Genesis of glauconite
Numerous scholars have conducted extensive research on the origin of autochthonous glauconite and have proposed various hypotheses. Among these, the layer-type lattice theory (Burst, 1958a), the grain glauconization theory (Odin and Matter, 1981), and the pseudomorphic displacement theory (Banerje et al., 2016) are three widely recognized genetic hypotheses. According to the lattice theory, during the evolution of glauconite, Fe and K are simultaneously absorbed into the interlayer lattice of clay minerals, leading to the formation of mature glauconite minerals. Consequently, glauconite of this origin exhibits a positive correlation between K2O and TFeO in its chemical composition. The grain glauconization theory emphasizes that glauconite forms at the sediment-water interface. Initially, potassium-poor glauconite fills the marginal pores of parent minerals such as limestone, fecal pellets, and rock cuttings. As particles exchange materials with seawater to acquire potassium ions, the montmorillonite expansion layer collapses, creating a pore structure that allows pore water to penetrate the particle core. Through this process, glauconitic montmorillonite evolves into glauconite and eventually recrystallizes into mature glauconite. Consequently, according to the grain glauconization theory, the K2O content of glauconite increases with maturity, while the TFeO content remains relatively stable. The pseudomorphic displacement theory Through this process, glauconitic montmorillonite evolves into glauconite and eventually recrystallizes into mature glauconite. Consequently, according to the grain glauconization theory, the K2O content of glauconite increases with maturity, while the TFeO content remains relatively stable. The pseudomorphic displacement theory primarily applies to glauconite formed in very small micropores in shale. By analyzing the covariant relationship between K2O and TFeO contents in glauconite minerals, the genetic types
of glauconite can be determined based on differences in K+ and Fe exchange patterns during glauconite formation. This analytical approach serves as a valuable tool for gaining a deeper understanding of glauconite genesis and evolution.
It is widely accepted that autochthonous glauconite is typically deposited in a slow, low-energy, and weakly reducing environment, while allochthonous glauconite is predominantly formed in high-energy and weakly oxidizing environments (Zhang et al., 2017) (Fig. 5). Amorosi (1997) suggested that the majority of glauconites found on the outer shelf are primary, whereas those formed in shallow and deep ocean environments are more likely to be allochthonous and clastic type. Particularly in limestone, glauconite often forms near bioclasts, with the growth and decomposition processes of these organisms providing essential materials such as iron, potassium, and silicon for glauconite formation (Yang et al., 2020).
The glauconite within the Dawangou Formation exhibits good sorting and roundness and is usually distributed along with mud bands. Some or all crystal forms may have disappeared, being associated with pyrite. These characteristics suggest that they are quasi-autochthonous glauconites, originating in a medium-outer gentle slope environment with bioclasts and carbonate particles as the parent material. According to the grain glauconitic theory, these glauconites were transported to the middle-outer gentle slope sedimentary environment by waves, tides, and storms after reaching a mature to over-mature stage in situ. Despite undergoing a brief transport process, quasi-autochthonous glauconites are still preserved in sediments of the same age, thus continuing to be significant indicators of the sedimentary environment (Qin et al., 2016).
The glauconite matrix in the Dawangou Formation primarily consists of bioclasts, and variations in the porosity and permeability of the matrix can affect the concentrations of Fe and K in glauconite. The relatively limited pore space within bioclasts typically results in lower Fe and K levels in glauconite compared to the coprolite matrix. Clay minerals have the capacity to absorb K from seawater, facilitating the enrichment of K in glauconite (Wang et al., 2011). The organic reef environment is characterized by high iron content, which is favorable for glauconite formation. The upward growth of coral reefs helps to resist burial processes during transgressive periods, facilitating long-term interactions between the sediment-water interface and seawater, providing a stable environment for glauconite growth.
