Page 20 - Petrology of Sedimentary Rocks

sands of the Gulf Coast show very little rounding taking place simply because the
shoreline fluctuates so rapidly that time is not adequate. Balasz and Klein report rapid
rounding in a “merry-go-round” tidal bar.

In studying roundness watch for two things: (I) an abnormal relation between
roundness and mineral hardness, e.g. if tourmaline is round and softer hornblende is
angular it means there is a multiple source area; and (2) an abnormal relation between
size and rounding, e.g. if coarse grains are angular and fine ones are round it again
usually means a multiple source with the angular grains being primary, the rounder ones
coming from reworked sediments. Also, poor roundness sorting (i.e. within the same
grade size there are both very rounded and very angular grains) indicates a multiple
source. To determine how much rounding is taking place in the last site of deposition,
look for the most angular grains; for example a sand consisting of 70% well rounded
grains and 30Ggular grains indicates little or no rounded grains have simply been
inherited from older sediments. Perhaps the I6 percentile of roundness is the best
parameter to evaluate the present rate of rounding.

Effect of transportation on grain size and morphology. Transportation does
reduce size of pebbles through chipping or rubbing and occasionally through
fracturing; but it is thought that very little size reduction of sand-sized quartz is
effected through transport. The differences in size between deposiisarexfly due to
selective sorting (where the finer particles, which travel in almost ali curren-is, outrun
the coarser particles, which can travel only in the strong currents), rather than to
abrasion. Thus one cannot say that a very fine sand has been abraded longer than a fine
sand: simply it has been carried farther or deposited by weaker curents. The effect of
abrasion on sphericity of sand is slight but noticeable. Crushed quartz and many
angular sands have W/L values of about .60-.64; very well rounded sands have W/L of
over .70. Selective sorting will also produce form and sphericity differences between
samples.

It can be certainly stated that abrasion does cause rounding of sand grains (albeit
very slowly), and that it even more rapidly will produce polish. Thus the smallest order
features are affected first: considering sand grains in water, starting initially with
crushed quartz, the first thing that happens is that the grains become polished; after
much more abrasion, they become rounded; after an extreme length of time they begin
to attain higher sphericity values; and still later their size is reduced. Really these
processes are all taking place at once, and the above list simply gives the rates at which
these changes are being effected. Surface features and, secondarily, roundness are the
important clues to the latest environment in which the sand was deposited; sphericity
and form are the clues to the earliest environment in which the sand was formed,
namely source rock.

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