Laboratory Manual for Introductory Geology 4e

6.3.3 Biochemical and Organic Sedimentary Rocks
While alive, some organisms build shells by extracting dissolved ions from their
environment. For example, clams, oysters, and some types of algae and plankton
use calcium, carbon, silicon, and oxygen ions dissolved in water to produce shells
made of calcite or quartz. Plants extract CO 2
from the air to produce the cellulose
of leaves and wood. And all organisms produce organic chemicals, meaning chemicals
containing rings or chains of carbon atoms bonded to hydrogen, nitrogen, oxygen,
and other elements. When these organisms die, the materials that compose their
shells or cells can accumulate, just as inorganic clasts do. Calcite and quartz shells
are quite durable—after all, they’re composed of relatively hard minerals compared
with the organism’s soft organic tissues, which commonly decompose and oxidize.
But in special depositional settings where there is relatively little oxygen and the
organisms are buried rapidly, organic chemicals can be preserved in rocks.
Rocks composed primarily of the hard shells of once-living organisms are classified
as biochemical sedimentary rocks, indicating that living organisms extracted
the dissolved ions. Rocks containing significant amounts of carbon-rich soft tissues
are called organic sedimentary rocks. Continuing diagenesis tends to modify both
biochemical and organic rocks significantly after initial lithification. For example,
rocks containing abundant calcite may undergo pervasive recrystallization, so that
the original grains are replaced by new, larger interlocking crystals. Diagenesis may
also dissolve some grains and precipitate new cements in pores, and it may drive the
transformation of calcite (CaCO 3
) into dolomite [CaMg(CO 3
) 2
]. Organic chemicals
tend to lose certain elements over time and are transformed into either pure carbon
or a hydrocarbon (carbon 1 hydrogen). The textures of biochemical and organic
rocks, like those of other sedimentary rocks, reflect the nature of the depositional
environment. In some settings, shells are transported by currents or waves and break
into fragments that behave like clasts. Geologists distinguish among many types of
biochemical and organic sedimentary rocks based on their composition and texture.
6.3.3a Biochemical Sedimentary Rocks
■ Limestone is a general class of sedimentary rock composed of calcite. Biochemical
limestone, which forms from calcite-containing shells, commonly tends
to form chunky blocks, light gray to dark bluish gray in color. Geologists recognize
distinct subcategories of biochemical limestone by their texture:
– Fossiliferous limestone contains abundant visible fossils in a matrix of fossil
fragments and other grains.
– Micrite (or micritic limestone) is very fine-grained limestone formed from
the lithification of carbonate mud. The mud may be made up of the tiny
spines of sponges or the shells of algae or bacteria, or it may form after
burial beneath sediment via diagenesis.
– Chalk is a soft white limestone composed of the shells of plankton.
– Coquina consists of a mass of shells that are only poorly cemented together
and have undergone minimal diagenesis.
■ Biochemical chert forms when shells of silica-secreting plankton accumulate
on the seafloor and partially dissolve to form a very fine-grained gel, which then solidifies.
We use the word biochemical to distinguish this chert from replacement chert,
which forms by diagenesis in previously lithified limestone.
6.3.3b Organic Sedimentary Rocks
■ Coal, composed primarily of carbon, is derived from plant material (wood,
leaves) that was buried and underwent diagenesis. Geologists distinguish three ranks
152 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY