Sedimentary Fabrics Research Articles

Deciphering the sedimentary imprint of tsunamis and storms in the Lesser Antilles (Saint Martin): A 3500-year record in a coastal lagoon

Our study aims at identifying and characterizing tsunami and storm deposits by combining sedimentological, geochemical, and radiocarbon dating analyses. X-ray computed microtomography (micro-CT) is used to characterize the sedimentary fabric of the deposits. The study was conducted on a transect of oriented short sediment cores from a small lagoon at Saint Martin (Lesser Antilles), where both event types could be observed. Using micro-CT data of different event deposits, we were able to derive paleo-flow orientations and directions and compare the dynamics of the events to their environmental setting. The results showed that seven extreme-wave event (EWE) deposits, originating from hurricanes or tsunamis, had the highest Ca/Fe ratios, distinctive for marine input to the lagoon, and were characterized by coarser grains. Some also presented above the sandy part of the event deposit a siliciclastic geochemical signal rich in fine siliciclastic sediment with organic matter, which might correspond to backwash deposits. The thickness and frequency of these sandy EWE deposits in our cores decreased from the proximal to the distal zone in the center of the lagoon, with high lateral and vertical variability in sediment thickness. The youngest EWE deposit relates to the unprecedentedly powerful Category 5 Hurricane Irma, which barely left a fine layer of sand in the lagoon, demonstrating the lagoon's resilience to tropical storms. Five of the six other EWEs, characterized by micro-CT-derived sedimentary fabric, likely originated from tsunamis and occurred over the last 3500 years BP with a recurrence interval of 300–400 years, based on radiocarbon dating. A well-studied tsunami deposit from Pre-Columbian times (∼400 years BP) showed oriented sedimentary fabric, which could be used for paleo-flow reconstruction and EWE-type distinction in the Lesser Antilles, suggesting the use of an additional method for identifying EWE-type deposits in the geological record rather than other methods alone. To develop effective strategies for mitigating the natural hazards faced by communities in the coastal areas of the Lesser Antilles, it will be crucial to examine EWEs in the Caribbean region beyond historic times successfully.

Marine anoxia events across the upper Cambrian (stage 10) of eastern Gondwana: Implications from paleoenvironmental geochemical proxies

The uppermost Cambrian succession (Stage 10) of the Wa'ergang section (∼300 m of rhythmites, South China) records three globally correlated negative δ13Ccarb excursions (N1, N2, and the HERB- Helenmaria – Red Tops Boundary), overlying the SPICE event (Steptoean Positive Carbon Isotope Excursion), indicating perturbations in the global carbon cycle. The trigger and process of these events still remain elusive. The petrographic and chemical screening of the investigated carbonates proves well preservation of sedimentary fabrics and at least near-primary geochemical signatures. The paleoredox proxy (UEF/TOC, VEF/TOC, and MoEF/TOC) and δ15Norg profiles exhibit positive excursions that correlate with the negative δ13Ccarb excursions, indicating pulses of expanded oceanic anoxia potentially resulted from anoxic deep-water upwelling. The correlated negative shifts in TOC and paleoproductivity proxy (NiEF and CuEF) profiles suggest suppressed primary productivity, possibly by upwelling-induced oceanic toxic events, aligning with mass extinctions particularly during the HERB event. Independently, the paired δ13Ccarb and δ13Corg (Δ13C) variations reveal decreased biological fractionation that aligns with the δ13Ccarb excursions, implying pulses of declining atmospheric O2 levels, which are consistent with the suggested expanded oceanic anoxia and suppressed primary productivity. The peaks of the negative δ13Ccarb excursions are correlated with the base of upward-shallowing sequences, consistent with the evidence from TOC, δ13Corg, δ15Norg, and trace element proxies, support a scenario of upwelling deep toxic waters during sea-level rise that likely resulted in expanded oceanic anoxia, and ultimately led to the negative excursions in δ13Ccarb values and mass extinction.

