Paleozoic sedimentation-structural paleogeography of Shandong Province

The Paleozoic strata of Shandong Province are distributed in the vast area west of the Yishu Fault Zone (Anqiu-Juxian Fault). The stratigraphic distribution of this stage is missing in the eastern Shandong area, but research shows that the eastern Shandong area is the same as the western Shandong area. The area was also once covered with Paleozoic sedimentary cover, which was completely eroded due to subsequent orogeny. During the Paleozoic, the western Shandong area belonged to the surface sea basin of the North China Plate, the northeastern Shandong area was on the passive continental margin, and the southeastern Shandong area and the northern Jiangsu area together formed the Dabie-Sulu rift basin (Figure 5.1).

Bounded by the unconformity between the Ordovician and Carboniferous, the sedimentary-tectonic paleogeographic pattern between the Upper and Lower Paleozoic has undergone major changes. The Lower Paleozoic is dominated by relatively stable marine facies. Mainly, the Upper Paleozoic was composed of marine and continental alternating phase oscillation deposits. The stratigraphic divisions and sedimentary phases of the Paleozoic are shown in Table 5.1.

Fig.5.1 The Paleozoic tectonic units of Shandong province

1—North China plate surface sea basin; 2—Dong Shandong Passive continental margin; 3-Dabie-Sulu Rift Basin

5.2.1 Early Paleozoic marine stable sedimentation stage

In the Early Paleozoic, the Shandong area ended the rifting at the end of the Neoproterozoic subsidence activity, entering a global synchronous subsidence period. The sedimentary facies are mainly shallow marine facies, and the coastal facies appeared in the Early Cambrian. The general geographical trend of the sedimentary-tectonic paleogeographic pattern is that it is deeper in the east and shallower in the west.

(1) Cambrian sedimentation-structural paleogeography

Early Cambrian sediments were distributed along the Yishu fault zone. During the Canglangpu period, the coastal land of the Liguan Formation was first deposited. Debris beach sandy conglomerate facies; from the late Canglangpu to the early Longwangmiao period, the scope of marine transgression expanded slightly and the seawater became relatively deeper. The early sediments were mainly thin-layered limestone and rich in trilobite fossils. At this stage, the coast was roughly along the line of Shouguang, Mengyin, Feixian, and Xuecheng, with the ancient continent to the west and the sea to the east (Figure 5.2). The transgression direction was from south-east to north-west. The Early Cambrian terrigenous clastic material originated from the Luxi Ancient Continent. Due to the abundant supply of terrigenous material, quartz sandstone deposits up to 100 meters thick were formed in the subsidence basin, which is a compensatory basin deposit. Especially in the northwest area of ??Zaozhuang, it contains relatively large amounts of quartz sandstone. There are many conglomerates, which should be deposits of ancient river channels, indicating that this area is the main channel for supplying terrigenous materials.

Early and middle stages of Longwangmiao: mainly carbonate platform sedimentation. The depositional environments include intertidal-supratidal zone Sabha, intertidal zone-shallow subtidal zone and shallow subtidal zone. environment. The marine transgression in the middle Longwangmiao period submerged the ancient land of Western Shandong for the first time (Figure 5.3), and the Western Shandong region was connected with the North China Sea. The seawater still spreads from south-east to northwest. The seawater in the eastern part of western Shandong is relatively deep and in the shallow subtidal carbonate rock high-energy environment; while the west of the Yutai-Ningyang-Xintai-Mengyin-Linqu-Shouguang line The ancient topography of the area was relatively high, coupled with the natural barrier of the surface sea, it was in a limited platform intertidal-supratidal Sabha depositional environment. During this period, the climate was dry and hot, and seawater evaporation was large. Gypsum formed in the relatively closed bay. deposition. Xuecheng, Tengzhou, Zouxian, Qufu, Sishui, Pingyi, Xintai areas and Weifang and Anqiu areas are in the transition zone between the two, belonging to the intertidal zone-shallow subtidal zone environment.

Middle and late Longwangmiao: The overall sedimentary environment is shallow subtidal to intertidal sand and mud flats. The sedimentary basins are divided into north and south. The sedimentary basin in Zaozhuang in the south is shallower than that in Jinan in the north. The two sedimentary basins are shallower than those in Jinan in the north. The sedimentary basins are separated by the relative underwater paleo-uplifts of Qufu-Pingyi-Yishan.

The Maozhuang period of the Middle Cambrian was dominated by fine clastic sediments, which were mainly formed in the intertidal sand flats. The terrigenous clasts originated from the ancient continent in the northwest region.

The Xuzhuang period is quite different from the Maozhuang period. It is characterized by sandstone deposition and is a coastal sand bar facies deposit. It is mainly distributed in the central and southern areas of Shandong and is distributed in a "semi-circular" shape. Its northern boundary is on the Linqing-Tai'an-Xintai-Channgle line. The area north of this line is inter-beach basin sedimentation, and the sediments are calcareous shale.

Table 5.1 Stratigraphic Units and associated Sedimentary Facies of the Shandong Paleozoic strata

The Zhangxia Period was a standard carbonate platform to medium-deep gentle slope sedimentation , transitioning from the platform margin slope phase to the platform reef shoal phase from east to west (Figure 5.4). The platform reef shoal facies is located in most areas west of the Binzhou-Tai'an-Dongping-Wenshang-Jining-Sishui-Feixian-Zaozhuang line. The area east of this line is the platform margin slope facies, with deeper seawater and more shale deposits. , there is a long and narrow transition zone between the two phase zones, which is equivalent to the front slope of the platform, and many slumped algal mud mounds are common in this zone. Judging from the thickness of the sediments, the eastern basin is of a compensatory nature, and the sediment subsidence rate is nearly equal to the structural subsidence rate. During the Zhangxia period, the transgression spread in two directions: the north branch and the south branch, from south east to north west, and from northeast to south west.

The Zhangxia Period was an important transition period in the Cambrian sedimentary environment, sedimentary facies, and sediment combinations. Before the Zhangxia Period, the water body was relatively turbid, and terrigenous clasts dominated the sediments, indicating that the overall deposition site was It is close to the ancient continent and the water body is relatively shallow; but after the Zhangxia Period, the Cambrian was generally deposited in relatively deep water areas, mainly characterized by the development of carbonate rocks, rarely containing terrigenous clastic materials, and storm deposition occurred.

Fig.5.2 The distribution of sedimentary facies and inferred paleogeographic map during the Changlangpu to Earlier Longwangmiao Stages, Early Cambrian[10]

The Gushan Stage of the Late Cambrian was the largest marine transgression in the Cambrian, mainly characterized by the deposition of calcareous shale, thin-layered limestone and nodular limestone, starting from the north Storm rock deposits gradually appear from east to south. The Gushan period is generally in a medium-deep gentle slope sedimentary facies area. The area east of the Huimin-Tai'an-Sishui-Yinan line is a deep gentle slope facies deposit; east of the Lingxian-Pingyin-Dongping-Juye-Yutai line, The semi-annular area west of the above-mentioned deep gentle slope sedimentary facies area belongs to the medium-deep gentle slope sedimentary facies area; the vast area to the west of this facies area belongs to the medium-shallow gentle slope sedimentary facies area. From the analysis of the paleogeographic environment, the sea water gradually becomes shallower from northeast to southwest, the calcareous shale representing deep water to still water deposition gradually decreases, while the nodular limestone, thin-layered limestone and storm rock gradually increase and thicken.

During the Changshan Period, the deposition of storm rock intercalated with argillaceous strips of limestone on medium-deep gentle slopes was dominated. The deposition was relatively uniform, and the differentiation of sedimentary facies zones was not obvious.

The Fengshan Period was dominated by lagoon facies dolomite deposits in the east, roughly in Qingzhou-Zichuan-Yiyuan-Mengyin-Laiwu-Taian-Sishui-Tengzhou-Zou County-Wenshang-Juye- The southeastern area of ??the Juancheng line is lagoon facies, while the vast area in the northwest is a shallow gentle slope sedimentary facies area; the area east of the Changle-Yishui-Zaozhuang line and Dongming, Dingtao, and Shanxian areas is a deep lagoon facies area; the two phases The transition zone between the zones is in the shallow lagoon facies zone.

Fig.5.3 The distribution of sedimentary facies and inferred paleogeographic map during the Middle Longwangmiao Stage, Early Cambrian [10]

(2) Ordovician sedimentation-structural paleogeography

The Xinchang period of the Early Ordovician and the Fengshan period of the Late Cambrian were continuous deposits, and their sea areas and paleogeographic outlines are consistent with The end of Fengshan period is basically the same. At the end of the Xinchang period, due to the influence of the Huaiyuan Movement, it rose into land and suffered erosion. During the Xinchang period, dolomite was deposited, and the overall environment was a confined platform lagoon. In particular, the strata in the early Xinchang period contained "small bamboo leaf" gravel dolomite, indicating that it was in a medium-deep lagoon environment at that time.

