Geological survey of Heyuan basin in Guangdong Province

The geological background of Heyuan Basin in Guangdong introduced here mainly summarizes the work of Guangdong Bureau of Geology and Mineral Resources (1988) and Zhang Xianqiu (under review). Heyuan basin is a faulted basin controlled by the main structure of Heyuan fault (Figure 2). Heyuan fault was caused by Indosinian movement in Middle Triassic, forming a series of large folds extending northeast and northeast. The old strata are accompanied by fault activity, magmatism and metamorphism, and faults are also deformed by Yanshan movement. The first stage of Yanshan movement occurred at the end of early Jurassic. In the Middle Jurassic, lacustrine detritus and volcanic eruption filled the eastern Guangdong fault basin, forming Zhangping Formation, and the corresponding stratum in northern Guangdong was Maziping Group with intermontane basin facies. The second stage of Yanshan Movement appeared at the end of Middle Jurassic, which caused extensive folding movement and considerable magmatic activity and metamorphism in Middle Jurassic and old strata. With the further strengthening of late Jurassic plate subduction, three stages of Yanshan movement were produced. Therefore, many deep and large faults in this area began to be active, and together with the newly generated faults, they formed a widely distributed volcanic activity zone in Northeast China. This eruption activity is particularly strong and widely distributed in the coastal areas of eastern Guangdong. Accompanied by volcanic movement and plutonic diagenesis, the fourth and fifth stages of Yanshan movement occurred in early Cretaceous and late Cretaceous respectively, but the degree was much weaker than that in late Jurassic, indicating that Yanshan movement was in a recession at that time. In the early Jurassic, with the subduction of the Kula plate under the Eurasian plate, seawater invaded the north-central part of Guangdong, resulting in extensive marine deposition of Jinji Formation (Guangdong Bureau of Geology and Mineral Resources, 1988).

Fig. 2 Location map of stratum profile in the north of Heyuan Basin

At the end of the late Jurassic, extensive plutonism took place, forming a high-temperature and low-pressure metamorphic belt and migmatite belt along the Changle-Nanwa deep fault zone. Since the early Cretaceous, this area has entered a relatively mild tectonic period, which is characterized by the emergence of a series of small and medium-sized fault basins controlled by tensile stress. At the same time, a set of red detritus containing volcanic rocks was deposited in the basin, which lasted until the formation of the red sequence containing gypsum and salt in the Late Cretaceous (Guangdong Bureau of Geology and Mineral Resources, 1988).

Heyuan Basin is one of the ten great basin in Guangdong Province, with an area of about 725 km2, which consists of Upper Cretaceous and Lower Tertiary. The basin extends to the southeast and northwest, showing an irregular "S" shape. The northern part of Heyuan Basin refers to the area north of Datangpu Village in the southern suburb of Heyuan City, which is approximately half-moon in shape and northeast-northeast. It is 2 1 km long from north to south and 6-8 km wide from east to west, with an area of about 160 square kilometers. The northwest side is Heyuan fault and the east side is reverse fault. The strike of the stratum is NE, and the dip angle is 24 ~ 38. Due to the long-term activity of the northwest river source fault, the sedimentary center is westward, constantly shifting from southeast to northwest, forming a monoclinal basin inclined to northwest. The Cretaceous-Paleogene strata in the northern part of Heyuan Basin are more than 4,000 meters thick, which are in fault or unconformity contact with the lower Jurassic Jinji Formation and Yanshan Ⅲ granite (Figure 3).

Stratigraphic profile from Shengli fluorite mine to Niugongzhai in northern Heyuan Basin.

The Cretaceous and Paleogene red beds in the northern Heyuan basin are a set of red coarse clastic rocks, which are characterized by the special development of conglomerate, glutenite and gravel-bearing unequal-grained sandstone. According to the lithologic characteristics, it can be divided into Taihō Formation, Main Field Formation and Mingshui Formation of Nanxiong Formation of Upper Cretaceous, and Shang Hu Formation and Danxia Formation of Paleocene-Eocene from bottom to top (Figure 4).

Fig. 4 Stratigraphic histogram of Heyuan Basin

Taihō Formation (K2d) is distributed in the areas of Datangpu, Tian Xin, Mazhuwo, Chen Tian, Wu Luo and Daoji, that is, the southern suburbs of Heyuan City and the east of Dongjiang River. Characterized by coarse-grained sediments, conglomerate and glutenite are developed and are rich in round dinosaur eggs. It consists of two sedimentary cycles from bottom to top and from coarse to fine, with a thickness of 837 meters. The bottom of Taihō Formation is dark purple massive conglomerate with coarse gravel, which has poor sorting. 80% of the gravel is micro-metamorphic sandstone shale, often sandwiched with gravel sandstone lenses, which is in fault contact with Yanshan III granite or lower Jurassic Jinji Formation, with a thickness of 237 meters ... The lower part is purplish red gravel unequal grained sandstone and medium fine sandstone, which is highly weathered, low and flat, often covered by vegetation, with a thickness of about 200m； meters; ; The middle part is purplish red conglomerate mixed with gravel, and Danxia landform is obvious, with a thickness of150m; ; The upper part is purplish red granite debris mixed with medium coarse gravel, coarse sandstone mixed with fine gravel, unequal grained sandstone mixed with gravel and calcareous sandstone. Gravel is mainly granite, followed by Jinji Formation sand shale, which is rich in round dinosaur eggs and about 250 m thick.

