With the unconformity between Ordovician and Carboniferous as the boundary, the sedimentary-tectonic palaeogeographic pattern of upper and lower Paleozoic has changed greatly. The lower Paleozoic is dominated by relatively stable marine facies, while the upper Paleozoic is characterized by land-sea interaction. The Paleozoic stratigraphic division and sedimentation are in accordance with Table 5. 1.
Fig. 5. 1 Paleozoic tectonic units in Shandong province
Fig. 5. 1 Paleozoic tectonic units in Shandong province
1- North China plate epicontinental sea basin; 2- passive continental margin in eastern Shandong; 3- Dabie-Sulu rift basin
5.2. 1 Early Paleozoic marine stable sedimentary stage
The early Paleozoic Shandong area ended the rifting activity at the end of Neoproterozoic and entered the global synchronous subsidence period. Sedimentary facies is mainly shallow sea facies, and coastal facies appeared in early CAMBRIAN. The general geographical trend of sedimentary-tectonic paleogeographic pattern is deep in the east and shallow in the west.
(1) CAMBRIAN sedimentary-tectonic palaeogeography
In the early Early Cambrian, sediments were distributed along the Yishu fault zone, and in Canglangpu period, the sandy gravel facies of the coastal continental clastic beach of Li Guan Formation was first deposited. From the late Canglangpu to the early Longwangmiao, the scope of transgression was slightly expanded and the sea water was relatively deep. The early sediments were mainly thin limestone, rich in trilobite fossils. At this stage, the coast is roughly along the line of Shouguang, Mengyin, Feixian and Xuecheng, with the ancient land in the west and the sea area in the east (Figure 5.2), and the transgression directions overlap from southeast to northwest. Early CAMBRIAN terrigenous clastic materials originated from the ancient land of western Shandong. Due to the abundant supply of terrestrial materials, timely sandstone deposits with a thickness of up to 100 meters were formed in the subsidence basin, which belongs to compensatory basin deposits, especially in the northwest of Zaozhuang, which should be the ancient river channel retention deposits, indicating that this area is the main channel for the supply of terrestrial materials.
Early and middle Longwangmiao: the main sedimentary environment is carbonate platform deposition, including intertidal zone-supratidal Sabha, intertidal zone-subtidal zone and subtidal zone environment. The transgression in the middle period of Longwangmiao flooded the ancient land of Luxi for the first time (Figure 5.3), and the Luxi area merged with the North Sea. Seawater is still flooding from southeast to northwest, and the eastern part of western Shandong is deep and in the high-energy environment of carbonate rocks in the subtidal zone. However, the western area of Yutai-Ningyang-Xintai-Mengyin-Linqu-Shouguang line is relatively high in paleotopography, and it is in the Sebha sedimentary environment of the limited platform intertidal zone-supratidal zone due to the natural barrier of the land surface sea. During this period, the climate was dry and hot, and the evaporation of seawater was large, forming gypsum deposits in relatively closed bays. Xuecheng, Tengzhou, Zouxian, Qufu, Sishui, Pingyi, Xintai, Weifang and Anqiu are in the transitional zone between them, belonging to the intertidal-subtidal environment.
Middle and late Longwangmiao: The overall sedimentary environment is from shallow subtidal zone to sand mud flat in intertidal zone, and the sedimentary basin is divided into two parts. Zaozhuang in the south is shallower than Jinan in the north, and the two sedimentary basins are separated by Qufu-Pingyi-Yishan relative underwater paleouplift.
The middle CAMBRIAN Maozhuang period was dominated by fine clastic deposits, which were mainly formed in the sand flat in the intertidal zone, and the terrigenous debris came from the ancient land in northwest China.
The Xuzhuang period is quite different from the Maozhuang period, characterized by sandstone deposits, belonging to coastal sand bar facies deposits, mainly distributed in the south-central part of Shandong Province, with a "semicircle" distribution, and its northern boundary is on the Linqing-Taian-Xintai-Changle line, and the area north of this line is inter-beach basin deposits with calcareous shale deposits.