The early Daobao Bay period was the product of rising and denudation following the Huaiyuan movement and then subsidence and transgression. In the early stage of transgression, conglomerates several centimeters thick mostly existed above the unconformity surface. The composition of the gravel is relatively complex, and more quartz gravel can be seen in some areas, indicating that the time interval of this discontinuity is longer. In the early stage, it was a limited platform lagoon, which generally contained gypsum-soluble breccia, with up to 4 layers in some areas; in the middle and late stages, it was mainly an open platform shallow subtidal zone, and some parts were intertidal zones.

Fig.5.4 The distribution of sedimentary facies and inferred paleogeographic map at the Zhangxia Stage, Middle Cambrian[10] ]

The depositional environment in the late Daobao Bay period is similar to that in the early Daobao Bay period. In the early stage, it was also a confined platform lagoon deposition, with obvious paste dissolution phenomena visible in various places. In the middle and late stages, it was an open platform shallow subtidal zone deposition environment. The center of the sedimentary basin at that time was in the central-eastern area near the Yishu fault zone. The basin was basket-shaped and opened to the southeast. The northwest edge of the basin had relatively steep terrain and a large slope (Figure 5.5).

The Middle Ordovician includes two periods: the Great Bay Period and the Dariwell Period. The Dawan period was a restricted platform lagoon facies deposit. The lagoon seawater was relatively deep. Shale and thin layers of dolomitic limestone were developed in some areas. Gypsum breccia was relatively undeveloped, indicating that the seawater flow at that time was relatively smooth. The early Darivian period was an open platform shallow subtidal sedimentary environment, dominated by medium-thick pure limestone with rich biological fossils. The late Darivian sedimentary environment was transformed into a restricted platform lagoon, with Algal mat dolomite and rock (gypsum) solution breccia dolomite are the main ones. Compared with the Dawan period, the seawater depth is much shallower and the seawater does not circulate. The seawater withdrew from Shandong in the late Darivil period, ending the sedimentary history of the Ordovician.

Fig.5.5 The distribution of sedimentary facies and inferred paleogeographic map at the Late Daobaowan Stage, Early Ordovician [10]

Overview of the Ordovician sedimentary environment in Shandong, limited platform and open platform appeared alternately. During the lagoon deposition period of the restricted platform, gypsum and dolomite were mostly formed, while during the open platform deposition period, pure gypsum and dolomite were mostly formed. The particulate matter in the thick layer of limestone is mostly algae debris, which has relatively low energy and the water body is clean and quiet.

5.2.2 The late Paleozoic sea-continental interaction oscillation sedimentation stage

Entering the late Paleozoic, the sedimentary-tectonic paleogeographic pattern underwent major changes, and tectonic active areas were activated and began to subside slowly. It quickly turned into a slow uplift, changing from marine and continental alternating phase sedimentation to continental phase sedimentation.

(1) Late Carboniferous sedimentary-structural paleogeography

In the Benxi period, the crust subsided and transgression occurred in the area. The transgression direction was roughly from south to north, and a large number of ancient weathering crusts After the material was deposited, due to the large amount of iron and aluminum minerals in the weathering crust, a set of iron-aluminous rocks, partially bauxite, was formed first, forming the Benxi Formation. Affected by the ancient topography at that time, the thickness of aluminous rock deposition in each subway varied. At this time, the sea water was relatively deep and belonged to a shallow marine depositional environment.

In the Late Carboniferous-Early Permian Jinci-Early Taiyuan Period, marine transgression reached its peak, depositing a layer of limestone equivalent to the largest marine flooding surface. Afterwards, it entered the regression period, and sedimentation The environment changed from shallow marine phase to tidal flat phase. The sediments were mainly mudstone and siltstone with several layers of limestone. Due to the humid climate and developed vegetation at that time, coal seams were formed in most areas. In the middle period, a large-scale marine transgression generally occurred in the entire region, depositing a thick layer of limestone, and then the sea retreated to a tidal flat-tidal delta depositional environment, forming a set of sand and mudstone combinations with widespread sandstone content. increased, forming multiple layers of coal in the delta plain environment. At the end of the period, the third marine transgression occurred in the area, forming lagoon facies limestone deposits at the top of the Taiyuan Formation. The sedimentary thickness is generally thicker in the south and thinner in the north, thicker in the east and thinner in the west. Its lithofacies and paleogeography are shown in Figure 5.6.

