Page 121 - Petrology of Sedimentary Rocks

Surely we recognize that there are deep troughs wherein enormous sections of
sediments accumulate, and other areas that are stable with only thin undeformed layers
of sediment. And among the sediment-filled troughs there are many different settings;
non-volcanic vs. volcanic, occurring in different positions with respect to continents,
plates, etc.

What we see in a thin section of a sandstone usually is the endpoint of an
incredibly long chain of events extending back into the dimmest geologic past. As we
trace causes back things get more and more hazy. Nevertheless it is worthwhile to try
to trace this chain of events as far back as we can, to as ultimate a cause as possible.
Beginning at the prime source, (I) convection currents in the mantle presumably cause
(2) movements of crustal plates. These movements produce (3) zones of surface rocks
having different types of tectonic activity--compression, tension, volcanism, etc.
These in turn (4) become different types of source terranes: metamorphics, granites,
andesi tes, etc. These in turn (5) produce different types of sandstones as modified by
weathering processes, relief, rate of erosion, and differential abrasion or weathering
during transport and deposition in various environments. For example, plate conver-
gence produces horizontal deformation (Dh), e.g., thrusts and folds, and this often
produces metamorphic rocks, which under rapid erosion, ineffective weathering and
lack of abrasion will produce fluvial sediments that are phyllarenites, or beach
sediments that will be quartzarenites with a lot of metamorphic quartz. As another
example, plate separation produces vertical deformation, i.e., tension and block faulting
and either uplift of plutonic basement (Dv) or granitic intrusion, thus common granitic
sources, hence alluvial fans and arkoses. These are ideal examples, providing the “main
sequence,” or the “guiding model.” Mother nature is not that simple and we have to
realize that variations exist because of random interfering causes, e.g., past geologic
history.

Reducing plate tectonics to its basic framework, one can establish the following
classes of plate behavior.

I. STABLE PLATE (symbolically, K for continent). Basins of sedimentation atop
an unbroken continental plate. Plate character may range from a shield of prolonged
neutral or mildly positive nature (Canadian shield), to a mildly depressed perikratonic
margin with widespread thin sheets of platform sediment (mid-Continent U.S.); it can
have local gentle basins (Michigan) or block-faulted troughs (late Paleozoic, Colorado).
Occurring in this framework are perikratonic shelves, and the parageosynclines of Stille
(audo-, zeugo-, and taphrogeosynclines of Kay). Sediments may also spill off the edge
of a stable continental plate, producing a geocline indistinguishable from KOA (see
below). Tectonic activity mainly Qk or Q, (giving carbonates, supermature quartz-
arenites orsubarkoses) locally R or even rarely Dv (arkoses). Continental plate is not

ruptured, hence is strong enough to resist later crumpling at that site, thus no Dh;
basins contain only mildly deformed sediments.

IIA. PLATES SEPARATING, Ensialic rift (KAK, read K split K). A continental
plate is beginning to split apart, oftenbecause a pair of oppositely-directed mantle
currents well up underneath it. However, simatic basement is not yet reached. But a
linear geosyncline forms upon the interior of a continent (miogeosyncline). This may be
filled by sediments swept in from the stable Kraton on either side (Qk); often thick
troughs of quartzarenite result. Continental plates, weakened along this join, may
osculate (repeated kissing-type activity): compressional spasms causing Dh (intense
thrusting, folding and even metamorphism with production of phyllarenite elastic

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