Page 139 - Petrology of Sedimentary Rocks

are usually continental sediments. These arkoses are usually red because of pink
feldspar and a red, hematite-stained clay matrix either derived from the red soil
developed under warm, humid climates; Walker believes that red color may form also in
dry climates by intrastratal oxidation of heavy minerals.
Variants from this polar type are fairly common. Many times the block faulting is
not of enough magnitude to expose “pure” granite or gneiss; areas of schist, metaquart-
zite or veins then contribute considerable strained and hydrothermal quartz, metamor-
phic rock fragments, micas, and typical metamorphic heavy minerals to the arkose and
it consequently grades into an impure ar kose. If faulting is of still smaller magnitude or
a thick sedimentary cover is present, this blanket may not be completely eaten through,
and reworked grains of older sandstones, cherts, and limestones are contributed; the
rock may even grade into a calclithite if carbonate detritus dominates. Infrequently, in
place of granite a more basic pluton furnishes detritus; here the feldspar will be
dominantly plagioclase. If climate is dry instead of humid, there will be much less clay,
the red hematitic matrix will be absent, and the feldspar will all be pretty fresh. Block
faulting bordering the sea (as in California) produces marine arkoses with different
textural features, and some of these are turbidites.

Climatic arkose, often Qk.G/s/CD(n). U d er some situations a granitic
n
land area becomes peneplaned during a period of tectonic quiesence. Such may happen
either to areas that have been stable for a very long time (e.g., Canadian shield), or to
areas that have been uplifted, undergone deep erosion, and then were planed down on
cessation of uplift. If the climate is humid, the feldspar will all be decomposed because
of the slow rate of erosion on such low-relief topography, and quartzarenites will result.
But if the climate is dry very little chemical decay of feldspar will take place and
supermature arkose results. Feldspar is eliminated by abrasion alone, because of its
inferior hardness. Because of tectonic quiescence and low topography, beaches and
dunes comprise the bulk of the sedimentary environments. No areas are subsiding or
being uplifted greatly, consequently, a sheet of sand is spread smoothly and evenly by
broadly transgressive or regressive seas; the very slow influx of detritus permits
uniform spreading of the sediment. Thus sands are of the blanket type. Sediments are
supermature because of the high abrasion and sorting efficiency of the beach-dune
environment and the great length of time available under such slow conditions of
erosion and deposition. The sediments are superbly rounded fine to very fine sands,
with the feldspar fresh and usually finer than the quartz (Folk). Feldspar content can
range up to 40 percent or more; fine-grained beds may carry abundant feldspar and
intercalated coarser ones very little. Common quartz is the only other major
constituent; heavy minerals are almost solely rounded tourmaline, zircon and occasion-
ally garnet because little else can stand such prolonged and effective abrasion.
Cements may be quartz, carbonates, evaporites, or quite frequently authigenic feldspar
overgrowths. There is little associated shale section because of the lack of soil in the
source area. The color is white, so that in the field they are very commonly mistaken
for quartzareni tes. Close study usually reveals whitish or light gray specks of feldspar.
With prolonged abrasion, these rocks pass into subarkose (the most common type of
climatic “arkose” is really a subarkose) or even into quartzarenites.

Volcanic Arkose and Plagioclase Arkose, often X.X/b/C. One type of plagioclase
arkose (that eroded from a basic pluton) has already been discussed. Most plagioclase
arkoses and subarkoses, however, are formed when volcanic activity contributes
material to rapidly depositing sediments. Deposition must be rapid because of the very
unstable nature of basic volcanic materials. If the volcanic materials are chiefly bits of
aphanitic ground-mass, then the term “volcanic-arenite” (or, if specific identification is
possible, “andesi te-areni te,” “rhyol i te-areni te, ” “basalt arenitic,” etc.) should be used; if

133