Basin Research - Clinoforms and Clinothems: Fundamental Elements of Basin Infill, 2020

Clinoforms and clinothems represent the dominant architectural style in many sedimentary basins and are widely recognized as one of the fundamental building blocks of the stratigraphic record.

This special issue dealing with the recent advances on modern and ancient clinoform‐stratified sedimentary successions arises from a European Geoscience Union (EGU) session “Clinoform drivers and stratigraphic products in siliciclastic and carbonate successions”, Vienna, April 2018. Clinoforms and clinothems represent a dominant architectural style of strata in many sedimentary environments, including deltaic and nondeltaic shorelines in both marine and lacustrine settings, and are one of the key building blocks of the sedimentary record. This Special Issue in Basin Research aspires to represent a step forward in understanding formation and preservation of these fundamental stratigraphic elements. As this Special Issue documents, a comprehensive understanding of clinoformal strata requires a multidisciplinary and multi‐scale approach. Sixteen papers present case studies from a variety of tectonic settings worldwide, investigated with an array of methods, including seismo‐stratigraphy, well logs, cores, high‐resolution biostratigraphy, outcrop studies and modern bathymetric data. While observations document sedimentary processes and products in sedimentary basins, numerical models are necessary to provide a quantitative basis for the extrapolation of these processes and strata at different temporal and spatial scales. The papers highlight at least five main research avenues that we briefly introduce and discuss below: (a) clinoforms and clinothems as sedimentary archives; (b) the nested nature of clinoformal strata and implications for the trajectory of the rollover point(s); (c) quantitative clinoform parameters and dynamic indices; (d) architecture, growth and sequence stratigraphy of marine versus lacustrine clinoformal strata; and (e) clinoforms and geological time. This introduction also contains brief descriptions of each paper of the Special Issue.

Though clinothem geometry represents a key control on fluid flow in reservoir modelling, tracing clinothem boundaries accurately is commonly limited by the lack of sufficiently precise outcrop or subsurface data. This study shows that in basin systems with strongly heterogeneous compositional signatures, the combination of bulk‐sediment geochemistry and benthic foraminiferal distribution can help identify clinothem architecture and generate realistic models of 3D deltaic upbuilding and evolution. Middle‐late Holocene deposits in the Po Delta area form an aggradational to progradational parasequence set that reveals the complex interaction of W–E Po Delta progradation, S‐directed longshore currents (from Alpine rivers) and Apennines rivers supply. Unique catchment lithologies (ophiolite rocks and dolostones) were used to delineate basin‐wide geochemical markers of sediment provenance (Cr and Mg) and to assess distinctive detrital signatures. The geochemical characterization of cored intervals across different components of the sediment routing system enabled a direct linkage between clinothem growth, transport pathways and provenance mixing to be established. On the other hand, abrupt microfaunal variations at clinothem boundaries were observed to reflect the palaeoenvironmental response to sharp changes in sediment flux and fluvial influence. This study documents the ability of an integrated geochemical and palaeoecological approach to delineate three distinct sources (Po, Alps and Apennines) that contributed to coastal progradation and to outline the otherwise lithologically cryptic geometries of clinothems that using conventional sedimentological methods it would be virtually impossible to restore.

Two nested clinoform set types of different scales and steepness are mapped and analysed from high‐resolution seismic data. Restoration of post‐depositional faulting reveals a persistent pattern of small‐scale, high‐angle clinoforms contained within platform‐scale, low‐angle clinothems, showing a combined overall progradational depositional system. The large clinoforms lack a well‐defined platform edge, and show a gradual increase in dip from topset to foreset. A consistent recurring stratal pattern is evident from the architecture, and is considered a result of interplay between relative sea‐level change and autocyclic switching of sediment delivery focal points that brought sediment to the platform edge. This un‐interrupted succession records how intra‐shelf platforms prograde. Quantitative clinoform analysis may assist in determining the most influential depositional factors. Post‐depositional uplift and erosion requires restoration with re‐burial to maximum burial depth. Backstripping, decompaction and isostatic correction was performed assuming a range of lithologic compositions, as no wells test the lithology. Nearby wells penetrate strata basinward of the clinoforms, proving mudstone content above 50%, which in turn guide restoration values. Typical restored platform heights are 250–300 m, with correspondingly sized platform‐scale clinoform heights. Typical large‐scale clinoform foreset dip values are 1.3°–2.4°. Small‐scale clinothems are typically 100 m thick, with restored foreset dip angles at 4.4° ‐ > 10°. The results suggest that intrashelf platform growth occurs in pulses interrupted by draping of strata over its clinoform profile. The resultant architecture comprises small‐scale clinoforms nested within platform‐scale clinothems.

