Abstract
Early Cretaceous desert sedimentary system was developed in Northwest Ordos Basin. At present, the research degree of desert sedimentary paleogeography is low. The research on paleogeographic environment of Early Cretaceous sedimentary sandstone has guiding significance for the exploration of uranium resource and underground water resource in Northwest Ordos Basin. Based on the previous basic geological data of the lower Cretaceous in the Northwest of Ordos, through field geological survey, drilling core observation, and paleocurrent measurement, the sedimentary paleogeographic environments in different periods of Early Cretaceous had been studied. The results show that the lower Cretaceous in Northwestern Ordos Basin can be divided into two sedimentary cycles, the lower sedimentary cycle composed of Luohe Formation and Huachi-Huanhe Formation, and the upper sedimentary cycle composed of Luohandong Formation and Jingchuan Formation. Alluvial, Aeolian, and desert lake depositional systems were developed. The main sedimentary facies were alluvial fan, braided river, sand dune, dry valley, and dry lake. Alluvial fan and braided river facies had mainly developed in Luohe Formation. Braided fluvial facies mainly had developed in Huachi-Huanhe Formation, and alluvial fans locally developed around it. The deposits in Luohandong Formation were mainly aeolian sandstone, and the alluvial fan facies were inherently developed at Western and Northern margins. The depositional environment of Jingchuan formation is mainly river and lake. The tectonic evolution of lower Cretaceous had controlled the development of paleogeographic environment, and sedimentary facies had controlled the development of aquifer system. The paleogeographic environment of Early Cretaceous is conducive to the reduction and enrichment of uranium and the development of metallogenic units.
1 Introduction
The continental Cretaceous desert deposits in China are unique [1], which were mainly distributed in the subtropical high-pressure arid zone at 20–40°N and the Westerly and Northeast trade wind zones to the South [2,3], belonging to standard zonal desert [4,5]. Ordos Basin is one of the four largest sandy lands in the Early Cretaceous in China, located between Ordos City and Yulin City, in the Northwest of which the desert developed during early Cretaceous [6,7]. The desert sedimentary system is composed of many genetically related desert sub-environments such as wind and water, sedimentary system units such as aeolian sedimentary system, desert lake system, and marginal alluvial system [8,9]. The aeolian sedimentary system is a typical system tract unit of the ancient desert, which is composed of sand dune, inter-dune, sand sheet, rocky desert, conglomerate desert, etc.
Predecessors have done some investigations on the early Cretaceous deposits in the Northwest of the Ordos Basin [10–12]. However, the degree of exploration is still shallow. In the 1950s and 1980s, The third Petroleum Survey brigade of the Ministry of Geology and mineral resources (1965) [13], Bureau of Geology and mineral resources of Inner Mongolia Autonomous Region (1991) [14], and Shaanxi Bureau of Geology and mineral resources (1991) [15] had carried out some basic regional geological surveys. At the beginning of twenty-first century, led by the Xi’an Geological Survey Center of China Geological Survey, the groundwater exploration had been carried out based on the good reservoir physical properties of the early Cretaceous deposits [16–18]. In 2019, the 208 Geological Brigade of Chinese nuclear industry made the first drilling in the lower Cretaceous Huachi-Huanhe formation of Hangjin Qi in the Northwest of Ordos Basin and obtained an important discovery of sandstone type uranium ore, which reached industrial grade [19]. This uranium ore occurrence was characterized by high grade, thick ore block and large reserves, which was the first major discovery in the early Cretaceous strata in this area, belonging to “new area and new interval.” However, the effect of uranium prospecting by several subsequent drilling wells was not ideal. The unclear paleogeography of the lower Cretaceous desert sedimentary sand body in the study area is one of the important factors restricting uranium resource and underground water resource exploration, at the same time, desert deposition in geological history is also one of the weak research fields of sedimentology [5,7]. The effort on research of paleogeographic environment of Early Cretaceous desert sediments is urgently needed.
2 Geological setting
2.1 Overview of the study area
The research area is located in the Northwest of Ordos Basin, the administrative division belongs to the West of Ordos City with Yulin City. It is a typical mid-temperate semi-arid plateau continental climate, and the landform type is desert plateau. The tectonic location of the study area belongs to the Yimeng uplift of the Ordos block in the North China plate, of which the internal structure is relatively simple, dominated by nose folds. The stratum is gentle, and the dip angle is generally less than 1° [20–22] (Figure 1). The strata in the study area are well developed from Archean to Mesozoic Cenozoic, including Archean to Early Proterozoic crystalline basement, Mesoproterozoic to Paleozoic marine sedimentary cover strata, and Mesozoic Cenozoic continental sedimentary cover. As part of the North China plate (Craton), the research area experienced some large tectonic movements, such as the late Caledonian tectonic movement, the late Indosinian tectonic Movement, and the late Yanshan tectonic Movement during its formation. The geological structure is famous for its stability and characterized by integral uplift, continuous sedimentation, and broad gentle monocline. The geological structure evolution process in the study area can be divided into six tectonic evolution stages [23,24]: the crystalline basement formation stage from Archean to early Proterozoic, the depression valley or rift stage in middle Late Proterozoic, the epicontinental sea of North China stage in early Paleozoic, the Craton depression stage of North China from late Paleozoic to Middle Triassic, the inland depression stage of Ordos from late Triassic to Cretaceous, and the peripheral fault depression stage of Ordos in Cenozoic.
