Page 126 - Petrology of Sedimentary Rocks

compression; resting or brief tensional phases (Dv) are possible. Two continental plates
may also shear sideways (K/K) as in southern California. Again, volcanism appears to
be suppressed. After continents are welded together and the scar tissue healed, a new
split may arise in a new location--thus progressing back to I or II.

IIIC. PLATES COLLIDING, Ocean/Ocean (0 SO). Forms chains of volcanic
oceanic islands with surrounding auieole of volcanic and carbonate sediments. Shear
motion also possible (O/O) as in the Cayman Trench. Often trenches do not contain
much sediment.

In summary, most geosynclinal nomenclature is worthy of retention. Plate
tectonics has simply provided the driving engine and revealed much better understand-
ing of the reason behind the geosynclines.

Genetic Code for Sandstones

In deciphering genesis of a sandstone, we need to consider at least five elemental
points. These are (I) tectonics of the source land, which controls erosion rate,
geomorphology, etc.; (2) paleogeology of the source terrane, which controls mineralogy;
(3) tectonics of the depositional site which controls thickness pattern, etc.; (4)
depositional environment, which controls texture, sedimentary structures, etc.; and (5)
degree of weathering of the material, which affects mineralogy and texture. All are to
some degree linked, but the latter two are more or less “accidental,” while the first
three are linked more closely to the diastrophic framework. To serve as a shorthand, or
at least an outline for meditation, any sandstone can be categorized by a symbol
involving all five aspects; for example, Dv.G/p/Cf(w), or Qk.S/s/L(n). If you don’t like
the shorthand, it can be written out in words. Krynine’s “Holy Trinity” of sandstone
types adopted the simple view that all five properties could be predicted from the
source area tectonics; the scheme presented here emphasizes that all combinations are
possible, though some are much more common than others. Now, to discuss each of
these five aspects in turn.

I. Tectonics of the Source Area. Five endpoints are recognizable:

Q, QUIESCENCE or Stability; low relief; “peneplanation,” slow sediment
influx; mild epeirogenic warping and eustatic changes may be involved
also.

Qk, Kratonic Source--example, Canadian shield during Paleozoic time.

Q, source in neokraton, stabilized welt, peneplaned orogen, etc.--
example, Atlantic coast Cretaceous (Schooley peneplane).

D, DEFORMATION of source area, with differential activity, folding,
faulting, etc. More rapid detrital influx and higher relief. A
gradational mess, but often two end-members can be recognized in a
given region:

Dh, Horizontal Deformation--strongly asymmetrical folding, over-
thrusts, rocks become squashed and sheared, (“Alpinotype”

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