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Geological time scale
The history of
the Earth can be displayed in the form of a calendar which is based on the
observation of rocks formed through time. Depending on the location, the
formation of rocks has recorded different (regional) histories describing the
magmatic, tectonic, hydrospheric, or biospheric evolutions, and thus different
calendars have been generated. Geologists (stratigraphers) are attempting to
unify these different calendars. Stratigraphers define rock units bracketed
between two boundaries that can be correlated worldwide. The succession of these
modern units is the global geological time scale.
Stratigraphical units
In general, the
evolution of the Earth is continuous, so fixing the location of the unit
boundaries can be achieved only through convention (decisions are made under the
aegis of the International Commission on Stratigraphy). Stratigraphers use a
variety of tools for characterizing the age of a rock, including physical ages,
fossils, magnetism, and chemical properties, all of which have evolved through
time. There are three intervals in the history of the Earth
(Archean-Proterozoic, Phanerozoic, Plio-Quaternary) with each one showing
distinct material available for characterizing rocks; thus, there are three
successive kinds of geological time scale. In the earliest time scale, during
Archean and Proterozoic eons, fossils are rare or absent in most rocks, and the
major tool is the physical dating process based on naturally unstable isotope
decay used by geochronologists. For this interval of time, the boundaries of the
calendar units are defined by selected numerical ages (Fig. 1). These ages are
conventionally abbreviated Ma (Mega anna, or million years, following
recommendation of the Subcommission on Geochronology).
Fig. 1 Agreed-upon geological time scale of the
pre-Phanerozoic history of the Earth. The beginning of the Earth is estimated to
be 4,470 to 4,570 Ma, with a preferred age at 4,550
Ma.
During the
Phanerozoic Eon certain animals and plants elaborated skeletons or exoskeletons
that, when preserved as fossils, provide an additional means of time correlation
(Fig. 2). Up to about 5 Ma, these fossils become the major tool for determining
the relative age of a rock. Together with other physicochemical characteristics,
fossils allow a definition of stages which cover an average duration of about 5
Ma each. For the last 5 Ma, Plio-Quaternary time, there is evidence that the
evolution of the environment on Earth is diversified and well represented in the
geological record.
Fig. 2 Simplified geological time scale of the
Phanerozoic Eon. From about 120 possible stages, only one-fourth are formally
defined in modern terms. Among others, many are mostly used regionally (in
parentheses) with less precise boundaries and content compared to formal ones.
Stages are not shown for some epochs for which no realistic subdivision can be
recommended today. Ages given in two columns are shown without error bar when
they are obtained from an interpolation procedure. For some boundaries, the
error bar is asymmetrical; for example, the Aptian-Albian boundary has a
preferred age at 108 Ma, but this age is constrained only between 111 and 107
Ma. (After G. S. Odin, Geological Time Scale, C. R. Acad. Sci., 318:59–71,
1994)
Most
stratigraphical tools can be used continuously in this young interval, each tool
providing a specific time scale. Because several tools can often be used to
characterize (that is, to date) a single rock, it is easy to connect all these
time scales. A variety of easily interchangeable time scales are useful.
However, the absolute time dimension for all time scales can be calibrated only
by using geochronology. The progress toward a common terminology and geological
time scale benefits from three factors: better definition of the conventional
units, better geochronological calibration, and a potential for extrapolating
between calibrated ages.
Geochronological
calibration
Progress in
geochronological calibration benefits from both new technology and new
geochronological information. During the 1990s, the precision and
reproducibility of the measurements obtained using mass spectrometry have
increased significantly. This improvement offers the possibility of dating
minute quantities of certain material with remarkable precision. Using the
uranium-lead (U-Pb) dating method, the age of a single crystal of zircon, 200
micrometers in length and 10 μm in diameter, can be measured. More
significantly, several portions of this same crystal can be dated separately,
allowing for verification of the internal consistency of the data obtained from
the single crystal. For the potassium-argon (K-Ar) dating method, developments
in the irradiation techniques which transform the original potassium into argon
allow single biotite mica flakes 500 μm in diameter and 10 μm thick to be dated.
Laser heating can also give several ages obtained from different points of a
single crystal.
