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System Series Stage Age (Ma)
Neogene Miocene Aquitanian younger
Paleogene Oligocene Chattian 23.03–28.4
Rupelian 28.4–33.9
Eocene Priabonian 33.9–37.2
Bartonian 37.2–40.4
Lutetian 40.4–48.6
Ypresian 48.6–55.8
Paleocene Thanetian 55.8–58.7
Selandian 58.7–61.7
Danian 61.7–65.5
Cretaceous Upper Maastrichtian older
Subdivision of the Paleogene Period according to the IUGS, as of July 2009.

The Eocene Epoch, lasting from about 56 to 34 million years ago (55.8 ± 0.2 to 33.9 ± 0.1 Ma), is a major division of the geologic timescale and the second epoch of the Palaeogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by the emergence of the first modern mammals. The end is set at a major extinction event called Grande Coupure (the "Great Break" in continuity), which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified,[1] though their exact dates are slightly uncertain.

The name Eocene comes from the Greek ἠώς (eos, dawn) and καινός (kainos, new) and refers to the "dawn" of modern ('new') mammalian fauna that appeared during the epoch.

Contents

Subdivisions

The Eocene epoch is usually broken into Early and Late, or - more usually - Early, Middle, and Late subdivisions. The corresponding rocks are referred to as Lower, Middle, and Upper Eocene. The Faunal stages from youngest to oldest are:

Priabonian (37.2 ± 0.1 – 33.9 ± 0.1 Ma)
Bartonian (40.4 ± 0.2 – 37.2 ± 0.1 Ma)
Lutetian (48.6 ± 0.2 – 40.4 ± 0.2 Ma)
Ypresian (55.8 ± 0.2 – 48.6 ± 0.2 Ma)

The Ypresian and occasionally the Lutetian constitute the Lower, the Priabonian and sometimes the Bartonian the Upper subsection; alternatively, the Lutetian and Bartonian are united as the Middle Eocene.

Climate

Earth's surface temperatures generally rose from the late Paleocene through the Early Eocene (~59 - 50 Ma), reaching maximum Cenozoic temperatures during the Early Eocene Climatic Optimum (EECO). Superimposed on this warming were a series of "hyperthermals". These are best described as geologically brief (<200 kyr) events characterized by rapid and extreme global warming. The most prominent of these is the Paleocene-Eocene Thermal Maximum or Initial Eocene Thermal Maximum (PETM or IETM), which began at the Paleocene-Eocene Boundary. This was an episode of rapid and intense warming (up to 7 °C at high latitudes) that lasted less than 200,000 years.[2] The Thermal Maximum coincided with a major mammalian turnover on land (that distinguishes Eocene fauna from Paleocene fauna), and an extinction of many benthic foraminifera species in the deep sea.

The Eocene global climate was perhaps the most homogeneous of the Cenozoic; the temperature gradient from equator to pole was only half that of today's, and deep ocean currents were exceptionally warm.[3] The polar regions were much warmer than today, perhaps as mild as the modern-day Pacific Northwest; temperate forests extended right to the poles, while rainy tropical climates extended as far north as 45°. The difference was greatest in the temperate latitudes; the climate in the tropics however, was probably similar to today's.[4] The recent discovery of a giant snake (estimated length 13 m) in Colombia that may have lived during the Eocene suggests, on the contrary, that the tropics were much warmer than today,[5][6] a conclusion in accord with numerical simulations of the climate during the Eocene.[7]

Although the global climate remained comparatively warm throughout the rest of the Eocene it was this epoch that marked the start of a slow global cooling trend, possibly triggered by the Arctic Ocean Azolla event and the formation of the Antarctic circumpolar current following the final break up of Gondwana. This trend would eventually lead to the Pleistocene glaciations.

Paleogeography

A global paleogeographic reconstruction of the Earth during the Eocene, some 50 million years ago.

During the Eocene, the continents continued to drift toward their present positions.

At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the planet and keeping global temperatures high. But when Australia split from the southern continent around 45 Ma, the warm equatorial currents were deflected away from Antarctica, and an isolated cold water channel developed between the two continents. The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north, reinforcing the cooling.

The northern supercontinent of Laurasia began to break up, as Europe, Greenland and North America drifted apart.

In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte.

In Europe, the Tethys Sea finally vanished, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar.

India continued its journey away from Africa and began its collision with Asia, folding the Himalayas into existence.

It is hypothesized that the Eocene hothouse world was caused by runaway global warming from released methane clathrates deep in the oceans. The clathrates were buried beneath mud that was disturbed as the oceans warmed. Methane (CH4) has ten to twenty times the greenhouse gas effect of carbon dioxide (CO2).

Flora

At the beginning of the Eocene, the high temperatures and warm oceans created a moist, balmy environment, with forests spreading throughout the Earth from pole to pole. Apart from the driest deserts, Earth must have been entirely covered in forests.

Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from the Eocene have been found on Ellesmere Island in the Arctic. The preserved remains are not fossils, but actual pieces preserved in oxygen-poor water in the swampy forests of the time and then buried before they had the chance to decompose. Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene have also been found in Greenland and Alaska. Tropical rainforests grew as far north as the Pacific Northwest and Europe.

Palm trees were growing as far north as Alaska and northern Europe during the early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well.

Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry out, with forests thinning out considerably in some areas. The newly-evolved grasses were still confined to river banks and lake shores, and had not yet expanded into plains and savannas.

The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By the end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America, Africa, India and Australia.

Antarctica, which began the Eocene fringed with a warm temperate to sub-tropical rainforest, became much colder as the period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra.

Fauna

Fossil nummulitid foraminiferans showing microspheric and megalospheric individuals; Eocene of the United Arab Emirates; scale in mm.

The oldest known fossils of most of the modern mammal orders appear within a brief period during the early Eocene. At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like artiodactyls, perissodactyls and primates, had features like long, thin legs, feet and hands capable of grasping, as well as differentiated teeth adapted for chewing. Dwarf forms reigned. All the members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Paleocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat.

Both groups of modern ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like Mesonyx. Early forms of many other modern mammalian orders appeared, including bats, proboscidians, primates, rodents and marsupials. Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt and southeast Asia. Marine fauna are best known from South Asia and the southeast United States.

Reptile fossils from this time, such as fossils of pythons and turtles, are abundant. The remains of a giant snake of the size of a school bus has recently been discovered;[8] such a massive snake would have not survived were the tropics as warm as today, contradicting previous conclusions drawn from other proxies for temperature.

During the Eocene, plants and marine faunas became quite modern. Many modern bird orders first appeared in the Eocene.

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Oceans

Basilosaurus

The Eocene oceans were warm and teeming with fish and other sea life. The first Carcharinid sharks appeared, as did early marine mammals, including Basilosaurus, an early species of whale that is thought to be descended from land animals that existed earlier in the Eocene, the hoofed predators called mesonychids, of which Mesonyx was a member. The first sirenians, relatives of the elephants, also appeared at this time.

Grande Coupure

The "end Eocene" event.

The Grande Coupure, or "great break" in continuity,[9] with a major European turnover in mammalian fauna about 33.5 Ma, marks the end of the last phase of Eocene assemblages, the Priabonian, and the arrival in Europe of Asian immigrants. The Grande Coupure is characterized by widespread extinctions and allopatric speciation in small isolated relict populations.[10] It was given its name in 1910 by the Swiss palaeontologist Hans Georg Stehlin,[11] to characterise the dramatic turnover of European mammalian fauna, which he placed at the Eocene-Oligocene boundary. A comparable turnover in Asian fauna has since been called the "Mongolian Remodelling".

The Grande Coupure marks a break between endemic European faunas before the break and mixed faunas with a strong Asian component afterwards. J. J. Hooker and his team summarized the break:[12]

"Pre-Grande Coupure faunas are dominated by the perissodact family Palaeotheriidae (distant horse relatives), six families of artiodactyls (cloven-hoofed mammals) (Anoplotheriidae, Xiphodontidae, Choeropotamidae, Cebochoeridae, Dichobunidae and Amphimerycidae), the rodent family Pseudosciuridae, the primate families Omomyidae and Adapidae, and the archontan family Nyctitheriidae.
"Post-Grande Coupure faunas include the true rhinoceros (family Rhinocerotidae), three artiodactyl families (Entelodontidae, Anthracotheriidae and Gelocidae) related respectively to pigs, hippos and ruminants, the rodent families Eomyidae, Cricetidae (hamsters) and Castoridae (beavers), and the lipotyphlan family Erinaceidae (hedgehogs). The speciose genus Palaeotherium plus Anoplotherium and the families Xiphodontidae and Amphimerycidae were observed to disappear completely.
"Only the marsupial family Herpetotheriidae, the artiodactyl family Cainotheriidae, and the rodent families Theridomyidae and Gliridae (dormice) crossed the faunal divide undiminished."

