second will not move 500 gram samples spaced one diameter apart while the same current will move 5000 gram samples
if their spacing is greater than 5 diameters. Thus gravel
accumulations and bars are a form of kinematic wave caused
by particle interaction. Leopold et al. (1966) also noted
that there was no relationship between transport distance and particle size for pebble and cobble clasts that can be entrained by the flow.
In very coarse material roundness changes are very rapid during turbulent flood surge conditions (Scott &
Gravlee, 1968). They noted that, with diorite clasts, changes from angular to subangular took place in a matter of 100 m and the clasts became rounded after travelling
2.5 Km. This very rapid rounding is partly due to the
lack of fine detritus for cushioning effects. The rapid
decrease in size downstream from -8 phi at 0.6 Km to -6.5 phi at 205 Km is mainly due to progressive sorting and only 10 percent of the size decrease is attributable to abrasion and breakage.
According to Ouma (1967) roundness evolves down stream at a medium rate in grades coarser than cobbles, faster in the cobble to granule range, and slowest in
sub-granule grades. Roundness increases to a maximum
downstream but with further transport it then declines as
size decreases. In Fivemile Creek, Wyoming, Hadley (1960)
has shown that gravel sized pebbles are rapidly rounded
during early transport. Thus in the first 30 Km of trans
port roundness increases from 0.34 to 0.51 and sphericity
increases from 0.52 to 0.65 on the Wadell scale. Beyond
30 Km increase in roundness and sphericity is attained
relatively slowly. The following table after Hadley (1960)
shows the variation of average roundness with distance, according to lithology:-
Limestone
8 Km 0.42
90 Km 0.62
Quartz 0.30 0.70 ?due to reworked
Chert 0.17 0.52 quartz Granite 0. 30 0.30 Siltstone 0o 45 0.63 Sandstone 0.50 disintegrates Quartzite 0.50 with abrasion
In humid regions limestone is generally weakly
resistant and is readily removed in solution. However,
this is not always the case and in mountainous terrains on the north-east coast of New Guinea limestone conglom erates are accumulating in tropical conditions due to
the very rapid erosion and deposition. In arid areas
limestone is generally a resistant rock because of the very reduced effect of chemical weathering.
Limestone pebbles become rounded in approximately 16 Km of fluvial transport (Sneed & Folk, 1958) while quartz
and chert pebbles require larger transport distances. In
this present study, limestone clasts are generally rounded
whereas chert fragments are subangular to rounded. Quartz
pebbles are all well rounded but they are rare and are
probably reworked from earlier formations. Thus transport
distance cannot be adequately defined although it was probably in excess of 16 Km during the deposition of most beds. At section AO sandstone and limestone clasts are
the clasts were probably only transported a short distance
from the source area. Fossil scree deposits are preserved
at section GC where a breccia with angular clasts up to
-11 phi lies at the base of a presumed fault scarp» The
deposit is unsorted and unstratified and probably re presents non-transported debris.
In general
,
studies of pebble morphology are ofvery limited value in the Pertnjara and Finke Groups because most of the clasts are sedimentary and they be come rounded very rapidly. Even the limestone and chert fragments are moderately rounded which may indicate trans
port distances of 30 to 110 Km. The use of the proportion
of quartz pebbles in conglomerates as a climatic indicator (Dal Cin, 1968) could not be attempted in this study
because the source area was not metamorphic and most of the quartz pebbles are probably reworked.
In the Colorado River, Bradley (1970) noted a distinct change in pebble size and composition with dis
tance of transport. He noted that coarse granite
diminished by 50 percent in size in about 250 Km of transport whereas quartz and chert gravel decreased by
30 and 20 percent respectively. In this reach the
Colorado River is essentially at grade and conditions are unfavorable for significant down-valley size sorting
of the detritus (Mackin, 1963). Fresh granite would
only be reduced in size by 10 percent in an equivalent distance of travel and Bradley (1970) explains the extra
40 percent reduction as due to more rapid abrasion of particles weathered during periods of temporary alluvial
storage.
Changes in composition and size of pebbles in a downstream direction can be inferred from studies con
ducted across the Missionary Plain. On the northern
margin pebbles are abundant in the Ljiltera Member and the range in composition is the same as that in the lower Brewer Conglomerate - namely a large variety of sandstone and siltstone clasts with less abundant limestone and chert
clasts and rare quartz pebbles. The clast size varies from
-2 to -8 phi with occasional sandstone cobbles being even
larger. Roundness values vary from subrounded to well
rounded and sphericity is moderate. At Gosses Bluff where equivalent horizons crop out, clast size rarely
exceeds -6 phi and sandstone fragments other than quartz
ite are less abundant than to the north. There is little
change in the average roundness or sphericity values. Still further south near the Gardiner Range pebble abundance has decreased considerably and they usually
only occur as isolated clasts on bedding surfaces near the
base of a coset. The size of the pebbles in these locali
ties rarely exceeds -4.5 phi and the southward decrease in pebble size can probably be accounted for best by sorting
as well as abrasion during transport. The mean roundness
of the pebbles is generally within the rounded class and thus there is no direct relationship between roundness
where most of the pebbles are ovoid to sub-spherical and
discoid pebbles are less abundant. This may be an effect
of sorting rather than abrasion since the pebbles were probably transported by flows in the upper part of the lower flow regime (Fahnestock & Haushild, 1962) where rolling is the main mode of movement of clasts of this size. Composition is also notably different in the south. The lithologies are less varied and quartzite, chert,
limestone and quartz pebbles are the main representatives. A few siltstone clasts survive this distance of travel
(50 Km) while non-cemented sandstone fragments are very
rare. The presence of limestone clasts and the rarity of
sandstone and siltstone clasts suggests that the latter were rapidly reduced by abrasion, and weathering (espec
ially acid weathering) was minimal.