Page 90 - Petrology of Sedimentary Rocks

Feldspars may become altered (I) in the source rock through deuteric or
hydrothermal activity; (2) in the soils of the source area by weathering; (3) post-
depositionally, either on deep burial by migrating connate waters, or when the
sedimentary rocks are exposed to the Recent surface and weathered. Post-depositional
alteration may be suspected if weathered feldspar grains tend to occur together, as in
the neighborhood of veins, stylolites, or other weathered surfaces; or if well-cemented
rocks contain fresh feldspar and porous portions contain weathered feldspar. Compari-
son of the freshness of feldspar overgrowth with the feldspar sand grains also helps. In
some sedimentary rocks, feldspars are partially to completely replaced by calcite;
rarely, they are replaced by quartz.

Considering the several kinds of feldspar, microcline is the most stable under
weathering conditions, with orthoclase next. Sodic plagioclase is apparently more
unstable under surface conditions, for in streams and soils on peneplaned igneous areas
there is very little plagioclase, even though it is common in the source rocks. In Llano
(Texas) plagioclase dissolves much more rapidly than microcline (Begle). Calcic
plagioclase is extremely unstable, thus in sediments usually occurs as the result of
volcanism, whereby it has short-circuited any soil-forming process.

Climatic, Tectonic and Physiographic Interpretation based on Feldspar. This is a
complex subject, because of the relative durability and chemical stability of quartz and
feldspar, together with the confusing effects of rate of weathering and rate of erosion.
P.D. Krynine has shown that the main clues are grain roundness, indicating length of
time available hence the rate of erosion, the ruggedness of the topography and
ultimately the tectonic framework; and average degree of alteration of the feldspar
together with homogeneity of alteration of the feldspar. By homogeneity is meant: “do
all the feldspars of one species show about the same degree of alteration, or does the
same species show a mixture of fresh and weathered grains?” It is normal in sediments
to find orthoclase more weathered than microcline or sodic plagioclase, but the thing
you must look for is a mixture of fresh plus weathered grains of the same mineral
species (Krynine).

A humid climate with rugged topography produces abundant feldspar in angular,
coarse grains with a wide variation from fresh to heavily weathered (because the
vigorous streams can cut through the intensely weathered mantle to fresh bedrock). If
the climate is dry the feldspar will all be pretty fresh, as was observed by Mackie
( 18991, Judd (I 885), and Wade (191 I, QJGS). Well-rounded, fresh feldspars if present in
moderate to abundant quantitites are excellent indicators of an arid climate and
peneplane conditions. These are commcn in some of our best ancient dune sand
blankets, associated with dune cross-bedding and evaporities. The criterion seems to
work so well that it may somewhat irreverently be referred to as the “Dogma of the
Immaculate Feldspar”. If topography is low, and the climate is humid and warm, nearly
all the feldspar will be weathered away. These relations are shown in the graph below,
in which “Intensity” is to be determined, given the other factors.

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