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The interplay between high-frequency relative sea-level changes, such as those connected to late Quaternary glacio-eustasy, and regional uplift usually produces staircases of marine terraces along coastal areas. These particular settings are characterized by variable topography and gradient of the landscape due to both tectonics and wave erosion, which result in the development of subaerial unconformities during both transgressive and forced regressive stages of the high-frequency relative sea-level curve. In particular, the increased topographic gradient due to tectonic uplift results in a coastal profile steeper than the transgressive shoreline trajectory and the consequent formation of a retreating coastal cliff fronted by a transgressive wave ravinement surface, whereas subaerial exposure occurs landwards. In contrast, the relatively low-gradient wave ravinement surface formed during transgression seaward of the coastal cliff affords the deposition of a highstand shallow-marine prograding wedge characterized by a relatively low-gradient top, gentler than the forced regressive shoreline trajectory, resulting in the formation of a new subaerial unconformity at the top of highstand and forced regressive deposits during relative sea-level fall. The overall uplifting conditions favor the persistent development of the already formed transgressive and forced regressive subaerial unconformities also during the highstand and lowstand stages of the high-frequency relative sea-level curve, excepting for the accumulation of continental deposits just seaward of the uplifted paleo-coastal cliffs. The repetition of such a motif for several high-frequency relative sea-level changes will result in the formation of a long-term, higher rank subaerial unconformity expanding seaward and recording the uplift of the coastal area. The way in which the subaerial unconformity develops in contexts dominated by regional uplift and superimposed glacio-eustatic sea-level changes is not described by classic sequence stratigraphic models and this needs to be taken into account in sequence stratigraphic analysis.

High-frequency sequences composed of mixed siliciclastic-carbonate deposits may exhibit either vertical or horizontal changes between siliciclastics and carbonates. Vertical facies shifts occur between systems tracts and define a ‘reciprocal sedimentation’ pattern, typically consisting of transgressive/highstand carbonates and forced regressive/lowstand siliciclastics, although variations from this rule are common. Mixed systems with lateral facies change, usually typifying transgressive and/or highstand systems tracts, may exhibit proximal siliciclastics and distal carbonates or vice-versa, although variations may also occur along depositional strike. The marked variability of mixed siliciclastic-carbonate sequences makes the definition of a universal sequence stratigraphic model impossible, as the composition and geometries of systems tracts may change considerably, and sequence stratigraphic surfaces and facies contacts may vary in terms of occurrence and physical expression. However, some resemblance exists between siliciclastic sequences and mixed sequences showing lateral facies changes between siliciclastics and carbonates. In particular, these mixed sequences display 1) a stratal architecture of the clastic part of the systems tracts that is comparable to that of siliciclastic deposits, 2) a dominant role of the inherited physiography and of erosional processes, rather than carbonate production, in shaping the shelf profile, and 3) a local lateral juxtaposition of siliciclastic sandstones and carbonate bioconstructions due to hydrodynamic processes. These observations are helpful in predicting the location of porous and potential sealing bodies and baffles to fluid flow at the intra-high-frequency sequence scale, and ultimately they are useful for both petroleum exploration and production.

The Pliocene and Pleistocene intervals of the Neogene to Quaternary succession of the Crotone Basin, southern Italy, offer good examples of shallow-marine high-frequency sequences (fourth-order or lower rank stratigraphic frameworks) composing higher-rank units. The analysis of some representative sections from this succession has allowed performing an integrated sedimentological and micropaleontological study aimed at defining new criteria to recognize high-frequency maximum flooding surfaces. In particular, field data have been compared with some parameters derived from the micropaleontological analysis, such as the abundance, diversity and % fragmentation of benthic foraminifera, as well as the ratio between distal and proximal benthic foraminifera species and the plankton/benthos ratio. This integrated study allows to define an uncertainty interval in the high-frequency sequences, in which the maximum flooding surface should lie. This uncertainty interval is usually characterized by lower values of the % fragmentation and higher values of the abundance, diversity and distal/proximal ratio, whereas the peak of the plankton/benthos ratio allows to define a surface of maximum water depth, placed above the maximum flooding surface. Usually, the greater the magnitude of the transgression, the thinner the uncertainty interval. These results highlight the cryptic nature of the maximum flooding surface within condensed sections, even in high-resolution outcrop studies, and allow to refine the criteria based on which the position of such surface can be approximated in the field and in sediment cores.

