1911 Encyclopædia Britannica/Cretaceous System
CRETACEOUS SYSTEM, in geology, the group of stratified rocks which normally occupy a position above the Jurassic system and below the oldest Tertiary deposits; therefore it is in this system that the closing records of the great Mesozoic era are to be found. The name furnishes an excellent illustration of the inconvenience of employing a local lithological feature in the descriptive title of a wide-ranging rock-system. The white chalk (Lat. creta), which gives its name to the system, was first studied in the Anglo-Parisian basin, where it takes a prominent place; but even in this limited area there is a considerable thickness and variety of rocks which are not chalky, and the Cretaceous system as a whole contains a remarkable diversity of types of sediment.
Classification.—The earlier subdivisions of the Cretaceous rocks were founded upon the uncertain ground of similarity in lithological characters, assisted by observed stratigraphical sequence. This method yielded poor results even in a circumscribed area like Great Britain, and it breaks down utterly when applied to the correlation of rocks of similar age in Europe and elsewhere. Study of the fossils, however, has elicited the fact that certain forms characterize certain “zones,” which are preceded and succeeded by other zones each bearing a peculiar species or distinctive assemblage of species. By these means the Cretaceous rocks of the world have now been correlated zone with zone, with a degree of exactitude proportional to the palaeontological information gained in the several areas of occurrence.
The Cretaceous system falls naturally into two divisions, an upper and a lower, in all but a few limited regions. In the table on page 288 the names of the principal stages are enumerated; these are capable of world-wide application. The sub-stages are of more local value, and too much importance must not be attached to them for the correlation of distant deposits. The general table is designed to show the relative position in the system of some of the more important and better-known formations; but it must be remembered that the Cretaceous rocks of Europe can now be classified in considerable detail by their fossils, the most accurate group for this purpose being the cephalopods. The smaller table was compiled by T. C. Chamberlin and R. D. Salisbury to show the main subdivisions of the North American Cretaceous rocks. The correlation of the minor subdivisions of Europe and America are only approximate.
Relation of the Cretaceous Strata to the Systems above and below.—In central and northern Europe the boundary between the Cretaceous and Tertiary strata is sharply defined by a fairly general unconformity, except in the Danian and Montian beds, where there is a certain commingling of Tertiary with Cretaceous fossils. The relations with the underlying Jurassic rocks are not so clearly defined, partly because the earliest Cretaceous rocks are obscured by too great a thickness of younger strata, and partly because the lowest observable rocks of the system are not the oldest, but are higher members of the system that have overlapped on to much older rocks. However, in the south of England, in the Alpine area, and in part of N.W. Germany the passage from Jurassic to Cretaceous is so gradual that there is some divergence of opinion as to the best position for the line of separation. In the Alpine region this passage is formed by marine beds, in the other two by brackish-water deposits. In a like manner the Potomac beds of N. America grade downwards into the Jurassic; while in the Laramie formation an upward passage is observed into the Eocene deposits. There is a very general unconformity and break between the Lower and Upper Cretaceous; this has led Chamberlin and Salisbury to suggest that the Lower Cretaceous should be regarded as a separate period with the title “Comanchean.”
Physiographical Conditions and Types of Deposit.—With the opening of the Cretaceous in Europe there commenced a period of marine transgression; in the central and western European region this took place from the S. towards the N., slow at first and local in effect, but becoming more decided at the beginning of the upper division. During the earlier portion of the period, S. England, Belgium and Hanover were covered by a great series of estuarine sands and clays, termed the Wealden formation (q.v.), the delta of a large river or rivers flowing probably from the N.W. Meanwhile, in the rest of Europe alternations of marine and estuarine deposits were being laid down; but over the Alpine region lay the open sea, where there flourished coral reefs and great banks of clam-like molluscs. The sea gradually encroached upon the estuarine Wealden area, and at the time of the Aptian deposits uniform marine conditions prevailed from western Europe through Russia into Asia. This extension of the sea is illustrated in England by the overlap of the Gault over the Lower Greens and on to the older rocks, and by similar occurrences in N. France and Germany.
