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<jats:title>ABSTRACT</jats:title> <jats:p>Generation of thick megabreccias along carbonate platforms apparently relies on the establishment of overpressured zones in the margin and upper slope deposits (particularly during relative sea-level lowstands), but the main triggering mechanism is thought to be seismic tremors. Here, we present a detailed sedimentological analysis of carbonate density-flow deposits south and north of the Mattinata Fault, a major strike-slip fault zone in the Gargano Promontory (Italy). The analysis shows that in the southern sector the deposits of Albian–Cenomanian age (Monte S. Angelo Formation) are made up predominantly of thick and amalgamated debrites (megabreccias), whereas some 25 km to the north they are composed predominantly of prograding high-density turbidites. Moreover, detailed analysis of Maastrichtian slope deposits (Monte Acuto Limestones) from the southern sector shows that they make up a N–S-prograding system of coalesced lobes composed of high-density turbidites and subordinate debrites. We infer that preconditional factors (e.g., platform progradation, tectonic oversteepening, and high pore pressures in the margin and upper-slope deposits) for triggering the density flows varied along strike of the platform, but the main controlling factors were the activity of the prominent strike-slip Mattinata Fault and the propagation of tremor energy to its near- and far-field regions: large earthquakes triggered thick (up to 40 m) and amalgamated Albian–Cenomanian debrites in proximity (south of) to the fault, whereas the subdued effect of the tremors triggered thinner debrites (5–10 m) and rock falls in the far-field region north of the fault. Moreover, predominantly high-density turbidite deposits were emplaced in the far-field region during lower-magnitude earthquakes. During the Maastrichtian the thick succession of high-density turbidites and scattered thick debrites south of and adjacent to the Mattinata Fault may record an overall quiescent period of the fault. The results are of relevance for understanding the spatiotemporal distribution of density-flow deposition along carbonate platforms in tectonically active regions—in particular with respect to the activity of large strike-slip faults.</jats:p>

<jats:p>Froude-supercritical bedforms and associated sedimentary structures are formed in turbulent flows when value of the Froude Number is Fr greater than 1. They have been increasingly studied in recent years, and while they were previously considered to be of rare preservation, they have been increasingly identified in modern settings and the rock record. In alluvial systems, these structures are being recognized as characteristic of rivers with high variability of discharge, especially in arid, semiarid and subhumid tropical and subtropical climates. However, the development of facies models for such rivers remains tentative, particularly for the rock record, and with the exception of Australia, examples in Gondwana are scarce. The Early Triassic Sanga do Cabral Formation represents an arid to semiarid ephemeral fluvial system cropping out in Southern Brazil, Southwestern Gondwana. The present study reinterprets the sedimentary structures within this formation as Froude-supercritical structures, and ident ifies three Fluvial Styles (FS). FS1 predominantly consists of fine-grained massive sandstone, with interruptions of intraclastic conglomerates, and occasionally-visible faint lamination and mud intraclast levels. It is interpreted as deposited by unconfined flows in the distal portion of a fluvial system, generating hyperconcentrated flows which resulted in thin beds of fine-grained sandstone with massive structure or plane-parallel lamination, and incipient antidunes. FS2 was deposited by flash floods occurring repeatedly within a short period during a wet season. This resulted in a fining-upwards succession of intraclastic conglomerates with supercritical-flow structures, through sandstones with supercritical-flow structures, to sigmoidal cross-stratification and ripple marks with diffuse lamination. FS3 was deposited by catastrophic flash floods characterized by high discharge and flow velocity, possibly generated by erratic storms, which poured in single events. These catastrophic flows generated lar ge-scale sandy antidunes and other Froude-supercritical bedforms with mud intraclasts, which deposited sandstone in undulating laminae, and other supercritical-flow structures. These floods waned extremely rapidly, bypassing the stability field of lower-flow regime bedforms. Measurements taken from undulating stratification, interpreted as antidune deposits, allowed for the estimation of paleoflow velocity and depth. The largest antidunes had a maximum estimated wavelength of 28.92 m (with a mean of 15.4 m) and maximum estimated height of 1.42 m (with a mean of 0.85 m), resulting in an estimated paleoflow velocity of up to 6.72 ms-1 (with a mean of 4.9 ms-1) and a maximum flow depth of 1.59 m (with a mean of 0.9 m). These parameters are comparable to those observed in modern fluvial floods. This study reinforces the significance of Froude-supercritical structures in enhancing our understanding of fluvial systems characterized by high variability in discharge, allowing for a finer interpretation of their d ischarge patterns. This approach can be applied to better understand the many arid, semiarid, or strongly seasonal environments of the Early Triassic period in Gondwana, and potentially other regions and geological times.</jats:p>