Table 5. 1 Paleozoic stratigraphic division and sedimentary facies table 5. 1 Shandong Paleozoic stratigraphic units and related sedimentary facies
The Zhangxia period is a standard carbonate platform to medium-deep gentle slope deposit, which transits from platform edge slope to platform reef beach from east to west (Figure 5.4). Platform reef-beach facies is located in most areas west of Binzhou-Taian-Dongping-Wenshang-Jining-Sishui-Feixian-Zaozhuang line, and the area east of this line is platform marginal slope facies with deep seawater and more shale deposits. There is a long and narrow transition zone between the two zones, which is equivalent to the front slope of the platform. Sluggish algal mud mounds are often seen in this zone. Judging from the thickness of sediments, the eastern basin belongs to compensation, and the deposition rate of sediments is almost equal to that of structures. In Zhangxia period, transgression spread from north branch to south branch, from southeast to northwest and from northeast to southwest respectively.
Zhangxia period is an important turning point of CAMBRIAN sedimentary environment, sedimentary facies and sedimentary assemblage. Before the Zhang Xia period, the water body was relatively turbid, and the sediments were mainly terrigenous debris, indicating that the deposition site was close to the ancient land and the water body was relatively shallow. However, after the Zhang Xia period, the Cambrian was generally deposited in relatively deep waters, characterized by the development of carbonate rocks, and rarely contained terrigenous clastic materials, forming storm deposits.
Fig. 5.2 Lithofacies Paleogeography from Late Canglangpu to Early Longwangmiao in Early Cambrian [10]
Fig. 5.2 Sedimentary facies distribution and inferred paleogeographic map from Changlangpu to Longwangmiao in early Cambrian [10]
The late Cambrian Gushan period was the biggest transgression in Cambrian, which was mainly manifested by calcareous shale, thin limestone and nodular limestone deposits, and storm rock deposits gradually appeared from northeast to southwest. The Gushan period is generally in the middle-deep gentle slope sedimentary facies area, belonging to the deep gentle slope sedimentary facies east of Huimin-Taian-Sishui-Yinan line. The semi-circular area east of Lingxian-Pingyin-Dongping-Juye-Yutai line and west of the above-mentioned deep gentle slope sedimentary facies area belongs to the middle-deep gentle slope sedimentary facies area; The vast area to the west of this facies area belongs to medium-shallow gentle slope sedimentary facies area. From the analysis of paleogeographic environment, the seawater becomes shallower from northeast to southwest in turn, and calcareous shale representing deep water and still water deposits gradually decreases, while nodular limestone, thin limestone and storm rock gradually increase and thicken.
In Changshan period, the medium-deep gentle slope storm rocks are mainly mixed with argillaceous limestone, and the deposition is uniform, but the differentiation of sedimentary facies belts is not obvious.
The lagoon dolomite is mainly deposited in the eastern part of Fengshan period, and the lagoon facies is in Qingzhou-Zichuan-Yiyuan-Mengyin-Laiwu-Taian-Sishui-Tengzhou-Zouxian-Wenshang-Juye-southeast of Juancheng, and the vast area in the northwest is a shallow gentle slope sedimentary facies area. It belongs to the deep lagoon facies area east of Changle-Yishui-Zaozhuang line and in Dongming, Dingtao and Shanxian areas. The transition zone between two-phase areas is located in shallow lagoon facies area.
Fig. 5.3 Lithofacies Paleogeography of Longwangmiao Middle Stage of Early Cambrian [10]
Fig. 5.3 Sedimentary facies distribution and inferred paleogeographic map of Longwangmiao Middle Stage of Early Cambrian [10]
(2) Ordovician sedimentary-tectonic palaeogeography
The Xinchang period of early Ordovician and Fengshan period of late Cambrian are continuous deposits, and their sea areas and paleogeographic contours are basically consistent with the end of Fengshan period. At the end of the new factory, due to the influence of Huaiyuan movement, it rose to land and suffered erosion. The Xinchang period is dolomite deposit, and the overall environment is limited platform lagoon, especially the early strata of Xinchang contain "small bamboo leaves" gravel dolomite, which indicates that it is in a medium-deep lagoon environment.
