Paleozoic Era

Paleozoic stratigraphy concerns the study of rock sequences that date from 544 to 245 million years before the present. The primary method by which the stratigraphic relationships of the Paleozoic are established is stratigraphic analysis of distinctive types of fossils.

Cambrian System

The Paleozoic is the oldest era of the Phanerozoic eon, ranging from approximately 544 to 251 million years before the present. In North America, it is divided into seven periods. Beginning with the oldest, these are the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian. Outside North America, the Mississippian and Pennsylvanian are combined to form the Carboniferous period. In stratigraphy, strata laid down during a particular period are referred to as a system. Systems are subdivided into series (the time-rock unit equivalents of epochs), which may be further subdivided into stages (the time-rock unit equivalents of ages). These subdivisions are very important in subdividing time-rock units of the Paleozoic and other eras of earth history.

The Cambrian is the oldest system in the Paleozoic, ranging from approximately 542 to 488 million years before the present. It was named by Adam Sedgwick in 1835 on the basis of rock exposures in northern Wales. The Cambrian system is subdivided globally into a Lower (Early), Middle, and Upper (Late) series. The earliest rocks of the Phanerozoic eon are stratigraphically difficult to interpret. One reason is that although most rocks of the Paleozoic are subdivided primarily on the basis of their included fossils, the record of Precambrian and Early Cambrian life is poorly known. By Middle Cambrian times, the composition of the fossil communities had changed greatly; this proliferation of forms preserved in the rock record has enabled paleontologists to establish fairly well the stratigraphy of the Middle and Upper Cambrian series. The primary tool used in the establishment of Middle and Upper Cambrian stages is the stratigraphic distribution of trilobites, marine arthropods that somewhat resembled modern pill bugs. Acritarchs—tiny planktonic algae that form a resistant, fossilizable covering—were another group that have become useful index fossils for the Proterozoic (Upper Precambrian) through Devonian. Another common Cambrian fossil group is the brachiopods; these two-valved suspension feeders were of relatively simple form during the Cambrian, a fact that lessens their utility as stratigraphic indicators. During the Late Cambrian, a series of events caused many types of trilobites to become extinct. These events of extinction and evolutionary radiation are especially useful in precise definition of Cambrian stratigraphy. The last of these extinction episodes, which occurred at the very end of the Cambrian, eliminated numerous trilobite species. After this event, the trilobites never rebounded; in post-Cambrian strata they cannot be widely used as index fossils.

Ordovician, Silurian, and Devonian Systems

The second Paleozoic system is the Ordovician. It ranges from approximately 488 to 444 million years before the present. The Ordovician was named in 1879 by Charles Lapworth, who combined portions of the Cambrian and Silurian systems as first defined in Wales, thus ending a debate that had developed concerning those sequences of strata. The Ordovician may also be subdivided into Early, Middle, and Late series, and for these the stages are much better established than in the Cambrian.

Strata of Ordovician age are often characterized by an abundance of carbonate (limestone and dolomite) rocks, which were deposited upon shallow seaways extending over many of the continents. Although trilobites declined in number throughout the Ordovician, many other animal groups became abundant or appeared for the first time. Especially important for biostratigraphy are graptolites, primitive hemichordate animals that lived in colonies; often they are preserved as black marks that resemble tiny hacksaw blades on rocks. Also of biostratigraphic importance are the acritarchs and the nautiloids. The latter group were predatory mollusks that at this time typically had straight shells, with tentacles emanating from one end. The chitinozoans are another group used in biostratigraphy for rocks of Ordovician through Devonian age. Although they are often classified as algae, the precise relationships of these microscopic, typically vase-shaped organisms are unknown. Finally, the evolution of more complex, hinged-shell groups of brachiopods with distinctive form enables stratigraphers to utilize this group in biostratigraphy from Late Ordovician through Early Pennsylvanian series rocks. The end of the Ordovician is marked by another mass extinction event, in which approximately one hundred families of marine animals were wiped out.

The Silurian system was named by Roderick Murchison in 1835 on the basis of rock exposures in southern Wales. The age of the Silurian ranges from 444 to 416 million years before the present. Worldwide, an Early and a Late series are recognized. In the best-known exposures of Silurian rocks, however, various series designations are used. Silurian rocks are common on all continents and were deposited in continental or marine environments. The most common rocks of the Silurian are limestones and dolomites, although many dark, clay-rich marine rocks containing graptolites are present. As in the Ordovician, this group is a useful index fossil for the Silurian. Brachiopods, acritarchs, and chitinozoans are other important fossils used in determining the age of Silurian strata.