X-ray tomography applied to tsunami deposits: Optimized image processing and quantitative analysis of particle size, particle shape, and sedimentary fabric in 3D

The analysis of particle size, shape, size, and their spatial arrangement (i.e. fabric) in a sedimentary deposit plays a significant role in their interpretation in terms of transport mechanism, flow conditions, and sediment source. The present work focuses on the use of X-ray computed tomography (X-CT) to analyze tsunami deposits (core samples), as examples of complex sedimentary assemblages. A workflow of image processing is proposed to measure 3D parameters of particle size, shape, and orientation using two different software packages (Fiji-ImageJ, and IPSDK) and two different methods of image segmentation. The semi-automated method provides better segmentation quality of the tsunami deposit, as compared with the poor performance of fully-automated image segmentation. We also provide some clues to reduce the computational time. This workflow is applied to two examples of tsunami deposits on the coasts of the Algarve (1755 Lisbon tsunami, southern Portugal). Optimized processing of X-CT images gives access to detailed vertical variations of grain size, grain shape, and componentry (clayey matrix, silicates, and carbonates, including marine bioclasts), as well as information on flow directions inferred from the sedimentary fabric in 3D. High-resolution analysis of the bedforms and vertical grain size grading allows determining the flow conditions during the inundation phase of the tsunami.

A data-driven hyperspectral method for sampling of diagenetic carbonate fabrics – A case study using an outcrop analogue of Jurassic Arab-D reservoirs, Saudi Arabia

Diagenetic alteration commonly overprints depositional fabrics in carbonate-dominated sediments and impact reservoir quality. Dolomitization is a prevalent diagenetic process observed in subsurface reservoirs that profoundly alters the depositional precursor's pore network, thereby influencing subsurface storage capacity and fluid flow behavior. Typical workflows to characterize the dolomitized sequences, textures, degree and extent of dolomitization rely on mapping, spatial sampling, and time-consuming geochemical, petrographic and petrophysical analysis. In this study, we propose a hyperspectral data-driven workflow for identifying dolomitized horizons and extracting sample sets optimized to characterize textural and chemical variations. Hyperspectral imaging (HSI) data was acquired with 1.5 mm spatial sampling along a 50 m long core drilled behind an outcrop of the Late Jurassic Jubaila-Arab sequence in Wadi Daqlah, Saudi Arabia. Spectral features in the visible (VNIR), shortwave (SWIR), mid-wave (MWIR), and long-wave (LWIR) infrared regions were then used to classify carbonate mineralogy, allowing for the rapid identification of dolomitized zones, and k-means clustering applied exclusively to the dolomitized areas used to identify intra-dolomite variations and suggest representative sample locations. Petrographic and geochemical analyses were carried out on these samples, revealing that clusters identified with the hyperspectral data represent four distinct diagenetic fabrics. These results demonstrate the value of HSI for objective and data-driven sampling, reducing the number of samples required for petrographic, geochemical and geophysical analysis and hence time and costs required to spatially characterize diagenetic alteration.

Identification of Growth Strata: New Insights From Anisotropy Corrected Paleomagnetic Data

AbstractGrowth strata are important for understanding regional tectonic evolution. However, it is difficult to identify growth strata in the field, because of subsequent intensive modification and erosion. To address this problem, we studied two paleomagnetic sections at the stable deposition setting and the growth end of the Miocene growth strata of the Gengma Basin. Average inclinations of the two sections show a systematic deviation of 4.7° ± 5.2°. The anisotropy of magnetic susceptibility results for the growth end of the section A demonstrate a sedimentary fabric. However, the tilt‐corrected K3 axis deviates to the southwest from the normal direction of the bedding by 5.4° ± 3.8°, the magnetic foliation deviates to the equatorial plane by 5.3° ± 4.7°. The K3 axis is perpendicular to the bedding at the stable deposition setting. The deviation of the K3 axis from the normal direction of bedding is interpreted as the growth angle, which can be used to correct the paleomagnetic inclination data.

Porosity characterization using medical computed tomography scans of two deeply buried paleokarst aquifers in Silurian carbonates, southern Ontario, Canada