![Figure 1
Distribution of Cretaceous desert in China (a) [5], diagram map of geotectonic location of Ordos Basin (b) (modified after Zhao et al. [23]), and geological sketch map of the study area (c) (modified after Zhang et al. [10] and Guo et al. [11]).](/document/doi/10.1515/geo-2022-0469/asset/graphic/j_geo-2022-0469_fig_001.jpg)
2.2 Stratigraphic correlation of the lower Cretaceous
The lower Cretaceous is widely distributed in the Northwest of Ordos Basin, belonging to the continental Cretaceous strata in the Northwest stratigraphic region [1]. Predecessors had carried out regional geological surveys in this area, and the lower Cretaceous had been divided. From 1948 to 1951, Tian Zaiyi, Zhang Chuangan, and Zhang Geng carried out regional survey in Ordos Basin and Longdong area, the lower Cretaceous Zhidan Group (formerly known as Bao’an System) was divided into Yijun formation, Luohe Formation, Huachi Formation, Huanhe Formation, Luohandong Formation, and Jingchuan Formation from bottom to top [11]. From 1955 to 1956, team 206 of North China Geological Bureau conducted a 1:200,000 petroleum geological survey from Yimeng Uplift to Zhuozi mountain, the Cretaceous stratum was divided into Dongsheng Formation, Atumushan Formation, and Xinzhao Formation from the bottom to the top, which were collectively called the Cretaceous to Tertiary Yimeng System [10]. From late 1950s to early 1960s, the third survey brigade of the Ministry of Geology (formerly called Ordos Petroleum Survey brigade) carried out a 1:200,000 petroleum geological survey in Ordos Area; the Cretaceous strata were systematically studied and the Cretaceous strata sequence K1 1–K1 7 were reestablished [13]. In 1978, the eighth detachment of the Inner Mongolia Geological Team carried out 1:200,000 regional geological survey of Hangjin Qi and Dongsheng City in Yimeng uplift. On the basis of previous work, the lower Cretaceous had been re-divided into three lithologic sections of the Yijinhuoluo Formation and two lithologic sections of the Dongsheng Formation. At this stage, the geological survey team of Shaanxi Provincial Bureau of Geology and Mineral Resources carried out 1:200,000 regional survey in the South of Ordos Basin, and the lower Cretaceous system was named Zhidan Group, which was divided into Yijun Formation, Luohe Formation, Huachi-Huanhe Formation, Luohandong Formation, and Jingchuan Formation from bottom to top [15]. Huang et al. believed that the age of the magnetic strata in the lower part of the Zhidan Group (Yijun Formation, Luohe Formation, and Huanhe Formation) was about 141–135 Ma [25]. Li made a comparison between the phyllopoda fossils in the upper part of Zhidan Group and the strata in Western Liaoning Province, and he thought that the age of Zhidan Group was Aptian period in lower Cretaceous [26].
