Sedimentary and Early Diagenetic Responses to the Huaiyuan Movement During the Early–Middle Ordovician Transition in the Ordos Basin, North China

Abstract

The early Paleozoic Huaiyuan Movement created a major unconformity in the Ordos Basin, significantly influencing sedimentation and early diagenesis in both the overlying and underlying strata near the unconformity. However, the origins of the associated dolomite and silica near this unconformity remain poorly understood. This study aims to reveal how this tectonic event controlled the Early–Middle Ordovician sedimentary environments and early diagenetic processes. The petrological and geochemical results indicate a progressive transition from a dolomitic tidal flat to an intra-platform depression, culminating in a mixed tidal flat during the Early-to-Middle Ordovician, driven by the Huaiyuan Movement. Furthermore, this movement, accompanied by intense weathering and erosion, increased the supply of marine dissolved silica (DSi) and terrestrial nutrients. Consequently, extensive tidal-edge biogenic silica accumulated, which later precipitated as siliceous-cemented dolomite during a shallow-burial stage. We propose a conceptual model of the sedimentary–early diagenetic processes in response to the Huaiyuan Movement, providing novel insights into the regional paleoenvironmental evolution across the Early–Middle Ordovician transition in the Ordos Basin.

1. Introduction

The Huaiyuan Movement was a significant regional tectonic event during the Early Paleozoic in the North China Craton [1,2,3,4], marking an early phase in the Caledonian tectonic cycle. This movement exhibited a multi-stage “episodic” nature, spanning from the late Middle Cambrian to the Early Ordovician [5]. It caused various changes in the geographic and sedimentary patterns of the North China Craton. Notably, the multi-stage crustal uplift during the Huaiyuan Movement resulted in extensive stratigraphic unconformities within the cratonic basin from the Late Cambrian to the Middle Ordovian [5,6,7]. Previous studies of the regional unconformity surfaces caused by the Huaiyuan Movement have focused on investigating the geological characteristics of the unconformity surfaces [8], distribution patterns [9,10], development of paleo-karst [11,12], and their significance in relation to oil and gas exploration [13].

The stratigraphic boundary between the Early Ordovician Liangjiashan Formation and the Middle Ordovician Majiagou Formation in the Ordos Basin represents an important unconformity surface formed during the Huaiyuan Movement [11]. Mao et al. (2024) reconstructed the pre-Ordovician paleo-geomorphology of the Ordos Basin, showing a west-high–east-low topographic gradient during the initial phase of the Huaiyuan Movement [9]. Driven by early-stage tectonic activity and concurrent global sea level fall, the Liangjiashan Formation was restricted to the southeastern area of the basin and localized western margins [14]. However, complex diagenetic overprinting has limited the study of the Liangjiashan Formation along the southeastern margin of the basin, particularly regarding the disparities among the sedimentary environments across the Huaiyuan Movement unconformity and their impact on early diagenetic processes. The study of the strata across this unconformity is essential for understanding how the Huaiyuan Movement influenced both the sedimentary environments and the early diagenetic evolution during the Early-to-Middle Ordovician transition in the Ordos Basin.

The Huaiyuan Movement generated a paraconformity, i.e., a depositional hiatus with little or no noticeable erosional features, along the southeastern Ordos Basin margin, separating the mudstone at the bottom of the Majiaogou Formation and the dolomite at the top of the Liangjiashan Formation. Siliceous-cemented dolomite developed in the uppermost area of the Liangjiashan Formation, which was initially identified in the Songshan area [15]. Tian (1989) attributed the unique lithology below the parallel unconformity to the Huaiyuan Movement, and identified two phases of dolomitization and two phases of silicification [15]. Furthermore, chert bands and nodules are pervasive in the dolomite beneath the unconformity [5,16,17], with enlarged dolomite crystals adjacent to the siliceous cements [6]. While dolomite occurs extensively on both sides of the unconformity, significant textural and compositional variations exist between them. Resolving the coupling between the siliceous diagenesis and the Huaiyuan Movement, as well as the contrasting dolomite characteristics across the unconformity, is critical for reconstructing the differential diagenetic evolution across the Huaiyuan Movement unconformity.

This study focuses on the Liangjiashan Formation–Majiaogou Formation of the Xipo section in the southeastern margin of the Ordos Basin. Based on field observations, thin-section studies, and geochemical analyses, we analyze the lithological variations, sedimentary environments, and the evolution of the early diagenetic processes during the transition from the Early-to-Middle Ordovician. Additionally, we investigate the formation mechanisms of the dolomite and explore the origin of the silica by using a binary mixing model of the Ge/Si ratios. Finally, we establish a response model of the sedimentary–early diagenetic processes in relation to the Huaiyuan Movement.

2. Geological Setting

The Ordos Basin, located in the southwestern North China Craton, is the second-largest sedimentary basin (~2.5 × 10⁵ km²) in China [18]. Bordered by the Helan–Yinshan Mountains (west/north), Lvliang Mountains (east), and Qinling Mountains (south) (Figure 1a), this stable polycyclic cratonic basin contains abundant petroleum, natural gas, and coal resources [19].

During the Early Paleozoic, the Ordos Block was dominantly influenced by the Paleo-Asian Ocean to the north and the Paleo-Qinling–Qilian Ocean system to the south [20]. During the deposition of the Liangjiashan Formation, the intensified northward subduction of the Paleo-Asian Ocean formed the North Margin Depression Belt (Figure 1b). The interplay between this northern belt and the southern/southwestern passive margins ultimately triggered the Huaiyuan Movement—a major Early Paleozoic crustal uplift event that significantly reshaped the North China Block [22,23]. Substantial evidence indicates that the Huaiyuan Movement involved polyphase tectonic uplift spanning the Late Cambrian to Early Ordovician [5,6,11]. The Huaiyuan Movement progressively expanded the central paleo-uplift in the basin, triggering a widespread marine regression and depositional hiatus, and culminating in a regional unconformity beneath the Middle Ordovician Majiagou Formation [24] (Figure 1c). Previous studies have demonstrated that the unconformity caused by the Huaiyuan Movement displays clear zonal variations. The paleo-uplift zone, which underwent maximum tectonic uplift, developed high-angle unconformities that progressively decreased eastward into low-angle unconformities and, ultimately, paraconformity zones [7] (Figure 1a).

The studied Xipo section, located on the southeastern Ordos Basin margin, recorded minimal Huaiyuan Movement influence, exhibiting only a stratigraphic hiatus between the Lower Ordovician Liangjiashan and Middle Ordovician Majiagou Formations, marked by a paraconformity [24]. Both formations are well-exposed in the outcrop, with basal conglomerates at their contact serving as a diagnostic paraconformity indicator (Figure 2a). The lithology below the unconformity primarily consists of cryptocrystalline-to-microcrystalline dolomite, sharply overlain by a 30 cm siliceous-cemented dolomite layer (Figure 2b). Above the unconformity, capped by a 1 m thick transgressive succession of mudstones interbedded with medium-thin sandstone layers (Figure 2c), sandy microcrystalline dolomite dominates the Majiagou Formation (Figure 2d).

