Page 112 - Petrology of Sedimentary Rocks

The range of maturity encountered in natural environments is estimated in the
following figure, thickness of the lenses corresponding to the most common stages of
maturity present in each environment. The next figure attempts to show typical grain
size for the various sedimentary environments.

It is important to realize that the maturity rating of an environment depends on
how much mechanical energy is exerted on a sediment after it has been moved
essentially to its final resting place, by currents and waves at the final site of
deposition; it does not depend on the energy expenditure required to move it from the
7
source area to the sr te of final deposition. A flood or turbidity current, for example,
may expend a tremendous amount of energy while it is transporting sediment, but once
the sediment is dropped it is simply buried by more sediment and never suffers any
further sorting or winnowing; thus such sediments have low maturity. Waves on a beach,
on the contrary, sort and rework the sediment continually as the tide goes in and out, or
as storms and seasonal changes chew up beach sediments and shift them repeatedly.

Krynine has shown that each sediment-modifying process operates at a certain
optimum energy level, and this philosophy is certainly true in concept of textural
maturity. Thus the amount of energy expended in modifying sediments must be within
certain limits; if it is too little, sorting and rounding do not operate efficiently; if too
much energy is exerted, the maturity,may be destroyed (such as when a sudden storm
destroys a well-sorted beach and mixes it with lagoonal clays, or when a mountain
cloudburst produces torrents that fracture boulders that had become rounded under a
more gentle stream regimen). Thus sudden excess bursts of energy are often
responsible for textural inversion. It is a curious thing that the best rounded sands are
usually not too well sorted; this may mean that rounding operates best in environments
with energy levels that are too high for optimum sorting, where currents, winds or
waves are too vigorous and repeatedly mix up and “unsort” previously sorted layers of
sands.

Tectonism and Textural Maturity. It has been pointed out above that textural
maturity is very largely the result of the environment of deposition (beach, lagoon,
floodplain, delta, alluvial fan, etc.) Some workers, however, claim that textural
maturity is a direct function of tectonism: intense tectonic activity with rapidly
subsiding geosynclines yielding stratigraphic sections composed entirely of immature
sediments, mild tectonic instability (unstable shelves) giving all submature sediments,
stable conditions (stable shelves) producing nothing but mature sediments, and periods
of prolonged tectonic stability resulting in supermature sediments. This is a gross over-
simplification because (at least in the lower three stages of maturity) environment of
deposition exerts a much greater control than tectonism on the sorting and rounding of
sediments. Thus a flood-plain or neritic sediment will probably contain just as much
clay if the depositional basin is sinking at the rate of one inch per hundred years
(violent subsidence), or one inch per ten thousand years (stable shelf). Similarly, a
beach sand may get just as well sorted if the shore line stays at one position for one
hundred years, as it would if the shoreline was stabilized for a hundred thousand years.
For example, Texas Gulf coast beaches (and their buried Tertiary equivalents) have
superb sorting values, with a in the range of 0.25-0.35~); yet no one would call this
rapidly subsiding geosynclinal region a “stable shelf.” The reason for that is that the
geologic processes of sorting and winnowing take place almost instantaneously when
compared with the geologic time scale-- sands can be sorted pretty well, and clay
removed, by a few swishes of a miner’s pan ! Thus it does not require any long period of
crustal stability to produce well-winnowed or well-sorted sediments.

106