CRETACEOUS PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. third period of the Mesozoic Era. It began about 144 million years ago and ended 66.4 million years ago (see Table). It is often divided into the Early Cretaceous Epoch (144 to 97.5 million years ago) and the Late Cretaceous Epoch (97.5 to 66.4 million years ago). As the final period of the Mesozoic Era, the Cretaceous Period follows the Jurassic Period and precedes the Tertiary Period. Spanning more than 77 million years, the Cretaceous is the longest of all the Phanerozoic periods; it represents more time than has elapsed since the last dinosaurs roamed the Earth. The rocks that were either deposited or formed during the Cretaceous Period make up the Cretaceous System. interval of geologic time from approximately 144 to 66.4 million years ago. It is often divided into the Early Cretaceous Epoch (144 to 97.5 million years ago) and the Late Cretaceous Epoch (97.5 to 66.4 million years ago). As the final period of the Mesozoic Era, the Cretaceous Period follows the Jurassic Period and precedes the Tertiary Period. The Cretaceous spans more than 77 million years, making it the longest period of Phanerozoic time. The rocks that originated during the period make up the Cretaceous System. At the start of the Cretaceous Period, South America was still connected to the western coast of Africa, and Australia was joined to Antarctica. India had split apart from these two landmasses and was drifting northward toward the Eurasian continent. North America had broken away from Africa, but it was still attached to western Europe. A deep seaway, the Tethys, extended eastward from Spain across southern Asia. In the mid-Cretaceous, South America split from Africa and began moving westward. Then North America began to separate from western Europe, and, by the beginning of the Tertiary, the Atlantic was open right up to the Arctic Ocean. From Late Cretaceous to Early Tertiary times, the Laramide orogeny occurred along the leading edge of the moving continent, forming the Rocky Mountains. Similar disturbances occurred in the circum-Pacific belt. Meanwhile, New Zealand separated from the Australia-Antarctica complex, and, at the end of the Cretaceous, Australia began to separate from Antarctica. Africa moved north against Europe, closing the Tethys Sea and initiating the Alpine orogeny that led to the building of the Alps in the Tertiary Period. The climate was warmer than today, but the northern hemispheric landmasses were moving away from the Equator. In the Late Cretaceous, extensive deposition of chalk (calcium carbonate) in deep water was probably paralleled by a decrease in carbon dioxide in the atmosphere, which would have lowered temperatures globally. In Early Cretaceous times, seas began to advance over the continental area. The rocks that were deposited resemble those of the Jurassic; marine deposits, including shallow- and deep-water limestones and marine clays, were laid down over northern Europe, the Mediterranean, and the circum-Pacific belt. Nonmarine deposits, more common than in the Jurassic, included sandstones and shales in northwestern Europe and clastic deposits (e.g., those of sand, clay, and mud) in Central Asia, Africa, South America, and Australia. The advancing seas reached a maximum extent in Late Cretaceous times, and marine deposits covered a far greater area than at any other time during the Mesozoic. Great thicknesses of chalk laid down in what is now western Europe, eastern Russia, the Gulf Coast of the United States, and Western Australia are especially notable. The seas retreated again at the close of the period. In Cretaceous seas, marine invertebrates such as rudist bivalves, ammonites, belemnites, brachiopods, and echinoids continued to flourish and evolve. Both ammonites and belemnites, however, mysteriously became extinct at the end of the period. Bony fishes vigorously evolved and were prominent in Late Cretaceous times. Among marine reptiles, ichthyosaurs became less common, but the long-necked plesiosaurs continued to thrive, and a new group of aquatic lizards, the mosasaurs, appeared, attaining great size and worldwide distribution in a relatively short period of time. On land, angiosperms (flowering plants) had a spectacular rise to supremacy in the Late Cretaceous, and the insects, bees in particular, began their thriving partnership with them. Mammals and birds remained inconspicuous throughout the Cretaceous, while the reptiles continued their spectacular dominance until the end of the period. In the air, the flying reptiles called pterosaurs were predominant, with some species reaching enormous size. On land, the dinosaurs reached the peak of their evolution. Of the saurischian dinosaurs, both sauropods and theropods persisted into the Cretaceous, but the period is chiefly remarkable for the evolution of new forms of ornithischian dinosaurs: heavily armoured ankylosaurs, horned ceratopsids, and such ornithopods as iguanodons and hadrosaurs (duck-billed dinosaurs), which were superbly adapted