Page 114 - Petrology of Sedimentary Rocks

occasionally-preserved fluvial sediments) may be just as clayey and poorly sorted as the
fluvial or neritic sediments produced in the other more active tectonic frameworks.

Thus grandiose tectonic conclusions should not be drawn from one or a few
samples, unless it can be shown that they are typical of a large stratigraphic interval.
A single bed of very well-sorted sand may represent a beach, but one cannot jump
immediately and conclude that the beach sand formed in a tectonically stable
framework; it may be just a local beach in a great mass of deltaic sediments formed in
a violently subsiding geosyncline. The decision on what type of tectonism is repre-
sented has to be made by examining the quantitative distribution of environments over
a large stratigraphic interval, and integrating this with other rock features such as
mineralogy, thickness distributions, and gross stratigraphy.

Thus, the textural maturity of sandstones is dependent on environment, but
volumetric importance of specific environments is determined by tectonic activity.
The mineral composition of sandstones is controlled by source area lithology, and this in
turn is also affected by tectonism (Krynine). Consequently both properties interact,
although they show a considerable degree of independent variation. Their interaction
produces what may be called a “main sequence” of sandstone types: most immature
rocks, usually formed during periods of crustal unrest, are rich in unstable constituents
like metamorphic rock fragments, micas, and feldspars, hence are usually arkoses or
litharenites. Most supermature rocks, formed during periods of crustal stability, have
suffered enough abrasion or weathering to remove the unstable constituents, and the
only thing that remains is quartz; thus they are usually quartzarenites. Sublitharenites
and subarkoses frequently represent transitional stages of maturity and mineralogy.
There are many exceptions to the main sequence, however, which are very important to
recognize as they represent unusual conditions of climate or depositional environment.
ivlature litharenite, supermature sublitharenites, immature and submature orthoquart-
zites, and mature and supermature arkoses are quite common examples falling off the
main sequence. Any mineral composition can occur in any stage of textural maturity,
but certain conditions are preferred in nature.

Krynine’s Theory of the Tectonic Control of Sandstone Properties

M. S. Shvetsov, in the 1920’s, was apparently the first one to realize that pure
quartz sandstones are characteristic of stable shelf areas and quiescent continental
shields, or kratons; and that sandstones in erogenic areas are much more complex
mineralogically--either arkoses (rich in feldspar), graywackes (rich in rock fragments)
or tuff ites (rich in volcanics). But Krynine went further and about 1940 set up his
system, a three-stage scheme for linking the tectonic development of a continent with
deposition of specific mineralogical sandstone types. These were, in chronological
order; (I) Quiescence or Peneplanation Stage, resulting in “quartzite”; (II) Moderate
Deformation or Geosynclinal Stage, resulting in “graywacke’‘--i.e., a sandstone rich in
metamorphic rock fragments, micas and micaceous clay matrix; and (III) Violent
Deformation or Post-geosynclinal Phase, resulting in arkose. For references in which
these ideas were presented, see PDK memorial in I966 JSP. The most accessible
references are JG I948 and AGU Trans. 1951. The following is an ultra-brief
condensation of Krynine’s ideas.

A typical segment of the earth’s crust is assumed to consist of three layers:
sediments on top, metamorphics and veins next, and plutonic igneous rocks at greatest
depth. Krynine’s idea is that increasing amounts of tectonic activity are required to
bring these successively deeper layers to the surface so that they can act as source
areas.

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