Cretaceous The third and youngest system of Mesozoic rocks takes its name from the Latin
creta, meaning chalk. The word
Cretaceous first appeared in an early book on the geology of England by W. D. Conybeare and William Phillips in 1822. It was based on the term
Terrain Cretacé which had recently been used by J. J. d'Omalius d'Halloy in France. Lasting for about 80 million years, the Cretaceous period terminated around 65 Ma ago with the ending of the Mesozoic Era and the occurrence of a major extinction event. Thus it is one of the longest of the Phanerozoic periods. It is customarily divided into two rather than three epochs, the formations being known as Upper and Lower Cretaceous. There are 12 stages, and 13 biozones based on ammonoid cephalopods are recognized. Biozones are also established in the Chalk (Upper Cretaceous) on the basis of foraminifera and coccolith assemblages. The Cretaceous system is very widely distributed about the world; great areas of the continental cratons are covered by the chalky limestones that mark the extent of the late Cretaceous very high sea level. In North America the
Zuni synthem (sequence) represents a major transgression from the margins of the craton towards the interior, and equivalent transgressions are found on other continents. It lasted from Mid-Jurassic time until the Paleocene.
Cretaceous palaeogeography was primarily influenced by the continuing dispersal of the continental fragments of Pangaea. Indeed, it seems that during this period the rate of ocean floor spreading increased. Africa became detached from South America and a seaway between the North and South Atlantic oceans opened up in the later part of the Early Cretaceous. The North Atlantic also was opening relatively fast (Fig. 1). The Bay of Biscay came into existence late in the period, as perhaps did the Rockall Trough. Tethys now ceased to exist as an open ocean with the movement of Africa and Arabia northward against the southern margin of Eurasia. The Alpine-Himalayan orogenic episode had begun. Madagascar became well separated from Africa and, probably before the end of the Early Cretaceous, India was detached from Antarctica and headed northward to make contact with the Lhasa Block by the middle of Late Cretaceous time. Orogeny on this margin was beginning in Kashmir and northern Afghanistan as the Tethyan Ocean closed. Accretion to the western margin of North America was operating as the Kula plate ‘docked’ against the mountains there, and then slid northwards through some 20 degrees of latitude.
The vigorous ocean-floor spreading of Cretaceous time is one aspect of the igneous activity that also affected the continents. There were also very large and complex intrusions emplaced in the Cordillera of South America, western USA, and Canada; the famous Boulder Batholith of Colorado is one of these. Associated with these intrusions were outpourings of andesitic and more acidic lavas and fine ashes. Some of the ashes are important as bentonite ash bands, which are of enormous geographic extent and yield good isotope late Cretaceous dates. High in the Canadian arctic and in Greenland tiny kimberlite pipes and small dykes were intruded as Greenland rifted away from the Canadian Arctic Islands. In India, the famous Cretaceous Deccan Traps are tholeiitic basalts, over 1200 m thick and attaining some 5000 00 km
2 in area. In South America the Parana basalts cover an area of 12 000 000 km
2 and seem to be associated with fissures near to the continental margin, which was under tensional stress as the south Atlantic spreading developed. On the opposing stable continent, Africa, kimberlite pipes were being punched through the crust in Angola.
As we noted above, global sea level rose throughout the period, beginning at a level rather lower than today's average and rising to a point in the Late Cretaceous where only 18 per cent of the Earth's surface remained as land, compared with 28 per cent today. A sharp fall began shortly before the end of the Cretaceous (Fig. 2).
Cretaceous climates seem to have been more varied and seasonal than those earlier in the Mesozoic. Clear evidence of a more conspicuous temperature gradient between high and low latitudes is provided by fossil plants. A cold climate flora and ice-rafted pebbles occur in Alaska and arctic Canada. Nevertheless there are indications that these were the products of temporary spells of colder climate. In detail, it seems that the cold snaps were more pronounced in the Early Cretaceous and the notably warmer phases were about equally distributed between the Early and Late Cretaceous. The Milankovitch climatic cycles found so clearly preserved in Quaternary deposits are also present in Cretaceous sediments, and presumably indicate similar short-term fluctuations of climate.
One of the fascinations of the Cretaceous system is the great variety of sediments that were deposited during that period. We are able to interpret the majority of them in the light of our knowledge of modern sedimentary environments. For many years the origin of the chalky limestones were, however, a puzzle, solved only since the invention of the electron microscope. They are the products of coccolith accummulation. Coccoliths are the extremely small calcareous parts of microscopic marine algae. In the Late Cretaceous they were extremely abundant in the warm clear seas that flooded the continents. As much as 90 per cent of the rock is commonly made of coccolithic debris. Other remarkable and typically Cretaceous limestones are the massive shelly limestones formed of the heavy rudist bivalves. The limestones are predominantly white, but are at several levels interrupted by think black shale bands. These are carbon-rich deposits, formed during brief phases of oxygen depletion in the bottom waters of the oceans. They are most common in the lower part of the Upper Cretaceous. Dark sediments are also characteristic of the main basin of the Early Cretaceous Atlantic. Most of the Cretaceous seas were organically highly productive: it is estimated that more than half of the world's oil and gas comes from reservoirs in Cretaceous rocks.