Whether this abrupt change was caused by climate change associated with the earliest polar glaciations[13] and a major fall in sea levels, or by competition with taxa dispersing from Asia, few argue for an isolated single cause. More spectacular causes are related to the impact of one or more large bolides in northern hemisphere at Popigai, Toms Canyon and Chesapeake Bay. Improved correlation of northwest European successions to global events[12] confirms the Grande Coupure as occurring in the earliest Oligocene, with a hiatus of about 350 millennia prior to the first record of post-Grande Coupure Asian immigrant taxa.

An element of the paradigm of the Grande Coupure was the apparent extinction of all European primates at the Coupure: the recent discovery[14] of a mouse-sized early Oligocene omomyid, reflecting the better survival chances of small mammals, further undercut the Grand Coupure paradigm.

See also

Notes

  1. ^ The extinction of the Hantkeninidae, a planktonic family of foraminifera became generally accepted as marking the Eocene-Oligocene boundary; in 1998 Massignano in Umbria, central Italy, was designated the Global Boundary Stratotype Section and Point (GSSP).
  2. ^ NASA GISS: Science Briefs: Ocean Burps and Climate Change?
  3. ^ http://www.ga.gov.au/odp/publications/tnotes/tn20-4/leg171c.html
  4. ^ Stanley, 508
  5. ^ Huber, M (February 2009). "Climate change: Snakes tell a torrid tale". Nature 457 (7230): 669–71. doi:10.1038/457669a. ISSN 0028-0836. PMID 19194439. 
  6. ^ Head, Jj; Bloch, Ji; Hastings, Ak; Bourque, Jr; Cadena, Ea; Herrera, Fa; Polly, Pd; Jaramillo, Ca (February 2009). "Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures". Nature 457 (7230): 715–7. doi:10.1038/nature07671. ISSN 0028-0836. PMID 19194448. 
  7. ^ Huber and Sloan 2001, Geophysical Research Letters, 28, 3481-3484.
  8. ^ Head, Jj; Bloch, Ji; Hastings, Ak; Bourque, Jr; Cadena, Ea; Herrera, Fa; Polly, Pd; Jaramillo, Ca (February 2009). "Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures". Nature 457 (7230): 715–7. doi:10.1038/nature07671. ISSN 0028-0836. PMID 19194448. 
  9. ^ also termed the MP 21 event.
  10. ^ Called "dispersal-generated origination" in Hooker et al. 2004
  11. ^ H.G. Stehlen, 1910. "Remarques sur les faunules de Mammifères des couches eocenes et oligocenes du Bassin de Paris," in Bulletin de la Société Géologique de France, 4'.9, pp 488-520.
  12. ^ a b Hooker, J.J.; Collinson, M.E.; Sille, N.P. (2004). "Eocene-Oligocene mammalian faunal turnover in the Hampshire Basin, UK: calibration to the global time scale and the major cooling event". Journal of the Geological Society 161: 161. doi:10.1144/0016-764903-091. 
  13. ^ A major cooling event preceded the Grande Coupure, based on pollen studies in the Paris Basin conducted by Chateauneuf (J.J. Chateauneuf, 1980. "Palynostratigraphie et paleoclimatologie de l'Éocene superieur et de l'Oligocene du Bassin de Paris (France)" in Mémoires du Bureau de Recherches Géologiques et Minières, 116 1980).
  14. ^ Köhler, M; Moyà-Solà, S (December 1999). "A finding of oligocene primates on the European continent" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 96 (25): 14664–7. doi:10.1073/pnas.96.25.14664. ISSN 0027-8424. PMID 10588762. PMC 24493. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=10588762. 

References

  • Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
  • Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

EOCENE (Gr. iicbs, dawn, Kacvos, recent), in geology, the name suggested by Sir C. Lyell in 1833 for the lower subdivision of the rocks of the Tertiary Era. The term was intended to convey the idea that this was the period which saw the dawn of the recent or existing forms of life, because it was estimated that among the fossils of this period only 31% of the species are still living. Since Lyell's time much has been learned about the fauna and flora of the period, and many palaeontologists doubt if any of the Eocene species are still extant, unless it be some of the lowest forms of life. Nevertheless the name is a convenient one and is in general use. The Eocene as originally defined was not long left intact, for E. Beyrich in 1854 proposed the term "Oligocene" for the upper portion, and later, in 1874, K. Schimper suggested "Paleocene" as a separate appellation for the lower portion. The Oligocene division has been generally accepted as a distinct period, but "Paleocene" is not so widely used.