to eating vegetation. The total extinction of the dinosaurs and the decimation of many other reptile groups occurred at the end of the Cretaceous. Many hypotheses have been proposed to explain their disappearance: deteriorating climate, with its attendant effects on food supply (e.g., seasonal declines in edible plant matter); alterations in habitat because of Earth movements; or reduction in numbers due to egg-eating reptiles or mammals. The most credible theory, however, is that a catastrophic collision of a comet or asteroid with the Earth raised a global dust cloud that enshrouded the planet in darkness for months or even years, resulting in the cessation of photosynthesis and the death of green plants on which herbivores and their predators were ultimately dependent. Given their abruptness, their worldwide scope, and the number of groups involved, the Cretaceous boundary extinctions have provided one of the great enigmas of geology and paleontology. Additional reading Studies of the stratigraphy of the period include T. Birkelund et al., Cretaceous Stage Boundaries: Proposals, Bulletin of the Geological Society of Denmark, 33(12):320 (1984); J.M. Hancock and E.G. Kauffman, The Great Transgressions of the Late Cretaceous, Journal of the Geological Society of London, 136:175186 (1979), comparing the transgressions in Europe, North America, and other areas; T. Matsumoto, Inter-regional Correlation of Transgressions and Regressions in the Cretaceous Period, Cretaceous Research, 1(4):359373 (1980), comparing the stable areas and relationships to tectonically active areas; and Richard A. Reyment and Peter Bengtson (compilers), Events of the Mid-Cretaceous: Final Report on Results Obtained by IGCP Project (1986), surveying mid-Cretaceous studies worldwide. Cretaceous rocks and environmental features are the subject of W.G.E. Caldwell (ed.), The Cretaceous System in the Western Interior of North America (1975), a series of papers on a Cretaceous epicontinental sea; C.R. Lloyd, The Mid-Cretaceous Earth: Paleogeography, Ocean Circulation and Temperature, Atmospheric Circulation, Journal of Geology, 90(4):393413 (1982), providing quantitative and qualitative synthesis of climate, circulation, and temperatures; and Walter Kegel Christensen, Upper Cretaceous Belemnites from the Vomb Trough in Scania, Sweden (1986), focusing on the biostratigraphy and biogeography of boreal Europe. Life-forms are examined by Norman F. Sohl, Cretaceous Gastropods: Contrasts Between Tethys and the Temperate Provinces, Journal of Paleontology, 61(6):10851111 (1987), a comparative faunal history; C.F. Koch and N.F. Sohl, Preservational Effects in Paleoecological Studies: Cretaceous Mollusc Examples, Paleobiology, 9:2634 (1983); and S.M. Stanley, Earth and Life Through Time, 2nd ed. (1989). Carl Fred Koch Cretaceous environment Paleogeography The major geographic subdivisions of the world for the Cretaceous Period are the northern boreal, southern boreal, and Tethyan regions. The Tethyan region separates the other two and is recognized by the presence of reef-forming rudist bivalves, corals, larger foraminiferans, and certain ammonites that inhabited only the warmer Tethyan waters. Early in the Cretaceous, North America and South America separated sufficiently for the marine connection between the Tethys and Pacific to deepen substantially. The Tethys to Pacific marine connection allowed for a strong westward-flowing current, which is inferred from faunal patterns. For example, as the Cretaceous progressed, the similarity between rudist bivalves of the Caribbean and western Europe decreased, while some Caribbean forms have been found on Pacific seamounts, in Southeast Asia, and possibly in the Balkans. The Cretaceous of the northern boreal realm in North America, Europe, Russia, and Japan has been extensively studied because of many years of geologic research and the generally wealthier political entities that exist in those areas. It is known, for instance, that sediments in the southwestern Netherlands indicate several temperature swings for the Late Cretaceous. These temperature swings imply that the paleogeographic boundary between the northern boreal areas and the Tethys was not constant with time. Russian workers recognized six paleobiogeographic zones: boreal, which in this situation is equivalent to Arctic; European region; Mediterranean region, including the central Asian province; Pacific region; and two paleofloristic zonations of land. At present, studies of the southern boreal areas and the rocks representing the southern Tethys margin lack this level of detail. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during Late As pointed out earlier, the positions of the landmasses changed significantly during the Cretaceous. This is not unexpected considering that the period lasted more than 77 million years. At the onset there existed the two supercontinents of Gondwana and Laurasia, which were barely attached at the junction of North and South America. When these enormous landmasses divided, the South Atlantic Ocean, Indian Ocean, Gulf of Mexico, and Caribbean came into being. By the end of the Cretaceous, the present-day continents were separate entities except for Australia, which was still joined to Antarctica. Also, India had not yet fused to Asia. The positions of the various continents were very nearly those shown in thefigure for the Maastrichtian shortly before the close of the Cretaceous. Very high sea stand was a major factor influencing the paleogeography of the Cretaceous. In general, the maximum was lower in the Early Cretaceous than in the Late Cretaceous, but detailed study indicates from 5 to 15 different episodes of rises and falls in sea level. The high sea stands were not expressed equally in all regions. Some regions were especially tectonically active during the Cretaceous. The Pacific margin of Canada, for one, shows evidence of an Early Cretaceous transgression, but by the Late Cretaceous much of the region had been uplifted some 800 to 2,000 metres. Japan also was very tectonically active, giving it a Cretaceous sedimentary record that varies from north to south, island to island, and time to time. The patterns for sea-level changes for the stable platforms throughout history are quite similar, although several differences are notable. During the Berriasian, Valanginian, Hauterivian, and Barremian, parts of Arctic Canada, Russia, and Western Australia were underwater, but most of the other platforms were not. During the Aptian and Albian, east-central Australia experienced major transgressions, but those of the Late Cretaceous are not recorded there. In the Late Cretaceous, most continental landmasses were transgressed but not always at the same time. One suggestion for the lack of a synchronous record is the concept of geoidal eustacy. It has been suggested based on observed patterns of Cretaceous rocks and physical calculations that, as the Earth's continents move about, the oceans bulge out at some places to compensate, and thus sea level rise is different from ocean basin to ocean basin. Paleoclimate In general, the climate of the Cretaceous Period was much warmer than at present, perhaps the warmest on a worldwide basis than at any other time during the Phanerozoic. The climate was also more equable in that the temperature difference from the poles to the equator was about one-half the present gradient. Floral evidence suggests that tropical to subtropical conditions existed as far north as 45, and temperate conditions extended to the poles. Temperatures were lower at the beginning of the period, rising to a maximum in the late Albian and then declining slightly with time until the decline was accentuated during the Campanian and Maastrichtian. Ice sheets and glaciers were almost entirely absent except in the high mountains. The late Maastrichtian was the coolest part of the Cretaceous, but the temperature was still much warmer than it is today. Models of the Earth's climate for the mid-Cretaceous based on the positions of the continents, location of water bodies, and topography suggest that winds were weaker than at present. Westerly winds were dominant in the lower to mid-latitudes of the Pacific for the entire year. Winds were, however, westerly in the North Atlantic during winter but easterly during summer. Surface water temperatures were about 30 C at the equator year round, but 14 C in winter and 17 C in summer at the poles. A temperature of 17 C is suggested for the ocean bottom during the Albian but may have declined to 10 C by the Maastrichtian. These temperature values have been calculated from oxygen-isotope measurements of the calcitic remains of belemnites, planktonic foraminiferans, and benthic foraminiferans. These temperatures support models that suggest diminished ocean circulation both vertically and latitudinally. Such circulation patterns could account for the periods of black shale deposition during the Cretaceous. In general, the Cretaceous would rank as the most arid period of the Phanerozoic. Not all scientists agree with this assessment, however. Evaporites are plentiful in the Early Cretaceous, a fact that seems to point to an arid climate. Yet, this situation may result more from constricted ocean basins than from climatic effects. The occurrence of evaporites mainly between 10 and 30 latitude suggests arid subtropics, but the presence of coals poleward of 30 indicates humid mid-latitudes. Occurrences of early Cretaceous bauxite and laterite, which are products of deep weathering in warm climates with seasonal rainfall, support the notion of humid mid-latitudes. Other paleontological indicators suggest details of ocean circulation. The occurrence of early and mid-Cretaceous rudists and larger Tethyan foraminiferans in Japan may very well mean that there was a warm and northward-flowing current in the region. A similar occurrence of these organisms in AptianAlbian sediments as far south as southern Tanzania seems to indicate a southward-flowing current along the east coast of Africa. The fact that certain warmwater life-forms