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<jats:title>ABSTRACT</jats:title><jats:p>Traditional mapping of bedforms in submarine canyons relied on vessel‐mounted and towed sensors, but their fine‐scale geomorphology and shallow structure requires higher resolution datasets. This study utilizes a high‐resolution dataset obtained from an autonomous underwater vehicle, combined with seismic reflection profiles and sediment cores, to analyze bedform sets within a 25.6 km long submarine canyon (canyon C14) in the northern South China Sea. A train of crescent‐shaped axial steps, indicative of cyclic steps formed by supercritical turbidity currents, is imaged along the canyon. Axial steps in the upper course show erosional truncations and sub‐horizontal reflectors on the lee and stoss sides, respectively, pointing to erosional–depositional cyclic steps formed by confined flows with high erosional capacity. This is facilitated by canyon narrowness and steeper axial gradient. After a transition segment, the lower course widens, with a gentler axial gradient, resulting in increased asymmetry and wavelength of axial steps. Backset bed deposits on the stoss sides of these steps indicate depositional cyclic steps with higher aggradation. Sediment filling, almost padding each cyclic step associated scour suggests the reworking of previously formed bedforms by gravity flows fed by destabilization processes on the canyon sidewalls and upstream lee faces and, possibly, by shelf‐edge and uppermost slope spillover into the canyon. At the lowermost course, cyclic steps transition to a furrow field, likely associated to flow velocity reduction facilitated by canyon floor widening and a further decrease in slope gradient. Flow braiding and re‐convergence, related to the erosion of fine‐grained deposits within the canyon floor, should have played a role to produce furrows under supercritical conditions. This work enhances our understanding of the detailed morphology and shallow relief configuration of bedforms in deep‐water submarine canyons, providing insights into their causative processes and evolution.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Lake Van, the world's largest alkaline lake, hosts some of the largest microbialite towers worldwide, which are considered as modern analogues of ancient stromatolites. This study investigates the links between microbialite evolution, geology, climate and hydrology, and the role of biotic and abiotic processes in microbialite growth and morphology. For these objectives, the northern shelf of Lake Van was surveyed by subbottom seismic profiling and diving, and two 9 m and 15 m high microbialite chimneys were sampled at 25 m water depth. Samples were analysed for stable oxygen and carbon isotopes, X‐ray diffractometry, scanning electron microscopy and U/Th age dating. Lake Van microbialites precipitate wherever focused Ca‐rich groundwater flows to the lake floor to mix with alkaline lake water. Variable columnar, conical and branching morphologies of the microbialites indicate various processes of formation by groundwater channelling within the chimneys. Collectively, our data suggest that the microbialite chimneys have formed within the last millennium, most likely during the warm and humid Medieval Climate Anomaly (<jats:italic>ca</jats:italic> AD 800–1300), when lake level rose approximately to the present level due to enhanced Inputs of riverine Ca‐rich freshwater and groundwater. Our new scanning electron microscopy observations indicate that the internal structure of the microbialites below the outer cyanobacteria‐covered crust is constructed by calcified filaments, globular aggregates and nanocrystals of algal, cyanobacterial and heterobacterial origins and inorganically precipitated prismatic calcite crystals. These textural features, together with dive observations, clearly demonstrate the important role of inorganic carbonate precipitation at sites of groundwater discharge, followed by cyanobacteria and algal mucilage deposition and microbially meditated calcification in the photic zone in the rapid growth of the microbialite chimneys. Considering the close similarities of some textures with those of ancient stromatolites and meteorites, the results of this study provide new insights into the environmental conditions associated with stromatolite formation and extra‐terrestrial life evolution.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Incised valleys are commonly investigated based on outcrop, modern setting and seismic data, which are often limited by data availability, especially for broad (<jats:italic>ca</jats:italic> &gt;100 km wide) shelf settings. Consequently, few have described complete depositional systems of the incised valleys, especially those linked to their corresponding modern rivers in a source‐to‐sink framework to determine comprehensive controlling factors. This study documents Late Quaternary incised valleys and their characteristics in the Balingian – Central Luconia shelf based on regionally‐extensive three‐dimensional seismic data, two‐dimensional high‐resolution seismic data and boreholes. The three‐dimensional seismic data show that the main Tatau incised valley and tributary Suai incised valley fed the Tatau–Suai shelf‐edge delta. The Tatau incised valley is interpreted as the main incised valley based on its broader width, thicker infill, two‐tier stratigraphic architecture and its larger drainage basin area compared to the tributary Suai incised valley. The Tatau incised valley shifted its direction towards