Sedimentary Geology最新文献

The mining of lower coal seams in the Huainan coalfield faces significant threats from water bursts from the Ordovician carbonate karst aquifer. Paleokarst breccias constitute significant water storage spaces, and comprehending their developmental characteristics and formational models is paramount for implementing effective prevention and control strategies against water inrush hazards. Through such methods as the field geological survey, core drilling, petrography, and geophysical exploration, the compositions and textures of carbonate breccias had been investigated. The results indicate that the Ordovician paleokarst breccias can be divided into three types. Syngenetic breccias formed during brief sedimentary breaks, consist of different size gravel particles, with poorly sorted and angular fragments. Pores between angular clasts are filled with calcium and mud cementitious materials, which reveal the significant influence of sedimentary and diagenetic processes. Epigenetic karst breccias occur within the interlayer between the Majiagou and Xiaoxian formations, which originated from the collapse and subsidence of tensioned karst caves in the interlayer. Angular clasts are typically subrounded-angular in shape, and poorly sorted, which development processes are influenced by carbonate lithology, tectonic evolution, and paleogeomorphology. Tectonic breccias occurred along major tectonic belts and consist of angular to subangular poorly sorted gravel size fragments, which development processes are primarily controlled by multiphase tectonic movements, particularly Indosinian-Yanshanian tectonic events. The above research results have provided an important foundation for the safety mining of deep coal seams in the Huainan coalfield, and other coalfields with similar geological condition in China.

The platform-marginal shoal carbonates of the Middle Permian Qixia Formation in the NW Sichuan Basin, as important ultra-deep (>5000 m) hydrocarbon reservoirs, experienced multi-phase dolomitization and recrystallization during the long-term and complex basin tectonic evolution; however, there is no consensus on the dolomitization mechanism. Four types of dolomites have been identified: medium- and coarsely-crystalline dolomites within patchy dolomitized limestone (LD) at the top of the carbonate strata, massive medium to coarsely-crystalline dolomites categorized as porous dolomites (MD1) and tight dolomites (MD2) at the bottom of the carbonate strata, and cement coarsely saddle dolomites (SD) in the vugs and fractures. All dolomites exhibit similar REE_SN patterns to the host limestone with an obviously positively Ce anomaly, and show Sr isotopes falling in the range of Permian seawater. However, the LD dolomites display a higher Sr but lower Mn concentration, and more positive δ¹³C and δ¹⁸O than other types of dolomites. The petrographic and geochemical results suggest the dolomites of the Qixia Formation replaced precursor grainstone by the seawater-derived fluids. A comprehensive dolomitization model is proposed, incorporating seawater-reflux, intermediate-burial and tectonic-squeegee, and hydrothermal dolomitization for forming different types of dolomites. LD dolomites are early diagenetic products after selective dissolution under a near-surface or shallow burial environment, in which the weak-evaporated Permian seawater mixed with minor meteoric water accounting for dolomitizing fluids. MD1 and MD2 dolomites are likely to form due to the residually buried seawater for widely dolomitization driven by the Late Triassic tectonic compression. Subsequently, the dolomitic strata underwent hydrothermal modification due to rejuvenation of the thrust belt to enable saddle dolomites filling pre-forming porosity. This study integrates outcrop, petrographic, geochemical and dating analysis with paleogeography and tectonic events, providing a new perspective to establish a conceptual model for multi-phase dolomitization as an analog in comparative tectonic settings worldwide.

Ripples on tidal flats significantly influence bedform roughness and near-bed turbulence, yet their dynamics in sandy and muddy environments remain incompletely understood. While laboratory studies have elucidated the effects of mud content and extracellular polymeric substances (EPSs) on ripple formation and stability, the interactions between ripple characteristics, EPS, and mud content in natural settings are more complex and not thoroughly explored. To address this gap, we conducted field studies on sand ripples at the central Jiangsu coast, China, using high-resolution drone photogrammetry to accurately measure ripple dimensions. We performed bedload sampling at both ripple crests and troughs to analyze median grain size, mud content, and EPS concentrations. Our results show that the investigated ripple wavelengths range from 34 to 46 mm, and heights vary between 2.7 and 5.3 mm. Ripples developed near tidal creeks show pronounced asymmetry. Significantly, EPS concentrations are markedly higher at the ripple crests than at the troughs, and it follows a power-law relationship with the median grain size. These results highlight the complex intricate relationship between the ripple morphology, environmental forcing conditions, and EPS content. Our findings enrich the understanding of ripple development and the factors influencing current-dominated ripples in natural environments. This research also adds to the better prediction of the bedform roughness and quantification of the near bed sediment transport.