The presence of abundant decaying organisms and organic matter, such as fecal particles, creates a low-oxygen environment that promotes glauconite formation. This weak reducing condition positively influences glauconite formation (Nicolas Tribovillard, et al. 2022). Moreover, sulfides produced by sulfate-reducing bacteria contribute to pyrite formation, illustrating that brackish water, microbial activity, weak reduction conditions, and iron-rich environments promote the formation of glauconite and pyrite. Figures 3a and 3b demonstrate a symbiotic relationship between pyrite and glauconite, indicating a weak reducing environment during the deposition of the Dawangou Formation in Well XSD1. Additionally, minor fluctuations in pore water and the bottom REDOX state promote the iron cycle, phosphate concentration, rare earth elements, and subsequently, the autogenesis of glauconite.
Glauconite maturity
Glauconitization primarily occurs at the contact interface between sediment and water, involving a gradual enrichment of potassium ions that leads to the evolution of glauconite from a potassium-poor montmorillonite state to a potassium-rich mica state. The K2O content is considered a key indicator of glauconite maturity. According to Odin and Matter (1981) and Odin (1988), the process of glauconite formation can be divided into four stages: 1. In the primary stage, the total iron content of the host increases significantly, and the glauconite exhibits potassium deficiency with a K2O content of approximately 2–4%. 2. During the low maturity stage, green glauconite streaks begin to precipitate in the host, leading to an increase in K2O content to 4–6%. 3. As the maturity progresses, host particles start to disintegrate, allowing dispersed K+ ions to diffuse to the core, resulting in the formation of new proglauconite organisms outward from the core. At this stage, the K2O content is around 6–8%. 4. In the high maturity stage, root-spheroid glauconite particles are fully precipitated, indicating the longest deposition duration and the highest K2O content of 8–10% (Li et al., 2011; Vika et al., 2007). In the present study, the K2O content of glauconite in the Dawangou Formation of XSC 1 well ranged from 6.27–8.73%, with an average value of 7.19%(Table 4). Based on the classification scheme mentioned above, these glauconites are categorized as being at the mature to high mature stage, suggesting that they have undergone a substantial period of sedimentation and maturation.
The maturity of glauconite can be determined by its color, which is influenced by the presence of octahedrally coordinated iron. This leads to increased light absorption and a reduction in the amount of iron-rich montmorillonite mixed with glauconite as it matures. As glauconite matures, the ratio of Fe2+/Fe3+ within its lattice increases, resulting in a darker color. Glauconite with a high Fe2+/TFe ratio typically indicates a more advanced stage of evolution. As maturity progresses, the color of glauconite transitions from yellow-green to light green, emerald green, and finally to dark green. The emerald green to dark green color of glauconite in the Dawangou Formation confirms its mature characteristics. This observation aligns with elemental tests, collectively suggesting a high level of maturity in the glauconite of the Dawangou Formation.
Moreover, the microscopic morphology observed under scanning electron microscopy is also one of the important indicators for assessing the maturity of glauconite. During the initial in-situ formation phase of glauconites, they typically manifest as amorphous fine particles smaller than 5µm, exhibiting unstable morphology. As maturity progresses into the low maturity stage, these small particles agglomerate to form 2 ~ 3µm caterpillar structures, displaying a degree of organization. As glauconite matures further, it transforms into slightly curved, parallel 1 ~ 3µm sheet structures, which densely cover the intergranular spaces and microcracks of the host minerals, establishing an orderly and compact structural framework. In the high maturity stage, glauconites develop into 4 ~ 5µm flowerlike structures, demonstrating advanced maturity and organizational complexity. However, the morphology of allochthonous glauconite particles often shows signs of partial or complete destruction after transportation, attributed to the physical and chemical influences on the particles during the transport process. Through thin section observations and SEM analysis, it was observed that glauconite particles from the Ordovician Dawangou Formation in XSC 1 Well exhibit large sizes ranging from 5µm to 40µm, displaying granular or flower-like structures. The presence of large particles and intricate structural morphology suggests that the glauconite in this region has undergone significant evolution and is currently in an advanced stage of maturity. This finding further supports the distinctive characteristics of this area as a basin with a low sedimentation rate (Vika et al., 2007).
