Page 145 - Petrology of Sedimentary Rocks

Petrology of Quartzareni tes

Krynine’s original scheme of the 1940’s used the term “quartzite” for sandstones
made of quartz f chert; as this was confused with metamorphic rock, Pettijohn (I 949)
used instead “orthoquartzite” (this term was apparently coined by Tieje (I 921, JG).
Even so, this still misled the average geologist into thinking metamorphically, so
gradually the term quartzarenite (Gilbert; McBride) has superceded it. Though less
euphonious and longer, it will be used throughout this book.

Quartzarenites originate in diverse ways, but their high degreee of purity usually
requires a long period of abrasion and/or weathering to remove the less stable feldspar,
metamorphic rock fragments, etc. Quartzarenites may be developed directly from a
granite or granitic gneiss under humid conditions if tectonic stability is sufficiently
prolonged (say of the order of one-fourth to one-eighth of a geologic period). If the
climate is arid, however, a quartzarenite may be made directly from a granite only
under an exceptional and very prolonged period of tectonic stability coupled with
intensive beach and dune action, because the feldspar must be removed by abrasion
alone, unassisted by much chemical decay. Quartzarenites may be derived from
metamorphic sources, but this also requires rather prolonged stability (a fraction of a
geologic period) to remove all the metamorphic rock fragments. Some are made largely
of vein quartz; these also require prolonged abrasion. If the source area consists of
older sediments, however, a quartzarenite may be developed very rapidly because the
source material is already quite enriched in quartz. Hence quartzarenites derived from
reworked sediments require no period of stability, in fact many of them are erogenic
sediments.

Plutonic source, humid climate, frequently Qk. G/s/L(w). To obtain a rock that has

less than 5 percent feldspar directly from a source granite-gneiss containing 70 percent
feldspar requires prolonged abrasive or weathering activity of the most intensive kind.
Such intense work takes place only on beaches or dunes that remain stabilized for a
very long time. Hence the production of this type of orthoquartzite requires stable
tectonic conditions, characterized by very mild and gradual epeirogenic upwarps and
downwarps to let the seas transgress and regress across the area. Under such stable
conditions the source area is worn down to a peneplane; influx of detritus is slowed very
markedly, so that winds, waves and currents are able to do their work thoroughly on
each grain. Consequently these sediments are practically always supermature and show
extreme roundness. Because detritus is contributed very slowly, and because no part of
the depositional area is going down rapidly, the mild epeirogenic warpings and continual
transgressions and regressions of the sea spread the deposits out into a thin blanket of a
large area; sheets of sand 100-200 feet thick may cover 10,000 square miles. These
sheets are made up of coalescing beaches and dune sands reworked into beaches or
peritidal and shallow marine sands. The humid climate, given such a long time in which
to work on the parent material in the deep soils of the source area, destroys all the
feldspar. Hence this rock type consists entirely of quartz, almost all “common” quartz.
If the source is deep within a batholith, there is very little vein quartz. Of course there
is no chert and little metamorphic quartz. The only heavy minerals that can stand this
prolonged abrasion are the ultra-stable tourmaline and zircon. The grain size is medium
to fine sand (the typical size of beaches and dunes), and there is no silt or clay; gravel is
rare, occurring only near the base. Quartz overgrowths are perhaps the most common
cement, but carbonates are common and many of these rocks (even older Paleozoic ones)
are uncemented. The color is very commonly white, but some are red due to hematite.
Sometimes thousands of feet of highly mature to immature quartzarenite accumulate in
narrow troughs bordering a stable Kraton. These sands, symbolically Qk.G/g/MBt may

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