During the Late Ordovician, abundant siliceous rocks interlayered with K‐bentonites were widely distributed in the Wufeng Formation in the north‐western part of the Sichuan Basin, South China. In order to analyse the material provenance, redox condition, sedimentary environment, and their formation, here we present mineralogical and geochemical investigations on siliceous rocks samples collected from three sections. The presence of massive radiolarian tests, high Si/Si + Al + Fe + Ca ratios (0.87–0.99) and Al–Fe–Mn diagram plotting at nonhydrothermal area, reflects that the deposition of the siliceous rocks was simultaneous, effected by biogenic and terrigenous origin. The TiO2–Zr, La/Th–Hf, and La/Yb‐∑REE bivariate diagrams indicate that the provenance of Late Ordovician siliceous rocks was primarily from felsic igneous rocks. According to the palaeoredox indicators (V/(V + Ni), V/Cr, V/Sc, U/Th, and Ni/Co) and the size of framboidal pyrite, it is considered that the water column conditions of the Late Ordovician siliceous rocks were likely to be anoxic in all three sections. The application of parameters for relatively immobile elements allowed us to establish a continental margin for most samples from the three sections. Finally, we propose that the deposition of siliceous rocks was related to the accretion of the Cathaysia Block to the Yangtze Block. During the Late Ordovician, huge amounts of volcanic ash were released by intensified volcanic eruptions and fell into the ocean, triggering the proliferation of radiolarians. The abundant radiolarians and the persistent continental detritus constituted the major components of siliceous rocks in the Upper Yangtze Block.
The Qixia Formation and Maokou Formation of Middle Permian in the southwestern Sichuan Basin were pervasively dolomitized during the diagenetic history. Petrographically, four types of dolomites, namely three replacive dolomites (Rd1, Rd2, and Rd3) and one dolomite cement (Cd), were distinguished. Rd1 dolomite occurs as very fine (<50 µm), planar-s to nonplanar crystals; Rd2 dolomite shows planar-e to planar-s crystal shapes with fine crystal sizes (50–250 µm) and is characterized by center-frog and margin-clear; Rd3 dolomite occurs as medium to coarse (250 µm–2 mm), nonplanar crystals; and Cd dolomite is characterized by saddle crystals filling dissolution pores and/or fractures, translucent white color in the hand samples, and strong sweeping extinction under cross-polarized light. In areas close to reactivated basement faults (Zhangcun outcrop and well Hanshen1), Rd3 (~65% by abundance) was the dominant type of replacement dolomite and minor amounts of Rd1 and Rd2 (~10%) were found in this area. Cd (~25%) was extensively developed in fractures and dissolution pores, whereas, in areas far away from the fault zones (Xinjigu outcrop), Rd1 (~20%) and Rd2 (~55%) were dominant replacement dolomites, and only a small portion of them were recrystallized to form Rd3 (~20%), with minor Cd (~5%) dolomite occurring in some dissolution pores. The δ13CV-PDB (−0.37‰ to 4.32‰) and δ18OV-PDB values (−7.41‰ to −5.19‰), 87Sr/86Sr ratios (0.707085 to 0.707795), and rare earth elements (REE) patterns (flat REE patterns with slight light rare earth element (LREE) enrichment and slight negative Ce anomalies) suggest that Rd1 dolomite was formed penecontemporaneously in an evaporitic tidal flat evaporation environment with salinities higher than seawater. The Rd2 dolomite, characterized by δ13CV-PDB (−0.18‰ to 4.89‰) and δ18OV-PDB values from −6.6‰ to −5.5‰, 87Sr/86Sr ratios from 0.707292 to 0.707951, and LREE enrichment and slight negative Ce anomalies, was interpreted as forming from the recrystallization of Rd1 at shallow burial. The δ18OV-PDB values (−12.01‰ to −8.23‰), the prominent positive anomaly of Eu, high 87Sr/86Sr ratios (0.7081–0.7198) and high fluid inclusion homogenization temperatures (149–255 °C) suggest that Rd3 and Cd dolomite were formed from hot fluids. Based on regional stratigraphic data, the Rd3 and Cd were likely formed at depths less than 1500 m; thus, the ambient burial temperature would be lower than 85 °C. The high fluid temperatures recorded by fluid inclusions, thus, indicate that the dolomitization was of hydrothermal nature. The δ18OV-SMOW values, homogenization temperatures, and salinities of the fluid inclusions of Rd3 and Cd in proximal areas were systematically higher than those in distal areas, suggesting that the hydrothermal fluid ascended along faults in proximal areas and then migrated laterally along the strata to distal areas. The dolomites of the Middle Permian carbonates in the southwestern Sichuan Basin, thus, resulted from different dolomitization phases, and the latter hydrothermal dolomitization was controlled by a combination of strata and structures.
