Clinoform development and topset evolution in a mud‐rich delta – the Middle Triassic Kobbe Formation,Norwegian Barents Sea

Clinoform surfaces are routinely used to mark transitions from shallow waters to deep basins. This concept represents a valuable tool for screening potential reservoir intervals in frontier basins where limited data are available. Variations in the character of clinoform geometries and shoreline and shelf‐edge trajectories are indicators of a range of different factors, such as palaeobathymetry, changes in relative sea‐level and sediment supply. Applications of conceptual and generalized models might, however, lead to erroneous assumptions about the supply of coarse‐grained material to the delta front and basin when superficial similarities between clinoform geometries are not treated holistically. The present study examines the mudstone‐dominated Middle Triassic Kobbe Formation – a potential hydrocarbon reservoir interval in the Barents Sea, where prodeltaic to deltaic deposits can be examined in cores, well logs and two‐dimensional and three‐dimensional seismic data. Despite pronounced acoustic impedance contrast to the surrounding shale, channel belt networks are not observed close to the platform edge in seismic datasets, even at maximum regressive stages. However, sub‐seismic prodeltaic deposits observed on the shallow platform indicate that prodeltaic deposits were sourced directly from the delta plain. Clinoform surfaces with different geometries and scale are observed basinward of the palaeoplatform edge of underlying progradational sequences, correlative to mudstone‐dominated prodeltaic core sections. Results indicate that platform‐edge deltas developed at discrete sites in the basin due to normal regression, but the positions of these deltas are not directly relatable to variations in clinoform geometries. Transitions from third‐order to fourth‐order clinoform geometries record discrete transgressive–regressive cycles but are not necessarily good indicators of sandstone deposition. Because of prolonged periods with high accommodation, channel avulsions were frequent and only very fine‐grained sandstone was deposited in heterolithic units at the delta front. Sandstones with good reservoir properties are predominantly found along basin margins.     

A sequence stratigraphic model for an intensely bioturbated shallow-marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.     

Facies architecture and sequentiality of nearshore and 'shelf'sandbodies; Haystack Mountains Formation, Wyoming, USA

The Haystack Mountains Formation (Campanian, Mesaverde Group, US Western Interior Basin, Wyoming) contains a series of shallow-marine sandbodies, extending tens of kilometres out from a basin margin. The study succession (around 200 m thick) is composed of eight major sandstone tongues (Bolten Ranch, O'Brien Spring, Seminoe 1–2–3–4, Hatfield 1 and 2 members), each partially encased within marine shale intervals. The Formation is ‘sequential’at several scales. At the largest scale, the whole succession presents an aggradational to basinward-stepping stacking pattern of the sandstone tongues. At a lower level, each tongue (member) is characterized internally by two different types of lithosome: the first represents shoreface progradation with hummocky cross-strata passing up to swaley and trough cross-stratified sandstones. This lithosome is erosively truncated at its top in most cases, and has a general sheet-like geometry along strike, whereas down dip it displays a series of sharp-bounded clinothems. The latter sometimes indicate a downward as well as a basinward shift through time, as suggested by the occurrence of coarser and/or shallower facies at a lower level in the shoreface profile. The second type of lithosome is sheet- or wedge-like and sharply overlies the shoreface deposits. The lithosome consists of laterally widespread units of planar tabular to trough cross-bedded medium sandstones passing laterally (in a dip direction) into bioturbated sandstones. The lower part of this lithosome is progradational, becoming retrogradational into the overlying shales. The facies within the cross-bedded lithosome suggest a tidally dominated delta front to estuarine depositional setting. The two types of lithosome are not related genetically. The erosion surface separating the two lithosomes is a sequence boundary separating forced-regressive (relative sea-level fall) shoreface deposits from lowstand to transgressive (early relative sea-level rise), cross-bedded deposits. The uppermost part of the cross-stratified lithosome shows a landward-stepping of component parasequences and is abruptly blanketed by open-marine shales. The most widespread cross-bedded lithosomes are apparently best developed in the lowermost members of the Haystack Mountains Formation, i.e. in the aggradational part of the large-scale progradational succession. In the uppermost, highly progradational sandstone tongues, the shoaling-upward shoreface lithosome dominates, whereas the cross-bedded lithosome occurs in narrow, lensoid belts, or is absent. The middle portion of the succession shows intermediate characteristics. The vertical variation in geometry, thickness and progradational extent of successive cross-bedded lithosomes results from greater confinement of the incised nearshore systems both in space (landward direction) and in time (from the aggradation to the progradation architecture). The latter is a consequence of a decreasing rate of accommodation creation through time.     

