Definition of Sediment Groups by Bounding Surfaces
3.3 Definition of Sediment Groups
by Bounding S11ofaces
For some clastic deposits, as noted ea·rlier, attempts
have been made to recognize the types of facies
composition, vertical succession, geometry, scale,
and bounding surfaces that characterize the deposits
formed over particular time scales. Where this can
be done it permits the deposits to be subdivided
empirically into a hierarchy of architectural units.
Characteristics of the bounding surfaces that enclose
these units may also include features useful for distinguishing the depositional units themselves, such
as the relationship of the surfaces to overlying and
underlying strata (erosional, gradational), their
shape (flat, irregular, concave- or convex-up), areal
extent) and the nature of associated facies (e.g., they
overlie mud drapes, or are followed by poorly sorted
lag deposits). However, sets of field criteria are not
yet available that can provide unambiguous interpretations of the time scale of the sedimentary processes for every field case. Many deposits, therefore,
cannot readily be classified into the groups listed in
Table 3.2, but could fall into any one of two or three
of the groups. For example, diurnal, seasonal, and
random meteorological processes may produce
similar resUlts in eolian (G. Kocurek, pers. comm.,
1989) and fluvial strata, particularly if preservation
of the products is incomplete. For these reasons, it is
not yet possible (and may never be) to establish an
all-encompassing classification of the architectural
subdivisions and bounding surfaces of all terrigenous clastic .deposits. A discussion of the progress
to date in this direction is the subject of the remainder of this section, and the fluvial bounding-surface
classification used in this book is described in Sect.
4.5.
Lamination and small�scale ripples develop rapidly beneath traction currents in a wide range of
sedimentary environments. These are group 1 and 2
deposits, respectively. The superimp�sition of trains
of ripples may generate surfaces of bedform climb,
or may be represented by virtually flat bounding
surfaces. These are set boundaries, in the terminology of McKee and Weir (1953). Those that occur in
fluvial sediments were classified as first-order surfaces by Miall (1988a,b). This type of surface is one
where little or no erosion is apparent, and the surface
records the virtually continuous sedimentation of
trains of similar bedforms. Changes in flow characteristics may lead to changes in the type of bedform,
with resulting changes in cross-bed style. Bounding
69
surfaces separating different assemblages of crossbed structures are coset boundaries (McKee and
W eir 1953), and were classified as second-order surfaces by Miall (1988a,b). Dynamic superimposition
of bedforms, such as the migration of megaripples
over sand waves (jackson 1975), may also lead to the
development of second-order surfaces.
In the tidal environment, the individual smallscale bedforms that make up class V and VI sand
waves are group 2 structures of the present classification. The boundaries between them were termed
E3 surfaces by Allen (1980).
In storm cycles containing hummocky crossstratification, two types of internal bounding surfaces have been recognized by Dott and Bourgeois
(1982). Individual laminae are separated by what
they termed third-order surfaces, and are thought to
represent lamination produced by individual wave
oscillations or pulsations of wave trains. This process represents periods of seconds or minutes, and
so the deposits are of group 1 or 2 in the present
classification.
Diurnal changes in bedform migration may develop discrete bundles of cross-strata, or rhythms,
commonly separated by minor reactivation surfaces.
Tidal bundles are particularly distinctive examples
of group 3 deposits. Boundaries between tidal
bundles are E2 surfaces, in Allen's (1980) terminology, and are fo rmed by erosion by the subordinate
current in the tidal cycle, according to him. However, Dalrymple (1984) showed that, at least in some
cases, the spacing between E2 surfaces is several
times the net bedform-migration distance per tidal
cycle. He interpreted E2 surfaces and variation in
tidal foreset thickness between the surfaces as a response to the arrival at the crest of the sand wave of
the peaks and troughs of superimposed megaripples.
The equivalent structures in fluvial environments
are simple reactivation surfaces, some of which
probably reflect diurnal changes in stage and therefore separate units of group 3 rank. The variations in
tidal-bundle thickness related to neap-spring
changes that were described by Visser ( 1980) are also
bounded by E2 surfaces. In this case, the surfaces are
useful for defining sediment packages of group 4
rank.