Discrimination of sedimentary environment
Rock mineralogy index
Authigenic glauconite grains typically develop in shallow marine environments during the Precambrian period, particularly in regions abundant in sandy sediments, leading to their common occurrence in sandstone. However, since the Phanerozoic eon, authigenic glauconites have been more frequently identified in shales (Banerjee et al., 2016). The reducing and acidic conditions created by organic matter decomposition facilitate the incorporation of iron ions into the glauconite lattice, resulting in pyrite often being found alongside glauconite (Udita Bansal et al., 2018; Lee et al., 2002; El Albani et al., 2005). In oxidizing sedimentary environments, the migration of iron ions is restricted, leading to authigenic minerals with reduced iron content, typically producing goethite. In our investigation, a significant association between pyrite and glauconite was observed, suggesting that glauconite formation of XSC 1 well occurred in the reducing environment. Furthermore, the composition of glauconite is closely related to its porosity and permeability characteristics. Cenozoic glauconites are predominantly found in fecal spherules, primarily due to the higher porosity and permeability of fecal spherules compared to the micropores of deep-water foraminifera, which can supply more Fe and K elements for glauconite formation. This factor may also explain the association of glauconite with bioclasts in XSC 1 well.
The matrix is predominantly composed of micrystalline cementation, a feature commonly found in calm water or low-energy environments with weak turbulence, such as lagoons, enclosed seas, and tidal flats. The presence of bioclastic fabric formed in shallow water environments suggests that these sediments were originally situated above the wave base and were subsequently transported by wave action to a relatively quiet water setting. In tidal flat environments, micrite bioclastic limestone within argillaceous rocks typically occurs in discontinuous, lens-like strips or thin layers, with a relatively simple biological community. However, in this study, the micrite bioclastic rocks exhibit thick layer development, with the entire Dawangou Formation being predominantly composed of such rocks, suggesting that the possibility of a tidal flat environment can be discounted.
Rocks formed in enclosed environments often display characteristics of a "facies mosaic," where low-energy muddy deposits alternate with deposits rich in biodetritus. This facies transition is commonly observed in shallow water settings, where minor fluctuations in sea level and sedimentary geomorphology can lead to significant changes in sedimentary environments. Conversely, in open environments with relatively flat terrain, muddy deposits may extend laterally for hundreds of kilometers and be several meters thick.
Paleoenvironment Indicators
In various depositional settings, the chemical interactions between sediments and the surrounding media are highly intricate and diverse. These interactions result in the deposition, segregation, and recombination of chemical elements within sediments, leading to distinct compositions and combinations of elements in different sedimentary environments. Based on this principle, sedimentary geochemistry has emerged as a powerful tool for delineating sedimentary facies and reconstructing paleoenvironments. Consequently, the paleoenvironment can be accurately reconstructed through the integration of sedimentary geochemistry with ratio analysis of trace elements and rare earth elements.
Murray (1994) introduced a methodology for identifying sedimentary environments based on the ratios of Fe2O3/TiO2, LaN/CeN, and Al2O3/(Al2O3 + Fe2O3), as well as their pairwise relationships. This method was developed following extensive geochemical analyses of rock samples from various sedimentary settings spanning the Early Paleozoic to Tertiary periods worldwide. A key finding was that an Al2O3/(Al2O3 + Fe2O3) ratio below 0.4 typically signifies deposition in a mid-ocean ridge environment, while ratios between 0.4 and 0.7 may suggest an ocean basin setting. Ratios exceeding 0.5 are more indicative of a continental margin depositional environment. Analysis of the data presented in Table 1 revealed that the Al2O3/(Al2O3 + Fe2O3) ratio for micrite bioclastic limestone from the Dawangou Formation ranges from 0.57 to 0.91, with an average of 0.78. This suggests a sedimentary environment consistent with a continental margin location.
