Ordovician Sedimentary Facies in the Xishan Area.Linfen.Shanxi Province

The Lower and Middle Ordovician in the Xishan area, Linfen, Shanxi Province consist of carbonate rocks intercalated with evaporites. The carbonate rocks in various formations and members are accounted as the restricted subtidal shallow-water doposits. The subsiding rates were once slowed down for several times during subsidence. The greater depositional rates than the subsiding rates are responsible for the shallowing-upwards sedimentary sequences from restricted subtidal carbonates to supratidal sabkhas and the sequences from restricted subtidal carbonates to intertidal lagoons. During the Yeli stage of the early Early Ordovician, the area was in periplatformal subtidal-intertidal environments in which tidal action operated intensely. More well-rounded "edgewise" gravel fragments and bioclasts with purplish red oxidized haloes were laid down in the scorching and strongly oxidizing environments. Up to the beginning of the Liangjiashan stage, the larger-scale transgression made the area a restricted shallow sea where water oscillation became weak, the algae flour- ished and the stromatolitic algae and the carbonates with the algal laminae were accumulated. And during the late stage, the area was graded into intertidal and supratidal environments and uplifted and eroded at last due to the regression. The uplifting has now been referred to as the "Huaiyuan Movement". Three periods of Middle Ordovician may be distinguished, i. e., the Lower Majiagou, Upper Majiagou and Fengfeng periods. The Lower Majiagou period may further be subdivided into three stages: Stage 1 during which slow-moving transgression resulted in the formation of intertidal and supratidal sabkha environments over a long period of time and the development of early diagenetic dolomitization and gypsification in the study area. The intertidal pools occurred in parts of the area where the gypsum deposits were formed. Stage 2 during which the enlargement of the transgression led to the formation of the restricted intertidal shallow sea in the area. As a result of rapid accumulation of the carbonates, the sedimentary rates were greater than the subsiding rates for three times in the course of the overall subsidence, thus resulting in the formation of the typical shallowing-upwards sedimentary cycles of the carbonates, i. e., the cycles from subtidal restricted shallow sea through intertidal sabkha to supratidal sabkha. Stage 3 during which the area was characterized by the long-term supratidal environments, well-developed algal mats, strong dolomitization and gypsification, so that various nodular gypsolith and algal laminated gypsolith were formed. The Upper Majiagou period may still be subdivided into three stages:stages 1 and 2 when the area displayed the transgressive-regressive cycles and was in the environments from subtidal restricted shallow sea to intertidal-supratidal lagoon in which carbonates and gypsum were precipitated. At the beginning of stage 3, the area began to descend in a short time and became a subtidal restricted shallow sea again. Afterwards, it ascended once again to develop into a locally-gypsificated intertidalsupratidal evaporite flat. During the Fengfeng period, the area sank into a restricted shallow sea. At the end of the period, it was subjected to erosion due to rising. Therefore, the regressive sediments failed to be preserved. The isotopic data have revealed that the δ¹⁸O values for the Ordovician carbonates are relatively low, and those for the Majiagou Formation exhibit more higher negative anomaly. The limestones affected by diagenetic dolomitization tend to increase in the δ¹⁸O values, whereas influenced by igneous rocks in the depth of the crust, the δ¹⁸O values tend to be still lower. The δ¹³C values are the lowest both in the Liangjiashan Formation where the algae were developed and in or beneath the gypsum series, and may not be affected by diagenetic dolomitization and hydrothermal metamorphism. The δ³⁴S values are generally higher, among which those for gypsum in the late Lower Majiagou period are higher than those in the middle Upper Majiagou period, indicating the different gypsum-forming enviroments. Compared with the eastern part of the North China Platform, the environments in the area in which subtidal shallow-marine carbonate sediments occur are apparently restricted, indicating that every member in the area might have been separated by submarine rises. This is of great importance to the evaporite deposits in the western platform. The shallowing-upwards subtidal restricted shallow-marine sedimentary sequences should occur principally on the margins of the basin and the gypsum on the top of the sequences should be regarded as the gypsificated deposits in the supratidal sabkha or the deposits in the intertidal haline pools. These environments, generally speaking, are not suitable to the formation of halite.