Elemental logging evidence for paleoenvironmental reconstruction of the ordovician strata in the Western Ordos basin, China

Input and preservation conditions of organic matter

Paleo-productivity

Paleo-productivity refers to the amount of organic carbon fixed by ancient marine or lacustrine ecosystems per unit of time and area⁵typically associated with the photosynthetic activity of planktonic organisms. During photosynthesis, phytoplankton can convert atmospheric CO₂ into organic matter²⁰and eventually settles and forms organic-rich sediments. High paleo-productivity often implies significant input of organic matter into the depositional environment. Previously, different proxies have been developed to indicate the paleo-productivity in ancient environments^21,22,23. P is an important limiting nutrient for primary producers in aquatic ecosystems and it acts as an essential element for the skeletal system of organisms²⁴. In general, the proliferation of primary producers leads to increased consumption of P, which consequently enhances the deposition of P in the sediments. However, Ti is a common element in the Earth’s crust, typically unaffected by biological processes. Hence, P/Ti ratio increases when higher paleo-productivity of the ancient marine or lacustrine ecosystem²⁵. Cu and Ni can also been obviously influenced by organism activities and Cu/Al (ppm/%) and Ni/Al (ppm/%) ratios can serve as indicators of paleo-productivity^{22,24,26,27,28}. In addition, the Al/Ti ratio minimizes the influence of variations in absolute terrigenous flux and instead acts as an indicator of the relative enrichment of nutrient-rich, clay-sized aluminosilicates within the total detrital sediment fraction. A higher ratio signifies a greater potential supply of crucial micronutrients and macronutrients associated with these clays, pointing to conditions conducive to enhanced biological productivity in the overlying water column during deposition.

In general, Ordovician sediments commonly experienced repeated redox oscillations driven by multiple sea-level fluctuations, leading to far more intense phosphorus remobilization than often appreciated. A critical complication arises from the thermal evolution of Ordovician source rocks. Samples having reached overmature stages may have experienced substantial Cu and Ni leaching during late-stage hydrocarbon generation and expulsion processes. Hence, as a baseline indicator of terrigenous flux and nutrient efficiency, Al/Ti is broadly applicable and reliable in Ordovician successions. In this study, Al/Ti ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 11.38, 13.18, 17.58, 22.33, and 19.41, respectively. Data from sedimentary proxies may suggest the Wulalike Formation had the highest paleoproductivity among the Ordovician strata.​.

Redox condition

Redox condition acts as an important factor influencing the preservation of the organic matters in sediments²⁹. In general, oxygen is a primary factor driving the oxidation of organic matter, promoting its decomposition. In a reducing environment, the absence of oxygen limits oxidative processes, thereby slowing the breakdown of organic matter^30,31. Apart from this, most microorganisms responsible for organic matter decomposition rely on oxygen for aerobic respiration. In anaerobic conditions, the activity of these microbes is significantly reduced, or the microbial community shifts to anaerobic organisms, which decompose organic matter at a slower rate, thus favoring its preservation^32,33. Trace elements are commonly used as indicators of redox conditions in aquatic environments during sedimentary processes, owing to their sensitivity to changes in oxidation-reduction states²⁴. This sensitivity arises from the distinct chemical behavior of trace elements under different redox conditions, which significantly alters their oxidation states, speciation, and concentration profiles.

U/Th ratio is a useful geochemical proxy for assessing the redox conditions of sedimentary environments and it has been widely applied in previous studies^7,34,35. Generally, U is immobile and tends to precipitate out as uranium minerals such as uraninite (UO₂) at reducing conditions, while it is more mobile in its oxidized form at oxidizing conditions²⁴. Th always exists as highly insoluble state in both oxidizing and reducing conditions³⁶. Hence, U/Th ratio of < 0.75 and > 1.25 indicates oxic and anoxic environments, respectively ³⁶.U/Th ratios provide a ​quantitative, diagenetically robust, and widely applicable​ tool for paleoredox reconstructions. When combined with complementary proxies (e.g., Mo, V, Fe speciation), they offer one of the most reliable frameworks for discriminating marine oxygenation states in ancient sedimentary systems. However, lithological differences across formations can also exert a significant influence on Al/Ti ratios. Carbonate-dominated intervals (e.g., Sandaokan and Zhuozishan formations) generally contain lower detrital clay fractions, which may dilute aluminosilicate-associated Al, resulting in lower Al/Ti values. In contrast, shale- or marl-rich intervals (e.g., Wulalike Formation) typically have higher proportions of fine-grained aluminosilicates, increasing the Al content relative to Ti and thereby elevating the Al/Ti ratio. Therefore, part of the observed stratigraphic variation in Al/Ti may reflect primary lithological shifts rather than solely changes in nutrient availability or productivity. Accounting for this lithological control is essential for a more robust interpretation of paleo-productivity trends. In this study, the average U/Th ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 1.02, 0.92, 0.71, 0.92, and 0.73, respectively. Data from sedimentary proxies suggest the Ordovician strata is generally characterized by a dysoxic condition.

