Abstract The Early Permian freshwater lacustrine sedimentary sequence in the vicinity of Bačov (Czechia) is world‐famous for its rich fossiliferous content, including amphibians, actinopterygians and insects. The detailed sampling of two quarries and nearby outcrops (129 samples) focused on the reasons for this sequence's extraordinary paleontological content. The mineral composition of all samples was determined by powder X‐ray diffraction analysis, accompanied by FESEM and WDS analysis, heavy mineral analysis and bulk geochemical analysis of selected samples. In both quarries, the rock‐forming minerals include quartz, albite, microcline, illite, chlorite and carbonates. The steady content of analcime (average 10 and 6 wt% in both quarries) and aegirine (average 4 and 5 wt% in both quarries) is surprising. The extreme enrichment of the 40‐cm‐thick layer in Quarry No. III in analcime (average 35 wt%, max. 56 wt%) indicates its transition to analcimolite, sedimentary rock with over 50% analcime, known from less than two dozen localities worldwide. The presence of authigenic aegirine, NaFe3+ clinopyroxene, in the sedimentary sequences makes them even more unique, as it has been reported from only about five localities. The combination of several clues, including the presence of analcimised glass shards, layers rich in microcline (up to 58 wt%), well‐developed prismatic apatite crystals, the presence of smectites and the geochemical signature of alteration‐resistant elements, indicates the presence of low‐temperature tephra systems altered during burial diagenesis at temperatures <240°C. The presence of aegirine could be explained by the reaction of Na+ and (SiO4)4− partially expelled during glass decomposition with Fe3+ brought to the basin from extrabasinal source areas during arid to semi‐arid weather periods, as evidenced by the accompanying hematite. Ashfall of rather intermediate volcanic rock source fits the synsedimentary volcanism of at least two European volcanic complexes. There is a direct link between ashfalls and fossil accumulations (Konzentrat‐Lagerstätten).
Abstract The Cretaceous Cauvery Basin along the eastern margin of India is associated with strong plume and rift‐controlled tectonic activity, causing highly variable sediment successions under fast relative sea level changes. The ~60 m thick fossil‐rich Kallankurichchi Formation comprises dominantly calcarenites, whereas its basal part contains mixed siliciclastic‐carbonate scree‐like conglomerates and dark grey limestone in the deepest section, possibly the result of a complex tectonic history. Due to the flow transformation, the clast‐supported scree‐like conglomerates grade into matrix‐supported debris flow conglomerates. One unit of these matrix‐supported debris flow conglomerates contains 6–14 μm sized calcite rhombs, which have not been previously reported for such deposits. Through integrated petrographic, geochemical and sedimentological analyses, we document the calcite rhomb formation and discuss their origin by either dedolomitisation or by meteoric diagenesis in shallow marine environments that were temporally subaerially exposed. The absence of any evaporites, absence of large quantities of originally high Mg components and absence of dolomite relicts limits the interpretation of dedolomite. Bored limestone clasts in the matrix‐supported debris flow conglomerates, and the occurrence of fully marine species such as Inoceramus is generally interpreted as scree‐like deposits in nearshore environments being deposited in fully marine conditions. An alternative explanation is tectonic uplift on one side of a fault causing scree deposits to accumulate in shallow marine environments, and subsequent subaerial exposure due to further uplift or a short interval of sea level fall, allowing infiltration of meteoric waters and rhombic calcite formation, followed by submersion. The findings imply a significant tectonic, post‐depositional readjustment event, leading to localised meteoric diagenesis in an otherwise full marine depositional regime. This study documents the complex strata, discusses potential formation mechanisms of rhombic calcite in scree‐like deposits and provides new insights into the interplay between diagenetic processes and basin tectonics.
