Wojciech Nemec

This study of the Aptian lower part of the Carolinefjellet Formation in Svalbard, Norwegian high Arctic, is based on well cores and outcrop section in the Adventdalen area of Spitsbergen and reports on the deposits and bioturbation structures of an ancient subpolar marine shelf from a well-known period of global greenhouse climate. The study documents the sedimentation conditions and benthic fauna activity on a warm-water aggrading shelf subject to harsh Arctic wave climate and eurybatic base-level changes, with episodic bottom incursions of cold polar water. Lithofacies associations and 38 observed ichnotaxa represent subenvironments ranging from offshore to lower shoreface and hosting the Cruziana ichnofacies in its distal to proximal expression, with a brief mid-Aptian encroachment of middle shoreface zone with a distal expression of the Skolithos ichnofacies. The ichnofacies are variously impoverished compared to their archetypes. The sediment bioturbation intensity varies, but similar lithofacies associations show a comparable intensity throughout the stratigraphic succession, which indicates an ichnofauna ecology controlled by the seafloor hydraulic regime and oxygenation, and thus mainly by the wave climate and relative sea-level changes. Sandstone tempestites indicate high-frequency storms, commonly exceeding the magnitude of largest modern hurricane events. The study confirms that a change in global climate mode, such as the Early Cretaceous warming, entails extreme weather conditions.

This study of the Aptian lower part of the Carolinefjellet Formation in Svalbard, Norwegian high Arctic, is based on well cores and outcrop section in the Adventdalen area of Spitsbergen and reports on the deposits and bioturbation structures of an ancient subpolar marine shelf from a well-known period of global greenhouse climate. The study documents the sedimentation conditions and benthic fauna activity on a warm-water aggrading shelf subject to harsh Arctic wave climate and eurybatic base-level changes, with episodic bottom incursions of cold polar water. Lithofacies associations and 38 observed ichnotaxa represent subenvironments ranging from offshore to lower shoreface and hosting the Cruziana ichnofacies in its distal to proximal expression, with a brief mid-Aptian encroachment of middle shoreface zone with a distal expression of the Skolithos ichnofacies. The ichnofacies are variously impoverished compared to their archetypes. The sediment bioturbation intensity varies, but similar lithofacies associations show a comparable intensity throughout the stratigraphic succession, which indicates an ichnofauna ecology controlled by the seafloor hydraulic regime and oxygenation, and thus mainly by the wave climate and relative sea-level changes. Sandstone tempestites indicate high-frequency storms, commonly exceeding the magnitude of largest modern hurricane events. The study confirms that a change in global climate mode, such as the Early Cretaceous warming, entails extreme weather conditions.

The deposition of a classic turbidite by a surge-type turbidity current, as envisaged by conceptual models, is widely considered a discrete event of continuous sediment accumulation at a falling rate by the gradually waning density flow. Here, we demonstrate, on the basis of a high-resolution advanced numerical CFD (computational fluid dynamics) simulation and rock-record examples, that the depositional event in reality involves many brief episodes of nondeposition. The reason is inherent hydraulic fluctuations of turbidity current energy driven by interfacial Kelvin-Helmholtz waves. The experimental turbidity current, with realistic grain-size composition of a natural turbidite, used only 26 to 33% of its in-place flow time for deposition, while the remaining time went to the numerous episodes of sediment bypass and transient erosion. The general stratigraphic notion of a gross incompleteness of sedimentary record may then extend down to the deposition time scale of a single turbidite.

This case study from the renowned Demänová Cave System in the Carpathians of Slovakia demonstrates that the conventional methods of fluvial sedimentology, combined with an allostratigraphic mapping and speleothem U‐series isotopic dating, can give unprecedented insights into the hydrological history of underground karst conduit. The deposits studied are a relic compound sidebar ranging from gravel to mud and encapsulating the conduit’s hydrological history from the middle Pleistocene to the present time. A succession of 10 allostratigraphic units, time‐constrained by speleothems, are recognized in the sidebar deposits, and the corresponding morphodynamics of an evolving cave‐floor sedimentation are reconstructed in considerable detail. The subterranean river water stages recognized from the deposits, time‐constrained by flowstone layers and stalagmites, correlate with and add to the regional record of climate changes. Two distinct episodes of flow ponding (high‐stage slackwater conditions) are recognized and attributed to the independently documented downstream cave‐roof collapses, probably triggered by the Carpathian post‐orogenic earthquakes. This multidisciplinary study may serve as a useful methodological guide for the analysis of fluviokarstic deposits in speleological research and reconstruction of their hosting cave hydrological history.