Fig.5.6 The distribution of sedimentary facies and inferred paleogeographic map during the< /p>

Jinci-Taiyuan stages, Late Carboniferous to Early Permian[10]

(2) Permian sedimentary-tectonic paleogeography

Early Permian Shanxi stage , manifested as a relatively rapid sea retreat, entering a shallow water delta depositional environment. Judging from the distribution of rocks in the area, the sea retreat direction is northwest, and the southern area is mainly a delta sandstone facies environment, forming mainly sandstone intercalated with mudstone and coal seam deposits. In the northern area, sediments are mainly composed of mudstone intercalated with sandstone and thin coal seams (Figure 5.7). The sedimentary thickness is generally thicker in the north and thinner in the south.

Fig.5.7 The distribution of sedimentary facies and inferred paleogeographic map at the Shanxi Stage, Early Permian

Fig.5.7 The distribution of sedimentary facies and inferred paleogeographic map at the Shanxi Stage, Early Permian

< p> After the Shanxi period sedimentation ended, the area completely entered the continental deposition period. When the Heishan sandstone section was deposited in the Shihezi period, the entire area showed fluvial facies deposition; in the early deposition period of the Wanshan mudstone section, the earth's crust sank as a whole and developed A set of lacustrine mudstone deposits, and then the lake retreated, forming a set of river delta sand and mudstone deposits; by the time the Kuishan sandstone section was deposited, the area was in a low-water period, and the river flushing was strong, and a large set of sand bars developed Coarse-grained quartz sandstone in the facies was deposited; when the Xiaofu River mudstone section was deposited, two lake expansion and lake retreat processes occurred in the area, forming lacustrine and coastal lake facies sediments dominated by mudstone and sand and mud respectively. It can be seen from the sediment composition and thickness that the continental basin as a whole tended to shrink during this period.

5.2.3 Paleozoic sedimentary-structural paleogeography in the eastern Shandong area

No exact Paleozoic sedimentary cover has been discovered in the eastern Shandong area. Some researchers once believed that brachiopod paleontological fossils were found in the Penglai Group [105], and therefore proposed that the Penglai Group was formed in the Devonian. Later, the Shandong Geological Survey Bureau concluded through a special study that the so-called "brachiopod" fossils were actually from Huang False fossils formed from iron ore nodules, so it is still maintained that the Penglai Group was formed in the Sinian Period [106]. Therefore, most predecessors believed that the eastern Shandong area was an ancient continent that continued to rise during the Paleozoic. Since the mid-1980s, some meaningful information has been discovered one after another, raising questions about the Ludong ancient continent.

Guo Zhenyi et al. [107] first reported the discovery of limestone and oolitic limestone gravels in the lower part of the Laiyang Group, Huanghuadian Town, Zhucheng City, and found Schubertella Lata in the gravel; foraminifera: Fossils such as Palaeotxtu?raria sp. (undetermined species), Climacammina sp. (undetermined species), Tetrataxis sp. (undetermined species), Eotubertina sp. (undetermined species), etc. are considered to belong to the genus The Carboniferous-Permian southern type paleontological fossil assemblage suggests that Sinian-Paleozoic cover deposits with a thickness of about 9 to 10 km existed on the Jiaonan Uplift. When China University of Geosciences (1991) carried out a 1:50,000 regional geological survey, in addition to a large number of foraminiferal fossils in the limestone gravel at the same layer, they also found fragments of brachiopods, echinoderms, soft bodies, ostracods and corals. It is believed that the characteristics of the fossils are similar to those found in Binhai County in northern Jiangsu, Jiangning County, Yixing County, and Jurong County in southern Jiangsu. Therefore, it is believed that the limestone gravels in the Laiyang Group come from the Carboniferous-Permian in northern Jiangsu. chronological strata. Niu Baoxiang et al. [108] compared the eight foraminiferal fossil species found in the Laiyang Group with the foraminiferal fossils in the Carboniferous strata in the western Shandong area and found that, except for Inequalis and Badongia, the former included Among the latter, it is therefore believed that the Eastern Shandong Massif and the Western Shandong Massif had the same sedimentary history before the Mesozoic. The Eighth Academy of Shandong Geology and Mineral Resources Bureau and the former Changchun Institute of Geology collected suspected micro-paleofossils from the limestone interlayers of the Laiyang Group in the Zhonglou Basin [109]. They believed that these fossils mainly appeared in the Paleozoic Era, and proposed that they were from the Paleozoic Era of the Sulu Orogenic Belt. The product of redeposition of stratigraphic fossils. Geological mapping organized by the Shandong Bureau of Geology and Mineral Resources found that there are many limestone gravels in the Laiyang Group on the southern edge of the Jiaolai Basin, and there are many gravels from the ultrahigh-pressure metamorphic zone of the metamorphic basement of the Jiaonan uplift, indicating that the material on the south side of the Jiaolai Basin The source area is the Sulu ultrahigh pressure metamorphic belt.

