With this general meaning, flooding surfaces' may be facies contacts within transgressive deposits, or may coincide with different types of sequence stratigraphic surfaces (maximum regressive, transgressive ravineament, or maximum flooding). Advances in high-resolution sequence stratigraphy show that the scales of sequences and parasequences are not mutually exclusive; the two types of units define different approaches to the delineation of stratigraphic cycles at high-resolution scales.
that record full cycles of relative sea-level change (Zecchin, 2010). This is the case in tectonically active basins where subsidence and uplift can occur at the same time along the shoreline of an interior seaway, leading to the coeval formation of depositional sequences and para- sequences (e.g., Catuneanu et al., 2002: forelands; Gawthorpe et al., 2003: half-graben rift basins). Beyond stereotypes, there is evidence of significant variability in the composition of parasequences, which may consist of successions dominated by either shallowing- or deepening- upward trends (e.g. Kidwell, 1997; Saul et al., 1999; Di Celma et al., 2005; Zecchin, 2005, 2007; Di Celma and Cantalamessa, 2007; Spence and Tucker, 2007; Amorosi et al., 2017; Bruno et al., 2017; Zecchin et al., 2017a, 2017b). The concept of parasequence is commonly applied at scales of 100–101 m and 102–105 yrs., which coincide with the scales of high- resolution sequence stratigraphy. In contrast, the sequences of seismic stratigraphyaretypicallyrecognizedatscalesof101–102mand 105–106 yrs.; Vail et al., 1991, Duval et al., 1998; Schlager, 2010; Catuneanu, 2019a, 2019b). The observation of sequences at seismic scales led to the proposal of a scale-variant hierarchy system which postulates orderly patterns in the sedimentary record (i.e., bedsets < parasequences < sequences; Van Wagoner et al., 1990; Sprague et al., 2003; Neal and Abreu, 2009; Abreu et al., 2010). However, this scheme does not provide a reproducible standard, as parasequences and de- positional sequences of equal hierarchical ranks can coincide (e.g., in the case of orbital cycles; Strasser et al., 1999; Fielding et al., 2008; Tucker et al., 2009), or form side by side in tectonically active basins (Catuneanu et al., 2002; Gawthorpe et al., 2003; Zecchin, 2010). As summarized by Schlager (2010),“data on sequences of 103–107 years duration, the interval most relevant to practical application of sequence stratigraphy, do not conform well to the ordered-hierarchy model. Particularly unsatisfactory is the notion that the building blocks of classicalsequences(approximatedomain105–106 years)arepara- sequences bounded byflooding surfaces (Van Wagoner et al., 1990; Duval et al., 1998)”. In spite of the progress made by the publication of formal guidelines for sequence stratigraphy (Catuneanu et al., 2011), confusion still persists with respect to a number of key issues, including the scale of sequences and the difference between high-frequency sequences and parasequences. Some of these confusions are rooted in the historical development of the method, and stem from the scales of observation imposed by the resolution of the data that were used to define the concepts (e.g., in the context of seismic stratigraphy in the 1970s, the scale of sequences, systems tracts and depositional systems had to ex- ceed, by default, the vertical resolution of seismic data). This paper revisits the reasons for this nomenclatural conundrum, the nature of parasequences as stratigraphic units, and the solution for a standard nomenclature that is in line with the modern principles and realities of sequence stratigraphy. Fig. 1.Schematic cross-section of a parasequence along depositional dip, and two vertical sections showing ideal parasequences in proximal (A) and distal (B) locations (vertical sections courtesy of Steven Holland; modified from Van Wagoner et al., 1990). O. Catuneanu and M. ZecchinEarth-Science Reviews 208 (2020) 103289 2
2. History of the parasequence concept The origin of the‘parasequence’can be traced back to the concept of ‘paracycle’of relative sea level, defined as“the interval of time occu- pied by one regional or global relative rise and stillstand of sea level, followed by another relative rise, with no intervening relative fall”(Vail et al., 1977). The corresponding stratal unit was termed‘parasequence’ (Van Wagoner, 1985; Van Wagoner et al., 1988), defined as“a rela- tively conformable succession of genetically related beds and bedsets bounded by marineflooding surfaces and their correlative surfaces”. This formulation emulates the earlier definition of a‘sequence’as“a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities”, coined in the context of seismic stratigraphy (Mitchum, 1977). Important to the classification of stratigraphic cycles, the scales of sequences and parasequences at any location were inferred to be mu- tually exclusive, with parasequences being the building blocks of se- quences and component systems tracts (Van Wagoner et al., 1988, 1990). Sequences were envisaged to represent full cycles of relative sea- level rise and fall, whereas parasequences were assumed to form during relative sea-level rise. The inferred link between the paracycle and the relative sea level implies an allogenic origin for parasequences. How- ever, it is now known that several allogenic and autogenic controls can interplaytogenerateparasequences,includingeustasy,tectonism, compaction-drivensubsidence,andautogenicchangesinsediment supply (e.g., autocyclic delta-lobe switching). The interplay of allogenic and