پالئوزوئیک
Paleozoic
During its first four decades,
natural groupings of strata were studied and named for easy reference. Thus the
several subdivisions of the Paleozoic, ultimately the six standard systems, were
established. The original basis for establishing sequence was superposition. The
operational stratigraphic hypothesis is that, in most instances, the strata at
the bottom of a sequence are the oldest and the overlying beds are progressively
younger. Thus, the basal system of the Paleozoic, in which primitive shelly
fossils are found, is the Cambrian. As younger and younger layers were studied,
their fossils collected, and the biological affinities suggested, the concept of
evolution from simpler to more complex life forms took shape in the minds of the
paleontologists and geologists who were studying the rocks. This process did not
take place in an orderly way, from oldest to youngest strata, but rather as a
consequence of fulfilling a need of the moment, whether to complete a geologic
map or to solve a problem of stratigraphic correlation. Consequently, the first
Paleozoic system to be named and studied in some detail was the
Carboniferous—the great “coal-bearing” sequence—given that name by W. D.
Conybeare and W. Phillips in 1822. These strata were to provide the world's
major energy resources during the next century and a half. Most of the Northern
Hemisphere's coal fields, and much of its oil and gas as well, were produced
from Carboniferous rocks. See also: Superposition principle
In the 1830s and 1840s two British
geologists, R. Murchison and A. Sedgwick, studied and named the natural
groupings of rock strata in the British Isles. Sedgwick named the Cambrian
System in 1835, for a sequence of strata that overlies the Primordial
(Precambrian) rocks in northwest Wales. Four years later, Murchison gave the
name Silurian to the early Paleozoic rocks found in the Welsh borderland.
However, there was an almost complete overlap of the Cambrian by Murchison's
Silurian. It was not until 1879, when C. Lapworth named the Ordovician System
for rocks intermediate between the Cambrian and the “upper” Silurian, that the
three early Paleozoic systems were sorted out in the correct order. In the
meantime, Murchison and Sedgwick managed to agree on the rocks above the
Silurian and, in 1839, they named the Devonian System for rocks exposed in
Devonshire, England. The final Paleozoic system, the Permian, was named by
Murchison in 1841, after an expedition to Russia, where he recognized the
youngest Paleozoic fossil assemblages in the carbonate rocks exposed in the
province of Perm. See also: Precambrian
Subdivisions
The Paleozoic Era is divided into
six systems; from oldest to youngest they are Cambrian, Ordovician, Silurian,
Devonian, Carboniferous, and Permian. The Carboniferous is subdivided into two
subsystems, the Mississippian and the Pennsylvanian which, in North America, are
considered systems by many geologists. The Silurian and Devonian systems are
closer to international standardization than others; all the series and stage
names and lower boundaries have been agreed upon, and most have been accepted.
Despite continuing revisions, the major subdivisions of the geologic time scale
have been relatively stable for nearly a century. See also: Cambrian;
Carboniferous; Devonian; Ordovician; Permian; Silurian
Paleotectonics
The Paleozoic oceans, just as those
today, surrounded a series of landmasses that formed the cores of ancient
plates, always in motion as are their modern counterparts. Sediments were
supplied to the seas through a network of river drainage systems and distributed
in the oceans, by currents and gravity, very like today. Clastic sediments were
supplied by the mountainous regions that were uplifted and eroded in cyclic
patterns as the major plates collided and parted; and subduction at the leading
edges of some plates produced volcanic highlands. The plate tectonic theory
provides a template for sorting out the periods of mountain building during the
Paleozoic. Like the discovery of the stratigraphic systems, periods of orogeny,
with their concurrent volcanic and intrusive igneous activities, were revealed
by field studies. Tectonic effects (folding and faulting) were analyzed by
geologic mapping, as were crosscutting igneous relations and unconformities in
the sedimentary sequence. Regional orogenic terranes were named and the general
time sequence assigned; these were sharpened as the use of isotopic age analyses
of the igneous components became possible in the twentieth century.
Because Alpine and Appalachian
mountain chains were among the first studied in detail, orogenies were first
named there. In eastern North America, mountain-building effects during the
early Paleozoic were ascribed to the Taconic orogeny (Middle and Late
Ordovician); middle Paleozoic events were assigned to the Acadian orogeny
(Middle and Late Devonian); and late Paleozoic movements were called Appalachian
(more accurately Alleghenian) for Permian and, perhaps, Triassic events.
