Borehole Cores Research Articles

Time-lapsed in situ monitoring of mechanical property in deep soil mixing with surface wave

Evaluating the mechanical properties of deep soil mixing (DSM) requires destructive borehole coring because they are mainly situated underground. Although the surface wave method offers potential for quality assurance, its complexity arises from multi-mode phenomena and the need to re-evaluate inversion results, often necessitating manual interpretation. This paper presents a data-driven surface wave framework to retrieve a field DSM profile Vs over time, incorporating a mode-free forward operator and the Monte Carlo tree search inversion. Validations using synthetic data affirmed the framework’s accuracy and efficiency in tracking subsurface shear wave velocity. In a real-world DSM site, the proposed method successfully captures the evolution of mechanical properties across two DSM layers over the curing period, aligning well with site investigations and borehole coring. This pioneering monitoring framework integrates geotechnical engineering with geophysics expertise, underscoring the value of non-destructive seismic methods for measuring subsurface property evolution.

Geochemical characteristics of borehole cores and their indicative significance for gas hydrates in the permafrost area, Qinghai-Tibet Plateau

Geochemical characteristics of borehole cores and their indicative significance for gas hydrates in the permafrost area, Qinghai-Tibet Plateau

The role of remote sensing to enlarge knowledge on health state of a historical building hit by earthquake: the case of Garisenda leaning tower (Bologna)

An evaluation of the health state of a fragile historical building like a leaning tower, especially if it is located in an environment characterized by significant seismic risk, requires the integration of data obtained through various types of observation techniques. This is the case of the Garisenda Tower in Bologna, which is a 48 m high leaning tower, whose tilt angle reaches 4°, and for which some concerns about its stability recently grown. Historical sources, past and current monitoring of both structure and subsoil with several instruments, geological, geophysical and geotechnical surveying (in particular, continuous borehole coring) and experimental modal analysis provide a large amount of information on the tower and its environment. This study is aimed at pointing out how remote sensing can contribute to the achievement of a reliable picture of the current health state of the tower, considering that this picture could be useful for planning future reinforcement/restoration strategies. In particular, the results of the multitemporal morphological analysis of the tower façades, based on remote sensing data acquired in 2010, during the 2012 Emilia Romagna earthquake and in 2023, are shown and discussed on the basis of some available information. The morphological analysis can provide real and interesting information on geometric deformations, also highlighting their possible partial or total recovery when the stress ceases, in particular when a seismic sequence ends, although an almost total recovery of such deformations does not necessarily imply that the structure is not affected by permanent damage.

Small Ripples in a Big Pond: Sea-level Change and Palaeoenvironmental Signatures at Formby, Merseyside

ABSTRACT This article presents the findings of a developer-funded, geoarchaeological study at Brackenway, Formby, Merseyside. The study comprised the extraction of borehole cores, which were subjected to deposit modelling, radiocarbon dating, and palaeoenvironmental assessment that considered a range of environmental proxies (pollen, plant macrofossils, diatoms, microfauna and insects). The study identified two terrestrial (peat) layers, separated by a perimarine deposit (the Downholland Silt), and these provide important data on the chronology of sea-level change in Merseyside, the formation of the Irish Sea, and of potential hunter-gatherer activity during the mid-late ninth and eighth millennia cal BC. It therefore highlights the way in which comparatively small-scale developer-funded investigations can produce important records of environmental change, which can be used to supplement and refine pre-existing models of sea-level and landscape change relevant to both the locale and wider regions.

An assessment of Rare Earth Elements in borehole cores from the Ermelo, Witbank and Waterberg Coalfields, South Africa: Focus on mode of occurrence