Explosive
volcanic eruptions producing ash clouds blown over large areas are common. The
ability to date small quantities of material has led to the search for minor
volcanic events within the stratigraphical record. This advance is important
because the same volcanic material covers both marine and continental areas,
sometimes at the scale of a continent, allowing correlation of distant deposits.
In addition, there is great interest in this method since explosive volcanism
often scatters a variety of minerals, including uranium-bearing and
potassium-bearing ones. Thus, the geochronologist can perform measurements on
independent isotopic systems in order to make a reciprocal check of the validity
of the calculated ages. The calibration of the geological time scale is mostly
realized through the study of crystal-bearing volcanic dust discovered in
sediments. Precise dating can also be achieved through a variety of other
datable materials. For example, a few tens of milligrams of microtektite
particles (scattered in wide areas when an asteroid collides with the Earth) can
be dated. Another example is calcite crystallized in paleosols during cyclic
deposition in shallow basins. Because this calcite is associated with organic
matter which favors uranium enrichment, and is formed essentially at the time of
deposition, calcite crystals become a potentially datable material using
uranium-lead methods.
Two examples of
refinement
One example of
recent refinement concerns the dating of the Eocene-Oligocene boundary. That
boundary has long been known as the Grande Coupure (great break) in the history
of European land-mammal evolution. There have been a few European studies which
documented an age at about 34 Ma. But the age of the boundary was assumed to be
about 37–38 Ma by some North American geologists. Paleontologists did not like
the latter age because, in North America, the 38-Ma-old mammals were dated using
contemporaneous volcanic flows and seemed less evolved than those known to be at
the stratigraphical boundary in
Another
significant example of the beneficial combination of improved stratigraphical
definition and modern geochronological dating is given by the
Precambrian-Cambrian boundary. The base of the Cambrian (and of the Phanerozoic
Eon) had long been placed at the first occurrence of skeletalized fossils
including trilobites (arthropods). Later, a Tommotian pretrilobitic stage was
added below it in view of the presence of older faunal remains, such as
archaeocythids (calcitized spongelike forms), which have been well documented on
the Siberian Platform. Before 1980, the earliest skeletalized faunas were
estimated to be between 570 and 590 Ma. However, independent geochronological
data were gathered from northern
Extrapolation
procedures
The direct
geochronological calibration method will never allow for the continuous
calibration of every point of geological history, since datable material is much
too scarce in rocks. However, continuous dating can be refined through the use
of interpolation procedures between geochronologically calibrated points. This
principle consists in combining those tie points with a continuous geological
phenomenon. Commonly used phenomena are rhythmic sedimentation and the oceanic
record of past magnetic fields. When the rhythmic deposition of sediments can be
related to the orbital (Milankovitch) parameters of the Earth, the time scales
of which are reasonably well understood, the duration of deposition can be
estimated when combined with nearby measured ages.
Another
procedure considers the aperiodic change (reversal) of the direction of the
Earth's magnetic field that is recorded in the oceanic plates being continuously
formed at midocean ridges (separating two tectonic plates). For a given plate,
the distance between two magnetic reversals is proportional to the time
durations between reversals and spreading rate (which can be calculated from two
geochronologically dated points). Thus, geological ages can be calculated for
each point of the record, though with some degree of uncertainty.
The geological
time scale is gradually becoming unified through an internationally agreed upon
scale which is replacing a variety of regional scales. It is ironic that such an
important improvement in calibrating this vast expanse of time is linked to the
discovery of volcanic dust interbedded in sedimentary rocks.
See also: Archeological chronology;
Dating methods; Fossil; Geochronometry; Geologic time scale; Index fossil;
Radiocarbon dating; Rock age determination; Sedimentary rocks; Stratigraphy
G. S. Odin
Bibliography
-
H. Blatt, W. B. N. Berry, and S. Brande, Principles of Stratigraphic Analysis, 1991
-
J. P. Grotzinger et al., Biostratigraphic and geochronologic constraints on early animal evolution, Science, 270:598–604, 1995
-
G. S. Odin, Geological time scale, C. R. Acad. Sci., 318:59–71, 1994
-
G. S. Odin et al., Numerical dating of Precambrian-Cambrian boundary, Nature, 301:21–23, 1983
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G. S. Odin and A. Montanari, Radio-isotopic age and stratotype of the Eocene/Oligocene boundary, C. R. Acad. Sci. Paris, 309:1939–1945, 198۹
Quaternary
A period that encompasses at least
the last 3 × 106 years of the Cenozoic Era, and is concerned with major
worldwide glaciations and their effect on land and sea, on worldwide climate,
and on the plants and animals that lived then. The Quaternary is divided into
the Pleistocene and Holocene. The term Pleistocene is gradually replacing
Quaternary; Holocene involves the last 7000 years since the Pleistocene.