Sand-sized quartz grains selected from sediments of a Pleistocene inland dune (eastern Poland) were subjected to a series of laboratory analyses, including grain-size distribution, morphoscopy analysis, microtextural analysis using scanning electron microscope (SEM) and nanostructural analysis using transmission electron microscope (TEM). The results indicate the presence of a crust on the surface of all of the studied quartz grains and a surprisingly low number of aeolian-induced mechanical microtextures. The TEM examination identifies illite-smectite assemblages as the main component of the crust, accompanied by amorphous silica, local accumulation of Fe, K, Ca, Mg oxides, and mineral particles (quartz, K-feldspar, chlorite). The thickness of the crust reaches approx. 0.1–0.2 μm and varies from the minimum on micro-protrusions up to the maximum in micro-cavities. Here, we present a new theory on the origin of the crust observed on the surface of cold-climate aeolian quartz grains. We postulate that the crust is formed within a near-surface saltation layer during active aeolian transport of sand-sized grains and clay-sized particles operating under cold-climate conditions. The development of abrasion features and the formation of the crust are interpreted here to occur simultaneously and continuously during the aeolian transport. The formation of the crust results from specific properties of aeolian transport (i.e. self-induced electrification and electrification mechanisms of quartz grains) and the quartz grains themselves (i.e. grain shape and surface microtopography).

Distinctive erosional and omission surfaces occur at several stratigraphic levels in the tidal carbonates of the 3rd-order Coniacian sequence (Iberian Basin). They are ancient analogs of omission surfaces developed on lithified carbonates in subaerial and coastal settings. Omission surfaces consist of (i) firmground Glossifungites ichnofacies (Balanoglossites-Thalassinoides); and (ii) hardground Trypanites ichnofacies (scalloped and planar surfaces and Gastrochaenolites-Entobia surfaces). Firmgrounds are also related to erosion or ferruginous crusts. Hardground surfaces are related to bioerosion, dissolution and physical erosion. Grain size and textural features in Balanoglossites and Thalassinoides firmground surfaces are essentially the same, suggesting that even bathymetry could be similar. Several stages in hardgrounds consist of different, scalloped or planar surfaces related to bioerosion, dissolution and physical erosion. Gastrochaenolites-Entobia borers represent a major change in the trace fossil associations and imply different processes in their origin, being originated at slightly different depths with Gastrochaenolites representing shallower environments. The studied field sections display a cyclicity on the scale of meters that tentatively reflects the presence of 4th-order parasequence sets. Two kinds of sedimentary discontinuities have been used for correlation: omission surfaces and ferruginous crusts representing regional sea level falls and rises. Part of the described surfaces does not appear to have been previously recognized in older carbonate deposits. Their common presence of similar surfaces along modern coasts and in karst terrains, as well as their abundance in the Coniacian sequence, suggests that they might also be abundant in the geologic record in other sedimentary basins for defining palaeoshorelines.

Quantitative identification of sedimentary processes' evolution trend has substantially influenced the exploration of lacustrine mudrocks. To explore the indicative applicability of the Hurst index (HI) for analyzing the sedimentary processes' evolution trend in a deep lacustrine sequence, lacustrine mudrocks from wells L69, FY1, and NY1 in the Jiyang Depression, eastern China are selected. The comparable fourth-order cycle is divided through lithofacies observation, total organic carbon (TOC), and natural gamma ray (GR) logs combined with previous studies. In addition, the variation of accommodation space is selected to represent the sedimentary processes' evolution trend in the lacustrine environment, characterized by the frequency of oscillation (FST) and directionality (D). Further, HI is introduced to identify the long-term persistence of the sedimentary processes' evolution trend in the lacustrine environment. Through the criteria for the series selections, the TOC series and GR logs passed the Hurst phenomenon test, and the computed HI of those two series was selected to evaluate its effectiveness toward the lacustrine sedimentary processes' evolution trend. The HI computed from the TOC series demonstrates strong negative linear relationships with FST and D, with R² in 0.833 and 0.814, respectively. Besides, the HI from the more accessible GR logs demonstrates robust negative linear relationships with FST and D, with R² in 0.981 and 0.976, respectively. The statistical analysis indicates that the content of deep lake subfacies, shale, and mudstone could affect the long-term persistence of the lacustrine mudrock sequence. Thus, the HI could be an alternative indicator to identify the imperceptible sedimentary processes' evolution trend in the deep lacustrine environment.