Nested clinoforms in intrashelf platforn strata record the platform progradation in pulses interrupted by flooding and draping. Restoration of their syn‐depositional geometries and architecture give insights to the dynamic balance between processes during sedimentation.

This article provides the first documentation of shelf‐edge clinoforms prograding landward and upslope (“upslope‐climbing clinoforms”). The upslope outbuilding of the Eridanos Shelf‐edge Delta took mainly place during an earliest Calabrian lowstand, possibly linked to Eburonian glacio‐eustasy. This architecture is due to high sediment supply combined with a pre‐existing regional depositional interface dipping ca. 0.12° contrary to the Eridanos progradation direction.

Clinoforms are basinward‐dipping and accreting palaeo‐bathymetric profiles that record palaeo‐environmental conditions and processes; thus, clinothems represent natural palaeo‐archives. Here, we document shelf‐edge scale clinoform sets which prograded through the entire width of an epicontinental marine basin (ca. 400 km), eventually encroaching onto the opposite basin flank, where they started to prograde upslope and landward, in defiance of gravity (“upslope‐climbing clinoforms”). The giant westward‐prograding Eridanos muddy shelf‐edge clinothem originated from the Baltic hinterland in the Oligocene and achieved maximum regression in the Early Pleistocene, on the UK Central Graben (CG) and Mid North Sea High (MNSH), after crossing the whole North Sea mesopelagic depocentre and causing near complete basin infill. Here we integrate well and seismic data through the MNSH and CG and examine the Eridanos final heyday and demise, identifying five clinothem complexes (A1, A2, A3, B and C) and six depositional sequence boundaries (SB1 to SB6) in the Miocene‐Recent section. Tectonic and climatic events drove the recent evolution of this system. Early Pleistocene climate cooling, in particular, resulted in a stepwise increase in sediment supply. This climaxed in the earliest Calabrian, following a likely Eburonian eustatic fall (=SB3) when the Eridanos clastic wedge was restructured from a 100–300 m thick compound shelf‐edge and delta system to a “hybrid” shelf‐edge delta at sequence boundary SB3 (ca. 1.75 Ma). In the ca. 40 kyr that followed SB3, a progradation rate peak (>1,000 m/kyr) is associated with clinoforms starting to accrete upslope, onto the east‐dipping slope between CG and MNSH. This “upslope‐climbing clinoform” phase was quickly followed by the maximum regression and final retreat of the Eridanos system in the Early Calabrian (=SB4), likely as the result of climate‐driven changes in the Baltic hinterland and/or delta auto‐retreat. To our knowledge, this contributions represents the first documentation of “upslope‐climbing clinoforms” recorded in the stratigraphic record.

Lacustrine basins and their deposits are good paleoclimate recorders and contain rich energy resources. Shelf‐margin clinoforms do exist in deep lacustrine basins, but with striking differences from those in deep marine basins, caused by a correlation between the river‐derived sediment supply and the lake level. This study uses empirical relationships to calculate the water and sediment discharge from rivers and coeval lake level during wet–dry cycles at 10 s of ky time scale. Sediment supply and lake‐level changes are used for a stratigraphic forward model to understand how lacustrine clinoforms develop under different climate conditions. The results show that both wet and dry cycles can be associated with thick deep‐water fan deposits, supporting the existing climate‐driven lacustrine model proposed based on field data (e.g. Neogene Pannonian Basin and Eocene Uinta Basin). The wet period with high sediment supply and rising lake level creates the highly aggradational shelf, progradational slope and thick bottomset deposits. This is contrary from marine basin settings where the presence of rising shelf‐margin trajectory commonly indicates limited deep‐water fan deposits. This work suggests marine‐based stratigraphic models cannot be directly applied to lacustrine basins.

