Page 106 - Petrology of Sedimentary Rocks

PETROLOGY OF SANDSTONES

According to Krynine (Jour. Geol. 1948) sedimentary rocks have three basic
properties: mineral composition, texture, and structure. Mineralogy and texture have
already been discussed from a piecemeal aspect; now it remains to see how they are
integrated in the classification and used in interpreting the genesis of sandstones.
Structure is not covered in this syllabus, many excellent texts now exist [e.g.
Pett ij ohn/Potter, ( I 963- 1964)] .

Textural Maturity. The texture of sediments has two aspects: (I) description of
properties, i.e., determination of the grain-size parameters and grain-size name,
measurement of grain shape and description of surface features, and (2) integration of
these properties into an assumed sequential development, the four stages of textural
maturity. (Folk, J. Sed. Pet. 1951, 1956). This concept proposes that, as sediments
suffer a greater input of mechanical energy thorugh the abrasive and sorting action of
waves or currents, they pass sequentially through the following four stages:

I. Immature stage. Sediment contains over 5 percent terrigenous clay matrix;
sand grains usually poorly sorted and angular.

II. Submature stage. Sediment contains under 5 percent clay, but sand grains are
still poorly sorted (a over 0.5~)) an1 clre not well rounded.

Ill. Mature stage. Sediment contains little or no clay, and sand grains are well
sorted (a under &SC)), but still not rounded.

IV. Supermature stage. Sediment contains no clay, sand grains are well sorted and
well rounded (Waddell roundness over .35; p over 3.0). This determination should be
made, if possible, on quartz grains of medium and fine sand size.

Textural maturity is thus one of the most important keys to the physical nature of
the environment of deposition, since it provides a descriptive scale that indicates the
effectiveness of the environment in winnowing, sorting, and abrading the detritus
furnished to it. Thus immature sediments accumulate in loci such as flood plains,
alluvial fans, or neritic or lagoonal environments where current action is either weak or
deposition is very rapid so that sediments do not have a chance to be subjected to input
of any mechanical energy after deposition. Supermature sediments, on the other hand,
indicate deposition in loci of intense abrasion and sorting, such as beaches or desert
dunes, where energy is constantly being expended on the grains.

Because it is supposed to be an index of original sedimentational conditions, one
ignores later effects of diagenesis--i.e., discounts authigenic clay minerals that have
crystallized in formerly open pore spaces, ignores chemical cements, and considers
shapes of grains underneath overgrowths if any. To determine textural maturity, follow
this key:

I. Determine the percentage of clay (including micaceous hash finer than
coarse-silt size, .03mm).

a. If the rock has over 5 percent clay, it is IMMATURE.

b. If the rock contains less than 5 percent clay, go to (2).

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