an active tectonic lineament (West Baram Line) and bypassed sediments to the deep‐water via a series of upper slope channels. The Tatau incised valley system comprises the following elements: (i) an upstream segment characterized by rivers dissecting an uplifted hinterland comprising Neogene sedimentary rocks, humid‐tropical vegetation, monsoonal climate, extremely high river discharge and sediment supply; (ii) a midstream segment comprising an emergent, vegetated (tropical rainforest to mangroves) and relatively narrow (<jats:italic>ca</jats:italic> 150 to 200 km wide) shelf, which was characterized by basinward‐increasing, tectonically‐driven accommodation space, valley incision and deposition; (iii) a downstream segment with increasing tidal‐influence where the Tatau and its tributary valley merged; and (iv) a short (15 to 20 km), narrow (10 to 15 km), tectonically‐controlled shelf‐edge delta and channelized upper slope. This study demonstrates the value of documenting an incised valley system in the context of its broader source‐to‐sink setting, which may provide a useful analogue for ancient incised valleys in comparable geological settings.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>The mechanisms controlling the stacking patterns of deep‐water turbidite lobes are currently open to a wide range of interpretations. A study of Turonian to Coniacian turbidite lobe complexes in the greater Marulk area (Dønna Terrace, Norwegian Sea) was undertaken to examine the balance and respective influences of various controlling factors using a large sediment core, well‐log and seismic dataset. A four‐tiered lobe hierarchy is described based on a detailed sedimentological study of three Cretaceous turbidite lobe systems, involving a variety of sedimentary processes and flow regimes. The inferred depositional stacking patterns were then used to identify autogenic and allogenic forcings on the large‐scale depositional architecture of turbidite lobes. Autogenic processes (best observed in core data) control the self‐regulation of sediment dispersal and the broad evolution of lobe sub‐environments. Conversely, allogenic forcings (best observed in well‐log data) regulate axial migration within the turbidite lobe succession, and control sediment pulses and ultimately the general evolutionary trend of the turbidite lobe complex. Beyond this, an updated approach is proposed here aiming at deciphering autogenic‐dominant and allogenic‐dominant processes at different hierarchical levels in both confined and unconfined turbidite lobe deposits, which may help with assigning appropriate inputs for geomodels for an improved understanding of the internal and external controls on lobe depositional architecture. It is advocated that this approach may eventually be used in other depositional systems, such as in deltaic complexes from coastal settings, both in actual and ancient sediments.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Strata produced by fluvial dunes can provide insight into the hydrological regime of ancient rivers. Recent experiments indicate that conditions of disequilibrium between bedforms and formative flows may be inferred from the coefficient of variation of preserved dune cross‐set thickness, suggesting that this quantity may act as a proxy for the flashiness of river floods relative to the time required for full bedform translation. To assess whether this idea is applicable to interpretations of the stratigraphic record, this study examines published data relating to more than 2600 cross‐sets from 53 sedimentary units of 19 river systems. The presented analyses must not be over‐interpreted, because the considered rivers span different environmental settings, the data sources are heterogeneous in terms of type and dimensionality, and some variables were established by applying empirical relationships. Yet, significant findings are revealed. Larger rivers exhibit discharge and bedform characteristics that are more conducive to disequilibrium; however, a modest increase in the coefficient of variation of cross‐set thickness, CV(<jats:italic>D</jats:italic><jats:sub>st</jats:sub>), as opposed to the expected decrease, is seen as a function of river size. Crucially, smaller CV(<jats:italic>D</jats:italic><jats:sub>st</jats:sub>) values are not systematically associated with conditions that should favour dune disequilibrium. Meanwhile, only <jats:italic>ca</jats:italic> 25% of the studied examples demonstrate cross‐set thickness statistics compatible with quantitative formulations of the autogenic control by variable dune topography – the notion of ‘variability‐dominated’ preservation. These findings indicate that the variability in cross‐set thickness may be a poor predictor of discharge variability, perhaps because of the multiplicity of factors controlling dune preservation, such as bedform hierarchy, transport stage and depth‐dependent variations in dune disequilibrium. To improve interpretations of cross‐stratified deposits, further research is needed: (i) to establish the value of process‐to‐product models for reverse product‐to‐process interpretations; and (ii) to define representative samples for preserved dune deposits accounting for temporal and spatial variability in preservation potential.