430
"A semi-arid climate during the deposition of at least part
of the Budleigh Salterton Pebble B!!ds is suggested by the
evidence for large variations of discharge (e.g. drapes of the
scour troughs of sand dunes and drapes of the lower portions of gravel macroforms). Also, rare aeolian sandstones
have been found in the upper part of the sequence, where
they occur as thin lenses capping bar tops."
The conglomerates consist entirely of stratified gravels, commonly with cross-bedding, organized into
large in-channel macroforms, and were deposited by
traction-currents in gravel-bed braided rivers (Figs.
6. 18, 8. 21). The term braid-delta would be more suitable for these deposits, following the recommendations of Blair and McPherson (1994).
12.5.1.2 Temperate-Humid Climates
Several studies have documented mass�flow processes in modern temperate-humid climates. Debris
flows are particularly important in mountainous regions of southwestern Japan (Ono 1990) and New
Zealand (Pierson 1980), where in many cases they
contribute to the reworking and redeposition of
glacial fan deposits. Kochel and Johnson (1984) described active fans in Virginia. The fan deposits consist of poorly sorted, mud-supported gravels with
inverse grading and poor stratification, with a bed
thickness varying from 50 em to 5 m. The main depositional process is interpreted as debris flows and
debris avalanches triggered by intense rainfall. From
20 to 70 % of the fan surface is affected at any one
time. The recurrence interval of flood events is estimated at 3- 6 ka, based on radiocarbon dating. Colluvial processes are also very important. This study was
expanded to other parts of the Appalachians by
Kochel (1990), who confirmed the widespread distribution of debris-flow-dominated fans but also documented the local occurrence of fans formed by
stream-flow and hyperconcentrated flow processes,
where lithologic controls on source sediment and
hydrologic controls on runoff prevented the generation of debris flows. Wells and Harvey (1987) described the results of a catastrophic cloudburst in
northern England. In a temperate�humid environment such rare catastrophic floods can generate erosional and depositional effects that may overshadow
the products oe
flows. In this particular study, which examined numerous fans, the debris-flow deposits were found to
constitute a greater part of the accumulated sediment
on the fans draining smaller, steeper drainage areas.
The description of a small modern fan by Blair (1987)
What Does Fluvial Lithofacies Reveal About Climate?
indicates an absence of debris-flow deposits, although he reported evidence of hyperconcentrated
flows.
It is perhaps significant that several recent
descriptions of ancient alluvial-fan/braid-delta deposits from interpreted temperate-humid climatic
regimes indicate that stream-flow deposits are
dominant, and that mass-flow deposits are of minor
importance. This depositional style is illustrated in
Fig. 12. 1B. In the study by Crews and Ethridge
(1993), the dominance of braided stream-flow traction processes was demonstrated, with evidence of
lateral channel migration. Debris flows were determined to be rare to absent. Likewise, DeCelles et al.
(1991) reported mass-flow deposits in only one of
numerous localities. In their study, Ridgway and
DeCelles (1993) drew comparisons with the modern
gravel sandurs of southern Alaska (Boothroyd and
Nummedal 197 8). Similar clast-supported gravels
deposited by stream-flow processes are the dominant facies in their rocks. Debris flows are rare. One
possible inference that could be drawn from the
studies noted here is that while mass-flow processes
may be very spectacular in modern humid-temperate settings, they may have low preservation
potential and are therefore of limited geological
significance. However, this ancient/modern contrast is based on only a few reports and should not
be regarded as definitive.
12.5. 1.3 Tropical-Humid Climates
Deposits containing significant mass-flow deposits
and examples with a lack of such deposits have both
been described in case studjes of the modern and
ancient record.
Modern fan/braid-delta deposits in Papua-New
Guinea were described by Brierley et a!. (1993). The
area has a monsoonal, savannah-type·climate, with
a significant dry season. In addition to stream-flow
deposits, debris flows and hyperconcentrated flows
are present, as indicated by gravel textures. The
mass flow deposits are limited to proximal reaches
of the fans, where they comprise up to about 30% of
the total. However, their preservation potential is
estimated to be low, because of continued incision
of the deposits, and this is aggravated by syndepositional tectonic uplift. The modern Yallahs braid
delta of J amaica lacks debris-flow deposits (Wescott
1990).