Similar, but not precisely correlative, orogenic episodes in western Europe are
ascribed to the early Paleozoic Caledonian and the late Paleozoic Variscan (or
Hercynian) orogenies. This regionalization, overlapping of timing of events and
lack of correlation of intrusive phases, tectonics, and sedimentation cycles
emphasize the universality of the ever-moving plates as the global mechanism
responsible for all tectonic events. See also: Dating methods; Isotope; Orogeny;
Plate tectonics; Unconformity
Fig. 1 Paleogeography of the Cambro-Ordovician
(Tremadoc) showing most of the northern plates spread east-west in the
equatorial regions. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic
Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society,
London, 1990)
Fig. 2 Paleogeography of Old Red Sandstone
continent (outlined in color) in Late Devonian. (After R. Goldring and F.
Langenstrassen, Open shelf and near-shore clastic facies in the Devonian, in M.
R. House, C. T. Scrutton, and M. G. Bassett, eds., The Devonian System, Spec.
Pap. Palaeont. 23, Palaeontological Association, London,
1979)
Fig. 3 Paleogeography of the Early
Carboniferous (Viséan), showing the Euro-American megaplate centered in
equatorial region. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic
Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society,
London, 1990)
Fig. 4 Paleogeography at the end of the Permian
showing the consolidation into Pangaea preparatory to the onset of the Mesozoic
tectonic cycle. (After W. S. McKerrow and C. R. Scotese, eds., Paleozoic
Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society,
London, 1990)
Lithofacies
The major changes in lithofacies
during the Paleozoic were also effected by biotic evolution through the era.
Limestone facies became more abundant and more diversified in the shallow warm
seas as calcium-fixing organisms became more diverse and more widespread.
Sediment input from the land was modified as plants moved from the seas to the
low coastal plains and, eventually, to the higher ground during the Devonian.
Primitive vertebrates evolved during the Cambro-Ordovician, but true fishes and
sharks did not flourish until the Devonian. Amphibians invaded the land during
the Late Devonian and early Carboniferous at about the same time that major
forests began to populate the terrestrial realm. These changes produced an
entirely new suite of nonmarine facies related to coal formation, and the
Carboniferous was a time of formation of major coal basins on all continental
plates.
Climate continually influenced
depositional patterns and lithofacies both on land and in the seas. In the major
carbonate basins and platforms, particularly from the Late Ordovician onward,
cyclic climatic changes resulted in changes from calcitic to magnesian
carbonates and, ultimately, to various saline deposits as the basins dried up.
There were great salt deposits in several systems, but spectacular thicknesses
developed in many basins during the Silurian and Permian. Major cycles of cold
and warm climates were overlaid on depositional and evolutionary patterns,
producing periods of continental glaciation when large amounts of the Earth's
water were tied up in ice during the Late Ordovician, the Late Devonian, and the
late Permian. During the earliest and latest of these periods, icesheets were
concentrated in the Southern Hemisphere on a single large Paleozoic continental
mass—Gondwana. See also: Depositional systems and environments; Facies
(geology); Paleoclimatology
Paleogeography
Paleogeographic changes naturally
followed the shifting of plates on a megacyclic scale during the Paleozoic. In
general, the Paleozoic featured a single southern landmass (Gondwana) for most
of the era. This megaplate moved relatively sedately northward during this
entire time interval (540–250 Ma) and always contained the magnetic and
geographic south poles. Consequently, many of the facies and biologic provinces
in the Gondwanan region were influenced by the cooler marine realms and
continental and mountain glaciers in nearly every Paleozoic period. Most of the
tectonic action that produced major periods of collision, mountain building,
carbonate platform building, back-arc fringing troughs with their distinctive
faunas and lithofaces, and formation of coal basins and evaporites took place in
the Northern Hemisphere. These pulsations produced combinations of Laurentian
(North American), Euro-Baltic, Uralian, Siberian, and Chinese plates at various
times during the Paleozoic; and these combined units, in turn, moved slowly
across the latitudes, producing climatic change; lithofacies changed in response
to both the climate and the plate tectonics.