Utilizing two outcrop data sets with dip direction exposures of shallow‐water (tens of meters) deltaic clinoforms, this paper quantifies sedimentary facies proportions and clinoform lengths and gradients, and links process regimes to delta clinoform dimensions. Both data sets are from foreland basins, the Cretaceous Chimney Rock Sandstone of the Rock Springs Formation from the US Western Interior, and the Eocene Brogniartfjellet Clinoform Complex 8 of the Battfjellet Formation from the Central Basin of Spitsbergen. Sedimentary facies indicate presence of both river‐ and wave‐dominated clinothems in each data set. Facies characteristics and distribution implies that river‐dominated clinothem progradation was primarily driven by deposition from weak hyperpycnal flow turbidity currents across the clinoforms, and minor slumps. Wave‐dominated clinothems were constructed by wave processes rather than alongshore currents, and are also progradational subaerial clinoforms, with one exception, where the formation of a compound subaqueous clinoform set indicates erosion and sediment bypass above the wave base. Sediment distribution and lithological heterogeneity in the river‐dominated clinothems is controlled by individual hyperpycnal flow events or mouth‐bar collapse events, and thus by self‐organization and minimal reworking that results in a heterogeneity that is difficult to predict (high entropy). The efficient reworking of river‐derived sediments in wave‐dominated clinothems results in predictable lithological sediment partitioning (low entropy). Clinoform dimension analyses show that although of similar sediment caliber, river‐dominated clinoforms in both data sets are on average 3–4 times steeper and 3–4 times shorter than the wave‐dominated clinoforms, with mean gradients of ca 4 degrees and ca 1 degree, respectively, and mean lengths of 150–230 m and 640–760 m. These results require corroboration from additional data sets, but do suggest that river‐ and wave‐dominated delta clinoforms are likely to have distinct downdip extents (lengths) and gradients for given clinoform heights. Clinoform shape can thus be a method for differentiating ancient river‐ vs. wave‐dominated deltaic clinoforms, in addition to their sedimentary facies, biogenic features and sandstone maturity, and helpful when incorporated into reservoir models.

Quantitative analyses of shallow‐water deltaic clinothems from two different foreland basins show that river‐dominated clinothems are consistently 3‐4 times shorter and steaper as compared to wave‐dominated clinothems.

The siliciclastic topset of a continental margin, or a shelf‐margin prism in subsiding nonplate‐margin deepwater basins, is the flat‐lying upper part of the margin succession; it is coeval basinwards with deepwater slope clinoforms. Topsets develop by the aggradation of repeated, cross‐shelf, shoreline regressions and transgressions, thereby hosting the shelf portion of stacked, fourth‐order stratigraphic sequences. Sediment spreading downdip and along strike during the cross‐shelf transit of the sediment delivery system, as well as process regime changes of deltas and shorefaces (regressive) and of estuaries, barrier–lagoon systems and shelf ridges (transgressive) are highly variable over short distances, so that correlation within a single stratigraphic sequence is far more difficult than correlation of the cross‐shelf maximum flooding surface boundaries. Thickness of individual regressive–transgressive, fourth‐order sequences is given by shelf accommodation, typically <10 m in embayment or on the inner shelf and up to 200 m on outer shelf. Tectonic subsidence and compaction will enhance this thickness only if rates are very high compared to shelf‐transit time. In very high subsidence rate settings, the transgressive tracts are well preserved and often thickly developed. Topset sequences in an Icehouse climate setting tend to have a high proportion and greater landward development of marine vs nonmarine deposits, compared to Greenhouse sequences, because of the importance of eustatic rise of sea level in the former. Previous numerical experiments show that even for very wide shelves and irrespective of Icehouse or Greenhouse conditions, deltas rarely take more than 10 s of ky to reach their shelf edge, suggesting that it is fourth‐order (or higher) sequences that are the fundamental ones in sequence stratigraphy.

Topsets are the flat‐lying, upper portion of clinoforms that build the sedimentary prisms of margins. They contain a sequential record of high‐frequency (100–300 ky) shoreline regressions and transgressions that are optimally preserved in settings of high tectonic subsidence and Icehouse climate. The former allows good preservation of the transgressive phases whereas the latter fosters the creation of wide marine shelves such as we see today.