In north-western Europe the close of the Cretaceous period was marked by an extensive emergence of the land, accompanied, in many places, by considerable erosion of the Mesozoic rocks; a prolonged interval elapsed before a relative depression of the land set in and the first Eocene deposits were formed. The early Eocene formations of the London-Paris-Belgian basin were of fresh-water and brackish origin; towards the middle of the period they had become marine, while later they reverted to the original type. In southern and eastern Europe changes of sealevel were less pronounced in character; here the late Cretaceous seas were followed without much modification by those of the Eocene period, so rich in foraminiferal life. In many other regions, the great gap which separates the Tertiary from the Mesozoic rocks in the neighbourhood of London and Paris does not exist, and the boundary line is difficult to draw. Eocene strata succeed Cretaceous rocks without serious unconformity in the Libyan area, parts of Denmark, S.E. Alps, India, New Zealand and central N. America. The unconformity is marked in England, parts of Egypt, on the Atlantic coastal plain and in the eastern gulf region of N. America, as well as in the marine Eocene of western Oregon. The clastic Flysch formation of the Carpathians and northern Alps appears to be of Eocene age in the upper and Cretaceous in the lower part. The Eocene sea covered at various times a strip of the Atlantic coast from New Jersey southward and sent a great tongue or bay up the Mississippi valley; similar epicontinental seas spread over parts of the Pacific border, but the plains of the interior with the mountains on the west were meanwhile being filled with terrestrial and lacustrine deposits which attained an enormous development. This great extension of non-marine formations in the Eocene of different countries has introduced difficulties in the way of exact correlation; it is safer, therefore, in the present state of knowledge, to make no attempt to find in the Eocene strata of America and India, &c., the precise equivalent of subdivisions that have been determined with more or less exactitude in the LondonParis-B elgian area.

It is possible that in Eocene times there existed a greater continuity of the northern land masses than obtains to-day. Europe at that time was probably united with N. America through Iceland and Greenland; while on the other side, America may have joined Asia by the way of Alaska. On the other hand, the great central, mediterranean sea which stretched across the Eurasian continents sent an arm northward somewhere just east of the Ural mountains, and thus divided the northern land mass in that region. S. America, Australia and perhaps Africa may have been connected more or less directly with the Antarctic continent. Associated, no doubt, with the crustal movements which closed the Cretaceous and inaugurated the Eocene period, there were local and intermittent manifestations of volcanic activity throughout the period. Diabases, gabbros, serpentines, soda-potash granites, &c., are found in the Eocene of the central and northern Apennines. Tuffs occur in the Veronese and Vicentin Alps - Ronca and Spelecco schists. Tuffs, basalts and other igneous rocks appear also in Montana, Wyoming, California, Oregon, Washington, Idaho, Colorado; also in Central America, the Antillean region and S. America.

It has been very generally assumed by geologists, mainly upon the evidence of plant remains, that the Eocene period opened with a temperate climate in northern latitudes; later, as indicated by the London Clay, Alum Bay and Bournemouth beds, &c., the temperature appears to have been at least subtropical. But it should be observed that the frequent admixture of temperate forms with what are now tropical species makes it difficult to speak with certainty as to the degree of warmth experienced. The occurrence of lignites in the Eocene of the Paris basin, Tirol and N. America is worthy of consideration in this connexion. On the other hand, the coarse boulder beds in the lower Flysch have been regarded as evidence of local glaciation; this would not be inconsistent with a period of widespread geniality of climate, as is indicated by the large size of the nummulites and the dispersion of the marine Mollusca, but the evidence for glaciation is not yet conclusive.

Eocene Stratigraphy

In Britain, with the exception of the Bovey beds and the leaf-bearing beds of Antrim and Mull, Eocene rocks are confined to the south-eastern portion of England. They lie in the two well-marked synclinal basins of London and Hampshire which are conterminous in the western area (Hampshire, Berkshire), but are separated towards the east by the denuded anticline of the Weald. The strata in these two basins have been grouped in the following manner London Basin. Upper Bagshot Sands. Middle Bagshot Beds and part of Lower Bagshot Beds.

Part of Lower Bagshot Beds, London Clay, Blackheath and Oldhaven Beds, Woolwich and ReadingBeds,Thanet Sands.