3. Materials and Methods

3.1. Sample Preparation

The dolomite samples in this study were obtained from the Xipo (XP) section located in the southeastern Ordos Basin, encompassing the Liangjiashan Formation and the Majiagou Formation. A total of 10 dolomite hand specimens were collected, including 7 samples from the Liangjiashan Formation and 3 samples from the Majiagou Formation. All the samples were taken from fresh outcrop surfaces and systematically sampled at regular intervals of approximately 30 cm. All the samples were cut into mirrored thin sections (30 μm) and thick sections (2 mm). The samples were stained with Alizarin Red S solution and examined under a microscope for a thin-section analysis to determine the lithology. The sample powder (approximately 50 mg) was obtained through micro-drilling at corresponding positions of the thick sections under the guidance of the thin-section observations. The carbonate components of the samples underwent an elemental analysis and a carbon and oxygen isotope analysis. Additionally, 10 samples of siliceous-cemented dolomite from the top of the Liangjiashan Formation were collected for major, trace, and rare earth elements (REEs) analyses of the siliceous detrital components.

3.2. Major, Trace, and Rare Earth Elements Analyses

3.2.1. Carbonate Component Analysis

The powdered samples were micro-drilled into a particle size of 200 mesh. The dissolution of the carbonate component was carried out as follows: Approximately 30 mg of the dolomite powder sample was placed into a 15 mL centrifuge tube, followed by the addition of 5 mL of 0.5 mol/L acetic acid. Subsequently, the centrifuge tube was placed in an ultrasonic bath for 30 min and then centrifuged at 3000 rpm for 10 min to guarantee the complete dissolution of the carbonate component. The supernatant was collected for an elemental composition analysis. Major, trace, and rare earth elements analyses were conducted at the Key Laboratory of Carbonate Reservoir of China National Petroleum Corporation using an ICP-MS. The analysis was calibrated by utilizing a set of standard samples with concentrations ranging from 0.1 × 10⁶ to 10 × 10⁶. These standards were tested before and after every 20 samples to ensure the accuracy and reliability of the measurements.

3.2.2. Siliceous Component Analysis

The pretreatment of the siliceous component was carried out as follows: After dissolving the carbonate components, the residual was washed with 5 mL of 1N HCl to remove any remaining carbonate residues. Following this, deionized water was added for rinsing, and the mixture was centrifuged and separated five times to thoroughly wash out any residual Cl⁻ ions, thereby preventing the formation of volatile GeCl_4, which could lower the measurement of Ge. After washing, the sample was wrapped in aluminum foil and placed in a 50 °C oven to dry for 12 h. Approximately 50 mg (30–50 mg) of the residue was weighed into a clean polytetrafluoroethylene (PTFE/Teflon, 7 mL) beaker, and then dissolved in 1 mL of concentrated HNO₃ and 3 mL of concentrated HF. After heating at 130 °C for 12 h, 5 mL of concentrated HNO₃ was added and heated on a hot plate for another 12 h. If the solution had clarified, 5 mL of 2% HNO₃ was added and transferred into a test tube for analysis. The solution was re-dissolved until clarification was achieved to see if any undissolved substance remained. The major, trace, and rare earth elements analyses of the siliceous components were conducted on an ICP-MS at the Key Laboratory for Carbonate Reservoir of China National Petroleum Corporation.

3.3. Carbon and Oxygen Isotope Analyses

The carbon and oxygen isotope analyses were performed by the orthophosphoric acid method on a Finnigan MAT252 stable isotope ratio mass spectrometer at the Geochemistry Laboratory of Yangtze University. The sampled carbonate powders were reacted with 100% phosphoric acid at 25 °C for 24 h to generate CO₂. The liberated CO₂ gas was then purified cryogenically for analysis. The δ¹³C and δ¹⁸O values were expressed as per mil (‰) V-PDB, with a precision of 0.1‰ and 0.05‰, respectively.

4. Results

4.1. Petrological Characteristics

The Lower Ordovician Liangjiashan Formation and the overlying Middle Ordovician Majiagou Formation are dominated by dolomite and mudstone. The lower part of the Majiagou Formation consists of approximately 1 m thick gray-green mudstone, interbedded with thin-to-medium-bedded gray-brown muddy sandstone. The dolomite can be classified into three types based on the rock fabric, including sandy dolomite, siliceous-cemented dolomite, and crystalline dolomite. Furthermore, the crystalline dolomite can be divided into cryptocrystalline–microcrystalline dolomite and fine-to-medium crystalline dolomite, according to the crystal morphology.

The cryptocrystalline–microcrystalline dolomite is characterized by a subhedral (planar-s)-to-euhedral (planar-e) dolomite with a crystal size ranging from 0.01 to 0.05 mm (Figure 3a). Most dolomite crystals interlock with each other with few visible pores and fractures. This type of dolomite developed at the top of the Liangjiashan Formation and the top of the Majiagou Formation in the studied section.

The fine-to-medium crystalline dolomite is dominated by anhedral rhombs of finely sized crystals ranging from 0.05 mm to 0.15 mm. The dolomites are tightly packed, with the development of straight, compromise boundaries; thus, the original rock fabric is normally eliminated (Figure 3b). This type of dolomite was significantly altered by late diagenetic fluids, with obvious recrystallization and secondary enlargement rims (Figure 3b,c). This type of dolomite occurs in the middle part of the Liangjiashan Formation.

Siliceous-cemented dolomite differs from other dolomites by its siliceous cementation. The dolomite crystals are highly euhedral with sizes ranging from 0.1 mm to 0.15 mm, and most have a limpid rim and a turbid core, displaying a zonation fabric (Figure 3d). The dolomite crystals are cemented by cryptocrystalline silica (Figure 3f). The euhedral dolomite rhombs are loosely scattered in the siliceous component. Additionally, distinct evidence of dolomite crystal replacement by siliceous is observable (Figure 3e), indicating that the recrystallization of the dolomite preceded the process of siliceous cementation. The intra-crystalline pores within the dolomite crystals are well-developed and, in some cases, filled with calcite (Figure 3g). This type of rock developed at the top of the Liangjiashan Formation, in direct contact with the unconformity surface.

Sandy dolomite contains a higher content of terrigenous clastic particles, mainly comprising well-rounded quartz grains, feldspar, and cryptocrystalline chert (Figure 3h). The dolomite crystals are primarily micritic, which can hardly be identified by optical microscopy. Well-formed euhedral dolomite crystals can be observed (Figure 3i). This kind of dolomite occurs at the base of the Majiagou Formation.

4.2. Geochemical Characteristics of Carbonate Components

4.2.1. Major and Trace Elements

The results of the major and trace elements analyses of the carbonate components are shown in

Table 1. Summary of statistics of δ¹³C, δ¹⁸O, and major and trace element measurements of dolomite in Liangjiashan Formation and Majiagou Formation. The δ¹³C and δ¹⁸O values are in ‰ VPDB.