The fourth system of the Paleozoic is the Devonian, ranging from 416 to 359 million years before the present. The Devonian system was named by Murchison and Sedgwick in 1839 for rock exposures in Devon and Cornwall, in southwest England. Globally, the Devonian is subdivided into Early, Middle, and Late series. Devonian rocks are very common; they are present on all continents, where they formed in a great variety of environments. Thick sequences of Devonian-age sediments accumulated along the edges of ancient continents and record a time of changing climate and environments. On land, red bed sequences, colored by oxidation of iron-bearing minerals, are widely distributed for the first time. Of these units, the Old Red Sandstone of the British Isles is best known. As in the Ordovician and Silurian, graptolites, brachiopods, and chitinozoans are important marine index fossils for the Devonian. The proliferation of different groups of organisms also adds to the list of fossils used in stratigraphic correlation. Here, for the first time, pollen and spores can be used to define strata; they are especially important in correlating Devonian continental rocks. Also of biostratigraphic utility are the strange, tiny, toothlike conodont fossils (from animals of unknown affinity) and ostracods (minuscule, two-valved crustaceans). In addition, for the first time, ammonoids (cephalopods with typically coiled shells) are of widespread use in biostratigraphy. Goniatite ammonoids are of major importance in defining rocks of the Devonian through Permian age.

Mississippian, Pennsylvanian, and Permian Systems

The term “Carboniferous” was first applied to rocks within the area of north-central England by William Conybeare and William Phillips in 1822. Yet no type section (the area in which rock units are first described) was ever designated. The Carboniferous lasted from approximately 359 to 299 million years before the present. Although the system is named for coal-bearing strata, only its upper portion contains large amounts of coal; the lower portion typically consists of carbonates. In North America, a different classification is employed, based largely on these rock differences. In 1870, American geologist Alexander Winchell proposed the name “Mississippian” for Lower Carboniferous exposures in the Mississippi River valley between southeastern Iowa and southern Illinois. Rocks of the Mississippian range from 359 to 299 million years before the present. In 1891, Henry Williams named Upper Carboniferous exposures the Pennsylvanian, after the widespread coal-bearing strata in that state. The Pennsylvanian ranges from 318 to 299 million years before the present. The widespread application of these names to Carboniferous-age rocks in North America led to the formal recognition of the Mississippian and Pennsylvanian as systems by the U.S. Geological Survey in 1953.

The Mississippian system (Lower Carboniferous series) is characterized by the widespread distribution of limestone and dolomite formed by marine incursions over many continents. Cyclic successions are developed in many places within Mississippian-age strata, especially within the upper portion of the system, in which sequences of limestones, sandstones, and shale were repetitively deposited one upon the other. Goniatites, brachiopods, spores, and pollen are all utilized in the Mississippian-age strata for correlation purposes.

Within North America, there is a widespread unconformity (a gap in the rock record) between the Mississippian and Pennsylvanian (Upper Carboniferous). The Pennsylvanian was a time of extensive coal deposition within low-lying swamps within North America, Europe, and many portions of Asia. Coal deposits are often found capping cyclical sequences consisting of limestone, shale, sandstone, and more shale (with the associated coal). Within the marine sequences, goniatite ammonoids are utilized for determining age and, at the base of the system, brachiopods are used for correlation. In addition, fusulinid foraminifers (small fossils resembling grains of rice but giants of this one-celled group) become important for Upper Pennsylvanian correlation. With the abundance of coal-producing plants, fossil spores and pollen serve as valuable index fossils for continental sediments of the Upper Carboniferous.

The Permian system was named for the region of Perm in the western Ural Mountains of Russia. The system, first proposed by Murchison in 1841, roughly corresponds to a period of earth history between 299 and 251 million years before the present. The Permian may be subdivided into a Lower and Upper series. Permian deposits are thick and widespread in many parts of the world and indicate a time of climatic complexity. Permian stratigraphic sequences are quite varied. Coal was still present but was no longer being extensively formed in North America or central Europe. Continental deposits of the Permian are primarily known for extensive red bed sequences created through the oxidation of iron-rich sediments. Extensive evaporites are also present, especially in Upper Permian-age strata, with thick layers of gypsum and salt. Large reefs are found, and, on the Gondwana continents, tillite deposits give evidence for extensive Permian glaciation. Major index fossils for the Permian include goniatite ammonoids and fusulinids for the marine sequences and spores and pollen for continental deposits.

