Sandst ones: The Significance of Sand Body Architecture and Sedimentary Structures
435
modern Rio Puerco, New Mexico. The point bars are
up to 5 m thick and contain interbedded eolian
lenses. Turning to the ancient record of ephemeral�
stream sedimentation, Stear (1983) described ribbon
and composite sheet sandstone bodies in the Lower
Beaufort Group (Permian) of the Karoo Basin, south
Africa, that were deposited in channels up to 5 m
deep and 100 m wide. Many of the sandstone bodies
consist of accretionary bundles of plane laminated
sandstone (lithofacies Sh) and trough cross-bedded
sandstone (St) separated by numerous internal
scour surfaces. Lateral-accretion surfaces within
many of the sheet sandstones indicate the development of point bars. Well-developed fining-upward
cycles are also present (Turner 1978). Various lines
of evidence, including numerous scour surfaces,
intraclast breccias, and desiccation features, indicate an ephemeral, flash-flood style of sedimentation. In a later paper, Stear (1985) was able to
compare these deposits with the products of a recent
flash flood in the arid Karoo Basin. Mufioz et al.
(1992) described accretionary bundles within Triassic lateral-accretion deposits in Spain. They commonly show a cyclic character, indicating that they
were formed by floods or seasonal high-discharge
events. The Westwater Canyon Member of the
Morrison Formation (Jurassic) in New Mexico consists largely of sandstone sheets several meters thick
organized into macroforms (Cowan 1991) and containing well-developed fining-upward cycles of &everal types (Godin 1991). This unit has also been
interpreted as having been deposited by flash-flood
sedimentation in moderate-sized channels (Cowan
1991) under arid climatic conditions (Parrish 1993).
In their study of a Devonian unit in Quebec,
Lawrence and Williams (1987) described lateral-accretion deposits which they compared with the
ephemeral Rio Puerco examples described by Shepherd (1987). These fluvial styles are described further in Sects. 8.2.8 and 8.2.14. In their study of
terminal fans, Kelly and Olsen (1993) erected a
«feeder zone" lithofacies assemblage for the deposits
of the main channel in the apical regions of the fan.
This channel may be ephemeral, or perennial with
widely fluctuating discharge. The lithofacies are
characterized by channel sand bodies up to several
meters thick (their examples indicate channel
depths up to 6 m) and may contain the evidence of
braid-bar complexes (Sect. 8.3).
The presence of channels seVeral meters deep or
cross-bedded assemblages organized into macroforms with evidence oflateral or downstream accre�
tion (architectural elements LA and DA) should not,
therefore, be interpreted as necessarily indicating
perennial flow conditions. The presence of numerous internal scour surfaces indicates frequent discharge fluctuations. Macroforms consisting of
accretionary bundles a few decimeters thick, separated by third�order erosion surfaces, as described
by Stear (1983), Shepherd (1987), Langford and
Chan (1989), and Mufioz et al. (1992), maybe indicative of ephemeral or strongly seasonal conditions
(Fig. 12.2B).
Another possible architectural indicator of highdischarge sedimentation, such as is characteristic of
flash floods, is the scour hollow (architectural element HO). Deep scours form at channel confluences,
as observed in flumes and in modern rivers, and a
facies model for the deposits of these scours has been
proposed by Bristow et al. (1993). This element is
described and illustrated in Sect. 6.9.
Numerous studies have confirmed the predominance of unchannelized sheet flow in the medial to
distal parts of arid braidplains, based on observations of modern floods (McKee et al. 1967; Williams
1971) and ancient deposits (Miall et al. 1978;
Tunbridge 1981, 1984; Smoot 1983; Mertz and
Hubert 1990; Hirst 1991; Dreyer 1993b; Kelly and
Olsen 1993), The proportion of interbedded finegrained deposits increases distally, depending upon
the amount of fines in the sediment load. The
presence of abundant sheet sandstone deposits decimeters to a few meters thick, showing very little
evidence of channelization and no macro forms, but
characterized by fining-upward cycles a few meters
thick or less (commonly only a few decimeters) and
interbedded with playa mudflats with abundant evidence of desiccation (mudcracks, evaporite nodules), is now regarded as a definitive indication of
ephemeral, flash-flood sedimentation (Sects. 8.2.15,
8.2.16). These observations have now been incorporated into the terminal fan model of Kelly and Olsen
(1993; Sect. 8.3). This style of sedimentation is illustrated in Fig. 12.2B.
Feeder streams along the margins of modern arid
basins commonly occupy narrow, steep-sided canyons. In the arid southwestern United States they
are termed box canyons or arroyos. The distinctive
shape is probably generated by erosion during sudden floods in unvegetated areas (Bendix 1992). Such
channels are rarely preserved (an example is illustrated in Figs. 6.11 and 6.12), but arroyo erosion may
deliver large quantitites of unconsolidated or loosely
consolidated sediment, including pedogenic nodules, into the dispersal system (Marriott and Wright
1993).