Sample Number	No.	Stratum	Lithology	Concentration (ppm)							δ13CPDB (‰)	δ18OPDB (‰)
				Mg (ppm)	Al (ppm)	Sr (ppm)	Ca (ppm)	Fe (ppm)	Mn (ppm)	Ba (ppm)
XPS1-1	S1-1	Liangjiashan Formation	Mud–Microcrystalline dolomite	133,897	300	89	230,368	1911	154	2	−3.26	−6.80
XPS1-2	S1-2		Fine–medium crystalline dolomite	134,242	90	83	228,109	2817	168	2	−2.97	−7.15
XPS1-3	S1-3		Fine crystalline dolomite	136,968	174	86	230,378	2277	160	2	−3.64	−7.22
XPS1-4	S1-4		Fine–medium crystalline dolomite	130,518	190	98	226,301	2329	183	2	−3.33	−6.86
XPS1-5	S1-5		Fine–medium crystalline dolomite	128,170	553	74	219,602	3010	165	3	−3.13	−7.52
XPS1-6	S1-6		Siliceous-cemented dolomite	87,222	1643	51	154,744	3710	215	7	−3.14	−8.21
	S1-7		Siliceous-cemented dolomite	90,296	1609	52	159,367	3675	216	5	−3.18	−8.20
XPS1-7	S1-8		Siliceous-cemented dolomite	105,674	805	58	180,361	3752	243	6	−2.91	−7.54
	S1-9		Siliceous-cemented dolomite	107,964	576	60	184,499	3679	256	3	−2.83	−7.63
XPS1-8	S1-10	Majiagou Formation	Sandy dolomite	70,982	3194	61	119,858	5604	207	4	−1.57	−8.69
	S1-11		Sandy dolomite	81,373	2856	65	137,522	5518	215	60	−1.46	−9.49
XPS1-9	S1-12		Sandy dolomite	87,690	7434	73	151,281	6931	261	57	−1.12	−7.61
	S1-13		Sandy dolomite	87,131	2360	64	147,010	4488	249	16	−1.16	−7.54
XPS1-10	S1-14		Sandy dolomite	94,129	2936	66	161,411	6327	275	8	−1.13	−7.45
	S1-15		Sandy dolomite	92,365	6814	77	160,741	6289	275	5	−1.20	−7.60

. The Sr values of the sandy dolomite range from 61 ppm to 77 ppm, with an average value of 68 ppm. The Sr values of the siliceous-cemented dolomite range from 51 ppm to 60 ppm, with an average value of 55 ppm. The Sr values of the crystalline dolomite range from 74 ppm to 98 ppm, with an average value of 86 ppm. The results show that the Sr content of the sandy dolomite is comparable to that of the siliceous-cemented dolomite, while the crystalline dolomite has a higher Sr content than the other two types of dolomites. The Sr content in the profile exhibits significant variations (Figure 4). Within the dolomite of the Liangjiashan Formation, a distinct pattern is observed where the Sr content initially increases and subsequently decreases, ultimately stabilizing upon reaching the upper-siliceous-cemented dolomite.

The Fe and Mn content exhibits disparities between the Liangjiashan Formation and the Majiagou Formation. The Fe and Mn values of the sandy dolomite range from 4488 ppm to 6931 ppm and 207 ppm to 275 ppm. The Fe and Mn values of the siliceous-cemented dolomite range from 3675 ppm to 3752 ppm and 215 ppm to 256 ppm. The Fe and Mn values of the crystalline dolomite range from 1911 ppm to 3010 ppm and 154 ppm to 183 ppm (Table 1). The results indicate that the Fe and Mn content of the siliceous-cemented dolomite is higher than that of the crystalline dolomite. Notably, the Fe and Mn content of the sandy dolomite is significantly higher than that of the other two rock types, which can be attributed to the elevated detrital component concentration within the rock itself.

4.2.2. Carbon and Oxygen Isotopes

The carbon and oxygen isotopic data are shown in Table 1 and plotted in Figure 5. The δ¹³C_VPDB values for the sandy dolomite, siliceous-cemented dolomite, and crystalline dolomite range from −1.57‰ to −1.12‰ (avg. −1.21‰), −3.18‰ to −2.83‰ (avg. −3.02‰), and −3.64‰ to −2.97‰ (avg. −3.27‰), respectively. The δ¹⁸O_VPDB values for these three types of dolomites range from −9.49‰ to −7.45‰ (avg. −8.06‰), −8.21‰ to −7.54‰ (avg. −7.89‰), and −7.52‰ to −6.80‰ (avg. −7.11‰), respectively. According to previous studies, the δ¹³C_VPDB values of Ordovician seawater have been found to range from −2.00‰ to 0.50‰, while the δ¹⁸O_VPDB values range from −6.60‰ to −4.00‰ [25]. The siliceous-cemented dolomite and crystalline dolomite in the Liangjiashan Formation exhibit more negative δ¹³C_VPDB and δ¹⁸O_VPDB values compared to the contemporaneous Ordovician seawater, whereas the sandy dolomite in the Majiagou Formation shows a comparable δ¹³C_VPDB value with the contemporaneous Ordovician seawater, but a more negative δ¹⁸O_VPDB value than that of the contemporaneous seawater (Figure 5).

4.2.3. Rare Earth Elements of Carbonate Components

To evaluate the REEs fractionation patterns, we used the Post-Archean Australian Shale (PAAS) normalization method to eliminate the interference of “even-odd” effects. The REEs compositions are presented in

Table 2. Rare earth elements test data for dolomite in Liangjiashan Formation and Majiagou Formation.

Sample Number	No.	Stratum	W (Rare Earth Elements)/μg/g															ΣREEs	ΣMREE	(Nd/Yb)N	Ce/Ce*
			La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
XPS1-1	S1-1	Liangjiashan Formation	1.09	0.83	0.52	1.65	0.54	1.63	1.56	1.92	2.04	2.09	4.83	5.13	6.51	4.48	5.09	7.30	0.75	0.94	1.11
XPS1-2	S1-2		2.66	2.34	1.35	3.50	1.26	6.37	6.23	8.08	8.44	8.84	15.44	16.63	18.04	13.48	14.53	4.90	0.34	1.04	1.56
XPS1-3	S1-3		0.28	0.14	0.13	0.38	0.15	0.58	0.56	0.61	0.63	0.64	1.78	1.74	1.99	1.51	1.57	3.54	0.35	0.91	1.20
XPS1-4	S1-4		1.09	0.51	0.50	1.39	0.67	2.44	2.32	2.48	2.50	2.57	7.54	7.08	7.29	5.85	5.94	9.78	0.99	1.02	1.03
XPS1-5	S1-5		0.23	0.10	0.10	0.32	0.20	0.62	0.59	0.64	0.64	0.65	2.08	1.87	1.67	1.50	1.43	4.24	0.59	0.92	1.00
XPS1-6	S1-6		0.05	0.02	0.02	0.06	0.04	0.13	0.13	0.14	0.15	0.14	0.45	0.40	0.33	0.31	0.31	17.58	2.12	0.76	1.48
	S1-7		0.22	0.11	0.11	0.31	0.18	0.65	0.63	0.66	0.70	0.72	2.13	1.92	1.60	1.48	1.47	16.99	2.06	0.74	1.50
XPS1-7	S1-8		0.04	0.02	0.02	0.05	0.03	0.11	0.10	0.11	0.12	0.12	0.34	0.31	0.26	0.25	0.24	20.14	2.21	0.63	1.70
	S1-9		1.15	0.63	0.56	1.53	0.84	3.66	3.47	3.91	4.36	4.30	9.89	9.76	7.54	7.32	7.55	21.93	2.40	0.53	1.74
XPS1-8	S1-10	Majiagou Formation	0.04	0.02	0.02	0.05	0.03	0.12	0.12	0.13	0.14	0.15	0.37	0.34	0.29	0.27	0.26	48.87	6.92	0.73	1.18
	S1-11		0.11	0.05	0.05	0.14	0.08	0.32	0.32	0.37	0.41	0.42	0.98	0.90	0.78	0.72	0.69	48.92	6.29	0.73	1.26
XPS1-9	S1-12		0.02	0.01	0.01	0.02	0.01	0.04	0.04	0.05	0.06	0.06	0.14	0.13	0.12	0.11	0.10	48.72	5.35	0.84	1.15
	S1-13		0.10	0.04	0.05	0.11	0.06	0.27	0.26	0.33	0.39	0.41	0.86	0.80	0.72	0.68	0.60	39.46	4.95	0.72	1.18
XPS1-10	S1-14		0.01	0.01	0.01	0.02	0.01	0.04	0.04	0.05	0.06	0.06	0.12	0.12	0.10	0.10	0.09	41.25	4.83	0.82	1.17
	S1-15		1.09	0.83	0.52	1.65	0.54	1.63	1.56	1.92	2.04	2.09	4.83	5.13	6.51	4.48	5.09	49.29	5.26	0.96	1.12

