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Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Parasequences: Allostratigraphic misfits in sequence stratigraphy
Octavian Catuneanua,, Massimo Zecchinb
aDepartment of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada
bIstituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico, TS, Italy
A R T I C L EI N F O
Keywords:
Sequence stratigraphy
Stratigraphic cycles
Sequences
Parasequences
A B S T R A C T
Parasequences were introduced as the building blocks of seismic-scale systems tracts in the context of low-
resolution seismic stratigraphy. Pitfalls of this concept relate to the definition of parasequence boundaries as
lithological discontinuities that mark episodes of abrupt water deepening. With this general meaning,flooding
surfacesmay be facies contacts within transgressive deposits, or may coincide with different types of sequence
stratigraphic surfaces (maximum regressive, transgressive ravinement, or maximumflooding). In all cases,
flooding surfaces are allostratigraphic contacts restricted to coastal and shallow-water settings, where evidence
of abrupt water deepening can be demonstrated. Flooding surfaces may also be absent from the shallow-water
systems, where conformable successions accumulate during gradual water deepening. It follows that (1) para-
sequences have smaller extent than systems tracts, and (2) systems tracts do not always consist of stacked
parasequences. These limitations prevent the dependable use of the parasequence concept in sequence strati-
graphy.
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. Sequences that develop at parasequence scales provide a more reliable alter-
native for correlation, both within and outside of the coastal and shallow-water settings, rendering para-
sequences obsolete. Every transgression that affords the formation of aflooding surface starts from a maximum
regressive surface and ends with a maximumflooding surface observed at the scale of that transgression. These
systems tract boundaries are invariably more extensive than any facies contacts that may form during the
transgression. Flooding surfaces remain relevant to the description of facies relationships, but their stratigraphic
meaning needs to be assessed on a case-by-case basis. The use of sequences and systems tracts in high-resolution
studies provides consistency in methodology and nomenclature at all stratigraphic scales, irrespective of geo-
logical setting and the types and resolution of the data available.
1. Introduction
The sequence stratigraphic framework records a nested architecture
of stratigraphic cycles that can be observed at different scales, de-
pending on the purpose of the study and the resolution of the data
available. Much discussion and controversy surrounded the classifica-
tion and nomenclature of these cycles, with opinions ranging from the
use of thesequenceconcept at all stratigraphic scales (Vail et al., 1977;
Posamentier and Allen, 1999) to the use of different terms at different
stratigraphic scales (Van Wagoner et al., 1990; Mitchum Jr. and Van
Wagoner, 1991; Sprague et al., 2003; Neal and Abreu, 2009). In the
latter approach, thesequenceis arelatively conformablesuccession
relative to which smaller cycles (parasequences) and larger cycles
(compositesequences,megasequences)havebeendefined.This
nomenclatural issue has implications for the methodology, and so it is
important to resolve.
Perhaps the most contentious type of stratal unit in the scale-variant
classification system is theparasequence, due to its allostratigraphic
rather than sequence stratigraphic affinity. Problems with the para-
sequence concept have been pointed out in numerous studies (e.g.,
Krapez, 1996; Posamentier and Allen, 1999; Strasser et al., 1999;
Catuneanu, 2006; Zecchin, 2007, 2010; Catuneanu et al., 2009, 2010,
2011; Miall, 2010). Parasequences are bounded by facies contacts
which may or may not coincide with sequence stratigraphic surfaces,
and are usually assumed as shallowing-upward units with only minor or
no transgressive deposits (Fig. 1). Most authors consider parasequences
as units developed without intervening stages of relative sea-level fall,
despite the fact that they may pass laterally into units of the same rank
https://doi.org/10.1016/j.earscirev.2020.103289
Received 21 April 2020; Received in revised form 5 July 2020; Accepted 10 July 2020
Corresponding author.
E-mail address:octavian@ualberta.ca (O. Catuneanu).
Earth-Science Reviews 208 (2020) 103289

Available online 15 July 2020

0012-8252/ © 2020 Elsevier B.V. All rights reserved.

T
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
100101 m and 102105 yrs., which coincide with the scales of high-
resolution sequence stratigraphy. In contrast, the sequences of seismic
stratigraphyaretypicallyrecognizedatscalesof101102mand
105106 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 103107 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(approximatedomain105106 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 theparasequencecan be traced back to the concept of
paracycleof relative sea level, defined asthe 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 termedparasequence
(Van Wagoner, 1985; Van Wagoner et al., 1988), defined asa 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 asequenceasa
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 smallestparasequencescales. 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 torelatively conformableandgenetically relatedpackages 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 berelatively
conformable. A solution to this inconsistency is the notion thatrela-
tively conformable successionscan be observed at different scales,
depending on the resolution of the stratigraphic study (Catuneanu,
2019b). In this case, the scale of arelatively 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 asequencewas 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, andtransgressiveregressive; 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 arelatively 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-
sequencesbelow the scale of sequences, andcomposite sequencesand
megasequencesabove 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., therelatively
conformablesequence) 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., asequence
defined with low resolution data becomes acomposite sequencewhen
higher resolution data are acquired, which strips this terminology of
stratigraphic meaning). In this context, the argument that the concept
ofsystems tractshould only be applied at one scale within a frame-
work of nested stratigraphic cycles (i.e., at the scale ofrelatively
conformablesequences; 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 conformablesequences may only be found at sub-
seismic scales, which would render seismic stratigraphy obsolete.
There is, however, a solution tosaveseismic stratigraphy. In a
more encompassing view, the scale ofrelatively conformablesucces-
sions is set by the scale of observation rather than the resolution of the
data available (Catuneanu, 2019b; Fig. 8). In this approach,relatively
conformablesuccessions 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 asequenceto 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 / SUflooding 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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