Fig. 5 Major fossil groups used for detailed
biostratigraphy of the Paleozoic and younger strata. These are not total ranges
of all groups. (After J. T. Dutro, R. V. Dietrich, and R. M. Foose, eds., AGI
Data Sheets, 3d ed., American Geological Institution, 1989)
Representative geographies that show
the range of change have been deduced (Figs. 1–4). The map for the
Cambro-Ordovician portrays the general early Paleozoic patterns (Fig. 1); these
hold for the entire span of time from the Early Cambrian (about 540 Ma), through
the Cambrian, Ordovician, and Silurian, into the early Devonian (about 400 Ma).
There were consolidations in the Northern Hemisphere in the Devonian, leading to
a northern landmass—the so-called Old Red Continent (Fig. 2)—the forerunner of
the Euro-American megaplate of the Carboniferous (Fig. 3), and culminating at
the end of the Permian in the Pangaean continental mass (Fig. 4). This, in turn,
set the stage for the breakup of Pangaea during the subsequent Mesozoic tectonic
megacyle. See also: Paleogeography
Biogeography and
biostratigraphy
The complexities of evolution from
relatively simple forms at the beginning of the era to more advanced faunas and
floras at the beginning of the Mesozoic produced a web of distributions in both
time and space during the Paleozoic. In general terms, there were fewer and
simpler life forms in the Cambrian—often termed the Age of Trilobites. All
groups of invertebrates and plants became more numerous through geologic time.
For example, 7 major invertebrate animal groups at the beginning of the Cambrian
doubled to 14 by the end of the period, 20 by the end of the Ordovician, 23 at
the end of the Devonian, and 25 at the end of the Paleozoic. The pattern for
plant diversification, although starting later, is similar. Three simple plant
groups became 5 by the end of the Silurian, 7 at the end of the Devonian, and 13
at the end of the Paleozoic. The vertebrates also diversified very slowly. From
one or two groups in the Cambro-Ordovician (conodonts are now considered
primitive vertebrates), the number of major kinds rose to 6 at the end of the
Devonian and 8 at the end of the Paleozoic.
Biostratigraphic usefulness of
fossils varies widely. Certain groups have been shown empirically to be more
useful than others, and the abundance and diversity within these groups change
from system to system during the Paleozoic. Groups with wide dispersal,
occurrences in several facies, and rapid rates of evolution have proved most
useful (Fig. 5). In the Paleozoic, trilobites are most valuable in the Cambrian
and Ordovician; conodonts are more widely studied and are providing detailed
biochronologic control for many system, stage, and zonal boundaries. Graptolites
are indispensible in the deeper-water facies of the Ordovician through Early
Devonian; goniatite cephalopods provide standards in the Devonian through the
Permian; and fusulinids have long been essential for detailed work in the
Carboniferous and Permian. Of course, all groups are useful for other kinds of
paleobiologic research. Paleoenvironmental, paleoecological, and
paleobiogeographic reconstructions use all appropriate biologic, chemical, and
physical data in developing models of ancient Paleozoic worlds. See also:
Biogeography; Cephalopoda; Conodont; Fusulinacea; Geologic time scale;
Graptolithina; Index fossil; Paleoecology; Stratigraphy; Trilobita
- A. F. Embry, B. Beauchamp, and D. J. Glass (eds.), Pangea: Global Environments and Resources, Canadian Society Petroleum Geologists Memoir 17, 1994
- F. M. Gradstein, J. G. Ogg, A. G. Smith (eds.), A Geological Time Scale 2004, 2005
- M. R. House, C. T. Scrutton, and M. G. Bassett (eds.), The Devonian System, Spec. Pap. Palaeont. 23, Palaeontological Association, London, 1979
- E. G. Kauffman and J. E. Hazel (eds.), Concepts and Methods of Biostratigraphy, 1977
- W. S. McKerrow and C. R. Scotese (eds.), Palaeozoic Palaeogeography and Biogeography, Geol. Soc. Mem. 12, Geological Society, London, 1990
- R. C. Moore et al., Treatise on Invertebrate Paleontology, Part A, 1979
- G. C. Young and J. R. Lauries (eds.), An Australian Phanerozoic Time Scale, Oxford University Press, 1996
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