3 Data and methods
In this research, seven sets of data from the 1:200,000 Regional Geological Surveying of Otog Qi J-48-12, Bulage J-48-18, Chahannao J-49-7, Xinjie J-49-8, Wushen Qi J-49-13, Dukoutang K-48-36, and Xinzhao J-48-6 were collected and carefully analyzed. We carried out field geological survey for about 50 days, and 130 km of field geological survey route had been completed, including 3 km measured geological profile, 68 geological observation points (Figure 1), ten groups of paleocurrent measurement, 35 sketches of geological phenomena, core logging of three exploratory wells, 82 samples collection, and about 1,600 photos were taken. In the research room, 45 rock slices were systematically identified, the instrument was a polarizing microscope, and the model was Nikon-DS-Ri2. There were 62 geological maps drawn based on collecting and mastering the basic geological data of the lower Cretaceous in the Northwest of Ordos Basin, through field geological survey, drilling core observation, and paleocurrent measurement. The name of sandstones has been adopted using the composition-genetic classification scheme (Figure 2). First, the rock samples are collected and rock slices are made. Second, the rock thin sections are observed under the polarizing rock microscope and the content of rock matrix is counted. If the rock matrix is less than 15%, it is classified as sandstone, otherwise it is classified as greywacke. Third, the whole rock thin section is glanced under polarizing microscope, and several representative horizons are selected. The percentage content of Q (quartz, chert, quartzite cuttings, and siliceous rock cuttings), F (feldspar, granite cuttings, and granitic gneiss cuttings), and R (debris except those in Q and F) have been counted under each view horizon. Take the average value, which is the percentage value of the Q, F, and R in the rock sample. Finally, put the percentage values of Q, F, and R into the triangle diagram to obtain the name of the rock sample. Take the percentage content of Q as an example to introduce the method of putting the percentage values into the triangle diagram. The vertical line QQ′ was made from the Q vertex to the straight line FR, the length of vertical line QQ′ was divided into 100 equal parts, and each equal part represents 1% of Q content value. The Q content value gradually decreases from point Q to point Q′, the Q value at point Q is 100%, and the Q value at point Q′ is 0%. When a Q content value is known, read the value at point Q″ on the vertical line QQ′, and line AB parallel to the line FR is made through the point Q″. According to this method, the straight line CD is made according to the content of F, and the straight line EF is made according to the content of R. Three straight lines meet to form a point G, and the name of the area where the G point is located is the name of the sample rock. Sedimentary facies histogram of observation points, regional sedimentary facies comparison map, and early Cretaceous sedimentary paleogeographic series maps in the northwest of Ordos Basin were drawn. The control of tectonic evolution on paleogeographic environment and the influence of Early Cretaceous sedimentary paleogeography on resource exploration have been discussed. The purpose of the work mentioned above is to provide certain geological basis for the study of ancient desert environment and the exploration of lower Cretaceous resources in Ordos Basin.
Triangular diagram of sandstone classification of Huachi-Huanhe Formation in Northwest of Ordos Basin. I: quartz sandstone, II: feldspathic quartz sandstone, III: lithic quartz sandstone, IV: arkose sandstone, V: lithic feldspar sandstone, VI: feldspar lithic sandstone, VII: lithic sandstone. Q: Quartz; F: feldspar; R: debris.
4 Results
4.1 Division results of the lower Cretaceous
Combined with 1:200,000 regional survey work and relevant basic geological data, according to the core logging work, from bottom to top, the lower Cretaceous had been divided into Luohe Formation (from middle Berriasian to Valanginian, about 140–135 Ma), Huachi-Huanhe Formation (Hauterivian, about 135–130 Ma), Luohandong Formation (from Barremian to middle Aptian, about 130–120 Ma), and Jingchuan Formation (from middle to late Aptian, about 120–115 Ma) in the Ordos Basin (Figure 3).
![Figure 3
Comprehensive stratigraphic histogram of lower Cretaceous in Northwest Ordos Basin (modified after Cheng et al. [9]).](/document/doi/10.1515/geo-2022-0469/asset/graphic/j_geo-2022-0469_fig_003.jpg)
Comprehensive stratigraphic histogram of lower Cretaceous in Northwest Ordos Basin (modified after Cheng et al. [9]).
4.1.1 Luohe Formation
Luohe Formation is a sedimentary combination of aeolian desert facies, fluvial lacustrine facies, alluvial (proluvial) sediments, debris flow, and weathering residuals. According to lithologic composition, sedimentary rhythm, and change of lithofacies combination, Luohe Formation was divided into two sections, and the regional calcareous sandstone tight layer and mudstone section were taken as the dividing mark. The stratum at bottom is gravelly sandstone and conglomerate, formerly known as “Yijun conglomerate” [27], which belongs to slope deposit, eluvial deposit, and alluvial deposit near the basin margin and the ancient highland in the basin. The stratum is locally distributed as wedge-shaped body and rapidly thins and pinches out into the basin. The lower section of Luohe Formation is dominated by aeolian medium fine sandstone with lacustrine thin layer or lenticular mudstone. The upper segment of Luohe Formation is dominated by medium fine grained aeolian sandstone, with small to medium crossbedding developed. The angular unconformable contact with the Jurassic Anding Formation can be seen in the East Woertugou area of Hangjin Qi. There is a conformity contact between Luohe Formation and the overlying Huachi-Huanhe Formation widespread (Figures 2, 3a and b).
