Chapter Overview

The rock cycle is a theoretical model of the constant recycling of rocks as they form, are destroyed, and then reform. We began our discussion of the rock cycle with igneous rock (chapters 3 and 4), and we now discuss sedimentary rocks. Metamorphic rocks, the third major rock type, are the subject of the next chapter.

You saw in chapter 5 how weathering produces sediment. In this chapter, we explain more about sediment origin, as well as the erosion, transportation, sorting, deposition, and eventual lithification of sediments to form sedimentary rock. Because they have such diverse origins, sedimentary rocks are difficult to classify. We divide them into clastic, chemical, and organic sedimentary rocks, but this classification is not entirely satisfactory. Furthermore, despite their great variety, only three sedimentary rocks are very common - shale, sandstone, and limestone.

Sedimentary rocks contain numerous clues to their origin and the environment in which they were deposited. Geologists determine this information from the shape and sequence of rock layers and from the sediment grains and the sedimentary structures such as a fossils, cross-beds, ripple marks, and mud cracks that are preserved in the rock.

Sedimentary rocks are important because they are widespread and because many of them, such as coal and limestone, are economically important. About three-fourths of the surface of the continents is blanketed with a relatively thin skin of sedimentary rocks. Concentrated in sedimentary rocks are important natural resources such as crude oil, natural gas, ground water, salt, gypsum, uranium, and iron ore.

Learning Objectives

1. Sedimentary rocks have highly diverse origins and are difficult to classify, but only three are very common: shale, sandstone and limestone. Seventy five percent of continental surfaces are covered by sedimentary rocks.

2. Sediment is unconsolidated particles of either preexisting rocks or chemical precipitates. It is classified by size: gravel> 2 mm< sand> 1/16 mm<"mud", without regard to composition, although most grains of"mud" size are clay minerals.

3. Rounding (grinding away sharp edges) and sorting (separation by size) occur during transportation, usually by streams. Size decreases downstream in a river. Deposition occurs when agents of transportation lose their energy. Preservation of sediments requires their burial and is favored in subsiding basins.

4. Lithification converts loose sediment to sedimentary rock, usually by compaction (reduces pore space) and cementation (fills remaining pore space). These rocks have a clastic texture.

5. Not all sedimentary rocks form from sediment. Some form through crystallization of minerals from solution (example, calcite). These rocks have a crystalline texture, but that texture can also result from recrystallization that has destroyed an originally clastic texture.

6. A section on types of sedimentary rocks expands the discussion of clastic versus crystalline textures, and includes organic rocks as well. Clastic sedimentary rocks are classified by grain size and composition. Breccia (angular) and conglomerate (rounded) (the term "till" is deleted in 9th edition) have a gravel fraction. Sandstones contain sand-size grains, distinguished as quartz sandstone (>90% quartz), arkose (>25% feldspar), and graywacke (>15% matrix = silt and clay). Graywackes result from deposition by turbidity currents. Lithified silt forms siltstone, while combinations of silt and clay form shale. Predominately clay-size particles form claystone and mudstone.

7. Limestone is composed mostly of calcite through the action of organisms or as an inorganic precipitate. Varieties include coquina (cemented shells), bioclastic limestone (coarse-grained fossils), chalk (very fine-grained bioclastic limestone), oolitic limestone (small spheres of calcite), tufa and travertine (crystalline precipitates) and recrystallized limestone (original texture lost). Dolomite is a mineral CaMg(CO3)2 and a rock (sometimes called dolostone) that occurs as a replacement of limestone and destroys its original texture.

8. Chert is a fine-grained, sedimentary rock composed almost entirely of silica. It
can be a replacement, inorganic precipitate, or bioclastic, and may be recrystallized. Evaporites are sedimentary rocks formed from evaporation of seawater. They have crystalline textures and include rock gypsum and rock salt. Coal forms from consolidation of plant material, originally as peat. Compaction transforms peat to coal and several varieties are recognized. Organic material preserved in marine muds change to oil and natural gas through increased heat and pressure provided by burial.

9. Sedimentary structures form before lithification. Horizontal bedding planes are the most common feature of sedimentary rocks and reflect original horizontality and superposition. Cross bedding is inclined and most common in sandstones as a reflection of wind or water currents. Graded bedding exhibits a vertical change in grain size and characterizes turbidity current deposition. Mud cracks require air-drying of very fine-grained sediments. Ripple marks are either symmetric (waves) or asymmetric (currents) and form in any clastic rock. Fossils are traces of plants or animals buried by sediment and preserved as unaltered original material, replacements, molds or carbon films. Fossils may occur in any sedimentary rock type, but are most common in limestones.

10. Formations are bodies of rock recognized as a convenient means to map, describe and interpret the geology of a region. The first name is a geographic location where it is well exposed, and the second name is its rock type. The bounding surfaces are called contacts.

11. Source area of the sedimentary rock is determined by the composition of its grains (for example feldspar, quartz and mica indicate a granitic source). Sedimentary deposits thin away from their source, and sedimentary structures may help determine direction of current flow.

12. Continental environments include glacial environments, alluvial fans, river channels, that usually have a gravel component. Flood plains and lakes usually develop shales. Dunes have high angle cross-bedding. Shallow marine environments include deltas (usually with thick siltstone and shale, cut by sandstone channels), beaches and barrier islands (well sorted, quartz sandstone), dunes (high angle cross bedding), lagoons (shales), shallow marine shelves (widespread sandstone, siltstone and shale), and reefs (massive limestone cores). Deep marine environments receive deposition from turbidity currents.

13. The distribution of sedimentary rocks may be controlled by plate tectonics. Convergent boundaries accumulate thick clastic deposits in sedimentary basins. Turbidity currents dominate forearc basins, while sediments derived from rising mountains fill backarc basins. These deposits may now be found in mountains marking those plate boundaries. Transform boundaries may have organic rich deposits, while diverging boundaries form rift valleys with gravels, lake deposits and evaporites.

Related Readings

Blatt, H. 1992. Sedimentary Petrology. 2nd ed. New York: W. H. Freeman.

Boggs, S. 1987. Principles of Sedimentology and Stratigraphy. Columbus, OH: Charles E. Merrill.

Boggs, S. 1992. Petrology of Sedimentary Rocks. New York: Macmillan.

Busby, C. J., and R. V. Ingersoll. 1995. Tectonics of Sedimentary Basins. Palo Alto, CA: Blackwell.

Davis, R. A. 1992. Depositional Systems. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall.

Pettijohn, F. J. 1975. Sedimentary Rocks. 3rd ed. New York: Harper and Row.

Pettijohn, F. J., and P. E. Potter. 1964. Atlas and Glossary of Primary Sedimentary Structures. New York: Springer-Verlag.

Prothero, D. R., and F. Schwab. 1996. Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy. New York: W. H. Freeman.

Reading, H. G., ed. 1986. Sedimentary Environments and Facies. 2nd ed. Oxford: Blackwell.

Scholle, P. A., D. G. Debout, and C. H. Moore, eds. 1983. Carbonate Depositional Environments. Tulsa: Amer. Assoc. of Petroleum Geologists Memoir 33.

Scholle, P. A., and D. Spearing, eds. 1982. Sandstone Depositional Environments. Tulsa: Amer. Assoc. of Petroleum Geologists Memoir 31.

Selley, R. C. 1982. An Introduction to Sedimentology. 2nd ed. New York: Academic Press.

Tucker, M. E. 1991. Sedimentary Petrology: An Introduction. 2nd ed. Oxford: Blackwell.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 6:

Boxed Readings