In recent years, the discovery of two gas fields in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China confirmed the exploration potential of microbial carbonates. The aim of the present study is to clarify the formation mechanism of the microbial reservoirs in the Leikoupo Formation. For this purpose, lithofacies, depositional environments, and diagenesis analyses were performed in samples collected from cores of 12 wells. The climate of study area was arid during Anisian time, and the water body was restricted. In such a climate, an evaporitic environment was developed, where ten types of lithofacies, dominated by microbial carbonates and gypsum rocks, were recognized. Thrombolites and stromatolites are the main high-quality reservoirs rock types in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China, which developed as microbial mounds, with reservoir space of microbial inter-clot pores, intra-clot pores, fenestral pores, inter-crystalline pores, and cracks. The microbial inter-clot pores are the main reservoir space, formed by trapping and binding of marls by benthic microbial communities. These pores were partially filled with evaporites because of the arid climate, which were subsequently dissolved (mainly gypsum) in the syn-depositional period, thus greatly improving the quality of reservoirs. Although some pores were occluded by multi-stage cements during the burial stage, major pores were well preserved own to the early dolomitization, rapid burial of the Leikoupo Formation, and early charging of hydrocarbon. The early dolomitization enhanced the anti-compaction ability of microbial carbonates during the burial stage. Rapid burial of the Leikoupo succession slowed down early cementation, and it also accelerated the maturation and expulsion process source rock to promote early charging of hydrocarbon in pores, which created a closed system, inhibiting strong burial cementation.
The Middle Triassic succession of the Sichuan Basin contains key information about the Early Indosinian Movement, during which it has changed from extensional to compressional. In order to clarify the relationship between the evolution of the Sichuan Basin during the Indosinian Movement, the Middle Triassic tectonostratigraphic environment should be determined. Detailed stratigraphy and facies analysis integrated with structural and basin analysis, allowed a full reconstruction of the tectonic and depositional environments for the Middle Triassic strata of the Sichuan Basin. Six facies associations from two rock systems of: (a) carbonate–evaporite system and (b) a mixed carbonate–siliciclastic system, were characterized. Four depositional stages highlighted by the lithostratigraphic members of the Leikoupo (Lei) and Badong (Ba) formations were divided: (a) stable subsidence and deposition of the Lei‐1/Ba‐1 Member, leading to an embryonic ‘uplifts and sags’ configuration; (b) inherited deposition of the Lei‐2/Ba‐2 Member, with the platform margin belt extended; (c) rapid deposition of the Lei‐3/Ba‐3 Member, characterized by an eastward shift of the depositional centre and formation of the central anhydrite‐rich lagoon; and (d) intensification of cratonic tectono‐sedimentary differentiation during the Lei‐4/Ba‐4 deposition, that resulted in subaerial exposure along the Luzhou–Kaijiang uplift zone and a hydrodynamically restricted depression along the western platform. The Middle Triassic tectonic framework of the Sichuan Basin was dominated by compression in the east and extension in the west. The western margin was a passive continental margin, inheriting an early rifting event, while the eastern area within the craton showed an ‘uplift‐sag’ pattern, which is a response to intracontinental tectonic movement of the Xuefengshan uplift belt. The basin evolved from incipient low‐relief uplifts and sags within the intracratonic depression to subsequent isolated foreland and restricted depression during the Middle Triassic.
Fluctuations in nitrogen (N) availability influence protein and starch levels in maize (Zea mays) seeds, yet the underlying mechanism is not well understood. Here, we report that N limitation impacted the expression of many key genes in N and carbon (C) metabolism in the developing endosperm of maize. Notably, the promoter regions of those genes were enriched for P-box sequences, the binding motif of the transcription factor prolamin-box binding factor 1 (PBF1). Loss of PBF1 altered accumulation of starch and proteins in endosperm. Under different N conditions, PBF1 protein levels remained stable but PBF1 bound different sets of target genes, especially genes related to the biosynthesis and accumulation of N and C storage products. Upon N-starvation, the absence of PBF1 from the promoters of some zein genes coincided with their reduced expression, suggesting that PBF1 promotes zein accumulation in the endosperm. In addition, PBF1 repressed the expression of sugary1 (Su1) and starch branching enzyme 2b (Sbe2b) under normal N supply, suggesting that, under N-deficiency, PBF1 redirects the flow of C skeletons for zein toward the formation of C compounds. Overall, our study demonstrates that PBF1 modulates C and N metabolism during endosperm development in an N-dependent manner.
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