Paleokarst and paleokarst aquifers are not as well-documented as shallow karst, and studies of pores and pore networks are rare. Paleokarst lacks the open conduits typical of karst due to compaction, infilling, and diagenetic recrystallization by deep burial, so groundwater movement is through matrix porosity. In this case study, porosity networks in two saline aquifers in carbonate paleokarst of the Silurian Guelph Formation and Salina A-1 Carbonate Unit in southern Ontario (Canada) have been studied in drill cores at microscopic to macroscopic scales, utilizing medical computed tomography (CT) scans, optical petrography, macroscopic core examination, and scanning electron microscopy. The CT scans provided nondestructive three-dimensional visualization and quantification of pore distribution, size and volume, pore connections, and estimates of total porosity, similar to gas porosimetry. In the A-1 Carbonate, 50% of the pore volume comprises layers of macropores associated with algal laminations, with good horizontal connections. In the Guelph Formation, vuggy macropores contribute most of the pore volume. They are connected through subvertical fractures and rubble-filled karst conduits and by abundant small pores and micropores with poor horizontal connections. The Guelph paleokarst represents a longer period of subaerial exposure as evidenced by its greater thickness and geographic extent, former karst conduits, and near-total destruction of primary sedimentary fabrics. The findings provide real-world examples for construction of laboratory scale models of paleokarst aquifers, demonstrate the value of a multi-scale approach to porosity studies, and showcase the value of medical CT scans with its unique ability to visualize pore connections within drill cores.

Frost-induced changes in the structure of sediments – Results after 500, 1 000, 1 500 experimental freeze–thaw cycles

Sediment samples composed of silt (<0.063 mm), fine- (0.125–0.250 mm) and coarse-sand (0.5–1.0 mm) fractions were subjected to freeze–thaw cycles under controlled laboratory conditions for a period of eight months. A total of 40 samples of different sediment arrangement, sediment packing, water content and mineralization were studied at 0 (prior to experiment) and after 500, 1 000, and 1 500 freeze–thaw cycles. Macroscopic and 3-D X-ray imaging observations of the sediment structure reorganisation were reported. The results indicate modifications in the vertical and horizontal sediment arrangement along with the development of frost-induced deformational structures and internal (inter-layer) inhomogeneities of the sediment fabric. The most commonly observed changes in the sediment structure originated from the experimental frost action are: modifications at the boundary between lithologically-different sediments, formation of cracks within sediment layers, changes in thickness of the individual layers, and development of sediment deformations. The vast majority of frost-induced modifications of the sediment structure are observed within the silt layer and at its boundaries with sand layers. The presence and frost susceptibility of silt sediment may be treated as a driving force that facilitates the process of sediment deformation during the freeze–thaw activity. Moreover, it was shown that changes in the sediment structure include both formation and obliteration of frost-originated features with the increase in the number of freeze–thaw cycles.

Petrological Characteristics and Rock Nomenclature of Sedimentary Bauxite Gas Reservoir: A Case Study of Bauxite in Taiyuan Formation of Ordos Basin

With the great breakthrough in natural gas exploration of Paleozoic Taiyuan formation in Longdong exploratory area in the southwestern part of Ordos Basin, it is urgent to solve the petrological nomenclature of sedimentary bauxite, so as to further study the pore formation mechanism, distribution law, and controlling factors of bauxite reservoirs. In this paper, X-ray diffraction, polarizing microscope, scanning electron microscope, and other methods are used to analyze the mineral composition and structure of bauxite rocks in the study area and give appropriate names. The results show that the sedimentary sequence of bauxite in the study area can be divided into five sections: A, B, C, D, and E. The main mineral components are diaspore (C section content can be more than 90%), illite, kaolinite, and chlorite; accessory mineral include anatase and pyrite; trace components include quartz, feldspar, rutile, hematite, and rock salt; and some of the pores are filled with calcite, siderite, and (iron) dolomite. The rock structure is mainly bedding and massive structure, and some of them have geopetal structure. The texture mainly consists of granular texture, grain (powder crystal) texture, gel texture, and algal bonding texture. Considering the special lithology of section A~C and the lack of existing nomenclature method, based on mineral composition and sedimentary fabric, a triangulation classification nomenclature method is established, which adopts structure+texture as secondary name and the main mineral components diaspore-mud-pyrite of the three end-member mineral components as primary name. It can not only highlight the mineral assemblage characteristics of sedimentary sequence of bauxite but also reflect the lithologic characteristics of reservoir development section and the influencing factors of reservoir formation, effectively reflecting the petrological characteristics of bauxite reservoir. Among them, the diaspore rocks with granular, bean-oolitic, and grain texture are the main lithologies forming the natural gas reservoir space of Taiyuan formation in Longdong area. The naming method is feasible and reliable for sedimentary bauxite rocks, which lays a good foundation for the study of natural gas reservoir, pore formation mechanism, and distribution law in Longdong area, southwest of the basin.

Jack Dwayne Farmer April 18, 1947-February 22, 2023.

Jack Dwayne Farmer April 18, 1947-February 22, 2023.

