Overbank Fines: The Significance of Bedding and Minor Sedimentary Structures
437
the scale of cross-bedding have been observed in
some of these deposits. Much work remains to be
done to document the details of the architecture and
vertical profile of these macro forms under different
flow conditions, and at present there are inadequate
data for jones' (1977) ideas regarding discharge regimes to be tested.
Luttrell (1993) suggested that ephemeral streams
are characterized by a paucity of bedforms formed at
intermediate flow strengths, notably two-dimensional dunes (lithofacies Sp). She suggested that
"the association of high-energy structures grading
vertically and laterally into waning-flow structures
reflects ephemeral flood events.'' It is a matter of
observation that fluvial systems containing abundant planar-tabular cross-bedding (Sp) are predominantlY shallow perennial streams, . or at least
show steady summer discharges, the modern Platte
and William rivers being excellent examples (Smith
1970; Blodgett and Stanley 1980; Smith and Smith
1984; Sect. 8.2.12). Planar-tabular cross-bedding also
forms a significant component of many deep, perellnial, sand-bed braided rivers, such as the modern
South Saskatchewan, where fields of three-dimensional dunes commonly occupy the deeper channels
and large, flat-topped two-dimensional dunes develop in shallower reaches (Cant and Walker 1978;
Sect. 8.2.13). This river was the focus of a useful
discussion of channel aggradation and the paleocurrent characteristics of different bedform types by
Cant and Walker (1978). Flume observations indicate that at any given flow depth two-dimensional
dunes form at lower flow velocities than threedimensional forms (Sect. 5.3.1).
Flood cycles in ephemeral streams commonly
consist of sets of St overlain by Sh or Sr, reflecting
high-energy sheet flow followed by a rapid decline in
discharge. Planar-tabular cross-bed sets (Sp) may be
present, but the large flat-topped bedforms are not
as likely to form as in perennial rivers. Reactivation
surfaces within bedforms are rare, most forms being
deposited rapidly in a single event and then completely abandoned. Many variations on this succession have been described following work in various
modern and ancient ephemeral systems (McKee et
al. 1967; Mia!! and Gibling 1978; Williams 1971;
Tunbridge 1981, 1984; Sneh 1983; Glennie 1987;
Luttrell 1993; Dreyer 1993b; Sects. 8.2.15, 8.2.16).
Godin (1991) documented three scales of autogenic cyclicity in the Westwater Canyon member of
the Morrison Formation, in northern New Mexico, a
unit probably formed under high-energy ephemeral
flow. As discussed earlier, this unit contains the distinctive "hollow" architectural element formed
by scour at channel confluences (Cowan 1991). The
three types of cycle are related to macro form accretion, channel filling, and overall sand body aggradation (Sect. 10.3.2). A common lithofacies association
is that between horizontally laminated sandstone
(Sh) and low-angle cross-bedded sandstone (Sl). The
low-angle cross-bedding is commonly asymptotic
with reference to upper and lower set bounding
surfaces. Parting lineation may be present, with an
orientation parallel or oblique to the dip of the crossbedding. Some occurrences of lithofacies Sl represent plane beds deposited on initially dipping
surfaces, such as scour hollows. In other cases the
lithofacies represents distinct bedform geometries.
The washed-out dunes and humpback dunes that
occur at the transition between subcritical and
supercritical flow typically form this lithofacies
(Saunderson and Lockett 1983).
Observations of modern floods by Frostick and
Reid (1977) and Stear (1985) indicated that they
oG:cur in pulses. The passage of storms across different parts of a catchment area triggers waves of highenergy flow from different tributary systems which
arrive in the depositional area at different times.
This may be recorded in the deposits in the form of
changes in the thicknesses of flood laminae (Frostick
and Reid 1977) or as heterogeneous lithologies
(Stear 1985).
12.7 Overbank Fines: The Significance
of Bedding ami MnnQr Sedimentai'j/ Structures
Overbank areas are the main sites for the accumulation of various climatically sensitive chemical and
biochemical deposits, which are discussed in a later
section. However, the detrital sediments can also
yield some clues as to discharge regime, which depends on climate (Fig. 12.3).
Interfluve areas flanking the large channels of
perennial, seasonal, or ephemeral rivers are areas
that are only spasmodically affected by high-discharge events. In tropical-humid areas the rivers
may be flanked by raised peat bogs that keep out
overbank flows (McCabe 1984) or by swamps that
absorb them (Stanistreet and McCarthy 1993). In
temperate-humid and arid settings rare flows
may spread over the floodplains, and normal overbank processes lead to the generation of crevasse
channels and crevasse splays (Sect. 7.2), the geometry and composition of which convey little useful
437
the scale of cross-bedding have been observed in
some of these deposits. Much work remains to be
done to document the details of the architecture and
vertical profile of these macro forms under different
flow conditions, and at present there are inadequate
data for jones' (1977) ideas regarding discharge regimes to be tested.