There is currently limited knowledge concerning South African coal deposits as hosts for rare earth elements (REE). This project aims to determine the concentration of REE, including yttrium and scandium (REY+Sc) in various coal seams and adjacent sediments in borehole cores from the Ermelo, Witbank, and Waterberg coalfields in South Africa. Coal petrography (maceral count and vitrinite reflectance), XRD, XRF, sequential chemical extraction procedure (SCEP) before ICP-MS, and mineral liberation analysis (MLA) were conducted on the coal and associated sediment samples. The coals were inertinite-rich, medium-rank bituminous coal, with moderate to high ash content. The two dominant minerals in all the coalfields are kaolinite and quartz, except for sample ZBS2M, where dolomite was dominant. The dominant major oxides are Al2O3, SiO2, Fe2O3, and CaO, which concur with the XRD results. The ICP-MS results following SCEP indicated that both coal and the associated sediments were light REY+Sc dominant. There was also an elevation of medium REY+Sc in the Witbank coal's associated sediment samples. Notably, the samples with the highest REY+Sc results were the sediment samples. The Waterberg samples had the lowest REY+Sc from both the coal (less than 2 μg/g to 7 μg/g) and associated sediments (2 μg/g to 15 μg/g). While the highest REY+Sc was yielded from the Witbank-associated sediments (500 μg/g to 2,500 μg/g). The carbonate fraction step proved to be the most successful leaching step, as most REY+Sc was recovered in that step for all coalfields. In addition to the carbonate fraction, the silicates and sulfide fraction in the Witbank samples, and the ion-exchangeable fraction for the Waterberg-associated sediments samples proved successful in leaching out the REY+Sc. MLA determined that monazite and xenotime in all the coal samples were the REY+Sc-bearing minerals.

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields, Zambezi Karoo Basin of Mozambique: Implications for provenance, sediment dispersal and basin evolution

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields in the Zambezi Karoo Basin of Mozambique provides key insights into the regional provenance, sediment dispersal pathways and basin evolution. Borehole cores from the two coalfields reveal a stratigraphy spanning the early Roadian (middle Permian) to the Carnian (Upper Triassic). The Permian sandstones of the Moatize Coalfield (MC) yield three detrital zircon populations, with ages ranging from 1150 to 950 Ma, 900–780 Ma and 650–490 Ma. In contrast, the Permian sandstones of the N'Condédzi Coalfield (NC) have only one population, which ranges from 1150 to 950 Ma. During the Permian, the provenance area for the NC was the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma) to the north-northeast. In the MC, the detrital zircon populations of the early lake delta depositional setting (Roadian to Wordian) indicate a main provenance in the Zambezi Belt (900–780 Ma) located to the south of the MC, with minor sourcing from the Nampula Block (1150–950 Ma and 650–490 Ma) to the east. The transition from a lake delta to an alluvial depositional setting is attributed to a major tectonic event in the MC, which involved the formation of a braided channel belt. Sandstones from this braided channel belt yield three detrital zircons populations (650–490 Ma, 900–780 Ma and 1150–950 Ma), indicating provenance from both the Zambezi Belt and the Nampula Block. The overlying sandstones in the MC show only a minor population from the Zambezi Belt (900–780 Ma), implying a shift in provenance to the Nampula Block that was likely induced by tectonics. The absence of detrital zircon populations of 900–780 Ma and 650–490 in the Permian sandstones of NC implies that the two coalfields were not connected during the mid to late Permian. It is likely that an intra-rift horst (the Mesoproterozoic Gabbro-Anorthosite Tete Suite) separated the two coalfields. The Lower Triassic sandstones of the NC yield a main detrital zircon population indicating provenance from the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma). A minor population at 650–490 Ma is linked to increased aridity at the Permian – Triassic boundary, which caused expansion of the watershed across the Tete-Chipata Terrane and the Malawi Complex. The Upper Triassic sandstones in the NC yield a 1150–950 Ma detrital zircon population, indicating provenance from the Tete-Chipata Terrane and Malawi Complex and a return to the source-to-sink conditions seen in the mid to late Permian.

Paleostress evolution of the Outer Kwanza basin (offshore Angola); comparison with the Congo basin and implications for the tectonic history of the Central segment of the West Africa passive margin