Fabric characteristics of in situ sand with/without liquefaction verified by anisotropy of magnetic susceptibility

It is well known that fabric of sand may significantly affect mechanical behaviors and liquefaction resistance of sand. Various optical techniques are currently utilized to visualize the fabric, especially the distribution of the long axis of soil particles. However, none of these methods provides an ideal solution in laboratory tests and in situ observation. In this study, anisotropy of magnetic susceptibility (AMS) was first proposed as a convenient and efficient way to evaluate the liquefaction of clean sand. At first, investigations with scanning electron microscopy (SEM) and AMS were simultaneously conducted on two groups of soil specimens with different initial fabrics to verify the feasibility of the AMS technique. Then, 80 in situ samples were collected to analyze the feature of liquefied and non-liquefied sand layers through AMS tests. It is clearly known from the test results that the natural sedimentary fabric was destroyed during liquefaction and the fabric anisotropy was greatly changed after liquefaction. The feasibility of evaluating soil fabric using the AMS survey was verified by the laboratory tests. Furthermore, the applicability of AMS in detecting liquefied layer in situ was confirmed for the first time. • Anisotropy of magnetic susceptibility (AMS) was proposed to evaluate liquefaction. • Feasibility of AMS was verified by comparing with an optical method in laboratory. • AMS was used on in-situ samples to detect the anisotropic fabric for the first time.

UNLOCKING THE ARCHITECTURE OF THE COLONIZATION WINDOW: ICHNOFABRICS FROM UPPER CRETACEOUS TIDE-INFLUENCED MEANDER-LOOP DEPOSITS

ABSTRACT Studies dealing with the colonization window typically emphasize two major features: duration (short term vs. long term) and frequency of colonization (episodic vs. continuous). However, our understanding of tide-influenced meander loops requires consideration of an additional feature, the architecture of the colonization window, which comprises not only the spatial dimension and geometry of the colonization surface, but also its evolution through time. Tide-influenced meander-loop systems show a heterogeneous trace-fossil distribution that reflects the variety of processes operating along the point-bar and overbank colonization surfaces. Ichnofabric analysis of tide-influenced meander-loop deposits from the Upper Cretaceous Tremp Formation (Pyrenees, Spain) provides valuable insights into the sedimentary and ichnological dynamics of these marginal-marine systems and allows the importance of stratal geometry controlling the colonization window to be evaluated. Six ichnofabrics are identified in point bars and associated overbank deposits. These ichnofabrics differ in bioturbation index (e.g., higher in the upper part than the lower-middle parts of point bars), preservation of primary sedimentary fabric (typically preserved in the lower-middle parts of point bars), inferred behavior and trophic types (e.g., dominance of dwelling or feeding structures in the lower-middle and upper parts of point bars, respectively), and other features such as depth of penetration, ichnotaxonomic composition, presence or absence of root trace fossils and/or mottling, or number of superimposed suites. The key environmental factor controlling the nature and distribution of ichnofabrics is the morphology of the point-bar lateral-accretion surfaces and their evolution through time. The architecture of the colonization window is here linked to the helicoidal flow and discharge changes in meandering channels, and the successive development of lateral accretion units with time.

Vestige of a subaerial rock avalanche deposit in an upper Cretaceous synorogenic wedge-top succession (Gosau Group, Eastern Alps)

Modern subaerial catastrophic mass-wasting (rockslides, rock avalanches) are identified as being huge bulky masses with a surface littered with boulders to blocks. The identification of partly preserved deposits of ‘fossil’ subaerial catastrophic mass-wasting, in contrast, requires assessment of deposit fabrics. In the Alps, within the synorogenic Gosau Group (Turonian to Ypresian) that accumulated in an oblique-convergent setting, we had identified a ‘megabed’ (Wäschkogel megabed) up to 95 m thick that is interpreted as a vestige of a subaerial rock avalanche deposit. This megabed is an extremely poorly sorted, disordered, clast-supported deposit of sand- to boulder-sized dolostone fragments from an upper Triassic succession that comprises the local bedrock. In the megabed, numerous clasts display crackle, jigsaw and mosaic fabrics with fitted subclast boundaries. The matrix is a lithified gouge of fragmented dolomite crystals. In its upper part, the megabed displays very thick planar parallel stratification. Across strata, no change in sediment fabric is discerned.The distinctive features of the megabed, i.e., the sharp intercalation between fluvial deposits, large bed thickness with a uniform fabric, abundant clasts fractured in situ, and the gouge matrix all indicate that the megabed and lateral correlatives accumulated from a rock avalanche. The planar stratification in the upper part of the megabed is interpreted as a dynamic feature produced by internal simple shear during a terminal phase of rock avalanche propagation. Bedrock geology and the distribution of other Gosau Group deposits in the area suggest that the rock avalanche propagated along a SSW–NNE striking incised valley. This is the first identification of a subaerial rock avalanche deposit in the Gosau Group. Our case study may help to consolidate the awareness of deposits of fossil subaerial catastrophic mass-wasting.