During the mapping process, the Changchun Institute of Geology discovered abundant red algae, green algae, dinoflagellates and fungal fossils in the quartzite and marble in the Wulian Kunshan area [110]. Among them, red algae Molecular fossils of the family Poriferaceae are quite abundant, indicating that the angiophytes were quite prosperous in the ocean at that time and were one of the important biological groups at that time. Trachophyta is one of the most important fossils of the Rhodophyta subclass Euryphyta. It was first found in the Cambrian strata in other parts of the world. It has been reported in the Doushantuo Formation of the Sinian in my country. This is the earliest fossil discovered so far. Fossilized Siphonophora. The tracheal algae in the Wulian area can be compared with the most typical tracheal algae. Both cell differentiation (possibly preserving reproductive organs) and morphological characteristics show the characteristics of advanced algae. Therefore, it is believed that this area may have Sinian-Cold The remains of Wu period strata.

Based on structural research on the Sulu orogenic belt, high-pressure metamorphic rock research, isotope dating and regional structural analysis, some researchers speculate that there may be sedimentary cover from the Paleozoic era in the Sulu orogenic belt [111, 112].

This book shows through the sedimentary-tectonic paleogeography analysis of the western Shandong area that the early Paleozoic seawater was the deepest near the Yishu fault zone, gradually became shallower to the west, and the direction of marine transgression was mainly in the southeast direction. The material isopach is obviously cut by the Yishu fault. This phenomenon was especially obvious during the Cambrian, and in the Early Cambrian most areas of western Shandong were ancient continental areas. In the late Paleozoic, although the sedimentary subsidence center gradually moved away from the Yishu fault, the sediment isopach was still cut by the Yishu fault. This phenomenon shows that during the Paleozoic, the eastern Shandong area was covered by seawater like the western Shandong area, and the seawater gradually became deeper and the sediments became thicker toward the southeast. Intense tectonic movements since the Mesozoic Era have completely denuded the Paleozoic strata in eastern Shandong.

To sum up, it is believed that there was a Paleozoic cover in the eastern Shandong area. During the Paleozoic, the eastern Shandong area was covered by vast seawater like the western Shandong area, and from the western Shandong area to the eastern Shandong area. The depth of seawater gradually increases, and the Paleozoic strata in eastern Shandong should be an extension of the North China stratigraphic system. Some studies believe that starting from the Neoproterozoic, the Qinling-Dabie Ocean formed between the Yangtze and North China plates [113]. The Sulu orogenic belt area in eastern Shandong is the eastern extension of the Qinling-Dabie orogenic belt. There is currently no definite evidence to confirm whether an ocean was formed during the Paleozoic, but it is at least located relatively close to the ocean and may have the nature of a trifurcated rift. The northeastern Shandong area is likely to be like the southeastern margin of the North China Plate [113]. It was in a passive continental margin basin in the early Paleozoic and transformed into a foreland flysch basin in the late Paleozoic. The general geographical trend of the sedimentary-tectonic paleogeographic pattern at that time was deep in the south and shallow in the north.

Research shows that China’s Central Orogenic Belt experienced secondary collision orogenesis in the Devonian and Triassic periods. The formation of the Early Paleozoic Orogenic Belt was the Paleolithic boundary of the Gondwana supercontinent in the Middle and Late Devonian. A product of the initial expansion of the Tees Ocean Basin [114]. The ultrahigh-pressure metamorphic rocks in the Sulu orogenic belt, like the Qinling-Dabie area, have an isotope age of late Early Paleozoic to early Late Paleozoic (435-313 Ma) [94], indicating that strong events occurred along the Sulu area before and after the Devonian. The effect of plate convergence.