In contrast to high-frequency sequences driven by high-magnitude relative sea-level changes, those controlled by short-term, minor relative sea-level and/or sediment supply changes may be difficult to discriminate from sedimentological cycles (i.e., bedsets) unrelated to shoreline shifts, especially in case of limited outcrop exposures. In fact, meter-scale, fully shallow-marine high-frequency sequences and typical meter-scale bedsets may share a similar, simple facies succession documenting either an upward increase or decrease of event beds. It is therefore necessary to define a set of criteria that allows to discriminate between thin high-frequency sequences and bedsets, based on sedimentological, stratigraphic, micropaleontological, mineralogical and diagenetic data. In particular, the sedimentological and stratigraphic criteria that aid discriminating between high-frequency sequences and bedsets include: 1) occurrence of environmental changes across bounding surfaces; 2) occurrence of water-depth changes across bounding surfaces; 3) physical appearance of bounding surfaces and associated substrate-controlled ichnofacies; 4) lateral extent of bounding surfaces; 5) presence of condensed deposits; 6) cycle thickness; 7) recognition of a set of clinoforms in a regressive shoreface-shelf succession. Moreover, the formation of wave-ravinement surfaces in the shoreface is usually associated with an increase in the percentage of benthic micro-foraminifera specimens documenting energetic conditions, and in the abundance of heavy minerals. Extensive cementation may also be found just below and/or above transgressive surfaces. However, the integration of more than one of the above criteria is necessary to reliably discriminate between sequence stratigraphic surfaces (and therefore high-frequency sequences) and bedset boundaries, the latter being only related to changes of energy level and/or local sediment supply without shoreline shifts. This work is essential to correctly reconstruct the sequence stratigraphic framework of a given succession and to interpret the factors that controlled the cyclicity.

The Northern Adriatic Sea is a shallow and very flat shelf area located between the northern Apennines, the southern Alps and the Dinarides; its present day physiography is the result of the filling of a relatively deep Quaternary foredeep basin, developed due to the northeastward migration of the Apennine chain. Multichannel seismic profiles and well data have allowed documenting the stratigraphic architecture, the depositional systems and the physiographic evolution of the Northern Adriatic sea since early Pliocene time. In particular, three main depositional sequences bounded by regional unconformities were recognized. The Zanclean Sequence 1 documents first the drowning of late Messinian incised valleys and then the southward progradation of a shelf-slope system, which is inferred to be related to a tectonic phase of the Apenninic front. The Piacenzian-Gelasian Sequence 2 records a relatively rapid transgressive episode followed by minor southward progradation; the top of the sequence is associated with a major late Gelasian drowning event, linked to the NE-ward migration of the Apennine foredeep. The Calabrian to upper Pleistocene Sequence 3 testifies the infilling of accommodation previously created by the late Gelasian drowning event, and it initially accumulated in deep-water settings and then in shallow-water to continental settings. The upper part of Sequence 3, consisting of the paleo-Po deltaic system, is composed of seven high-frequency sequences inferred to record late Quaternary glacio-eustatic changes. These high-frequency sequences document the stepwise filling of the remaining accommodation, resulting in the development of the modern shelf.

The sequence stratigraphic architecture includes a complex array of stratal geometries with different degrees of stratigraphic significance. The ‘non-unique’ variability of the sequence stratigraphic framework (i.e., stratal geometries which are not diagnostic for the definition of systems tracts and bounding surfaces) is irrelevant to the workflow of sequence stratigraphy. What is relevant is the observation of the ‘unique’ stratal geometries that are diagnostic for the definition of units and surfaces of sequence stratigraphy. In downstream-controlled settings, these unique stratal stacking patterns relate to the forced regressive, normal regressive, and transgressive shoreline trajectories. Multiple controls interact during the formation of each type of stacking pattern, including accommodation, sediment supply, and the energy of the sediment-transport agents. This interplay explains the non-unique variability, but does not change the unique criteria that afford a consistent application of sequence stratigraphy. The distinction between unique and non-unique stratal geometries is critical to the sequence stratigraphic methodology. Failure to rationalize the non-unique variability within the context of unique stratal geometries is counterproductive, and obscures the simple workflow of sequence stratigraphy. This is the case with uncalibrated numerical modeling, which may overemphasize non-unique or even unrealistic stratigraphic scenarios. While useful to test the possible controls on stratigraphic architecture, modeling requires validation with real data, and plays no role in the sequence stratigraphic methodology.