Geometry and facies of Tertiary clinothems, Spitsbergen

Sandstone clinothems from the Battfjellet Formation (Palaeogene) on Spitsbergen are locally exceptionally well preserved along depositional dip-parallel mountainsides. The clinothems are more than 1 km wide and more than 100 m thick. Superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression. Deposition took place partly on‘post-transgressional’ depositional shelves, and partly by contributing seaward-sloping wedges, or clinothems, to a ramp progradation. Shorelines dominated both by mouth bar and shoreface environments have been identified. The clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies.     

Seismic‐stratigraphic record of a deglaciation sequence: from the marine Laflamme Gulf to Lake Saint‐Jean (late Quaternary,Québec,Canada)

The stratigraphy of the last deglaciation sequence is investigated in Lake Saint‐Jean (Québec Province, Canada) based on 300 km of echo‐sounder two dimensional seismic profiles. The sedimentary archive of this basin is documented from the Late Pleistocene Laurentidian ice‐front recession to the present‐day situation. Ten seismic units have been identified that reflect spatio‐temporal variations in depositional processes characterizing different periods of the Saint‐Jean basin evolution. During the postglacial marine flooding, a high deposition rate of mud settling, from proglacial glacimarine and then prodeltaic plumes in the Laflamme Gulf, produced an extensive, up to 50 m thick mud sheet draping the isostatically depressed marine basin floor. Subsequently, a closing of the water body due to glacio‐isostatic rebound occurred at 8.5 cal. ka BP, drastically modifying the hydrodynamics. Hyperpycnal flows appeared because fresh lake water replaced dense marine water. River sediments were transferred towards the deeper part of the lake into river‐related sediment drifts and confined lobes. The closing of the water body is also marked by the onset of a wind‐driven internal circulation associating coastal hydrodynamics and bottom currents with sedimentary features including shoreface deposits, sediment drifts and a prograding shelf‐type body. The fingerprints of a forced regression are well expressed by mouth‐bar systems and by the shoreface–shelf system, the latter unexpected in such a lacustrine setting. In both cases, a regressive surface of lacustrine erosion (RSLE) has been identified, separating sandy mouth‐bar from glaciomarine to prodeltaic muds, and sandy shoreface wedges from the heterolithic shelf‐type body, respectively. The Lake Saint‐Jean record is an example of a regressive succession driven by a glacio‐isostatic rebound and showing the transition from late‐glacial to post‐glacial depositional systems.     

An example of a highly bioturbated,storm‐influenced shoreface deposit: Upper Jurassic Ula Formation,Norwegian North Sea

Integrated ichnological and sedimentological analyses of core samples from the Upper Jurassic Ula Formation in the Norwegian Central Graben were undertaken to quantify the influence of storm waves on sedimentation. Two main facies associations (offshore and shoreface) that form a progradational coarsening upward succession are recognizable within the cores. The offshore deposits are characterized by massive to finely laminated mudstones and fine‐grained sandstones, within a moderately to highly bioturbated complex. The trace fossil assemblage is dominated by deposit‐feeding structures (for example, Planolites, Phycosiphon and Rosselia) and constitutes an expression of the proximal Zoophycos to distal Cruziana ichnofacies. The absence of grazing behaviours and dominance of deposit‐feeding ichnofossils is a reflection of the increased wave energies present (i.e. storm‐generated currents) within an offshore setting. The shoreface succession is represented by highly bioturbated fine‐grained to medium‐grained sandstones, with intervals of planar and trough cross‐bedding, thin pebble lags and bivalve‐rich shell layers. The ichnofossil assemblage, forming part of the Skolithos ichnofacies, is dominated by higher energy Ophiomorpha nodosa ichnofossils and lower energy Ophiomorpha irregulaire and Siphonichnus ichnofossils. The presence of sporadic wave‐generated sedimentary structures and variability in ichnofossil diversity and abundance attests to the influence of storm‐generated currents during deposition. As a whole, the Ula Formation strongly reflects the influence of storm deposits on sediment deposition; consequently, storm‐influenced shoreface most accurately describes these depositional environments.     