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Identification of the processes producing soft‐sediment deformation structures, common in siliciclastic deposits and less abundant in carbonate successions, is complex, because different processes may produce similar structures. Thus, interpreting the origin of these structures may be challenging: it requires both a detailed sedimentological study and the knowledge of the depositional environment and stratigraphic evolution, in order to provide hints to identify the processes affecting sediments after deposition. Among the potential causes of the formation of soft‐sediment deformation structures, seismic shock is one of the possibilities, but their origin could be also related to other triggering mechanisms, such as volcanic activity, sediment loading, salt tectonics, fluid expulsion, meteorite impacts and mass movements. Although it is a plausible option, the interpretation of these structures as ‘seismites’ is not obvious: it must be supported by different lines of evidence, considering that the correct interpretation of soft‐sediment deformation structures as a consequence of seismic shocks acquires important implications in palaeoseismology studies.</jats:p><jats:p>The occurrence of diverse soft‐sediment deformation structures in a fault‐controlled basin (i.e. in a geological setting characterized by syndepositional tectonics) preserved in different subenvironments of a Norian carbonate system in the Southern Alps of Italy provides the chance to characterize different types of soft‐sediment deformation structures along a platform‐to‐basin depositional profile. Presence of pseudonodules in basinal resedimented limestone, sedimentary dykes and clinostratified breccias with unlithified clasts in slope settings and liquefaction of inner platform facies at the platform top testify to an origin compatible with multiple seismic shocks, repetitively affecting the same stratigraphic intervals. The diverse types of soft‐sediment deformation structures in the studied carbonate system provide a rich catalogue of structures related to seismic shocks, representing a possible reference for other similar settings.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Upper Palaeozoic lacustrine basin deposits not only record local tectonism but are also an archive to evaluate global palaeoclimate changes linked to the Late Palaeozoic Gondwanan ice age. The Tournaisian Horton Group of Nova Scotia, south‐east Canada, accumulated in rift basins following the final accretion of peri‐Gondwanan terranes to the Appalachians. Sedimentology, mineralogy and geochemistry of the well‐exposed sandstones and shales at the classic Blue Beach section (<jats:italic>ca</jats:italic> 353.5 to 352 Ma) reveal the interplay of local tectonism and global climatic controls on lacustrine sedimentation. The lacustrine depositional environment gradually transitioned from deep water offshore at the base of the section to wave‐dominated and fluvial‐dominated nearshore at the top. Multiple small transgressive–regressive sedimentation cycles have an average 21 ka duration, likely related to Milankovitch cyclicity. Unusually abundant soft‐sediment deformation structures and landslides are the sedimentary responses to frequent earthquakes during the most active phase of rift subsidence. The overall succession shows changes from a shallowing‐up balanced‐filled to an overfilled lacustrine basin. The chemical weathering intensity index and the Th/K ratio show a longer‐term trend from dry and cool conditions low in the section to humid and warm conditions near the top, with rapid change in the transition period. The section records the peak of the global mid‐Tournaisian carbon isotope excursion and the corresponding cooling event (354 Ma to approximately 351 Ma). The sedimentary succession is a response to long‐term and short‐term climatic cycles influencing lake level and sediment supply during the time of maximum rift basin subsidence recorded by the soft‐sediment deformation structures.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>Dolomite is globally present in past geological records, but rare in modern environments. The mechanisms favouring its precipitation under ambient conditions remain highly debated. This study investigates sediments, containing high concentrations of early diagenetic calcian dolomite, from alkaline Lake Van (Republic of Türkiye, formally Turkey) dating back to 252 ka BP. Powder X‐ray diffraction and scanning electron microscopy evidence suggests that dolomite formation is associated with prior dissolution of aragonite and low‐Mg calcite and a subsequent co‐precipitation with, and/or partial transformation of, high‐Mg calcite into dolomite. The infrequent presence of diatom frustules encapsulated by dolomite suggests, for Lake Van, unusually low pore‐water pH at the time of dolomite formation. Conditions facilitating the preservation of silica, as well as dissolution and subsequent reprecipitation of carbonate phases, could result from periodic reventilations of Lake Van's deep water and an advection of pore fluids with contrasting redox potential and chemical concentration gradients. This continental analogue of the coastal ‘mixing‐zone’ dolomitization model argues that not overcoming a single specific hydrochemical threshold, but highly dynamic and fluctuating conditions trigger dolomite formation in Lake Van.</jats:p>

