Lacustrine Phosphorite in Late Cretaceous Nenjiang Formation of Songliao Basin and the Paleoenvironment Significance

Abstract

Phosphorus is crucial for reconstructing long-term feedback mechanisms between climate, the environment and ecology, as well as for assessing global biogeochemical changes. This study documents two thin yet laterally continuous phosphorite beds from the lower Nenjiang Formation (Late Cretaceous) of the Songliao Basin in NE China and evaluates their mineralogical characteristics and paleoenvironmental significance. The phosphorite beds occur in sharp contact with adjacent black shale and contain well-preserved Ostracoda and conchostracan fossils, providing biological constraints on the depositional conditions. Bulk rock compositions indicate elevated P₂O₅ contents, ranging from approximately 20 to 30 wt%. Mineralogical analyses reveal that the dominant phosphate mineral is carbonate-fluorapatite (CFA), accompanied by minor quartz, hydromica, goethite and pyrrhotite. Integrated fossil, sedimentological, and geochemical evidence suggests that CFA precipitated in a deep, stratified, eutrophic lacustrine environment. Enhanced productivity, biological enrichment and microbial decomposition of organic matter likely promoted phosphorus enrichment in bottom waters, facilitating CFA precipitation at or near the sediment-water interface during deposition and early diagenesis. Variations in physicochemical conditions, including pH and Ca²⁺ concentrations, may have further influenced mineral precipitation and subsequent diagenetic processes. These findings contribute to our understanding of phosphorus precipitation mechanisms in lacustrine basins and provide new constraints on the Late Cretaceous paleoenvironment of the Songliao Basin.

1. Introduction

Phosphorus is a fundamental nutrient and energy carrier in life, and a prime element in the intermediation between living and inanimate parts of the biosphere. The phosphorus cycle thus plays a crucial role in regulating the Earth’s climate and affecting biodiversity [1,2,3,4]. Phosphate deposits are widely distributed throughout geological time, ranging from the Paleoproterozoic to the Holocene [1,5,6], and have been extensively studied in marine sediments [5,7], where interpretations of phosphogenesis have evolved from single factor explanations such as the classic upwelling current theory [1,8,9], biogenesis [10], chemical genesis [8,11] to multi-factorial regulation of phosphorus cycle in earth history, e.g., [3,12,13,14,15].

Phosphates-bearing strata are also discovered in lacustrine environments, including the Eocene Green River Formation [16,17], the Pliocene Glenns Ferry Formation [18], the Miocene lacustrine deposits in the Cerdanya Basin [19], the Neogene clayey diatomite deposits in the Sarantaporo-Elassona Basin [20], and the Quaternary phosphate sediments in eastern Africa [21,22] and Lake Baikal in Russia [23]. These occurrences indicate that the enrichment of phosphates in lake basins may reflect complex interactions among biological productivity, water column stratification, redox dynamics and early diagenetic mineralization [16,17,18,19,20,21,22,23], thereby providing valuable evidence of environmental conditions and changes in continental basins.

In China, phosphate deposits are predominantly associated with marine sediments of the Yangzi platform and the Tarim Basin [24,25,26], whereas phosphate-bearing strata constitute critical records for interpreting paleoenvironmental conditions within lacustrine systems. The Songliao Basin, the largest Cretaceous non-marine basin in East Asia, is characterized by a long-lived lake system and well-preserved stratigraphic continuity [27]. With the accumulation of research achievements over several decades, an integrated stratigraphical framework has been established [28,29,30], underpinning studies of paleoclimate, paleoenvironment, biotic evolution, and major geological events, e.g., [31,32,33,34]. Within the lower part of the second member of the Nenjiang Formation, two thin layers of phosphorite with varying thickness and lateral continuity have been identified, yet their depositional origin and paleoenvironmental significance have not been systematically evaluated.

In this study, we mainly investigate the phosphorite layers and associated fossil assemblages from Houjingou and Yuewangcheng sections in the southeastern uplift of Songliao Basin through detailed mineralogical and geochemical analyses. Our aim is to elucidate the mechanisms of phosphorus enrichment and clarify the depositional environment in which the phosphorite formed. The findings provide new insights into phosphorus cycling and paleoenvironmental changes in Late Cretaceous lacustrine systems.

2. Geological Setting

The Songliao Basin, covering approximately 260,000 km², is located in northeast China at mid-latitudes in East Asia [27]. During the Late Cretaceous, it occupied a paleogeographic position along the eastern Asian continental margin, in proximity to the western Pacific marginal realm (Figure 1b) [27,35].