The Thanet sands have not been recognized in the Hampshire basin; they are usually pale yellow and greenish sands with streaks of clay and at the base; resting on an evenly denuded surface of chalk is a very constant layer of green-coated, well-rounded chalk flint pebbles. It is a marine formation, but fossils are scarce except in E. Kent, where it attains its most complete development. The Woolwich and Reading beds (see Reading Beds) contain both marine and estuarine fossils. In western Kent, between the Woolwich beds and the London Clay are the Oldhaven beds or Blackheath pebbles, 20 to 40 ft., made up almost entirely of wellrounded flint pebbles set in sand; the fossils are marine and estuarine. The London Clay, 500 ft. thick, is a marine deposit consisting of blue or brown clay with sandy layers and septarian nodules; its equivalent in the Hampshire area is sometimes called the Bognor Clay, well exposed on the coast of Sussex. The Bagshot, Bracklesham and Barton beds will be found briefly described under those heads.

Crossing the English Channel, we find in northern France and Belgium a series of deposits identified in their general characters with those of England. The anticlinal ridge of the English Weald is prolonged south-eastwards on to the continent, and separates the Belgian from the French Eocene areas much as it separates the areas of London and Hampshire; and it is clear that at the time of deposition all four regions were intimately related and subject to similar variations of marine and estuarine conditions. With a series. of strata so variable from point to point it is natural that many purely local phases should have received distinctive names; in the Upper Eocene of the Paris basin the more important formations are the highly fossiliferous marine sands known as the "Sands of Beauchamp" and the local fresh-water limestone, the "Calcaire de St Ouen." The Middle Eocene is represented by the well-known "Calcaire grossier," about 90 ft. thick. The beds in this series vary a good deal lithologically, some being sandy, others marly or glauconitic; fossils are abundant. The Upper Calcaire grossier or "Caillasses" is a fresh-water formation; the middle division is marine; while the lower one is partly marine, partly of fresh-water origin. The numerous quarries and mines for building stone in the neighbourhood of Paris have made it possible to acquire a very precise knowledge of this division, and many of the beds have received trade names, such as "Rochette," "Roche," "Banc franc," "Banc vert," "Cliquart," "Saint Nom;" the two last named are dolomitic. Below these limestones are the nummulitic sands of Cuise and Soissons. The Lower Eocene contains the lignitic plastic clay (argile plastique) of Soissons and elsewhere; the limestones of Rilly and Sezanne and the greenish glauconitic sands of Bracheux. The relative position of the above formations with respect to those of Belgium and England will be seen from the table of Eocene strata. The Eocene deposits of southern Europe differ in a marked manner from those of the Anglo-Parisian basin. The most important feature is the great development of nummulitic limestone with thin marls and nummulitic sandstones. The sea in which the nummulitic limestones were formed occupied the site of an enlarged Mediterranean communicating with similar waters right round the world, for these rocks are found not only in southern Europe, including all the Alpine tracts, Greece and Turkey and southern Russia, but they are well developed in northern Africa, Asia Minor, Palestine, and they may be followed through Persia, Baluchistan, India, into. China, Tibet, Japan, Sumatra, Borneo and the Philippines. The nummulitic limestones are frequently hard and crystalline, especially where they have been subjected to elevation and compression as in the Alpine region, to,000 ft. above the sea, or from 16,000, to 20,000 ft., in the central Asian plateau. Besides being a wide-spread formation the nummulitic limestone is locally several thousand feet thick.

While the foraminiferal limestones were being formed over most of southern Europe, a series of clastic beds were in course of formation in the Carpathians and the northern Alpine region, viz. the Flysch and the Vienna sandstone. Some portions of this Alpine Eocene are coarsely conglomeratic, and in places there are boulders of Areas in which Middie Eocene Rocks are found ,y Non-marine Eocene deposits of N. America (Mid. Eocene, Lutetian) /; I JEocsne or unknown  ?,. -Suggested limits of tand Sea about Mid. Eocene time e w .s Distribution of Eocene Rocks f// e?! Upper Middle Lower Hampshire Basin. Headon Hill and Barton Sands. Bracklesham Beds and leaf beds of Bournemouth and Alum Bay.

London Clay and the equivalent Bognor Beds, Woolwich and Reading Beds.

non-local rocks of enormous dimensions included in the argillaceous or sandy matrix. The occurrence of these large boulders together with the scarceness of fossils has suggested a glacial origin for the formation; but the evidence hitherto collected is not conclusive. C. W. von Giimbel has classified the Eocene of the northern Alps (Bavaria, &c.) as follows: Upper Eocene Flysch and Vienna sandstone, with younger nummulitic beds and Haring group.

Middle „ Kressenberg Beds, with older nummulitic beds.

Lower „ Burberg Beds, Greensands with small nummulites. The Haring group of northern Tirol contains lignite beds of some importance. In the southern and S.E. Alps the following divisions are recognized.