The Quaternary includes four principal
glacial stages, each with subdivisions, in the western Rocky Mountains and
Sierra Nevada and in the north-central
Tertiary
The older major
subdivision (period) of the Cenozoic Era, extending from the Cretaceous (top of
the Mesozoic Era) to the beginning of the Quaternary (younger Cenozoic Period).
The term Tertiary corresponds to all the rocks and fossils formed during this
period. Although the International Commission on Stratigraphy uses the terms
Paleogene and Neogene (pre-Quaternary part) instead, Tertiary is still widely
used in the geologic literature. Typical sedimentary rocks include widespread
limestones, sandstones, mudstones, marls, and conglomerates deposited in both
marine and terrestrial environments; igneous rocks include extrusive and
intrusive volcanics as well as rocks formed deep in the Earth's crust
(plutonic). See also: Cretaceous;
Fossil; Rock
The Tertiary
Period is characterized by a rapid expansion and diversification of marine and
terrestrial life. In the marine realm, a major radiation of oceanic
microplankton occurred following the terminal Cretaceous extinction events. This
had its counterpart on land in the rapid diversification of multituberculates,
marsupials, and insectivores—holdovers from the Mesozoic—and primates, rodents,
and carnivores, among others, in the ecologic space vacated by the demise of the
dinosaurs and other terrestrial forms. Shrubs and grasses and other flowering
plants diversified in the middle Tertiary, as did marine mammals such as
cetaceans (whales), which returned to the sea in the Eocene Epoch. The pinnipeds
(walruses, sea lions, and seals) are derived from land carnivores, or fissipeds,
and originated in the Neogene temperate waters of the
Fig. 1 Baluchitherium, the largest land mammal
known, from the Tertiary (Oligocene Epoch) of
Geography
The modern
configuration of continents and oceans developed during the Cenozoic Era as a
result of the continuing process known as plate tectonics. Mountain-building
events (orogenies) and uplifts of large segments of the Earth's crust
(epeirogenies) alternated with fluctuating transgressions and regressions of the
seas over land. This resulted in a complex alternation of marine and terrestrial
sediments and their contained records of the passage of life (fossils). Some
modern inland seas (for example,
The middle to
late Tertiary Alpine-Himalayan orogeny and the late Tertiary Cascadian orogeny
led to the east-west and north-south mountain ranges, respectively, which are
located in Eurasia and western
Rocks
Tertiary
sedimentary rocks occur as a relatively thin veneer of marine rocks on the
margins of continents around the world. In the petroliferous province of the
Gulf of Mexico, Tertiary rocks attain thicknesses in excess of 30,000–40,000
(9000–12,000 m); whereas in the more tectonically active borderlands around the
Pacific Ocean, such as the Santa Barbara–Ventura Basin of California, and the
flanks of the uplifted Himalayan-Alpine chain of Eurasia, thicknesses in excess
of 50,000 ft (15,000 m) have been recorded. Terrestrial (nonmarine) strata are
generally thinner, are more patchy in distribution, and occur predominantly in
the internal basins of the continents (for example, the Basin and Range Province
of North America and the Tarim Depression of
Stratigraphy and
history
Although the
ancient Greeks recognized the shells of mollusks far inland from the Aegean Sea
as fossil marine organisms, as did Leonardo da Vinci some 2000 years later, it
was not until the era of enlightenment in the eighteenth century that the first
attempt was made to place the Earth's rock record into a historical context. The
term Tertiary is derived from Giovanni Arduino, who in 1759 formulated a
threefold subdivision of the Earth's rock record in Primary, Secondary, and
Tertiary. While the first two terms have long since disappeared from geologic
hagiography, the term Tertiary persists in modern scientific literature. In its
more modern sense the term Tertiary is defined by its usage in 1810 by the
French Scientists Alexandre Brogniart and Georges Cuvier for all the rock
formations in the Paris Basin that lay above the Cretaceous chalk sequence.