Cretaceous palaeontology is very rich, with well-preserved plants, invertebrates, and vertebrates from a wide range of ancient habitats. The marine coccolith-bearing algae have been mentioned; on land a new floral feature was the spectacular rise of the angiosperms, so that by mid-Late Cretaceous time they were dominant in many of the world's floras. They seem to have differentiated into three main floral provinces.
Marine communities also seem to have established themselves within geographical provinces. The Boreal province persisted from the Jurassic but was less marked, the Tethyan province was well populated with characteristic benthos. Molluscan faunas are rich, but with declining populations of cephalopods. Ultimately the ammonites, belemnites, inoceramid bivalves, and rudists became extinct. Among the microfaunal elements, the planktonic foraminifera suffered almost complete annihilation, and catastrophic extinction of almost all coccolithic algae also occurred at the end of the period.
The vertebrates of the Cretaceous made many evolutionary advances, but towards the end of the period the dinosaurs, pterosaurs, and giant marine reptiles died out. Dinosaur populations in a wide range of habitats were large and included both large and small herbivors and carnivores. They appear to have been present on most continents—even at high latitudes—during this period. The pterosaurs produced the largest-ever flying animals, while the birds achieved success in the air and as swimmers. Mammals remained small in size but increased in variety over their Jurassic ancestors.
The progressive disappearance of so many forms of life towards the end of the Cretaceous period has been the subject of intense research and debate. Several arguments have been put forward to account for the demise of creatures both on land and in the sea. They include arguments about late Cretaceous climatic change, the impact of some extraterrestrial object, cosmic radiation, and prodigious volcanic activity. The general trend of opinion is towards a combination of several of these factors operating over a short but decisive interval of time to accomplish a colossal extinction event.
All but about a dozen genera of dinosaurs were extinct before the end of the period, and their final days were seemingly under climatic stress. It had been getting progressively colder and drier during the last 10 Ma of Cretaceous time as sea-level fell. This change in climate must have affected the vegetation, including food for the dinosaurs. Added to this there is the possibility that the herbivorous mammals, being smaller but more prolific breeders and competing for food, were able to survive on seeds, nuts, and the new vegetation that was insufficient for the larger reptiles.
The impact scenario is based upon the occurrence of considerable widespread geochemical and mineralogical evidence of a large bolide striking the Earth. An event of this kind would create a huge crater, send out shock waves, and create tremendous atmospheric disturbance. Traces of a possible end-Cretaceous impact crater perhaps 300km in diameter have been found in the Yucatán area of Mexico. Critically, and apart from local blast effects, immense volumes of dust would have been carried high, far, and wide, screening out the sun's heat and creating a cold snap lasting several years. Added to this would have been chemical consequences, the generation of poisonous hydrogen cyanide and nitrous oxide in the atmosphere. Late Cretacous volcanism is known to have been on an unusually large scale, again with the injection of huge quantities of dust into the atmosphere and acidification of the air by aerosols. Almost certainly this would have attacked the ozone layer to admit larger doses of harmful ultraviolet radiation.
Other cosmic agencies have also been called in as possible sources of additional doses of radiation. Whatever the actual cause, any combination of these factors would seriously have affected the global environment for some time. The end of the period was marked by a major crisis in the history of life, the like of which had not been seen since the end of the Permian period, 185 Ma earlier.
That the end of the Cretaceous period marked a turning-point in the history of life was first recognized by the early geologists and palaeontologists in Europe, who noted that the widespread and conspicuous Chalk is overlain unconformably in Cenozoic sands and clays. The unconformity itself represents a period of uplift and non-deposition, short but immensely significant. Only in certain areas, such as parts of Denmark and Italy, is the sequence continuous; and it was in the Italian succession that a geochemical anomaly—a relatively high proportion of the element iridium in a clay band at the top of the Cretaceous—was discovered in the 1970s. The discovery led to the hypothesis of a cosmic impact and to the successful search for this or a similar anomaly elsewhere in the world. The discovery of the Yucatán crater seems to provide the site from which the Cretaceous catastrophe originated. The original Catastrophists would be gratified at this development.
D. L. Dineley
Bibliography
Kennedy, W. J. (1978) Cretaceous. In McKerrow, W. S. (ed.) The ecology of fossils, pp. 280–322. Duckworth, London.
Moullade, J. and and Nairn, A. (1978) The Phanerozoic geology of the world. II. The Mesozoic A. Elsevier Scientific Publications, Amsterdam.
Moullade, J. and and Nairn, A. (1983) The Phanerozoic geology of the world. II. The Mesozoic B. Elsevier Scientific Publications, Amsterdam.
Stanley, S. M. (1987) Extinction. Scientific American Books, New York.
