Laboratory Manual for Introductory Geology 4e

FIGURE 6.3 The five steps in clastic sedimentary rock formation.
Weathering
Erosion
Solid particles and ions
are transported in
surface water (in river).
New sediment arrives
Water
Deposition
Escaping water
Weight of
overburden
Coarse
Ions enter
the sediment.
Substrate
Ions are transported
in solution in
groundwater.
Fine
Ions in moving
groundwater
Compaction and
cementation occur.
(a) Clasts produced by (1) weathering undergo (2) erosion,
(3) transportation, and (4) deposition. Dissolved ions may eventually
become cement.
(b) The process of (5) lithification takes place
during progressive burial.
majority are derived from silicate rocks and thus contain clasts composed of silicate
minerals. (To emphasize this, geologists sometimes refer to them as siliciclastic rocks.)
Formation of clastic sedimentary rock involves the following processes (FIG. 6.3a):
1. Weathering: The process of weathering reduces solid bedrock to loose clasts,
also known as detritus.
2. Erosion: Erosion occurs as moving water (streams and waves), moving air
(wind), and moving ice (glaciers) pluck or pick up the clasts.
3. Transportation: Moving water, wind, and ice carry clasts away from their source.
4. Deposition: When moving water or wind slows, or when ice melts, clasts settle
out and accumulate. The places where this occurs (such as the land surface,
the seafloor, or a riverbed) are called depositional environments.
5. Lithification: This process transforms loose sediment into solid rock through
compaction and cementation (FIG. 6.3b). Over time, accumulations of clasts are
buried. When this happens, the weight of overlying sediment squeezes out air and
water, fitting the clasts more tightly together. This process is called compaction. As
ion-rich groundwater passes through the compacted sediment, minerals precipitate
and bind, or “glue,” the clasts together. This process is called cementation,
and the mineral glue between clasts is called cement. Quartz and calcite are the
most common cements in sedimentary rock, along with limonite and hematite.
In a final process, the application of pressure and the circulation of fluids over
time may gradually change the characteristics of a sedimentary rock (e.g., grain
size and composition of cement). Any chemical, physical, or biological process that
alters the rock after it is formed is called diagenesis. It is important to note that
diagenesis occurs at relatively low temperatures—below those at which the changes
are considered to be metamorphism.
The names of clastic sedimentary rock types are based primarily on the size of
their clasts, described in the Wentworth scale (TABLE 6.1). From coarsest to finest,
clastic sedimentary rocks (and their grain sizes) include the following:
■ Conglomerate and breccia consist of pebbles and/or cobbles, often in a matrix
of sand and finer-grained material. Clasts in conglomerate are rounded (have no
148 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY