Clinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1-10 4 m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or 'delta-scale compound clinoforms'), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60 m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich. This study analyses a large dataset of modern and ancient delta-scale, shelf-prism-and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of deltascale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws. The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4-6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained. Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics.(1) They are formed during relative sea-level stillstands (e.g., Late Holocene); (2) their stratigraphic architecture and facies character are dominated by basinal processes, and are quite uniform; (3) their plan-view morphology is shore-parallel and laterally extensive; (4) their sigmoidal cross-sectional geometry contrasts with the oblique profiles of most subaerial deltas. Holocene-age, delta-scale, sand-prone subaqueous clinoforms occur on steep (≥0.26°) and narrow (5-32 km) shelves, at typical distances of 0.6-7.2 km from the shoreline break. That contrasts with mud-prone subaqueous deltas, which form clinoforms on gently-sloping (0.01-0.38°), wide (23-376 km) shelves, at usual distances of 7.5-125 km from the shoreline. Delta-scale sand-prone subaqueous clinoforms have diagnostically steep foresets (0.7-23°). Similarly steep gradients were observed in much larger shelf-prism-and continental-margin-scale clinoforms. Gentler foreset gradients are shown by sand-prone subaerial deltas (0.1-2.7°), and mud-prone subaqueous and subaerial deltas (0.03-1.50°). Due to the lack of connections with river mouths, Holocene delta-scale sa...

A B S T R A C TClinoforms are inclined and normally basinward-dipping horizons developed over a range of spatial and temporal scales in both siliciclastic and carbonatic systems. The study of clinoform successions underpins sequence stratigraphy and all efforts to reconstruct the relative partitioning of reservoir, seal and source rocks along shoreline to basin-floor profiles.Here, we review clinoform research and propose a more systematic description and classification of clinoforms. This is a crucial step to improve predictions of facies and lithology distribution within shoreline to continental shelf and abyssal plain successions, together with the genesis, drivers and dynamics of their constituent sedimentary units.Four basic clinoform types are here distinguished in delta/shorelines, lacustrines and marine environments, on the basis of their overall spatial and temporal scale, morphology, outbuilding dynamic and geodynamic and depositional setting: (1, 2) delta-scale clinoforms, which in turns are sub-divided into shoreline and delta-scale subaqueous clinoforms; (3) shelf-edge clinoforms; and (4) continental-margin clinoforms. Delta-scale clinoform sets are tens of metres high and typically represent 1-10 3 kyr, with progradation rates ranging from 1,000-100,000 m/kyr for shorelines and "subaerial deltas" to 100-20,000 m/kyr for subaqueous deltas; shelfedge clinoform sets are hundreds of metres high and are nucleated and accreted in 0.1-20 Myr (usual progradation rates of 1-100 m/kyr) by successive cross-shelf transits of delta-scale clinoforms; continental-margin clinoform sets are thousands of metres high, hallmark key geodynamic/crustal boundaries (e.g., continent/ocean transition) and slowly prograde basinwards in ca. 5-100 Myr, with typical rates of 0.1-10 m/kyr. As a consequence of the very different progradation rates and of the difficulty of large-scale clinothems to backstep during transgressions, shorelines are the most dynamic clinoforms with regards to position, continental margins the least, and shelf-edges are intermediate. Shortly after a transgression, therefore, the four clinoform types may prograde synchronously along shoreline-to-abyssal plain transects, forming "compound clinoform" systems. During the subsequent regressive cycle, however, due to the dissimilarity in progradation rates, different clinoform types will normally merge progressively with each other, giving rise to "hybrid clinoforms" (e.g., shelf-edge deltas), and fewer depositional breaks-in-slope are distinguished along a single shoreline-toabyssal plain transect. Overall, all clinoform systems are the result of the dynamic evolution of compound and hybrid clinoforms along a temporal and spatial continuum, regulated by the cyclical backstepping of the smallerscale system within natural progradational-retrogradational cycles of larger-scale clinothem outbuilding.All clinothem types may show either an accretionary/active or draping/passive style, depending on the proximity to the sediment source. Draping clinothems are nearly...