This study aims at reconstructing the paleostress history of the Outer (offshore) Kwanza basin (West African passive margin) and at comparing it to stress results acquired further north in the Congo basin. Three oriented borehole cores provided by TotalEnergies and reaching the syn-rift, Barremian-Aptian pre-salt carbonates offshore Angola were investigated. Paleopiezometry based on the Stylolite Roughness Inversion Technique (SRIT) and Calcite Twin Inversion Technique (CSIT) was combined with fracture analysis, U–Pb geochronology of carbonates and burial modelling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the pre-salt carbonates over time. Calcite twins were measured from a primary sparite matrix, and the inversion process unravelled a polyphase stress history, comprising ∼ E-W and NE-SW extensional trends that we associate to the rifting (130-112 Ma) that led to the opening of the South Atlantic ocean. The ∼ E-W extension is consistent with the early occurrence of N-S striking normal faults which developed in relation to the reactivation of inherited basement structures. This ∼ E-W extension evolved during the Barremian-Aptian (?) into the dominant regional NE-SW extension marked by large-scale NW-SE striking normal faults. The stress history also comprises compressional and strike-slip stress regimes associated with a ∼N-S trending σ1 which can be related to the transfer of orogenic stresses from the distant Africa-Eurasia plate boundary at ∼67-60 Ma. Finally, compressional and strike-slip stress regimes associated with a ENE-WSW to ∼E-W trending σ1 dominated since at least ∼17-15 Ma (possibly ∼34 Ma); they are interpreted as the expression of the mid-Atlantic ridge push. These (paleo)stress results are compared and combined with earlier paleostress reconstructions in the northern offshore Lower Congo basin (also belonging to the Central segment of the margin) and in the onshore Congo basin in order to refine the stress record and the timing of tectonic events since the early Cretaceous, thus providing unprecedented constraints on the tectonic history of the West Africa passive margin. This tectonic history includes both extensional and compressional events, and was driven mainly by far-field stresses, either gravitational or tectonic in origin, which are related to interactions between the African plate and surrounding plates.

Syndepositional and diagenetic processes in the pigmentation of Middle Ordovician carbonate red beds in South China

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.

Integrated Remote Sensing Investigation of Suspected Landslides: A Case Study of the Genie Slope on the Tibetan Plateau, China

The current deformation and stable state of slopes with historical shatter signs is a concern for engineering construction. Suspected landslide scarps were discovered at the rear edge of the Genie slope on the Tibetan Plateau during a field investigation. To qualitatively determine the current status of the surface deformation of this slope, this study used high-resolution optical remote sensing, airborne light detection and ranging (LiDAR), and interferometric synthetic aperture radar (InSAR) technologies for comprehensive analysis. The interpretation of high-resolution optical and airborne LiDAR data revealed that the rear edge of the slope exhibits three levels of scarps. However, no deformation was detected with differential InSAR (D-InSAR) analysis of ALOS-1 radar images from 2007 to 2008 or with Stacking-InSAR and small baseline subset InSAR (SBAS-InSAR) processing of Sentinel-1A radar images from 2017 to 2020. This study verified the credibility of the InSAR results using the standard deviation of the phase residuals, as well as in-borehole displacement monitoring data. A conceptual model of the slope was developed by combining field investigation, borehole coring, and horizontal exploratory tunnel data, and the results indicated that the slope is composed of steep anti-dip layered dolomite limestone and that the scarps at the trailing edges of the slope were caused by historical shallow toppling. Unlike previous remote sensing studies of deformed landslides, this paper argues that remote sensing results with reliable accuracy are also applicable to the study of undeformed slopes and can help make preliminary judgments about the stability of unexplored slopes. The study demonstrates that the long-term consistency of InSAR results in integrated remote sensing can serve as an indicator for assessing slope stability.

Study on automatic lithology identification based on convolutional neural network and deep transfer learning

Automatic and fast rock classification identification is an important part of geotechnical intelligent survey system. Image based supervised deep learning analysis, especially for convolutional neural networks (CNN), has potential in optimizing lithologic classification and interpretation using borehole core images. However, the accuracy and efficiency of lithology identification models are low at present. In this work, a systematic and enormous rock data framework based on the geological rock classification system is firstly established to provide rock learning datasets. The dataset is composed of approximately 150,000 images of rock samples, which covers igneous rocks, sedimentary rocks, and metamorphic rocks. Secondly, based on CNN-deep transfer learning algorithm, an end-to-end, image-to-label rock lithology identification is established. Finally, the generalization of the proposed model and the field drilling core verification test show that the constructed intelligent rock recognition model has an ability to identify rocks quickly and accurately, and the recognition accuracy of 12 kinds of common engineering rocks is more than 95%. The proposed rock intelligent classification model provides a convenient and fast tool for field geologists and scientific researchers.

Low Bacterial Diversity and Nitrate Levels in Cores from Deep Boreholes in Pristine Karst.