4.1.2 Huachi-Huanhe Formation
Huachi-Huanhe Formation is widely exposed in the study area, which is generally distributed in NNW direction, and the strata dip from east to west, integrated contact with the overlying Luohandong Formation. Scour surface was commonly seen at the bottom of Huachi-Huanhe Formation. The lower part of which is grayish white and purplish red gravelly arkose sandstone and glutenite, with plant debris, carbon chips, and erect wormholes visible. The upper part is composed of soil purple red, gray white, and yellow green interbedded lithic sandstone with mudstone and conglomerate (Figure 2). Under the oxidation–reduction process, the sandstone in some areas appears fancy shade. The lithology of this formation is relatively stable, and the thickness gradually increased from East to West. The stratum thickness is only 33 m at Zhujiagedu in the East, the stratum thickness is more than 300 m at Aolun bulage area in the West, and the maximum sedimentary thickness is 915 m in Xinzhao Sumu. This formation is a desert marginal sedimentary assemblage, the main sedimentary environment was hydrostatic water condition, and the secondary sedimentary environment was eolian environment. The sedimentary facies were dominated by braided river deposits, which were thick in the East and North and thin in the West and South area. Trough crossbedding and tabular crossbedding had developed. According to 1:200,000 regional survey data of Hangjin Qi and Ordos City, the fossils are rich in Huachi-Huanhe Formation, and fossils of fish, phyllopods, ostracods, Protoceratops, and vertebrates were found [11] (Figures 3, 4c and d).
Field outcrop photos of Early Cretaceous layers in Northwest Ordos Basin: (a) alluvial fan conglomerate of Luohe Formation in the western margin, (b) glutenite of Luohe Formation in well ZKW2019-1, (c) variegated sandstone of Huachi-Huanhe Formation with trough bedding, (d) plant debris in the lower part of Huachi-Huanhe Formation in well ZKW2019-1, (e) variegated aeolian sandstone of Luohandong Formation in Otog Qi, with large crossbedding, purple mud gravel, and calcareous nodules visible, (f) purple red sandstone of Luohandong Formation in Xinzhao Sumu, with large aeolian crossbedding, (g) variegated mudstone with mud cracks of Jingchuan Formation in Shuang Obo, Otog Qi, and (h) grey sandstone with ripples of Jingchuan Formation in Shuang Oboo, Otog Qi.
4.1.3 Luohandong Formation
The Luohandong Formation exposed in a large area in the shape of “厂” in the study area, and integrated contact with the underlying Luohe Formation. The upper part is mainly Earthy red, purple red sandstone, argillaceous siltstone, and mixed with purple black iron nodules. The middle part is interbedded with orange red, orange yellow, and gray medium coarse grained sandstone and argillaceous siltstone. The lower part is red massive sandstone, partially mixed with mudstone, containing calcareous nodules. Tabular crossbedding and trough crossbedding developed in the sandstone, in which thin salt beds (white), calcareous nodules, iron nodules, etc. can be seen. The sedimentary facies are mainly aeolian (Figures 3, 4e and f).
4.1.4 Jingchuan Formation
The Jingchuan Formation is mainly distributed from North to south in the west of the study area, such as Wujia Temple, Ulan Abdel, Princess Qihar, Tegai Temple, etc. Jingchuan Formation was mainly composed of variegated argillaceous rocks with a thickness of 50–233 m, which was a set of river lake facies with horizontal beddings developed. The stratum contains abundant bivalve fossils, ostracods, phyllopods, mollusc fossils, fish, and sporopollen fossils, and conforming contact with underlying strata. The mud cracks developed in mudstone, and symmetric ripple marks developed in red argillaceous siltstone. In the stratum exposed in the south of Shuang Oboo, a large number of bivalve fossils were found at the top of the mudstone and at the bottom of siltstone. The siltstone was calcareous cemented, and the sedimentary facies are lakeside facies (Figures 3, 4g and h).
4.2 Division of sedimentary cycles
During the Early Cretaceous, there was a paleoclimatic condition dominated by drought, accompanied by short-term semi-arid paleoclimate in the Northwest of Ordos Basin. According to the sedimentary facies, the Early Cretaceous desert sequence can be divided into two sedimentary cycles, the lower sedimentary cycle is from Luohe Formation to Huachi-Huanhe Formation and the upper sedimentary cycle is from Luohandong Formation to Jingchuan Formation (Figure 3). There are basically no fossils in the red coarse clastic Luohe formation at the bottom of the lower cycle, while there are a large number of animal and plant fossils in Huachi-Huanhe Formation, carbonized plant debris can be seen in sandstone in Otog Qi (Figure 4d). In addition, a large number of animal fossils and sporopollen fossils of Lycopodiaceae, Selaginella spp., Cyatheaceae, Pteridiumconiforme, Cycads spp., Ginkgo spp., Cephalotaxus spp., Picea spp., Abies spp., and Ephedra spp. were also found in Huachi-Huanhe Formation. In the upper cycle, there were few fossils in Luohangdong Formation, and only a few wormholes were found in the field survey. Jingchuan Formation was another fossil-rich stratum, with fish, ostracods, plants, sporopollen, and other fossils. Cycads, ginkgo biloba, conifers, etc. were dominant in sporopollen fossils, while conifer ferns and selaginella were secondary [11].