The integration of core sedimentology, seismic stratigraphy and seismic geomorphology has enabled interpretation of delta-scale (i.e. tens of metres high) subaqueous clinoforms in the upper Jurassic Sognefjord Formation of the Troll Field. Mud-prone subaqueous deltas characterized by a compound clinoform morphology and sandy delta-scale subaqueous clinoforms are common in recent tide-influenced, wave-influenced and current-influenced settings, but ancient examples are virtually unknown. The data presented help to fully comprehend the criteria for the recognition of other ancient deltascale subaqueous clinoforms, as well as refining the depositional model of the reservoir in the super-giant Troll hydrocarbon field. Two 10 to 60 m thick, overall coarsening-upward packages are distinguished in the lower Sognefjord Formation. Progressively higher energy, wave-dominated or current-dominated facies occur from the base to the top of each package. Each package corresponds to a set of seismically resolved, westerly dipping clinoforms, the bounding surfaces of which form the seismic 'envelope' of a clinoform set and the major marine flooding surfaces recognized in cores. The packages thicken westwards, until they reach a maximum where the clinoform 'envelope' rolls over to define a topset-foreset-toeset geometry. All clinoforms are consistently oriented sub-parallel to the edge of the Horda Platform (N005-N030). In the eastern half of the field, individual foresets are relatively gently dipping (1°to 6°) and bound thin (10 to 30 m) clinothems. Core data indicate that these proximal clinothems are dominated by fine-grained, hummocky crossstratified sandstones. Towards the west, clinoforms gradually become steeper (5°to 14°) and bound thicker (15 to 60 m) clinothems that comprise mediumgrained, cross-bedded sandstones. Topsets are consistently well-developed, except in the westernmost area. No seismic or sedimentological evidence of subaerial exposure is observed. Deposition created fully subaqueous, near-linear clinoforms that prograded westwards across the Horda Platform. Subaqueous clinoforms were probably fed by a river outlet in the north-east and sculpted by the action of currents sub-parallel to the clinoform strike.

This article presents a new numerical inversion method to estimate progradation rates in ancient shallow-marine clinoform sets, which is then used to refine the tectono-stratigraphic and depositional model for the Upper Jurassic Sognefjord Formation reservoir in the super-giant Troll Field, offshore Norway. The Sognefjord Formation is a 10-200-m thick, coarse-grained clastic wedge, that was deposited in ca. 6 Myr by a fully marine, westward-prograding, subaqueous delta system sourced from the Norwegian mainland. The formation comprises four, 10-60-m thick, westerly dipping, regressive clinoform sets, which are mapped for several tens of kilometres along strike. Near-horizontal trajectories are observed in each clinoform set, and the sets are stacked vertically. Clinoform age and progradation rates are constrained by: (i) regionally correlatable bioevents, tied to seismically mapped clinoforms and clinoform set boundaries that intersect wells, (ii) exponential age-depth interpolations between bioevent-dated surfaces and a distinctive foreset-to-bottomset facies transition within each well, and (iii) distances between wells along seismic transects that are oriented perpendicular to the clinoform strike and tied to well-based stratigraphic correlations. Our results indicate a fall in progradation rate (from 170-500 to 10-65 km Myr À1) and net sediment flux (from 6-14 to ≤1 km 2 Myr À1 ) westwards towards the basin, which is synchronous with an overall rise in sediment accumulation rate (from 7-16 to 26-102 m Myr À1). These variations are attributed to progradation of the subaqueous delta into progressively deeper waters, and a concomitant increase in the strength of alongshore currents that transported sediment out of the study area. Local spatial and temporal deviations from these overall trends are interpreted to reflect a subtle structural control on sedimentation. This method provides a tool to improve the predictive potential of sequence stratigraphic and clinoform trajectory analyses and offers a greater chronostratigraphic resolution than traditional approaches.

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.