The basin preserves a well-developed Cretaceous stratigraphic succession comprising, from oldest to youngest, the Huoshiling (K₁h), Shahezi (K₁sh), Yingcheng (K₁yc), Denglouku (K₁d), Quantou (K₂q), Qingshankou (K₂qn), Yaojia (K₂y), Nenjiang (K₂n), Sifangtai (K₂s) and Mingshui (K₂m) formations (Figure 1) [36]. Structurally, it is subdivided into six first-order units: the north plunge zone, central depression zone, northeast uplift zone, southeast uplift zone, southwest uplift zone and west slope zone. The study area lies within the southeast uplift zone (Figure 1a).

Members 1 and 2 of the Nenjiang Formation are mainly exposed in the study area and consist of dark mudstone, black shale and oil shale interlayers formed in a deep-lake environment [37]. A marine incursion occurred in the lower K₂n, leading to a large-scale lake anoxic event [38,39,40], which promoted significant organic matter enrichment and formed the most important source rock segment in the Songliao basin. The age of the Nenjiang formation is constrained to Santonian–Campanian based on new high-resolution magnetostratigraphic results and SIMS U–Pb zircon analyses [41,42,43,44]. The phosphorite layers investigated in this study are restricted to the lower part of the K₂n² and are laterally extensive but thin, with thicknesses of approximately 1 cm or 3–4 cm (Figure 2).

3. Materials and Methods

Two sections containing phosphorite in the Songliao basin were sampled in this study: the Yuewangcheng section (two samples; YWC-P1, YWC-P2) in Baliyingzi Village, Huangyuquan Town, Nong’an County, Jilin Province (44°52′17.01″ N, 125°30′17.41″ E) (Figure 2a,c,e) [45], and the Houjingou section (two samples; HJG-P1, HJG-P2) in Qingshankou village, Nong’an County, Jilin Province (44°52′29.4″ N, 125°30′8″ E) (Figure 2b,d,f) [46].

Thirteen thin sections were prepared for petrographic and mineralogical analyses. Rock samples were cleaned, dried, cut and polished to obtain smooth surfaces for microscopic and electron microscopic observations. Petrographic and microstructural analyses were conducted at China University of Geosciences (Beijing) using a Zeiss Axio Scope A1 polarizing microscope and a Zeiss Supra 55 Field Emission Scanning Electron Microscope (FESEM) (Carl Zeiss, Oberkochen, Germany) operated at 20 kV with a working distance of 15 mm. Secondary electron (SE2) and AsB detectors were used for morphological and compositional imaging, respectively. Prior to SEM analysis, the thin sections were coated with an approximately 8 nm thick carbon layer. Chemical compositions were determined using an Oxford X-act energy-dispersive spectrometer (EDS) (Oxford Instruments, Abingdon, UK) attached to the FESEM, with an analytical spot size of approximately 2 μm. Fluorine was measured by EDS together with other elements. Elemental totals were normalized to 100 wt% on an elemental basis; oxide totals were calculated from elemental concentrations without oxygen correction for F substitution.

Four samples (HJG-P1, HJG-P2, YWC-P1, and YWC-P2) were analyzed by whole-rock X-ray diffraction (XRD). The samples were dried and ground to 200 mesh using an agate mortar. Phase identification and semi-quantitative mineral analysis were performed using a Rigaku SmartLab diffractometer (9 kW) (Rigaku, Tokyo, Japan) at China University of Geosciences (Beijing), equipped with a Cu-target X-ray source operating at 45 kV and 200 mA. Diffraction data were collected over a 2θ range of 4–70° with a step size of 0.02° and a scanning rate of 1°/min. Only whole-rock powder samples were analyzed. Clay fractions were not separated, and no ethylene glycol solvation or thermal treatment was applied.

Major element compositions of 11 samples, including phosphorite and black shale, were determined by X-ray fluorescence (XRF) analysis. Samples from the Houjingou section were analyzed by using an ARL ADVANT XP+ spectrometer (Thermo Fisher Scientific, Ecublens, Switzerland) at the laboratory of Orogenic Belts and Crustal Evolution, Peking University, whereas samples from the Yuewangcheng section were analyzed using a wavelength-dispersive XRF spectrometer (Axios-mAX; PANalytical, Almelo, The Netherlands) equipped with an AB104L fusion system at the Analysis and Testing Research Center, Beijing Research Institute of Uranium Geology.

Two sections of phosphorite samples (YWC-P1, YWC-P2, HJG-P1, and HJG-P2) were fragmented and soaked in water or hydrogen peroxide for 12 h for microfossil examination. The residues were washed through a two-layer sieve (1 mm and 0.1 mm), dried, and subsequently examined under a microscope. Representative specimens were then imaged using the Zeiss Supra 55 FESEM (Carl Zeiss, Oberkochen, Germany).