Almost throughout the Upper Cretaceous period the marine invasion continued, varied here and there by slight movements in the opposite sense which did not, however, interfere with the quiet general advance of the sea. This marine extension made itself felt over the old central plateau of France, the N. of Great Britain, the Spanish peninsula, the Armorican peninsula, and also in the Bavarian Jura and Bohemia; it affected the northern part of Africa and East Africa; in N. America the sea spread over the entire length of the Rocky Mountain region; and in Brazil, eastern Asia and western Australia, Upper Cretaceous deposits are found resting directly upon much older rocks. Indeed, at this time there happened one of the greatest changes in the distribution of land and water that have been recorded in geological history.
We have seen that in early Cretaceous times marine limestones were being formed in southern Europe, while estuarine sands and muds were being laid down in the Anglo-German delta, and that beds of intermediate character were being made in parts of N. France and Germany. During later Cretaceous times this striking difference between the northern and southern facies was maintained, notwithstanding the fact that the later deposits were of marine origin in both regions. In the northern region the gradual deepening and accompanying extension of the sea caused the sandy deposits to become finer grained in N.W. Europe. The sandy beds and clays then gave way to marly deposits, and in these early stages glauconitic grains are very characteristically present both in the sand and in the marls. In their turn these marly deposits in the Anglo-Parisian basin were succeeded gradually and somewhat intermittently by the purer, soft limestone of the chalk sea, and by limestones, similar in character, in N. France, extra-Alpine Germany, S. Scandinavia, Denmark and Russia. Meanwhile, the S. European deposits maintained the characters already indicated; limestones (not chalk) prevailed, except in certain Alpine and Carpathian tracts where detrital sandstones were being laid down.
The great difference between the lithological characters of the northern and southern deposits is accompanied by an equally striking difference between their respective organic contents. In the north, the genera Inoceramus and Belemnitella are particularly abundant. In the south, the remarkable, large, clam-like, aberrant pelecypods, the Hippuritidae, Rudistes, Caprotina, &c., attained an extraordinary development; they form great lenticular banks, like the clam banks of warm seas, or like our modern oyster-beds; they appear in successive species in the different stages of the Cretaceous system of the south, and can be used for marking palaeontological horizons as the cephalopods are used elsewhere. Certain genera of ammonites, Haploceras, Lytoceras, Phylloceras, rare in the north, are common in the south; and the southern facies is further characterized by the peculiar group of swollen belemnites (Dumontia), by the gasteropods Actionella, Nerinea, &c., and by reef-building corals. The southern facies is far more widespread and typical of the period than is the chalk; it not only covers all southern Europe, but spreads eastwards far into Asia and round the Mediterranean basin into Africa. It is found again in Texas, Alabama, Mexico, the West Indies and Colombia; though limestones of the chalk type are found in Texas, New Zealand, and locally in one or two other places. The marine deposits are organically formed limestones, in which foraminifera and large bivalve mollusca play a leading part, marls and sandstones; dolomite and oolitic and pisolitic limestones are also known.
|European Classification.||Britain.||Germany, &c., several
other parts of Europe.
(placed by some
in the Tertiary).
Chalk of Trimingham.
Upper Chalk with
Middle Chalk without
Marls and pisolitic
Limestone of Meudon.
Limestone of Saltholm
and Faxö (Denmark).
Upper Quader Sandstone.
Quader Marls and
Lr. Pläner and Lr.
Tourtia of Mons, &c.
Gault and Upper
Flammen mergel. Clay
of N. Germany.
|Upper Cretaceous.||Lower Cretaceous.|
|Alpine Region.||Aptychenkalk in E. Alps ... Cretaceous Flysch...
Biancone of S. Alps.
|... Cretaceous Flysch ...|
Carpathian and Vienna Sandstones,
Gosau formation of E. Alps.
Seewan beds of N. Alps.
Scaglia of S. Alps.
|Africa.||Nubian Sandstone of ...