Intra‐parasequence architecture of an interpreted asymmetrical wave‐dominated delta

Although modern wave‐dominated shorelines exhibit complex geomorphologies, their ancient counterparts are typically described in terms of shoreface‐shelf parasequences with a simple internal architecture. This discrepancy can lead to poor discrimination between, and incorrect identification of, different types of wave‐dominated shoreline in the stratigraphic record. Documented in this paper are the variability in facies characteristics, high‐resolution stratigraphic architecture and interpreted palaeo‐geomorphology within a single parasequence that is interpreted to record the advance of an ancient asymmetrical wave‐dominated delta. The Standardville (Ab1) parasequence of the Aberdeen Member, Blackhawk Formation is exposed in the Book Cliffs of central Utah, USA. This parasequence, and four others in the Aberdeen Member, record the eastward progradation of north/south‐trending, wave‐dominated shorelines. Within the Standardville (Ab1) parasequence, distal wave‐dominated shoreface‐shelf deposits in the eastern part of the study area are overlain across a downlap surface by southward prograding fluvial‐dominated delta‐front deposits, which have previously been assigned to a separate ‘stranded lowstand parasequence’ formed by a significant, allogenic change in relative sea‐level. High‐resolution stratigraphic analysis of these deposits reveals that they are instead more likely to record a single episode of shoreline progradation characterized by alternating periods of normal regressive and forced regressive shoreline trajectory because of minor cyclical fluctuations in relative sea‐level. Interpreted normal regressive shoreline trajectories within the wave‐dominated shoreface‐shelf deposits are marked by aggradational stacking of bedsets bounded by non‐depositional discontinuity surfaces. Interpreted forced regressive shoreline trajectories in the same deposits are characterized by shallow incision of fluvial distributary channels and strongly progradational stacking of bedsets bounded by erosional discontinuity surfaces that record enhanced wave‐base scour. Fluvial‐dominated delta‐front deposits most probably record the regression of a lobate delta parallel to the regional shoreline into an embayment that was sheltered from wave influence. Wave‐dominated shoreface‐shelf and fluvial‐dominated delta‐front deposits occur within the same parasequence, and their interpretation as the respective updrift and downdrift flanks of a single asymmetrical wave‐dominated delta that periodically shifted its position provides the most straightforward explanation of the distribution and relative orientation of these two deposit types.     

Expanding the spectrum of shallow‐marine,mixed carbonate–siliciclastic systems: Processes,facies distribution and depositional controls of a siliciclastic‐dominated example

Most of the present knowledge of shallow‐marine, mixed carbonate–siliciclastic systems relies on examples from the carbonate‐dominated end of the carbonate–siliciclastic spectrum. This contribution provides a detailed reconstruction of a siliciclastic‐dominated mixed system (Pilmatué Member of the Agrio Formation, Neuquén Basin, Argentina) that explores the variability of depositional models and resulting stratigraphic units within these systems. The Pilmatué Member regressive system comprises a storm‐dominated, shoreface to basinal setting with three subparallel zones: a distal mixed zone, a middle siliciclastic zone and a proximal mixed zone. In the latter, a significant proportion of ooids and bioclasts were mixed with terrigenous sediment, supplied mostly via along‐shore currents. Storm‐generated flows were the primary processes exporting fine sand and mud to the middle zone, but were ineffective to remove coarser sediment. The distal zone received low volumes of siliciclastic mud, which mixed with planktonic‐derived carbonate material. Successive events of shoreline progradation and retrogradation of the Pilmatué system generated up to 17 parasequences, which are bounded by shell beds associated with transgressive surfaces. The facies distribution and resulting genetic units of this siliciclastic‐dominated mixed system are markedly different to the ones observed in present and ancient carbonate‐dominated mixed systems, but they show strong similarities with the products of storm‐dominated, pure siliciclastic shoreface–shelf systems. Basin‐scale depositional controls, such as arid climatic conditions and shallow epeiric seas might aid in the development of mixed systems across the full spectrum (i.e. from carbonate‐dominated to siliciclastic‐dominated end members), but the interplay of processes supplying sand to the system, as well as processes transporting sediment across the marine environment, are key controls in shaping the tridimensional facies distribution and the genetic units of siliciclastic‐dominated mixed systems. Thus, the identification of different combinations of basin‐scale factors and depositional processes is key for a better prediction of conventional and unconventional reservoirs within mixed, carbonate–siliciclastic successions worldwide.     