<jats:p>The present paper introduces an algorithm for automating the analysis of quartz sandstones and siltstones petrographic thin sections images. The images of thin sections are obtained in polarized light at magnifications providing good image quality. In addition, the images for each section are obtained at different angles of rotation of the microscope stage. Augmentation is applied to the obtained photographs: the number of images increases due to rotations, shifts and rescaling of the image. For training the neural network of the Mask R-CNN architecture, transfer learning is used, with initial weights obtained from a huge variety of non-geological images. The results of image segmentation using Mask R-CNN are compared versus the Watershed algorithm results and the U-Net network for two metrics. According to the standard Intersection over Union metric, U-Net for high quality images and Watershed for blurry images show the best results with a slight superiority. However, according to the Grain Size Metri c, which evaluates the accuracy of grain size measurement, the best accuracy (over 95%) is shown by Mask R-CNN. The grain size analysis is done, and the porosity of the studied petrographic sections is determined. The use of the proposed approaches in the study of thin sections will significantly reduce the time for obtaining the results of grain size distribution analysis and porosity determination.</jats:p>

<jats:p>Shale diagenesis is not well understood, and the cracks in shale contain important information about the diagenetic conditions. The way these cracks open reveals physical changes in the sediment, and the infilling minerals provide insight into the chemical conditions of the formation water. Typically, the authigenic minerals filling the folded cracks are consistent with the chemical composition of the host rocks. For example, folded calcite cracks are found in limestone. This paper, however, focuses on a set of folded calcite cracks in non-calcareous black shales. The goal is to improve our understanding of shale diagenesis by deciphering the origins of these cracks. The cracks are sinuously to ptygmatically folded in a vertical view and weakly sinuous on the bedding plane. They are filled with calcite, bitumen, pyrite, or a combination of them. Evidence of bioturbation and low redox-sensitive trace elemental ratios suggest suboxic to oxic depositional conditions of the shale. The cracks were likely op ened by gas expansion in the unconsolidated mud. The main mineral filling the cracks, calcite, was sourced from hydrothermal fluid that passed through the underlying dolomite. Hydrothermal influences are indicated by the presence of bornite and microcrystalline pyrite in the cracks, as well as Fe and Mn enrichment in the host sediments. Hydrothermal activity can also explain the presence of buddingtonite, an ammonium feldspar in the shale. The results of this study suggest that folded cracks filled by minerals, gradually narrowing towards the top, and lacking internal detrital matrix are likely formed during early diagenesis. The inconsistency between the chemical compositions of the infilling material and the host sediment requires further exploration to identify the source. Hydrothermal fluid passing through the underlying dolomite may be the source of folded calcite cracks in non-calcareous sedimentary rocks. These cracks resemble molar-tooth structures (MTS), which are sinuous cracks filled with micro crystalline calcite mostly found in Precambrian limestone and calcareous shales. If these cracks are indeed MTS, they may be an exception to the age and host-rock lithology constraints of MTS.</jats:p>

<jats:p>Quantifying paleodischarge from geological field observations remains a key research challenge. Several scaling relationships between paleodischarge and channel morphology (width; depth) have been developed for rivers and river deltas. Previous paleodischarge scaling relationships were based on discharge-catchment area scaling and an empirical flow velocity estimate (e.g. Chézy, Manning formulae) multiplied by channel cross-sectional area to derive discharge. In deltas, where marine (wave, tide) energy causes bidirectional flow within distributary channels, the available paleodischarge scaling relationships are not applicable due to their unidirectional flow assumption. Here, the spatial variability of distributary channel widths from a database of 114 global modern river deltas is assessed to understand the limit of marine influence on distributary channel widths. Using 6213 distributary channel width measurements, the median channel widths of distributary channels for each delta were correlated with bankfull discharge for river-, tide- and wave-dominated deltas, the latter two including the effect of bidirectional flow. Statistically significant width-discharge scaling relationships are derived for river- and wave-dominated deltas, with no significant relationships identified for tide-dominated deltas. By reverse bootstrapping the channel widths measured from modern deltas, the minimum number of width measurements needed to apply width-discharge scaling relationships to ancient deltaic deposits is estimated as 3 and 4 for the upstream parts of river- and wave-dominated deltas, respectively, increasing to 30 in the downstream parts of river-dominated deltas. These estimates will guide sedimentological studies that often have limited numbers of distributary channel widths exposed in the rock record. To test the reliability of these alternative width-discharge scaling relationships in the rock record, paleodischarges were estimated for the well-studied Cretaceous lower Mesa Rica Sandstone Formation, USA . Comparison of these results with the more complex Chézy-derived method suggests that these new scaling relationships are accurate. Hence, it is proposed that the scaling relationships obtained from modern deltas can be applied to the rock record, requiring fewer, and easier to measure, data inputs than previously published methods.