Sedimentary Geology最新文献

The lithofacies architecture and depositional evolution of the Lower Mississippian (Tournaisian) Pekisko Formation in the subsurface of the Hawk Hills area in northwestern Alberta have been established by integrating detailed core work and well log data. The formation is composed of skeletal-peloidal limestones and argillaceous limestones that were deposited along the northern flank of the Peace River Embayment, a semi-restricted and tectonically active oceanic re-entrant located along the western margin of Laurasia at low paleolatitude. Lithofacies associations recognized in the study area include the outer ramp to slope (LA 1), outer ramp (LA 2) and mid ramp (LA 3), which are stacked into three decameter-scale, deepening-upward and aggradational cycles that are of regional extent and have meter-scale deepening and shallowing-upward trends. A previously unrecognized paleosol horizon at the top of decameter-scale cycle 2, indicating widespread subaerial exposure of the ramp, is interpreted as a sequence boundary that divides the Pekisko and Shunda formations in the study area (and possibly elsewhere in the Peace River Embayment) into two third-order sequences, each consisting of transgressive and highstand systems tracts. The Pekisko Formation in the study area is interpreted to represent a low-energy, temperature-stratified ramp that was mainly homoclinal, but with transient, distal steepening occurring in the southern part of the study area. Ramp deposition was strongly affected by basement-fault reactivation causing differential subsidence and uplift in the Peace River Embayment. The paleogeography and paleoceanographic conditions of the embayment favored upwelling currents and development of a temperature-stratified ramp, as well as the formation of heterozoan carbonate deposits and mid ramp facies of predominantly packstones and wackestones. This depositional scenario is atypical, as most other documented examples of the Pekisko Formation and other Lower Mississippian ramp successions in western North America and western Europe are characterized by moderate to high-energy, mid to inner ramp facies deposited in open-ocean conditions. The results of this study contribute to an improved understanding of the range of depositional settings along the western margin of Laurasia during the Early Mississippian and demonstrate the applicability of the thermocline-stratified ramp model, with some modification, to ramps in semi-restricted embayments and other low energy settings.

Epicontinental basins are extremely prone to major paleogeographic changes, and this will directly affect any organic matter (OM) preserved in the depositional record. In this study the Middle–Late Miocene successions in the northern Pannonian Basin System were investigated via sedimentological, petrographic, and geochemical analyses of cores from the Danube Basin to reveal the interplay of factors driving the character of the OM. In the late Middle Miocene (~12.3 Ma), the Central Paratethys Sea maintained normal marine salinity, with dysoxic bottom waters in a distal basin floor environment rich in aquatic OM. The last rifting phase followed during the Late Miocene and led to formation of the deep Lake Pannon. Like seawater, the brackish lake water still also contained sulfate. These open lacustrine deposits (~11.6–10.0 Ma) reveal OM sourced from submerged/floating macrophytes and algae, and humid conditions are indicated by the preponderance of deciduous trees and shrubs on shores. The study identifies hybrid event beds (HEBs) on Lake Pannon's floor (~10.0–9.3 Ma), with currents redepositing mud and OM, resulting in similarities between the Middle Miocene and Late Miocene successions. Turbidite deposition (~9.3–9.0 Ma) from the paleo-Danube induced a shift in OM, replacing algae with terrestrial input. Complete isolation from the main water masses of Lake Pannon (~9.0–8.9 Ma) altered its sources of OM, transitioning from algae to macrophytes, and caused a drop in salinity, likely associated with a humidity peak. The subsequent deltaic dominance (~8.9–8.6 Ma) features well-developed topset lakes, swamps, and floodplain forests, reflecting warm temperate to subtropical climates. The Middle–Upper Miocene deposits studied here are source rocks with fair to very good richness and poor to fair generative potential, and contain kerogen types III and IV, while type II is rare. The rapid paleoenvironmental changes observed over the order of ~100 kyr caused the complete switching of OM type and delivery, giving an indication of the complexity of epicontinental basins.

Marine red beds (MRBs) are often attributed to specific redox environments during syndepositional and early diagenetic phases. During the Middle Ordovician, a succession of reddish, deeper-water nodular argillaceous limestones (i.e., Zitai and Kuniutan formations) were deposited along the margin of Yangtze Platform in South China. However, the origin of their color remains enigmatic. In this study, we investigated the Middle Ordovician MRBs from a borehole core newly drilled in the Lower Yangtze area of South China whose stratigraphy frameworks are constrained by carbon isotope and biostratigraphy. This study investigates the pigmentation of these MRBs by integrating petrographic observations, elemental geochemistry, diffusive reflectance spectrometry (DRS), and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometry (EDS). DRS results show that the red pigment is caused by hematite particles in submicron- to micron-level size. SEM demonstrates that the hematite grains are either detrital grains with traces of physical transport from terrestrial source, or flaky amorphous hematite aggregates situated within the calcite crystal interstices, implicating both syndepositional and early diagenetic origins, respectively. In terms of the geochemical result of the bulk rocks, the close positive correlation between Al₂O₃ and Fe₂O₃ indicates that the iron pigment materials may mainly originate from terrestrial Fe-bearing phyllosilicates. These observations are also consistent with the distribution patterns of rare earth elements (REEs) in carbonate leachates. The MRB limestones are characterized by MREE-bulged patterns and close to ~1 Ce anomalies, suggesting active reductive dissolution and subsequent reprecipitation of iron oxides during diagenesis in pore systems. This study proposes that the coloration of Middle Ordovician MRBs in Lower Yangtze Platform was linked to the enhanced input of terrestrial clay minerals rich in iron. The reductive dissolution released iron ions from terrestrial detrital and allowed subsequent reprecipitation of iron-oxides in pore water system during the fluid-buffered diagenesis. In this light, hematites formed during both the syndepositional and diagenetic processes thus could have involved the coloration of the Middle Ordovician carbonate red beds in this case.

Fine-grained rocks have historically been interpreted as a product of the settling of fine fractions in low-energy environments. However, recent studies have suggested that in these environments, more dynamic and complex processes operate. These processes involve a much more diverse set of sedimentary processes than those previously assumed. The lithological homogeneity of clayey successions, associated with the obliteration of their primary characteristics by diagenetic and/or weathering processes, makes it difficult to interpret depositional processes and understand the paleoenvironment. To contribute to a better comprehension of the mechanisms of transport and deposition of fine-grained rocks in a siliciclastic mud-dominated succession, as well as their depositional site, the present study aims at the detailed analysis of a continuous drill core. This core contains a thick Devonian succession consisting essentially of shale from the Ponta Grossa Formation (Paraná Basin, Brazil). Throughout the analyzed succession, a wide variety of sedimentary structures have been identified, such as parallel laminations, wave-ripple cross-stratification, hummocky cross-stratification, normal grading, and gutter casts. These structures show the constant performance of high-energy flows, which are configured as important deposition and/or rework agents. The integrated analysis of facies and trace fossils has proven to be more efficient for paleoenvironmental characterization than isolated approaches. This is because facies and ichnofacies (e.g., Cruziana, with its proximal, impoverished, archetypal, and distal expressions; Skolithos and Glossifungites) can be easily correlated, facilitating the identification of facies associations. This approach not only allows for the interpretation of a shallow marine platform with a gentle slope (ramp shelf) paleoenvironment of moderate to high energy, subject to storm wave action, but also enables the identification of the particular characteristics of deposits from different sub-environments (proximal to distal offshore, offshore transition, shoreface to offshore transition, and prodelta) and their deposition mechanisms. Furthermore, the facies associations are useful for highlighting the variations formed in response to oscillations of the relative sea level and changes in sedimentary input.

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.