The early Bao Dao Bay was the product of transgression after the uplift and denudation of Huaiyuan movement, and there were many conglomerates several centimeters thick on the unconformity surface in the early transgression. Gravel composition is relatively complex, and more timely gravel can be seen locally, indicating that the interval is long. In the early stage, it was a limited platform lagoon, generally containing gypsum-dissolved breccia, reaching 4 layers locally; In the middle and late stage, the shallow tidal zone of the open platform is the main one, and the intertidal zone is local.
Fig. 5.4 Lithofacies Palaeogeography of Zhangxia Stage in Middle Cambrian [10]
Fig. 5.4 Distribution of Sedimentary Facies and Inferred Paleogeographic Map in Zhangxia Period of Middle Cambrian [10]
The sedimentary environment in the late Bao Dao Bay is similar to that in the early Bao Dao Bay. In the early stage, it was also a limited platform lagoon deposit, and obvious gypsum dissolution phenomenon could be seen everywhere. The middle and late stage is an open platform subtidal sedimentary environment. At that time, the center of the sedimentary basin was in the central and eastern areas near the Yishu fault zone, and the basin was dustpan-shaped and opened to the southeast. The northwest edge of the basin is relatively steep with a large slope (Figure 5.5).
The Middle Ordovician consists of two periods: Dawan period and Dariwell period. The Dawan period was a limited platform lagoon facies deposit, and the lagoon seawater was relatively deep. Shale and thin dolomite limestone developed in some areas, and gypsum-dissolved breccia was relatively undeveloped, indicating that the seawater circulation was relatively smooth at that time. In the early Darrell period, the sedimentary environment was open platform and shallow tidal zone, which was mainly composed of medium-thick layered pure limestone and rich in biological fossils. In the late Darrell period, the sedimentary environment changed into a limited platform lagoon, which was mainly composed of algal mat dolomite and rock (gypsum) dissolved breccia dolomite. Compared with the Dawan period, the depth of seawater is much shallower, and seawater does not circulate. In the late Darrell period, seawater withdrew from Shandong, ending the sedimentary history of Ordovician.
Fig. 5.5 Lithofacies Paleogeography of Late Early Ordovician in Bao Dao Bay [10]
Fig. 5.5 Sedimentary facies distribution and inferred paleogeographic map of the late Early Ordovician Daobao Bay [10]
Throughout the Ordovician sedimentary environment in Shandong, limited platforms and open platforms appear alternately. Gypsum and dolomite were mostly formed in the lagoon deposition period of limited platform, while thick limestone was mostly formed in the open platform deposition period. Most of the particulate matter is algae chips, and the energy is relatively low, so the water body is clean and quiet.
5.2.2 Late Paleozoic marine-terrestrial interactive oscillation sedimentary stage
The late Paleozoic sedimentary-tectonic palaeogeographic pattern changed greatly, and the tectonic active area was active, starting from slow subsidence, quickly turning into slow uplift, and turning from land-sea accretion to continental deposition.
(1) Late Carboniferous Sedimentary-Tectonic Paleogeography
Crustal subsidence occurred in Benxi period, and the direction of transgression was generally from south to north, and a large number of ancient weathering crust materials were deposited. Because there are many Fe and Al minerals in the weathering crust, a set of Fe-Al rocks, some of which are bauxite rocks, was first formed, forming the benxi formation. Influenced by the ancient topography at that time, the thickness of bauxite deposits in each subway is different. At this time, the seawater is relatively deep and belongs to the shallow sea sedimentary environment.
During the late Carboniferous-early Permian Jinci-Taiyuan period, transgression reached its peak and a layer of limestone equivalent to the maximum flood surface was deposited. Then it entered the regression period, and the sedimentary environment changed from shallow sea facies to tidal flat facies. The sediments are mainly mudstone and siltstone, with several layers of limestone. Due to the humid climate and developed vegetation, coal seams were formed in most areas. In the middle period, a large-scale transgression occurred in the whole area, and a thick layer of limestone was deposited, and then it retreated to the tidal flat-tidal delta sedimentary environment, forming a set of sandstone and mudstone combinations, and the sandstone content generally increased, forming multiple layers of coal in the delta plain environment. At the end, the third transgression occurred in this area, and lagoon limestone deposits were formed at the top of Taiyuan Formation. The sedimentary thickness is generally thick in the south and thin in the north, thick in the east and thin in the west, and its lithofacies palaeogeography is shown in Figure 5.6.