that existed in the area of present-day Argentina are absent from the west coast of Africa suggests a counterclockwise gyre in the South Atlantic. In addition, the presence of larger foraminiferans in Newfoundland and Ireland points to the development of a proto-Gulf Stream by the mid-Cretaceous. Cretaceous life The lengthy Cretaceous Period constitutes a major portion of the interval of transition between ancient life-forms and those forms that dominate the Earth today. Many of these modern animals and plants, as, for example, the placental mammals and angiosperms, made their first appearance during the Cretaceous. (Some authorities maintain that certain varieties of angiosperms belonging to the class Monocotyledoneae evolved in the Late Jurassic based on fossil evidence.) Other groups, such as clams and snails, snakes and lizards, and teleost fishes (the ray-finned variety considered to be the most advanced of the bony fishes) developed distinctively modern characteristics by the end of the Maastrichtian. Characteristic fauna and flora The marine realm can be divided into two paleobiogeographic regions, the Tethys and the boreal. The division is based on the occurrence of rudist-dominated organic reeflike structures. Rudists were large, rather unusual bivalves that had one valve shaped like a cylindrical vase and another that resembled a flattened cap. The rudists were generally dominant over the corals as framework builders. They rarely existed outside the Tethyan region, and the few varieties found elsewhere did not create reeflike structures. Rudist reeflike structures of Cretaceous age serve as reservoir rocks for petroleum in Mexico, Venezuela, and the Middle East. Other organisms that were almost entirely restricted to the Tethys region were actaeonellid and nerineid snails, colonial corals, calcareous algae, larger benthic foraminiferans, and certain kinds of ammonites and echinoids. In contrast, belemnites were apparently confined to the colder boreal waters. Important bivalve constituents of the Cretaceous boreal marine biota were the reclining forms (e.g., Exogyra and Gryphaea) and the inoceramids, which were particularly widespread and therefore useful for biostratigraphic zonation. Ammonites were numerous and were represented by a variety of forms ranging from the more usual coiled types to straight forms. Some of the more unusual ammonites, called heteromorphs, were shaped like fat corkscrews and fat hairpins. Such aberrant forms most certainly had difficulty moving about. Ammonites preyed on other nektonic and benthic invertebrates and were themselves prey to many larger animals, including the marine reptiles called mosasaurs. Other marine reptiles were the long-necked plesiosaurs and more fishlike ichthyosaurs. Sharks and rays also were marine predators, as were the teleosts. One Cretaceous fish, Xiphactinus, grew to more than 4.5 metres and is the largest known teleost. In the air, the flying reptiles called pterosaurs dominated. One pterosaur from the latest Cretaceous of what is now Texas, Quetzalcoatlus, had a wingspan of about 15 metres. While it has been determined that birds developed from a reptilian ancestor during the Jurassic and Cretaceous, the fossil record for birds is too sparse to accurately document their evolution. Hesperornis was a genus of Cretaceous flightless, diving bird that had large feet and sharp backward-directed teeth adapted for preying on fish. Although the fossil record is irregular in quality and quantity during the Early Cretaceous, it is obvious that dinosaurs continued their lengthy dominance on land. The Late Cretaceous record is much more complete, particularly in the case of North America and Asia. It is known, for instance, that during the Late Cretaceous many dinosaur types lived in relationships not unlike the present-day terrestrial mammal communities. Although the larger dinosaurs such as the carnivorous Tyrannosaurus and the herbivorous Iguanodon are the best known, many smaller forms also lived in Cretaceous times. Triceratops, a large three-horned dinosaur, inhabited western North America during the Maastrichtian age. The land plants of the Early Cretaceous were similar to those of the Jurassic. They included the cycads, ginkgoes, conifers, and ferns. The angiosperms appeared by the Barremian, became common by the end of the Albian, and came to represent the major component of the terrestrial flora by the mid-Late Cretaceous. This flora included figs, magnolias, poplars, willows, sycamores, and herbaceous plants. With the advent of many new plant types, insects also diversified. Cretaceous rocks Occurrence and distribution The occurrence and distribution of Cretaceous rocks resulted from the interplay of many forces. The most important of these are the position of the continental landmasses, level of the sea relative to these landmasses, local tectonic and orogenic activity, climatic conditions, availability of source material (for example, sands, clays, and even the remains of marine animals and plants), igneous activity, and the history of the rocks and sediments after intrusion or deposition. Many Cretaceous sedimentary rocks have been eroded since their deposition, while others are merely covered by younger sediments or are presently underwater or both. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during Late The figure shows the position of the present-day continental landmasses during the Maastrichtian approximately 70 million years ago. At the very beginning of the Mesozoic these landmasses had been together. As was mentioned earlier, South America, Africa (including the adjoining pieces of what are now the Arabian Peninsula and Middle East), Antarctica, Australia, India, Madagascar, and several smaller landmasses were joined as Gondwana in the south, while North America, Greenland, and Eurasia (including Southeast Asia) formed Laurasia in the north. The breakup of the supercontinent of Pangaea, which had started more than 100 million years earlier during the Early Jurassic, showed major developments in the Cretaceous. Africa split from South America, the last land connection being that between Brazil and Nigeria. This separation was complete by about AptianAlbian time, resulting in the joining of the South Atlantic Ocean with the widening North Atlantic. In the region of the Indian Ocean, Africa and Madagascar separated from India, Australia, and Antarctica in Late Jurassic to Early Cretaceous times. Once separated from Australia and Antarctica, India began its journey northward, which culminated in a collision with Eurasia during the Cenozoic. Madagascar broke away from Africa during the later Cretaceous. During the Late Cretaceous, Greenland separated from North America. A graphic example of the influence of continental fragmentation on the Cretaceous rock record can be seen in the stratigraphic column for southeastern Nigeria in the Table. The stratigraphic record begins in the Albian only after the South Atlantic opened. Sea level was higher during most of the Cretaceous than at any other time in Earth history. In general, the world oceans were about 100 to 200 metres higher in the Early Cretaceous and roughly 200 to 250 metres higher in the Late Cretaceous than at present. The high Cretaceous sea level is thought to have been primarily the result of water in the ocean basins being displaced by the enlargement of the mid-oceanic ridges. The Table gives the overall trend in sea level, but minor peaks and troughs are known to have occurred. The rocks shown in the Table illustrate well the effect of the high Cretaceous sea level. Chalks and limestones, for example, were deposited in the western interior of North America only during the early Late Cretaceous, when sea levels were at their highest. As a result of higher sea levels during the Late Cretaceous, marine waters inundated the continents, creating relatively shallow epicontinental seas in North America, South America, Europe, Russia, Africa, and Australia. In addition, all continents experienced diminution of land area adjacent to the major oceans. The effects of these higher sea levels were not felt to the same extent by each continent, because the various continents experience movement of their crustal level due to isostasy and tectonism. For example, if a continent is emergent due to isostatic rebound, the vertical movement could exceed the rise in sea level and so the continent would not experience transgression of marine water but rather regression. When a platform is underwater, sedimentation occurs; when it is not, erosion takes place. The rock record for the Great Artesian Basin in east-central Australia (see Table), for instance, shows marine rocks for much of the AptianAlbian but nonmarine sediments during the Cretaceous maximum transgression near the end of the Cenomanian. A comparison of the rock record for the North American western interior with that for eastern England (see Table) reveals chalk deposition in eastern England from Cenomanian to Maastrichtian time, but chalks and marine limestone are limited to late Cenomanian through early Santonian time in North America. The two areas have nearly identical histories of transgression. It has been noted that the land areas of western Europe during the Late Cretaceous were limited to a few stable regions that represent low-lying islands within a chalk sea. Furthermore, sedimentological evidence indicates an arid climate that would minimize erosion of these islands and limit the input of sands and clays into the basin. In contrast, the North American western interior was receiving abundant clastic sediments that were being eroded from the new mountains along its western margin created by the Sevier orogeny of Cretaceous time. In addition to the areas that have been mentioned above, Cretaceous rocks crop out in the Arctic, Greenland, central California, the Gulf and Atlantic coastal plains of the United States, central and southern Mexico, and the Caribbean islands of Jamaica, Puerto Rico, Cuba, and Hispaniola. In Central and South America, Cretaceous rocks are found