Study on Depositional Fabric and Environmental Characteristics of Dendrolites in the Cambrian Zhangxia Formation: An Example from the Houziyu Section of Ezhuang Town, Zibo, Shandong Province, Eastern China

The dendrolites composed of light gray massive micrite limestone occur at the top of the Cambrian Zhangxia Formation in Houziyu Section, Zibo, Shandong Province. Because it is very difficult to study the genesis of microbial rocks through “the function of diagenesis filter,” this paper analyzes its genetic mechanism by characterizing the sedimentary fabric and formation environment of dendrolites and it is of great significance to study microbial carbonate deposited in geological history in oil exploration. The Cambrian Zhangxia Formation in the Houziyu section is a marine deposit. The macroscopic shrub-like fabric of centimeter scale can be seen on the surface of the tree-shaped stone, with small biological burrows and remnants of calcareous mud components after filling. The microstructure observation shows that the main fabric of the dendrolites is a dark micrite matrix, bright calcite cement, clastic particles, and a calcified cyanobacteria biological community. Detailed supplementary information on the diversity of calcified microorganisms in dendrolites has been made through the study of various types of calcified cyanobacteria and calcified residual substances in the microbial membrane of calcified cyanobacteria in the dendrolites of the study area. It is clear that dendrolites are the product of the calcification of biofilms or microbial mats dominated by cyanobacteria. With the deepening of this kind of research, it is bound to raise the research of microbial carbonate to a new level in China and provide great help to find oil and gas resources in marine strata of microbial origin in China.

Heterogeneity of a Sandy Conglomerate Reservoir in Qie12 Block, Qaidam Basin, Northwest China and Its Influence on Remaining Oil Distribution

In view of the key geological factors restricting reservoir development, the reservoir heterogeneity of an alluvial fan sandy conglomerate reservoir in the Qie12 block of Qaidam Basin, Northwest China, and its influence on remaining oil distribution, were studied according to geology, wireline logging data, and dynamic production data. This study illustrates that the difference in pore structures, which are controlled by different sedimentary fabrics, is the main cause of reservoir microscopic heterogeneity. Besides, the temporal and spatial distribution of architectural units in the alluvial fan controls reservoir macroheterogeneity. Our results show that the thick sandy conglomerate develops two types of pores, two types of permeability rhythms, two types of interlayers, two types of interlayer distribution, two types of effective sand body architecture, and four types of sand body connecting schemes. The strongest plane heterogeneity is found in the composite channel unit formed by overlapping and separated stable channels of the middle fan, and the unit’s permeability variation coefficient is &gt;0.7. However, the variation coefficient in the range of 0.3–0.5 is found in the extensively connected body unit sandwiched with intermittent channels of the inner fan. The distributions of the remaining oil vary significantly in different architectural units because of the influence of reservoir heterogeneity, including distribution patterns of flow barriers, permeability rhythm, and reservoir pore structures. The composite channel unit formed by overlapping and separated stable channels, or the lateral alternated unit with braided channel and sheet flow sediment of the middle fan, is influenced by the inhomogeneous breakthrough of injection water flowing along the dominant channel in a high-permeability layer. The microscopic surrounding flow and island-shaped remaining oils form and concentrate mainly in the upper part of a compound rhythmic layer. Meanwhile, in the extensively connected body unit sandwiched with intermittent channels of the inner fan, poor injector–producer connectivity and low reservoir permeability lead to a flake-like enrichment of the remaining oil.