Upper Eocene Macigno or Tassello - Vienna Sandstone, conglomerates, marls and shales.

Middle „ Nummulitic limestones, three subdivisions. Liburnian stage (or Proteocene), foraminiferal limestones with fresh-water intercalations at the top and bottom, the Cosina beds, fresh-water in the middle of the series.

In the central and northern Apennines the Eocene strata have been subdivided by Prof. F. Sacco into an upper Bartonian, a middle Parisian and a lower Suessonian series. In the middle member are the representatives of the Flysch and the Macigno. These Eocene strata are upwards of 5500 ft. thick. In northern Africa the nummulitic limestones and sandstones are widely spread; the lower portions comprise the Libyan group and the shales of Esneh on the Nile (Flandrien), the Alveolina beds of Sokotra and others; the Mokattam stage of Egypt is a representative of the later Eocene. Much of the N. African Eocene contains phosphatic beds. In India strata of Eocene age are extensively developed; in Sind the marine Ranikot beds, 1500 to 2000 ft., consisting of clays with gypsum and lignite, shales and sandstones; these beds have, side by side with Eocene nummulites, a few fossils of Cretaceous affinities. Above the Ranikot beds are the massive nummulitic limestones and sandstones of the Kirthar group; these are succeeded by the nummulitic limestones and shales at the base of the Nari group. In the southern Himalayan region the nummulitic phase of Eocene deposit is well developed, but there are difficulties in fixing the line of demarcation between this and the younger formations. The lower part of the Sirmur series of the Simla district may belong to this period it is subdivided into the Kasauli group and the Dagshai group with the Subathu group at the base. Beneath the thick nummulitic Eocene limestone of the Salt Range are shales and marls with a few coal seams. The marine Eocene rocks of N. America are most extensively developed round the coast of the Gulf of Mexico, whence they spread into the valley of the Mississippi and, as a comparatively narrow strip, along the Atlantic coastal plain to New Jersey.

The series in Alabama, which may be taken as typical of the Gulf coast Eocene, is as follows: Upper Jacksonian, White limestone of Alabama (and Vicksburg?).

Middle Claibornian 1 Claiborne series. Buhrstone series.

Lower 3 Chickasawan Sands and lignites.

Midwayan or Clayton formation, limestones.

The above succession is not fully represented in the Atlantic coast states.

On the Pacific coast marine formations are found in California and Oregon; such are the Tejon series with lignite and oil; the Escondido series of S. California (7000 ft.), part of the Pascadero series of the Santa Cruz Mountains; the Pulaski, Tyee, Arago and Coaledo beds - with coals - in Oregon. In the Puget formation of Washington we have a great series of sediments, largely of brackish water origin, and in parts coal-bearing. The total thickness of this formation has been estimated at 20,000 ft. (it may prove to be less than this), but it is probable that only the lower portion is of Eocene age. The most interesting of the N. American Eocene deposits are those of the Rocky Mountains and the adjacent western plains, in Wyoming, Nevada, Nebraska, Colorado, &c.; they are of terrestrial, lacustrine or aeolian origin, and on this account and because they were not strictly synchronous, there is considerable difficulty in placing them in their true position in the time-scale. The main divisions or groups are generally recognized as follows: Mammalian Zonal Forms.

'1' Diplacodon. Telmatotherium. Middle 2 Bridger Group, 2000 ft. (?= Clai bornian) .

((3 Wind River Group, 800 ft.. Lower j ' Wasatch Group, 2000 ft. (? = Chickasawan) .

5 5 Torrejon Group, 300 ft. .

s Puerco Group, 500 to woo ft.

1 South of the Uinta Mts. in Utah. 3 Wind river in Wyoming.

6 Torrejon in New Mexico.

The Fort Union beds of Canada and parts of Montana and N. Dakota are probably the oldest Eocene strata of the Western Interior; they are some 2000 ft. thick and possibly are equivalent to the Midwayan group. But in these beds, as in those known as Arapahoe, Livingston, Denver, Ohio and Ruby, which are now often classed as belonging to the upper Laramie formation, it is safer to regard them as a transitional series between the Mesozoic and Tertiary systems. There is, however, a marked unconformity between the Eocene Telluride or San Miguel and Poison Canyon formations of Colorado and the underlying Laramie rocks.

Many local aspects of Eocene rocks have received special names, but too little is known about them to enable them to be correctly placed in the Eocene series. Such are the Clarno formation (late Eocene) of the John Day basin, Oregon, the Pinyon conglomerate of Yellowstone Park, the Sphinx conglomerate of Montana, the Whitetail conglomerate of Arizona, the Manti shales of Utah, the Mojave formation of S. California and the Amyzon formation of Nevada.