The main mine stratum (K2z) is distributed in Heyuan City, Huangsha, Dongyuan County and Tang Xian Town. The granularity of stratum sediments in the main mine field is relatively fine, which is different from the underlying Taihō Formation and the overlying Mingshui Formation (Sheet I). The main strata are purplish red and light brick red fine conglomerate, medium-coarse grained feldspar timely sandstone and unequal grained sandstone, which are interbedded with unequal thickness, and the lower part is mixed with silty fine sandstone and gravelly iron mudstone. Sandstone is moderately layered with gray-green spots, stripes and gray-green calcareous nodules. Rocks are easily weathered and denuded, and the terrain is low and flat. Most of them are gentle slopes, low mountains, plain hills, producing soft-shelled turtles, Yellow River source dragons (Lu, 2003) and long dinosaur eggs, with a thickness of1.200m..

Mingshui Formation (K2zs) is distributed in Liantangping, Weibei, Bugouyan, Majingding, Yangmeiping and Hezitian, south of Baitouling-Abby line of National Highway 205. The sediment is thick, with white and purple thick layers, and massive fine conglomerate is mixed with medium gravel, glutenite, gravelly coarse sandstone mixed with brown-purple feldspar, timely sandstone, siltstone and sandstone with a small amount of ash.

Shanghu Formation (E 1s) is distributed in Meishixia, Huangguotang, Pingshan, Changlong, Maopu and dog days. It is purplish red and orange, containing a large amount of granite gravel, gravelly coarse sandstone and feldspar timely sandstone, with argillaceous fine sandstone and sandy carbonaceous siltstone locally. Gravel composition is mainly granite, followed by timely sandstone and underlying Nanxiong Formation red sandstone, with good roundness, mostly semicircular shape, moderately developed oblique bedding in sandstone, with the thickness below 1 m, and no fossils found, with a thickness of 820 m.

The Danxia Formation (E2dx) is distributed in Meishizhai, Goubuli, Niuzhizhai and Shimen in the northwest margin of the basin, and it is purplish red, brick red and brownish red massive conglomerate and granite clastic fine conglomerate, containing medium and coarse gravel. The upper part is dominated by fine conglomerate, while the lower part is dominated by giant conglomerate. The diameter of gravel is larger, which can reach more than1m. The gravel is dominated by granite, with poor sorting and grinding roundness, iron, argillaceous and sandy cementation. The silicification near the fault is obvious and hard, and cliffs are often formed, such as Meishizhai, Niuzhizhai and Shimen, forming a typical Danxia landform, and no fossils are found, with a thickness of 534 m.

Nanxiong Group in the north of Heyuan Basin can be divided into three parts according to the grain size of sediments, from coarse to fine and then to coarse. The Lower Taihō Formation has coarse lithology and developed conglomerate, which is the early stage of basin development. The terrain difference is obvious and the erosion is strong, which is the product of rapid accumulation; The main field formation in the middle is fine in grain size, rich in gray-green calcareous nodules or calcareous patches and stripes, and deposited in the environment where the basin develops into a wide and deep water body, with far provenance and small elevation difference around it. The lithology in the upper part of Mingshui Formation becomes thicker again, and conglomerate and glutenite are very developed. The above characteristics are similar to Nanxiong Group in Nanxiong Basin in the standard area and can be compared with it (Figure 5). The main difference between them is that the grain size of sediments in the main Tian Formation and Mingshui Formation in Heyuan Basin is coarser than that in Nanxiong Basin, and fine conglomerate, gravelly sandstone and unequal grain sandstone are more developed, while silty mudstone and argillaceous siltstone are less. Fine clastic rocks and argillaceous rocks are well developed in Nanxiong basin, especially in the main mine field, which are mainly a set of argillaceous siltstone and silty mudstone, and fine sediments are mixed with thin layers, lenticular limestone and marl. The argillaceous rocks of Mingshui Formation are also well developed. In terms of biota, the Nanxiong Formation in both basins produces round dinosaur eggs, while the main Tian Formation produces egg stealing dragons and long dinosaur eggs. There are many kinds of fossils in Nanxiong Group in Nanxiong Basin, including ostracods, foraminifera, Charophyta (Zhang Xianqiu, 1987), gastropods, bivalves, arachnids, insects and other microfossils.

Fig. 5 Stratigraphic correlation between northern Heyuan Basin (A) and Nanxiong Basin (B)

Shanghu Formation in Nanxiong Basin is mainly purplish red and dark purplish red silty mudstone and argillaceous siltstone, rich in calcareous nodules, rich in fossils, mixed with fine sandstone, locally mixed with gravelly sandstone and fine conglomerate, and rich in fossils (Zhang Xianqiu, 1992a). Shanghu Formation in the north of Heyuan Basin is mainly composed of fine conglomerate, gravelly coarse sandstone and unequal-grained feldspar chronological sandstone, with a small amount of silty mudstone and siltstone. The sediment is obviously thicker than Nanxiong basin, and the lithology difference between them is obvious. Danxia Group is a set of coarse clastic rocks with obvious Danxia landform characteristics. It is very similar to Danxia Group in Danxia Mountain section of Danxia Basin, and its lithologic and geomorphological characteristics can be compared. Danxia Group in Weidanxia Basin belongs to Late Cretaceous (Zhang Xianqiu, 1992b). The age basis of Danxia Group in Heyuan Basin is insufficient, which may be later than Danxia Group in Danxia Basin. The Danxia Group on both sides of Nanxiong Basin is a phase transition between Nanxiong Group and Luofozhai Group, and it is an obvious diachronic lithostratigraphic unit, and its age is from the middle and late Late Cretaceous to the early Eocene. From west to northeast, the horizon of Danxia Group in Shitouzhai is getting higher and higher, and the times are getting newer and newer (Zhang Xianqiu et al., 2000a, b). In a word, no fossils have been found in Shang Hu Formation and Danxia Group in the north of Heyuan Basin, but they are all located above Nanxiong Group, with clear sequence and should be later than Nanxiong Group.