435
modern Rio Puerco, New Mexico. The point bars are
up to 5 m thick and contain interbedded eolian
lenses. Turning to the ancient record of ephemeral�
stream sedimentation, Stear (1983) described ribbon
and composite sheet sandstone bodies in the Lower
Beaufort Group (Permian) of the Karoo Basin, south
Africa, that were deposited in channels up to 5 m
deep and 100 m wide. Many of the sandstone bodies
consist of accretionary bundles of plane laminated
sandstone (lithofacies Sh) and trough cross-bedded
sandstone (St) separated by numerous internal
scour surfaces. Lateral-accretion surfaces within
many of the sheet sandstones indicate the development of point bars. Well-developed fining-upward
cycles are also present (Turner 1978). Various lines
of evidence, including numerous scour surfaces,
intraclast breccias, and desiccation features, indicate an ephemeral, flash-flood style of sedimentation. In a later paper, Stear (1985) was able to
compare these deposits with the products of a recent
flash flood in the arid Karoo Basin. Mufioz et al.
(1992) described accretionary bundles within Triassic lateral-accretion deposits in Spain. They commonly show a cyclic character, indicating that they
were formed by floods or seasonal high-discharge
events. The Westwater Canyon Member of the
Morrison Formation (Jurassic) in New Mexico consists largely of sandstone sheets several meters thick
organized into macroforms (Cowan 1991) and containing well-developed fining-upward cycles of &everal types (Godin 1991). This unit has also been
interpreted as having been deposited by flash-flood
sedimentation in moderate-sized channels (Cowan
1991) under arid climatic conditions (Parrish 1993).
In their study of a Devonian unit in Quebec,
Lawrence and Williams (1987) described lateral-accretion deposits which they compared with the
ephemeral Rio Puerco examples described by Shepherd (1987). These fluvial styles are described further in Sects. 8.2.8 and 8.2.14. In their study of
terminal fans, Kelly and Olsen (1993) erected a
«feeder zone" lithofacies assemblage for the deposits
of the main channel in the apical regions of the fan.
This channel may be ephemeral, or perennial with
widely fluctuating discharge. The lithofacies are
characterized by channel sand bodies up to several
meters thick (their examples indicate channel
depths up to 6 m) and may contain the evidence of
braid-bar complexes (Sect. 8.3).
The presence of channels seVeral meters deep or
cross-bedded assemblages organized into macroforms with evidence oflateral or downstream accre�
tion (architectural elements LA and DA) should not,
therefore, be interpreted as necessarily indicating
perennial flow conditions. The presence of numerous internal scour surfaces indicates frequent discharge fluctuations. Macroforms consisting of
accretionary bundles a few decimeters thick, separated by third�order erosion surfaces, as described
by Stear (1983), Shepherd (1987), Langford and
Chan (1989), and Mufioz et al. (1992), maybe indicative of ephemeral or strongly seasonal conditions
(Fig. 12.2B).
Another possible architectural indicator of highdischarge sedimentation, such as is characteristic of
flash floods, is the scour hollow (architectural element HO). Deep scours form at channel confluences,
as observed in flumes and in modern rivers, and a
facies model for the deposits of these scours has been
proposed by Bristow et al. (1993). This element is
described and illustrated in Sect. 6.9.
Numerous studies have confirmed the predominance of unchannelized sheet flow in the medial to
distal parts of arid braidplains, based on observations of modern floods (McKee et al. 1967; Williams
1971) and ancient deposits (Miall et al. 1978;
Tunbridge 1981, 1984; Smoot 1983; Mertz and
Hubert 1990; Hirst 1991; Dreyer 1993b; Kelly and
Olsen 1993), The proportion of interbedded finegrained deposits increases distally, depending upon
the amount of fines in the sediment load. The
presence of abundant sheet sandstone deposits decimeters to a few meters thick, showing very little
evidence of channelization and no macro forms, but
characterized by fining-upward cycles a few meters
thick or less (commonly only a few decimeters) and
interbedded with playa mudflats with abundant evidence of desiccation (mudcracks, evaporite nodules), is now regarded as a definitive indication of
ephemeral, flash-flood sedimentation (Sects. 8.2.15,
8.2.16). These observations have now been incorporated into the terminal fan model of Kelly and Olsen
(1993; Sect. 8.3). This style of sedimentation is illustrated in Fig. 12.2B.
Feeder streams along the margins of modern arid
basins commonly occupy narrow, steep-sided canyons. In the arid southwestern United States they
are termed box canyons or arroyos. The distinctive
shape is probably generated by erosion during sudden floods in unvegetated areas (Bendix 1992). Such
channels are rarely preserved (an example is illustrated in Figs. 6.11 and 6.12), but arroyo erosion may
deliver large quantitites of unconsolidated or loosely
consolidated sediment, including pedogenic nodules, into the dispersal system (Marriott and Wright
1993).