Moreover, Murray (1994) highlighted that the LaN/CeN ratio exhibited variations corresponding to changes in sedimentary environments, typically demonstrating a progressive increase from continental margins to deep-sea basins and then to mid-ocean ridges. Specifically, rocks deposited at continental margins generally display LaN/CeN ratios ranging from 0.5 to 1.5, while those in deep-sea basins fall between 1.0 and 2.5. In contrast, rocks deposited at mid-ocean ridges typically exhibit LaN/CeN ratios exceeding 3.5. Analysis of the data presented in Table 3 reveals that the LaN/CeN ratio of bioclastic limestone from the Ordovician Dawangou Formation ranges from 0.59 to 0.92, with an average value of 0.81, supporting the interpretation that the sedimentary environment corresponds to a continental margin setting. By applying the research methodology outlined by Murray (1994) and integrating the relevant data on micritic bioclasts from the Ordovician Dawangou Formation in this study, Fig. 5 is constructed, providing initial evidence that the depositional environment of the Dawangou Formation in XSD1 and XSC 1 Wells is situated at a continental margin(Fig. 6).
Paleosalinity index
The ratio of strontium to barium is a key parameter used to differentiate between marine and continental sedimentary environments. Due to the lower solubility of barium compared to strontium, barium is more likely to precipitate as barium sulfate (BaSO₄) with increasing seawater salinity. As salinity continues to rise, strontium, which is relatively enriched, precipitates as strontium sulfate (SrSO₄). This sequential sedimentation process makes the Sr/Ba ratio in sediments a valuable tool for quantitatively assessing paleosalinity in aquatic systems. A Sr/Ba value greater than 1 indicates a marine sedimentary environment, while a value between 0.6 and 1.0 suggests a brackish water environment. The Sr/Ba ratios of micrite bioclastic limestone samples from the Dawangou Formation in this area range from 1.15 to 108.69, with an average of 56.9. These values exceed the typical Sr/Ba ratio range for a brackish water environment (0.6 ~ 1.0), indicating that the area was predominantly in a marine brackish water environment during the deposition period.
Paleoclimate index
Previous research has demonstrated that the aridity of sedimentary climates plays a significant role in the precipitation of Sr elements in water. As the climate becomes drier, the water tends to become more alkaline, leading to easier precipitation of elements like Sr. Consequently, there exists a positive relationship between the degree of aridity in sedimentary environments and the concentrations of Sr and other elements. Particularly, the Sr/Cu ratio serves as a crucial indicator of the humidity of the climate. In regions with high humidity, the Sr/Cu ratio typically falls between 1 and 10. Conversely, a dry climate is characterized by a Sr/Cu ratio exceeding 10. Analysis of micrite bioclastic limestone samples from the Dawangou Formation in this area revealed Sr/Cu ratios ranging from 1108 to 2116, with an average of 1381. These values significantly surpass the typical Sr/Cu ratios observed in humid climates, suggesting that the sedimentary period was indeed arid.
Previous research has typically suggested that glauconite is a distinctive mineral formed during transgression, and its growth is favored by warm shallow marine environments rich in organic matter. Glauconite formation is commonly found at depths of up to 250 m (Porrenga, 1968), although occurrences in deeper waters have been documented (McRae, 1972), which are believed to be of transported origin rather than formed in situ. The absence of marine glauconite in deep-sea settings is attributed to the low iron content in deep waters, leading to the majority of glauconite formation in continental shelf environments (Banerjee et al., 2016). Furthermore, the formation of glauconite typically occurs at temperatures above 15℃ (Baldermann A, et al., 2013), indicating that cold environments are not conducive to glauconite formation. This could be due to a combination of factors such as limited silicate availability, decreased reaction rates, and reduced microbial activity under cold conditions.
