Page 146 - Petrology of Sedimentary Rocks

be littoral to turbidite sediments, and often form as the initial stage of KAK, as a
continental plate begins to split.

Plutonic Source, Arid Climate (or rarely, cold climate), frequently Qk.
G/s/Cd,L(n). These require much longer tectonic stability than the preceding type and
are consequently rare. They also form thin blankets of supermature sand, formed in
desert dune and beach environments (dune more important here) neighboring a pene-
planed source land and are of course an extension of the climatic arkose. The
distinguishing mark is the presence of extremely well rounded and frosted quartz and
feldspar grains, the feldspar are smaller than the quartz and usually perfectly fresh
(the “Dogma of the Immaculate Feldspar”). Little else but common quartz and well-
rounded zircon and tourmaline comprise the sand. Cement may be authigenic quartz,
feldspar overgrowths, carbonate, or evapori tes. In other properties these rocks
resemble the preceding type. There is likely to be little shale section associated with
these rocks because of the lack of soil mantle in the source; the plutonic humid
orthoquartzite frequently is accompanied by shale section (either stratigraphically
adjacent or in a lateral facies development) because of the thick kaolinitic clay soils
developed on intense weathering.

Metamorphic Source, often Q.M/s/L(w). These also develop under quiescent
tectonic conditions and form widespread blanket sands, chiefly of beach-dune environ-
ments. Metamorphic materials are eliminated by prolonged abrasion, the only re-
minders being a high proportion of metamorphic quarts types and undulose quartz; hence
these grade into the subphyllarenite. Pebbles are common, and consist of vein quartz
and various types of metaquartzite. Sands are supermature to submature and of fine to
medium grain. Immature quartzarenites form if beach sand is blown or carried into
low-energy environments such as lagoons. No feldspar or chert is present. If the quartz
is not rounded, heavies may contain some less stable metamorphic minerals like
amphiboles, garnet or epidote, but often abrasion is long enough so that only rounded
tourmaline and zircon remain. Associated shale section is rich in mica and micaceous
paste, derived from breakdown of slate, schist, etc. Quartz or carbonate cement are
common. The color is generally white or light gray, but is occasionally red.

Vein Quartz Source. Many areas of soft, low-rank metamorphic rock are shot
through with abundant quartz veins. If such areas are eroded under quiescent tectonic
conditions, the beach-dune activity is sufficient to wear away the soft MRF’s so that
about the only resistant particles left are the sand grains and pebbles composed of vein
quartz, recognized by their richness in bubbles. This is an uncommon rock type, but
there are a few conspicuous examples.

Older Sedimentary Source. Since no period of tectonic stability and no period of
beach-dune action is required to attain a quartzarenite by reworking older, already
quartz-rich sediments, these can form under any tectonic framework and almost any
environment. Some are erogenic sediments, deposited in thick wedges and eroded from
source areas consisting of older sandstones, mudrocks, and limestones; some develop on
gentle uplift of a large area blanketed by older sedimentary rocks, hence form blanket
sands. Maturity is generally low (because again, beach-dune action is not necessary for
their production), and they are characterized primarily by many textural inversions
(poor sorting and high rounding; or, more commonly, a lack of correlation between
roundness and size, with mixture of angular and rounded grains within the same size, or
small round grains plus large angular ones). The erogenic sediments are poorly sorted,
having the size-distribution of a tectonic arkose, but the mineralogy of a quartzarenite
to chert-arenite and include reworked rounded grains. Conglomerates are frequent,
with pebbles of chert, sandstone, perhaps limestone, and inherited pebbles of vein

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