Page 94 - Petrology of Sedimentary Rocks

most characteristic and defining components of phyllarenites, formerly called “gray-
wackes.” They are composed of fine-grained quartz, micas, and sometimes clays. It is
somewhat difficult but necessary to distinguish them from clay pellets or shales; this
can be done by the fact that the MRF’s are coarser grained, better oriented and harder
due to recrystallization; shale fragments tend to be deformed by neighboring grains to a
greater extent. Blatt believes it is almost impossible to distinguish shale fragments
from slate fragments, and calls them both URF’s unless definite evidence of mica
recrystallization is visible. For convenience I would lump the whole spectrum under
MRF’s, even though this does somewhat diminish the “source-area” concept of rock
classification. No data has yet been collected, but it is suspected that fragments of
slate are more abundant than fragments of shale because of their superior hardness;
shale disappears rapidly during transport (Blatt).

Availability. Metamorphic rock fragments form 5-15% of the terrigenous fraction
of sediments, and are derived from low-rank to high-rank metamorphic rocks exposed in
areas of intense folding.

Durability. Metamorphic rock fragments are quite durable if composed largely of
quartz, but quite soft and easily abraded if composed chiefly of fine micas. As abrasion
continues on an assemblage of MRF’s, the softer ones become rounded rapidly and
reduced in size, eventually being broken down into clay which is winnowed away, while
the hard metaquartzite fragments persist. Todd (U.T.M.A., 1956) shows in the Carrizo
and Newby sands that MRF content may fluctuate from 0 to 30% in successive thin
beds, the result of differential abrasion and sorting. Beach sands consequent I y have
much less abundant MRF’s than fluvial sands from the same source. Upon very long
abrasion, it is possible that metaquartzite may abrade faster than single grains of
quartz, and supermature sediments hence may contain relatively less metaquartzite
(Blatt). For durability of RF’s, see Cameron and Blatt, ‘7 I JSP.

Normally, softer micaceous rock fragments abrade so rapidly that if they are
found together with well-rounded quartz it indicates a mixed metamorphic and older
sedimentary source, because they could not persist under the prolonged abrasion
required to round the quartz.

Since MRF’s are usually derived from foliated rocks, the fragments are generally
elongated or platy.

Stability. Metamorphic rock fragments are moderately stable chemically because
they are made up of quartz and mica, two stable minerals. However, under intense
weathering (e.g. Georgia) the MRF’s disaggregate into their components, quartz silt and
fine micas and then disappear from the sand fraction.

Depositional Characteristics. Metamorphic rock fragments have essentially the
same specific gravity and shape as quartz, thus travel with it. On long transport, MRF’s
wear down faster than quartz, hence finer sands may have more MRF’s than coarser
coarser sands. Compaction tends to squash some of the softer, more micaceous
fragments, reducing porosity significantly.

Sedimentary Rock Fragments (SRF’s)

Fragments of older sedimentary rocks are common in conglomerates. Many sands,
also, may contain reworked fragments of siltstone, shale, limestone, or fine sandstone.
Generally these fragments are not resistant to abrasion, so if present in important

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