Shelf edge and base of slope facies variability in Lajas‐Los Molles outcrops in Neuquen Basin. (a) Vertical logs with possible facies changes across the shelf edge (three types) and the base of slope (two types). (b) Cross section with possible facies changes encountered along the clinothem components in the Lajas ‐ Los Molles. (c) Clinoform height changes under different accretion rates of the basin floor or shelf.

Seismic‐reflection data show that most deepwater (>200 m water depth) basins are filled by sand and mud dispersed across clinoformal geometries characterized by gently dipping topsets, steeper foresets and gently dipping bottomsets. However, the entire geometry of these ubiquitous clinoforms is not always recognized in outcrops. Sometimes the infill is erroneously interpreted as “layer cake” or “ramp” stratigraphy because the topset‐foreset‐bottomset clinoforms are not well exposed. Regional 2‐D seismic lines show clinoforms in the Lower to Middle Jurassic Challaco, Lajas, and Los Molles formations in S. Neuquén Basin in Argentina. Time equivalent shelf, slope and basin‐floor segments of clinoforms are exposed, and can be walked out in hundreds of metres thick and kilometres‐wide outcrops. The studied margin‐scale clinoforms are not representing a continental‐margin but a deepwater shelf margin that built out in a back‐arc basin. Lajas‐Los Molles clinoforms have been outcrop‐mapped by tracing mudstones interpreted as flooding surfaces on the shelf and abandonment surfaces (low sedimentation rate) in the deepwater basin. The downslope and lateral facies variability in the outcrops is also consistent with a clinoform interpretation. The Lajas topset (shelf) is dominated by fluvial and tidal deposits. The shelf‐edge rollover zone is occasionally occupied by a 40–50‐m‐thick coarse‐grained shelf‐edge delta, sometimes incising into the underlying slope mudstones, producing oblique clinoforms expressing toplap erosion on seismic. A muddy transgressive phase capping the shelf‐edge deltas contains tidal sandbodies. Shelf‐edge deltas transition downslope into turbidite‐ and debris flow‐filled channels that penetrate down the mud‐prone Los Molles slope. At the base‐of‐slope, some 300m below the shelf edge, there are basin‐floor fan deposits (>200 m thick) composed of sandy submarine‐fan lobes separated by muddy abandonment intervals. The large‐scale outcrop correlation between topset–foreset–bottomset allows facies and depositional interpretation and sets outcrop criteria recognition for each clinoform segment.

Ocean bottom currents shape shelf‐edge clinoforms offshore Brazil.

In southeastern Brazil, the Serra do Mar coastal mountain range blocks the sediment influx from arriving at a ca. 1,500 km long continental margin comprising Santos and Pelotas basins. Despite this deprivation, the margin accumulated a ca. 1 km thick sedimentary succession since the Mid‐Miocene. Examination of seismic reflection and oceanographic data indicates that shelf‐margin clinoform formation exhibits a regional variability, with major sigmoidal clinoforms developed in the transitional area between both basins. Laterally, poorly developed oblique clinoforms constitute isolated depocenters along the shelf margin. The continuous clinoform development in the transitional area is attributed to the major influence on sediment transport patterns of several ocean bottom currents flowing along the margin, such as the Brazil Coastal Current, the Brazil Current and the Intermediate Water Brazil Current. These currents erode, transport and distribute sediments across the shelf break and upper slope from distant sediment sources located either north or south of the study area. The progressive southward strengthening of the Brazil Current could be responsible for a major southward sediment redistribution from the northern Campos Basin, and/or for sediment entrainment from northward‐induced transport by the Brazil Coastal Current, originally derived from the De la Plata Estuary. In the transition between Santos and Pelotas basins, the Intermediate Water Brazil Current splits forming the Santos Bifurcation, allowing for a continuous depositional process and clinoform generation. We suggest that ocean bottom currents may shape other shelf‐edge ‘contouritic clinoforms’ in continental margins mainly constructed by along‐strike sediment transport largely driven by long‐term geostrophic currents.