Uitenhage Beds S. Africa.
|... N. Africa and Syria.|
Pondoland Beds S. Africa.
|India.||Oomia and Utatur Group.||Arialoor Beds (Deccan Trap).|
|Australia.||Rolling Down Formation.||Desert Sandstone.|
|New Zealand.||Thick conglomeratic Series with Bitumous coals.||Waipara Beds and Limestones, Chalk,|
with Flints, Marls and Greensand.
|S. America.||Puegiredon Series. Belgrano ...||... Series. San Martin Series.|
|Japan.||Torinosa Limestone and Ryoseki Series.||Izumi Sandstone and Hokkaido Series.|
|Greenland.||Kome Group.||Atani Group. Patoot Group (part).|
|Note to Table.|
|Montian||from||Mons in Belgium.|
|Danian||”||Denmark = Garumnien of Leymerie.|
|Emscherian||”||Emscher river in Westphalia.|
|Senonian||”||Sens in department of Yonne.|
|Cenomanian||”||Le Mans (Cenomanum).|
|Albian||”||dept. of Aube.|
|Selbornian||”||Selborne in Hampshire.|
|Aptian||”||Apt in Vaucluse.|
|Gargasian||”||Gargas near Apt.|
|Bedoulian||”||la Bedoule (Var) = Rhodanien of Renevie|
|Barremian||”||Barrême in Basses Alpes.|
|Hauterivian||”||Hauterive on Lake of Neuchâtel.|
|Valangian||”||Château de Valangin near Neuchâtel.|
|Berriasian||”||Berrias (Ardéche) near Besseges.|
|Urgonian||”||Orgon near Arles.|
The Cretaceous seas were probably comparatively shallow; this was certainly the case where the deposits are sandy, and in the regions occupied by the hippuritic fauna. Much discussion has taken place as to the depth of the chalk sea. Stress has been laid upon the resemblance of this deposit to the modern deep-sea globigerina-ooze; but on the whole the evidence is in favour of moderate depth, perhaps not more than 1000 fathoms; the freedom of the deposit from detrital matter being regarded as due to the low elevation of the surrounding land, and the main lines of drainage being in other directions. Sandy and shore deposits are common throughout the system in every region. Besides the Weald, there were great lacustrine and terrestrial deposits in N. America (the Potomac, Kootenay, Morrison, Dakota and Laramie formations) as well as in N. Spain, and in parts of Germany, &c. The general distribution of land and sea is indicated in the map.
Earth Movements and Vulcanicity.—During the greater part of the Cretaceous period crustal movements had been small and local in effect, but towards the close a series of great deformative movements was inaugurated and continued into the next period. These movements make it possible to discriminate between the Cretaceous and Tertiary rocks, because the conditions of sedimentation were profoundly modified by them, and in most parts of the world there resulted a distinct break in the sequence of fossil remains. Great tracts of our modern continental land areas gradually emerged, and several mountainous tracts began to be elevated, such as the Appalachians, parts of the Cordilleras, and the Rocky Mountains, and their northern continuation, and indeed the greater part of the western N. American continent was intensely affected; the uplifting was associated with extensive faulting. Volcanic activity was in abeyance in Europe and in much of Asia, but in America there were many eruptions and intrusions of igneous rock towards the close of the period. Diabases and peridotites had been formed during the Lower Cretaceous in the San Luis Obispo region. Great masses of ash and conglomerate occur in the Crow’s Nest Pass in Canada; porphyries and porphyritic tuffs of later Cretaceous age are important in the Andes; while similar rocks are found in the Lower Cretaceous of New Zealand. It is, however, in the Deccan lava flows of India that we find eruptions on a scale more vast than any that have been recorded either before or since. These outpourings of lava cover 200,000 sq. m. and are from 4000 to 6000 ft. thick. They lie upon an eroded Cenomanian surface and are to some extent interbedded with Upper Cretaceous sediments.
|Atlantic Coast.||Eastern Gulf
|Western Gulf |
|Western Interior.||Pacific Coast.||European.|
(possibly Eocene). &c.
1. Eagle Ford
2. Fox Hills.
1. Fort Pierre and
|U n c o n f o r m i t y.|
Morrison (or Como).