Normal faults grow either by radial propagation and segment linkage or by accruing displacement without a proportional increase in fault length. To test these competing models of fault growth, a novel 2D seismic stratigraphic interpretation of the recentFucino Basin of the central Apennines (Italy) has been performed. The Fucino is a major Pliocene-Quaternary non-marine 'extensional collapse basins', developed immediately after the Apenninic compressional strain had locally abated, and bounded by seismogenic faults that generate strong (Mw = 6-7) and destructive earthquakes.The Fucino is an overall dual polarity half-graben, built around two border fault systems: the northern one lies along the east-northeast striking Avezzano-Bussi regional fault-zone; the other, south-eastern trending, bounds the basin to the east. Two separate fault-driven depocentres of fluvio-lacustrine sequences (maximum thicknesses of ~1,750 m) are present. One is associated to the northern border faults, with mainly Late Pliocene activity; the second is to the hanging-wall of the eastern border-faults and reveals a stepwise Pleistocene-Recent activity. Tectonic depocentres have migrated clockwise through time, and new-born fault systems have developed at the south-eastern basin periphery (Gioia dei Marsi area). This, together with the progressive accumulation of throw in time without significant fault lengthening, suggests that the model of fault growth by segment linkage is not the best explanation for this basin. Instead, the stepwise onset and growth of the Fucino extensional collapse faulting, and its ongoing earthquake hazard, may have been promoted by polyphase inversion tectonics of inherited deep-seated zones of weaknesses (e.g. Avezzano-Bussi Line).

The Caledonian and Variscan orogens in northern Europe and the Alpine-age Apennine range in Italy are classic examples of thrust belts that were developed at the expense of formerly rifted, passive continental margins that subsequently experienced various degrees of post-orogenic collapse and extension. The outer zones of orogenic belts, and their adjoining foreland domains and regions, where the effects of superposed deformations are mild to very mild make it possible to recognize and separate structures produced at different times and to correctly establish their chronology and relationships. In this paper we integrate subsurface data (2D and 3D seismic reflection and well logs), mainly from the North Sea, and structural field evidence, mainly from the Apennines, with the aim of reconstructing and refining the structural evolution of these two provinces which, in spite of their different ages and present-day structural framework, share repeated pulses of alternating extension and compression. The main outcome of this investigation is that in both scenarios, during repeated episodes of inversion that are a characteristic feature of the Wilson cycle, inherited basement structures were effective in controlling stress localization along faults affecting younger sedimentary cover rocks.

The Mid North Sea High (MNSH) is located on the UKCS in quadrants 35–38 and 41–43. It is a large structural high that is flanked by the mature hydrocarbon provinces of the Central North Sea (CNS) to the NE and the Southern North Sea (SNS) to the SE. In the MNSH region, the source and reservoir intervals that characterize the SNS (Westphalian, Lower Permian) are absent and therefore the area is relatively underexplored compared to the SNS Basin (c. one well per 1000 km2). Nevertheless, two discoveries in Dinantian reservoirs (Breagh and Crosgan) prove that a working petroleum system is present, potentially charged either via lateral migration from the SNS or from within the lower Carboniferous itself. Additionally, gas was found in the Z2 carbonate (lower Zechstein Group) in Crosgan, with numerous other wells in the area reporting hydrocarbon shows in this unit. The results of the interpretation of recently acquired 2D and 3D seismic reflection datasets over parts of UKCS quadrants 36, 37 and 42 are presented and provide insight into both the geology and prospectivity of this frontier area.This study suggests that intra-Zechstein clinoform foresets represent an attractive, hitherto overlooked, exploration target. The Zechstein Group sits on a major unconformity, probably reflecting Variscan-related inversion and structural uplift. Below it, fault blocks and faulted folds occur, containing pre-Westphalian Carboniferous and Devonian sediments, both of which contain potential reservoirs. In the lower Zechstein, a large build-up is observed, covering a total area of 2284 km2. This is bounded on its margins by seismically defined clinoforms, with maximum thicknesses of 0.12 s two-way time (c. 240–330 m). This rigid, near-tabular unit is clearly distinguished from the overlying deformed upper Zechstein evaporites. In map-view, a series of embayments and promontories are observed at the build-up margins. Borehole data and comparisons with nearby discoveries (e.g. Crosgan) suggest this build-up to represent a Z1–Z2 sulphate–carbonate platform, capped by a minor Z3 carbonate platform. Interpreted smaller pinnacle build-ups are observed away from the main bank. The seismic character, geometry, size and inferred composition of this newly described Zechstein platform are similar to those of platforms hosting notable hydrocarbon discoveries in other parts of the Southern Permian Basin. The closest of these discoveries to the study area is Crosgan, which is characterized by the Z2 carbonate clinothem (Hauptdolomit Formation) as a proven reservoir.