Paleozoic Era

Paleozoic stratigraphy concerns the study of rock sequences dating from 544 to 245 million years before the present. The primary method for establishing the Paleozoic stratigraphic relationships is the 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's history.

The Cambrian is the oldest system in the Paleozoic, ranging from approximately 542 to 440 million years before the present. It was named by Adam Sedgwick in 1835 based on 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 challenging to interpret. One reason is that although most rocks of the Paleozoic are subdivided primarily based on 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 the stratigraphy of the Middle and Upper Cambrian series fairly well. The primary tool used in establishing 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 extinction events and evolutionary radiation are instrumental in precisely defining 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 concerning those strata sequences. The Ordovician may also be subdivided into Early, Middle, and Late series, and the stages are much better established than in the Cambrian.

Strata of the Ordovician Age is often characterized by an abundance of carbonate (limestone and dolomite) rocks, which were deposited upon shallow seaways extending over many 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 the Ordovician Age through the 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 forms 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 based on 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. However, various series designations are used in the best-known exposures of Silurian rocks. 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 to determine 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 recorded 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 in 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. A different classification is employed in North America 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 US 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 in 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 the Upper Carboniferous continental sediments.

The Permian system was named after the Perm region in the western Ural Mountains of Russia. The system, first proposed by Murchison in 1841, roughly corresponds to a period of Earth’s 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 no longer 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

Paleozoic stratigraphy studies primarily utilize rock unit analyses, paleontological studies, and radiometric dating techniques. Lithostratigraphy defines strata (rock layers) based on rock or lithologic characteristics. Distinctive rock layers are instrumental 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 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 based on their fossil content. The basic unit of biostratigraphic classification is the biozone, in which the occurrence of a certain fossil or fossils characterizes strata. These index or guide fossils are abundant, distinctive forms with a wide geographic distribution but a narrow stratigraphic range. They are useful in correlation and in determining the age of strata. The fossils most often used are planktonic (floating) or nektonic (swimming) forms. Trilobites, brachiopods, and graptolites are especially important in studying Paleozoic stratigraphy. 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

One of the most important applications of Paleozoic stratigraphy is in the search for natural resources. The exploration geologist's job would be impossible without knowing the age of the rocks being evaluated or the economic products potentially included within them. The widespread presence of coal in the Pennsylvanian and Permian Ages rocks is of particular significance. 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, searching for them is 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 hydrocarbons have been found in Devonian- and Carboniferous-age rocks, and discoveries of huge oil and gas fields in the Permian Basin of west Texas resulted from exploration of Upper Paleozoic marine strata. Arid conditions, especially common during the Permian, resulted in huge deposits of sodium and potassium salts, gypsum, and anhydrite. Mountain-building events during the Paleozoic period also created tremendous deposits of precious metals and metals that are important to the industry. With an understanding of the stratigraphic occurrence of these valuable resources, the recovery of these materials is possible.

Studying the Paleozoic Era remains essential in the twenty-first century for other reasons. Scientists have discovered continuous changes in oxygen levels, beginning in the Devonian period, leading to considerable explosions in Earth's biodiversity. Scientists at the Massachusetts Institute of Technology have created a detailed timeline of temperature changes during the Paleozoic Era based on the analysis of carbonate mud. These findings show that life flourished in warmer temperatures and highlight the climate's significant impact on early animal life. Finally, paleontologists working in Canada's Yukon Territory discovered fossils and rock layers dating to the Paleozoic Era, highlighting the emerging biodiversity of the Earth during that time.

Principal Terms

biostratigraphy: defining rock layers based on 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

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