This study is a reconstruction of the Coniacian palaeogeographic and palaeoenvironmental development in the North Sudetic Basin, a synclinal trough within the Late Cretaceous Central European seaway linking the Boreal and Tethyan marine provinces. The basin formed as an early side effect of the Alpine orogeny combined with the mid‐Cretaceous eustasy, and crucial stages of its evolution occurred during the Coniacian. The basin in the early Coniacian was a long and narrow shallow‐marine embayment with a hypothetical (non‐preserved) bayhead strait funnelling tidal currents. Coalescing tidal sand ridges formed a littoral platform that prograded from the bayhead zone along the basin axis, impinged on laterally by the basin‐margin shoreface and local river deltas. A mid‐Coniacian forced marine regression and closure of the bayhead strait, attributed to the Alpine tectonism combined with eustasy, brought about a dramatic change in the basin, whereby the basin‐wide littoral sand platform emerged and turned briefly into a denudated coastal plain. The late Coniacian eustatic marine transgression formed an in‐place growing coastal sand barrier at the outer edge of the former littoral platform, sheltering a paralic limno‐lagoonal plain with peat‐forming mires. The coastal barrier was eventually drowned by the sea and maximum marine flooding occurred, followed by a normal regression recorded as a rapidly upwards‐shallowing succession of offshore‐transition to fluvio‐deltaic deposits. This case study of the sedimentation pattern in an evolving, tectonically controlled marine embayment contributes to the existing facies models for estuarine embayments formed by a passive marine drowning of large fluvial or glacial valleys.

Sea‐floor topography of deep‐water folds is widely considered to have a major impact on turbidity currents and their depositional systems, but understanding the flow response to such features was limited mainly to conceptual notions inspired by small‐scale laboratory experiments. High‐resolution three‐dimensional numerical experiments can compensate for the lack of natural‐scale flow observations. The present study combines numerical modelling of thrusts with fault‐propagation folds by Trishear3D software with computational fluid dynamics simulations of a natural‐scale unconfined turbidity current by MassFlow‐3D™ software. The study reveals the hydraulic and depositional responses of a turbidity current (ca 50 m thick) to typical topographic features that it might encounter in an orthogonal incidence on a sea‐floor deep‐water fold and thrust belt. The supercritical current (ca 10 m sec⁻¹) decelerated and thickened due to the hydraulic jump on the fold backlimb counter‐slope, where a reverse overflow formed through current self‐reflection and a reverse underflow was issued by backward squeezing of a dense near‐bed sediment load. The reverse flows were re‐feeding sediment to the parental current, reducing its waning rate and extending its runout. The low‐efficiency current, carrying sand and silt, outran a downslope distance of >17 km with only modest deposition (<0·2 m) beyond the fold. Most of the flow volume diverted sideways along the backlimb to surround the fold and spread further downslope, with some overspill across the fold and another hydraulic jump at the forelimb toe. In the case of a segmented fold, a large part of the flow went downslope through the segment boundary. Preferential deposition (0·2 to 1·8 m) occurred on the fold backlimb and directly upslope, and on the forelimb slope in the case of a smaller fold. The spatial patterns of sand entrapment revealed by the study may serve as guidelines for assessing the influence of substrate folds on turbiditic sedimentation in a basin.

The infilling history of the Adventdalen fjord‐valley in central Spitsbergen is reconstructed, with a focus on permafrost development, based on sedimentological and cryostratigraphic evidence from drilling cores. The techniques of optically stimulated luminescence and radiocarbon accelerator mass‐spectrometry dating were used to establish sediment chronology. The fjord‐fill sedimentary succession includes the fjord‐bottom late Weichselian subglacial till of the Last Glacial Maximum, the early Holocene muddy glaciomarine deposits with ice‐rafted debris formed during the fjord deglaciation, and the younger Holocene deposits of a fjord‐head Gilbert‐type delta of which the fluvial distributary plain shows raised alluvial terraces hosting aeolian sedimentation. This sedimentary record of the last glaciation/deglaciation cycle is interpreted in terms of sequence stratigraphy. Zones of epigenetic and syngenetic permafrost are recognized from the vertical distribution of cryofacies, with a conclusion that the formation of permafrost commenced and extended down‐fjord as the fluvio‐deltaic fjord‐fill was gradually reaching subaerial exposure. The upwards‐grown syngenetic permafrost and the top part of downwards‐grown epigenetic permafrost below contain excess ice in a suite of cryofacies indicating ground‐ice segregation and segregative intrusion. The deeper epigenetic permafrost is ice‐poor and contains cryofacies formed solely by segregation processes. This case study may serve as an analogue for other similar Arctic fjord‐valleys where the fjord‐head shoreline was established during the post‐Weichselian deglaciation.