Considerable attention has been dedicated to the sedimentological processes associated with carbonate drift and contourite deposits, but a noticeable gap exists in the understanding of the diagenetic aspects of those deposits, especially dolomitization. This study presents an examination of dolomites from Middle to Late Miocene drift deposits collected during IODP Expedition 359 to the Maldives archipelago. Multiple geochemical parameters and petrographic analysis are used to explore the potential role of closed versus open system dolomitization in the two oldest drift sequences, which are overlain by a multimillion year-long hiatus. Overall, dolomite abundance is variable, but 50 % to nearly 100 % in a 30 m thick interval below the unconformity surface at one of the examined sites. The dolomite in the study interval consists of very fine-to-fine-sized crystalline dolomite cements and mimetically replaced dolomite grains. All dolomite is non-stoichiometric (mean 42.7 ± 2.0 mol% MgCO₃) and mostly poorly ordered. Geochemical attributes include relatively invariant δ¹³C (+1.3 ‰ to +1.7 ‰ VPDB) and relatively high Sr concentrations in dolomite cements (mean 505 ppm) and dolomitized grains (mean 784 ppm). δ¹⁸O values and the constraints of burial histories indicate dolomitization in normal marine seawater at burial depths of 0 to 300 m and temperatures of ∼10 °C–14 °C below an ocean water column 100 to 400 m deep. Sr-isotope ages suggest dolomitization of the most extensively dolomitized interval below the unconformity occurred between 12.3 Ma and 6.7 Ma. Overall, the geochemical data and previously published δ³⁴S_CAS data suggest a closed, diffusion-dominated system created most of the dolomite. However, the youngest dolomite's age and bulk rock Sr isotope ages of calcitic rocks at the base of the drift deposits can only be explained by the advective flux of seawater through all the drift deposits. Furthermore, the geological context, including permeable facies, ocean current patterns, and other indicators, such as the absence of hardground or permeability barriers, suggests that some open system dolomitization may have also occurred.

The continental red beds, encompassing a broad spectrum of genetic types, can serve as important palaeoclimatological and palaeoenvironmental archives. The origin of sediment colouration is a complex process involving abiotic processes (e.g., breakdown of original and precipitation of newly-formed minerals), which, together with biogenic factors, lead to mobilisation of redox-sensitive elements and precipitation of Fe- and Mn-(oxy)hydroxides. There is still discussion about the interpretation of the continental red beds as palaeoclimatological archives or the colour patterns reflecting ancient redox gradients. The layers coloured in red, yellow or black can be found in the Quaternary glaciofluvial sediments in the Czech Republic. We are using a combination of field study with multi-spectral petrophysical, petrological and geochemical analyses to investigate the mechanism and timing of the origin of coloured coatings in glaciofluvial sediments, and causes of cycling of Fe, Mn, and other redox sensitive elements and isotopes. The results show that both syn-depositional and early diagenetic processes are responsible for the origin of colour patterns in the Quaternary glaciofluvial sediments. The stable molybdenum and iron isotope fractionation is primarily driven by the breakdown of the primary Fe and Mn-bearing silicates and the precipitation of the secondary Fe- and Mn-(oxy)hydroxides, such as goethite and birnessite. These precipitates are the main components of colouring coatings on the detrital grains and are able to bind other redox-sensitive elements, such as Cu, As, Mo, U, and REEs. The textural patterns and geochemistry suggest that the colour features were developed in the time range of decades to several thousand years after the deposition along ancient subsurface redox gradients due to changes in groundwater flow associated with primary lithology, glaciotectonics, and seasonal changes in the active layer of permafrost. The coatings show morphological features (rods, botryoids) and geochemical signatures (e.g., increased P contents) suggesting involvement of microorganisms to their precipitation.

Mixed siliciclastic‑carbonate depositional systems are prone to differential diagenesis due to lithological heterogeneity. However, unlike pure carbonate and/or siliciclastic counterparts, the diagenesis within mixed clastic‑carbonate successions remains poorly known. This study integrates sedimentological and petrographic analysis with porosity-permeability measurements to understand diagenetic variability and its impact on fluid flow in the Oligocene-Miocene mixed clastic‑carbonate system (lower part of Musayr Fm) in Midyan Basin, Red Sea, Saudi Arabia. Sedimentological observations highlight that siliciclastic intervals comprise conglomerates, coarse- to fine-grained sandstones and subordinate mudstones whereas mixed siliciclastic‑carbonate intervals are composed of shell, ooids and microbialites-dominated facies. Petrographic analysis indicates presence of several diagenetic processes in siliciclastic intervals including dissolution of unstable silicate grains (feldspar), formation of pore filling and/or grain-replacive kaolinite, precipitation of calcite and/or dolomite, and replacement of dolomite by silica. The siliciclastic strata exhibit minimal compaction fabrics with no stylolites and are characterized by higher permeability (average = 1884 md) and porosity (average = 18.7 %). On the other hand, mixed siliciclastic‑carbonate intervals underwent micritization, sparry calcite formation, dolomitization of micrite and bioclasts, and replacement of sparry calcite by pyrite and Fe-oxides. Wavy stylolite seams were also observed in ooid facies reflecting moderate chemical compaction. In addition, the porosity (average = 6.6 %) and permeability (average = 93 md) are magnitudes lower than siliciclastic counterparts. Significant differences between porosity and permeability of siliciclastic and mixed intervals are partly linked to relatively intense diagenetic alteration (higher cementation and chemical compaction) in mixed units. Understanding such diagenetic heterogeneity in mixed systems has important implications for identification of reservoir and non-reservoir zones and may provide useful insights for hydrocarbon exploration and carbon sequestration in the analogous strata in the subsurface.