Page 140 - Petrology of Sedimentary Rocks

it is chiefly plagioclase, use “plagioclase-arenite” as the clan name. These rocks imply
unstable tectonic conditions and rapid deposition of volcanic materials plus other
diluting terrigenous detritus in fans, floodplains and river channels; hence maturity is
immature to mature. Grain size varies from conglomerates to silts. Volcanic biotite is
a common accessory as is apatite, together with basaltic hornblende and other more
common ferromagnesians. Sometimes these constitutents may be so abundant that the
rock can be called “hornblende-arenite,” etc. Clays may be chloritic or montmoril-
lonitic, occasionally grown as authigenic fringes; a chloritic matrix is very common if
the rock fragments are basic. Cements, if any, are usually calcite, opal or zeolite.
Volcanic-arenites are usually dark colored in the field (if derived from andesites, or
basal ts); plagioclase-arenites are light gray. If volcanic glass is a major component
(implying a siliceous magma), the rock may be classed as a “vitric-arenite”; these rocks
are light gray and, on the Texas Gulf coast, contain much idiomorphic zircon
(Callender).

Petroloav of Phvllarenites and Related Rocks

Metamorphic rock fragments, the chief essential constituent of the phyllarenite,
are soft and quite susceptible to abrasion. Therefore their abundance in sand
(consequently theclan designation--phylIareni te vs. subphyllareni te vs. quartzareni te)
varies quite enormously, with environment of deposition, distance from source, and
tectonic activity. The former term for this sandstone type was “graywacke,” now
abandoned as completely prostituted. The term “phyllarenite,” coined in 1965, was
suggested by an older term, “schist-arenite” as used by Knopf in the 1930’s. “Phyllare-
nite” implies that the major constituents are foliated metamorphic rock fragments:
slate, phyllite and schist.

Krynine (unpublished notes) divided his “graywackes” into “Drowned Graywackes”
(mio- and eugeosynclinal) and “Floating Graywackes,” mainly post-erogenic, with
transport direction outward from the kraton instead of inward from an offshore welt.
This is the basis for the classification used here: (I) Orthogeosynclinal deposits,
including (IA) Miogeosynclinal Phyllarenite, generally the product of crustal plate
juncture, KAK in compression phases, or K K, and (IB) Eugeosynclinal rocks (compo-
sition variable, volcanic and metamorphic); and (II) Rejuvenation Phyllarenite. Ortho-
geosynclinal phyllarenites tend to be deposited in long, narrow geosynclines al,ong the
continental margin with detritus derived from a welt beyond the margin and moving
dominantly inward, toward the kraton (as in the Appalachian geosyncline). Near the
continent, the geosyncline subsides less rapidly (miogeosyncline) to produce “normal”
phyllarenites lacking feldspar, while the outer geosyncline subsides more rapidly and
also receives volcanic contributions (eugeosyncline of Stille) to produce rocks that
range widely in composition from phyllarenite to volcanic-arenite to plagioclase-
areni te. Rejuvenation Phyllarenites can be produced any place; an ancient metamor-
phic terrane is simply uplifted (without further squashing) to produce a rejuvenated
source area, and they need have no connection at all with orthogeosynclines. A fine
example (first pointed out by H. Blatt) is the southern Appalachians, an area
metamorphosed in the Paleozoic but which provided a flood of Rejuvenation Phyllare-
nites that spread all the way to Texas in the Eocene (Todd, AAPG 1957) and which, in
the year 1979, is still providing phyllarenite sands to the local rivers.

Miogeosynclinal Phyllarenite, often Dh. MS/g/PCM(w). Intense horizontal defor-
mation on the proto-continental margin raises a welt or series of welts of considerable
relief, separated from the cratonal mass by a rather rapidly subsiding geosyncline. The
intense folding (sometimes isoclinal) and major overthrusting that accompany horizontal
deformation often convert older shales, sandstones and other rocks into low-rank

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