The South China Sea continental margin in the Qiongdongnan Basin (QDNB) area has incrementally prograded since 10.5 Ma generating a margin sediment prism more than 4km‐thick and 150–200 km wide above the well‐dated T40 stratigraphic surface. Core and well‐log data, as well as clinoform morphology and growth patterns along twenty‐eight 2D seismic reflection lines, illustrate the evolving architecture and margin morphology; through 5 main seismic‐stratigraphic surfaces (T40, T30, T27, T20, T0) frame 15 clinothems in the southwest that reduce over some 200 km to 8 clinoforms in the northeast. The overall margin geometry shows a remarkable change from sigmoidal, strongly progradational and aggradational in the west to weakly progradational in the east. Vertical sediment accumulation rate increased significantly across the entire margin after 2.4 Ma, with a marked increase of mud content in the succession. Furthermore, an estimate of sediment flux across successive clinoforms on each of the 3 selected seismic cross sections indicate an overall decrease of sediment discharge west to east, away from the Red River depocenter, as well as a decrease in the percentage of total discharge crossing the shelf break in this same direction. The QDNB Late Cenozoic continental margin growth, with its overall increased sediment flux, responded to the climate‐induced, gradual cooling and falling global sea level during this ice‐house period.

The South China Sea continental margin in the Qiongdongnan Basin (QDNB) area has incrementally prograded since 10.5 Ma generating a margin sediment prism more than 4km‐thick and 150–200 km wide above the well‐dated T40 stratigraphic surface. Core and well log data, as well as clinoform morphology and growth patterns along 28 2D seismic reflection lines, illustrate the evolving architecture and margin morphology; through five main seismic‐stratigraphic surfaces (T40, T30, T27, T20 and T0) frame 15 clinothems in the southwest that reduce over some 200 km to 8 clinoforms in the northeast. The overall margin geometry shows a remarkable change from sigmoidal, strongly progradational and aggradational in the west to weakly progradational in the east. Vertical sediment accumulation rate increased significantly across the entire margin after 2.4 Ma, with a marked increase in mud content in the succession. Furthermore, an estimate of sediment flux across successive clinoforms on each of the three selected seismic cross sections indicate an overall decrease in sediment discharge west to east, away from the Red River depocenter, as well as a decrease in the percentage of total discharge crossing the shelf break in this same direction. The QDNB Late Cenozoic continental margin growth, with its overall increased sediment flux, responded to the climate‐induced, gradual cooling and falling global sea level during this icehouse period.

The Neogene Paratethyan basins, strung together by the Danube from the North Alpine Foreland basin (NAFB) downstream to the Euxinian (Black Sea) basin (EB) shelf, were transformed from several‐hundred‐meter‐deep marine or lacustrine basins to shallow marine and to fluvial environments during the Neogene. As the shelf‐edge clinoform sets of these basins along the present‐day course of the Danube indicate, this process took place generally from NW to SE within each basin, and with a general temporal younging between the basins from NW to SE. In spite of this geographical and temporal pattern, however, only the Danube (Kisalföld) basin (DKB), the Central Pannonian basin (CPB), and the younger part (<4 Myr) of the EB shelf were filled by the shelf advance of the proto‐Danube; the NAFB and the Vienna basin (VB) were filled by sediment transport systems that had no temporal continuity with the modern Danube, whereas the Dacian basin (DB) had been filled by local sediment systems by the time the proto‐Danube found its way to the DB ca. 4 Mys ago.

The Miocene marine basins of Central and Southeast Europe, once comprising the Paratethys Sea, were gradually filled with sediments during the Neogene and turned to be the catchment area of the proto‐Danube and finally that of the modern Danube. Seismic data from various parts of the large Danube catchment area show that these several hundred meter deep basins were filled by lateral accretion of river‐transported sediments, appearing as shelf edge scale clinoform sets in seismic profiles. The direction of shelf edge progradation is NW to SE (N to S, W to E) in each basin, except for the Dacian basin where NE to SW direction prevails. The age of the clinoform sets is generally younging downstream: 19–18 Ma in the North Alpine Foreland basin, 14–13 Ma in the Vienna basin, 10–9 Ma in the Danube (Kisalföld) basin, 8.6–4 Ma in the Central Pannonian basin (Alföld), ?9–5 Ma in the Dacian basin, and 6–0 Ma in the Euxinian (Black Sea) basin. In spite of this geographical and temporal pattern, only the Danube (Kisalföld) and the western and central part of the Central Pannonian basin were filled by the proto‐Danube shelf accretion. Formation of the Danube, as a longitudinal river of the Alpine foreland that gradually elongated to the east and followed the retreating shoreline of the Paratethys, most probably took place at the beginning of the Late Miocene, ca. 11 Ma ago, thus the Early and Middle Miocene shelf advance in the North Alpine Foreland and Vienna basins, respectively, cannot be attributed to a „paleo‐Danube”. The clinoform systems of the Dacian basin are coeval with those of the upstream Central Pannonian basin, indicating that by the time the Danube sedimentary system reached the Dacian basin, it was already a shallow basin. The vast clinoforms of the northwestern Euxinian shelf also significantly overlap in age with the Pannonian basin ones; only the <4 Ma part of the shelf accretion can be attributed to the Danube sensu stricto.