Economic Products of Cretaceous Rocks.—Coal is one of the most important products of the rocks of this system. The principal Cretaceous coal-bearing area is in the western interior of N. America, where an enormous amount of coal—mostly lignitic, but in places converted into anthracite—lies in the rocks at the foot of the Rocky Mountains; most of this is of Laramie age. Similar beds occur locally in Montana. Coal seams of Lower Cretaceous age are found in the Black Hills (S. Dakota), Alaska, Greenland, and in New Zealand; and the “Upper Quader” of Löwenberg in Silesia also contains coal seams. Coals also occur in the brackish and fresh-water deposits of Carinthia, Dalmatia and Istria, while unimportant lignitic beds are known in many other regions. The Fort Pierre beds are oil-bearing at Boulder, Colorado; and the Trinity formation bears asphalt and bitumen. Important clay deposits are worked in the Raritan formation of New Jersey, &c., and pottery clays are found in the Löwenberg district in Germany. The Washita beds yield the well-known hone stone. Great beds of gypsum exist in the Cretaceous rocks of S. America. Near Salzburg a variety of the hippuritic limestone is quarried for marble. Lithographic stone occurs in the Pyrenees. The economic products peculiar to the chalk are mentioned in the article Chalk. Beds of iron ore are found in the Lower Cretaceous of Germany and England.
The Life of the Cretaceous Period.—The fossils from the Cretaceous series comprise marine, fresh-water and terrestrial animals and plants. Foremost in interest and importance is the appearance in the Lower Potomac (Lower Cretaceous) of eastern and central N. America of the earliest representatives of angiospermous dicotyledons, and undoubted monocotyledons, the progenitors of our modern flowering plants. The angiosperms spread outward from the Atlantic coast region of N. America, and first appeared in Europe in the Aptian of Portugal; towards the close of the Lower Cretaceous period they occupied parts of Greenland, the remaining land areas of N. America, and were steadily advancing in every quarter of the globe. At first the Jurassic plants, the Cycads, ferns and conifers, lived on and were the dominant plant forms. Gradually, however, they took a subordinate place, and by the close of the Cretaceous period the angiosperms had gained the upper hand. The earliest of these fossil angiosperms is not in a true sense a primitive form, and no records of such types have yet been discovered. Some of the early forms of the Lower Cretaceous are distinctly similar to modern genera, such as Ficus, Sassafras and Aralia; others bore leaves closely resembling our elm, maple, willow, oak, eucalyptus, &c. Before the close of the period many other representatives of living genera had appeared, beech, walnut, tamarisk, plane, laurel (Laurus), cinnamon, ivy, ilex, viburnum, buckthorn, breadfruit, oleander and others; there were also junipers, thujas, pines and sequoias and monocotyledons such as Potamogeton and Arundo. This flora was widely spread and uniform; there was great similarity between that of Europe and N. America, and in parts of the United States (Virginia and Maryland) the plants were very like those in Greenland. The general aspect of the flora was sub-tropical; the eucalyptus and other plants then common in Europe and N. America are now confined to the southern hemisphere.
The marine fauna comprised foraminifera which must have swarmed in the Chalk and some of the limestone seas; their shells have formed great thickness of rock. Common forms are the genera Alveolina, Cristellaria, Rotalia, Textularia, Orbitolina, Globigerina. Radiolarians were doubtless abundant, but their remains are rare. Sponges with calcareous (Peronidilla, Barroisia) and siliceous skeletons (Siphonia, Coeloptychium, Ventriculites) were very numerous in certain of the Cretaceous waters. Corals were comparatively rare, Trochosmilia, Parasmilia, Holocystis being typical genera; reefs were formed in the Maestricht beds of Denmark and Faxoe, in the Neocomian and Turonian of France, in the Turonian of the Alps and Pyrenees, and also in the Gosau beds and in the Utatur group of India. Sea-urchins were a conspicuous feature, and many nearly allied forms are still living; Cidaris, Micraster, Discoidea are examples. Crinoids were represented by Marsupites, Uintacrinus and Bourgueticrinus; starfish (Calliderma and Pentagonaster) were not uncommon. Polyzoa were abundant; brachiopods were fairly common, though subordinate to the pelecypods; they were mostly rhynchonellids and terebratulids, which lived side by side with the ancient forms, like Crania and Discina. The bivalve mollusca were very important during this period, Inoceramus, Ostrea, Spondylus, Gervillia, Exogyra, Pecten, Trigonia being particularly abundant in the northern seas, while in the southern waters the remarkable Hippurites, Radiolites, Caprotina, Caprina, Monopleura and Requienia prevailed. Gasteropods were well represented and included many modern genera. Cephalopods were important as a group, but the ammonites, so vigorous in the foregoing period, were declining and were assuming curious degenerate forms, often with a tendency to uncoil the shell; Baculites, Hoplites, Turrilites, Ptychoceras, Hamites are some of the typical genera, while Belemnites and Belemnitella were abundant in the northern seas.
