An interval through the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) from the Vocontian Basin in southeastern France was examined using integrated sedimentology and ichnology at both macro- and microscopic scales to elucidate subtle bottom water and pore water oxygen changes. This study provides the first comprehensive trace fossil record for the region, confirming a diverse assemblage of ichnogenera and revealing evidence of microscopic reworking even in mudstones previously considered unbioturbated. Relative dissolved oxygen changes were interpreted through temporal fluctuations in trace fossil attributes (diversity, burrow size, and ichnoguilds) to illuminate the dynamic redox history. The onset of the OAE2 marked a rapid decline from pre-event oxic conditions to sustained suboixa. Peak deoxygenation occurred just below the Cenomanian-Turonian Boundary, marked by oscillations between suboxic and temporarily anoxic conditions. Prior to peak deoxygenation, a significant reoxygenation event (the Plenus Cold Event) resulted in fluctuating oxic and dysoxic conditions. Post-OAE2 recovery was protracted and gradual, transitioning from dysoxic to weakly oxic bottom waters. Integrating these ichnological findings with existing geochemical and micropaleontological data corroborates the interpretation that OAE2 in this region was a dynamic event characterized by fluctuating redox conditions, rather than persistent anoxia. This work underscores the critical value of ichnological analysis in paleoredox studies for resolving subtle changes within the oxic-dysoxic-suboxic spectrum, demonstrating that the absence of macroscopic bioturbation is an unreliable proxy for anoxia.

Research on travertines has increased over the past decade due to their importance to local hydrology, connections with microbial ecosystems, economic value as mineral resources, and potential as reservoir rocks. There are many unanswered questions about isotopic fractionation in different travertine settings worldwide. We use a petrological-geochemical approach to investigate the evolution of the Botijuela travertine system, positioned over ~3380 m above sea level, in the southwestern Antofalla Graben of the Argentinean Puna, Central Andes. This travertine system offers a complete set from proximal to distal zones for understanding the origin and development of carbonate precipitation. Fluid-source analysis indicates a mixture of fresh water and hydrothermal influx, reflected in the system's mineralogical dominance of calcite, with notable variations in its geochemical composition. A conical mound in the proximal zone consists of mudstone and boundstone and preserves a geochemical halo characterized by high iron concentrations and non-equilibrium calcite precipitation. Intermediate and distal zones are dominated by high-energy facies (grainstone and rudstone), with low iron and arsenic contents, where the isotopic composition occasionally approaches equilibrium. Isotopic values (δ 13 C: 2.0 to 10.4‰; δ 18 O: −8.9 to −0.7‰ VPDB) indicate fractionation triggered by CO 2 degassing and evaporation with precipitation cycles in which distal-zone isotopic signatures partially overlap those of the proximal zone. The high altitude may indicate a unique condition that promotes calcite precipitation under isotopic non-equilibrium. This setting is marked by low atmospheric pressure, sudden hydraulic exhumation triggering fluid decompression and enhanced CO 2 degassing, episodic hydrothermal influx, and the deposition of high-energy facies.

Fallout pyroclasts from large explosive volcanic eruptions consist of loose material that accumulates at significant distances from volcanic vents, i.e., primary tephra; they can become secondary tephra if reworked by sedimentary processes. This discrimination is challenging for old, lithified tephra due to limited outcrops and the inability to use sieving techniques. Here, a multi-analytical protocol is adopted and validated on a 5.5 Ma old, lithified (tuff-like), thick, and widely distributed tephra in Central Italy (the Maccarone section). The deposit and mesoscopic samples are mainly massive, lacking erosive surfaces and allochthonous lithics. Geochemical and mineralogical (XRPD) analyses reveal a calc-alkaline, crystal-poor (feldspars and biotite) deposit, rich in both glass and bulk SiO 2 . Sub-μm secondary sheet-silicates are widespread, whereas sedimentary calcite is significant in specific horizons. EPMA and SEM show rhyolitic, stubby, and poorly vesiculated glassy ashes, with 2D (by image analysis) grain-size distributions from ~200 to a few μm and highly sorted; ashes are also elongated and angular and poorly rounded. These microscopic shape attributes, coupled with the massive, structure-free mesoscopic character, indicate primary tephra deposition; conversely, the minor horizons with plane-parallel, curvilinear, or cross-layered aspects are instead ascribed to secondary and local remobilisations, which are unable to alter the textures of the ashes. The last hypothesis may represent a common intermediate depositional process between primary ashes settled in seawater and secondary pyroclast-rich turbidites. Trace elements indicate a volcanic-arc origin, possibly from a large eruption at approximately 5.5 Ma.

Depositional processes and early grain-scale properties exert a primary control on compositional heterogeneity and diagenetic evolution in sandstone bodies, yet their relative influence at the field scale remains difficult to quantify. As a result, regional petrographic datasets that explicitly link spatial variability in grain attributes and authigenic mineral assemblages to depositional environments are still limited. This study presents a field-wide, quantitative petrographic analysis of the Slochteren Formation of the Upper Rotliegend Group in the Groningen area (the Netherlands), which provides exceptional subsurface coverage and stratigraphic continuity. Petrographic data from fifteen wells are used to define spatial trends in detrital composition, grain size and sorting, and cement distribution, and to relate these patterns to depositional environments and structural position. A south–north transition from proximal fluvial to distal playa deposits is accompanied by smaller grain size and better sorting toward the north, whereas aeolian dune deposits are most prominent in the central and western parts of the study area. Grain size and sorting are the dominant controls on porosity and permeability, with the highest reservoir quality occurring in well-sorted, fine to medium-grained sandstone bodies of distal and aeolian origin. Compositional variations reflect both depositional and provenance controls: lithic fragments and polycrystalline quartz dominate medium and coarse southern sandstone bodies, whereas feldspar gets more common northwards and stratigraphically upward, consistent with a shift in sediment supply, likely related to either unroofing or pre-depositional reducing weathering conditions due to increasing aridity. Authigenic mineral assemblages record the combined influence of depositional texture, detrital input, and burial evolution with many cement constituents being internally sourced from local dissolution of more unstable precursor minerals. Illite and chlorite formation is closely associated with clay-coated grains in fine-grained playa facies, whereas quartz cementation is limited and controlled by burial depth, local silica availability, and inhibition by grain coatings. Dolomite locally exerts a strong control on porosity reduction. These results highlight the fundamental role of hydraulic sorting influenced by the interaction of aeolian and fluvial processes and early diagenetic processes in generating spatial heterogeneity in sandstone successions and demonstrate how grain-scale attributes control subsequent diagenetic pathways in mixed depositional systems. • Field-wide petrography reveals depositional controls on texture and composition. • Detrital composition covaries with grain size and depositional environment. • Feldspar increases stratigraphically upward, indicating changing sediment supply. • Burial diagenesis amplifies inherited depositional heterogeneity. • Authigenic minerals are largely internally derived, not externally sourced.