!50
nels (Fig. 6.22). Aggradation and progressive abandonment of these channels occur slowly or during
single flood events. In either case, fining-upward
cycles are commonly the typical result. Williams
(1971) and Hardie et al. (1978) described modern
examples, and Miall and Gibling (1978) documented
an ancient example. In the latter case, cycles are
mostly between 1 and 3 m in thickness, and show
an upward transition from a scoured base through
Sh, Sp, or St, to Sr and Fl, or directly to Fl omitting
Sr. Similar deposits characterize the arid «terminal
fan" deposits of northern India (Parkash et al. 1983)
and elsewhere, a depositional style that is gaining
increasing recognition in the ancient record (Sect.
8.3).
Other Occurrences OfE/ementSB. In the pre-Devonian, the
lack of vegetation is thought to have resulted in a
predominance of weakly channelized bed load
streams (Schumm 1968a; Cotter 1978). The architecture and composition of the resulting fluvial deposits were probably in many cases similar to the distal
braid-plain sand sheets described here. Long (1978)
discussed some Proterozoic examples.
Fig. 6.23. Example of a midchannel sand flat in a modern
sandy-braided river. These compound bars develop by
lateral and downstream accretion, and contain internal
Architectural Elements Formed Within Channels
Particularly vigorous flood events in ephemeral
channels may produce a distinctive type of lithofacies assemblage and sand body geometry, described below under the heading of element LS.
A few workers have documented the ocurrence of
giant cross-bedding, indicating the former existence
of deep channels which migrated down very large
bedforms. Such bedforms are mesoforms. They are
internally of simple structure, with steep foreset dips
(e.g., Fig. 5.21), and are not to be confused with the
more complex macroforms described in the next
section. Giant bedforms are particularly common in
the giant sandy rivers draining the Himalayas, where
sand waves up to 15 m high have been recorded
(Coleman 1969; Singh and Kumar 1974). However, as
noted below, at least some of these sand waves are
probably macro forms. Possible ancient analogs were
described by Conaghan and jones (1975). McCabe
(1977) and jones and McCabe (1980) analyzed cross :
bed sets up to 40 m thick and 1 km wide that they
interpreted as the deposits oflarge prograding alternate bars in a major delta distributary (Fig. 2.27).
Reactivation surfaces (third-order surfaces) were interpre t ed in terms of fluctuating water depth. If, as in
bounding surfaces of first- to third-order dipping in
the direction of accretion. (Photograph courtesy of D.G.
Smith)
nels (Fig. 6.22). Aggradation and progressive abandonment of these channels occur slowly or during
single flood events. In either case, fining-upward
cycles are commonly the typical result. Williams
(1971) and Hardie et al. (1978) described modern
examples, and Miall and Gibling (1978) documented
an ancient example. In the latter case, cycles are
mostly between 1 and 3 m in thickness, and show
an upward transition from a scoured base through
Sh, Sp, or St, to Sr and Fl, or directly to Fl omitting
Sr. Similar deposits characterize the arid «terminal
fan" deposits of northern India (Parkash et al. 1983)
and elsewhere, a depositional style that is gaining
increasing recognition in the ancient record (Sect.
8.3).
Other Occurrences OfE/ementSB. In the pre-Devonian, the
lack of vegetation is thought to have resulted in a
predominance of weakly channelized bed load
streams (Schumm 1968a; Cotter 1978). The architecture and composition of the resulting fluvial deposits were probably in many cases similar to the distal
braid-plain sand sheets described here. Long (1978)
discussed some Proterozoic examples.
Fig. 6.23. Example of a midchannel sand flat in a modern
sandy-braided river. These compound bars develop by
lateral and downstream accretion, and contain internal
Architectural Elements Formed Within Channels
Particularly vigorous flood events in ephemeral
channels may produce a distinctive type of lithofacies assemblage and sand body geometry, described below under the heading of element LS.
A few workers have documented the ocurrence of
giant cross-bedding, indicating the former existence
of deep channels which migrated down very large
bedforms. Such bedforms are mesoforms. They are
internally of simple structure, with steep foreset dips
(e.g., Fig. 5.21), and are not to be confused with the
more complex macroforms described in the next
section. Giant bedforms are particularly common in
the giant sandy rivers draining the Himalayas, where
sand waves up to 15 m high have been recorded
(Coleman 1969; Singh and Kumar 1974). However, as
noted below, at least some of these sand waves are
probably macro forms. Possible ancient analogs were
described by Conaghan and jones (1975). McCabe
(1977) and jones and McCabe (1980) analyzed cross :
bed sets up to 40 m thick and 1 km wide that they
interpreted as the deposits oflarge prograding alternate bars in a major delta distributary (Fig. 2.27).
Reactivation surfaces (third-order surfaces) were interpre t ed in terms of fluctuating water depth. If, as in
bounding surfaces of first- to third-order dipping in
the direction of accretion. (Photograph courtesy of D.G.
Smith)