4. Results

4.1. Petrography, Mineralogy, and Chemical Composition of Phosphorite

Two phosphorite beds exhibit sharp contacts with black shale. The phosphorite beds are brownish in the interior and gray at the margins. The beds display a massive (structureless and non-laminated) fabric, with average thicknesses of 1 cm or 3–4 cm (Figure 2c–f).

Under plane-polarized light, the phosphorite appears dark brownish to yellow and displays a cryptocrystalline to microcrystalline texture (Figure 3d,f,g). The rock is matrix-supported, with microcrystalline carbonate-fluorapatite (CFA) forming the dominant matrix that encloses detrital quartz grains, clay minerals and fossil fragments (Figure 3c–g). Quartz occurs as fine-grained angular to subangular fragments with low roundness. Clay minerals are present as fine platy flakes disseminated within the matrix and were optically identified as hydromica based on morphology and optical properties. Opaque components appear as irregular patches and disseminated grains, interpreted as ferruginous and/or organic-rich material (Figure 3d–g). Rounded aggregates composed of Fe–S sulfides occur within the phosphorite matrix (Figure 3i,k), with EDS spectra indicating Fe–S composition, interpreted as pyrrhotite and related Fe–S sulfides.

Whole-rock XRD analyses of phosphorite samples from the Yuewangcheng (YWC-P1, YWC-P2) and Houjingou (HJG-P1, HJG-P2) sections indicate that the dominant phosphate mineral is CFA (Figure 3l; Table S2). CFA is characterized by reflections at approximately 25.9° and a typical apatite peak within 31–34° 2θ, with an additional reflection near ~49.5° 2θ. Quartz is present in all samples, as indicated by reflections at approximately 20.8°, 26.6°, 36.5°, 50.1° and 59.9° 2θ. Weak low-angle reflections near ~8.9° and ~17.8° 2θ occur in YWC-P1 and are interpreted as minor clay minerals (possibly illite). The XRD data reported in Table S2 represent relative peak intensities.

Bulk chemical compositions, recalculated on a F-free basis and normalized to 100 wt%, are presented in Table S3 for direct comparison with EDS-derived compositions. The bulk chemical compositions show clear enrichment of CaO and P₂O₅ in phosphorite samples (Table S3). P₂O₅ ranges from 20.45 to 36.43 wt% (mean = 29.55 wt%), while CaO varies from 29.68 to 61.21 wt% (mean = 47.30 wt%). SiO₂ contents are generally low to moderate (2.76–31.44 wt%, mean = 9.11 wt%), with elevated values observed in YWC-P1. Al₂O₃ concentrations range from 0.62 to 8.12 wt% (mean = 2.50 wt%), and total Fe₂O₃ ranges from 1.19 to 7.58 wt% (mean = 3.44 wt%).

Quantitative EDS spot analyses of phosphatic domains (Table S1) show high Ca and P contents consistent with CFA. Most analyses show P₂O₅ values between 30 and 45 wt%, and CaO contents range from 45 to 58 wt%, consistent with apatite-dominated compositions, with mean values of 35.89 wt% and 48.10 wt%, respectively. Al₂O₃ contents vary widely (0.72–53.09 wt%; mean = 9.53 wt%), reflecting variable incorporation of clay minerals or aluminosilicate phases. SiO₂ ranges from 1.05 to 59.06 wt% (mean = 7.00 wt%), indicating heterogeneous admixture of detrital components. Fluorine is readily detectable in the majority of domains, generally ranging from 1.72 to 4.44 wt%, with an average of 3.06 wt%, consistent with the presence of fluorapatite.

Specifically, while fluorine is detectable by XRF, the measured F content exceeds the expected range for fluorapatite. This is likely due to matrix effects in XRF analysis, particularly in Ca–P-rich phosphorite samples, where fluorine is often overestimated. Therefore, we adopted the F content derived from EDS analysis, as it provides a more accurate measurement, with an average of approximately 3 wt%.

4.2. Comparison of Mineralogy and Chemical Composition Between Phosphate and Black Shale

The bulk major-element compositions of the black shale samples are listed in Table S3 (15YWC-2-M1, 15YWC-3-M3, 15YWC-5-M1). Compared to the phosphorite samples, the black shale exhibits relatively higher contents of Si, Fe, Al, K and Na oxide, but lower CaO and P₂O₅ contents. The average P₂O₅ content is approximately 0.19 wt%, suggesting that little apatite formed during sedimentation and early diagenesis of the black shale.