Sandy clinothems are of interest as hydrocarbon reservoirs but there is no proven, economic, clinothem reservoir in the Norwegian Barents Sea. We used high‐resolution, 2D and 3D seismic, including proprietary data, to identify a previously untested, Barremian, clinoform wedge in the Fingerdjupet Subbasin (FSB). Data from recent well 7322/7‐1 plus seismic have been used to characterize this wedge and older Lower Cretaceous clinoforms in the FSB. In the latest Hauterivian – early Barremian, during post‐rift tectonic quiescence, shelf‐edge clinoforms (foreset height > 150 m) prograded into an under‐filled basin. Increased sediment input was related to regional uplift of the hinterland (northern Barents Shelf). Early Barremian erosion in the north‐western FSB and mass wasting towards the SE were followed by deposition of delta‐scale (<80 m high), high‐angle (c. 8°) clinoform sets seaward of older shelf‐edge clinoforms. This may be the local expression of a regional, early Barremian, regressive event. By the close of the Barremian, clinoforms had prograded, within a narrow, elongate basin, across the FSB and towards the uplifted Loppa High. A seismic wedge of high‐angle (10–12°), low‐relief, delta‐scale (25–80 m) clinoform sets occurs between shelf‐edge clinoforms to the NW and the uplifted area to the SE. Well 7322/7‐1, positioned on a direct hydrocarbon indicator, <1 km NNW of the high‐angle, low‐relief, delta‐scale clinoforms, found upward coarsening siltstone‐cycles linked to relative sea‐level fluctuations on a marine shelf. Sand may have accumulated, offshore from the well, in high‐angle, low‐relief foresets of the delta‐scale clinothems (which are typical geometries elsewhere interpreted as ‘delta‐scale, sand‐prone subaqueous clinoforms’). Deposition was controlled by the paleosurface, storms and longshore currents on an otherwise mud‐dominated shelf. The study highlights challenges associated with exploration for sandstone reservoirs in seismic wedges on an outer shelf.

A seismic wedge (red) which is part of a clinoform complex (orange, brown and yellow) was recently drilled for the first time. The well encountered gascharged marine siltstones. Quantitative clinoform characterization, seismic stratigraphy and regional geology support the interpretation of further potential sandtone reservoirs within the targeted clinform complex.

Most slope‐channel outcrop studies have been conducted at continental margin‐scale on seismic data. However, in foreland and back‐arc deepwater settings, sub‐seismic scale slope channels hold equally important information on deepwater sediment delivery, often in hydrocarbon‐bearing provinces. One such slope‐channel system is examined in Lower Jurassic prograding shelf‐margin clinoforms in Bey Malec Estancia, La Jardinera area, southern Neuquén Basin, Argentina. In a 4 km wide, 300 m tall, slightly oblique‐ to depositional‐dip section of Jurassic Los Molles Formation deepwater slope deposits, seven clinoform timelines were identified by isolated slope‐channel fills with thicknesses less than 50 m. Sedimentary logs, satellite images, a digital elevation model and drone photogrammetry were used to map variations in downslope channel geometry and infill facies. The slope channels are filled with sediment density flow deposits: poorly sorted conglomeratic debrites, structureless sandy high‐density turbidites and well‐sorted, fine‐grained, graded low‐density turbidites. The debrite portion decreases downslope, whereas high‐ and low‐density turbidites increase. A grain‐size analysis reveals a broad downslope fining trend of turbidite and debrite beds within slope channels with increasing water depth, and some notable bypass of conglomeratic facies to the lowermost slope channels and basin floor fans. The architecture of the slope channels changes from lateral to aggradational infill downstream. The Bey Malec clinoforms and its slope channels add new knowledge on downslope changes for sediment delivery in relatively shallow (<500 m water depth), prograding‐dominant deepwater basins. They also highlight one of very few outcropping examples of oblique‐type clinoforms.