. The total REEs (ΣREEs) in the dolomite of the Majiagou Formation range from 39.46 μg/g to 48.92 μg/g (avg. 46.09 μg/g). In contrast, the ΣREEs in the dolomite of the Liangjiashan Formation vary between 3.54 μg/g and 21.93 μg/g (avg. 12.77 μg/g). The dolomite of the Majiagou Formation exhibits a higher ΣREEs content compared to that of the Liangjiashan Formation. Notably, significant variations in the ΣREEs are observed among the different types of dolomite within the Liangjiashan Formation. The ΣREEs in the crystalline dolomite vary between 2.54 μg/g and 9.78 μg/g (avg. 5.95 μg/g). The ΣREEs in the siliceous-cemented dolomite vary between 16.99 μg/g and 21.93 μg/g (avg. 19.60 μg/g), which is much higher than that in the crystalline dolomite, and may also be related to the alteration and metasomatism of the upper rocks by later-stage silica-rich fluids. The variation extent of the LREEs/HREEs ratio ((Nd/Yb)_N) in the three types of dolomite ranges from 0.92 to 1.04, 0.72 to 0.96, and 0.53 to 0.76, respectively, thus featuring a typical LREEs deficiency and HREEs enrichment. The anomalous Ce/Ce* ratio for the siliceous-cemented dolomite is obvious, as it varies between 1.48 and 1.74, with an average of 1.63 (Table 2), which is higher than that of the sandy dolomite (avg. 1.18) and the crystalline dolomite (avg. 1.18), indicating the Ce/Ce* values of the siliceous-cemented dolomite differ significantly from those of the other two types of dolomite.

All three types of dolomite exhibit relatively flat rare earth elements distribution patterns, with the middle rare earth elements (MREEs) being relatively high in concentration. However, there are significant differences in the ΣMREEs, showing a trend of ΣMREEs (sandy dolomite) > ΣMREEs (siliceous-cemented dolomite) > ΣMREEs (crystalline dolomite) (Figure 6). Vertically, the ΣMREEs gradually increase upwards in the Liangjiashan Formation, while the ΣMREEs in the Majiagou Formation are higher but show a downward trend towards the top (Figure 4).

4.3. Geochemical Characteristics of Siliceous Components

4.3.1. Major and Trace Elements and Ge/Si Ratio

The elemental content data for the siliceous components of the dolomite at the top of the Liangjiashan Formation are shown in

Table 3. Major and trace element test data for siliceous components from siliceous-cemented dolomite at the top of the Liangjiashan Formation.

No.	Major Element Content (%)								Zr/%	Ge/ (μg/g)	(Ge/Si)/ (μmol/mol)
	Al2O3	TFe2O3	K2O	MgO	CaO	MnO	Na2O	SiO2
XPS1-6-1	5.88	0.80	1.54	0.65	0.072	0.0012	0.12	89.45	0.035	1.83	1.66
XPS1-6-2	6.08	0.98	1.65	0.68	0.079	0.0017	0.12	88.76	0.038	1.32	1.20
XPS1-6-3	5.94	1.03	1.55	0.67	0.086	0.0019	0.12	88.75	0.040	1.32	1.21
XPS1-6-4	5.83	0.75	1.55	0.65	0.083	0.0016	0.12	89.35	0.039	1.31	1.19
XPS1-6-5	5.94	1.03	1.56	0.67	0.083	0.0019	0.12	88.97	0.039	1.39	1.27
XPS1-7	7.85	2.14	2.25	0.89	0.088	0.0034	0.12	84.93	0.056	1.63	1.56
XPS1-7-2	7.76	2.44	2.19	0.90	0.086	0.0049	0.12	84.56	0.055	1.56	1.50
XPS1-7-3	6.64	1.20	1.79	0.74	0.077	0.0033	0.12	87.69	0.041	1.47	1.36
XPS1-7-4	6.65	1.33	1.81	0.75	0.086	0.0037	0.12	87.34	0.043	1.45	1.35
XPS1-7-5	6.74	1.28	1.80	0.75	0.074	0.0043	0.12	87.61	0.044	1.53	1.42

. The SiO₂ values vary from 84.56% to 89.45% (avg. 87.74%), the Al₂O₃ values vary from 5.83% to 7.85% (avg. 6.53%), the Fe₂O₃ values vary from 0.75% to 2.44% (avg. 1.30%), the K₂O values vary from 1.54% to 2.25% (avg. 1.77%), the MgO values vary from 0.65% to 0.90% (avg. 1.12%), the CaO values vary from 0.072% to 0.088% (avg. 0.08%), and the MnO values vary from 0.0012% to 0.0049% (avg. 0.0028%). The Ge/Si ratio of the samples (μmol/mol) varies from 1.19 to 1.66 (avg. 1.37). Fe₂O₃ and MnO have a positive correlation (Figure 7a).

4.3.2. Rare Earth Elements of Siliceous Components

The rare earth elements (REEs) were normalized using the Australian Archaean Shale (PAAS) standard (

Table 4. Rare earth elements test data for siliceous components from siliceous-cemented dolomite at top of Liangjiashan Formation.

No.	W (Rare Earth Elements)/μg/g															ΣREEs	Eu/Eu*	Ce/Ce*
	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Y	Ho	Er	Tm	Yb	Lu
XPS1-6	0.27	0.24	0.22	0.20	0.11	0.07	0.13	0.08	0.08	0.07	0.09	0.13	0.16	0.16	0.16	44.32	0.60	0.98
XPS1-6-2	0.31	0.28	0.25	0.23	0.14	0.09	0.15	0.10	0.10	0.09	0.11	0.14	0.18	0.19	0.19	51.59	0.62	0.99
XPS1-6-3	0.32	0.28	0.26	0.23	0.14	0.09	0.15	0.10	0.10	0.09	0.11	0.15	0.17	0.19	0.19	52.97	0.62	0.98
XPS1-6-4	0.30	0.27	0.25	0.22	0.13	0.09	0.14	0.10	0.09	0.09	0.11	0.14	0.16	0.18	0.19	50.73	0.63	0.98
XPS1-6-5	0.31	0.28	0.25	0.23	0.14	0.09	0.15	0.10	0.10	0.09	0.11	0.14	0.17	0.19	0.19	51.67	0.61	0.99
XPS1-7	0.46	0.43	0.40	0.38	0.25	0.15	0.25	0.16	0.15	0.13	0.16	0.21	0.25	0.27	0.27	80.34	0.62	1.00
XPS1-7-2	0.45	0.42	0.40	0.38	0.26	0.15	0.25	0.17	0.16	0.14	0.17	0.21	0.25	0.26	0.27	79.52	0.60	1.00
XPS1-7-3	0.36	0.33	0.30	0.28	0.17	0.11	0.19	0.12	0.11	0.10	0.12	0.16	0.18	0.20	0.20	60.66	0.60	0.99
XPS1-7-4	0.38	0.34	0.32	0.30	0.18	0.11	0.19	0.13	0.12	0.11	0.13	0.16	0.20	0.21	0.21	64.48	0.59	0.99
XPS1-7-5	0.39	0.36	0.33	0.31	0.20	0.13	0.20	0.13	0.12	0.11	0.14	0.17	0.20	0.22	0.23	66.87	0.63	1.00