Quantifying Centimeter- to Microscale Heterogeneities in Sedimentary, Compositional, and Geomechanical Properties of Siltstone Deposits in the Lower Triassic Montney Formation, Northeastern British Columbia, Canada

Abstract Geological and geomechanical heterogeneities exist at multiple scales in fine-grained rocks; however, the complexity of characteristics at the centimeter- to microscale heterogeneities remains poorly understood. In this study, 10 representative samples composed of three centimeter-scale sedimentary fabrics (massive siltstone (F1), stratified siltstone (F2), and bioturbated siltstone (F3)) were analyzed from the Lower Triassic Montney Formation in the Western Canada Sedimentary Basin to describe sedimentological heterogeneity based on sedimentary fabric, compositional, and geomechanical properties. Sedimentary fabric was determined based on grainsize and the distribution of bedforms, which subdivide the facies into four μm- to mm-scale microfacies (massive siltstone (MF1), pinstriped laminated siltstone (MF2), planar- to cross-stratified siltstone (MF3), and bioturbated siltstone (MF4)). Microscale analysis using a scanning electron microscope was used to characterize microfacies and their respective mineralogical makeup (matrix, cement, and framework grains). To quantify heterogeneity, sedimentary fabric was assessed using a CT scan complemented by elemental composition (using X-ray fluorescence), and geomechanical hardness (using Equotip Piccolo Bambino handheld microhardness tool) was collected within a 1 cm by 1 cm grid within each sample. Datasets were compared using a discriminant analysis (DA) to recognize trends between multiple properties and suggest that sedimentary fabric with the highest centimeter-scale aluminum content from XRF (avg. 11%) comprises microfacies that are comparatively matrix-rich consisting of micas, negligible calcite cement, and exhibit the lowest handheld hardness values (&amp;lt;770). Alternatively, sedimentary fabric with a higher elemental calcium component (avg. 18%) comprises microfacies that are matrix-poor, cemented by carbonate (calcite and dolomite) and quartz, and overall exhibit a positive trend with hardness measurement (770–850). Furthermore, to relate the elemental and geomechanical proxies to controls on rock mechanics, natural calcite-filled fractures within the studied core intervals were characterized. Fractures were subdivided into three types—brecciated, bed-parallel, and vertical to subvertical fractures with each type being constrained to a specific sedimentary fabric. Based on centimeter gridding, microscale analysis and the degree of fabric interbedding play a primary role on the variability in mechanical hardness and the geometry and termination of natural fractures. Collectively, this dataset provides insight into the influence that sedimentary fabric and the distribution of elemental composition has on mechanical properties and natural fractures below well log resolution. These findings can be used to better model and predict fine-grained deposit characteristics before undergoing hydraulic stimulation.

Tectonic evolution of the Eocene–Oligocene Lushi Basin in the eastern Qinling belt, Central China: Insights from paleomagnetic constraints

Abstract The eastern Qinling orogenic belt, located in the conjoint region between the Neo-Tethys tectonic domain and the western Pacific tectonic domain, is key to understanding the tectono-sedimentary evolution of the East Asian interior. Extensive Late Cretaceous–Cenozoic fluvial and lacustrine deposits within the eastern Qinling orogen provide ideal archives of tectono-sedimentary evolution in Fenwei, the eastern Qinling orogen. We present an integrated rock magnetism and anisotropy of magnetic susceptibility (AMS) study of a Middle Eocene–Early Oligocene succession in the Lushi Basin, eastern Qinling region. Our rock magnetic results suggest that hematite and magnetite are the main magnetic carriers of remanence, while χ−T curves, hysteresis loops, and low values of bulk susceptibility indicate that paramagnetic minerals are major contributors to AMS in the Lushi Basin. Nevertheless, the anomalous magnetic fabrics in the Zhangjiacun and Lushi Formations may result from the contribution of the iron-bearing carbonates in deposits. The clustering of the minimum principal axes nearly vertical to the bedding plane and the well-defined NW–SE magnetic lineation almost parallel to the dip of the bounding fault suggest that the AMS of the Dayu Formation is not a sedimentary fabric but an incipient deformation magnetic fabric. The pronounced NW–SE magnetic lineation indicates the NW–SE stretching of the Lushi Basin during the Late Eocene–Earliest Oligocene, which may have been caused by the combined effects of the India–Eurasia collision and the subduction of the western Pacific plate.