Several ancient conglomerate deposits have been
interpreted to have been formed in a tropical-
"A semi-arid climate during the deposition of at least part
of the Budleigh Salterton Pebble B!!ds is suggested by the
evidence for large variations of discharge (e.g. drapes of the
scour troughs of sand dunes and drapes of the lower portions of gravel macroforms). Also, rare aeolian sandstones
have been found in the upper part of the sequence, where
they occur as thin lenses capping bar tops."
The conglomerates consist entirely of stratified gravels, commonly with cross-bedding, organized into
large in-channel macroforms, and were deposited by
traction-currents in gravel-bed braided rivers (Figs.
6. 18, 8. 21). The term braid-delta would be more suitable for these deposits, following the recommendations of Blair and McPherson (1994).
12.5.1.2 Temperate-Humid Climates
Several studies have documented mass�flow processes in modern temperate-humid climates. Debris
flows are particularly important in mountainous regions of southwestern Japan (Ono 1990) and New
Zealand (Pierson 1980), where in many cases they
contribute to the reworking and redeposition of
glacial fan deposits. Kochel and Johnson (1984) described active fans in Virginia. The fan deposits consist of poorly sorted, mud-supported gravels with
inverse grading and poor stratification, with a bed
thickness varying from 50 em to 5 m. The main depositional process is interpreted as debris flows and
debris avalanches triggered by intense rainfall. From
20 to 70 % of the fan surface is affected at any one
time. The recurrence interval of flood events is estimated at 3- 6 ka, based on radiocarbon dating. Colluvial processes are also very important. This study was
expanded to other parts of the Appalachians by
Kochel (1990), who confirmed the widespread distribution of debris-flow-dominated fans but also documented the local occurrence of fans formed by
stream-flow and hyperconcentrated flow processes,
where lithologic controls on source sediment and
hydrologic controls on runoff prevented the generation of debris flows. Wells and Harvey (1987) described the results of a catastrophic cloudburst in
northern England. In a temperate�humid environment such rare catastrophic floods can generate erosional and depositional effects that may overshadow
the products oe
flows. In this particular study, which examined numerous fans, the debris-flow deposits were found to
constitute a greater part of the accumulated sediment
on the fans draining smaller, steeper drainage areas.
The description of a small modern fan by Blair (1987)
What Does Fluvial Lithofacies Reveal About Climate?
indicates an absence of debris-flow deposits, although he reported evidence of hyperconcentrated
flows.
It is perhaps significant that several recent
descriptions of ancient alluvial-fan/braid-delta deposits from interpreted temperate-humid climatic
regimes indicate that stream-flow deposits are
dominant, and that mass-flow deposits are of minor
importance. This depositional style is illustrated in
Fig. 12. 1B. In the study by Crews and Ethridge
(1993), the dominance of braided stream-flow traction processes was demonstrated, with evidence of
lateral channel migration. Debris flows were determined to be rare to absent. Likewise, DeCelles et al.
(1991) reported mass-flow deposits in only one of
numerous localities. In their study, Ridgway and
DeCelles (1993) drew comparisons with the modern
gravel sandurs of southern Alaska (Boothroyd and
Nummedal 197 8). Similar clast-supported gravels
deposited by stream-flow processes are the dominant facies in their rocks. Debris flows are rare. One
possible inference that could be drawn from the
studies noted here is that while mass-flow processes
may be very spectacular in modern humid-temperate settings, they may have low preservation
potential and are therefore of limited geological
significance. However, this ancient/modern contrast is based on only a few reports and should not
be regarded as definitive.
12.5. 1.3 Tropical-Humid Climates
Deposits containing significant mass-flow deposits
and examples with a lack of such deposits have both
been described in case studjes of the modern and
ancient record.
Modern fan/braid-delta deposits in Papua-New
Guinea were described by Brierley et a!. (1993). The
area has a monsoonal, savannah-type·climate, with
a significant dry season. In addition to stream-flow
deposits, debris flows and hyperconcentrated flows
are present, as indicated by gravel textures. The
mass flow deposits are limited to proximal reaches
of the fans, where they comprise up to about 30% of
the total. However, their preservation potential is
estimated to be low, because of continued incision
of the deposits, and this is aggravated by syndepositional tectonic uplift. The modern Yallahs braid
delta of J amaica lacks debris-flow deposits (Wescott
1990).
Several ancient conglomerate deposits have been
interpreted to have been formed in a tropical-