Page 35 - Petrology of Sedimentary Rocks

Suggested Outline for Detailed
Study of Texture

In routine field examination the determination of a grain-size name is probably
sufficient; but for detailed work the following model for textural description is offered.
Following the specimen number, stratigraphic and geographic position, and other
pertinent information, the method of study is recorded, whether it be by hand-lens,
binocular-microscope, thin-section, or mechanical analysis. A space is left for the
grain-size name, which, of course, is filled in after the description has been made.
Next, the size characteristics of the sediment as a whole and then of each of the three
size fractions are reported. For the entire sediment, the range between the sixteenth
and eighty-fourth percentiles of the size distribution should be given, as this gives a
much truer picture of the nature of the sediment than simply mentioning the extreme
range. Possible discontinuities in the distribution (bimodality) are very important and
should be described in detail. Following this information appears a discussion of the
relation of the three textural end-members of one another, including the following
characteristics: (I) homogeneity (Does the rock have coarse and fine layers, or are the
size fractions thoroughly intermixed?); (2) thickness and type of bedding (is it massive,
random, or cyclic parallel, hummocky, graded-bedded, or cross-bedded? Is the bedding
caused by a physical parting or a change in composition, texture, or color?); (3)
orientation of grains; and, finally, (4) particle shapes. Mention is then made of the
correlation of size with composition (for example, the gravel fraction might consist
entirely of shell fragments, whereas the sand fraction was entirely quartz). An example
of this method follows:

I. Sample number and location: (Give pertinent date)

II. Method of study: Hand lens, in outcrop

Ill. Grain-size name: Pebbly muddy fine sandstone

IV. Detailed size description

A. Entire sediment
I. Median: 0.2-0.3 mm.
2. Sorting: Poor
3. 16-84 per cent range: Clay size to about 5 mm.
4. Extreme range: Clay size to 20 mm.

B. Gravel f rat tion
I. Per cent: I5
2. Median: About 6 mm.
3. Range: 3-20 mm., in itself poorly sorted

C. Sand fraction
I. Per cent: 55-65
Median: Probably in fine-sand class, about 0.2 mm.
:: Range: Up to 0.5 mm., rather well sorted

D. Mud fraction
I. Per cent: About 20-30
2. Silt versus clay: About half clay(?)
3. Median of silt portion: Not determined in the field

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