High-frequency clastic shelf sequences deposited in high-latitude settings display marked differences, in term of facies and stratigraphic architecture, with respect to their lower latitude counterparts. This is due to the presence of ice which (1) leads to the accumulation of glacigenic and glacimarine deposits; (2) provides an additional control on accommodation; and (3) determines the position of the shoreline. Transgressions and regressions in glaciated settings are controlled respectively by the retreat and advance of the ‘ice’ shoreline (i.e., the water/ice contact) irrespective of relative sea-level changes; once the ice retreats across the land, the traditional ‘land’ shoreline is exposed and the control on sequence architecture is exerted by the interplay between relative sea-level changes and sediment supply as in low- and middle-latitude settings. A general model that includes both glacial and non-glacial climatic regimes is provided by this paper. In this frame, the classic sequence stratigraphic model represents one (ice-free) end member, which is opposed to an ice-permanent end member. Between these end members, sequences may accumulate in part under ice-free conditions and in part under conditions dominated by ice on the shelf. The main implication of this is that the classic sequence stratigraphic model may be viewed as only a possible scenario in the stratigraphic record rather than the rule.

In the geological record, high-frequency sequences at the scale of fourth-order and lower rank stratigraphic frameworks are usually stacked to compose higher rank (lower frequency) systems tracts (i.e., sequence sets) along dip-oriented transects. Higher rank transgressive, normal regressive (highstand and/or lowstand) and falling-stage systems tracts are defined by retrogradational, upstepping progradational, and downstepping progradational stacking patterns of component high-frequency sequences, respectively. However, these stacking patterns may vary markedly along depositional strike due to lateral changes in the rates of accommodation and sedimentation, resulting in the coeval deposition of different types of higher rank systems tracts. In this case, surfaces bounding such systems tracts are diachronous along depositional strike. Moreover, if the high-frequency sequences that compose the lower frequency systems tracts are laterally continuous, the higher rank bounding surfaces are discontinuous along depositional strike, consisting of a series of stepped surfaces that can be observed in high-resolution studies. These aspects have important implications for petroleum exploration and need to be considered in high-resolution sequence stratigraphic analysis.

The Crotone Basin is a Neogene-Quaternary depocenter developed along the Ionian margin of Calabria, southern Italy. The basin opening was concomitant with the onset of spreading of the Tyrrhenian backarc Basin during late Serravallian and the subduction of the Ionian lithosphere below the Calabrian Arc. The adoption of modern sequence stratigraphic principles to subdivide the basin fill into stratigraphic sequences of different rank, bounded by subaerial unconformities, has allowed cycles of creation and loss of accommodation mainly related to tectonics to be recognized. In particular, an alternation between phases of basin subsidence, concomitant with spreading of the Tyrrhenian backarc Basin and forward migration of the Arc, and phases of compressional or transpressional tectonics, accompanied by basin uplift and stopping of Arc migration, are well recognizable in the basin fill, in particular for the late Messinian to late Pleistocene part of the succession. The present study highlights the effectiveness of the sequence stratigraphic method in basin analysis to decipher the effects of large-scale tectonic events on sedimentation and ultimately to reconstruct the geodynamic evolution of the central Mediterranean.

The aim of this study is to better define the scale, timing and driving mechanisms of a seaward-gliding landmass currently affecting the Crotone Basin, a forearc basin on the Ionian side of Calabria (southern Italy). An integrated approach of seismic, well and field data, together with literature information, have revealed that the slide includes an onshore updip extensional domain compatible with the presence of a curved seaward-dipping fault system to the NW, which is connected southward to an offshore downdip compressional domain through a buried detachment surface lying at a Messinian halite layer. The landmass, which involves Messinian-Pleistocene deposits, has started to slide since the Zanclean, and experienced a paroxysmal episode between late Zanclean and early Piacenzian linked to transpressional tectonics. After a phase of inactivity during the Calabrian, the gravitational collapse underwent a second reactivation since middle Pleistocene due to regional uplift, which seems to be restricted to the offshore sector, the Crotone peninsula and probably the eastern coastal area.