Fig. 5.6 Lithofacies Paleogeography of Jinci-Taiyuan Period from Late Carboniferous to Early Permian [10]
Fig. 5.6 Sedimentary facies distribution and inferred paleogeographic map during sedimentary period
Jinci-Taiyuan Period, Late Carboniferous to Early Permian [10]
(2) Permian sedimentary-tectonic palaeogeography
The early Permian Shanxi period was characterized by rapid regression and entered the sedimentary environment of shallow water delta. Judging from the distribution of rocks in the area, the direction of regression is northwest, and the delta sandstone facies environment is mainly formed in the southern area, mainly composed of sandstone with mudstone and coal seam deposits, while the northern area is mainly composed of mudstone with thin coal seam deposits (Figure 5.7). The sedimentary thickness is generally thick in the north and thin in the south.
Fig. 5.7 Lithofacies Palaeogeography of Shanxi Stage of Early Permian
Fig. 5.7 Distribution of Sedimentary Facies and Inference of Paleogeographic Map in Shanxi Period of Early Permian
After the end of Shanxi period, the area completely entered the continental sedimentary period, and when the Heishan sandstone section was deposited in Shihezi period, the whole area showed river deposition; At the early stage of Wanshan mudstone section, the whole crust sank, and a set of lacustrine mudstone deposits developed, and then the lake retreated, forming a set of river delta sandstone mudstone deposits; By the time the Kuishan sandstone section was deposited, the area was in the low water level period, with strong river scouring, and a large set of coarse-grained timely sandstone deposits developed in the sandbar period. When the mudstone section of Xiaofuhe River was deposited, the area experienced two processes of lake expansion and lake retreat, and lake facies-lakeside facies deposition was formed, mainly mudstone and sand mud. From the composition and thickness of sediments, it can be seen that the whole continental basin has a shrinking trend during this period.
5.2.3 Paleozoic Sedimentary-Tectonic Paleogeography in Eastern Shandong
Up to now, no exact Paleozoic sedimentary cover has been found in eastern Shandong. Some researchers thought that brachiopod fossils were found in Penglai Group [105], so it was suggested that Penglai Group was formed in Devonian. Later, Shandong Geological Exploration Bureau thought that the so-called brachiopod fossils were actually pseudofossils formed by pyrite nodules, so it still insisted that Penglai Group was formed in Sinian [106]. Therefore, many predecessors thought that eastern Shandong was an ancient land that continued to uplift in Paleozoic. Since the mid-1980s, some meaningful information has been discovered one after another, questioning the ancient land of eastern Shandong.
Guo [107] reported for the first time that limestone and oolitic limestone gravel were found in the lower part of Laiyang Group in Huanghuadian Town, Zhucheng City, and loose Schubert zone was found in the gravel. Foraminifera: ancient beaded worm (undetermined species) ancient Turk? Larrea sp. , ladder (undetermined species) Climacammina sp., Tetradactylum. And Eotubertina sp. are considered to belong to the Carboniferous-Permian southern paleontological assemblage, so it is considered that there was an Sinian-Paleozoic cover with a thickness of about 9 ~10 km on Jiaonan uplift. When China University of Geology (199 1) conducted a regional geological survey of1∶ 50,000, in the limestone gravel of the same horizon, besides many foraminifera fossils, brachiopods, echinoderms, ostracods and coral biological detritus were also found, and it was considered that the fossil characteristics were the same as those of Binhai County, Jiangning County, Yixing County and Jurong County in the same period. Niu Baoxiang et al. [108] compared the 8 foraminiferal fossil species found in Laiyang Group with those found in Carboniferous strata in western Shandong, and found that all the former were included in the latter except coccidia and Badong, so it was considered that the eastern Shandong block and the western Shandong block had the same sedimentary history before Mesozoic. Suspected microfossils were collected in limestone interlayer of Laiyang Group in Zhonglou Basin by the Eighth Institute of Shandong Bureau of Geology and Mineral Resources and the former Changchun Institute of Geology [109]. It is considered that these fossils mainly appeared in Paleozoic, and it is suggested that they are the products of redeposition of Paleozoic stratigraphic fossils in Sulu orogenic belt Geological mapping organized by Shandong Bureau of Geology and Mineral Resources found many limestone gravels in Laiyang Group in the southern margin of Jiaolai Basin, many of which came from ultrahigh pressure metamorphic belt of metamorphic basement in Jiaonan Uplift, indicating that the provenance of Jiaolai Basin was Sulu ultrahigh pressure metamorphic belt.
