Page 113 - Petrology of Sedimentary Rocks

The role of tectonic stability in producing supermature sediments is less clear.
Certainly those times in earth’s history that were characterized by very stable and
quiescent conditions (upper Cambrian, Ordovician, and Cretaceous for example) have
produced abundant supermature sediments; and relatively stable areas of the earth’s
crust have produced supermature sediments throughout much of geologic time (Cana-
dian and Australian shield sediments, for example). But contrary evidence shows that
supermature sands can be produced in one cycle, and in very brief geologic time if the
environment is potent enough. This happens in the Ellenburger of West Texas, where
beds of supermature sand alternate repeatedly with angular sands of the same grain
size, all coming from the same primary granitic-gneissic source area. The identical
thing happens in the Silurian of West Virginia, where quartz sands from the same
metamorphic and vein source may be very well rounded in one bed and subangular in a
bed two feet higher. If periods of tectonic stability are called upon to explain these
sequences, then those “stable” periods must have lasted only a small fraction of a
gologic period. In the opinion of this writer, then, these rapid alternations are caused by
shifts in environment (beach or dune vs. fluvial or neritic, for example) and do not
require any prolonged stability of the crust. They probably would not form in a period
of such rapid shoreline shifts as we have had in the Pleistocene or Recent (since very
little rounding of sand-size grains is going on in present U.S. ocean beaches), but might
be formed during non-glacial times in places where the environment was right. The
beach-dune environment would have to be relatively stationary at one place (or else
sweep through the same spot repeatedly) in order to produce high rounding, but the
point is that it need not be stabilized for more than a fraction of a geologic period.

Although the environment of deposition is apparently the immediate controlling
factor in textural maturity, the tectonic framework exercises an indirect control by
determining which environments shall be volumetrically dominant and which environ-
ments shall be rare in a given region or stratigraphic section. As Krynine has shown,
the degree and type of tectonic activity does determine a certain preferred (but not
necessary) association of source area lithology, relief, geomorphic processes, and rate
of subsidence of the depositional basin. These factors in turn integrate to produce
preferred associations of environments, because the rate of influx of detritus into a
basin, combined with the rate of subsidence of that basin, determines the quantitative
distribution of the environments, e.g., proportion of continental to marine facies, or
relative importance of deltaic vs. each sediments. And these environments in turn
control the textural maturity. For example, in a tectonically active geosynclinal area
sediment deposition is rapid with buildup of extensive floodplains, aggrading river
channels, large deltas, and thick masses of neritic sediments, possibly in part deposited
by turbidity flows and submarine mudslides. In such a tectonic framework, immature
sediments will dominate because of the prevalence of low-maturity environments in the
region. But local beaches in this complex, although volumetrically minor, may contain
extremely well-sorted sediments of high maturity, and some of the river channel and
shallow neritic sediments may be pretty well sorted. When intense block-faulting
occurs in the continental interior (e.g., Basin and Range province), very rapid erosion
and presence of abrupt scarps leads to the production of immense thicknesses of
continental sediments, huge alluvial fans, mudflows, bahadas, bolson fill, and fluvial
sediments. As a whole then, these deposits will be of low maturity, but occasional river
channel sands or lake beaches (e.g., Lake Bonneville) may be quite well sorted. In a
tectonically stable shield area, the shoreline transgresses and regresses over large areas
because of the flat topography, while subsidence is very slow and little sediment influx
occurs. Thus most of the sediments are beach (or dune) deposits, and fluvial sands have
only a temporary existence because they are destroyed and reworked by the next
marine transgression. The beach sands are packed one against the other by shoreline
shifts, to produce a wide sheet of highly matured sand. But neritic sediments (and

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