Page 153 - Petrology of Sedimentary Rocks

DESCRIPTION AND NOMENCLATURE
FOR TERRI GENOUS SEDIMENTS

For super-detailed description of sedimentary rocks of all types in thin section,
the following outline is used. This form sheet covers in complete detail every rock
property, and for many rocks, parts of it may be omitted.

Before starting with the microscope, hold the slide up and look at it with a hand
lens to see the major structures. Then start with the lowest microscopic power. It is
very useful to look at slides using direct sunlight as a reflected light source; much
valuable detail can often be seen in otherwise featureless opaques like limonite or
hematite.

Point-counting technique and theory are discussed by Chayes (1956). A point
count is most conveniently accomplished by laying out an equispaced grid to cover most
of the microscopic slide, but theoretically any random distribution of points is
satisfactory, just so one does not cheat in the location of points. All points that the
crosshair lands on should be tallied, even “unknowns” or “garbage”. The interval
between points should be larger than the largest grain; but if this cannot be managed
and one grain is hit twice, it should be counted twice, etc. For most sandstones a
spacing of about I mm is satisfactory. In a thin section, the traverse should not run
parallel to bedding because a string of points might run along a placer of micas, heavy
minerals, etc. For coarse rocks, a grid of lines can be ruled on plastic film and placed
over the sawn surface.

I. REFERENCE NUMBER, Geologic age, formation, stratigraphic level within
that formation, locality, regional geology and structure (for example, if the
rock comes from a very tight fold or from vicinity of a fault that will affect
its properties and should be noted).

II. NAME OF THE ROCK (following the pattern, grain size: prominent ortho-
chemical cements, textural maturity, notable or unusual transported con-
stituents, main rock name. This of course is done last).

Ill. MEGASCOPIC PROPERTIES (hardness, color, freshness, sedimentary struc-
tures, fossils, sorting and grain size, composition of the terrigenous and
orthochemical minerals).

IV. MICROSCOPIC DESCRIPTION
A. Brief summary of the important features of the rock (so the reader
doesn’t have to wade through a great mass of detail to find the
features you think are very important--give a brief summary of the
major, unusual or interesting features of the rock just like you’d write
an abstract of a paper).

B. Texture
I. Fundamental end-members
Percent terrigenous materials
t : Percent allochemical materials
Percent orthochemical materials
:: Main rock group, based on the above (See page 2, syllabus).

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