The Serra Mulara canyon is a middle Pleistocene submarine canyon developed along the Ionian coast of Calabria, southern Italy, now exposed on land due to the long-term regional uplift of the area. New data have allowed recognizing three depositional sequences in the upper part of the succession, probably glacio-eustatic in origin, documenting an evolution of depositional systems that reflect the emergence of the canyon. Sequence 1 shows a rapid shift from turbidites and hemipelagites accumulated in the upper part of the canyon fill to forced regressive deltaic deposits, which reflect a marked shallowing of the area. Sequence 2 consists of estuarine transgressive and deltaic highstand deposits, whereas Sequence 3 is fully composed of fluvial deposits, testifying the emergence of the area. The upper part of the Serra Mulara succession is an useful example showing the transition from submarine canyon to continental settings, a situation that is not adequately documented in the literature, and is also relevant to better reconstruct the timing of the uplift of this part of Calabria.

High-frequency clastic shelf sequences contain porous deposits, consisting of coastal to shallow-water sandstones and conglomerates, which may represent very good reservoirs, and therefore critical targets for exploration and production, as well as for fluid (hydrocarbon, water or CO2) storage. The study of these reservoirs, following a sequence stratigraphic approach, is essential to predict their features and distribution within systems tracts forming high-frequency sequences, which in turn compose higher-rank systems tracts and sequences. A critical aspect to improve the exploitability of the reservoirs is their heterogeneity, which directly affects the volume of the reservoirs and determine internal compartmentalization. The heterogeneity can be primary, related to depositional processes influencing grain size and facies distribution, and secondary, related to diagenesis. In particular, the primary heterogeneity is referred to the facies variability from distal to proximal settings, to the accumulation of condensed shell beds, and to facies characteristics of bedsets, whereas the secondary heterogeneity derives from diagenetic processes at sequence stratigraphic surfaces and facies contacts, and within sandbodies. Both the distribution of reservoirs and their heterogeneity can be predicted following a high-resolution sequence stratigraphic approach. Ultimately, the integration of subsurface data with the analysis of outcrop analogs is essential to better understand the features of clastic shelf reservoirs and to develop 3D models of fluid migration.

The Plio-Pleistocene succession of the Venice area represents part of the infill of a foreland region located between three mountain chains: the Northern Apennines, the Southern Alps and the Dinarides. This structural setting favored the development of a complex stratigraphic architecture of the succession, mostly due to the conveying of sediments from the Southern Alps to the north and the Northern Apennines to the south, in particular since the activation of strong subsidence related to the NE-ward migration of the Apennine foredeep in the early Pleistocene. Accordingly, the studied succession is composed of five third-order sequences mostly controlled by tectonics, the most recent of which display complex patterns due to the interfingering of sedimentary bodies showing contrasting directions of progradation and pinch-out. Despite this, the sequence stratigraphic method still can be applied in the present context, allowing to recognize diagnostic stratal architectures and reconstruct the relative sea-level history of the region. Moreover, the recognized peculiar stratigraphic architecture of the basin fill may serve as an analogue that needs to be taken into account to predict the distribution of porous coarse-grained sedimentary units in similar contexts, aiding for a profitable exploration and production of reservoirs and source/sealing rocks.

The upper Messinian to lower Zanclean succession of the Crotone Basin, a forearc basin located along the Ionian side of the Calabrian Arc (southern Italy), consists of a turbidite-bearing succession (the Petilia Policastro Formation) overlying evaporites and abruptly passing upward into coarse-grained continental deposits (the Carvane Conglomerate), which are in turn overlain by distal shelf to slope mudstones (the Cavalieri Marl). The marked environmental changes from subaqueous to continental and vice versa documented by the studied succession are inferred to be the expression of basin-scale tectonics, leading to uplift and exposure, superposed on the Mediterranean-scale base-level rise that followed the acme of the Messinian Salinity Crisis. The upper part of the succession documents a transgressive trend probably initiated within the Carvane Conglomerate, and further testified by the presence of a ravinement surface marked by a transgressive lag at the base of the Zanclean Cavalieri Marl, pointing to a non-catastrophic deepening occurred near the end of the Messinian.