The end of the Permian is marked by an extinction event of major significance. Fusulinids, tabulate and rugose corals, and trilobites became extinct; brachiopods, bryozoans, and echinoderms sustained huge losses, and substantial numbers of bivalve and gastropod mollusks also became extinct. On land, many groups of the larger coal-swamp trees became extinct, and several major groups of vertebrates also died out. These huge losses in plant and animal types mark the stratigraphic boundary between the Paleozoic era (“the era of ancient life”) and the subsequent Mesozoic era.

Study of Paleozoic Stratigraphy

Studies of Paleozoic stratigraphy primarily utilize analyses of rock units, paleontological studies, and radiometric dating techniques. Lithostratigraphy defines strata (rock layers) on the basis of rock, or lithologic, characteristics. Distinctive rock layers are very useful in determining the position and relative age of local outcroppings of Paleozoic strata. Most Paleozoic systems tend to be dominated by specific rock types. For example, carbonate rocks (limestones and dolomites) are characteristic of many Ordovician, Silurian, and Mississippian sequences. Clastic rocks, including sandstones and mudrocks with their included coals, are diagnostic of Pennsylvanian-age deposits. Red bed deposits, consisting of oxidized iron-rich sediments, are often associated with clastic sequences in Devonian- and Permian-age strata. Unconformities—gaps in the rock record representing periods of erosion or nondeposition—often separate the Paleozoic systems and form natural boundaries for their separation. A major unconformity also typically separates Precambrian rocks from those of the earliest Paleozoic.

Biostratigraphy defines rocks on the basis of their fossil content. The basic unit of biostratigraphic classification is the biozone, in which strata are characterized by the occurrence of a certain fossil or fossils. These index or guide fossils are abundant, distinctive forms with a wide geographic distribution but narrow stratigraphic range and are therefore useful in correlation and in determining the age of strata. The fossils most often used are planktonic (floating) or nektonic (swimming) forms. Of special importance for the study of Paleozoic stratigraphy are trilobites, brachiopods, and graptolites. For certain portions of the Paleozoic, acritarchs, conodonts, chitinozoans, fusulinids, pollen, and spores, as well as nautiloid and ammonoid cephalopods, are widely utilized in biostratigraphic studies.

Lithostratigraphic and biostratigraphic studies can establish only the relative ages of strata. Determination of the absolute age of Paleozoic rocks is done primarily through radiometric dating techniques. These studies utilize the known decay rates of radioactive isotopes to establish the age of rocks in terms of years. Isotopes used for studies of Paleozoic rocks are uranium-235, uranium-238, and thorium-232, all of which decay to lead. Also used are potassium-40, which decays to argon-40, and rubidium-87, whose daughter isotope (product of decay) is strontium-87. Igneous and metamorphic rocks are generally utilized in radiometric dating techniques.

Significance

Probably the most important application of Paleozoic stratigraphy is in the search for natural resources. Without knowing the age of the rocks being evaluated or the economic products potentially included within them, the exploration geologist's job would be impossible. Of particular significance is the widespread presence of coal in rocks of Pennsylvanian and Permian age. Most of the coal produced in North America and central Europe has been mined from Pennsylvanian-age rocks. As these coal beds often occur within the same portion of a cyclical sequence of sediments, the search for them is made easier through knowledge of Paleozoic stratigraphy. Permian-age coal sequences are found in Australia, China, the former Soviet Union, India, and South Africa. There are also great quantities of oil and natural gas within rocks of Paleozoic age. Significant finds of hydrocarbons have been made in Devonian- and Carboniferous-age rocks, and discoveries of huge oil and gas fields in the Permian Basin of west Texas were a result of exploration of Upper Paleozoic marine strata. Arid conditions, especially common during the Permian, resulted in huge deposits of sodium and potassium salts as well as gypsum and anhydrite. Mountain-building events during the Paleozoic also created tremendous deposits of both precious metals and metals that are important for industry. Without an understanding of the stratigraphic occurrence of these valuable resources, the recovery of these materials would be impossible.

