The Grouping of Architectural Units in Ciastic Rocks According to Depositional Time Scale
63
Group 4 deposits represent time periods of a few
days to a few months. Many of the depositional
events that occur over such a time frame were
termed "dynamic events" by jackson (1975). They
are events that move disproportionately large volumes of detritus in relatively short time periods.
Many marine storm events fall into this category.
Hummocky cross-stratification (HCS) sequences
are thought to form in a few days to a few weeks
during major storms. This time span also includes
the lunar neap-spring tidal cycle. Periodicity in tidal
bundles was observed by Visser (1980) and in tidal
ripples by Dalrymple and Makino (1989) and
equated with the lunar cycle. In fluvial systems, complete trains of mesoforms, such as dunes, may be
deposited during such time. spans.
Group 5 deposits are those that represent time
spans of 1 year to a few tens of years. Seasonal
deposits, such as spring runoff events and glacial
varves, are among the most important types of deposit in this category. The deposits of occasional
major storms (shelf environment) and flash floods
(fluvial environment), including those of strengths
occurring only every few years (e.g., McKee et al.
1967), are also group 5 deposits. Annual changes in
wind strength and direction lead to cyclic deposition
of eolian dunes, as has been recognized in several
ancient deposits (Hunter and Rubin 1983; Kocurek
et al. 1991).
Growth increments on macroforms in subaerial
and subaqueous environments are typical depositional products, and may be separated by large-scale
reactivation surfaces showing low-angle truncations
of underlying bedding (bounding surfaces are discussed later). Examples are shown in Fig. 3.5.
Coleman (1969) showed that in the Brahmaputra
River giant "sand waves" (possibly more accurately
classified as macroforms) are active primarily during seasonal floods. jackson's (1976) studies of the
Wabash River showed that meander bends in that
river migrated up to 2 km in 50 years, generating an
equivalent width of a new point bar. The Ucayali
River, a tributary of the upper Amazon in Peru,
shows comparable migration rates of 40-60 m/year
(Dumont and Fournier 1994). lseya and Ikeda (1989)
demonstrated that, in the ridge-and-swale topography of a point bar in a meandering river in Japan, the
recurrence interval of the ridges is estimated to be 25
years. However, preservation of point-bar deposits
depends on the direction and style of meander migration, which could include meander expansion,
translation, and rotation. Net sedimentation-accu�
mulation rates are highest during phases of meander
expansion (Jackson 1976b). Studies of the Atchafalaya delta in Louisiana indicate that sedimentation
during high-discharge events may add as much as
1000 km2 of new land to the delta during the next 50
years (Roberts et al. 1980).
Many storm sequences containing hummocky
cross-stratification, although developing in a few
days or weeks, may also represent the action of violent storms or storm groups that occur only every
few years (Dott and Bourgeois 1982). Separation of
such deposits from storm deposits of group 4 is
arbitrary in this classification, because there is probably a continuum of depositional variability between
the sequences that represent individual storms during an annual storm season, and those that represent
the "10-year" or "100-year" event. As noted later,
work on hurricane deposits is beginning to suggest
some criteria for discriminating storm magnitude in
carbonate environments, but there are no such data
relating to hummocky cross-stratification or other
terrigenous clastic deposits.
Group 6, in this classification, represents deposits
accumulating over time spans of hundreds to a
few thousands of years. Particularly violent dynamic events are thought to occur infrequently in
many environments, and are commonly termed for
convenience (if not accuracy) the "100-year events".
These would include violent hurricanes, catastr9phic flash floods, and sediment gravity-flow
events in many environments (in some settings,
sediment gravity-flows are infrequent enough to be
group 7 events). Many of the resulting depositional
sequences are very similar to those of group 5, such
as those described earlier, and could be distinguished only by such clues as the large volume or
thickness of the deposit, or the scale of the resulting
bedforms. For example, Wanless and others (1988)
took advantage of the passage of a major hurricane across the Caicos Platform in the British West
Indies to compare its effect on carbonate tidalflat sediments with that of "normal" winter storms.
Hurricanes, which are group 5 or 6 events in
this area, deposit centimeter-scale-thick beds of
peloidal grainstone, whereas winter storms (group
5) generate millimeter-thick laminae. Much more
work of this type is needed to provide criteria for the
recognition of event magnitude in the sedimentary
record.
Complete macro forms are group 6 deposits (Figs.