In 2009, the National Institute of Oceanography and Experimental Geophysics (OGS, Italy) has performed a geophysical survey in the northernmost sector of the Northern Adriatic Sea, between the Tagliamento and Po river deltas, with R/V OGS Explora. About 820 km of 2D multichannel seismic and Chirp profiles, together with multibeam data along the ship tracks, have been acquired, with the aim to reconstructing the Plio-Quaternary stratigraphic and tectonic setting of the study area. Data have been also analyzed to verify whether this region is suitable for CO2 geological storage. Sites eligible for the application of Carbon Capture and Storage (CCS) techniques have been already identified in the Adriatic Sea, except in the northernmost sector, due to the scarcity of available data. The analyses of the new OGS seismic dataset highlighted the occurrence of peculiar seismic anomalies, represented by sub-vertical wipe-out zones, that have been interpreted as due to gas leakages affecting the Plio-Quaternary sequence. They could be possibly related to the formation of rock outcrops interpreted as methane-derived carbonates. It is suggested that gas migrates along sub-vertical chimneys throughout the Plio-Quaternary sequence. The correlation of these sub-vertical features among the seismic profiles reveals that two main alignments could be recognized: a NW–SE trend offshore the Venice Lagoon and a NE–SW in the northernmost part of the investigated area. It leads to hypothesize that the preferential conduits for gas migration are associated to tectonic lineaments. The analysis of the tectonic setting has been then addressed at defining the role of fault/fracture zones, which can affect the integrity of a CO2 storage complex in this area. Tectonic features have been identified in the Mesozoic–Paleogene succession, but their relationship with the shallow faults/fractures representing the paths for gas leakages still need further work and additional data in order to be clearly constrained.

The high-resolution sequence stratigraphy tackles scales of observation that typically fall below the resolution of seismic exploration methods, commonly referred to as of 4th-order or lower rank. Outcrop- and core-based studies are aimed at recognizing features at these scales, and represent the basis for high-resolution sequence stratigraphy. Such studies adopt the most practical ways to subdivide the stratigraphic record, and take into account stratigraphic surfaces with physical attributes that may only be detectable at outcrop scale. The resolution offered by exposed strata typically allows the identification of a wider array of surfaces as compared to those recognizable at the seismic scale, which permits an accurate and more detailed description of cyclic successions in the rock record. These surfaces can be classified as ‘sequence stratigraphic’, if they serve as systems tract boundaries, or as facies contacts, if they develop within systems tracts. Both sequence stratigraphic surfaces and facies contacts are important in high-resolution studies; however, the workflow of sequence stratigraphic analysis requires the identification of sequence stratigraphic surfaces first, followed by the placement of facies contacts within the framework of systems tracts and bounding sequence stratigraphic surfaces. Several types of stratigraphic units may be defined, from architectural units bounded by the two nearest non-cryptic stratigraphic surfaces to systems tracts and sequences. The need for other types of stratigraphic units in high-resolution studies, such as parasequences and small-scale cycles, may be replaced by the usage of high-frequency sequences. The sequence boundaries that may be employed in high-resolution sequence stratigraphy are represented by the same types of surfaces that are used traditionally in larger scale studies, but at a correspondingly lower hierarchical level.

Both allogenic and autogenic processes may contribute to the formation of sequence stratigraphic surfaces, particularly at the scale of fourth-order and lower rank cycles. This is the case with all surfaces that are associated with transgression, which include the maximum regressive surface, the transgressive ravinement surfaces and the maximum flooding surface, and, under particular circumstances, the subaerial unconformity as well. Not all autogenic processes play a role in the formation of sequence stratigraphic surfaces, but only those that can influence the direction of shoreline shift. Any changes in shoreline trajectory, whether autogenic or allogenic in origin, influence the stratal stacking patterns in the rock record which sequence stratigraphic interpretations are based upon. The discrimination between the allogenic and autogenic processes that may control changes in shoreline trajectory is a matter of interpretation and is tentative at best in many instances. For this reason, the definition and nomenclature of units and bounding surfaces need to be based on the observation of stratal features and stacking patterns rather than the interpretation of the controlling mechanisms. In this light, we extend the concept of ‘sequence’ to include all cycles bounded by recurring surfaces of sequence stratigraphic significance, irrespective of the origin of these surfaces. The updated sequence concept promotes a separation between the objective observation of field criteria and the subsequent interpretation of controlling parameters, and stresses that a sequence stratigraphic unit is defined by its bounding surfaces and not by its interpreted origin. The use of high-frequency sequences eliminates the need to employ the concepts of parasequence or small-scale cycle in high-resolution studies, and simplifies the sequence stratigraphic methodology and the nomenclature.