32
Chapter 2 Continental Sediments
FLUVIAL CHANNELS
NARROW
ISOLATED
a BROAD ISOlA TED
RIBBON
OVERlAPPING RIBBONS
SAND SHEET
,. &::::::- .. ,,- ,. - .. - , . '-~ - , : - . - : : - , : . . . ,:
~. : . ~, , : ~- : - , : -,-- - : : - , -- : - .. ~. ~. ~_ ~~-:.~~-=-~= -=-~=7---M : . I
UL
TI . STOREY
b
VERT ICAL STACKING
(RAPID $UB$IDENC EI
LA TERAL ST ACK ING
(SLOW SUBSIDENCEI
CHANNE LS "jb==~·' ;]z '.:: Z ... ;::;: ··:2 -.l ::;;
•.. ::::; ... :=;:-;::; . ' :2:::::=t..Y
Fig. 2.8. Main types
of sand and sandstone reservoir geometries generated
by channel tills in
fluvial systems. a
Single sandstone
bodies. b Different
types of stacked
channel sands related to slow or
rapid subsidence.
Note that the active
channels visible at
the surface do not
necessarily indicate
the nature of the
buried channel
complex. (After
Ethridge 1985;
Miall 1985, modified). c Channel
fills of bedload,
mixed-load, and
suspended-Ioad
rivers. (Based on
Galloway 1985)
LA TERAl CHANNEL MIGRA nON
tLiTTLE CONTE MPOAANEO US SUBSIDENCEI
,;i;j
MIXED lOAD CHANNELS
(SOLATED STACKING
~
'"-'-'"
SUPENDED· LOAD CHANNELS
SAND AND GRAVEL
SAND LATERAL ACCRET(ON
MUD
SAND
lying finer material from further reworking. Gravel
exposed to currents, which are only capable of eroding
and transporting sand, roll into developing scours
pools. Rapid vertical aggradation can occur only if
sediment input into the system is high. This may lead
to climbing ripples and ripple drift cross-stratification.
Sand waves and antidunes form in the upper flow
regime. They are more or less destructive features and
are associated with sediment loss rather than with
aggradation of fluvial material. Sand waves and
antidunes are rarely preserved because they are frequently truncated or completely reworked by subsequent "normal" current action.
The larger bed forms and their interna I structures
result from fluctuating water stages and current velocities, and therefore cannot be attributed to certain flow
conditions.
Fig. 2.9. Relationship between current velocity (hydraulic regime), grain-size, minor and medium-sized
bed forms, and internal sedimentary structures of
fluvial deposits (overview). (Drawn after different
sources, e.g., Harms and Fahnestock 1965; Collinson
1996). The boundary between 10wer and upper flow
regime (in terms of mean current velocity) strongly
Coarse particles such as gravel increase in roundness downstream if they originate from angular rock
debris and not from older sedimentary rocks with prerounded material. They often form characteristic fabrics in wh ich flat pieces dip upstream, with their long
axes perpendicular to the flow direction (Fig. 2.9).
Less comrnon is the orientation of gravel and pebbles
with their long axes parallel to flow. This type of fabric appears to be restricted to high energy flow conditions.
Both criteria, the downstream growth of cross-bedding and the imbrication of gravel, indicate the current
direction which was responsible for the formation of
the bed. Variations of these directions from a general
trend are one of the means of discriminating between
different fluvial systems in the ancient record (see below).
depends on water depth. The larger bed forms and
their internal structures result from fluctuating water
stages and current velocities. In different river systems (A through C), certain flow conditions. and bed
forms prevaiL See text and Table 2.1 for further explanation
Chapter 2 Continental Sediments
FLUVIAL CHANNELS
NARROW
ISOLATED
a BROAD ISOlA TED
RIBBON
OVERlAPPING RIBBONS
SAND SHEET
,. &::::::- .. ,,- ,. - .. - , . '-~ - , : - . - : : - , : . . . ,:
~. : . ~, , : ~- : - , : -,-- - : : - , -- : - .. ~. ~. ~_ ~~-:.~~-=-~= -=-~=7---M : . I
UL
TI . STOREY
b
VERT ICAL STACKING
(RAPID $UB$IDENC EI
LA TERAL ST ACK ING
(SLOW SUBSIDENCEI
CHANNE LS "jb==~·' ;]z '.:: Z ... ;::;: ··:2 -.l ::;;
•.. ::::; ... :=;:-;::; . ' :2:::::=t..Y
Fig. 2.8. Main types
of sand and sandstone reservoir geometries generated
by channel tills in
fluvial systems. a
Single sandstone
bodies. b Different
types of stacked
channel sands related to slow or
rapid subsidence.
Note that the active
channels visible at
the surface do not
necessarily indicate
the nature of the
buried channel
complex. (After
Ethridge 1985;
Miall 1985, modified). c Channel
fills of bedload,
mixed-load, and
suspended-Ioad
rivers. (Based on
Galloway 1985)
LA TERAl CHANNEL MIGRA nON
tLiTTLE CONTE MPOAANEO US SUBSIDENCEI
,;i;j
MIXED lOAD CHANNELS
(SOLATED STACKING
~
'"-'-'"
SUPENDED· LOAD CHANNELS
SAND AND GRAVEL
SAND LATERAL ACCRET(ON
MUD
SAND
lying finer material from further reworking. Gravel
exposed to currents, which are only capable of eroding
and transporting sand, roll into developing scours
pools. Rapid vertical aggradation can occur only if
sediment input into the system is high. This may lead
to climbing ripples and ripple drift cross-stratification.
Sand waves and antidunes form in the upper flow
regime. They are more or less destructive features and
are associated with sediment loss rather than with
aggradation of fluvial material. Sand waves and
antidunes are rarely preserved because they are frequently truncated or completely reworked by subsequent "normal" current action.
The larger bed forms and their interna I structures
result from fluctuating water stages and current velocities, and therefore cannot be attributed to certain flow
conditions.
Fig. 2.9. Relationship between current velocity (hydraulic regime), grain-size, minor and medium-sized
bed forms, and internal sedimentary structures of
fluvial deposits (overview). (Drawn after different
sources, e.g., Harms and Fahnestock 1965; Collinson
1996). The boundary between 10wer and upper flow
regime (in terms of mean current velocity) strongly
Coarse particles such as gravel increase in roundness downstream if they originate from angular rock
debris and not from older sedimentary rocks with prerounded material. They often form characteristic fabrics in wh ich flat pieces dip upstream, with their long
axes perpendicular to the flow direction (Fig. 2.9).
Less comrnon is the orientation of gravel and pebbles
with their long axes parallel to flow. This type of fabric appears to be restricted to high energy flow conditions.
Both criteria, the downstream growth of cross-bedding and the imbrication of gravel, indicate the current
direction which was responsible for the formation of
the bed. Variations of these directions from a general
trend are one of the means of discriminating between
different fluvial systems in the ancient record (see below).
depends on water depth. The larger bed forms and
their internal structures result from fluctuating water
stages and current velocities. In different river systems (A through C), certain flow conditions. and bed
forms prevaiL See text and Table 2.1 for further explanation