Page 11 - Petrology of Sedimentary Rocks

the pure clay range (IO@). Thus the general size vs. sorting trend is a distorted sine
curve of two cycles. Work so far indicates that the apparent reason for this is that
Nature produces three basic populations of detrital grains to rivers and beaches
(Wentworth). ( I) A pebble population, resulting from massive rocks that undergo blocky
breakage along joint or bedding planes, e.g. fresh granite or metaquartzite outcrops,
limestone or chert beds, vein quartz masses. The initial size of the pebbles probably
depends on spacing of the joint or bedding planes. (2) A sand-coarse silt population,
representing the stable residual products liberated from weathering of granular rocks
like granite, schist, phyll ite, metaquartzite or older sandstones whose grains were
derived ultimately from one of these sources. The initial size of the sand or silt grains
corresponds roughly to the original size of the quartz or feldspar crystal units in the
disintegrating parent rocks. (3) A clay population, representing the reaction products of
chemical decay of unstable minerals in soil, hence very fine grained. Clays may also be
derived from erosion of older shales or slates whose grain size was fixed by the same
soil-forming process in their ultimate source areas. Under this hypothesis, a granite
undergoing erosion in a humid climate and with moderate relief should produce (I)
pebbles of granite or vein quartz from vigorous erosion and plucking of joint blocks
along the stream banks, (2) sand-size quartz grains, and (3) clay particles, both as
products formed in the soils during weathering.

Because of the relative scarcity in nature of granule-coarse sand (0 to -2$)
mean
particles, and fine silt [6 .to 84 grains, sediments with -- sizes in these ranges must
be a mixture of either (I) sand with pebbles, or (2) sand or coarse silt with clay, hence
will be more poorly sorted than the pure end-members (pure gravel, sand, or clay)]. This
is believed to be the explanation of the sinusoidal sorting vs. size trend. Of course
exceptions to this exist, e. g. in disintegration of a coarse-grained granite or a very fine
phyllite which might liberate abundant quartz grains in these normally-rare sizes. If a
source area liberates grains abundantly over a wide range of sizes, sorting will remain
nearly constant over that size range (Blatt) and no sinusoidal relation will be produced.
Shea (I 974 JSP) denies existence of “gaps” in natural particle sizes.

Although it appears that all sediments (except glacial tills) follow this sinusoidal
relation, there is some differentiation between environments. It is believed that given
the same source material, a beach will produce better sorting values for each size than
will a river; both will produce sinusoidal trends, but the beach samples will have better
sorting values all along the trend because of the “bean spreading” type of deposition.
Considering only the range of sands with mean sizes between I$ and 3$., most beach
sands so far measured here have sorting VI) values between .25-.50$, while most river
sands have values of .35-l -OO@. Thus there is some averlap, and of course there are
some notable exceptions; beach sands formed off caving cliffs are more poorly sorted
because the continual supply of poorly sorted detritus is more than the waves can take
care of, and rivers whose source is a well-sorted ancient beach or marine sand will have
well-sorted sediments. Coastal dune sands tend to be slightly better sorted than
associated beaches, though the difference is very slight, but inland desert dunes are
more poorly sorted than beaches. Near shore marine sands are sometimes more poorly
sorted than corresponding beaches, but sometimes are better sorted if longshore
currents are effective. Flood-plain, alluvial fan, and offshore marine sediments are
still more poorly sorted although this subject is very poorly known and needs a great
deal more data. Beach gravels between 0 4 and - 8 4, whether made of granite, coral,
etc., have characteristic sorting values of 0.4 -0.6 $ (if they are composed mainly of one
type of pebble); there seems to be no difference in sorting between beaches with gentle
wave action vs. those with vigorous surf. (Folk).

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