Page 149 - Petrology of Sedimentary Rocks

quartz or metaquartzi te. Because these are derived from erosion of many previous
formations, quartz types are diverse and some reworked overgrowths may be found. In
addition to inherited well-rounded grains, angular quartz may come from older, non-
rounded sandstones. Abundant chert is the chief diagnostic material; it is quite
commonly angular and associated with rounded and reworked quartz grains. There may
be a very little feldspar, mica, etc., so that these are not quartzarenites of high purity,
and grade into adjacent clans. The heavy minerals are also diverse but characteristi-
cally contain some recycled, rounded tourmaline and zircon. The color of these rocks is
usually dirty white or light gray with dark specks (chert).

Petrology of Chert-Arenites

These rocks are members of the litharenite family, in which chert fragments are
important. Some sandstones and conglomerates contain 50-90% chert grains, most of
these are dark gray in hand specimens. They are usually erogenic sediments eroded
from tectonically disturbed older sedimentary sequences, D.S/p/Cf. Mesozoic sand-
stones of the Rockies are of ten chert-areni tes (Ballard). However, some chert-areni tes
are residual gravels and sands developed on extensively weathered cherty limestone
terranes-Q.S/s/C(w).

Petrology of Calclithites, typically D. Sc/p/Cf(n).

Calclithites are terrigenous rocks of the Litharenite group, in which carbonate
rock fragments dominate; often they contain over 50 percent carbonate fragments,
obtained by erosion of limestones or dolomites outcropping in a source land. The source
rocks are usually (though not necessarily) much older than the calclithite itself; thus the
Miocene Oakville formation of Texas consists of grains of Cretaceous limestone; some
Triassic beds in New Mexico consist of pebbles of Pennsylvanian Carbonate rock;
Pennsylvanian conglomerates in Oklahoma consist of Ordovician limestone pebbles.
Hence these are to be distinguished from intrasparite limestones, which are made up of
a penecontemporaneous intraclasts torn up from the basin of depositon itself. Calcli-
thites are terrigenous rocks like arkoses, graywackes, or orthoquartzites; it is indefen-
sible to consider them as limestones although they have the composition of a
limestone--it would be just as bad as calling an arkose a “granite” because they both
contain quartz and feldspar.

In order for a rock to be made largely of a constituent so soft and so soluble as
limestone, the rate of erosion must have greatly overbalanced the rate of chemical
decay. Hence these rocks are exactly like arkoses, whose unstable and soft feldspar can
be preserved only by rapid erosion or a dry (or cold) climate. Therefore calclithites also
form chiefly in areas of intense faulting, but where the sedimentary cover is so thick or
faulting is not of enough magnitude to get down to the granitic basement rock. It is
possible that some calclithites are the result of a cold or arid climate alone, though
these also require rather rapid erosion and deposition to preserve the fragments from
much abrasion. Calclithites are common in the post-erogenic sandstones of thrusted
cabonate regions such as the calcareous Alps and Carpathians of Slovakia and Poland
(Siedlecka). In these areas of vigorous deformation, they have formed even under a
humid climate, and are still accumulating today. Calclithites are forming today even in
tropical New Guinea as a result of violent uplift (see Creaser, 1977, Austr. Atnl. Univ.
for huge review).

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