Principal Terms

biostratigraphy: defining rock layers on the basis of their fossil content

correlation: matching rock units of equivalent age

index (guide) fossil: the remains of an ancient organism that are useful in establishing the age of rocks; index fossils are abundant and have a wide geographic distribution, a narrow stratigraphic range, and a distinctive form

series: a time-rock unit representing rock deposition during a geologic epoch

stage: a time-rock unit representing rock deposition during a geologic age

stratigraphy: the study of rock layers (strata)

system: a time-rock unit representing rock deposition within a geologic period

Bibliography

Briggs, D. E., and R. A. Fortey. “Wonderful Strife: Systematics, Stem Groups, and the Phylogenetic Signal of the Cambrian Radiation.” Paleobiology 31 (2005): 94-112. The authors discuss the unresolved issues of systematics in reference to the morphological disparity and divergence times of the Cambrian period. The article provides specific examples to support the broad concepts. Best suited for graduate students and professionals interested in theories of evolutionary biology.

Brookfield, Michael E. Principles of Stratigraphy. Hoboken, N.J.: Wiley-Blackwell, 2004. Written for undergraduate students, this text provides an overview of the principles and applications of stratigraphy. Includes stratigraphic techniques and case studies. It is organized into three sections, beginning with foundational material, followed by data collection and research topics, and completed with interpretations and analysis.

Bruton, David L., ed. Aspects of the Ordovician System. New York: Oxford University Press, 1984. A symposium volume from the Fourth International Symposium on the Ordovician system, held in Norway in 1982. The introductory paper and a few of the papers on the stratigraphic framework and sea-level changes of the Ordovician would be understandable to a college-level student with some training in geology. The sections on Ordovician environments and their included faunas are written by and for specialists in those areas.

Dineley, D. L. Aspects of a Stratigraphic System: The Devonian. New York: John Wiley & Sons, 1984. A well-written and well-illustrated book covering many aspects of Devonian stratigraphy, sedimentology, and fossils. Because it is a single-authored text, it possesses much more continuity than do most symposium volumes. The text would be understandable and very readable for a college student with some training in geology.

Grotzinger, John, and Tom Jordan. Understanding Earth. 6th ed. New York: W. H. Freeman, 2009. This comprehensive physical geology text covers the formation and development of the earth. Readable by high school students, as well as by general readers. Includes an index and a glossary of terms.

Levin, Harold L. The Earth Through Time. 9th ed. Hoboken, NJ: Wiley, 2009. An introductory college-level text that covers aspects of historical geology. A large section is devoted to Paleozoic-age rocks. Well written and profusely illustrated.

Ogg, James G., Gabi Ogg, and Felix M. Gradstein. The Concise Geologic Time Scale. New York: Cambridge University Press, 2008. This book is a complete overview of the geological time scale, including stratigraphy topics such as chronostratigraphy and magnetic stratigraphy. The book is organized by geological time periods. Also contains a reference appendix, the geological time scale table, and indexing.

Prothero, Donald R. Bringing Fossils to Life. 2d ed. Boston: McGraw-Hill, 2004. This well-illustrated and entertaining text covers a broad range of paleontological topics, including biostratigraphy and the use of fossils in the study of Paleozoic rocks. Glossary, bibliography, and index.

Prothero, Donald R., and Robert H. Dott, Jr. Evolution of the Earth. 8th ed. New York: McGraw-Hill, 2009. This basic textbook on historical geology is aimed at students of geology. However, it is very readable by anyone with a background in science. Presents an up-to-date account of the earth's history from the viewpoint of plate tectonics. Includes a glossary.

Stanley, Steven M. Earth System History. 3rd ed. New York: Freeman, 2008. An introductory undergraduate-level text integrating coverage of life and the environment. Good coverage of the Mesozoic.

Whittington, H. B. The Burgess Shale. New Haven, Conn.: Yale University Press, 1985. Whittington reviews the beautifully preserved animals from the Middle Cambrian of the Rocky Mountains in Canada and gives a detailed look at the often strange animals from that famous locale. A review of the stratigraphy and sedimentology of the locality is given, along with comments on the evolutionary significance of the Early Paleozoic animals. The work is written for general audiences.

Wicander, Reed, and James S. Monroe. Historical Geology. Belmont Calif.: Brooks/Cole, Cengage Learning, 2010. This textbook is accessible to university students with or without a background in geology. It covers earth's geological systems and plate tectonics, physical and biological history, geological time, and evolution. There are many diagrams, images, and charts along with a CD-ROM to supplement the text.