Three‐dimensional bounding surface architecture and lateral facies heterogeneity of a wet aeolian system: Entrada Sandstone, Utah

ABSTRACTAeolian deposits form noteworthy reservoirs (for example, Norphlet Formation and Rotliegend Group) in hydrocarbon extraction and carbon capture and storage contexts, but stratigraphic architecture imparts significant heterogeneity. Bounding surfaces result from autogenic and allogenic controls and can represent important changes in dune‐field dynamics. To further evaluate the impacts of facies heterogeneity and flow‐inhibiting bounding surfaces on reservoir performance and reconstruct ancient erg evolution, the stratigraphic architecture of aeolian systems must continue to be studied at multiple scales. This study pairs traditional methods (for example, measured stratigraphic sections) with advanced technologies (for example, drone‐derived outcrop models) to precisely resolve the metre to kilometre‐scale three‐dimensional stratigraphic architecture of wet aeolian Middle Jurassic Entrada Sandstone outcrops located at Rone Bailey Mesa near Moab, Utah, USA. Five facies are identified, primarily based on sedimentary fabrics, and are grouped into three associations named dune, sabkha and sand sheet. Statistical analyses of gamma‐ray spectrometer and automated mineralogy data indicate a distinct mineralogical difference between dune (quartz‐rich) and sabkha (more feldspathic) packages, suggesting that gamma‐ray logs may be used to better predict facies distribution in the subsurface. Seven modelled super bounding surfaces are planar to undulatory, with no perceived spatial trends. Five modelled interdune migration surfaces are undulatory but exhibit an average 0.09° angle of climb roughly parallel to the palaeocurrent direction. Two modelled superposition surfaces are linear to sinuous in plan‐view. Laterally discontinuous sabkha packages observed are interpreted to be remnants of closed, damp, interdune flats located between ca 8.5 to 17.0 m tall, sinuous, transverse bedforms or patches of such bedforms. Based on stratigraphic architecture interpretations, the Entrada Sandstone preserves signals of allogenic forcing and localized autogenic bedform cannibalization of the substrate. The findings of this study, some of which are not commonly recognized in wet aeolian facies models, enhance the understanding of erg evolution and can parameterize static models of aeolian reservoirs.

Thrust Structure and Bidirectional Paleocurrent in the Jingzhushan Formation during the Late Cretaceous in the Nyima Basin, Tibet Plateau, China: Approach of Magnetic Fabric and Zircon Chronology

(1) Background: The widely developed thrust structures in the Jingzhushan Formation of late Cretaceous in Nyima Basin are related to the collision which leads to orogeny between the Lhasa and Qiangtang blocks. Clarifying the tectonic properties of the Nyima Basin is of great significance to characterize the evolution of the collision between the Lhasa block and the Qiangtang block. (2) Methods: Combined with mineralogy and petrology and U-Pb zircon dating of extrusive rocks, a detailed magnetic fabric test and rock magnetic experiment were carried out on the red sandstones of the Late Cretaceous Jingzhushan Formation in Nyima Basin. (3) Results: The sedimentary magnetic fabrics developed at the three sampling locations in the southern part of the basin are related to paleocurrents. The strained magnetic fabrics developed at one sampling site in the north, including pencil-like fabrics and tensile linear fabrics, are related to SE-NW tectonic stress. The average magnetic susceptibility value of the strained fabric with sandstone is lower than that of the sedimentary fabric, and its size is controlled by mica paramagnetic minerals. Ferromagnetic minerals are magnetite or hematite. In addition, the age of sandstone in the Jingzhushan Formation from the Nyima Basin matches the age of the eruptive rock in the Jingzhushan Formation from Coqen Basin, which is 92 Ma. (4) Conclusions: The strained fabric is developed near the thrust fault, which can explain the widely developed thrust structures in the basin. The Nyima Basin of the Jingzhushan Formation in the Late Cretaceous was an ocean-continent transitional foreland basin developed under the background of arc-arc “soft collision”.