This study of Eocene carbonate succession in the Dinaric Foreland Basin of northern Dalmatia, Croatia, integrates palaeontological and sedimentological data to document a range of carbonate ramps formed intermittently during the basin tectonic development. The end‐Cretaceous basal erosional unconformity records the coupling of Adria and Eurasia crustal plates, with an antiformal uplift along their suture zone. The overlying late Ypresian carbonate ramp, spanning biozones SBZ 11–12, developed on the forebulge flank of a shallow‐marine early synclinal basin. Basal grainstone/packstone facies, dominated by encrusting foraminifers with alveolinids and miliolids, pass upwards into packstones dominated by miliolids and rotaliids with bryozoan and echinoid fragments, indicating an increased bathymetry of the retreating forebulge flank. Deposition of grainstone facies preceded an end‐Ypresian (SBZ 12/13 transition) subaerial exposure due to post‐subductional isostatic uplift. The younger, middle to late Eocene carbonate ramps (SBZ 13–19) formed episodically as perched isolated features on blind‐thrust anticlines in a bathymetrically diversified wedge‐top basin, where phases of clastic and skeletal biogenic sedimentation alternated due to disharmonic thrusting and relative sea‐level changes. Clastic sedimentation reflects anticline crest erosion and a forced‐regressive progradation of gravelly foreshore and sandy shoreface facies over heterolithic offshore‐transition and muddy offshore facies on the anticline flank. Biogenic sedimentation represents inner‐ to middle‐ramp environments, with the latter terminating bluntly in muddy offshore environment. An outer‐ramp environment, known from classic ramp models, was lacking due to bathymetric threshold. Analysis of larger benthic foraminifers (LBF), as biostratigraphic age indicators and palaeobathymetric proxies, helped distinguish systems tracts and determine their time span. A comparison of local and global sea‐level changes allowed the interplay of tectonic and eustatic forcing to be deciphered for the study area.

The Palaeogene Isparta Basin of southwestern Anatolia formed between two convergent arms of the Isparta Bend orocline of the Tauride orogen. The origin of this tightening orocline is hypothetically explained in plate‐tectonic terms. Basin sedimentation commenced on a down‐warped Mesozoic carbonate platform of a crustal block accreted at the end of Cretaceous to the southern margin of the Anatolian plate. The basin earliest deposits are Palaeocene reddish mudstones with a fossil‐barren condensed basal part and increasingly interspersed with thin calcarenitic turbidites towards the top. The supply of turbiditic sediment to the basin plain subsequently increased, as the upper‐bathyal basin plain became surrounded from both sides by a narrow littoral shelf with an advancing turbiditic slope ramp. A major forced regression occurred at the end of Bartonian, causing incision of subaerial to submarine valleys up 600 m deep, filled in with gravelly to sandy turbidites and debrisflow deposits during the subsequent rise of relative sea level. The half‐filled valleys were re‐incised due to a Rupelian forced regression and were fully filled with fluvio‐deltaic bayhead deposits during a final marine transgression that re‐established the basin‐margin biocalcarenitic shelf. The littoral environment then expanded across the shallowing basin, as the basin axial zone was up‐domed and eroded to bedrock level at the end of Oligocene and the basin was tectonically inverted in Miocene. The pattern of intra‐orocline foreland sedimentation documented by this case study provides tentative criteria for the recognition of synorogenic oroclines and for their distinction from post‐orogenic oroclines.

This natural‐scale experimental study combines structural modelling of soft‐linked normal‐fault relays with a CFD (computational fluid dynamics) numerical simulation of a range of unconfined turbidity currents overrunning the relay‐system topography. The flow, released from an upslope inlet gate 2000‐m wide and 50‐m to 100‐m high, rapidly expands and adjusts its thickness, velocity and sediment load to the substrate slope of 1.5°. A lower initial sediment concentration or smaller thickness renders the quasi‐steady flow slower and its sediment‐transport capacity lower. A 3D pattern of large interfering Kelvin‐Helmholtz waves causes fluctuations of the local flow velocity magnitude and sediment concentration. Four zones of preferential sediment deposition are recognized: a near‐gate zone of abrupt flow expansion and self‐regulation; a flow‐transverse zone on the counter‐slope of fault footwall edges; a flow‐transverse zone at the fault‐scarp toes and a similar transverse zone near the crest of the hanging wall counter‐slopes. The sand deposited on the counter‐slope tends to be re‐entrained and fed back to the current by a secondary reverse underflow. The spatial extent and sediment accumulation capacity of depozones depend upon the released current volume. The impact of relay system on an overrunning current depends upon the fault separation distance and stage of tectonic evolution. An early‐stage relay system, with small vertical displacement and little overlap of faults, is bypassed by the current with minimum flow disturbance and no pronounced deposition. An advanced‐stage system, with greater fault displacement and overlap, gives a similar hydraulic effect as a single fault segment if the fault separation is small. If the separation is relatively large, the flow tends to be internally redirected sideways from the ramp into the hanging wall synclinal depressions. Since normal‐fault relays are common features in extensional basins, the study bears important implications for turbiditic slope‐fan models and for the spatial sand prediction in subsurface exploration of faulted submarine slopes.