai Sea area, deposited between 56 and 42 Ma. Their sedimentation commenced with the deposition of coarse alluvial and subaqueous fan clastic rocks, generally red at their base, containing dark lacustrine mudstones in their middle and upper parts (Hou et al., 2000). In this period, the lavas were well developed in these narrow grabens (Hou et al., 2003).

Figure 2. The Cenozoic evolution of the North China Basin

The Cenozoic evolution of the North China Basin

The Cenozoic evolution of the North China Basin

Extension estimates in the Late Eocene and Early Oligocene were calculated from 7.0% to 31.7% with a mean of 18.1%. From 42 Ma, the North China Basin entered new period: the basin became much wider and extension rates increased (Figure 2b). The area of sedimentation expanded, resulting in the development of an interconnected rhomboidal shaped lake. The late Eocene Shahejie-3 Member (Es3) consists of alluvial fans, subaqueous clastic fans, submerged delta and lacustrine mudstones deposited between 42 and 38 Ma. These periods of sedimentation were controlled by the north-northeast or west-northwest trending growth faults. The depocentrers of Es3 were in the Liaohe Bay (north of the Bohai Sea) and Bohai Bay (west of the Bohai Sea). In this period, the volcanic rocks were well developed in the basin. The Shahejie-3 Member was deposited during a major tectonic subsidence and extension stage of the development of the North China Basin. The Shahejie-1/2 Members (38-32 Ma) consist of grey to dark mudstones interbeded with some carbonates and shales (Hou et al., 2000). In the period of their deposition, the basin was wide and deep, but tectonic subsidence was much reduced. The Shahejie-1/2 Members are considered as a period of tectonic quiescence during which the basin experienced its first stage of thermal subsidence.

The extension estimates for the Late Oligocene range from 4.3% to 21.7% with a mean of 11.8%. This period represents the second stage of strong extension in the basin during the Cenozoic interpreted as tectonically driven by major rifting of the basin. The Dongying Formation (Ed) (32-25 Ma) consists of delta and subaqueous fan facies associations lying conformably on the Shahejie Formation (Es1-3) with the basin wider than it was during deposition of the Shahejie Formation. The uplifts appear as several isolated islands in a large rhomboidal lake (Figure 2c). Sediments were deposited in a delta-type sedimentary environment and many deltas were developed in major depressions along the north-northeast and west-northwest trending growth faults. Two large deltas are identified in the Basin, namely the Paleo-Liaohe delta and Paleo-Huanghe delta (Hou et al., 2000), while two large delta fronts filled the center of Bohai Sea. This suggests that the depocenter of the basin migrated from the margins to the center of Bohai Sea in the late Oligocene (Ed) (Hou et al., 2000). Volcanic rocks are well developed in the late Oligocene, but activity was weaker than Shahejie-3 Member (Es3) (Hou et al., 2003).

Extension estimates for the Miocene and Pliocene range from 2.4% to 5.5% (Table 2), making extension during this period almost negligible. Few volcanic rocks were developed in the Miocene and Pliocene and rather than developing large growth faults, only minor east-west trending faults were developed (Figure 2d). Neogene formations lie unconformably over the Paleocene strata in the whole North China Basin. A boundary is easily identified between the Miocene and Oligocene Formations. The Miocene Guantao Formation (Ng) (25-12 Ma) is a plains facies sedimentary sequence, which covered most of the North China Basin, the thickness of which is relatively uniform suggesting that the whole basin experienced thermal subsidence. The Pliocene Minghuazhen Formation (Nm) (12-2 Ma) lies conformably on the Guantao Formation. It is also a plains facies sedimentary sequence consisting of sandstones and mudstones. In the Miocene and Pliocene, the depocenter was located in the center of Bohai Sea, where shallow-lake facies sediments were filled by the rivers flowing in from the margins of the basin (Hou et al., 2000).

Table 2. The extension calculation of four balanced-across sections in the offshore portion of the North

Balanced-across sections Periods Length of sections (km)   Extension in each period Extension ratio in each period (%) Accumulated extension Accumulated extension ratio (%) Extension factor (β)
  Formations Before extension After extension          
  Q + Nm 63.9 65.5 1.6 2.6 18.7 39.8 1.4
  Ng 62.2 63.9 1.7 2.7 17 36.3  
L1 Ed 59.6 62.2 2.5 4.3 15.3 32.7  
  Es1-3 55.7 59.6 4 7 12.8 27.3  
  Es4-Ek 53.3 55.7 2.4 4.5 8.8 18.8  
  Mz 46.8 53.3 6.4 13.7 6.4 13.7  
  Q + Nm 55.8 57.7 1.9 3.4 24 71.2 1.71
  Ng 54 55.8 1.8 3.3 22.1 65.5  
L2 Ed 44 54 10 21.7 20.3 60.2  
  Es1-3 36.7 44 7.3 19.9 10.3 30.5  
  Es4-Ek 33.7 36.7 3 8.9 3 8.9  
  Q + Nm 38 38.9 0.9 2.4 14.3 58.4 1.58
  Ng 37.1 38 0.9 2.4 13.4 54.8  
L3 Ed 35 37.1 2.2 6.2 12.6 51.2  
  Es1-3 26.1 35 8.9 31.7 10.4 42.4  
  Es4-Ek 24.6 26.1 1.5 6.2 1.5 6.2  
  Q + Nm 98.8 104 5.2 5.2 35 50.6 1.51
  Ng 93.7 98.8 5.2 5.5 29.8 43.1  
L4 Ed 81.4 93.7 12.3 15.1 24.6 35.7  
  Es1-3 71.5 81.4 9.9 13.9 12.4 17.9  
  Es4-Ek 69 71.5 2.5 3.6 2.5 3.58