Page 36 - Petrology of Sedimentary Rocks

E. Chemical precipitates
Per cent: Probably under 5
:: Type: Mostly calcite, a little quartz

V. Textural relations

A. Gravel tending to be segregated in vague bands parallel with the
bedding, whereas sand and mud are intimately intermixed; bedding 3
feet thick, random parallel to hummocky, a response to grain-size
variation; gravel shows weak imbrication, dip southeast; pebbles
subround, sand grains subangular

B. Distribution strongly bimodal, with little material between 0.5 and 3
mm.; separate modes in pebble and fine sand size

C. Gravel fraction composed entirely of chert, dull white; sand is chiefly
quartz; mud not identifiable in the field

Every terminology has its weak points, and this one is certainly no exception. For
example, a unimodal specimen consisting of a well-sorted mixture of coarse silt and
very fine sand should, according to the ternary diagram, be placed in one of the fifteen
textural groups by determining the ratio of sand to mud (the latter in this case
consisting entirely of silt, with no clay); yet this proportion would be exceedingly
difficult to determine, and the rock might be termed anything from a “well-sorted
siltstone” to a “well-sorted silty very fine sandstone.” The difficulty in placing such
boundary specimens is present in every classification but should not be considered a
serious handicap.

It may be argued that by far the greatest bulk of sediments and sedimentary rocks
lies in the bottom, nongravelly tier of the diagram, which includes only four major
classes; nevertheless, by virtue of the fact that the grain size of the sand fraction (five
grades) and the silt:clay ratio (three divisions) must be determined, a very large number
of subclasses is available (fig. lb)--and this does not even include the mention of
sorting within groups and fissility, which should be apart of the rock name. Thus the
grain-size classification is capable of considerable “lumping,” if one wishes to confine
himself to major group names, or to a much more intricate subdivision, if one wishes to
study a series of rocks that are closely related in grain-size distribution.

Making Distribution Maps Showing Grain Size of Sediments

A very useful way to summarize data from recent sediments studies that involve
sampling large areas (bays, offshore areas, deltas, etc.) is to make a map of the
geographic distribution of grain size classes. This can be done very easily using the
grain size triangle nomenclature for sand-silt-clay mixtures. One takes his sampling
points and records for each one the percent of sand in the sediment. This data is
contoured using values of IO%, 50%, and 90% sand as contour lines (these are the
boundary lines of the four “sandiness” classes of the grain size triangle). Next, one
records at each sampling point the percent silt in the mud fraction (i.e. percent silt
divided by percent silt plus clay). These values are contoured at the 33% and 67% lines.
The map is now covered with two independent, intersection contour systems, and each
“cell” bounded by these contours is a grain size class (e.g. the sector bounded by the 50
and 90% sandiness contours and the 33 and 67% siltiness contours is termed “muddy
sand”). These different cells may not be labeled, colored or shaded to bring out the

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