Although many subdivisions of the Tertiary exist that developed in the
succeeding two centuries, only five major time-rock units generally are
recognized. In 1833, Charles Lyell made the first systematic hierarchical
subdivision of the Tertiary Period based upon the observations of his Parisian
colleague M. Deshayes and other contemporary European conchologists that the
percentages of living species in the fossil record increased as the Tertiary
stratigraphic record ascended (Fig. 2). Lyell's Tertiary subdivisions include,
in ascending order, the Eocene, Miocene, Older Pliocene, and Newer Pliocene. The
last term was subsequently (1839) changed to Pleistocene. Heinrich Ernst von
Beyrich later defined the term Oligocene for rocks exposed in the North German
Basin and the Rhine Basin that had been previously allocated to a part of either
the Eocene or Miocene by Lyell. The paleobotanist W. P. Schimper added the term
Paleocene in 1874 based on the oldest Tertiary terrestrial strata exposed in the
east
W. A. Berggren
Fig. 2 Mollusk fossils used by Lyell to zone
the Tertiary. (a) Miocene. (b) Eocene. (After C. Lyell, Principles of Geology,
1833)
Bibliography
-
R. H. Dott, Jr., and D. R. Prothero, Evolution of the Earth, 7th ed., 2003
-
B. M. Funnell and W. R. Riedel, The Micropaleontology of Oceans, 1971
-
S. J. Gould, Time's Arrow, Time's Cycle, 1987
-
H. L. Levin, The Earth Through Time, 7th ed., 2003
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Cretaceous
In geological time, the last period
of the Mesozoic Era, preceded by the Jurassic Period and followed by the
Tertiary Period. The rocks formed during Cretaceous time constitute the
Cretaceous System. Omalius d'Halloys first recognized the widespread chalks of
The parts of the Earth's crust that
date from Cretaceous time include three components: a large part of the ocean
floor, formed by lateral accretion; sediments and extrusive volcanic rocks that
accumulated in vertical succession on the ocean floor and on the continents; and
intrusive igneous rocks such as the granitic batholiths that invaded the crust
of the continents from below or melted it in situ. The sedimentary accumulations
contain, in fossils, the record of Cretaceous life. The plutonic and volcanic
rocks are the chief source of radiometric data from which actual ages can be
estimated, and suggest that the Cretaceous Period extended from 144 million
years to 65 ± 0.5 million years before present (Fig. 1).
Fig. 1 Stages of the Cretaceous Period, their
estimated ages in years before present, and polarity chrons representing the
alternation between episodes of normal (black) and reversed (white) orientations
of the Earth's magnetic field. (After F. M. Gradstein et al., in W. A. Berggren
et al., eds., Geochronology, Time Scales and Global Stratigraphic Correlations,
SEPM Spec. Publ., no. 54, 1995)
Subdivisions and
time markers
Changes in life, by the development
of new species and by the loss of old, are not uniform but somewhat steplike.
Larger steps led to the recognition of periods such as the Cretaceous, while
lesser steps within the Cretaceous delineate 12 globally recognized
subdivisions, or stages (Fig. 1). In marine sediments the appearance and
disappearance of individual, widely distributed species allows further time
resolution by so-called zones. Initially these zones were largely based on
ammonites, a species-rich and rapidly evolving, now extinct group of
cephalopods, closest to the squids and octopuses but resembling the pearly
nautilus in possession of a coiled calcareous hydrostatic shell. Over 100
successive ammonite zones have been recognized, but due to the provinciality of
some species and the rarity of many, the dating of Cretaceous marine sediments
now mainly devolves on microscopic fossils of the calcareous plankton. The most
important of these are protozoans—the infusorial (tintinnid) calpionellids and
the sarcodine planktonic foraminiferans. Their shells, in the range of 0.1– 1
mm, occurring by hundreds if not thousands in a handful of chalk, have furnished
about 38 pantropical zones. Next in importance are the even tinier (0.01-mm)
armor plates of “nanoplanktonic” coccolithophores—prymnesiophyte algae, best
studied by electron microscopy. Thousands may be present in a pinch of chalk,
and 24 zones have been recognized.
