کرتاسه
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, and in the Cretaceous two
major elements of this, the prymnesiophyte coccolithophorids and the planktonic
foraminiferans, reach maximal species diversity. Bony fishes evolved extensively
in Cretaceous time, but prominent among marine predators were reptiles, some of
them gigantic. Marine turtles have persisted, the dolphinlike ichthyosaurs died
out in Turonian-Coniacian time, while the massive, sea-lion-like plesiosaurs and
the lizard-derived mosasaurs lasted to the end of the period. Flightless birds
populated some of the late Cretaceous seas. See also: Bivalvia; Gastropoda; Reptilia
On land, flowering plants
(angiosperms) first appeared in early Cretaceous time, as opportunistic plants
in marginal settings, and then spread to the understory of woodlands, replacing
cycads and ferns. In late Cretaceous time, evergreen angiosperms, including
palms, thus came to dominate the tropical rainforests. Evergreen conifers
maintained dominance in the drier midlatitude settings, while in the moist
higher latitudes forests of broad-leaved deciduous trees dominated. Grasslands,
however, were not developed until Tertiary time. Insects became highly diverse,
and many modern families have their roots in the Cretaceous. Amphibians and
small reptiles were present. Larger land animals included crocodiles and
crocodilelike reptiles, turtles, and dinosaurs. Dinosaurs, derived from
reptilian stock in Triassic time, rose to become the largest land animals of all
time. The very size of these animals implies complex circulatory, digestive, and
respiratory systems unlike those of reptiles. Evidence for temperature control
is emerging, and it is speculated that some dinosaurs were feathered. Thus,
there is growing support for classification of dinosaurs not as a branch of the
class Reptilia but as a separate class. The mammals, another reptilian offshoot
in the Triassic, remained comparatively minor elements in the Cretaceous faunas.
In early Cretaceous time, egg-laying and marsupial mammals were joined by
placentals, but Cretaceous mammals were in general small, and lack of color
vision in most modern mammalians suggests a nocturnal ancestry and a furtive
existence in a dinosaurian world. Birds had arisen, from dinosaurs in Jurassic
time, but their fossil record from the Cretaceous is poor and largely one of
water birds. More common are the remains of flying reptiles, the pterosaurs,
which in Cretaceous time reached a wingspan of 35 ft (11 m), the largest known
flying animals by far. See also:
Dinosauria; Mammalia; Marsupialia; Pterosauria
K/T boundary
crisis
Most species of the inoceramid and
rudistid bivalves died out in a crisis within the Maastrichtian Stage. Some
millions of years later, during the reversed magnetic interval known as chron
29R, a collision with an asteroid or comet showered the entire Earth with impact
debris, preserved in many places as a thin “boundary clay” enriched in the trace
element iridium. This event coincided with the great wave of extinctions—the K/T
crisis—which serve to bound the Cretaceous (Kreide) Period against the Tertiary.
The Chicxulub crater is located at
the northwestern edge of what was then the Yucatán Carbonate Platform and now
the
The iridium content associated with
the fallout implies an impactor with a composition similar to that of planetary
interiors. The size, estimated at 6 mi (10 km), implies an asteroid or a
correspondingly larger comet. Traveling at 16 mi/s (25 km/s), the body would
have struck with the explosive force of 1014 tons of TNT. Quite aside from local
and regional devastation, global effects must have included earthquake shock
many orders of magnitude greater than any found in human history; associated
land slips and tidal waves; a dust blackout of sunlight that must have taken
many months to clear; a sharp drop in temperatures that would have brought frost
to the tropics; changes in atmospheric and water chemistry; and disturbance of
existing patterns of atmospheric and oceanic circulation.
It is possible that earthquakes
influenced volcanic eruptions. The first flows of the
Different biotic communities were
affected to different degrees. The pelagic community, sensitive to
photosynthetic productivity, was severely struck, with coccolithophores and
planktonic foraminiferans reduced to a few species, while ammonites, belemnites,
plesiosaurs, and mosasaurs were eliminated. Yet dinoflagellates, endowed with
the capacity to encyst under stress, suffered no great loss. Benthic life was
only moderately damaged beyond the loss of species, excepting destruction of the
reef community. While North American trees underwent far more extinction at the
specific level than formerly believed, land floras escaped with little damage,
presumably because they were generally equipped to handle stress by dormancy and
seed survival. The plant-fodder-dependent dinosaurs perished, as did their
predators and scavengers. The fresh-water community, buffered by ground water
against temperature change and food-dependent mainly on terrestrial detritus,
was little affected. While a great many individual organisms must have been
killed by the immediate effects of the impact, the loss of species and higher
taxa must have occurred on land and in shallowest waters mainly in the aftermath
of darkness, chill, and starvation, and in the deeper waters in response to
changed regimes in currents, temperatures, and nutrition.
The Cretaceous crash led above all
to an evolutionary outburst of the mammals, which in the succeeding tens of
millions of years not only filled and multiplied the niches left by dinosaurs
but also invaded the seas, to become successors to plesiosaurs and mosasaurs.
Humans, among others, owe their existence to that devastating event of 65
million years ago.
Alfred G. Fischer
Bibliography
-
W. Alvarez et al. (eds.), Geochronology. Time Scales and Global Stratigraphic Correlation, SEPM Spec. Publ., no. 54, 1995
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R. H. Dott, Jr., and D. R. Prothero, Evolution of the Earth, 1992
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L. A. Frakes, J. E. Francis, and J. I. Syktus, Climatic Modes of the Phanerozoic, 1992
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J. L. Powell, Night Comes to the Cretaceous, 1998
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