Luttrell (1993) suggested that ephemeral streams
are characterized by a paucity of bedforms formed at
intermediate flow strengths, notably two-dimensional dunes (lithofacies Sp). She suggested that
"the association of high-energy structures grading
vertically and laterally into waning-flow structures
reflects ephemeral flood events.'' It is a matter of
observation that fluvial systems containing abundant planar-tabular cross-bedding (Sp) are predominantlY shallow perennial streams, . or at least
show steady summer discharges, the modern Platte
and William rivers being excellent examples (Smith
1970; Blodgett and Stanley 1980; Smith and Smith
1984; Sect. 8.2.12). Planar-tabular cross-bedding also
forms a significant component of many deep, perellnial, sand-bed braided rivers, such as the modern
South Saskatchewan, where fields of three-dimensional dunes commonly occupy the deeper channels
and large, flat-topped two-dimensional dunes develop in shallower reaches (Cant and Walker 1978;
Sect. 8.2.13). This river was the focus of a useful
discussion of channel aggradation and the paleocurrent characteristics of different bedform types by
Cant and Walker (1978). Flume observations indicate that at any given flow depth two-dimensional
dunes form at lower flow velocities than threedimensional forms (Sect. 5.3.1).
Flood cycles in ephemeral streams commonly
consist of sets of St overlain by Sh or Sr, reflecting
high-energy sheet flow followed by a rapid decline in
discharge. Planar-tabular cross-bed sets (Sp) may be
present, but the large flat-topped bedforms are not
as likely to form as in perennial rivers. Reactivation
surfaces within bedforms are rare, most forms being
deposited rapidly in a single event and then completely abandoned. Many variations on this succession have been described following work in various
modern and ancient ephemeral systems (McKee et
al. 1967; Mia!! and Gibling 1978; Williams 1971;
Tunbridge 1981, 1984; Sneh 1983; Glennie 1987;
Luttrell 1993; Dreyer 1993b; Sects. 8.2.15, 8.2.16).
Godin (1991) documented three scales of autogenic cyclicity in the Westwater Canyon member of
the Morrison Formation, in northern New Mexico, a
unit probably formed under high-energy ephemeral
flow. As discussed earlier, this unit contains the distinctive "hollow" architectural element formed
by scour at channel confluences (Cowan 1991). The
three types of cycle are related to macro form accretion, channel filling, and overall sand body aggradation (Sect. 10.3.2). A common lithofacies association
is that between horizontally laminated sandstone
(Sh) and low-angle cross-bedded sandstone (Sl). The
low-angle cross-bedding is commonly asymptotic
with reference to upper and lower set bounding
surfaces. Parting lineation may be present, with an
orientation parallel or oblique to the dip of the crossbedding. Some occurrences of lithofacies Sl represent plane beds deposited on initially dipping
surfaces, such as scour hollows. In other cases the
lithofacies represents distinct bedform geometries.
The washed-out dunes and humpback dunes that
occur at the transition between subcritical and
supercritical flow typically form this lithofacies
(Saunderson and Lockett 1983).
Observations of modern floods by Frostick and
Reid (1977) and Stear (1985) indicated that they
oG:cur in pulses. The passage of storms across different parts of a catchment area triggers waves of highenergy flow from different tributary systems which
arrive in the depositional area at different times.
This may be recorded in the deposits in the form of
changes in the thicknesses of flood laminae (Frostick
and Reid 1977) or as heterogeneous lithologies
(Stear 1985).
12.7 Overbank Fines: The Significance
of Bedding ami MnnQr Sedimentai'j/ Structures
Overbank areas are the main sites for the accumulation of various climatically sensitive chemical and
biochemical deposits, which are discussed in a later
section. However, the detrital sediments can also
yield some clues as to discharge regime, which depends on climate (Fig. 12.3).
Interfluve areas flanking the large channels of
perennial, seasonal, or ephemeral rivers are areas
that are only spasmodically affected by high-discharge events. In tropical-humid areas the rivers
may be flanked by raised peat bogs that keep out
overbank flows (McCabe 1984) or by swamps that
absorb them (Stanistreet and McCarthy 1993). In
temperate-humid and arid settings rare flows
may spread over the floodplains, and normal overbank processes lead to the generation of crevasse
channels and crevasse splays (Sect. 7.2), the geometry and composition of which convey little useful