Slope channels are found in an exposed prograding shelf‐margin clinoform outcrop in Bey Malec Estancia, southern Neuquén Basin, Argentina. Several clinoform timelines are mapped on a 4 km wide, 300m tall, slightly dip‐oblique section of Jurassic Los Molles Formation. Seven depositional environments are identified. Sedimentary logs, satellite images, a digital elevation model, and drone photogrammetry were used to track variations in downslope channel geometry and infill facies. The slope channels are all less than 50m in thickness and are filled with sediment density flow deposits. The debrite portion decreases downslope while high and low density turbidites increase. A grain‐size analysis reveals a broad downslope fining trend of turbidite and debrite beds within slope channels with increasing water depth, and some notable bypass of conglomeratic facies to the lowermost slope channels and basin‐floor fans. The architecture of the slope channels changes from lateral to aggradational infill downstream as the aspect ratio of the slope channels increase.

Clinoforms with a range of scales are essential elements of prograding continental margins. Different types of clinoforms develop during margin growth, depending on combined changes in relative sea level, sediment supply and oceanographic processes. In studies of continental margin stratigraphy, trajectories of clinoform ‘rollover’ points are often used as proxies for relative sea‐level variation and as predictors of the character of deposits beyond the shelf‐break. The analysis of clinoform dynamics and rollover trajectory often suffers from the low resolution of geophysical data, the small scale of outcrops with respect to the dimensions of clinoform packages and low chronostratigraphic resolution. Here, through high‐resolution seismic reflection data and sediment cores, we show how compound clinoforms were the most common architectural style of margin progradation of the late Pleistocene lowstand in the Adriatic Sea. During compound clinoform development, the shoreline was located landward of the shelf‐break. It comprised a wave‐dominated delta to the west and a barrier and back‐barrier depositional system in the central and eastern area. Storm‐enhanced hyperpycnal flows were responsible for the deposition of a sandy lobe in the river mouth, whereas a heterolithic succession formed elsewhere on the shelf. The storm‐enhanced hyperpycnal flows built an apron of sand on the slope that interrupted an otherwise homogeneous progradational mudbelt. Locally, the late lowstand compound clinoforms have a flat to falling shelf‐break trajectory. However, the main phase of shelf‐break bypass and basin deposition coincides with a younger steeply rising shelf‐break trajectory. We interpret divergence from standard models, linking shelf‐break trajectory to deep‐sea sand deposition, as resulting from a great efficiency of oceanographic processes in reworking sediment in the shelf, and from a high sediment supply. The slope foresets had a large progradational attitude during the late lowstand sea‐level rise, showing that oceanographic processes can inhibit coastal systems to reach the shelf‐edge. In general, our study suggests that where the shoreline does not coincide with the shelf‐break, trajectory analysis can lead to inaccurate reconstruction of the depositional history of a margin.

The Po River Lowstand Wedge (PRLW) of the central Adriatic Sea is made up of stacked alternated clinothems (Type A, B and C clinothems) that show different amount of aggradation of their topsets (e.g. Pellegrini et al., 2018). In this paper, we present novel sedimentological and paleontological analyses and discuss the processes responsible for the observed successions. We detail the character of the clinoforms deposited at the shelf‐edge, discussing the importance of hyperpycnal and storm‐enhanced sediment‐gravity flows in controlling the lithologic assemblage of the different tracts of the compound clinoforms. More in general, we show that, when the fine details of prograding clinoform units are investigated, the processes that lead to margin outbuilding are more complex than usually envisaged.

Grade index (Gindex) is a dimensionless number given as the volume‐in‐unit‐time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the Gindex, using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive‐to‐transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (At*) encounters a critical value that is expressed in terms of Gindex: regression and transgression are sustained when At* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both At* and Gindex decrease as the relative sea‐level rises. Eventually, the depositional system experiences autodrowning when At* = Gindex = 0. If γ < β; on the other hand, both At* and Gindex increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which At* = Gindex = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.

