148
Channel-Floor Dune Fields. Fields of 3-D dunes (lithofacies St), characteristically occupy the deeper por�
tions of active channels wherever the bed load is
predominantly sand (e.g., Fig. 6.20). Lenses or lobes
of trough cross-bedded sand a few meters thick
and tens to hundreds of meters wide may result.
They may be cut by broad, shallow scours and
erosion surfaces of second and third order, indicating stage fluctuations (Harms et al. 1963; Harms
and Fahnestock 1965; McGowen and Garner 1970;
jackson 1976a; Cant and Walker 1978; Nijman
and Puigdefabregas 1978; Plint 1983; Buck 1983;
Stear 1983; Tyler and Ethridge 1983; Lawrence and
Williams 1987). In the shallower parts of channels,
including the tops and flanks of macroform elements (such as point bars and sand flats), transverse
bedforms and sand waves (2-D dunes) are common.
These generate sheets of planar-tabular crOssbedding (lithofacies Sp) (N.D. Smith 1970, 1971,
1972; Levey 1978; jackson 1976a; Nijman and
Puigdefabregas 1978; Cant and Walker 1978;
Blodgett and Stanley 1980; D.G. Smith 1983).
Fig. 6.21. Sandstone sheet (element SB) resting on reces�
sive coal bed, at base of outcrop. The sandstone unit con�
tains gently dipping accretion surfaces at left, indicating
that it developed in part by lateral progradation, but domiArchitectural Elements Formed Within Channels
Shallow Channel-Fill Assemblages. Miall (1977) defined a
distinctive lithofacies assemblage dominated by Sp
and showing little or no internal cyclicity (Fig. 5.20).
This was named the Platte type of braided river
deposit, after the Platte River, Nebraska. This assem�
blage represents the migration of fields of linguoid
or transverse bedforms, many of which are capped
or draped by Sr or Fl during falling water (N.D.
Smith 1970, 1971, 1972; Blodgett and Stanley 1980).
The "cross-bedded simple bars" of Allen (1983a)
are similar. Many ancient examples of this assemblage occur, for example, in the Cretaceous and
Lower Cenozoic deposits of Arctic Canada (MiaU
1976, 1984b) and the Nubian Sandstone (JnrassicCretaceous) of North Africa (Harms et a!. 1982;
Bhattacharyya and Lorenz 1983).
Crowley (1983) suggested that in at least some
cases these bedforms form part of much larger
macroform structures 200-400 m in length and
0.7-1.5 m high. The architecture of these structures
is discussed in the next section. One of their diagnos�
tic characteristics may be an upward-coarsening
nantly by vertical aggradation. It is interpreted as a crevasse�splay deposit in an upper delta-plain setting.
Pocahontas Basin, Carboniferous, Kentucky. (Horne and
Perm 1976)
Channel-Floor Dune Fields. Fields of 3-D dunes (lithofacies St), characteristically occupy the deeper por�
tions of active channels wherever the bed load is
predominantly sand (e.g., Fig. 6.20). Lenses or lobes
of trough cross-bedded sand a few meters thick
and tens to hundreds of meters wide may result.
They may be cut by broad, shallow scours and
erosion surfaces of second and third order, indicating stage fluctuations (Harms et al. 1963; Harms
and Fahnestock 1965; McGowen and Garner 1970;
jackson 1976a; Cant and Walker 1978; Nijman
and Puigdefabregas 1978; Plint 1983; Buck 1983;
Stear 1983; Tyler and Ethridge 1983; Lawrence and
Williams 1987). In the shallower parts of channels,
including the tops and flanks of macroform elements (such as point bars and sand flats), transverse
bedforms and sand waves (2-D dunes) are common.
These generate sheets of planar-tabular crOssbedding (lithofacies Sp) (N.D. Smith 1970, 1971,
1972; Levey 1978; jackson 1976a; Nijman and
Puigdefabregas 1978; Cant and Walker 1978;
Blodgett and Stanley 1980; D.G. Smith 1983).
Fig. 6.21. Sandstone sheet (element SB) resting on reces�
sive coal bed, at base of outcrop. The sandstone unit con�
tains gently dipping accretion surfaces at left, indicating
that it developed in part by lateral progradation, but domiArchitectural Elements Formed Within Channels
Shallow Channel-Fill Assemblages. Miall (1977) defined a
distinctive lithofacies assemblage dominated by Sp
and showing little or no internal cyclicity (Fig. 5.20).
This was named the Platte type of braided river
deposit, after the Platte River, Nebraska. This assem�
blage represents the migration of fields of linguoid
or transverse bedforms, many of which are capped
or draped by Sr or Fl during falling water (N.D.
Smith 1970, 1971, 1972; Blodgett and Stanley 1980).
The "cross-bedded simple bars" of Allen (1983a)
are similar. Many ancient examples of this assemblage occur, for example, in the Cretaceous and
Lower Cenozoic deposits of Arctic Canada (MiaU
1976, 1984b) and the Nubian Sandstone (JnrassicCretaceous) of North Africa (Harms et a!. 1982;
Bhattacharyya and Lorenz 1983).
Crowley (1983) suggested that in at least some
cases these bedforms form part of much larger
macroform structures 200-400 m in length and
0.7-1.5 m high. The architecture of these structures
is discussed in the next section. One of their diagnos�
tic characteristics may be an upward-coarsening
nantly by vertical aggradation. It is interpreted as a crevasse�splay deposit in an upper delta-plain setting.
Pocahontas Basin, Carboniferous, Kentucky. (Horne and
Perm 1976)