Dolomite concretions are widely developed in the Nenjiang Formation of the Songliao Basin and have commonly been used as archives of depositional and diagenetic environments. However, their formation mechanisms and the extent to which they preserve primary environmental signals remain poorly constrained. In this study, we investigate dolomite concretions and their host rocks from a well-exposed outcrop section using petrography, major and trace element geochemistry, carbon and oxygen isotopes, strontium isotopes, and clumped isotope thermometry. Clumped isotope results infer formation temperatures of 25.3–27.2 °C, suggesting that the dolomite concretions formed under shallow-burial early diagenetic conditions. Compared with the surrounding rocks, the concretions show relatively enriched δ13C values and less negative δ18O values, indicating modification by early diagenetic pore-water processes rather than direct preservation of primary depositional signals. Trace-element ratios and strontium isotopic compositions further suggest that the concretions formed within relatively reducing pore-water environments and were influenced by external Sr-bearing fluids during early diagenesis. Together with the calculated pore-water oxygen isotopic compositions, these results indicate that concretion formation occurred within a lacustrine pore-water system characterized by isotopically light fluids and limited external fluid modification. These findings demonstrate that dolomite concretions in the Nenjiang Formation primarily record localized diagenetic pore-fluid evolution rather than primary depositional signals. Consequently, caution should be exercised when using carbonate concretions as indicators of paleosalinity, marine incursions, or other paleoenvironmental conditions in lacustrine basins. This study provides insights into the origin of dolomite concretions and their significance for reconstruction of paleoenvironmental conditions in continental sedimentary systems.
The stability of tunnel surrounding rock in karst areas is controlled by multiple factors and is therefore a critical issue for safe tunnel construction under complex geological conditions. This review systematically examines research on the stability of tunnel surrounding rock in karst areas. First, the main research methods, including theoretical analysis, laboratory testing, and numerical simulation, are summarized, and their complementary roles in deriving critical conditions, reproducing disaster processes, and analyzing complex working conditions are clarified. Second, the framework of numerical simulation methods is reviewed in detail. These methods can be classified into continuum-based methods, discontinuum-based methods, coupled continuum–discontinuum methods, and multi-field coupling methods. On this basis, the applicability of different methods to the analysis of surrounding-rock deformation, fracture propagation, and related processes is further compared. In addition, instability criteria based on displacement, strength, plastic zone, energy, and comprehensive indicators are summarized, and the effects of cave filling conditions, cave location, and cave morphology on surrounding-rock stability are analyzed. Overall, this review aims to integrate currently fragmented research findings and provide more reliable theoretical support for the safe construction of tunnels in karst areas.
The Lower Sub member of the 4th Member of the Shahejie Formation (Es4x) in the Bonan Depression of the Bohai Bay Basin shows a distinctive continental red bed succession characterized by alternating red and gray layers, mainly composed of red stratigraphy. This differs from earlier views of the unit as being uniformly red and indicates a more dynamic depositional environment than previously thought. A better understanding of the sedimentary structure in this interval is essential for reconstructing the early Eocene paleogeographic and paleoclimatic the Bonan Depression, for detailed understanding of its evolution, and for guiding exploration for deep hydrocarbon resources in the Bonan Depression. This study integrates detailed core observations, mud-logging records, laboratory-based geochemical and petrographic analyses supported by high-resolution seismic data and wireline logs to investigate the depositional processes and sedimentary evolution of the lower succession of the Eocene Shahejie Formation. The findings identify a wide range of depositional facies, such as alluvial fans, braided fluvial, floodplains, ephemeral saline lakes, perennial lacustrine, nearshore subaqueous fans, and storm-influenced lacustrine deposits. These facies indicate diverse hydrodynamic conditions and a dynamic interaction among climate, tectonics, and sediment supply. During predominantly arid climatic periods, alluvial fan–floodplain–saline lake facies associations developed, with coarse-grained alluvial fan deposits along basin margins transitioning basinward into fine-grained floodplain and ephemeral saline lake sediments. Intermittent humid phases promoted the expansion of perennial lakes, resulting in the deposition of gray-green bar sands and interbar mudstones along gentle slopes, while the northern basin evolved into semi-deep to deep lacustrine environments. Seismic and sedimentological evidence further indicates the development of subaqueous fan systems within incised gullies along steep northern slopes, controlled by syn-depositional tectonism and episodic high-energy events. The refined depositional model highlights the critical role of ephemeral lakes in the formation of continental red beds, providing new insights into early Eocene paleoclimate variability and offering a valuable framework for red-bed reservoir prediction and hydrocarbon exploration in the Bohai Bay Basin.
The Neogene Kechabta foreland basin of northern Tunisia contains well-exposed Messinian outcrops, including the Oued Bel Khedim (OBK) and Chaabet Et Tabbela (CET) formations. Through bed-by-bed logging and comprehensive analyses that combined biostratigraphy, sedimentology, and petrography, the age and depositional environments were established. The Messinian succession of the studied sections includes four distinct lithological units that have produced a rich and well-preserved ostracod fauna. Species such as Cyprideis ruggierii, Callistocythere antoniettae, Dorukella aff. bireticulata, Leptocythere sanmarinensis, and Costa batei collected from the OBK1 and OBK2 units provide biostratigraphic evidence supporting an Early Messinian attribution for the first time. In contrast, the OBK3 unit of the Oued Bel Khedim Formation, along with the overlying CET unit of the continental Chaabet Et Tabbela Formation, exhibits a significant fauna scarcity, tentatively indicating a Late Messinian age for these units. The facies analysis enabled the characterisation of eight distinct facies (F1-F8) organised into four associations (FA1, FA2, FA3, and FA4), representing different depositional environments. FA1 and FA2 include gypsum, clay, as well as oolitic and peloidal limestone mixed with predominantly clay-rich sediments containing both brackish and marine benthic foraminifers (Ammonia tepida and Elphidium, along with ostracods including Cyprideis, Loxoconcha, and Xestoleberis). These sediments likely formed in shallow lagoonal settings that experienced fluctuations between restricted and fully marine conditions. In contrast, FA3 features varved micritic to peloidal limestone and gypsum/clays, with very few bivalves and ostracods, and was formed in a saline lake environment that later shifted to a sebkha setting. Lastly, FA4, composed of conglomerates and coarse-grained sandstones, indicates continental environments dominated by alluvial fan and fluvial deposits. The correlation of lithostratigraphic units and their associated facies across the Kechabta foreland basin, located in front of the Maghrebian chain, allows reconstruction of the depositional history and emphasises the main local tectonic and regional eustatic factors that influenced the evolution of the Mediterranean basin during the Messinian. The revised stratigraphic framework for Messinian deposits in the studied section, when compared with the Gulf of Tunis and the Cap Bon Peninsula in northeast Tunisia, suggests that the three-stage model consisting of Lower Evaporites, Upper Evaporites/Terminal Carbonate Complex, and Lago Mare, which is proposed for the central Mediterranean basin, is applicable in northern Tunisia. This study also shows that the Messinian deposits in the Kechabta foreland basin, despite being formed at the foot of the Maghrebian chain and in a proximal margin, may record several major phases associated with the Messinian salinity crisis across the Mediterranean.
