Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 41

2.3.4 Rock Formation: Diagenesis
After sediments are formed, the processes of diagenesis turn the sediment into rock.
Diagenesis describes all the physical, chemical, or biological changes sediment undergoes
after it has been deposited.
The types of diagenesis a sedimentary deposit may experience include (Pettijohn 1975)
• Compaction
• Deformation
• Dissolution
• Cementation
• Authigenesis
• Replacement
• Recrystallization
• Hydration
• Bacterial action
• Concretion development
The most important diagenetic processes acting upon a sedimentary deposit are compac-
tion and cementation (lithification). Compaction occurs over time within a sedimentary
deposit where individual grains are rearranged to form a more tightly packed sediment.
The process of compaction forces out water that may be trapped in pore spaces between
mineral grains. Compaction is a function of burial depth, and is the first step to a sedi-
ment's journey of becoming a rock.
Cementation involves the precipitation of minerals at grain-to-grain contacts and within
the deposit's pore spaces. Only after cementation has occurred can a sedimentary deposit
be considered a rock. Although compaction may have significantly reduced the amount
of pore space within a sedimentary deposit, some pore space usually remains. Fluids con-
taining minerals migrate through the remaining pore spaces and precipitate cementing
materials within the remaining pore spaces and finalize the conversion of a sediment to a
rock. The most common cements in sedimentary deposits are calcium carbonate (CaCO 3 ),
silica (SiO 2 ), and iron oxides (Fe x O x ) (Flint and Skinner 1974).
2.4 Identification and Classification of Sedimentary Deposits
To this point, sedimentary rocks have been subjected to significant stress. They have
been torn from their original home, broken apart and perhaps changed chemically, and
then put into nature's equivalent of the trash compactor and cement mixer. We now
have them right where we want them—in an observable form near the Earth's surface—
so we can proceed with their identification and classification. The knowledge gained
from these procedures allows the investigator in urban areas to identify sedimentary
deposits of potential commercial value; including the sand and gravel used in con-
struction and consumer products, and the clay used in the manufacture of bricks. The