4.3 Main sedimentary facies
Based on the comprehensive analysis of rock mineral composition, grain texture, sedimentary structure, rock association with its vertical change, as well as geological background of the lower Cretaceous clastic rocks, the alluvial, aeolian, and desert lake sedimentary systems were developed in Early Cretaceous in the study area. The sedimentary facies of alluvial fan, braided river, dune, dry valley, and dry lake facies had developed in lower Cretaceous (Figures 3 and 4).
4.3.1 Alluvial fan facies
The alluvial fan facies can be divided into fan top subfacies, fan subfacies, and fan margin subfacies in the study area. The fan top subfacies are mainly found on the outcrop profile of Luohe Formation in the basin margin, and its lithology is coarse gravel or pebbles mixed with fine gravel and sandy argillaceous materials, which are poorly sorted and supported by complex foundation (Figure 3a). The fan subfacies are mainly developed in the Luohe Formation on the northern and western margins of the basin, of which the rock particles are relatively fine, mostly composed of medium and fine gravels. The number of sandstones is increased, and they are lenticular or thin layers. The particle sorting and roundness are relatively good, and the particles are supported by particles or heterogeneous bases (Figure 4b).
4.3.2 Braided fluvial facies
The braided river facies composed with a binary structure of fine upper and coarse lower in profile, with sand-rich channel subfacies in the lower part and mud-bearing flood plain subfacies in the upper part. Braided river facies were widely developed in Huachi-Huanhe Formation in the study area, with trough crossbedding developed, forming an upward fining positive grain sequence sedimentary sequence. Taking the braided river sediments of Huachi-Huanhe Formation at Albas Sumu in the west of the study area as an example, at the bottom, gravelly sandstone and conglomerate were commonly seen, the gravels are arranged directionally, and the scour phenomenon at the bottom can be seen. At upper part, it is the central beach sandstone. On the top, the flood plain subfacies were mainly composed of thin fine siltstone and mudstone, with horizontal bedding and sand ripple bedding. Due to the intense frequent migration and erosion of the river channel, the flood plain subfacies were generally less preserved and thinner in the profile. From the edge to the interior of the basin, from the bottom to the top of the sedimentary profile, the braided river facies often gradually evolved into meandering river facies or desert dune subfacies and shore shallow lake subfacies (Figures 4c and 5a).
Typical sedimentary facies of Early Cretaceous in the Northwest of Ordos Basin: (a) braided river sediments of Huachi-Huanhe Formation in Albas Sumu, (b) lake front sediments of Jingchuan Formation in Bayinwendur, and (c) braided river sediments of Jingchuan Formation and aeolian sediments of Luohandong Formation in Yihewusu.
4.3.3 Lacustrine facies
The lacustrine facies were mainly developed in Jingchuan Formation in the Southwest of the study area, which was mainly shore shallow lake subfacies. The sedimentary rock assemblage was mainly composed of purple mudstone, brown mudstone, light grey mudstone, grey green mudstone, variegated mudstone, silty mudstone, argillaceous siltstone, interbed of siltstone and fine sandstone, mud cracks, and asymmetric ripples are visible (Figure 4g and h). There were large three toed dinosaur footprint fossils found in the gray fine feldspathic quartz sandstone of the Jingchuan Formation in the Northwest of Albas Sumu, Otog Qi. The sedimentary facies of Jingchuan Formation were shore shallow lacustrine facies in Bayinwendur of Otog Qi, the stratum lithology was thick purple mudstone mixed with gray argillaceous siltstone, and a large number of bivalve fossils were developed near the interface between mudstone and argillaceous siltstone, the size of which was about 1–2 cm (Figure 5b).
4.3.4 Desert facies
The desert facies, widely developed in Luohe Formation and Luohandong Formation, was one of the most distinctive sedimentary facies type of the lower Cretaceous in the study area, mainly including dry valley, dune, dune, desert lake, and other sedimentary types. In the lateral direction, desert facies gradually transited with fluvial facies, alluvial fan facies, and even shore shallow lake facies to form a relatively complete desert sedimentary system. The stratum lithology was mainly brick red, brownish red, purple red feldspathic quartz sandstone. The clastic grains were sub-rounded, with rough surface, dish-shaped pits and other aeolian sand morphology characteristics. The granularity is well sorted, the sandstone texture was loose, and the cementation was poor. Large giant aeolian crossbedding and wedge-shaped crossbedding were developed (Figure 4e and f). Various types of inter-dune deposits were developed in the relatively low-lying zone between the dunes. Evaporative sedimentation developed in small inter-dune ponds with short-term ponding, and the sediments were thin mudstone or clay, accompanied by mud cracks, calcareous nodules, and gypsum salt deposits. The mud cracks were filled with eolian sand and formed sand veins. Due to frequent dry and wet changes between dunes in the Northwest of Ordos Basin in Early Cretaceous, various inter-dune deposits appear temporarily and alternate repeatedly, and mud fissures were well developed.