Paleo-climate

Paleo-climate can influence temperature, precipitation, sea-level fluctuations, ocean circulation, and nutrient supply, thereby impacting the accumulation and preservation of organic matter. Sr/Cu ratio is always used as an effective proxy to reflect paleo-climate^37,38. It is generally believed that Sr/Cu > 10 indicates a dry climate, 5 < Sr/Cu < 10 suggests a semi-humid to semi-arid climate, and 1 < Sr/Cu < 5 indicates a warm and humid climate³⁹. In this study, Sr/Cu ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 15.95, 18.49, 32.22, 13.50, and 17.83, respectively.

Rb/Sr ratio also serves as an indicator of paleo-climate^40,41. Rb is relatively stable, while Sr is more susceptible to leaching and loss in humid environments⁴². Hence, the Rb/Sr ratio is generally higher in humid environments, and arid climate conditions are typically associated with a lower Rb/Sr ratio. In this study, Rb/Sr ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.87, 0.82, 0.54, 0.71, and 0.50, respectively.

The classification of paleo-climate by Sr/Cu and Rb/Sr ratios is shown in Fig. 6. The results generally reveal a dry climate of the Ordovician strata. Meanwhile, among the different formations in the Ordovician, the Kelimoli Formation is generally characterized by the driest condition. Paleomagnetic constraints indicate that the western margin of the Ordovician Ordos Basin resided in a low-latitude position proximal to the paleoequator. This latitudinal setting aligns precisely with the basin’s diagnostic paleoclimatic signatures—specifically, the dominance of evaporitic facies and climatically sensitive geochemical proxies—consistent with the persistent influence of tropical atmospheric circulation patterns that govern global climate through latitudinal and altitudinal controls.

Paleo-salinity

Optimal conditions for organic matter accumulation and preservation include anoxia, reducing sedimentary environments, rapid burial, and the presence of silica-rich lithologies that limit microbial degradation. Among these conditions, paleo-salinity could influence the input and preservation of organic matter by controlling the growth of organisms⁴³. In general, Sr/Ba is a reliable proxy in revealing characteristics of aqueous medium and the ratio increases from freshwater to saline water⁴⁴. Sr/Ba ratios of < 0.5, 0.5-1, and > 1 reveal fresh water, brackish water and salt water, respectively^6,42. In this study, ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 3.02, 3.21, 6.46, 0.77, and 0.85, respectively. The results indicate a salt water environment during the deposition of the Sandaokan, Zhuozishan, Kelimoli formaitons, and a brackish water environment during the deposition of Wulalike and Lashizhong formations.

Paleo-water depth

Paleo-water depth acts as an important factor influencing the organic matter enrichment and decomposition. Different elemental proxies were developed to reveal the paleo-water depth due to their different behavior in accumulation and dispersion at varying water depths during deposition⁴⁵. As a stable element in sediments, K primarily resides in coarse detrital minerals (e.g.-feldspar, muscovite). In high-energy hydrodynamical settings (shallow water), coarse-grained minerals (sand fraction) are preferentially enriched, elevating bulk sediment K content. However, Rb is more reactive and prone to being adsorbed and its content tends to increase in deep lake sediments⁴⁶. Therefore, Rb/K ratio was widely applied in determining paleo-water depth and the ratio increases with increasing water depth. In this study, Rb/K ratios of the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.005, 0.006, 0.009, 0.004, and 0.004, respectively. Previous studies demonstrate distinct Rb/K ratio trends across bathymetric gradients in both modern and ancient sedimentary systems. Analysis of shelf-to-basin transects in the Atlantic Ocean reveals Rb/K values of 0.004–0.006 in shallow-marine sandy sediments, increasing to 0.008–0.015 in deep-marine muddy deposits (Tribovillard et al., 2006). In addition, in Ordovician strata of the Appalachian Basin, deep-water graptolitic shale facies exhibit elevated Rb/K ratios (0.012–0.018), contrasting with lower values (0.003–0.005) in coeval shallow-water shelly limestone facies (Rimmer et al., 2004). Collectively, paleobathymetric proxies record significant shifts in water depth during Ordovician deposition in the Ordos Basin. Analysis of sedimentary facies and geochemical indices demonstrates a shallowing-upward mega sequence across key formations: Deposition of the Sandaokan​and Zhuozishan formations​occurred under relatively shallow-water conditions. A pronounced deepening event is registered during sedimentation of the Kelimoli Formation. Subsequent shallowing is documented through the Wulalike Formation to the Lashizhong Formation, reflected by transitional facies associations culminating in peritidal carbonates and evaporites characteristic of very shallow subtidal to supratidal environments.

