Page 144 - Petrology of Sedimentary Rocks

Variants are common. It has already been mentioned that under more-than-usual
abrasion, subphyllarenite or quartzarenites develop with few MRF’s and some meta-
quartzite grains; in hand specimen these look like clean, white, well-sorted quartz
sandstones but they may contain a few scattered black or white specks. These rocks
may occur either far from the source area or along local, temporary strand lines nearer
the source. During brief periods of quiescence which occasionally interrupt geosynclinal
subsidence, such sands may spread as blankets entirely across the basin; frequently
these sands are supermature, and although usually classifiable as subphyllarenites,
intensive abrasion may make them pass into quartzarenites. Often these sands contain
pebbles of vein quartz or metal quartzite. Continental deposition under a humid
climate in the source area may produce red phyllarenites, developed if uplift overbal-
ances the rate of subsidence so that detritus spills into the basin faster than sinking can
take care of it, and a great delta (exogeosyncline) is built up. Uplift or erosion may be
so great (especially, late in the history of the orogeny) that higher-rank metamorphics
or even the upper fringe of the granitized zone are exposed; potash feldspar is then
contributed in small amounts and the rock grades into a feldspathic subphyllarenite. If
any volcanic materials enter the sediment, small amounts of plagioclase, volcanic rock
fragments, biotite, apatite, and montmorillonitic clay may appear. These, however, are
more typical of the eugeosynclinal phyllarenites.

Eugeosynclinal rocks, often DhX. MX/g*/Bt. These appear to be mainly products

of simatic activity along trenches and subduction zones, K$K or KSO. Accumulated in
the very rapid subsiding troughs or in the arc-trench gap with accompanying extensive
volcanism, they are very poorly sorted and immature, and deposits may consist of
interbedded muddy conglomerate, muddy sandstone, deep water black shale and
radiolarian chert. Shoreline facies are present but rare (Dott). Turbidity currents
operate par excellence in this environment. Basically, they are composed of a gamut of
metamorphic debris-- slates to schists and gneisses to which is added vein quartz, chert,
a little K-feldspar from granitized zones and pegmatites and, most important, plagio-
clase and volcanic rock fragments. Compositionally, they show a very wide range;
many could be called “feldspathic polylitharenites” because they contain such a variety
of rock fragments-- but mostly they range from phyllarenite through volcanic-arenite.
In some areas, plagioclase-areni tes are common. Typically they are made very hard by
a blackish-green matrix of chlorite--partly caused by basic volcanic material, partly of
metamorphic origin; sometimes zeolite cement appears. Inasmuch as many of these
rocks have suffered later metamorphism (forming as they do in such active tectonic
zones) their original characteristics are obscured, and burial metamorphism is common
(epidote, pumpellyi te, etc.). Some so-called eugeosynclinal rocks may be simply
tectonic arkoses metamorphosed to chlorite grade (W. Harris).

Rejuvenation Phyllarenite, often R. M/s/CP. These rocks are produced by uplift
(without further deformation) of older metamorphic sources (often neokratons). They
may range from local valley fills to fairly large basins, to coastal-plain geosynclines
like the Gulf and Atlantic coast Tertiary of the USA. They tend to be low in MRF’s
(actual1 y subphyllareni tes) because sources are mixed (older sediments, granites, etc.),
and the ratio between uplift and abrasion plus weathering is low, so that a lot of the
weak MRF’s are removed. It appears that most of these rocks, if continental, are of
subphyllarenite composition with some chert, feldspar, and other contaminants; littoral
or marine equivalents tend to be quartzarenites instead. Heavy minerals may be very
complex because of mixed source.

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