Paleoredox index
Throughout its extensive geological history, the ancient ocean has undergone numerous global transitions between anoxic and oxygen-rich environments with the fluctuation of sea level. During these periods, the distribution and abundance of elements were significantly influenced by REDOX reactions in the ancient marine setting. In particular, the content and proportion of REDOX sensitive elements such as U, Th, V, Cr and Ni can provide important clues for us to reveal the REDOX state of the ancient Marine environment. Through detailed analysis of bioclastic limestone samples from the Dawangou Formation in this region, it was observed that the Th/U ratio ranged from 0.45 to 8.34, with an average value of 2.74. Previous research suggests that a Th/U ratio below 2 typically indicates an anoxic environment, while a ratio exceeding 3.8 represents an oxidizing environment. Based on these criteria, the environment represented by the bioclastic limestone of the Dawangou Formation is more inclined to the weak oxidation-weak reduction environment. Furthermore, the Ni/Co ratio serves as a crucial indicator for assessing the REDOX conditions of the ancient ocean. A Ni/Co ratio above 7 indicates an anoxic environment, while a ratio between 5 and 7 suggests an oxygen-deficient environment. A ratio below 5 points to an oxidative environment. The bioclast samples from the Dawangou Formation exhibit elevated Ni levels, with the Ni/Co ratio ranging from 9.86 to 11.02, and an average value of 10.54, indicating an anoxic environment during the deposition period. Additionally, the V/(V + Ni) ratio and V/Sc ratio of the bioclastic limestone samples from the Dawangou Formation were also examined. The V/(V + Ni) ratio ranged from 0.01 to 0.04, with an average of 0.02, while the V/Sc ratio varied between 0.11 and 0.24, with an average of 0.16. These ratios further support the conclusion that the region experienced a weak oxidation-reduction environment.
In-situ glauconite often retains its original form without obvious signs of damage from transportation. In contrast, transported glauconite tends to become more rounded and abrasive. Additionally, during evolution, the potassium content gradually increases while the iron content decreases, leading to a deepening of color (Zhang et al., 2017). In this study, we observed that some glauconites exhibit a rounded shape, while most have an irregular crystal shape and commonly intrude along fractures with clay minerals. This suggests that these glauconites and clay minerals were deposited simultaneously in a low-energy reducing environment. Building on previous research on the sedimentary facies of the Lower Ordovician Yijianfang Formation in the northwest Tarim Basin, we tentatively conclude that the sedimentary environment of the Dawangou Formation represents slope-basin facies situated below the wave base with relatively weak hydrodynamic forces. Based on this interpretation, we have updated the depositional facies map of the Dawangou Formation in the northwest Tarim Basin, providing valuable insights for further exploration of the stratigraphic and sedimentary characteristics in the region (Fig. 7).
IMPLICATIONS
The findings of this study hold significant implications for mineral identification and environmental analysis (Raphael et al., 2022). This paper demonstrates the semi-quantitative determination of the elemental composition of glauconite and other minerals in thin sections using scanning electron microscopy and energy spectrum analysis. By comparing the major elements of glauconite from various sources and formation environments, a deeper understanding of the genetic mechanism and evolutionary process of glauconite can be achieved.
Glauconite serves as a crucial indicator mineral for environmental assessments. The sedimentary environment of glauconite can be inferred from its color, micromorphology, and elemental composition. Autochthonous glauconite typically forms in slow, low-energy, and weakly reducing environments, while allochthonous glauconite is predominantly found in high-energy and weakly oxidizing environments. In this study, the presence of glauconite alongside mud strips and in association with pyrite suggests the glauconite is quasi-autochthonous and formed in a weakly oxidizing and reducing environment. Furthermore, the presence of bioclasts and carbonate particles as parent material indicates transportation to the middle-outer gentle slope sedimentary environment by waves, tides, and storms.