4.4 Sedimentary paleogeography
4.4.1 Sedimentary paleogeography of Luohe Formation
During the sedimentary period of Luohe Formation in the early Cretaceous, the climate in the study area was dry, and the geological processes controlling the sediments were mainly flowing water and wind. It was the early development period of fluvial facies and desert facies in the Early Cretaceous, belonging to the first sedimentary evolution stage. The alluvial fan facies and braided river facies were widely developed in the study area, and desert lake sediments developed in the Southwest. From bottom to top, there were two types of evolutionary sequences, one was from alluvial fan facies to braided river facies, the other was from braided river facies to desert facies.
In the early sedimentary period of Luohe Formation, the dominated sedimentary system was alluvial system in the study area. In the vast area from Hangjin Qi at North to Otog Qi at South, the alluvial fan facies, braided river facies, and delta plain distributary channel facies had developed. The lithology of stratum was mainly gravelly sandstone and medium-coarse sandstone, with a thickness of 175–235 m.
In the Middle to Late sedimentary period of Luohe Formation, the Aeolian system and desert lake system had developed. There was a phase of ancient desert basin with the largest area of desert landscape, largest sand dunes, and maximum deposition thickness of aeolian sand, which was dominated by aeolian thick-layered medium-fine grain sandstone, and the sand bodies had strong connectivity and extensibility. The sand bodies in Luohe Formation were mostly developed Cretaceous stratum in Ordos Basin. In the South of the study area, the desert lake sediments were inherently developed, which were main channel sandstones and thin mudstones with mud cracks and small crossbedding, the facies were delta plain subfacies and delta front subfacies (Figure 6a).
Sedimentary paleogeography of Early Cretaceous in Northwest of Ordos Basin: (a) sedimentary paleogeography of Luohe Formation, (b) sedimentary paleogeography of Huachi-Huanhe Formation, and (c) sedimentary paleogeography of Luohandong Formation.
4.4.2 Sedimentary paleogeography of Huanhe-Huachi Formation
After the sedimentary period of Luohe Formation, the climate became relatively humid, and the basin underwent tectonic subsidence in the sedimentary period of Huachi-Huanhe Formation, with fluvial facies and lake facies developed. In this period, clastic rocks with gray color were developed. The paleogeography was dominated by braided river facies, and the alluvial fans developed at the North edge and West edge of the study area (Figure 6b).
During the sedimentary stage of Huachi-Huanhe Formation, the structural conditions were relatively stable in the study area. The sediment was coarse in the North and fine in the South, coarse in the East and West sides, and fine in the middle. The eastern boundary of the lake had shrinked toward West, slope in the East had changed from simple westward tilt to multiple tongue shaped westward prominent wavy tilt, and the sedimentary center generally migrated toward West and North. The sedimentary center was located in the South of Otog Qi, where desert lake sediments developed with mainly mudstone and silty mudstone. The thickness of sandstone layer was thin, and most sand bodies were lenticular. The thickness of sediment near Xinzhao Sumu was the largest, up to 915.5 m, but the distribution range of lacustrine sediments was small.
The fluvial system in Huachi-Huanhe Formation was still dominant, according to the profile structure, which can be divided into upper sedimentary cycle and lower sedimentary cycle. The upper sedimentary cycle was distributed in Yihewusu to Hangjin Qi and other places in the North area. The sand bodies in delta plain distributary channel facies and front underwater distributary channel facies had developed, and the thickness of single sand body ranged from 20 to 50 m. The sandstones in the lower sedimentary cycle had relatively developed, and the sedimentary environment consisted of alluvial fan and delta plain distributary channels.
4.4.3 Sedimentary paleogeography of Luohandong Formation
After the first sedimentary evolution stage in Early Cretaceous, the climate of the sedimentary period of Luohandong Formation turned to drought again, starting the second sedimentary evolution stage. Under the joint influence of unbalanced uplift and tectonic denudation in the study area, the sedimentary range of Luohandong Formation was obviously smaller, and stratum thickness decreased sharply. The strata of Luohandong Formation were mainly aeolian deposits, with large-scale crossbedding developed, which directly covered on the gray Huachi-Huanhe Formation braided river sediments. The sandstone was loosely bonded, and the particles were mostly pinkish-color quartz. The impact pits and rough-cast glass surfaces can be seen on the surface of quartz grains. At the end of the sedimentary period of Luohandong Formation, the climate gradually converted to humid and braided river facies developed. Alluvial fan facies had inherently developed in Western margin and Northern margin (Figures 6c and 7).
Schematic diagram of desert sedimentary model in Northwest Ordos.