In the process of mapping in Changchun Institute of Geology, abundant red algae, green algae, dinoflagellate and fungal fossils [1 10] were found in quartzite and marble in Wulian Kunshan area, among which the molecular fossils of red algae and porpoises were quite rich, indicating that Porphyra was quite prosperous in the ocean at that time and was one of the important biological groups at that time. Porphyridium is one of the most important fossils in the subclass Rhodophyta. It was first discovered in Cambrian strata in other parts of the world, and has been reported in the Sinian Doushantuo Formation in China. It is the earliest discovered Porphyridium fossil so far. Compared with the most typical laver, the laver in Wulian area shows the characteristics of advanced algae in both cell differentiation (possibly preserving reproductive organs) and morphological characteristics, so it is considered that there may be Sinian-Cambrian stratigraphic residues in this area.
According to the structural study, high-pressure metamorphic rocks, isotopic dating and regional structural analysis of Sulu orogenic belt, some researchers speculate that Paleozoic sedimentary caprocks may exist in Sulu orogenic belt [11,1 12].
Through the analysis of sedimentary-tectonic paleogeography in western Shandong, the book shows that the seawater in the early Paleozoic was deepest near the Yishu fault zone and gradually became shallower to the west. The direction of transgression is mainly southeast, and the sedimentary contour line is obviously cut by Yishu fault zone. This phenomenon is particularly obvious in Cambrian, and most of the early Cambrian western Shandong is an ancient land area. In the late Paleozoic, although the center of sedimentary subsidence gradually moved away from Yishu fault, the sedimentary isoline was still cut by Yishu fault. This phenomenon shows that in Paleozoic, eastern Shandong and western Shandong were covered by seawater, and the seawater gradually deepened to the south of the East China Sea and the sediments became thicker. The intense tectonic movement since Mesozoic has completely eroded the Paleozoic strata in eastern Shandong.
To sum up, it is considered that there are Paleozoic caprocks in eastern Shandong. In Paleozoic, eastern Shandong was covered by vast sea water as western Shandong, and the depth of sea water gradually increased from western Shandong to eastern Shandong. The Paleozoic strata in eastern Shandong should be an extension of the stratigraphic system in North China. Some studies believe that since Neoproterozoic, Qinling-Dabie Ocean [1 13] was formed between the Yangtze plate and the North China plate. The Sulu orogenic belt in eastern Shandong is the eastern extension of the Qinling-Dabie orogenic belt. There is no definite evidence to prove whether the ocean was formed in Paleozoic, but it is at least close to the ocean and may have the nature of a trigeminal rift. However, northeastern Shandong was probably in a passive continental margin basin in the early Paleozoic, just like the southeastern margin of the North China plate [1 13], and it was transformed into a foreland flysch basin in the late Paleozoic. At that time, the general geographical trend of sedimentary-tectonic paleogeographic pattern was deep in the south and shallow in the north.
The study shows that the central orogenic belt in China experienced secondary collision orogeny in Devonian and Triassic, and the formation of the early Paleozoic orogenic belt was the product of the initial expansion of the Paleo-Tethys ocean basin on the supercontinent edge of Gondwana in the middle and late Devonian [1 14]. Like Qinling-Dabie area, ultrahigh-pressure metamorphic rocks in Sulu orogenic belt have isotopic ages from late early Paleozoic to early late Paleozoic (435 ~ 3 13 Ma) [94], indicating that there was strong plate convergence along Sulu area around Devonian.