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The giant Aptian reservoirs and associated lacustrine rocks of the South Atlantic pre-salt section present a series of unique characteristics, which make the systems created for marine carbonate rocks quite inadequate for their classification. Based on our experience with the characterization of thousands of samples of the pre-salt reservoirs and associated deposits from the Brazilian basins, and on a wide literature evaluation, we propose an objective and operational system for the classification of the unusual yet extremely important pre-salt lacustrine rocks. The system allows the coherent record of structure, fabric, primary texture and composition, and main diagenetic modifications of the in situ and resedimented rock types. The in situ rocks are directly classified according to the original proportion among calcite spherulites, fascicular shrubs, and mud matrix. The resedimented rocks are classified according to the original volume of &amp;gt; 2 mm particles and the proportion between sand and mud, using calcirudite, calcarenite, and calcilutite as unbiased names. Such an approach can also be applied to the classification of other particulate carbonate rocks, avoiding the conceptual problems and dubious interpretation of depositional environment and reservoir quality of currently used classification systems. The direct, objective, and intuitive proposed classification system shall contribute to the understanding, exploration, and production of the extraordinary South Atlantic pre-salt petroleum province.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Subglacial sediments are subject to erosion, transport, and deposition in active, ephemeral, and spatially localized glacial environments. It is critical to determine how these mobilized sediments become immobilized in a time-transgressive process and can be frequently remobilized and reimmobilized. Microscopic sedimentary structural signatures provide invaluable information on subglacial processes and contribute to understanding till formation. Data were obtained from a series of field sites in Canada and Austria investigating the microsedimentological aspects of both alpine and continental glaciation tills to construct a conceptual model of subglacial deformation. Microstructures in these tills indicate rheological behaviors that can be summarized into a potential model for soft deforming subglacial sediments. Most microstructures noted in these subglacial till examples highlight the development of subglacial interface kinematics providing clues to till deposition mechanics, subglacial bedform development, and the processes involved in till provenance distributions. A conceptual process model of subglacial interface conditions in soft mobile sediments is developed that uses microsedimentological evidence and highlights how an active ice mass integrates with ongoing substrate deformation. In the model, interaction occurs between the ice and its sediment bed with internal sediment microstructures evolving where multiple transient shear deformation processes cause localized deformation linked to pervasive and nonpervasive sediment deformation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Dolomite textures are widely interpreted to reflect physical, mineralogical, and geochemical conditions of crystal growth. In particular, nonplanar dolomites, which display non-faceted crystal boundaries and a low percentage of crystals with compromise boundaries with preserved crystal-face junctions, have long been cited as evidence of crystal growth in fluids warmer than a theoretical dolomite critical roughening temperature (CRT) of ∼ 50–100°C. No direct experimental evidence across this temperature range exists, however, to confirm the theory that nonplanar dolomite should form exclusively above the CRT. The present study offers new Δ47 clumped-isotope data from nonplanar dolomites from the Paleocene–Eocene Umm er Radhuma Formation (Qatar) that show that nonplanar dolomite can form below the theoretical CRT. These dolomites are interpreted to have experienced only near-surface to shallow-burial conditions since deposition, and lack common burial features such as two-phase liquid-vapor inclusions, stylolites, compaction-reduced porosity, and burial cements. Scanning electron microscope images reveal that relatively large dolomite crystals (typically &amp;gt; 100 µm) comprise non-faceted mosaics with indistinct crystal boundaries, indicating a nonplanar texture. Thin-section petrographic measurements confirm the nonplanar texture, as the proportion of dolomite