Page 33 - 6200

1. Clastic Sedimentary Rocks-formed as a result of the weathering or fragmentation of pre-
existing rocks and minerals and classified on the basis of their textures, primarily the sizes of the
grains. Sedimentary rocks are divided into coarse-grained: conglomerates, medium-grained:
sandstones, and fine-grained: siltstones, mudstones, and shales. Within each textural category,
clastics are further subdivided by mineralogy, which reflects the parent rock, for example, a quartz-
rich sandstone or a feldspar-rich sandstone.
2. Chemical or Biochemical Sedimentary Rocks-formed as a result of chemical processes.
Primary carbonate deposition results from the precipitation and deposits formed by plants and
animals that utilize carbonates in their life processes. The most abundant mineral chemically or
biochemically precipitated in the oceans is calcite, most of it the shelly remains of organisms and
the main constituent of limestone. Many limestones also contain dolomite, a calcium-magnesium
carbonate precipitated during lithification. Gypsum and halite are formed by the chemical
precipitation during the evaporation of seawater.
There are five types of sedimentary rocks that are important in the production of
hydrocarbons:
Sandstones
Sandstones are clastic sedimentary rocks composed of mainly sand size particles or grains
set in a matrix of silt or clay and more or less firmly united by a cementing material (commonly
silica, iron oxide, or calcium carbonate). The sand particles usually consist of quartz, and the term
“sandstone”, when used without qualification, indicates a rock containing about 85-90% quartz.

Speaking Section
12 Make up a dialogue on the following situation and talk to your friend. Let one of your group
mates translate the dialogue consecutively.
You are visiting a geological museum. Ask your guide to tell you about the basic
classifications and type of sedimentary rock, etc.

9