4.4.4 Sedimentary paleogeography of Jingchuan Formation
The climate in the sedimentary period of Jingchuan Formation had changed from dry to wet again, and the sedimentary environment of rivers and lakes began to develop. From field outcrops and previous data, the sediments of Jingchuan Formation were mainly shore shallow lake subfacies in the study area. They experienced a strong overall tectonic uplift at the end of Late Cretaceous, and the large-scale sedimentation of the Ordos Basin was over. Since then, the strata have entered a stage of full exposure and denudation in the Ordos Basin.
5 Discussion
5.1 Control of lower Cretaceous tectonic evolution on paleogeography
In the early Cretaceous, influenced by Yanshan Movement, moderate tectonic uplift and tilt movement occurred in the Northwest area of Ordos Basin [28–30]. During this period, there were two times paleoclimatic changes from dry to wet, and the sedimentary environment includes lake, river, alluvial fan, delta, and eolian desert. At different stages of tectonic evolution in the study area, with the change of sedimentary environment, the types of sedimentary facies and lithologic association characteristics were changing in early Cretaceous. Under drought conditions, the appearance of coexisting facies of alluvial fan, river, lake, and desert was shown, and the Luohe Formation and Luohandong Formation sediments developed characterized by red coarse clastic rocks. Under the relatively humid climatic condition, the area of the lake expanded, while the aeolian deposits reduced, the sediments of Huachi-Huanhe Formation and Jingchuan Formation were mainly characterized by gray fine clastic rocks mixed with chemical sedimentary rocks. Based on the analysis mentioned above, controlled by Early Cretaceous tectonic evolution, there were two major sedimentary cycles developed in the study area. The lower sedimentary cycle was composed of Luohe Formation and Huachi-Huanhe Formation, including at least three secondary sedimentary cycles. The upper sedimentary cycle was composed of Luohandong Formation and Jingchuan Formation, including at least three secondary sedimentary cycles. The sedimentary system in the sedimentary cycle gradually regressed to the source area, which was generally shown as a process of gradual expansion of the area of one lake basin.
During the preliminary stage of Early Cretaceous, controlled by the late tectonic evolution of Yanshan movement, the strata in most areas of the study area were slightly inclined, forming wide-gentle folds, while the tectonic rise and fall range was large in some areas, which caused the uneven basement of the basin, the thickness and lithology of sedimentary bodies. There were residual glutenites and proluvial glutenites developed in the lower section of Luohe Formation at the margin and the ancient highlands of the basin. The glutenites appeared fan-shaped, mound-shaped, and lenticular in different sizes, and because of the change of sedimentary facies, the glutenites underwent rapidly, or pinched out toward the interior of the basin.
During the early of Early Cretaceous, the undulation of the basin basement was gradually flattened. Along with basin subsidence, there were aeolian deposits, braided river deposits, and desert lake deposits developed in Luohe Formation and Huachi-Huanhe Formation, with thick sediments, wide distribution, and developed sand bodies. The structural evolution in this period controlled the aeolian sandstone deposition in the upper of Luohe Formation. There were two types of sedimentary assemblages developed, the desert sedimentary assemblage was mainly composed of banded sand dunes, locally with muddy sediments between dunes and in small desert lakes. The other one was desert margin sedimentary assemblage. The sedimentary sequence was composed of dry valley alluvial fan glutenite, gravelly braided channel, and interbed series of aeolian sheet sand with conglomerate desert sedimentary, which were seen at slope edges on the North and South (Figure 7).
During the middle of Early Cretaceous, affected by Yanshan movement, the basin further subsided. Under the control of tectonic stability and settlement during this period, the water body gradually deepened, the Huachi-Huanhe Formation was dominated by alluvial fan and braided river deposits, followed by desert lake and delta deposits. The sediment color was generally reductive gray, and the sediment gradually transited to light red in the area near Eastern edge. There were many transient desert lakes in the Southern part of the study area, and the sediments were mainly lacustrine variegated mudstone and grayish green siltstone.
During the late of Early Cretaceous, affected by unbalanced uplift at the end of Yanshan movement, tilting and uplift occurred in the study area, caused high in the east and low in the west [29,31,32]. There were alluvial fan deposits developed in Luohandong Formation at the Northwest edge of the study area, while they were dominated by red aeolian deposits in the rest of the vast areas with large-scale aeolian crossbedding. During the sedimentary period of Jingchuan Formation, under the influence of regional thermal subsidence and the remote effect of Pacific plate subduction, the phenomenon of strongly differential settlements occurred in some parts of the study area. There were distinct separations between uplifts and depressions in the study area, with multiple settlement centers. There were freshwater bivalve fossils, ripple marks, variegated mudstones, and mud cracks found in Shuang Oboo, Yihewusu, Bayinwenduer, which represented lake sediments. After that, the rapid uplift and denudation began in the study area.