Depositional facies,environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline,northeast Sinai,Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.     

Stratigraphic evolution of an outcropping continental slope system, Tres Pasos Formation at Cerro Divisadero, Chile

Depositional slope systems along continental margins contain a record of sediment transfer from shallow‐water to deep‐water environments and represent an important area for natural resource exploration. However, well‐preserved outcrops of large‐scale depositional slopes with seismic‐scale exposures and tectonically intact stratigraphy are uncommon. Outcrop characterization of smaller‐scale depositional slope systems (i.e. < 700 m of undecompacted shelf‐to‐basin relief) has led to increased understanding of stratigraphic packaging of prograding slopes. Detailed stacking patterns of facies and sedimentary body architecture for larger‐scale slope systems, however, remain understudied. The Cretaceous Tres Pasos Formation of the Magallanes Basin, southern Chile, presents a unique opportunity to evaluate the stratigraphic evolution of such a slope system from an outcrop perspective. Inherited tectonic relief from a precursor oceanic basin phase created shelf‐to‐basin bathymetry comparable with continental margin systems (～1000 m). Sedimentological and architectural data from the Tres Pasos Formation at Cerro Divisadero reveal a record of continental margin‐scale depositional slope progradation and aggradation. Slope progradation is manifested as a vertical pattern exhibiting increasing amounts of sediment bypass upwards, which is interpreted as reflecting increasing gradient conditions. The well‐exposed, seismic‐scale outcrop is characterized by four 20 to 70 m thick sandstone‐rich successions, separated by mudstone‐rich intervals of comparable thickness (40 to 90 m). Sedimentary body geometry, facies distribution, internal bedding architecture, sandstone richness and degree of amalgamation were analysed in detail across a continuous 2·5 km long transect parallel to depositional dip. Deposition in the lower section (Units 1 and 2) was dominated by poorly channellized to unconfined sand‐laden flows and accumulation of mud‐rich mass transport deposits, which is interpreted as representing a base of slope to lower slope setting. Evidence for channellization and indicators of bypass of coarse‐grained turbidity currents are more common in the upper part of the > 600 m thick succession (Units 3 and 4), which is interpreted as reflecting increased gradient conditions as the system accreted basinward.     

Shelf‐margin clinothem progradation,degradation and readjustment: Tanqua depocentre,Karoo Basin (South Africa)

Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.     

Sedimentary architecture and depositional controls of a Holocene wave‐dominated barrier‐island system