The vertebrate fauna of the Cretaceous period differed in many features from that of the present day; mammals appear to have been only poorly represented by puny forms, related to Triassic and Jurassic types; they were mainly marsupials (Batodon, Cimolestes) with a few monotreme-like forms; carnivores, rodents and ungulates were still unknown. As in Jurassic times, reptiles were the dominant forms, and not a few genera lived on from the former period into the Cretaceous; but, on the whole, the reptilian assemblage was no longer so varied, and most of the distinctive mesozoic types had passed away before the close of this period. Dinosaurs were represented by herbivorous and carnivorous genera as in the Jurassic period, but the latter were less abundant than before. The Iguanodon of the Sussex-Weald and Bernissart in Belgium is perhaps the best-known genus; but there were many others, their remains being particularly abundant and well-preserved in the Cretaceous deposits of N. America. Titanosaurus, Acanthopholis, Megalosaurus and Hypsilophodon may be mentioned, some of these being of great size, while Diclonius was a curious duck-billed creature; but most remarkable in appearance must have been the horned Dinosaurs, Ceratops and Triceratops, gross, unwieldy creatures, 25 to 30 ft. long, whose huge heads were grotesquely armed with horns and bony frills.
Coincident, perhaps, with the widespread extension of the sea was the development of aquatic habits and structures suitable thereto amongst all the reptilian groups including also the birds. The foremost place was undoubtedly taken by the pythonomorphs or sea-serpents, including Mosasaurus and many others; these were enormously elongated creatures, reaching up to 75 ft., with swimming flappers and powerful swimming tails, and they lived a predatory life in the open sea. Ichthyosaurians soon disappeared from Cretaceous waters; but the plesiosaurians (Cimoliosaurus and others) reached their maximum development in this period. The remarkable flying lizards, pterosaurs, likewise attained their great development and then passed away; they ranged in size from that of a pigeon to creatures with a wing-spread of 25 ft.; notable genera are Pteranodon, Ornithocheirus, Nyctiosaurus. Ordinary lizard-like forms were represented by Coniosaurus, Dolichosaurus, &c.; and true crocodiles, Goniopholis, Suchosaurus, appeared in this period, and continued to approximate to modern genera. The earliest known river turtles are found in the Belly River deposits of Canada; marine turtles also made their first appearance and were widely represented, some of them, Archelon and Protostega, being of great size. True snakes appeared later in the period.
The birds, as far as existing evidence goes, were aquatic; some, like Ichthyornis, were built for powerful flight; others, like Hesperornis, were flightless. Enaliornis is a form well known from the Cambridge Greensand. They were toothed birds having structural affinities with the Dinosaurs and Pterodactyles.
Fish remains of this period show that a marked change was taking place; teleosteans (with bony internal skeleton) were taking a more prominent place, and although ganoids were still represented (Macropoma, Lepidotus, Amiopris, &c.) they had quite ceased to be the dominant types before the close of Cretaceous times. Sharks and rays were of the modern types, though distinct in species. Amongst the early forms of Cretaceous teleosteans may be mentioned Elopopsis, Ichthyodectes, Diplomystus (herring), Haplopteryx and Urenchelys (eel).
For further information see the articles Chalk; Greensand; Wealden. Sir A. Geikie’s Text-book of Geology, vol. ii. (4th ed., 1903), contains in addition to a full general account of the system very full references to the literature.