Abstract Investigating fluvio‐marine depositional environments from early geological times is of great importance, as they provide key insights into the palaeoenvironmental conditions of the early Earth. The present study aims to reconstruct the deposition and evolution of these systems from the Palaeoarchean Moodies Group in the Stolzburg Syncline of the Barberton Greenstone Belt, South Africa. It is based on a detailed investigation of the new, ~490 m‐long research drillcore BASE‐5B, using sedimentological logging, facies analysis and palaeocurrent measurements. Thirteen sedimentary facies were recognised and grouped into five facies associations. The stratigraphically lower section of the core, from ~490to ~294 m drilled depth, with an overall coarsening‐upward trend, is mainly characterised by unidirectionally cross‐stratified sandstones with occasional conglomerates and silicified sulphate concretions; this section was interpreted as a regressive, river‐dominated fluvio‐deltaic environment. The stratigraphically upper section, from ~294 to ~17 m, with an overall fining‐upward trend, is characterised by cross‐stratified sandstones with mudstone drapes and bidirectional palaeocurrents; this section was interpreted as a transgressive, fluvio‐estuarine environment dominated by combined tidal and riverine processes and with a well‐defined turbidity maximum. The vertical stacking pattern implies an evolution from a fluvio‐deltaic to a fluvio‐estuarine system with an overall regressive to transgressive trend due to an increase in basin subsidence and/or decrease in sediment supply. Two orders of transgressive–regressive cycles are superimposed on this general trend, reflecting relative sea‐level fluctuations, most probably driven by changes in sediment supply. These results help in understanding the sedimentary dynamics of Archean fluvial‐marine systems and their response to external forcings.
Abstract The sedimentary provenance and tectonic setting of the Youjiang Basin in southwestern China remain debated, particularly regarding the relative contributions of the Indochina Block and the southeastern margin of the South China Block (SCB) during the Indosinian Orogeny. This study integrates detrital zircon U–Pb geochronology and tourmaline geochemistry from Middle Triassic turbidites of the Baifeng Formation to constrain sediment provenance and tectonic evolution during the Indosinian Orogeny. A total of 189 concordant zircon ages reveal four major age populations at 2100–1500 Ma, 1200–730 Ma, 570–410 Ma and 375–234 Ma, accounting for approximately 85% of the concordant zircon population. The coexistence of abundant Late Palaeozoic–Early Mesozoic zircons with substantial Neoproterozoic and Early Palaeozoic components suggests input from a source region characterised by both Late Palaeozoic–Early Mesozoic arc‐related magmatism and older continental crust. Tourmaline compositions further indicate source contributions from Li‐poor granitoids, pegmatites, alaskites and metasedimentary rocks. Zircon CA–DA characteristics indicate significant input from relatively young magmatic sources, consistent with sediment generation in a convergent continental‐margin setting. Integrated provenance evidence suggests that the Middle Triassic Baifeng Formation sediments were predominantly derived from a Late Palaeozoic–Early Mesozoic continental arc along the southeastern margin of the SCB. These results provide new constraints on sediment dispersal patterns and tectonic evolution of South China during the Indosinian Orogeny and highlight the role of Paleo‐Pacific‐related continental arc processes in basin development.
Abstract Microbialites are valuable archives of environmental change, but their interpretation is often complicated by differential preservation and post‐depositional modification. Fossil Late Pleistocene microbialites preserved along former shorelines of palaeo‐Lake Maquinchao (northwestern Patagonia, Argentina) provide an opportunity to investigate the relationships between microbialite fabrics, preservation and erosion in a lacustrine setting. Petrographic observations were combined with cathodoluminescence, μ‐XRF mapping, scanning electron microscopy, stable isotope analyses, X‐ray diffraction and radiocarbon dating to characterise the internal structure and composition of these fossil microbialites. Two dominant microfabrics were identified: (i) porous micritic shrub fabrics containing filament moulds and microbial‐like structures, and (ii) denser laminated spar–micrite fabrics. These fabrics show a systematic spatial distribution within the build‐ups, with shrub fabrics occurring preferentially in interior portions and laminated fabrics concentrated along outer and upper surfaces. Mineralogical analyses indicate that both fabrics are composed predominantly of low‐Mg calcite, whereas radiocarbon ages constrain carbonate formation between approximately 21 and 14 ka BP. Field observations and microfabric analyses indicate that preservation is strongly influenced by porosity distribution. Dense laminated fabrics form resistant outer layers that enhance preservation, whereas porous shrub fabrics are more susceptible to weathering and erosion. The present morphology of the carbonate build‐ups therefore reflects both primary accretionary processes and subsequent differential preservation. These results demonstrate that differential preservation exerts a major control on the morphology and internal architecture of fossil microbialites and highlight the importance of preservation biases when interpreting lacustrine microbial carbonate records.