This study investigates the nitrate gradients within the deep biosphere of karst carbonate rocks and their resident microbiota. Samples were taken from borehole cores at depths down to 350 m below the surface, collected during geological site investigations for proposed railway tunnels and analysed using 16S rRNA amplicon sequencing. 16S rRNA amplicon sequencing analysis revealed relatively low microbial diversity, which can serve as a reliable indicator of the pristine nature of deep karst. However, some local hotspots of diversity are independent of depth. Pseudomonadota dominated the samples, with Gammaproteobacteria dominating at the class level. The low nitrate content in deep karst, in contrast to higher values closer to the surface, serves as an additional marker of its undisturbed and unpolluted status. Based on the prediction of functional profiles from 16S rRNA sequencing data, nitrates remain low due to indigenous microbial denitrification and assimilatory nitrate reduction. Pathways related to nitrogen fixation, ammonia assimilation, and nitrification were not confirmed. When elevated nitrate levels are observed in karst, they are most probably related to anthropogenic activities. Environmental factors other than depth and nitrate content play an important role in shaping bacterial communities.

Inverting the rock mass P-wave velocity field ahead of deep buried tunnel face while borehole drilling

Imaging the wave velocity field surrounding a borehole while drilling is a promising and urgently needed approach for extending the exploration range of the borehole point. This paper develops a drilling process detection (DPD) system consisting of a multifunctional sensor and a pilot geophone installed at the top of the drilling rod, geophones at the tunnel face, a laser rangefinder, and an onsite computer. A weighted adjoint-state first arrival travel time tomography method is used to invert the P-wave velocity field of rock mass while borehole drilling. A field experiment in the ongoing construction of a deep buried tunnel in southwestern China demonstrated the DPD system and the tomography method. Time-frequency analysis of typical borehole drilling detection data shows that the impact drilling source is a pulse-like seismic exploration wavelet. A velocity field of the rock mass in a triangular area defined by the borehole trajectory and geophone receiving line can be obtained. Both the borehole core and optical image validate the inverted P-wave velocity field. A numerical simulation of a checkerboard benchmark model is used to test the tomography method. The rapid convergence of the misfits and consistent agreement between the inverted and observed travel times validate the P-wave velocity imaging.

Tectonic and high-latitude climate controls on Quaternary sedimentary processes on the northern coast of Bohai Bay

Separating the relative roles of tectonics and climate change in geological evolution is complicated. The northern coast of Bohai Bay (NCBB), with a thick accumulation of Quaternary lacustrine sediments, is a subsiding area and is well suited to studying tectonic versus climatic controls on sedimentary processes. We used magnetostratigraphy and sedimentological analyses of borehole core JL01 from the NCBB to provide new insights into the effects of tectonic processes and climate changes in this region. The results show the following: (1) A shallow lacustrine delta, a deep-water lacustrine sedimentary system and a subaqueous fan-delta sedimentary system developed during 2.1–1.4 Ma, 1.4–0.78 Ma, and from 0.78 Ma to the present, respectively, corresponding to the rift-initiation, rift-climax, and late-rift tectonic stages of the evolution of an extensional basin, respectively. These stages constitute a low-order tectono-sedimentary cycle. (2) Tectonic and orbital-scale climate changes can be distinguished, particularly the 2.5 Ma climatic transition and a significant shift from 41- to 100-kyr cyclicity across the Mid-Pleistocene climatic transition. These changes indicate that the sedimentary processes of the NCBB were influenced by high-latitude climate forcing since the Late Pliocene. Our results demonstrate that detailed sedimentary facies analyses can provide valuable information about the effects of tectonics and climate changes on basin sedimentary evolution. This approach can be applied to other regional basins worldwide.

The First Application of Laser-Induced Breakdown Spectroscopy: A Fast-Analytical Technique in Targeted Search for Elements in Geological Samples from Deep Boreholes in Latvia

Abstract Laser-induced breakdown spectroscopy (LIBS) provides a rapid, cost-effective, and extra-sensitive analysis of geological samples to make preliminary conclusions about the presence of valuable elements up to the trace levels in the ore. We present the first results of a highly sensitive qualitative analysis of the core samples of geological ore from two boreholes in Latvia (Staicele 1, from a depth range of 794–802 m, and Garsene (Subate) 2A, from a depth range of 1102–1103 m) using LIBS. Our measurements using this technique confirmed the high iron content and indicated traces of rare and high in-demand metals (such as Ti, V, Co, Sm, etc.) in the sample from Staicele, renewing interest in studying boreholes across Latvia. The presented pilot studies demonstrated effectiveness and unique possibility in performing a very sensitive and time-saving qualitative analysis of the composition of samples of ores from the old but still valuable borehole cores by using the LIBS method. We compare these measurements with other methods of sample analysis.