autogenic processes has been documented at multiple stratigraphic scales, starting with the smallest‘parasequence’scales. For this reason, the sequence stratigraphic methodology is now decoupled from the interpretation of underlying controls (Catuneanu, 2019a, 2020). The definitions of both sequences and parasequences make re- ference to‘relatively conformable’and‘genetically related’packages of strata, implying that any interruptions in deposition during their ac- cumulation are not significant enough to breach Walther's Law; i.e., the strata that comprise sequences and parasequences accumulate in lateral continuity to one another, in agreement with Walther's Law. Abrupt facies shifts that violate Walther's Law are expected at parasequence boundaries (i.e.,flooding surfaces, at the contact between coastal or shallow-water facies below and deeper water facies above) and at the unconformable portions of sequence boundaries. However, if the scales of sequences and parasequences are mutually exclusive, and the latter are nested within the former, sequences could no longer be‘relatively conformable’. A solution to this inconsistency is the notion that‘rela- tively conformable successions’can be observed at different scales, depending on the resolution of the stratigraphic study (Catuneanu, 2019b). In this case, the scale of a‘relatively conformable succession’ cannot be used as a reproducible reference for the classification of stratigraphic cycles. As envisaged by Van Wagoner et al. (1990), parasequences occupy a specific place within a hierarchical system of classification of strata, at the limit between sedimentological units (beds and bedsets; Campbell, 1967) and stratigraphic units observed at larger scales (systems tracts; Brown Jr. and Fisher, 1977). In this view, parasequences, which consist of beds and bedsets, would define the building blocks of systems tracts, and would represent the smallest stratigraphic units at any location. The sedimentological makeup of parasequences may be described in terms of beds and bedsets (Campbell, 1967) or in terms of facies and facies successions (Walker, 1992). More important for stratigraphic analysis is the identification of parasequence boundaries, which may provide the means to subdivide stratigraphic successions into geneti- cally related packages of strata separated by sharp facies contacts (Fig 1). Parasequences may consist of variable facies successions, depending on depositional setting and the location within the basin, with the component facies accumulated in the order prescribed by Walther's Law (Figs. 1, 2, 3). Theearlyhypothesesabouttheoriginsandrelativescalesof sequences and parasequences proved to be contentious (Posamentier and Allen, 1999; Catuneanu, 2006; Catuneanu et al., 2009, 2011; Miall, 2010; Schlager, 2010). Both parasequence boundaries (i.e.,flooding surfaces) and sequence boundaries (e.g., subaerial unconformities in the case of depositional sequences, or maximumflooding surfaces in the case of genetic stratigraphic sequences) can form at the same strati- graphic scales, in relation to the same cycles of relative sea-level change. Accommodation cycles are recorded at all scales, starting from the sedimentological scales of tidal cycles, and exposure surfaces are as common asflooding surfaces in the rock record (Vail et al., 1991; Schlager, 2004, 2010; Sattler et al., 2005; Fig. 4). Moreover, every transgression that leads to the formation of aflooding surface ends with amaximumfloodingobservedatthescaleofthattransgression. Therefore, the distinction between sequences and parasequences is not based on scale or accommodation conditions atsyn-depositional time, but on the nature of their bounding surfaces. 3. Stratigraphic sequences 3.1. Definition The definition of a‘sequence’was revised and improved over time, in response to conceptual advances, the increase in the resolution of stratigraphic studies, and the need to accommodate all sequence stra- tigraphic approaches (Fig. 5). Stratal stacking patterns are at the core of the sequence stratigraphic methodology, as they provide the criteria to define all units and surfaces of sequence stratigraphy, at scales defined by the purpose of study and/or by the resolution of the data available. In the most general sense, sequences correspond to stratigraphic cycles of change in stratal stacking patterns, defined by the recurrence of the same type of sequence stratigraphic surface in the sedimentary record (Fig. 5; Catuneanu and Zecchin, 2013). This definition is inclusive of all types of stratigraphic sequences (i.e.,‘depositional’,‘genetic strati- graphic’, and‘transgressive–regressive’; see Catuneanu, 2019a for a review). The definition of sequences and component systems tracts is independent of temporal and physical scales, age, and inferred under- lying controls. Sequences are subdivided into systems tracts, which are stratal units that can be mapped from continental through to deep-water settings on the basis of specific stacking patterns. In coastal to shallow-water set- tings, where parasequences may form, the stacking patterns that are diagnostic to the definition and identification of systems tracts are linked to the trajectory of subaerial clinoform rollovers (i.e., shoreline trajectories: progradation with upstepping, progradation with down- stepping, and retrogradation; Fig. 6). Systems tract boundaries are surfaces of sequence stratigraphy, irrespective of their physical ex- pressionandconformableorunconformablecharacter(Catuneanu et al., 2009, 2011). The attribute that they all have in common is the fact that they mark a change in stratal stacking pattern; e.g., a max- imumflooding surface is mapped at the limit between retrogradational strata below and progradational strata above, even though it may be lithologically cryptic within a conformable succession. The same types of sequence stratigraphic surfaces can be observed at different scales; e.g., maximumflooding surfaces of different hierarchical ranks form in relation to transgressions of different magnitudes (Fig. 7). 