Page 10 - Petrology of Sedimentary Rocks

said about the significance of grain size. Still, it is an important descriptive property,
and only by collecting data on grain size will we be able to learn the meaning of it.

Mean size is a function of (I) the size range of available materials and (2) amount
of energy imparted to the sediment which depends on current velocity or turbulence of
the transporting medium. If a coastline is made up of out-crops of soft, fine-grained
sands, then no matter how powerful the waves are no sediments coarser than the fine
sands will ever be found on the beach. If a coastline is made up of well-jointed, hard
rocks which occasionally tumble down during rains, then the beach sediment will be
coarse no matter how gentle the waves of the water body. Once the limitations of
source material are understood, though, one can apply the rule that sediments generally
become finer in the direction of transport; this is the case with most river sands, beach
sands, spits and bars. This is largely the result not of abrasion, but of selective sorting
whereby the smaller grains outrun the larger and heavier ones in a downcurrent
,direction. Pettijohn and students have made excellent use of maximum pebble size in
predicting distance of transport quantitatively. Sediments usually become finer with
decrease in energy of the transporting medium; thus, where wave action is dominant
sediments become finer in deeper water because in deep water the action of waves on
the sea bottom is slight, whereas this turbulence is at a maximum in shallow waters at
the breaker zone. Where current action dominates, particularly in tidal channels,
coarses sediments occur in deeper waters, largely because of scour. Research is needed
to quantify these changes so that the rate of grain-size change with depth can be
correlated with wave energy expenditure or other environmental factors.

Sorting is another measure which is poorly understood. It depends on at least
three major factors: (I) Size range of the material supplied to the environment--
obviously, if waves are attacking a coastline composed of glacial till with everything
from clay to room-sized boulders, the beach sediments here will not be very well
sorted; or if a turbulent river is running through outcrops of a friable well-sorted
Tertiary sand then the river bars will be well sorted. (2) Type of deposition--“bean
spreading”, with currents working over thin sheets of grains continuously (as in the
swash and backwash of a beach) will give better sorting than the “city-dump” deposition
in which sediments are dumped down the front of an advancing series of crossbeds and
then rapidly buried by more sediment. (3) Current characteristics--currents of
relatively constant strength whether low or high, will give better sorting than currents
which fluctuate rapidly from almost slack to violent. Also very weak currents do not
sort grains well, neither do very strong currents. There is an optimum current velocity
or degree of turbulence which produced best sorting. For best sorting, then, currents
must be of intermediate strength and also be of constant strength. (4) Time--rate of
supply of detritus compared with efficiency of the sorting agent. Beach sediments
where waves are attacking continually caving cliffs, or are battling great loads of
detritus brought to the shore by vigorous rivers, will be generally more poorly sorted
than beaches on a flat, stable coast receiving little sediment influx.

It is probable that in every environment, sorting is strongly dependent on grain
size. This can be evaluated by making a scatter plot of mean size versus sorting
(standard deviation). In making many of these plots, a master trend seems to stand
revealed: the best sorted sediments are usually those with mean sizes of about 2 to 3$1
(fine sand) (Griffiths; Inman). As one measures coarser sediments, sorting worsens until
those sediments with a mean size of 0 to -IQ (I to 2 mm) show the poorest sorting
values. From here sorting improves again into the gravel ranges (-3 to -5@), and some
gravels are as well sorted as the best-sorted sands (Folk and Ward). Followed from fine
sand into finer sediments, the sorting worsens so that sediments with a mean size of 6
to 8$ (fine silts) have the poorest sorting values, then sorting gradually improves into

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