See also: Cephalopoda; Coccolithophorida; Foraminiferida; Marine
sediments; Micropaleontology; Paleontology; Phytoplankton; Protozoa; Zooplankton
The Earth's magnetic field reversed
about 60 times during Cretaceous time, and the resulting polarity chrons have
been recorded in the remanent magnetism of many rock types (Fig. 1). The actual
process of reversal occurs in a few thousand years and affects the entire Earth
simultaneously, providing geologically instantaneous time signals by which the
continental and volcanic records can be linked to marine sequences and their
fossil zonation. Noteworthy here is the occurrence of a very long
(32-million-year) interval during which the field remained in normal
polarity. See also: Paleomagnetism
Geography
During Cretaceous time the breakup
of Gondwana, the great late Paleozoic-Triassic supercontinent, became complete
(Fig. 2). For pre-Cretaceous time the positions of continents can be roughly
derived from the remanent magnetic vectors in their rocks, which record the
directions toward and the distance to the poles. But for Cretaceous and later
times the positions may be more precisely tracked by the “growth lines” of
crustal plates, visible in magnetic maps of the oceanic areas. As the Earth's
magnetic field reversed, the continuous growth of ocean floors along the
mid-oceanic ridge system occurred alternately in normal and in reversed
polarity. Imprinted in the rocks, the remanent magnetism reinforces the present
magnetic field above the crust grown in normal polarity and diminishes it over
that grown in reversed fields.
Fig. 2 Late Cretaceous global tectonics,
showing inferred lithosphere plate configurations with spreading, subduction,
and transform boundaries; generalized continental outlines are shown for
reference. (After R. H. Dott, Jr., and R. L. Batten, Evolution of the Earth, 3d
ed., McGraw-Hill, 1988)
Laurasia had already separated from
Africa by the development of Tethys and became split into North America and
Eurasia by the opening of the North Atlantic (though tenuous land connections
continued to exist in the far north). These new, deep oceanic areas continued to
grow in Cretaceous time. India broke away from Australia and Australia from
Antarctica. South America tore away from Africa by the development of the South
Atlantic Ocean, while India brushed past Madagascar on its way north to collide
with southeast Asia. As these new oceanic areas grew, comparable areas of old
ocean floor plunged into the mantle in subduction zones such as those that still
ring the Pacific Ocean, marked by deep oceanic trenches and by the development
of mountain belts and volcanism on adjacent continental margins (Fig. 3). See also: Continents, evolution of;
Plate tectonics; Subduction zones
Fig. 3 Schematic view of North America in
Cretaceous time. Cretaceous seas are shaded, and Cretaceous sediments in cross
section (in the black-and-white strip) are dotted. Note the counterclockwise
rotation of the continent shown by the lines of Cretaceous
latitude.
The face of the globe was also
affected by changes in sea level. Sea level at times in the early Cretaceous
stood at levels comparable to the present, but subsequently the continents were
flooded with relatively shallow seas to an extent probably not attained since
Ordovician-Silurian times. Maximal flooding, in the Turonian Stage, inundated at
least 40% of present land area. Cretaceous seas covered most of western Europe,
though old mountain belts such as the Caledonides of Scandinavia and Scotland
remained dry and archipelagos began to emerge in the Alpine belt. In America
(Fig. 3), seas flooded the southeastern flank of the Appalachian Mountains,
extended deep into what is now the Mississippi Valley, and advanced along the
foredeep east of the rising Western Cordillera to link at times the Gulf of
Mexico with the Arctic Ocean. In the far west, accretionary prisms of subducted
Cretaceous deep-water fans and oceanic sediments, partly mixed with ophiolites
in the Franciscan melange, became juxtaposed with forearc sediments.
Large seas extended over parts of
Asia, Africa, South America, and Australia. The wide spread of the chalk facies
is essentially due to this deep inundation of continents, combined with the
trapping of detrital sediments near their mountain-belt sources, in deltas or in
turbidite-fed deep-water fans such as the classical Alpine Flysch. At the same
time, carbonate platforms were still widespread, and the paratropical dry belts
were commonly associated with evaporite deposits. See also: Paleogeography; Saline
evaporites
Tectonic and
igneous activity
The North American plate, in the
process of separating from Europe, continued to have a western convergent margin
of growing mountains, and a southeastern passive margin. Convergent or active
margins such as those surrounding the Pacific Ocean (then as now a “ring of
fire”) became intruded by great batholiths of granitic composition, such as
those of the British Columbia Coast Range, the Sierra Nevada, the Peninsular
Ranges of southern and Baja California, and the great Andean batholiths. Above
these there accumulated superstructures of andesitic volcanics. Exotic island
arc terranes riding on the Pacific plate but resisting subduction came to be
welded to this margin of North America. The eastern margin of this Western
Cordillera overthrust and incorporated the margin of the subsiding Western
Interior Foredeep, presaging development of the Rocky Mountains (Fig. 3).