crystals with compromise boundaries with preserved crystal-face junctions ranges from 9% to 20% with an average of 14%, defining these dolomites as nonplanar sensu stricto (≤ 30%). The new Δ47 clumped-isotope data from these nonplanar dolomites reveals average crystallization temperatures ranging from 38.8 to 54.2°C and overall averaging 43.6°C. Calculated uncertainties, however, indicate the nonplanar dolomites could have formed at temperatures as low as 29.1°C or as high as 65.3°C. More than three quarters (∼ 78%) of the samples have mean temperatures that fall below 50°C, and all samples have calculated uncertainties indicating possible temperatures below 50°C, but not all indicate possible temperatures above 50°C. Furthermore, these calculated uncertainties overlap with the crystallization temperatures of planar and mimetic dolomites higher in the section, suggesting that all dolomites formed under similar temperature conditions, and therefore texture is unlikely driven solely by crystallization temperature. Cumulatively, these results indicate that the nonplanar dolomite formed in a shallow-burial setting at temperatures near or below the proposed dolomite CRT. The new Δ47 data, in conjunction with textural observations from natural dolomites and hundreds of published high-temperature experiments, suggest that nonplanar dolomite cannot be reliably used as an indicator of high-temperature environments of dolomitization.</jats:p>

<jats:p>The Santa Clara Abajo and Santa Clara Arriba formations host a diverse assemblage of trace fossils that record a wide range of behaviors and a broad array of ecological niches during the Middle Triassic, a critical period in the evolution of continental fauna with the diversification of both synapsids (cynodont and dicynodont) and archosauromorphs (dinosaurs, pterosaurs, and crocodilians) that represent post-Permian faunal recovery. The Santa Clara formations are part of the continental infill of the Cuyana rift Basin in Argentina and represent a lacustrine system with fluvial input and delta development. Sedimentological characteristics of these units as well as their stacking patterns characterize a “fluctuating profundal” facies association typical of a balanced-fill lake basin. The lacustrine and associated terrestrial environments preserve a rich record of invertebrate traces with 26 ichnogenera from ethological classes of fodichnia, domichnia, repichnia, pascichnia, and cubichnia occupying all co ntinental tiers (subaerial and subaqueous, surficial/very shallow, shallow, mid, and deeper) and ecological niches (epiterraphilic, terraphilic, hygrophilic, and hydrophilic). In association with the invertebrate traces, two taphonomic modes of tetrapod footprints have been found: a moderate fidelity mode and a high fidelity mode. Physical sedimentary features, burrows, trails, and tracks, and their stratigraphic positions are integrated to interpret the main factors involved in footprint preservation in these subsettings. The most significant and variable preservational factor found is water-table fluctuations controlled by the paleohydrology of a balanced-fill lake system. These data show that in balanced-fill lake systems, diverse trace assemblages occur in the lake and associated subsettings such as delta plains, and lake-margin settings whereas trace fossils can be totally absent in coeval lake-center strata, particularly if anoxic lake bottom conditions occur, as probably occurred in the meromictic Santa Clara lake system.</jats:p>

<jats:p>The ammonites in spherical carbonate concretions often preserve their original three-dimensional (3D) shell shapes and detailed fragile structures. However, the formation process of spherical ammonite concretion is not fully understood. Herein, the ammonite concretions identified in the Cretaceous (Campanian) Osoushinai Formation, Yezo Group, Japan, are examined to understand their formation process during the soft tissue decomposition after burial in marine sediments. In the Osoushinai Formation, almost all observed ammonites in concretions preserve their 3D form without phragmocone deformation. The calcite filling in the remaining body chamber of ammonites (BC1) shows that shells were buried with soft tissues. These occurrences, negative delta13C values, and the near-zero delta18O values of BC1 as well as the concretions indicate that both BC1 and concretions rapidly formed from dissolved inorganic carbon derived from organic matter, including the soft tissue of dead organisms, in the shallow part of the sediments. The increasing Fe concentration in BC1 shows that BC1 formed in the iron reduction (FeR) zone, where organic matter was decomposed owing to the activity of iron-reducing microorganisms. The similarity of the elemental and isotopic compositions of BC1 and concretions show that they concurrently formed in the FeR zone. In the Osoushinai Formation, an abundant influx of Fe(III) and intense bioturbation during the deposition of the formation promoted organic decomposition in the FeR zone, causing rapid formation of BC1 and concretions. Such rapidly formed calcite fillings and concretions protected fossils from deformation and dissolution during diagenesis to preserve their 3D form. Overall, the findings of this study provide a new insight into the relation between sedimentary environments and the fossil preservation process via rapid concretion formation.</jats:p>