) [26,27]. The ΣREEs in the siliceous components of the siliceous-cemented dolomite range from 44.32 μg/g to 80.34 μg/g, with an REEs partition pattern characterized by enriched LREEs and a relatively high La_N/Yb_N ratio (avg. 1.7, n = 10). The REEs partition patterns of all the samples present a negative Eu anomaly (Eu_N/Eu*_N average value of 0.6), with no Ce anomaly (Ce_N/Ce*_N average value of 0.99, n = 10) (Table 4).

5. Discussion

5.1. Evolution of Sedimentary Environments

The lithology, sedimentary structure, and geochemical characteristics of the strata above and below the unconformity in the study area exhibit significant disparities, further indicating distinct depositional environments. Based on the previous research and a petrological analysis of the study area, we provide a comprehensive interpretation of the depositional environments for both the Liangjiashan and Majiagou Formations in the Xipo section.

The crystalline dolomite in the lower part of the Liangjiashan Formation is pure carbonate without discernible terrigenous clastics, indicating a stable depositional environment without terrigenous inputs. However, intense recrystallization and a near-complete diagenetic alteration of these rocks have obscured their original lithology, complicating the reconstruction of their sedimentary environments. Previous studies on the Liangjiashan Formation in the Ordos Basin have provided critical insights into its depositional setting. Li et al. (2012, 2021) identified the formation as predominantly medium-to thick-bedded dolomites intercalated with relict grainstone dolomites, locally featuring silicified stromatolites [19,21]. Guo et al. (2019) further documented sedimentary structures, such as microbial laminations, stromatolites, and cross-bedding, in the outcrops [22]. These features collectively support a tidal flat environment.

Although the first member of the Majiagou Formation (Ma-1) was also deposited in a marine environment [22], its depositional environment changed significantly compared to that of the Liangjiashan Formation. The base of the Ma-1 member is characterized by a 1 m thick layer of dark mudstone (Figure 2), representing a rapid transgression at the onset of the Majiagou Formation. This interval is inferred from the recorded sedimentation in an intra-platform depression during the early transgression. As the sea level gradually fell, a succession of sandy micritic dolomite was deposited, showing an upward-decreasing terrigenous clastic content and transitioning into relatively pure micritic dolomite. This trend reflects the progressively clearer seawater conditions. Based on these observations, we propose that the Ma-1 member was deposited in a mixed siliciclastic–carbonate tidal flat environment near the paleocontinent, characterized by predominant marine carbonate deposition with intermittent influxes of terrigenous clastic inputs.

During the Early–Middle Ordovician, the depositional environment in the study area evolved from a dolomitic tidal flat to an intra-platform depression, culminating in a mixed siliciclastic–carbonate tidal flat system. This environmental evolution reflects the global eustatic regression during the terminal Early Ordovician, driven by the Huaiyuan Movement-induced sea level fall. The Huaiyuan Movement at the end of the Early Ordovician created a paraconformity between the Majiagou and Liangjiashan Formations in the study area [7]. The movement significantly altered the paleogeographic framework of the basin, causing seawater to retreat southeastward and transforming the southeastern margin from a dolomitic tidal flat to an intra-platform depression. Consequently, the Huaiyuan Movement profoundly influenced the sedimentary evolution of the Ordos Basin during the Early–Middle Ordovician transition.

5.2. Characteristics of Early Diagenesis

Early diagenesis refers to a series of physical, biological, and chemical changes among the sediments, pore water, and ambient water during deposition and shallow burial under conditions where the pore water remains fully saturated [28]. The petrographic and geochemical analyses of the study section reveal two distinct early diagenetic processes, silica cementation and dolomitization.

5.2.1. Siliceous Cementation

Source of Silica

(1)

Identification of silica source

Here we employ the Ge/Si (germanium/silicon) ratios of the siliceous components as a proxy to investigate the source of the silica. The utilization of the Ge/Si ratio for discerning the source of silica has been widely applied in previous studies [29,30]. Ge and Si, both belonging to Group 4 of the periodic table, have similar chemical properties and ionic radii. The similarity in the bond lengths between tetrahedral Ge-O (1.75 μm) and Si-O (1.64 μm) bonds allows Ge to substitute for Si in silicate minerals [31]. The Ge/Si ratio varies significantly across different geological reservoirs [32]. During the weathering of silicate rocks, the formation of metal oxides and aluminosilicate clay minerals can incorporate Ge through the isomorphic substitution of Si or adsorption, resulting in higher Ge/Si ratios (approximately 4–6 μmol/mol) in secondary minerals, such as clay minerals [33]. Consequently, the concentrations of dissolved germanic acid and silicic acid (or Ge(OH)₄ and Si(OH)₄) in river water are relatively low, with Ge/Si ratios typically ranging from 0.1 to 3 μmol/mol [34]. In hydrothermal fluids, the Ge/Si ratio is largely influenced by the thermodynamic differences between Ge and Si compounds, requiring high-temperature water–rock interaction equilibria. As a result, hydrothermal fluids often exhibit significantly higher Ge/Si ratios, generally ranging from 4 to 25 μmol/mol [35]. Thus, it is possible to determine the source of Si in siliceous sediment by the Ge/Si ratio.

In this study, the primary elemental composition of the siliceous-cemented dolomite is derived from a mixture of four components, including silica, dolomite, clay minerals, and metal oxides. The preprocessing steps effectively removed the carbonate components, thereby excluding the influence of dolomite on the Ge/Si ratio. However, given that clay minerals and metal oxides have higher Ge/Si ratios and cannot be distinguished from the siliceous component during measurement, it is necessary to evaluate the effect of the clay minerals’ Ge/Si ratios before determining the source of the silica. In the siliceous-cemented dolomite from the top of the Liangjiashan Formation, a positive correlation is observed between the Ge/Si ratio and Al₂O₃ content (Figure 7b), indicating the effect of clay minerals. Additionally, K₂O, TFe₂O₃, MnO, and Al₂O₃ show strong positive correlations (Figure 7c–e) and significant intercorrelations with each other, implying that apart from the terrestrial silica input, there was also an influx of iron and manganese oxides. However, the total mass of the silica and clay components ranges from 95.53 wt% to 97.65 wt% (avg. 96.90 wt%), with negligible metal oxide concentrations (Table 3); therefore, the influence of the metal oxide on the Ge/Si ratio can be ignored. By using a mass balance model, the Ge/Si ratio of the siliceous component can be estimated by regressing the Al₂O₃ content to zero. According to this method, the calculated Ge/Si ratio of the siliceous component is determined to be 0.63, which is similar to the Ge/Si ratio of modern seawater (~0.72) [36,37,38]. Thus, the contribution of the hydrothermal-derived silica can be excluded.

Moreover, previous studies have demonstrated that the Ge/Si ratio of clay minerals can serve as an indicator of silicate weathering [37,39]. Kurtz et al.’s (2002) calculation of the net losses of Ge and Si in soils of varying ages suggests a positive correlation between the Ge/Si ratios and weathering intensity [34]. Therefore, the Ge/Si ratio of clay components is a robust proxy for tracing silicate weathering during the Huaiyuan Movement.

To quantify the Ge/Si system of the siliceous-cemented dolomite, we employ a binary mixing model of clay mineral and siliceous cement to reconstruct the Ge/Si ratios of the silica and clay [40]:

$$\begin{array}{c}{(Ge/Si)}_{\mathrm{s}iliceous component}\hfill \\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} ={\displaystyle \frac{f\times {(Ge/Si)}_{clay}\times {Si}_{clay}+\left(1-f\right)\times {(Ge/Si)}_{clay}\times {Si}_{{SiO}_2}}{f\times {Si}_{clay}+\left(1-f\right)\times {Si}_{{SiO}_2}}}\hfill \end{array}$$

(1)

where the subscriptions siliceous component, clay, and SiO2 denote the siliceous component of the siliceous-cemented dolomite, clay mineral component, and siliceous cement component, respectively.

Given that Al is exclusively present in clay components, it serves as a representative element for the clay mineral component, namely,

$${Al}_{silica}=f\times {Al}_{clay}$$

(2)

Montmorillonite and illite, being the most prevalent clay minerals, are taken as end-members in the calculations [40], and assuming a Ge/Si ratio of 0.4 (wt%/wt%) for quartz, which represents the maximum estimated value of the silica Ge/Si in the siliceous-cemented dolomite, the Ge/Si ratio for the clay endmember can be derived from Equation (1). As shown in Figure 8, the Ge/Si ratio for the clay mineral components ranges from 5 to 10, which is higher than that of fresh granite (3.1) [41]. The stability of these values suggests a consistent terrestrial input during that time, indicating enhanced weathering compared to previous periods.

In addition to the Ge/Si ratio, rare earth elements (REEs) can also serve as important indicators for determining the source of silica [42,43]. Murray et al. highlighted that Ce/Ce* (0.5La_N + 0.5Pr_N) can be used to indicate the source of silica, with marine-derived silica typically exhibiting negative Ce anomalies and an average Ce/Ce* value of 0.29 [44]. In this study, the average Ce/Ce* value for the silica component is 0.99 (Table 4), indicating that the silica did not directly originate from seawater. Additionally, the distribution patterns of the rare earth elements exhibit a noteworthy negative anomaly in Eu (Eu_N/Eu*_N < 1), which contrasts with the typical positive Eu anomaly observed in hydrothermal fluids (Eu_N/Eu*_N > 1) [45,46] (Figure 9). Furthermore, according to the Al-Fe-Mn ternary diagram [47], the Al, Fe, and Mn data do not fall within the region of hydrothermal origin (Figure 10). Therefore, based on an integrated analysis of the Ge/Si ratios, rare earth elements (REEs) signatures, and Al-Fe-Mn ternary relationships, we infer that the silica is of non-hydrothermal origin.

Building on this conclusion, we employ SiO₂-Zr correlations to further constrain the potential silica sources. Zirconium (Zr), primarily hosted in the heavy mineral zircon, serves as a robust proxy for silt-sized terrigenous input [48]. A negative SiO₂-Zr correlation suggests a biogenic silica origin, whereas a positive correlation implies detrital sources [48,49]. In the siliceous-cemented dolomite at the top of the Liangjiashan Formation, the observed negative SiO₂-Zr correlation (Figure 7f) supports a biogenic origin for the silica.

(2)

Origin of biogenic silica

The siliceous rocks at the top of the Liangjiashan Formation show regional stability [17], likely associated with the Huaiyuan Movement during the late Liangjiashan period. This tectonic event enhanced the dissolved silica (DSi) concentrations in seawater, stimulating the productivity of siliceous organisms. The subsequent diagenesis gradually transformed these biogenic accumulations into the bedded siliceous rocks observed at the top of the Liangjiashan Formation. This phenomenon exhibits global comparability. Kidder et al. (2016) compiled comprehensive data on Ordovician biogenic siliceous rocks, demonstrating a marked decline in their abundance from the Early-to-Middle Ordovician [50]. This reduction primarily reflects the diminished occurrence of shallow-water peritidal biogenic siliceous rocks, coinciding with the global sea level fall at the Early Ordovician’s termination. The extreme lowstand likely restricted the development of peritidal environments, while the consequent depletion of the shallow-water DSi potentially forced siliceous sponges to migrate into deeper, DSi-enriched waters. These combined factors would have substantially limited the formation of peritidal biogenic siliceous deposits.

The development of siliceous cements at the top of the Liangjiashan Formation in this study is controlled by the concentration of dissolved silica (DSi) in the ocean. Previous studies have demonstrated that the concentration of DSi in Precambrian oceans significantly exceeded that observed in Mesozoic oceans [51]. This disparity can be attributed to the substantial depletion of the DSi content caused by the proliferation of diatoms during the Mesozoic. Therefore, the concentration of DSi in the ocean controls the development and distribution of marine siliceous organisms [50]. There are three primary pathways for DSi input into the ocean, including the continental fluvial system, the dissolution of aeolian dust, and seafloor weathering and hydrothermal fluid circulation [52]. Among these pathways, river inputs and submarine groundwater discharge are the main sources of DSi in surface seawater [53]. Therefore, intense terrigenous weathering probably supplied biologically usable DSi and nutrients to the oceans, promoting the abundance of siliceous organisms.

Paleozoic organisms played a crucial role in regulating marine silica concentrations, consequently controlling the spatial distribution of siliceous sediments [54]. The enhanced riverine input of silica and nutrients would theoretically promote the nearshore accumulation of biogenic siliceous facies. This predicted pattern is exemplified by the deposits in the Pennsylvanian epicontinental sea of North America [55]. During the terminal Early Ordovician, the Huaiyuan Movement triggered intense tectonic uplift, enhancing continental weathering and erosion that substantially increased the dissolved silica (DSi) flux into marine environments. This uplift, combined with prolonged weathering and terrestrial nutrient inputs, promoted oceanic nutrient enrichment, facilitating the proliferation of peritidal radiolarians and siliceous sponges [56]. While these organisms generated significant biogenic silica, the scarce preservation of siliceous fossils in the rock records for this period can be ascribed to their dissolution [57].

Siliceous Cementation Process

As discussed above, during the deposition of the Early Ordovician Liangjiashan Formation, the study area was characterized by a dolomitic tidal flat environment where dolomitic mud sediments accumulated. By the end of the Liangjiashan Formation, the Huaiyuan Movement had promoted biogenic silica production within the tidal flat environment, resulting in its mixing with the dolomitic mud deposits. During the subsequent shallow burial stage, the biogenic silica-enriched micritic dolomite underwent early diagenesis in a closed-to-semi-closed system, ultimately forming the laminated siliceous-cemented dolomite presently observed at the top of the Liangjiashan Formation (Figure 2c).

The petrological analysis revealed that the siliceous-cemented dolomite consists of dolomite crystals and siliceous cement (Figure 3d). The dolomite crystals underwent recrystallization, as they are characterized by well-developed euhedral crystals with limpid rims and turbid cores. Understanding the genetic mechanism of silica-cemented dolomite requires clarification of the relationships between dolomite recrystallization and siliceous cementation processes. Silica replacement of the dolomite crystals is evident (Figure 3e), indicating that dolomite recrystallization preceded siliceous cementation.

Extensive studies have demonstrated that dolomite recrystallization is a dynamic dissolution–reprecipitation process [58,59,60], while siliceous sediments undergo diagenesis from opal-A to quartz via dissolution–reprecipitation [61]. Greenwood and O’Grady [62] outlined three stages: (1) opal-A dissolution, (2) opal-CT/C precipitation, and (3) microcrystalline quartz formation. Biogenic silica, initially deposited as amorphous opal-A by marine organisms (e.g., radiolarians, sponges, and siliceous flagellates) in low-temperature seawater, undergoes progressive diagenetic maturation. This transformation follows the stability sequence opal-A (unstable) → opal-CT (metastable) → cryptocrystalline quartz or chalcedony (stable) → microcrystalline quartz (stable), via a dissolution–reprecipitation mechanism [63].

The siliceous-cemented dolomite at the top of the Liangjiashan Formation formed through coupled dolomite and biogenic silica dissolution–reprecipitation processes. In the shallow burial stage, rising temperatures promoted organic matter degradation, generating organic acids that lowered the porewater’s pH [64]. These acidic conditions triggered dolomite dissolution–reprecipitation, transforming the protodolomite into ordered dolomite. Although this process caused local pH fluctuations, the system trended toward increasing alkalinity [65]. As diagenesis progressed, a higher pH and temperature dissolved the unstable biogenic opal-A. This dissolution created space for dolomite recrystallization, yielding well-developed rhombohedral crystals, and released dissolved silica (DSi) that reduced the Mg²⁺ hydration kinetic barriers, further enhancing dolomitization [66]. With an increasing burial depth, opal-CT was precipitated in the dolomite crystal interstices, forming cement and replacing the earlier dolomite. Over time, the disordered opal-CT transformed into cryptocrystalline quartz via solid-state mechanisms [61]. The burial compaction forced the pore fluids to migrate laterally, as the overlying transgressive mudstones acted as an impermeable cap. This fluid flow pattern produced homogeneous silicification, ultimately forming the layered siliceous-cemented dolomite at the top of the Liangjiashan Formation.

5.2.2. Dolomitization

The dolomitization of the Liangjiashan Formation has been suggested to be associated with the regional regression induced by the Huaiyuan Movement [11]. At the end of the Cambrian, the Huaiyuan Movement occurred in the North China Plate, resulting in a gradual shallowing of the seawater. Until the end of the Early Ordovician, there was a complete regression of the seawater in this basin, which left behind only shallow water deposits at its southeastern margin. The sedimentary environment of the study area is characterized by a dolomitic tidal flat, forming a sequence of micritic dolomite formations in an evaporative environment [22]. The two types of dolomites in the Liangjiashan Formation, namely siliceous-cemented dolomite at the top and crystalline dolomite at the bottom, have both undergone alternations due to late-stage dolomitization.

Carbon and oxygen isotopes play an important role in investigating the formation mechanism of dolomite. As shown in Figure 5, the carbon and oxygen isotopic compositions of the two types of dolomite in the Liangjiashan Formation are lighter than the seawater of the same period, indicating two potential causes—(1) the influence of meteoric water [67] and (2) the burial thermal effects during diagenesis [68]—but the former should be excluded due to the significant recrystallize observed in the Liangjiashan dolomites. Therefore, it can be inferred that the dolomites of the Liangjiashan Formation underwent successive stages of burial dolomitization facilitated by multiple episodes of diagenetic fluid interaction.

The source of the dolomitizing fluids can be inferred from the composition, distribution patterns, and anomalies of the rare earth elements [69,70,71]. The rare earth elements’ distribution pattern in the dolomites in the Liangjiashan Formation shows a relatively flat “hat-shaped” feature, resembling the distribution pattern of pore water, suggesting the potential origin of the dolomitizing fluids from pore water. The siliceous-cemented dolomites display a similar distribution pattern to that of the crystalline dolomites, also characterized by a “hat-shaped” pattern with MREEs enrichment. However, a conspicuous positive Ce anomaly is observed in the siliceous-cemented dolomites, with a Ce/Ce* value ranging from 1.48 to 1.74 (Table 2). Its distribution pattern is similar to that of the ferromanganese crust (Figure 11). Relevant studies have shown that under hypoxic or anoxic conditions, particles rich in Fe-Mn oxide/hydroxide undergo reductive dissolution [72], resulting in the reduction of Ce⁴⁺ to Ce³⁺ and its subsequent release into the aqueous environment. This process promotes the enrichment of Ce in water bodies and leads to a positive Ce anomaly. Therefore, the presence of a positive Ce anomaly in the siliceous-cemented dolomite suggests that its dolomitization fluid was influenced by iron–manganese oxides. Meanwhile, there is a gradual decrease in the Fe, Mn, and MREEs content of the dolomite from the top to bottom of the Liangjiashan Formation (Figure 4), indicating that the predominant source of pore water fluid may have originated from the top of the Liangjiashan Formation.

Based on the petrological and geochemical characteristics, both types of dolomites in the Liangjiashan Formation experienced penecontemporaneous dolomitization during the early diagenetic stage (Figure 12e). Towards the end of the Liangjiashan period, the Huaiyuan Movement introduced a substantial influx of terrigenous iron–manganese oxides. During the subsequent shallow burial stage, under reducing conditions, the iron–manganese oxides at the top of the Liangjiashan Formation dissolved, preferentially releasing MREEs into the pore water and exerting a superimposed effect on the burial dolomitization in the underlying strata (Figure 12h).

The dolomites of the first member of the Majiagou Formation are the products of penecontemporaneous evaporative pumping dolomitization. A considerable amount of terrigenous clastics are observed in the thin sections. The dolomite of the Majiagou Formation exhibits significantly elevated contents of Fe, Mn, and middle rare earth elements (MREEs) compared to those found in the Liangjiashan Formation (Figure 4), indicating an enhanced influx of terrigenous materials. In an oxic environment, MREEs are preferentially adsorbed by iron and manganese oxides from terrestrial sources [73]. However, during the early diagenetic shallow burial in anoxic conditions, the reduction of iron and manganese oxides to Fe²⁺ and Mn²⁺ occurred, leading to their incorporation into the dolomite lattice while releasing the previously adsorbed MREEs [74]. In addition, the δ¹³C value is consistent with that of the contemporaneous seawater, indicating a marine origin for the dolomitizing fluid. As discussed before, the sedimentary environment of the Majiagou Formation is a mixed siliciclastic–carbonate tidal flat, allowing for the occurrence of evaporative pumping dolomitization. At the same time, it is suggested that meteoric water may have influenced the dolomite in the Majiagou Formation, as indicated by the lower δ¹⁸O content.