3.5, 3.6). Examples include point bars, sand flats, and
crevasse splays in fluvial systems, and similar elements in submarine channels; major segments of
coastal barriers and spits, and large washover fans in
63
Group 4 deposits represent time periods of a few
days to a few months. Many of the depositional
events that occur over such a time frame were
termed "dynamic events" by jackson (1975). They
are events that move disproportionately large volumes of detritus in relatively short time periods.
Many marine storm events fall into this category.
Hummocky cross-stratification (HCS) sequences
are thought to form in a few days to a few weeks
during major storms. This time span also includes
the lunar neap-spring tidal cycle. Periodicity in tidal
bundles was observed by Visser (1980) and in tidal
ripples by Dalrymple and Makino (1989) and
equated with the lunar cycle. In fluvial systems, complete trains of mesoforms, such as dunes, may be
deposited during such time. spans.
Group 5 deposits are those that represent time
spans of 1 year to a few tens of years. Seasonal
deposits, such as spring runoff events and glacial
varves, are among the most important types of deposit in this category. The deposits of occasional
major storms (shelf environment) and flash floods
(fluvial environment), including those of strengths
occurring only every few years (e.g., McKee et al.
1967), are also group 5 deposits. Annual changes in
wind strength and direction lead to cyclic deposition
of eolian dunes, as has been recognized in several
ancient deposits (Hunter and Rubin 1983; Kocurek
et al. 1991).
Growth increments on macroforms in subaerial
and subaqueous environments are typical depositional products, and may be separated by large-scale
reactivation surfaces showing low-angle truncations
of underlying bedding (bounding surfaces are discussed later). Examples are shown in Fig. 3.5.
Coleman (1969) showed that in the Brahmaputra
River giant "sand waves" (possibly more accurately
classified as macroforms) are active primarily during seasonal floods. jackson's (1976) studies of the
Wabash River showed that meander bends in that
river migrated up to 2 km in 50 years, generating an
equivalent width of a new point bar. The Ucayali
River, a tributary of the upper Amazon in Peru,
shows comparable migration rates of 40-60 m/year
(Dumont and Fournier 1994). lseya and Ikeda (1989)
demonstrated that, in the ridge-and-swale topography of a point bar in a meandering river in Japan, the
recurrence interval of the ridges is estimated to be 25
years. However, preservation of point-bar deposits
depends on the direction and style of meander migration, which could include meander expansion,
translation, and rotation. Net sedimentation-accu�
mulation rates are highest during phases of meander
expansion (Jackson 1976b). Studies of the Atchafalaya delta in Louisiana indicate that sedimentation
during high-discharge events may add as much as
1000 km2 of new land to the delta during the next 50
years (Roberts et al. 1980).
Many storm sequences containing hummocky
cross-stratification, although developing in a few
days or weeks, may also represent the action of violent storms or storm groups that occur only every
few years (Dott and Bourgeois 1982). Separation of
such deposits from storm deposits of group 4 is
arbitrary in this classification, because there is probably a continuum of depositional variability between
the sequences that represent individual storms during an annual storm season, and those that represent
the "10-year" or "100-year" event. As noted later,
work on hurricane deposits is beginning to suggest
some criteria for discriminating storm magnitude in
carbonate environments, but there are no such data
relating to hummocky cross-stratification or other
terrigenous clastic deposits.
Group 6, in this classification, represents deposits
accumulating over time spans of hundreds to a
few thousands of years. Particularly violent dynamic events are thought to occur infrequently in
many environments, and are commonly termed for
convenience (if not accuracy) the "100-year events".
These would include violent hurricanes, catastr9phic flash floods, and sediment gravity-flow
events in many environments (in some settings,
sediment gravity-flows are infrequent enough to be
group 7 events). Many of the resulting depositional
sequences are very similar to those of group 5, such
as those described earlier, and could be distinguished only by such clues as the large volume or
thickness of the deposit, or the scale of the resulting
bedforms. For example, Wanless and others (1988)
took advantage of the passage of a major hurricane across the Caicos Platform in the British West
Indies to compare its effect on carbonate tidalflat sediments with that of "normal" winter storms.
Hurricanes, which are group 5 or 6 events in
this area, deposit centimeter-scale-thick beds of
peloidal grainstone, whereas winter storms (group
5) generate millimeter-thick laminae. Much more
work of this type is needed to provide criteria for the
recognition of event magnitude in the sedimentary
record.
Complete macro forms are group 6 deposits (Figs.
3.5, 3.6). Examples include point bars, sand flats, and
crevasse splays in fluvial systems, and similar elements in submarine channels; major segments of
coastal barriers and spits, and large washover fans in