Barrier‐island system evolution is controlled by internal and external forcing mechanisms, and temporal changes in these mechanisms may be recorded in the sedimentary architecture. However, the precise role of individual forcing mechanisms is rarely well understood due to limited chronological control. This study investigates the relative role of forcing conditions, such as antecedent topography, sea‐level rise, sediment supply, storms and climate changes, on the evolution of a Holocene wave‐dominated barrier‐island system. This article presents temporal reconstruction of the depositional history of the barrier‐island system of Rømø in the Wadden Sea in unprecedented detail, based on ground‐penetrating radar profiles, sediment cores, high‐resolution dating and palynological investigations, and shows that ca 8000 years ago the barrier island formed on a Pleistocene topographic high. During the initial phase of barrier evolution, the long‐term sea‐level rise was relatively rapid (ca 9 mm year⁻¹) and the barrier was narrow and frequently overwashed. Sediment supply kept pace with sea‐level rise, and the barrier‐island system mainly aggraded through the deposition of a ca 7 m thick stack of overwash fans. Aggradation continued for ca 1700 years until sea‐level rise had decreased to <2 mm year⁻¹. In the last ca 6000 years, the barrier prograded 4 to 5 km through deposition of a 10 to 15 m thick beach and shoreface unit, despite a long‐term sea‐level rise of 1 to 2 mm year⁻¹. The long‐term progradation was, however, interrupted by a transgression between 4000 years and 1700 years ago. These results demonstrate that the large‐scale morphology of the Danish Wadden Sea shoreline influences the longshore sediment transport flux and the millennial‐scale dispersal of sediment along the shoreline. On decadal to centennial timescales, major storms induced intense beach and shoreface erosion followed by rapid recovery and progradation which resulted in a highly punctuated beach and shoreface record. Major storms contributed towards a positive sediment budget, and the sustained surplus of sediment was, and still is, instrumental in maintaining the aggradational–progradational state of the barrier island.     

Anatomy of an Upper Triassic continental to marginal‐marine system: the mixed siliciclastic–carbonate Travenanzes Formation (Dolomites,Northern Italy)

The Travenanzes Formation is a terrestrial to shallow‐marine, siliciclastic–carbonate succession (200 m thick) that was deposited in the eastern Southern Alps during the Late Triassic. Sedimentary environments and depositional architecture have been reconstructed in the Dolomites, along a 60 km south–north transect. Facies alternations in the field suggest interfingering between alluvial‐plain, flood‐basin and shallow‐lagoon deposits, with a transition from terrestrial to marine facies belts from south to north. The terrestrial portion of the Travenanzes Formation consists of a dryland river system, characterized by multicoloured floodplain mudstones with scattered conglomeratic fluvial channels, merging downslope into small ephemeral streams and sheet‐flood sandstones, and losing their entire discharge subaerially before the shoreline. Calcic and vertic palaeosols indicate an arid/semi‐arid climate with strong seasonality and intermittent discharge. The terrestrial/marine transition shows a coastal mudflat, the flood basin, which is usually exposed, but at times is inundated by both major river floods and sea‐water storm surges. Locally coastal sabkha deposits occur. The marine portion of the Travenanzes Formation comprises carbonate tidal‐flat and shallow‐lagoon deposits, characterized by metre‐scale shallowing‐upward peritidal cycles and subordinate intercalations of dark clays from the continent. The depositional architecture of the Travenanzes Formation suggests an overall transgressive pattern organized in three carbonate–siliciclastic cycles, corresponding to transgressive–regressive sequences with internal higher‐frequency sedimentary cycles. The metre‐scale sedimentary cyclicity of the Travenanzes Formation continues without a break in sedimentation into the overlying Dolomia Principale. The onset of the Dolomia Principale epicontinental platform is marked by the exhaustion of continental sediment supply.     

Deltaic sedimentation in the Devonian of Western Libya

Middle and Upper Devonian deposits from the Aouinet Ouenine Formation in the southern Ghadames Basin of western Libya provide a well exposed example of a deltaic complex containing both progradational and transgressive facies. Progradational facies comprise both laterally accreting and incised distributary channels overlying prodelta deposits. Also present is a progradational beach environment showing build-up from an offshore shelf through nearshore shelf to shoreface and foreshore sub-environments. Over-lying these progradational facies are transgressive tidal-flat, washover-fan, foreshore and nearshore deposits.The characteristics and interrelationships of the different facies are explained by two sedimentation models: progradational facies existed contemporaneously during phases of active sediment supply whereas the transgressive facies existed contemporaneously during periods of diminished or absent detrital influx.     