Model of Pleistocene geomorphological evolution in active Alpine neotectonics controlled margins in the western Mediterranean area: The case of SE Iberian Peninsula

At the northern tip of the Betic realm (SE Iberian Peninsula), some troughs (synclines) and elevations (anticlines) alternate, marking the present-day coastal lobed morphology of cape-bounded bays, where subsidence and uplift conditions prevailed, respectively. In this study, we were able to establish a clear coastal evolution. To this end, we considered the sedimentological and palaeoenvironmental conditions, the palaeogeographical reconstruction, and recent tectonics until Middle Pleistocene times (MIS 5) through the interpretation of onshore cores, raised marine deposits and geophysical profiles. In this regard, as reflected by both onshore and offshore information, there seems to be a stratigraphical gap from the end of the Pliocene to MIS 15 (Middle Pleistocene). In areas under uplift conditions, linked to tardive Alpine tectonics, the deposits of ancient shorelines and raised beaches were located at different post-depositional elevations, which were dated from odd MIS 15 to MIS 5 using amino acid racemization. Only deposits aged MIS 7 and MIS 5 are roughly at the present-day sea level or some meters above. In the troughs, which remain mostly as lagoons and salt marshes, subsidence did not allow the sedimentary record to be discerned. However, many borehole cores were recovered, attesting lagoonal, marsh, sabkha, or alluvial environmental conditions, which were usually unconnected from the sea. Micropaleontological and amino acid racemization dating revealed these cores to be of MIS15 to MIS5 age. Offshore seismic research revealed five erosive-bounded deposits that are stacked accretionary prisms corresponding to highstands between odd MIS 15 and MIS 5. In contrast, even MISs can be correlated to the erosive horizons that separated the seismic units, reflecting lowstands. In this regard, some bars, at a range of distances from the present-day coastline, protected wetlands from marine influence, allowing the development of diverse sub-environments under changing paleogeographical and paleoclimatological conditions.

Detecting fractures and monitoring hydraulic fracturing processes at the first EGS Collab testbed using borehole DAS ambient noise

Enhanced geothermal systems (EGS) require cost-effective monitoring of fracture networks. We validate the capability of using borehole distributed acoustic sensing (DAS) ambient noise for fracture monitoring using core photos and core logs. The EGS Collab project has conducted 10 m scale field experiments of hydraulic fracture stimulation using 50–60 m deep experimental wells at the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The first EGS Collab testbed is located at 1616.67 m (4850 ft) depth at SURF and consists of one injection well, one production well, and six monitoring wells. All wells are drilled subhorizontally from an access tunnel called a drift. The project uses a single continuous fiber-optic cable installed sequentially in the six monitoring wells to record DAS data for monitoring hydraulic fracturing during stimulation. We analyze 60 s time records of the borehole DAS ambient noise data and compute the noise root-mean-square (rms) amplitude on each channel (points along the fiber cable) to obtain DAS ambient noise rms amplitude depth profiles along the monitoring wellbore. Our noise rms amplitude profiles indicate amplitude peaks at distinct depths. We compare the DAS noise rms amplitude profiles with borehole core photos and core logs and find that the DAS noise rms amplitude peaks correspond to the locations of fractures or lithologic changes indicated in the core photos or core logs. We then compute the hourly DAS noise rms amplitude profiles in two monitoring wells during three stimulation cycles in 72 h and find that the DAS noise rms amplitude profiles vary with time, indicating the fracture opening/growth or closing during the hydraulic stimulation. Our results demonstrate that borehole DAS passive ambient noise can be used to detect fractures and monitor fracturing processes in EGS reservoirs.

Sandstone Layer Connectivity and Its Control on Coalbed Methane (CBM) Accumulation Based on Sequence Stratigraphic Analysis: A Case Study of the Lower Shihezi Formation in Qinan Coal Mine, Xuzhou–Suzhou Region, China

