Page 143 - Petrology of Sedimentary Rocks

metamorphics such as metaquartzite, slate, phyllite and schist. Furthermore, uplift
attendant on the rise of the welt brings deeper crustal areas to the surface, and some
intrusion and volcanism may occur. But dominantly the source area consists of low-
rank metamorphic rocks, carved into considerable relief as it is continually deformed
and raised. The resulting great flood of detritus is dumped rapidly in a series of
coalescing alluvial fans, flood plains, river channels, large and small deltas, coastal loci
such as beaches, swamps, lagoons, and estuaries, and neritic environments. Since this
type of deformation produces linear welts, sediments accumulate in a linear trough
which may be quite thick in the middle and thins to both sides. Climatic evidence is
difficult to obtain, but it seems likely that a rugged welt surrounded on all sides by seas
would usually have considerable rainfall.

Because of the softness of the slate and phyllite fragments, environment of
deposition has a tremendous influence on the composition of the phyllarenite. Near the
source where sediments are deposited rapidly, sands are immature--simply choked by
the flood of fine-grained, clay-mica paste which is produced by abrasion of the low-rank
micaceous source rocks (some “matrix” may readily be squashed MRF’s--it is hard to be
sure). These sands may contain up to 70 percent of metamorphic rock fragments
(MRF%). Traced farther away, the softer fragments are abraded out and the sands
become more mature and may pass into subphyllarenites, although mature phyllarenites
are common in high-energy loci such as river channels. Rapid abrasion of slate and
phyllite fragments produces great volumes of clay-mica mush, which are winnowed out
of the sands to produce thick shale section. At intermediate distances from the source,
environment plays the strongest role; beach sands (although not abundant in such a
rapidly subsiding basin) may have lost all their soft MRF’s and contain only stretched
rnetaquartzite as a reminder of their origin; some of these may pass into quartzarenites
or supermature subphyllarenites if a brief period of quiescence stabilizes the beach line.
But fluvial sands at the very same distance from the source may still contain
considerable MRF’s, and floodplain or deltaic sediments may be extremely rich in them.
Stratigraphic sections that have been deposited in this type of coastal region consist
chiefly of shales (marine, estuarine, lagoonal, or floodplain), alternating with lenticular
beds of texturally mature sands ranging from subphyllarenites to phyllarenites. But
sand beds and clay beds are cleanly differentiated, which is not the case nearer the
source. Far from the welt, most of the softer MRF’s disappear and only metaquartzite
and strained quartz remain. Dilution with detritus contributed from the kratonal side
of the geosyncline also occurs. Texturally, the phyllarenites average fine to very fine
sand (the finest-grained clan, on the average) because much of the source material
breaks down into fine particles, and the bulk of the phyllarenites are deposited in low
energy environments like deltas or sluggish rivers. Maturity depends on the environ-
ment of deposition but is usually rather low. Mineralogically they contain abundant
metamorphic quartz, metamorphic rock fragments, and micas (especially muscovite).
Shales and silts are especially micaceous. Pebbles are not uncommon in high-energy
loci and consist of vein quartz and metaquartzite. These rocks often contain some
chert. Potash feldspar is usually lacking or present only in very small amounts. Clays
are chiefly sericite, illite, and chlorite because they are derived from breakdown of
MRF’s, but kaolin (sometimes authigenic) is common from heavily weathered source
areas. Heavy minerals are simple (hornblende, zircon, tourmaline) because metamor-
phism in the source area is still of the low rank type which produces few fancy
minerals. Winnowed sands are cemented with carbonates or quartz. These phyllare-
nites are usually gray or gray-green (because of the dark color of particles of slate,
etc.) but staining by hematite may give red or purple colors. Tertiary phyllarenites of
Texas contain light-colored MRF’s which make the rock appear to be full of whitish
flour specks.

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