98
4 TRANSPORTATION AND SEDIMENTATION
Stream
power
Antidunes
Upper flow regime
............
=
.......
, ..........
Ripples
.L~, .... •
9 ..Lower
flow regime
t
9
Median fall-diameter
~0.65
Fig. 4.8. The relationship between stream power, fall diameter, bed form, and sedimentary structure in a unidirectional traction current system. (After Simons et al., 1965. Courtesy of the Society for Sedimentary
Geology.)
clay may settle out of suspension. The Hjulstrom effect (see Section 4.1) may allow
these clay laminae to be preserved from erosion during the subsequent tidal cycle. Thus
clay drapes on ripples and foresets may be indices of past tidal activity. Attempts have
even been made to identify tidal cycles in ancient sediments (Visser, 1980; Allen, 1981;
Fleming and Bartholoma, 1995). Sand will be transported when the ebb and flood currents are running sufficiently strongly. During these high-velocity intervals, bed forms
Grain size (rnm)
I 0
05
025
::
)i!!::: ::::!::::; : ...... ...............
i ):..... :....i":. ....
..... . ..... '.. :.:."., ...........
9 "...
?.....""
.....
" ...- .'.'....i .......
" .....
~
.........
9 .
"..
.
i.
.. "-... ....
)-:.. .., ...........
20%
005
...........
-..
...
"....
9 ........'.-....
9 ....
.
..:.:
.":..'; ': :. i': :"..... : "". ......
" ..........
.....................
i.
-
.L"
.<~<~
....
Limit of cross-bedding ~
Limit of crosslamination
I ....
] Trough
t .... " ~ -i crOss -
~bedding
9 ~lPlanar
"
J bedding
bedding
I _ . lM~
•
Lamination
i
1
":,~.,~-,,,~ Cross -
lamination
Fig. 4.9. Relationship between grain size and sedimentary structure in alluvial sandstones of the Torridon
Group (PreCambrian). Scotland. (Data from Selley, 1966. Courtesy of the Geologists' Association.)