Page 7 - Petrology of Sedimentary Rocks

INTRODUCTION TO SEDIMENTARY ROCKS

Sedimentary rocks cover some 80 percent of the earth’s crust. All our knowledge
of stratigraphy, and the bulk of our knowledge of structural geology are based on
studies of sedimentary rocks. An overwhelming percentage of the world’s economic
mineral deposits, in monetary value, come from sedimentary rocks: oil, natural gas,
coal, salt, sulfur, potash, gypsum, limestone, phosphate, uranium, iron, manganese, not
to mention such prosaic things as construction sand, building stone, cement rock, or
ceramic clays. Studies of the composition and properties of sedimentary rocks are vital
in interpreting stratigraphy: it is the job of the sedimentary petrologist to determine
location, lithology, relief, climate, and tectonic activity of the source area; to deduce
the character of the environment of deposition; to determine the cause for changes in
thickness or Iithology; and to correlate beds precisely by mineral work. Sedimentary
studies are also vital in prospecting for economic mineral reserves, especially as new
deposits become harder to locate. Study of sediments is being pursued intensely by oil
companies, phosphate, uranium, and iron mining companies in order to locate new
deposits and explain the origin of those already known.

Fundamental Classification of Sedimentary Rocks. Sediments consist fundamen-
tally of three components, which may be mixed in nearly all proportions: (I) Terrige-
nous components, (2) Allochemical components, and (3) Orthochemical components.

a. Terrigenous components are those substances derived from erosion of a land
area outside the basin of deposition, and carried into the basin as som
examples: quartz or feldspar sand, heavy minerals, clay minerals, chert or
limestone pebbles derived from erosion of older rock outcrops.

b. Allochemical constituents (Greek: “allo” meaning different from normal)
are those substances precipitated from solution within the basin of deposi-
tion but which are “abnormal” chemical precipitates because in general they
have been later moved as solids within the basin; they have a higher degree
of organization than simple precipitates. Examples: broken or whole shells,
oolites, calcareous fecal pellets, or fragments of penecontemporaneous
carbonate sediment torn up and reworked to form pebbles.

C. Orthochemical constituents (Greek: “ortho” meaning proper or true) are
“normal” chemical precipitates in the customary sense of the word. They
are produced chemically within the basin and show little or no evidence of
significant transportation or aggregation into more complex entities. Exam-
ples: microcrystalline calcite or dolomite ooze, probably some evaporites,
calcite or quartz porefi llings in sandstones, replacement minerals.

Classes (b) and (c) are collectively referred to as “Chemical” constituents; classes
(a) and (b) can be collectively termed “Fragmental.” Some people use “detrital” or
“elastic” as equivalent to “terrigenous”; other people use “detrital” or “elastic” as a
collective term including both “terrigenous” and “allochemical” above.

Sedimentary rocks are divided into five basic classes based on the proportions of
these three fundamental end members, as shown in the triangular diagram:

I