3.2. Scale of sequences A key aspect of the methodology and nomenclature is the scale at which sequences can be defined. In the context of seismic stratigraphy, the definition of a sequence as a‘relatively conformable succession’ (Mitchum, 1977; Fig. 5) inadvertently linked the scale of a sequence to the resolution of the data available. The subsequent definition of other types of stratigraphic cycles at smaller and larger scales (e.g.,‘para- sequences’below the scale of sequences, and‘composite sequences’and ‘megasequences’above the scale of sequences; Van Wagoner et al., O. Catuneanu and M. ZecchinEarth-Science Reviews 208 (2020) 103289 3
1988, 1990; Mitchum and Van Wagoner, 1991; Sprague et al., 2003; Neal and Abreu, 2009; Abreu et al., 2010) led to nomenclatural in- consistency, since the scale of the reference unit (i.e., the‘relatively conformable’sequence) varies with the resolution of the data available (Fig. 8). In reality, sequences do not occupy any specific niche within a framework of nested stratigraphic cycles. Sequences can be observed at all stratigraphic scales, depending on the geological setting (i.e., local conditions of accommodation and sedimentation), the resolution of the data available (e.g., seismic vs. well data or outcrops), and the scope of the study (e.g., petroleum exploration vs. production development) (see full discussion on sequence scales in Catuneanu, 2019b). A hierarchy system that is anchored to the resolution of the data available is superfluous, as it promotes a complex but volatile nomen- clature that changes with the acquisition of new data (e.g., a‘sequence’ defined with low resolution data becomes a‘composite sequence’when higher resolution data are acquired, which strips this terminology of stratigraphic meaning). In this context, the argument that the concept of‘systems tract’should only be applied at one scale within a frame- work of nested stratigraphic cycles (i.e., at the scale of‘relatively conformable’sequences; Neal and Abreu, 2009) isflawed by the fact that the scale of such units is tied to data resolution, and hence, it is variable. Sequences of any scale may include unconformities, whose identification depends on the resolution of the data available. Internal unconformities that are not resolvable with a low-resolution data set become bounding surfaces for smaller scale sequences in higher re- solution studies (Fig. 8). If high-resolution data were available in every study,‘relatively conformable’sequences may only be found at sub- seismic scales, which would render seismic stratigraphy obsolete. There is, however, a solution to‘save’seismic stratigraphy. In a more encompassing view, the scale of‘relatively conformable’succes- sions is set by the scale of observation rather than the resolution of the data available (Catuneanu, 2019b; Fig. 8). In this approach,‘relatively conformable’successions sensulargocan be observed at all strati- graphic scales, as stratal units whose internal unconformities are neg- ligiblerelativetothescaleoftheunitandofitsboundingun- conformities(Fig.8).Atthelargeststratigraphicscales,basin-fill sequences offirst order are relatively conformable successions in the sense that the internal unconformities are negligible relative to the scale of the sequence (i.e., they do not break the tectonic significance of thefirst-order sequence and the continuity in the paleogeographic evolution observed at the basin scale; Fig. 9). This scale-independent approach to the classification of stratigraphic cycles expands the ap- plication of Mitchum's (1977) definition of a‘sequence’to all strati- graphic scales, independently of data resolution. The use of a scale-variant nomenclature for stratigraphic cycles that developatdifferentscales(e.g.,parasequences < sequences < compositesequences < megasequences)isimpededfurtherbythe Fig.2.Coarsening-upwardparasequences inashallow-watersetting(Upper Cretaceous,WoodsideCanyon,Utah). Parasequences are commonly dominated by progradationaltrends,meterstotensof meters thick, but exceptions may occur in terms of scales and internal makeup. More importanttothedefinitionofpara- sequences,flooding surfaces mark abrupt increases in water depth (arrows). In this example, parasequences are c. 10 m thick, andflooding surfaces coincide with max- imum regressive surfaces. Fig. 3.Fining-upward parasequences in a tidalflat setting (Ordovician Juniata Formation,GermanyValley,WestVirginia;examplecourtesyofSteven Holland). Jacob staffis 1.5 m. In this example,flooding surfaces coincide with transgressive surfaces of erosion that replace maximum regressive surfaces. Fig. 4.Stratigraphic cycles in peritidal carbonates, driven by orbital forcing of different scales (Triassic, The Dolomites, Italy). In this example, the strati- graphic cycles satisfy the definition of both depositional sequences and para- sequences. Abbreviations: FS / SU–flooding surface (FS) superimposed on an exposure surface (subaerial unconformity, SU). O. Catuneanu and M. ZecchinEarth-Science Reviews 208 (2020) 103289 4
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