The sandstone layer connectivity in coal measure strata is one of the key factors in CBM escape in underlying coal seams, which lacks systematic research currently. This study aimed to explore sandstone layer connectivity and its control on CBM accumulation, taking the Lower Shihezi Formation in Qinan Coal Mine, Xuzhou–Suzhou Region, China, as a case study; to do so, we studied the No. 7 coal CBM unit, the pore-rich sandstone layers, and their connectivity modes by performing sequence stratigraphic analysis on the borehole cores, the logging data, and the theory of sequence stratigraphy and sedimentology, combined with the accumulation characteristics of the No. 7 coal seam CBM. This study shows the following: (1) The sequence stratigraphic framework of the Lower Shihezi Formation in the research region consists of two third-class sequences and six system tracts. (2) The No. 7 coal seam CBM unit includes the CBM formation layer, the connectivity layer, and the stable capping layer. (3) There are 10 types of parasequences in the No. 7 coal connectivity layer, and the pore-rich sandstone layers are all located in the connectivity layer and connected in three modes (vertical connectivity, lateral connectivity, and non-connectivity). (4) The connectivity modes and the thickness of pore-rich sandstone layers control the CBM accumulation in the region. Where the pore-rich sandstone layers are thickest and display vertical connectivity, the strong CBM desorption and escape lead to low CBM; where the pore-rich sandstone layers are thinnest and unconnected, the weak to no CBM desorption and escape result in high CBM. (5) Three models for sandstone layer connectivity and its control on CBM accumulation include the CBM weak accumulation model with a strong source supply, large basin subsidence, and undercompensation deposition; the CBM moderate accumulation model with a moderate source supply, moderate basin subsidence, and overcompensation to isostatic compensation deposition; and the CBM strong accumulation model with a weak source supply, small basin subsidence, and undercompensation deposition.

Sedimentary Characteristics and Depositional Model of the Paleogene Dawenkou Formation in Eastern China: Insights from the Huanggang Depression

The Huanggang depression in eastern China is a significant Cenozoic salt-bearing basin that formed during the alternating dry and wet climate periods from the Eocene to the Oligocene. Despite the economic importance of the Huanggang depression, its saliferous model remains controversial. To address this issue, we conducted comprehensive analyses of the sedimentology and elemental geochemistry on the YZR1 borehole core, which hosts a relatively complete sedimentary record of the Huanggang depression, consisting of five lithofacies’ assemblages. The combined lithofacies and geochemical ratios, including B/Ga, Sr/Ba, and V/(V + Ni), provide insights into the paleolake’s evolution in the Huanggang depression. Our analyses indicated that the paleolake underwent a transition from a freshwater lake to a brackish water/saline lake, subsequently transforming into a salt lake, reverting back to a brackish water/saline lake, and ultimately returning to its original state as a freshwater lake. These changes are reflected in the sedimentary record and inform the six stages of evolution of the paleolake of the Dawenkou Formation. We propose two metallogenic models to explain the accumulation of the thick halite (LA1) and thin halite layers (LA2), respectively. LA1 is primarily dominated by halite deposition, forming in an extremely shallow water environment under arid climate conditions. In contrast, LA2 records the alternating deposition of halite, anhydrite, and mudstone, and formed in a shallow water environment under arid to semi-arid climatic conditions. LA1 has a much drier climate and higher salinity than LA2. Our results suggest that the salt-forming period in the Huanggang depression occurred from the late Eocene to the early Oligocene. The halite in the Huanggang depression formed in a shallow water environment, providing the basis for the halite deposition model of the depression. This study sheds light on the formation mechanism of halite in the Paleogene in eastern China.

Deformation and recrystallization of sulfide interlayers at the Golets Vysochaishii Gold-Ore Black-Shale Deposit

Research subject. The relationship of sulfide interlayers in silt-pelitic rocks with the cleavage of the axial surface at the Golets Vysochaishii gold-ore black-shale deposit (Bodaibo ore district). Methods. The structure of rocks was studied in out-crops, borehole cores, sanded samples, transparent and polished sections. The chemical composition of minerals was determined. Results. It was established that sulfide and sulfide-containing interlayers undergo deformation and recrystallization together with the host silt-pelitic rocks during plastic dynamometamorphic transformations of rocks. Conclusions. Metamorphic transformations of rocks took place in two stages. During the folding-thrust deformations of the first stage, a cleavage of rocks is formed, which leads to the splintering of sulfide interlayers with the orientation of the lenses along the cleavage. Metamorphic transformations of the second stage are manifested in sulfide and sulfide-containing interlayers in the collective recrystallization of their composing minerals with the replacement of surrounding rocks deformed at the previous stage. The second stage is characterized by changes in the mineral composition of rocks, manifested in the substitution of pyrrhotite with pyrite, increase in the iron content of chlorites and carbonates.

Measurement and analysis of the thermal conductivity of borehole core samples from Huizhou, Guangdong, China

Measurement and analysis of the thermal conductivity of borehole core samples from Huizhou, Guangdong, China