Page 395 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 359
(a) Tilted Early Ordovician shale at Green Point in Gros Morne National Park, western Newfoundland.
(b) Limestone landscape; inset samples show biochemical limestone with shells and clasts cemented together.
Sandstone Limestone
▲Figure 12.8 Sedimentary rock types. [(a) All Canada Photos/Alamy. (b) Bobbé Christopherson. (c) nASA. Sandstone: michal812/Shutterstock. Limestone: Bobbé Christopherson.]
these size classes, combined with their composition, sorting, and cement characteristics, determine the com- mon sedimentary rock types. For example, pebbles and gravels become conglomerate, silt-sized particles become siltstone or mudstone, and clay-sized particles become shale.
Chemical Sedimentary Rocks Some sedimentary rocks are formed not from pieces of broken rock, but instead from the shells of organisms that contain calcium carbonate (a biochemical process) or from dissolved minerals that precipitate out of water solutions (a chemical process) and build up to form rock. Chemical precipitation is the formation of a separate solid sub- stance from a solution, such as when water evaporates and leaves behind a residue of salts. These processes are especially important in oceanic environments, as well as in areas of karst topography (discussed in Chapter 14).
The most common chemical sedimentary rock is limestone, and the most common form of limestone is biochemical limestone from marine organic origins. As discussed in Chapter 11, many organisms extract dis- solved CaCO3 from seawater to construct solid shells. When these organisms die, the solid shell material builds up on the ocean floor and is then lithified to become limestone (Figure 12.8b).
(c) Ancient sediment deposition is evident on Mars, in the western Arabia Terra.
Table 12.3
Clast Sizes and Related Sedimentary Rocks
Clast Size
Sediment Type
rock Type
>80 mm (very coarse) Boulders,
cobbles (breccia, if pieces
>2 mm (coarse)
Pebbles, gravel
Conglomerate
0.5–2.0 mm Sand (medium to coarse)
0.0039–0.062 mm (fine) Silt
Conglomerate are angular)
Sandstone
Siltstone (mudstone)
0.062–0.5 mm (fine to medium)
Sand
Sandstone
<0.0039 mm (very fine)
Clay
Shale
oxides and silica. Drying (dehydration) and heating can also unite particles.
The different sediments that make up sedimentary rock range in size from boulders to gravel to sand to mi- croscopic clay particles (Table 12.3). After lithification,