5.3. The Responses of Sedimentation and Early Diagenesis to the Huaiyuan Movement at the Turn of the Early–Middle Ordovician

The occurrence of the Huaiyuan Movement led to a transformation of the sedimentary patterns, from a dolomitic tidal flat during the Early Ordovician to an intra-platform depression during the Middle Ordovician, and eventually to the mixed siliciclastic–carbonate tidal flat. The evolution of the sedimentary environment inevitably resulted in changes to the early diagenetic conditions. The development of siliceous-cemented dolomites at the top of the Liangjiashan Formation was controlled by intensified weathering induced by the Huaiyuan Movement [17]. Additionally, iron and manganese oxides derived from continental weathering influenced the origin of burial dolomitizing fluids [74]. Therefore, both sedimentation and diagenesis during the Early-to-Middle Ordovician period in the Ordos Basin exhibited a strong response to the Huaiyuan Movement. The schematic response model is illustrated in Figure 12 and can be divided into the following stages.

The first stage corresponds to the early stage of the Early Ordovician, during which a normal shallow marine environment prevailed along the southeastern margin of the Ordos Basin with the development of the dolomitic tidal flat [21,22] (Figure 12a). Intense evaporation caused the concentration of the seawater, resulting in high salinity and magnesium-rich fluids seeping down to replace the lime mud, forming dolomitic mud. Consequently, penecontemporaneous dolomitization occurred, leading to the formation of micritic dolomite (Figure 12b).

The second stage took place at the end of the Liangjiashan Formation sedimentary period, coinciding with the occurrence of the Huaiyuan Movement. Intense tectonic activity resulted in the widespread uplift of the Ordos Basin [5,11,20]. This period witnessed the culmination of continental denudation, with a significant influx of DSi and iron–manganese oxides into the seawater. The Huaiyuan Movement also caused a paleogeographic change, wherein the basin interior witnessed the retreat of seawater towards the southeast (Figure 12c) [9]. The influx of terrigenous material, coupled with the significant presence of DSi, facilitated the flourishment of silica-rich organisms in the seawater [50,56], resulting in the abundant formation of biogenic silica, which subsequently mixed with the lime mud sediments on the seabed (Figure 12d). During the later shallow burial process, dolomitization and siliceous cementation occurred (Figure 12e). In the shallow burial environment, the dissolution of biogenic silica and the reduction of iron–manganese oxides led to an enrichment of silica and MREEs in the pore water. Meanwhile, the dissolution of the biogenic silica created space for the growth of euhedral dolomite crystals. Subsequently, silica reprecipitation occurred, predominantly cementing the recrystallized dolomite crystals to form a layer of siliceous-cemented dolomite at the top of the Liangjiashan Formation.

The third stage represents the deposition of the Majiagou Formation, including two processes, rapid transgression and slow regression. The early Majiagou period inherited the geographical pattern caused by the Huaiyuan Movement, where the southeastern margin constituted an intra-platform depression. The rapid transgression resulted in a further deepening of the water body, characterized by a low-energy environment, and subsequently led to the deposition of a sequence of mudstone at the bottom of the Majiagou Formation (Figure 12f). During the subsequent slow regression period, as the water became shallower, the energy increased. Simultaneously, with the supply of terrigenous detritus, the sedimentary environment transformed into a mixed siliciclastic–carbonate tidal flat [75], resulting in penecontemporaneous evaporative pumping dolomitization and forming a series of sandy micritic dolomites (Figure 12g,h).

Figure 12. Conceptual model showing sedimentation–early diagenesis response to Huaiyuan Movement. (a) Stage 1: depositional environment of Lower Ordovician Liangjiashan Formation—dolomitic tidal flat; (b) Stage 1: early-diagenesis tidal flat environment—evaporative pumping dolomitization; (c) Stage 2: occurrence of Huaiyuan Movement at end of Early Ordovician, with massive input of DSi and nutrients into ocean, promoting proliferation of tidal margin siliceous organisms; (d) Stage 2: depositional period featuring mixed sedimentation of siliceous organisms and dolomitic mud; (e) Stage 2: shallow burial period characterized by dolomite recrystallization, siliceous cementation, and burial dolomitization of underlying strata; (f) Stage 3: depositional environment during rapid transgression of Middle Ordovician Majiagou Formation, where uplift due to Huaiyuan Movement transformed study area into intra-platform depression environment, depositing set of mudstones; (g) Stage 3: slow regression depositional environment of Majiagou Formation—mixed tidal flat; (h) Stage 3: evaporative pumping dolomitization in tidal flat environment.

Figure 12. Conceptual model showing sedimentation–early diagenesis response to Huaiyuan Movement. (a) Stage 1: depositional environment of Lower Ordovician Liangjiashan Formation—dolomitic tidal flat; (b) Stage 1: early-diagenesis tidal flat environment—evaporative pumping dolomitization; (c) Stage 2: occurrence of Huaiyuan Movement at end of Early Ordovician, with massive input of DSi and nutrients into ocean, promoting proliferation of tidal margin siliceous organisms; (d) Stage 2: depositional period featuring mixed sedimentation of siliceous organisms and dolomitic mud; (e) Stage 2: shallow burial period characterized by dolomite recrystallization, siliceous cementation, and burial dolomitization of underlying strata; (f) Stage 3: depositional environment during rapid transgression of Middle Ordovician Majiagou Formation, where uplift due to Huaiyuan Movement transformed study area into intra-platform depression environment, depositing set of mudstones; (g) Stage 3: slow regression depositional environment of Majiagou Formation—mixed tidal flat; (h) Stage 3: evaporative pumping dolomitization in tidal flat environment.

This model illustrates how the Huaiyuan Movement controlled sedimentation–early diagenesis across the tectonic unconformity, advancing the theoretical framework for understanding Early–Middle Ordovician tectonic–sedimentary interactions in the Ordos Basin. Moreover, the model has significant practical implications, enabling the prediction of dolomite reservoirs associated with tectonic unconformities in the Ordos Basin.

6. Conclusions

The Early-to-Middle Ordovician sedimentary environments and early diagenetic processes along the southeastern Ordos Basin margin were controlled by the Huaiyuan Movement. During the Early Ordovician, a dolomitic tidal flat environment occupied the southeastern margin of the Ordos Basin, where evaporative pumping dolomitization occurred. By the terminal end of the Early Ordovician, the Huaiyuan Movement had triggered a major paleogeographic reorganization, and the basin’s interiors seawater experienced a southeastward regression while the southeastern margin evolved into an intra-platform depression. A brief sedimentary gap occurred along the southeastern margin, forming a paraconformity. This was followed by a rapid transgression, evidenced by the basal Majiagou Formation mudstones, indicating water deepening, and a subsequent gradual regression that established a mixed siliciclastic–carbonate tidal flat system.

Furthermore, the Huaiyuan Movement controlled two key early diagenetic processes, including syngenetic dolomitization and siliceous cementation. Dolomitization occurred via two processes:, initial evaporative pumping followed by burial dolomitization sourced from pore fluids. The uppermost dolomite in the Liangjiashan Formation experienced extensive siliceous cementation during shallow burial, with silica derived from biogenic sources. This biotic silica production was directly facilitated by the Huaiyuan Movement, which triggered enhanced terrigenous input and dissolved silica (DSi) enrichment in the seawater. These conditions promoted the prolific growth of siliceous organisms, resulting in the abundant formation of biogenic silica.