in Panama, Venezuela, Colombia, Ecuador, Peru, eastern and northeastern Brazil, and central and southern Argentina. Most European countries have Cretaceous rocks exposed at the surface. North Africa, West Africa, coastal South Africa, Madagascar, Arabia, Iran, and the Caucasus all have extensive Cretaceous outcrops, as do eastern Siberia, Tibet, India, China, Japan, Southeast Asia, New Guinea, Borneo, Australia, New Zealand, and Antarctica. Types The rocks and sediments of the Cretaceous System show considerable variation in their lithologic character and the thickness of their sequences. Mountain-building episodes accompanied by volcanism and plutonic intrusion took place in the circum-Pacific region and in the area of the present-day Alps. The erosion of these mountains produced clastic sediments, such as conglomerates, sandstones, and shales, on their flanks. The igneous rocks of Cretaceous age in the circum-Pacific area are widely exposed. The Cretaceous Period was a time of great inundation by shallow seas that created swamp conditions favourable for the accumulation of fossil fuels at the margin of land areas. Coal-bearing strata are found in some parts of Cretaceous sequences in Siberia, Australia, New Zealand, Mexico, and the western United States. Farther offshore, chalks are widely distributed in the Late Cretaceous. Another rock type called the Urgonian limestone is similarly widespread in the Upper BarremianLower Aptian. This massive limestone facies, whose name is commonly associated with rudists (a reef-building bivalve of the Mesozoic), is found in Mexico, Spain, southern France, Switzerland, Bulgaria, the southern Soviet Union, and North Africa. The mid-Cretaceous was a time of extensive deposition of carbon-rich shale with few or no benthic fossils. These so-called black shales result when there is severe deficiency of oxygen in the bottom waters of the oceans. Poor ocean circulation is suggested as the cause, and the poor circulation is thought to have resulted from the generally warmer climate that prevailed during the Cretaceous, the much smaller than present temperature difference between the poles and the equator, and the restriction of the North Atlantic, South Atlantic, and Tethys. Cretaceous black shales are extensively distributed on various continental areas, such as the western interior of North America, the Alps, the Apennines of Italy, western South America, Western Australia, western Africa, and southern Greenland. They also occur in the Atlantic Ocean, as revealed by the Deep Sea Drilling Program (a scientific program initiated in 1968 to study the ocean bottom), and in the Pacific, as noted on several seamounts. In North America the Nevadan orogeny took place in the Sierra Nevada and Klamath Mountains from Late Jurassic to Early Cretaceous times; the Sevier orogeny produced mountains in Utah and Idaho in the mid-Cretaceous; and the Laramide orogeny, with its thrust faulting, gave rise to the Rocky Mountains and Sierra Madre Oriental during the Late Cretaceous to Early Tertiary. In the South American Andean system, mountain building reached its climax in mid-Late Cretaceous. In Japan the Sakawa orogeny proceeded through a number of phases during the Cretaceous. In typical examples of circum-Pacific orogenic systems, regional metamorphism of the high-temperature type and large-scale granitic emplacement occurred on the inner continental side, whereas sinking, rapid sedimentation, and regional metamorphism predominated on the outer oceanic side. The intrusion of granitic rocks, accompanied in some areas by extrusion of volcanic rocks, had a profound effect on geologic history. This is exemplified by the upheaval of the Sierra Nevada, with the intermittent emplacement of granitic bodies and the deposition of thick units of Cretaceous shales and sandstones with many conglomerate tongues in the Great Valley of California. Volcanic seamounts of basaltic rock with summit depths of 1,300 to 2,100 metres are found in the central and western Pacific. Some of them are flat-topped, with shelves on their flanks on which reef deposits or gravels accumulated, indicating a shallow-water environment. Some of the deposits contain recognizable Cretaceous fossils. Although the seamounts were formed at various times during the late Mesozoic and Cenozoic eras, a large number of them were submarine volcanoes that built up to the sea surface during the Cretaceous. They sank to their present deep levels some time after the age indicated by their youngest shallow-water fossil. In west-central India, the Deccan traps consist of more than 1,200 metres of basaltic lava flows that erupted from the Late Cretaceous to the Eocene Epoch of the Tertiary over an area of some 500,000 square kilometres. Volcanic activity on the western margin of the North American epicontinental sea frequently produced ashfalls over much of the western interior seaways. One of these, the X bentonite near the end of the Cenomanian, can be traced more than 2,000 kilometres from central Manitoba to north Texas.