4.3. Micropaleontology

In this study, ostracod fossils were identified in all four phosphorite samples collected from the Yuewangcheng and Houjingou sections, and a total of 9 ostracod specimens were obtained (Figure 4). Due to poor preservation and extensive erosion, the fossils can only be classified at the genus level, including Mongolocypris sp., Cypridea sp., and Lycopterocypris sp. Additionally, several conchostracan fossils belonging to the Estheritilis mituishi fossil assemblage (Figure 3c, Figure 4: 10–11) and a few unidentified spherical-like fossils (Figure 4: 12) were also discovered. All specimens illustrated in this study are housed at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Beijing).

5. Discussion

5.1. Depositional Environment

The Songliao Lake was a large, deep-water, eutrophic lake during the deposition of the K₂n¹ to K₂n² [29,36,47,48]. In the late sedimentary stage of K₂n¹, the subsidence rate of the lake slowed down, whereas the sediment supply was sufficient, so that the lake reached a maximum area of 20 × 10⁴ km², covering almost the entire basin (Figure 5a) [36,49]. During the deposition period of the lower K₂n², the lake expanded again and the lake level rose, forming a deep-lake environment and resulting in the stratification of water (Figure 5b) [36,48,50]. Black mudstone and oil shale were preserved in the whole basin, which became an important indicator of regional stratigraphic correlation [48]. Phosphorite layers are distributed within the study area. The lake level dropped as the subsidence began to decrease after the middle to upper K₂n² interval [36,50]. Previous studies have proposed that seawater intrusion promoted the release of nutrients in the lake, based on analysis of marine fossil records and algal biomarkers (Figure 5c) [39,40], leading to the enrichment of organic matter from the K₂n¹ to the lower K₂n². Moreover, the discovery of ostracods (e.g., Mongolocypris sp.) and spores and pollen in the black shale [51,52,53] suggests that organisms flourished in this period, which is interpreted as a eutrophic stage [54,55,56].

Phosphorus in most lakes is present primarily as inorganic phosphate, organic phosphate, or combined with or absorbed by inorganic and dead particulate organic matter [57]. During the period of maximum lake incursion, the lake experienced rapid expansion, introducing substantial terrigenous nutrient fluxes, including weathered phosphorus. This phosphorus pool exhibited dual partitioning: one fraction remained as dissolved inorganic phosphorus (DIP) forms within the water column, whereas a significant proportion was assimilated into organic phosphorus (OP) reservoirs through biological uptake by plankton. More than 90% of total phosphorus exists in the form of organic matter in eutrophic lakes [57]. Consequently, the Songliao lake, which was a eutrophic lake during the upper K₂n¹ to the lower K₂n², likely contained a substantial proportion of phosphorus in organic form. Additionally, the phosphate concentration tends to increase with depth [58]. In such environments, algal blooms, planktonic organisms, and microbial activities facilitate the cycling and accumulation of phosphorus, e.g., [1,59,60,61,62,63,64]. Surface water shows preferential depletion of dissolved phosphorus due to biological assimilation, whereas the bottom organic matter deposition leads to phosphorus accumulation. In deep-lake and semi-deep-lake environments near the edge of the basin, phosphorus enrichment occurs rapidly through dual mechanisms: (1) fluvial input of terrigenous phosphorus and (2) intensified biogeochemical cycling mediated by biota. Furthermore, anoxic conditions in the bottom waters of the stratified lake likely suppressed phosphorus reactivation. The upward surge of bottom water along the edge, where it agitates near the stratified water interface, prompts phosphorus precipitation in the bottom waters [65,66,67].

5.2. Formation of Phosphate

Phosphorites formed in lacustrine environments are predominantly iron-rich phosphates [19,20,23,68], which originate from organic processes during diagenesis. The primary minerals in these iron-rich phosphates include vivianite, Fe and Ca-Fe-bearing phosphate minerals, which typically exhibit irregular or spherical morphologies. According to relevant studies, the genesis of phosphates in sedimentary basins is predominantly attributed to biogenic contributions, exemplified by diatom frustules, fecal pellets, fish teeth and bones [20,69,70]. However, in the Songliao Basin, no conclusive evidence has been found to support these biogenic components as primary sources of phosphate. Notably, the phosphorite deposits in this basin exhibit exceptional lateral continuity (Figure 2) coupled with remarkably low iron contents (Table S3), contrasting sharply with conventional iron-rich phosphate systems. This may suggest a fundamentally different genetic mechanism governing phosphogenesis in the Songliao Basin compared with ferruginous phosphate depositional systems.