Evolution of an ancient (Lower Cretaceous) marginal‐marine system from tide‐dominated to wave‐dominated deposition,McMurray Formation

Regionally extensive parasequences in the upper McMurray Formation, Grouse Paleovalley, north‐east Alberta, Canada, preserve a shift in depositional processes in a paralic environment from tide domination, with notable fluvial influence, through to wave domination. Three stacked parasequences form the upper McMurray Formation and are separated by allogenic flooding surfaces. Sediments within the three parasequences are grouped into three facies associations: wave‐dominated/storm‐dominated deltas, storm‐affected shorefaces to sheltered bay‐margin and fluvio‐tidal brackish‐water channels. The two oldest parasequences comprise dominantly tide‐dominated, wave‐influenced/fluvial‐influenced, shoreface to bay‐margin deposits bisected by penecontemporaneous brackish‐water channels. Brackish‐water channels trend approximately north‐west/south‐east, which is perpendicular to the interpreted shoreline trend; this implies that the basinward and progradational direction was towards the north‐west during deposition of the upper McMurray Formation in Grouse Paleovalley. The youngest parasequence is interpreted as amalgamated wave‐dominated/storm‐dominated delta lobes. The transition from tide‐dominated deposition in the oldest two parasequences to wave‐dominated deposition in the youngest is attributed mainly to drowning of carbonate highlands to the north and north‐west of the study area, and potentially to relative changes in accommodation space and deposition rate. The sedimentological, ichnological and regional distribution of the three facies associations within each parasequence are compared to modern and Holocene analogues that have experienced similar shifts in process dominance. Through this comparison it is possible to consider how shifts in depositional processes are expressed in the rock record. In particular, this study provides one of few ancient examples of preservation of depositional process shifts and showcases how topography impacts the character and architecture of marginal‐marine systems.     

Mud‐rich delta‐scale compound clinoforms in the Triassic shelf of northern Pangea (Havert Formation,south‐western Barents Sea)

In recent years it has become clear that many shallow‐marine heterolithic and mudstone‐dominated successions are deposited as mud belts forming part of subaqueous deltas that are related to major fluvial sources either upstream or along shore. Here the Havert Formation is presented as an ancient example of this kind of system. The Havert Formation in the south‐western Barents Sea represents shelf margin clinoforms consisting predominantly of heterolithic deposits. Sediments were mainly derived from the east (Ural Mountains), but a smaller system prograded northward from Fennoscandia. The Havert Formation holds a lot of interest due to: (i) its stratigraphic position, directly above the Permo–Triassic boundary and contemporaneous to the emplacement of the Siberian Traps; (ii) the fact that it represents the first siliciclastic input in the south‐western Barents Sea and it shows interaction between Uralian‐derived and Fennoscandian‐derived sediments; and (iii) its hydrocarbon potential. This study is focused on a detailed sedimentological analysis of cored intervals of the (Ural‐derived) Havert Formation, in combination with seismic interpretation, well‐log correlations and palynological analysis of the Havert and overlying Klappmyss formations. The cored intervals belong to the shelf environment of the Havert shelf‐margin clinoforms (300 to 500 m thick). This sedimentological analysis distinguishes six facies associations, spanning from tidally‐influenced channels at the shoreline to mud‐rich subaqueous platform and foresets of the subaqueous delta. Seismic lines and well‐log correlations show the larger‐scale evolution of the Ural‐derived Havert Formation, characterized by episodes of low‐accommodation and high‐accommodation. The palynological analyses provide the first detailed study of the Havert Formation in the Nordkapp Basin, revising its depositional age in the region as Induan to early Olenekian (Smithian). Furthermore, they strengthen the environmental interpretation; palynofacies present on the shelf record flora of tidally‐influenced coastal plains, whereas the palynofacies in the deep‐water slope contain only amorphous organic matter.     

A tidal-inlet complex in the Cretaceous epeiric sea of North America: Virgelle Member, Milk River Formation, southern Alberta, Canada