Of the Eocene of other countries little is known in detail. Strata of this age occur in Central and S. America (Patagonia-Megellanian series - Brazil, Chile, Argentina), in S. Australia (and in the Great Australian Bight), New Zealand, in Seymour Island near Graham Land in the Antarctic Regions, Japan, Java, Borneo, New Guinea, Moluccas, Philippines, New Caledonia, also in Greenland, Bear Island, Spitzbergen and Siberia.

Organic Life of the Eocene Period

As it has been observed above, the name Eocene was given to this period on the ground that in its fauna only a small percentage of living species were present; this estimation was founded upon the assemblage of invertebrate remains in which, from the commencement of this period until the present day, there has been comparatively little change. The real biological interest of the period centres around the higher vertebrate types. In the marine mollusca the most noteworthy change is the entire absence of ammonoids, the group which throughout the Mesozoic era had taken so prominent a place, but disappeared completely with the close of the Cretaceous. Nautiloids were more abundant than they are at present, but as a whole the Cephalopods took a more subordinate part than they had done in previous periods. On the other hand, Gasteropods and Pelecypods found in the numerous shallow seas a very suitable environment and flourished exceedingly, and their shells are often preserved in a state of great perfection and in enormous numbers. Of the Gasteropod genera Cerithium with its estuarine and lagoonal forms Potamides, Potamidopsis, &c., is very characteristic; Rostellaria, Voluta, Fusus, Pleurotoma, Conus, Typhis, may also be cited. Cardium, Venericardia, Crassatella, Corbulomya, Cytherea, Lucina, Anomia, Ostrea are a few of the many Pelecypod genera. Echinoderms were represented by abundant sea-urchins, Echinolampas, Linthia, Conoclypeus, &c. Corals flourished on the numerous reefs and approximated to modern forms (Trochosmilia, Dendrophyllia). But by far the most abundant marine organisms were the foraminifera which flourished in the warm seas in countless myriads. Foremost among these are the Nummulites, which by their extraordinary numerical development and great size, as well as by their wide distribution, demand special recognition. Many other genera of almost equal importance as rock builders, lived at the same time: Orthophragma, Operculina, Assilina, Orbitolites, Miliola, Alveolina. Crustacea were fairly abundant (Xanthopsis, Portunus), and most of the orders and many families of modern insects were represented.

When we turn to the higher forms of life, the reptiles and mammals, we find a remarkable contrast between the fauna of the Eocene and those periods which preceded and succeeded it. The great group of Saurian reptiles, whose members had held dominion on land and sea during most of the Mesozoic time, had completely disappeared by the beginning of the Eocene; in their place placental mammals made their appearance and rapidly became the dominant group. Among the early Eocene mammals no trace can be found of the numerous and clearly-marked orders with which we are familiar to-day; instead we find obscurely differentiated forms, which cannot be fitted without violence into any of the modern orders. The early placental mammals were generalized types (with certain nonplacental characters) with potentialities for rapid divergence and development in the direction of the more specialized modern orders. Thus, the Creodonta foreshadowed the Carnivora, the Uintatherium. Bathyopsis. Coryphodon. Pantolambda. Polymastodon. Lower „ Upper ' Uinta Group, 800 ft. (? = Jacksonian).

Basal s Fort Bridger Basin.

' Wasatch Mts. in Utah.

8 Puerco river, New Mexico.

Stages.

Paris Basin.

England.

Belgian Basin.

Mediterranean

regions and

Great Central

sea.

Flysch

Phase.

North America.

3artonien.'

Limestone of Saint-Ouen.

Barton beds.

Uinta Group and

Sands of Mortefontaine.

Sands of Lede.

Jacksonian.

Sands of Beauchamp.

Upper Bagshot sands.

2

Sands of Auvers.

1)

o

a.)

'

an

Bracidesham and Bourne-

Laekenien.

Lutetien.

Calcaire grossier.

mouth beds.

Lower Bagshot sands.

Bruxellien.

Paniselien.

.zi

cd c

'm

laib

al

o

>, z

Bri Group

Caibornian.

Wind River Group.

Ypresien.

Nummulitic sands of

Alum Bay leaf beds.

Sands of Mons en

a)

z

w gi

Wasatch Group

Soissons and Sands of

Cuise and Aizy.

Plastic Clay and lignite

beds.

London Clay.

Oldhaven beds.

Woolwich and Reading beds.

