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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 Europe as a stratigraphic unit. W. O. Conybeare and W. Phillips (1822) formally established the period, noting that whereas chalks were remarkably widespread deposits at this time, the Cretaceous System includes rocks of all sorts and its ultimate basis for recognition must lie in its fossil remains.  See also: Chalk; Fossil; Jurassic; Rock age determination; Stratigraphy; Tertiary

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)

 

 

 

fig 1

 

 

 

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)

 

 

 

fig 2

 

 

 

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.

 

 

 

fig 3

 

 

 

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 Yucatán Peninsula. Buried under some thousands of feet of Tertiary limestone, it forms a geophysical anomaly explored for oil by a number of boreholes. With a diameter of 110 mi (180 km) excluding possible peripheral rings, this is the largest crater known from Phanerozoic time. Like comparable craters on the Moon and Mars, it has a central peak and peripheral ejecta blanket. It is filled with partly melted impact debris (suevite) derived from sediments and the underlying igneous-metamorphic complex. Quartz grains with shock lamellae confirm the impact origin, and abundant glass yields the K/T boundary age of 65 ± 0.5 million years. The ejecta blanket, where exposed at the surface in the Mexico-Belize border country 300 mi (400 km) distant, consists mainly of middle to late Cretaceous carbonates in blocks up to 10 ft (3 m), mixed with carbonate lapilli and altered glass lumps. Rare cobbles of limestone and chert were melted and quenched. In surrounding regions, deeper-water marls of latest Cretaceous and earliest Tertiary age are separated by a few feet of unusual sediment: a thin bed contains chips of limestone, blebs of glass generally altered to clay, and shocked quartz. In the Gulf of Mexico region this is followed by cross-beds interpreted as deposits of a “tidal wave” (tsunami) that swept repeatedly across the Gulf. A centimeters-scale layer of clay, extending around the world, contains microscopic silicates, while electron microscopy reveals crystals of spinel with excessive degrees of oxidation. Shocked quartz, abundant and relatively coarse in America, becomes scarce and fine with distance.

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 Deccan basalts predate polarity chron 29R and the impact, but the great body of basalt poured out within that brief chron, possibly emitting enough sulfur to add to the crisis.

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
  • R. H. Dott, Jr., and D. R. Prothero, Evolution of the Earth, 1992
  • L. A. Frakes, J. E. Francis, and J. I. Syktus, Climatic Modes of the Phanerozoic, 1992
  • J. L. Powell, Night Comes to the Cretaceous, 1998
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