<jats:title>ABSTRACT</jats:title><jats:p>The source and pumping mechanism of magnesium play crucial roles in dolomitization. The preferential dolomitization of burrows has been extensively documented in geological archives. Although burrows are abundantly preserved in Cretaceous carbonates, burrow‐selective dolomitization is uncommon in normal salinity marine environments due to the low Mg/Ca ratio of seawater. However, burrow‐selective dolomitization occurred in the Cenomanian Rumaila Formation carbonates of the Arabian Platform providing an excellent example to further clarify the mechanism and explore other potential Mg sources of burrow‐elective dolomitization. Integrated studies of petrography, stable isotope geochemistry and laser ablation–inductively coupled plasma–mass spectrometry‐based <jats:italic>in situ</jats:italic> element geochemistry were conducted. It was found that the burrow‐selective dolomitization exclusively occurred in echinoderm fragment‐filled <jats:italic>Thalassinoides</jats:italic> networks which occurred as <jats:italic>Glossifungites</jats:italic> ichnofacies. Burrow dolomites showed fine to medium crystalline and planar subhedral to euhedral textures with cloudy centres and clear rims. They exhibited relatively greater Mn, lower Sr and Fe concentrations, no or very weak negative Ce anomaly and middle rare earth element‐bulge patterns, and are slightly enriched with occasionally depleted δ<jats:sup>13</jats:sup>C and comparable δ<jats:sup>18</jats:sup>O relative to the surrounding calcite matrix. The initial high‐Mg calcite echinoderm fragments in burrows have been stabilized to low‐Mg calcite, and echinoderm syntaxial overgrowth calcite cement was practically nonexistent. Echinoderm fragments were frequently replaced by dolomite in part or whole. Undolomitized echinoderms have negative Ce anomaly and seawater‐like rare earth element patterns, as well as very low Mn, Fe and relatively greater Sr concentrations. These suggest that echinoderm stabilization occurred in fluid unsaturated with respect to high‐Mg calcite driven by aerobic decomposition of organic matter in oxic seawater near the sediment–water interface, meanwhile, Mg ions were liberated into pore water. This process pre‐dated the dolomitization allowing the Mg derived from echinoderm to raise the Mg/Ca ratio of burrow interstitial water. The dolomites in burrows were generated by initial replacement and subsequent overgrowth cementation associated with bacterial sulphate reduction and methanogenesis in low‐temperature and suboxic to anoxic fluids in the near‐surface realm, and faintly recrystallized as burial depth increased. This study sheds light on the echinoderm stabilization process, links the early diagenesis of skeletons to burrow dolomitization, and proposes a conceptual model illustrating that high‐Mg calcite skeletons could act as a major Mg source for burrow‐selective dolomitization, which compensates for the deficiency of Mg in normal low Mg/Ca ratio Cretaceous seawater. This study implies the interaction effect and element cycle among components in early diagenetic systems, and verifies that high‐Mg calcite is indeed a non‐negligible potential Mg source for partial or elective dolomitization.</jats:p>

<jats:title>Abstract</jats:title><jats:p>Mass‐transport deposits are products of resedimentation phenomena involving a broad spectrum of gravity‐driven processes, and commonly have a high preservation potential in deep‐marine environments. This study documents various types of mass‐transport deposits that are interbedded with intensely bioturbated shallow‐marine calciclastic sediments deposited along a reflective coast during the middle and late Eocene. The sedimentary succession, located in the vicinity of Novigrad in northern Dalmatia, Croatia, comprises sediments deposited in a range of nearshore and carbonate ramp environments, and represents the infill of a thrust‐top (piggyback) basin of the North Dalmatian foreland basin. Five types of mass‐transport deposits, ranging in thickness from 13 cm to 6 m, have been identified: (i) calcilutite and calcarenite slumps; (ii) conglomeratic slump‐debrites with a ‘dough‐like’ appearance; (iii) blocky‐flow deposits bearing large blocks of beachface and/or shoreface deposits; (iv) rockfall deposits comprising scattered blocks of beachface conglomerates and shoreface calcarenites; and (v) ‘classical’ matrix‐supported debrites. Calciturbidites are rare and mainly comprise Ta and Tb divisions. Conglomeratic slump‐debrites are mostly found in association with offshore‐transition deposits, suggesting that mass flows were triggered above the storm‐wave base likely due to a combined effect of: (i) strong earthquakes related to the tectonic development of the basin; (ii) sediment destabilization due to pore‐water overpressure during forced regressions; and (iii) storm‐wave loading affecting the shallow seabed. Progressive deepening likely favoured mass‐flow transformations, although the overall paucity of turbidites suggests relatively short mass‐flow transport distance and turbidity current bypass to deeper realms. Multiple erosion phases and resedimentation processes from the Cretaceous to the late Eocene contributed to the diverse suite of extraformational clasts in the mass‐transport deposits studied. The mass‐transport deposits may be triggered and emplaced in shallow‐marine settings mainly during regressive stages of basin development, as the diverse gravel clast composition suggests significant tectonic influence. Although the mass‐transport deposits reported herein are relatively small, some of their peculiar sedimentary features and occurrence within shallow‐marine calciclastic deposits render them rather unique and suitable for a re‐assessment of the nature and evolutionary continuum of processes involved in subaqueous sediment mass transport, as well as the preservation potential of sedimentary features in high‐energy wave‐reworked environments.</jats:p>