The laterally extensive phosphorite beds exhibit sharp contacts with the host rocks, indicating that phosphogenesis in the Songliao Basin occurred at or near the sediment-water interface during deposition and early diagenesis [10]. Overall, the phosphorites examined in this study are similar to those reported in the marine environment and the lacustrine Glenns Ferry Formation, where they formed through precipitation of apatite [1,18]. Thus, the formation mechanism of the phosphorites in the study area may involve the direct precipitation of CFA. This precipitation could be triggered by a rise in temperature [71,72], an increase in pH [73], elevated calcium concentrations [74], increased salinity due to evaporation [17], and adsorption of dissolved orthophosphate onto ferric hydroxide (goethite) Fe (OOH).x H₂O [75], or the development of oxygenated microzones at the sediment-water interface (Figure 5d) [57,76]. Among these factors, variations in pH likely played a particularly significant role. Previous research has demonstrated that alkaline conditions are favorable for phosphate precipitation [77,78,79].

During the deposition of the lower K₂n², lake productivity increased [80], and the decomposition of organic matter resulted in the release of diffused phosphorus. This process consequently led to elevated phosphorus concentrations in the bottom waters. Compared with the host rocks, the phosphorites exhibit higher CaO contents and a lower proportion of detrital minerals, suggesting that phosphate rocks are products of chemical precipitation rather than clastic deposition. The presence of well-preserved ostracods within the phosphorite beds (Figure 4) indicates that the bottom waters were likely enriched in Ca²⁺ and characterized by pH values greater than 7.0. In contrast, slight dissolution features observed in ostracod shells from the overlying black shale suggest comparatively more acidic bottom water conditions [81]. Therefore, it is plausible that bottom water conditions may have shifted toward more alkaline values during the transition from black shale deposition to phosphorite formation. Based on this integrated analysis, we propose that hydrochemical fluctuations—particularly elevated pH and enhanced Ca²⁺ concentrations—may constitute the predominant precipitation mechanism governing phosphate enrichment in the Songliao basin. Phosphorus may have been adsorbed onto ferric hydroxides and/or precipitated as apatite through interactions with Ca²⁺ [75]. The well-preserved fossils of Ostracods and conchostracans (Figure 4) suggest rapid burial during sediment deposition [1]. Given that phosphate is a critical biomineralization component of the Crustacea fossils [82,83,84,85], it is reasonable to hypothesize that the formation of phosphorite may be closely linked to the preservation of conchostracan shells. In addition, bacteria may have played an important role in the formation of phosphate [1,10]. Subsequent diagenesis of organic matter releases phosphate ions, which may have been immobilized through ionic substitution or direct precipitation as authigenic phosphate minerals [19]. Some Ostracod shells within the phosphorites are phosphatized (Figure 4), indicating the existence of displacement in early diagenesis.

Figure 5. (a) The sedimentary environment of K₂n¹. (b) The sedimentary environment of K₂n². (c) Map showing the vertical evolution of major elements from K₂n¹ to K₂n² in the SK-1s drill cores, TOC = total organic carbon, Pr/Ph = pristane/phytane ratio, (a–c) modified from Wang et al. [86]; (d) A proposed model of biogeochemical cycle of phosphorus in the Nenjiang Formation of the Songliao Basin.

Figure 5. (a) The sedimentary environment of K₂n¹. (b) The sedimentary environment of K₂n². (c) Map showing the vertical evolution of major elements from K₂n¹ to K₂n² in the SK-1s drill cores, TOC = total organic carbon, Pr/Ph = pristane/phytane ratio, (a–c) modified from Wang et al. [86]; (d) A proposed model of biogeochemical cycle of phosphorus in the Nenjiang Formation of the Songliao Basin.

6. Conclusions

Two thin but laterally continuous phosphorite beds occur in the lower K₂n² of the Songliao Basin, with thicknesses of approximately 1 cm and 3–4 cm, respectively. Bulk-rock compositions show that P₂O₅ contents in the phosphorite range from 20 to 30 wt%. The dominant phosphate mineral is CFA, with minor associated minerals including quartz, hydromica, goethite and pyrrhotite.

Fossil, sedimentological and geochemical evidence suggest that phosphorite precipitation formed in a deep, anoxic and stratified eutrophic lake. Phosphogenesis likely took place at or near the sediment–water interface during deposition and early diagenesis.

High productivity and microbial decomposition of organic matter likely enhanced phosphorus enrichment in bottom waters, facilitating phosphorite accumulation. Changes in water chemistry, particularly increases in pH and Ca²⁺ concentrations, may have promoted the precipitation of CFA. Subsequent diagenetic processes might also play an important role on the formation of CFA.