Variation of paleo-environments across the ordovician

The variation of different paleo-environmental proxies across the Ordovician was presented in Fig. 5. Paleosalinity reconstructions based on Sr/Ba ratios reveal a progressive increase from the Sandaokan to Kelimoli Age, followed by a decline through the Kelimoli to Wulalike Age, indicating an initial salinity rise and subsequent freshening of paleoseawater. Concurrently, Sr/Cu and Rb/Sr ratios remained stable from the Sandaokan to Zhuozishan Age, reflecting persistently arid and hot climatic conditions, but showed a gradual decrease from the Kelimoli to Wulalike Age, signaling a transition toward warmer and more humid conditions. Synchronous trends are observed in redox-sensitive, bathymetric, and productivity proxies: U/Th ratios, Rb/K ratios, and Al/Ti ratios collectively indicate dominantly suboxic conditions, slow water deepening, and progressively enhanced paleoproductivity during the Sandaokan-Zhuozishan interval. This shifted markedly to anoxic/euxinic conditions, rapid deepening, and intensified productivity from the Kelimoli to Wulalike Age. These integrated paleoenvironmental trends align with global Ordovician eustatic records, demonstrating an oscillatory transgressive phase (overall sea-level rise) spanning the Sandaokan through Wulalike stratigraphic succession.

Variations of paleoenvironmental proxies across the Ordovician strata in the western Ordos Basin.

Under the constraints of the reconstructed paleoenvironmental evolution and integrating previous sedimentological and petrological investigations, the Ordovician sedimentary facies in the western Ordos Basin have been classified⁴⁷. The stratigraphic succession from the complete transgressive-regressive cycle dominated by an overall sea-level rise. This trend was punctuated by high-frequency sea-level oscillations, particularly pronounced during Zhuozishan Formation deposition. Seven principal sedimentary facies types have been identified as platform margin shoal facies, platform margin reefal facies, fore-slope facies, basin slope (or basin margin) facies, open marine shelf facies, and basin facies.

The depositional systems within key Ordovician successions of the western Ordos Basin—spanning the Sandaokan, Zhuozishan, Kelimoli, and Wulalike formations—exhibit complex lateral transitions between multiple facies belts. Despite these variations, the overall stratigraphic trend registers progressive water deepening, punctuated by numerous relative sea-level fluctuations. Spatially, the Sandaokan Formation demonstrates a westward-to-eastward progression from limestone-dominated basin margin facies through dolomitic limestone fore-slope deposits to platform interior and marginal facies (including shoal complexes and bioconstructed reefal banks). The overlying Zhuozishan Formation transitions similarly from basin margin limestones and dolomitic fore-slope sequences to dolomitized ridge-like paleohighs (interpreted as dolomitized platform margin buildups). In the Kelimoli Formation, facies evolve westward-to-eastward from thin-bedded open-marine shelf limestones to basin margin limestones, dolomitic fore-slope deposits, and peritidal dolomitic flats with circum-continental affinities. Finally, the Wulalike Formation is dominated by carbonaceous shale basin facies, with subordinate argillaceous dolomitic limestones of the open marine shelf and localized marl-rich shelf sequences.​.

Depositional model of the Ordovician strata in the western margin of the Ordos Basin.

The paleogeographic evolution of the Ordos Basin can be reconstructed. In general, the basin commences with the well-consolidated North China Platform (NCP) in the Precambrian, which formed the unified Paleo-North China Continent. Through the Early-Middle Cambrian, the Yimeng Paleocontinent (north) and Qingyang Paleocontinent (south) experienced successive uplift, establishing the tectonic framework of the Central Paleo-High that persisted into the Late Cambrian-Ordovician. During the Sandaokan Age (Early Ordovician), marine waters gradually transgressed the Ordos Basin, initiating the first Ordovician-wide transgression characterized by shallow depths. The western basin margin developed predominantly platform margin facies. By the Zhuozishan-Kelimoli Ages (Middle Ordovician), progressive deepening facilitated a depositional transition along the western margin to​fore-slope, basin margin, and open marine shelf facies. This evolution culminated in the Wulalike Age with accelerated deepening, establishing a carbonaceous shale-dominated basin fill facies as the principal paleogeographic environment.