5.2 Influence of Early Cretaceous sedimentary paleogeography on resource exploration
5.2.1 Control of Early Cretaceous sediments on aquifer system
The composition, spatial distribution, and changing characteristics of sedimentary facies determined the hydrogeological structure in lower Cretaceous in Northwest of Ordos Basin [17,33], and Luohe Formation and Luohandong Formation are the main aquifer systems in the Northwest area of Ordos Basin. From Northwest to Southeast and from bottom to top, the change of sedimentary facies is from alluvial fan facies to braided river facies, to desert facies or meandering river facies. The braided river facies and aeolian facies were absolutely dominant in the study area, while desert facies and meandering river facies distributed locally. The medium coarse grained sandstone is the main rock type of Luohe formation and Luohandong formation, the interlayer is thin, lenticular, discontinuous mudstones, and argillaceous siltstones, which formed sedimentary geological structure units, characterized by stable distribution and huge thickness. The geological structure units can be regarded as good aquifer system, providing good space for groundwater storage [34].
5.2.2 Influence of Early Cretaceous paleogeography on uranium enrichment and mineralization
Ordos Basin is one of the important areas with rich sandstone type uranium resources in China, the previous discoveries were mainly found in Jurassic strata [16,35]. An aeolian sandstone-hosted uranium deposit had been found in the southwest of Ordos Basin [36], while the sandstone type uranium deposits at lower Cretaceous in the Northwest of Ordos Basin is an important discovery of new horizons in the Ordos Basin. The study showed that the paleogeographic environment of the Early Cretaceous had an important influence on uranium enrichment and mineralization, and one uranium mineralization unit contained the lower water-resisting layers, ore-rich sand bodies and upper water-resisting layers. There were mainly alluvial system and desert lake system in Huachi-Huanhe Formation, which controlled the three sub-cycles and dozens of tertiary sedimentary cycles. There were 12 tertiary sedimentary cycles constituting the uranium mineralization unit in the study area. The lower water-resisting layers were grayish green silty mudstone with a thickness of about 1 m. The upper water-resisting layers were composed of 2.8 m thick grayish green fine-grained sandstone, 0.1 m grayish green mudstone, 0.3 m grayish green mudstone, and other fine-grained sediments. The ore bodies were enriched in sandstones at middle cycles, and mineralization tended to weaken upward and downward. The rocks of upper water-resisting layers and lower water-resisting layers were fine-grained clastic sediments, which horizontally transformed into thick argillaceous rocks.
The sandstone type uranium was mainly enriched in Huachi-Huanhe Formation in the study area. The mineralized rocks were light gray, light grayish brown, and light grayish yellow fine sandstone, with good permeability, characterized by shallow burial depth, large thickness, and high uranium grade, etc. The reduction environment, represented by grey sandstone and carbonaceous detritus of braided fluvial facies in Huachi-Huanhe Formation, was favorable for uranium mineralization. The uranium ore was generally associated with organic matter such as carbonaceous detritus, and uranium ore existed in layers rich in reductants such as plant debris and carbon chips (Figure 4d).
6 Conclusions
The lower Cretaceous desert sediments, from bottom to top, can be divided into Luohe Formation, Huachi-Huanhe Formation, Luohandong Formation, and Jingchuan Formation in the northwest of Ordos Basin. The lower Cretaceous had been divided into two large sedimentary cycles, the lower sedimentary cycle was from Luohe Formation to Huachi-Huanhe Formation and the upper sedimentary cycle was from Luohandong Formation to Jingchuan Formation. There developed three ancient desert sedimentary systems, such as alluvial system, aeolian system, and desert lake system.
There were two dry to wet paleoclimatic changes in Early Cretaceous, which controlled the sedimentary paleogeographic environment. The tectonic evolution controlled paleogeographic evolution. The sedimentary facies had a control effect to the development of aquifer system. The paleogeographic environment in Early Cretaceous was favorable for uranium enrichment and the development of metallogenic units.
Acknowledgements
The authors thank the journal editors and reviewers.
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Funding information: This research was supported by Key R&D and promotion projects in Henan Province (No. 232102320319), Doctoral program of Nanyang Normal University (No. 2022ZX040), Cultivation Fund of National Natural Science Foundation of Nanyang Normal University (No. 2022PY009), China’s Ministry of Education University-Industry Cooperative Education Program (No. 220902284303850), and Jiangxi Provincial Natural Science Foundation (No. 20202BABL211019).
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Author contributions: Yangwei Feng conceptualized the research idea, performed statistical analysis, and interpreted the result and discussion; Yan Ren and Ting Jiang helped to obtain the data and perform the calculation; Yangwei Feng, Fengxian Lu, and Fei Xia participated in field geological survey; Yan Ren, Jiazeng Guo and Yangwei Feng drew the map and joined the revision. All authors have read and agreed to the published version of the manuscript.
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Conflict of interest: The authors declare no conflict of interest.
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Institutional review board statement: Not applicable.
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Informed consent: Not applicable.
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Data availability statement: Not applicable.
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