The upper portion of the Virgelle Member (Upper Cretaceous Milk River Formation) at Writing-on-Stone Provincial Park of southern Alberta preserves evidence of tidal processes along an otherwise wave-dominated, progradational shoreline in the Cretaceous Interior Seaway of North America. The upper Virgelle Member is underlain by offshore transition to lower shoreface deposits of the Telegraph Creek Member and the lower Virgelle Member, respectively, and is overlain by the non-marine shales and sandstones of the Deadhorse Coulee Member. The sediments of the upper Virgelle Member were deposited along a prograding shoreline and are interpreted here as those of a tidal-inlet complex. Most inlet sections consist of an erosional base overlain by a shale-pebble conglomerate, followed by cross-bedded sandstones which become finer-grained and decrease in scale upwards. Indicators of tidal processes include palaeocurrent distributions, mud couplets, tidal bundles, re-activation surfaces and herringbone cross-beds. The sequence through the tidal-inlet complex can be differentiated, according to prevalent palaeoflow directions and sedimentary structures, as ebb-dominated, flood-dominated, or mixed-tidal influence. Ebb-dominated sections commonly contain lateral accretion surfaces whereas flood-dominated sections contain tidal-ramp deposits. Back-barrier lagoon deposits are dominated by sandstones of an extensive flood-tidal delta with only thin shales preserved locally at the top of the inlet complex. Deposits of ebb-tidal deltas are absent, presumably due to the effective sediment dispersal by waves and wave-induced longshore currents acting on the regionally wave-dominated shoreline.     

Facies model for a coarse‐grained,tide‐influenced delta: Gule Horn Formation (Early Jurassic), Jameson Land,Greenland

Tide‐dominated deltas have an inherently complex distribution of heterogeneities on several different scales and are less well‐understood than their wave‐dominated and river‐dominated counterparts. Depositional models of these environments are based on a small set of ancient examples and are, therefore, immature. The Early Jurassic Gule Horn Formation is particularly well‐exposed in extensive sea cliffs from which a 32 km long, 250 m high virtual outcrop model has been acquired using helicopter‐mounted light detection and ranging (LiDAR). This dataset, combined with a set of sedimentological logs, facilitates interpretation and measurement of depositional elements and tracing of stratigraphic surfaces over seismic‐scale distances. The aim of this article is to use this dataset to increase the understanding of depositional elements and lithologies in proximal, unconfined, tide‐dominated deltas from the delta plain to prodelta. Deposition occurred in a structurally controlled embayment, and immature sediments indicate proximity to the sediment source. The succession is tide dominated but contains evidence for strong fluvial influence and minor wave influence. Wave influence is more pronounced in transgressive intervals. Nine architectural elements have been identified, and their internal architecture and stratigraphical distribution has been investigated. The distal parts comprise prodelta, delta front and unconfined tidal bar deposits. The medial part is characterized by relatively narrow, amalgamated channel fills with fluid mud‐rich bases and sandier deposits upward, interpreted as distributary channels filled by tidal bars deposited near the turbidity maximum. The proximal parts of the studied system are dominated by sandy distributary channel and heterolithic tidal‐flat deposits. The sandbodies of the proximal tidal channels are several kilometres wide and wider than exposures in all cases. Parasequence boundaries are easily defined in the prodelta to delta‐front environments, but are difficult to trace into the more proximal deposits. This article illustrates the proximal to distal organization of facies in unconfined tide‐dominated deltas and shows how such environments react to relative sea‐level rise.     

Deep-water Depositional Features of Miocene Zhujiang Formation in Baiyun Sag,Pearl River Mouth Basin

Eleven lithofacies and five lithofacies associations were indentified in the Miocene Zhujiang Formation on the basis of detailed core analysis.It could be determined that three depositional types developed,namely submarine fan,basin and deep-water traction current.Six microfacies were further recognized within the fan,including main channels in the inner fan,distributary channels in the middle fan,inter-channels,levees and the outer fan.The lower Zhujiang Formation,mainly sandstone associations,was inner fan and inner-middle fan deposits of the basin fan and the slope fan. The middle part,mainly mudstone associations,was outer fan deposits.With the transgression,the submarine fan was finally replaced by the basinal pelagic deposits which were dominated by mudstone associations,siltstone associations,and deep-water limestone associations.During the weak gravity flow activity,the lower channels,the middle-upper outer fans and basin deposits were strongly modified by the deep-water traction current.The identification of the deep-water traction deposition in Miocene Zhujiang Formation would be of great importance.It could be inferred that the deep-water traction current had been existing after the shelf-break formation since the Late Oligocene (23.8 Ma) in the Baiyun sag,influencing and controlling the sediment composition,the distribution, and depositional processes.It would provide great enlightenment to the paleo-oceanic current circulation in the northern South China Sea.