Given steadily rising relative sea level, a deltaic clinoform grows in a nonequilibrium pattern. These nonequilibrium behaviours include autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which can be captured by the recently‐proposed grade index model. The regressive‐to‐transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain’s dimensionless basal area (At*, which can be reflected by the dimensionless delta plain radius, x^*) encounters a critical value that is expressed in terms of grade index (Gindex): regression and transgression are sustained when At* is below and above the threshold, respectively. During transgression, the transgression mode depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both At* and Gindex decrease as the relative sea‐level rises. Eventually, the depositional system experiences autodrowning when At* = Gindex = 0. If γ < β; on the other hand, both At* and Gindex increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which At* =Gindex = 1.

We developed a seismic geomorphology‐based procedure to enhance traditional trajectory analysis with the ability to visualize and quantify lateral variability along carbonate prograding‐margin types (ramps and rimmed shelves) in 3D and 4D. This quantitative approach analysed the shelf break geometric evolution of the Oligo‐Miocene carbonate clinoform system in the Browse Basin and delineated the feedback between antecedent topography and carbonate system response as controlling factor on shelf break rugosity. Our geometrical analysis identified a systematic shift in the large‐scale average shelf break strike direction over a transect of 10 km from 62° to 55° in the Oligo‐Miocene interval of the Browse Basin, which is likely controlled by far‐field allogenic forcing from the Timor Trough collision zone. Plotting of 3D shelf break trajectories represents a convenient way to visualize the lateral variability in shelf break evolution. Shelf break trajectories that indicate contemporaneous along‐strike progradation and retrogradation correlate with phases of autogenic slope system re‐organization and may be a proxy for morphological stability of the shelf break. Shelf break rugosity and shelf break trajectory rugosity are not inherited parameters and antecedent topography does not dictate long‐term differential movement of the shelf margin through successive depositional sequences. The autogenic carbonate system response to antecedent topography smooths high‐rugosity areas by filling accommodation and maintains a relatively constant shelf break rugosity of ~150 m. Color‐coding of the vertical component in the shelf break trajectory captures the creation and filling of accommodation, and highlights areas of the transect that are likely to yield inconsistent 2D sequence stratigraphic interpretations.

Map view and seismic dip lines visualizing differential progradation of SBT7‐8 along bin 60 and bin 100 (=2 km lateral separation). (a) Map view of SB7 and SB8. (b) Map view of SBT 7–8 color‐coded for aggradation (blue) and downstepping (red). (c) Uninterpreted depth‐converted seismic dip line along bin 60. Vertical exaggeration = 3×. (d) Uninterpreted depth‐converted seismic dip line along bin 100. (e) Interpreted depth‐converted seismic dip line along bin 60 with shelf breaks SB7 and SB8, indicating an aggrading shelf break trajectory with minor progradation. (f) Interpreted depth‐converted seismic dip line along bin 100 with shelf breaks SB7 and SB8, indicating a prograding shelf break trajectory with minor aggradation.

Reconstructions of ancient delta systems rely typically on a two‐dimensional (2D) view of prograding clinothems but may miss their three‐dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high‐resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high‐resolution multi‐beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth‐bar) and muddy seaward‐dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high‐resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one‐year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut‐and‐fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.

Conceptual stratigraphic section of the Po di Pila clinothem throughout the 4‐years bathymetric surveys. Bottom, year 2013: the Po di Pila shows compound progradation in the central sector, with a subaqueous Rollover Point (RP) at 8 m, whereas in the northern sector a single RP develops at an average of 3 m water depth. Center, year 2014: the Po di Pila shows compound progradation in both (central and northern) sectors, but a destruction phase in the central sector (eroded topset and RPs separated by a collapse depression) and a constructional phase in the northern sector (normal regression with compound clinothems) occur simultaneously. Top, year 2016: the Po di Pila shows a subaqueous RP at an average 5 m of water depth of the transverse bar in the central sector, whereas compound progradation is recorded in the northern sector. Note that in a subaerial delta, such as the modern Po Delta, subaerial RPs correspond to breaks in slope associated with the presence of mouth bars, whereas subaqueous nearshore RPs correspond to breaks in slope due to the presence of transverse bars and delta lobes.