Geology and geochemistry of the high-grade Zankan magnetite ore, Western Kunlun Mountains, NW China

Over the past decade, several magnetite-rich iron deposits have been discovered in the Taxkorgan region of the western Kunlun Mountains of northwestern China. These include the Zankan, Jiertieke, Laobing, Yelike, and Taaxi iron deposits. Prospective iron ore reserves are estimated at over 1 billion tons, with an average magnetite grade of 28% (Feng et al., 2018). The largest of the deposits, the Zankan Iron Deposit (ZID), is estimated to contain 628 million tons of iron ore, with at least 10% of the ores being high-grade (TFe > 50 wt%). However, the geological processes by which the high-grade ores in the ZID were formed remains poorly constrained. In this study, we combine whole rock and ore geochemistry data, oxygen and sulfur stable isotopes, and uranium-lead and hafnium isotopes from zircons to elucidate the plausible mechanisms underpinning the formation of the ZID ores. Based on mineralogy, we define two ore types. Type 1 is essentially comprised of quartz and magnetite, interlayered with schist, and it retains the sedimentary fabrics of the precursor iron-rich sediments which were an iron formation (IF). Type 2 formed by the intrusion of a dacite-porphyry into the IF which led to magnetite remobilization and recrystallization as manifest in magnetite-ferro actinolite veining. Rare earth element (REE) characteristics of the high-grade ores differs from those of the precursor IF, with chondrite normalized REE patterns showing enrichment of LREE and significant negative europium anomalies close to those of the dacite-porphyry. The oxygen and sulfur isotopic composition of magnetite and pyrite in the high-grade ores suggests a further meteoric influence on the ZID with silica enrichment driven by leaching of the precursor IF from elsewhere in the sequence. Circulation of the ore forming fluid is attributable to the development of faults and fracture zones that resulted from the intrusion of the dacite-porphyry. Laser ablation uranium-lead analyses of zircons from the dacite-porphyry yield two groups with ages of 551.25 ± 3.32 Ma and 497.82 ± 3.81 Ma, representing captured detrital zircons and the intrusive age of dacite porphyry, respectively. Given the intimate geological relationship between the dacite-porphyry and T2 ore, we suggest that the high-grade ores are formed after 497.82 Ma. Collectively our field observations and geochemical data reveal the detailed alteration processes that led to the development of the high-grade iron ores in the ZID, identifying distinctive features that may help to guide future exploration for additional high-grade ore deposits in the Taxkorgan region.

Impact of diagenesis on the microbial reservoirs of the terminal Ediacaran Dengying Formation from the Central to Northern Sichuan Basin, SW China

The central part of the Sichuan Basin shows an excellent exploration potential in the dolomitized microbial reservoirs of the Dengying Formation. However, the influence of diagenesis on the formation of high-quality reservoirs is unclear. This study aims to establish the parasequence of the diagenesis in microbial reservoirs and decipher its control on the quality of microbialite-dominated reservoirs. The microbialite consists of stromatolites and thrombolites in the Dengying Formation, which firstly underwent meteoric alteration in the early diagenetic stage. Subsequently, the microbialites experienced fibrous and bladed dolomite cementation and dolomitization. Laminated and clotted sedimentary fabrics are well-preserved in dolomite with less than 100 μm in crystal-sizes. The carbon (δ13C), oxygen (δ18O), and strontium (87Sr/86Sr) isotopes in the microbial dolomite are similar to the Ediacaran seawater, indicating the marine origin of the dolomitizing fluid. The carbon, oxygen and strontium isotopes of the fibrous and bladed dolomite cements are closed to the microbial matrix, which means that the diagenetic fluid is originated from the coeval seawater. The medium-coarse crystalline granular dolomite cement filled later than the fibrous cement, and the oxygen isotope of this cement was affected by the recrystallization caused by the temperature rise after burial. Finally, the saddle dolomite cement filled in fractures and veins. The δ13C (1.1 ± 1.3‰) value of saddle dolomite is similar to the matrix and early cement, while the δ18O value is significantly negative (−10.9 ± 1.0‰). The high Mn (751.4 ± 570.0 ppm), Fe (3872.5 ± 3347.9 ppm) concentrations, homogenization temperature (194 ± 26 °C) derived from the fluid inclusion and concomitant silicification suggest its formation of Fe-rich high-temperature burial fluid. For the microbialite-dominated reservoir, its initial porosity is high. The porosity and permeability characteristics show dissolution-specific behavior, suggesting a key factor of meteoric diagenesis in enhancing the reservoir. The early dolomitization enhance the resistance to the compaction and pressure dissolution. For the geophysical properties, the cementation of marine-derived fluids occupies the most pore space (∼28.1%), followed by compaction and stylolitization that reduce porosity by 10–13%. The hydrothermal alteration, organic acid dissolution, and thermochemical sulfate reduction slightly influence the reservoir quality. This study provides further understanding on the mechanism of microbial reservoirs and enlightens the further exploration of Precambrian successions.