Passive continental margins, such as those bordering the Atlantic, came to be
sites of massive sediment accumulations, the classical geosynclines of older
literature, from which mountain systems have developed locally as in the
Caribbean or have yet to arise. See
also: Geosyncline
Flood basalts of Cretaceous time
include the early Aptian outpourings of basalts on the mid-Pacific floor,
perhaps the largest on record. Early Cretaceous emplacement of the Rajmahal
basalts of India and Bangladesh was followed by the eruption of the Deccan
basalts of India, emplaced during the short magnetic interval (chron 29R) that
includes the Cretaceous/Tertiary (K-T) boundary. The beginnings of the
“Brito-Arctic” basalts also go back to Maastrichtian time.
Climate and
oceans
In parts of early Cretaceous time,
ice extended to sea level in the polar regions, as shown by glendonites,
concretionary calcite pseudomorphs of the cold-water mineral ikaite; and in
marine mudstones containing exotic pebbles, dropstones melted out of icebergs.
But during most of Cretaceous time, climates were in the hothouse or greenhouse
mode, showing lower latitudinal temperature gradients. Tropical climates may
have been much like present ones, and paratropical deserts existed as they do
now, but terrestrial floras and faunas suggest that nearly frost-free climates
extended to the polar circles as did abundant rainfall, and no ice sheets appear
to have reached sea level.
The cause for these climatic changes
may be sought in variations in the heat-retaining character of the atmosphere
(abundance of greenhouse gases such as carbon dioxide or methane) or in
oscillations of solar luminosity. The hydrologic cycle appears to have been
intensified, with more heating and therefore more water evaporation in the
tropics, leading to massive transport of water vapor to the polar regions warmed
by its condensation.
With a mean temperature of about
38°F (3°C), the present ocean lies 27°F (12°C) below the mean surface
temperature of the Earth. This refrigerator is maintained by the sinking of cold
waters in the circumpolar regions. Temperatures calculated from the oxygen
isotope ratios in Cretaceous deep-water foraminiferans yield values as high as
62°F (16°C), and suggest an ocean much closer to mean surface temperature. In
this ocean circulation, visualized long ago by T. C. Chamberlin, warm saline
waters of the dry paratropics were the main source of dense waters and sank to
the bottom when only moderately cooled. The implication is not only altered
current directions and deep-water temperature regimes but also a diminished
oxygen supply to the depths. This diminished oxygen is documented by the
widespread development of black shales, deposited on bottoms from which most or
all scavengers were excluded by lack of oxygen. The resulting excess burial of
organic matter yielded an abundance of petroleum source beds, reflected in the
large petroleum reserves in Jurassic and Cretaceous rocks. Occasional episodes
of black shale deposition, such as that of the Bonarelli event near the end of
the Cenomanian Stage, became circumglobal and buried enough organic matter to
alter the carbon isotope ratios in the entire ocean-atmosphere system, but the
circumstances of their origin remain unknown. See also: Oceanography; Oil shale;
Paleoclimatology; Petroleum reserves
Life
A walk along a Cretaceous beach or a visit to a surf-beaten cliff would have yielded many snail and bivalve shells and sea urchins differing from living ones only at the level of species. Yet there are some notable differences. Two families of bivalves rose to particular prominence in Cretaceous time. The inoceramids were a widespread and diversified group that developed species 5 ft (1.5 m) wide, the largest of all known clams, while the rudistids came to resemble corals in form and largely replaced corals as reef builders in Cretaceous time. Among cephalopods, ammonites were prominent swimmers in Cretaceous seas, as were belemnites—squids with a heavy calcareous skeleton. The calcareous microplankton had only in latest Jurassic time reached an abundance sufficient to produce widespread chalks, an