Pulses within individual storm events may occur
over periods of hours, days, or weeks, giving rise to
bundles of hummocky bedforms separated by second-order bounding surfaces (Dott and Bourgeois
classification), and representing group 3 deposits
(present classification). Storm events characteristically develop sequences that rest on scoured
3.3 Definition of Sediment Groups
by Bounding S11ofaces
For some clastic deposits, as noted ea·rlier, attempts
have been made to recognize the types of facies
composition, vertical succession, geometry, scale,
and bounding surfaces that characterize the deposits
formed over particular time scales. Where this can
be done it permits the deposits to be subdivided
empirically into a hierarchy of architectural units.
Characteristics of the bounding surfaces that enclose
these units may also include features useful for distinguishing the depositional units themselves, such
as the relationship of the surfaces to overlying and
underlying strata (erosional, gradational), their
shape (flat, irregular, concave- or convex-up), areal
extent) and the nature of associated facies (e.g., they
overlie mud drapes, or are followed by poorly sorted
lag deposits). However, sets of field criteria are not
yet available that can provide unambiguous interpretations of the time scale of the sedimentary processes for every field case. Many deposits, therefore,
cannot readily be classified into the groups listed in
Table 3.2, but could fall into any one of two or three
of the groups. For example, diurnal, seasonal, and
random meteorological processes may produce
similar resUlts in eolian (G. Kocurek, pers. comm.,
1989) and fluvial strata, particularly if preservation
of the products is incomplete. For these reasons, it is
not yet possible (and may never be) to establish an
all-encompassing classification of the architectural
subdivisions and bounding surfaces of all terrigenous clastic .deposits. A discussion of the progress
to date in this direction is the subject of the remainder of this section, and the fluvial bounding-surface
classification used in this book is described in Sect.
4.5.
Lamination and small�scale ripples develop rapidly beneath traction currents in a wide range of
sedimentary environments. These are group 1 and 2
deposits, respectively. The superimp�sition of trains
of ripples may generate surfaces of bedform climb,
or may be represented by virtually flat bounding
surfaces. These are set boundaries, in the terminology of McKee and Weir (1953). Those that occur in
fluvial sediments were classified as first-order surfaces by Miall (1988a,b). This type of surface is one
where little or no erosion is apparent, and the surface
records the virtually continuous sedimentation of
trains of similar bedforms. Changes in flow characteristics may lead to changes in the type of bedform,
with resulting changes in cross-bed style. Bounding
69
surfaces separating different assemblages of crossbed structures are coset boundaries (McKee and
W eir 1953), and were classified as second-order surfaces by Miall (1988a,b). Dynamic superimposition
of bedforms, such as the migration of megaripples
over sand waves (jackson 1975), may also lead to the
development of second-order surfaces.
In the tidal environment, the individual smallscale bedforms that make up class V and VI sand
waves are group 2 structures of the present classification. The boundaries between them were termed
E3 surfaces by Allen (1980).
In storm cycles containing hummocky crossstratification, two types of internal bounding surfaces have been recognized by Dott and Bourgeois
(1982). Individual laminae are separated by what
they termed third-order surfaces, and are thought to
represent lamination produced by individual wave
oscillations or pulsations of wave trains. This process represents periods of seconds or minutes, and
so the deposits are of group 1 or 2 in the present
classification.
Diurnal changes in bedform migration may develop discrete bundles of cross-strata, or rhythms,
commonly separated by minor reactivation surfaces.
Tidal bundles are particularly distinctive examples
of group 3 deposits. Boundaries between tidal
bundles are E2 surfaces, in Allen's (1980) terminology, and are fo rmed by erosion by the subordinate
current in the tidal cycle, according to him. However, Dalrymple (1984) showed that, at least in some
cases, the spacing between E2 surfaces is several
times the net bedform-migration distance per tidal
cycle. He interpreted E2 surfaces and variation in
tidal foreset thickness between the surfaces as a response to the arrival at the crest of the sand wave of
the peaks and troughs of superimposed megaripples.
The equivalent structures in fluvial environments
are simple reactivation surfaces, some of which
probably reflect diurnal changes in stage and therefore separate units of group 3 rank. The variations in
tidal-bundle thickness related to neap-spring
changes that were described by Visser ( 1980) are also
bounded by E2 surfaces. In this case, the surfaces are
useful for defining sediment packages of group 4
rank.
Pulses within individual storm events may occur
over periods of hours, days, or weeks, giving rise to
bundles of hummocky bedforms separated by second-order bounding surfaces (Dott and Bourgeois
classification), and representing group 3 deposits
(present classification). Storm events characteristically develop sequences that rest on scoured