Pevele.

Flanders Clay.

o

_.

.-

a

E

c,

a

U c,

o - - ,9

u

o. 0

1..

R, .b°'

a

and

Chickasawan.

Torrejon Group

and

Midwayan.

Puerco Group.

a)

Upper Landenien

sands

Sands of Ostri-

court.

L andenien tuff-

eau.

Marls of Gelinden.

a

ct

H

Limestones of Rilly and

Sezanne.

Sands of Rilly and Bra-

cheux.

Thanet sands.

Condylarthra presaged the herbivorous groups; but before the close of this period, so favourable were the conditions of life to a rapid evolution of types, that most of the great orders had been clearly defined, though none of the Eocene genera are still extant. Among the early carnivores were Arctocyon,Palaeonictis, Amblyctonus, Hyaenodon, Cynodon, Provivera, Patriofelis. The primitive dog-like forms did not appear until late in the period, in Europe; and true cats did not arrive until later, though they were represented by Eusmilus in the Upper Eocene of France. The primitive ungulates (Condylarths) were generalized forms with five effective toes, exemplified in Phenacodus. The gross Amblypoda, with five-toed stumpy feet (Coryphodon), were prominent in the early Eocene; particularly striking forms were the Dinoceratidae, Dinoceras, with three pairs of horns or protuberances on its massive skull and a pair of huge canine teeth projecting downwards; Tinoceras, Uintatherium, Loxophodon, &c.; these elephantine creatures, whose remains are so abundant in the Eocene deposits of western America, died out before the close of the period. The divergence of the hoofed mammals into the two prominent divisions, the odd-toed and even-toed, began in this period, but the former did not get beyond the three-toed stage. The least differentiated of the odd-toed group were the Lophiodonts: tapirs were foreshadowed by Systemodon and similar forms (Palaeotherium, Paloplotherium); the peccary-like Hyracotherium was a forerunner of the horse, Hyrochinus was a primitive rhinoceros. The evolution of the horse through such forms as Hyracotherium, Pachynolophus, Eohippus, &c., appears to have proceeded along parallel lines in Eurasia and America, but the true horse did not arrive until later. Ancestral deer were represented by Dichobune, Amphitragulus and others, while many small hog-like forms existed (Diplopus, Eohyus, Hyopotamus, Homacodon). The primitive stock of the camel group developed in N. America in late Eocene time and sent branches into S. America and Eurasia. The edentates were very generalized forms at this period (Gan Bartonien from Barton, England.

Lutetien „ Lutetia = Paris.

Ypresien „ Ypres, Flanders.

Landenien „ Landen, Belgium.

Thanetien „ The Isle of Thanet.

Sparnacien „ Sparnacum = Epernay.

Laekenien „ Laeken, Belgium.

Bruxellien „ Brussels.

Paniselien Mont Panisel, near Mons.

Other names that have been applied to subdivisions of the Eocene not included in the table are Parisien and Suessonien (Soissons);. Ludien (Ludes in the Paris basin) and Priabonien (Priabona in the Vicentine Alps); Heersien (Heer near Maastricht) and Wemmelien (Wemmel, Belgium); very many more might be mentioned.

odonta); the rodents (Tillodontia) attained a large size for members of this group, e.g. Tillotherium. The Insectivores had Eocene forerunners, and the Lemuroids - probable ancestors of the apes - were forms of great interest, Anaptomorphus, Microsyops, Heterohyus, Microchaerus, Coenopithecus; even the Cetaceans were well represented by Zeuglodon and others.

The non-placental mammals although abundant were taking a secondary place; Didelphys, the primitive opossum, is noteworthy on account of its wide geographical range.

Among the birds, the large flightless forms, Eupterornis, Gastornis, were prominent, and many others were present, such as the ancestral forms of our modern gulls, albatrosses, herons, buzzards, eagles, owls, quails, plovers. Reptiles were poorly represented, with the exception of crocodilians, tortoises, turtles and some large snakes.

The flora of the Eocene period, although full of interest, does not convey the impression of newness that is afforded by the fauna of the period. The reason for this difference is this: the newer flora had been introduced and had developed to a considerable extent in the Cretaceous period, and there is no sharp break between the flora of the earlier and that of the later period; in both we find a mixed assemblage - what we should now regard as tropical palms, growing side by side with mildtemperate trees. Early Eocene plants in N. Europe, oaks, willows, chestnuts (Castanea), laurels, indicate a more temperate climate than existed in Middle Eocene when in the Isle of Wight, Hampshire and the adjacent portions of the continent, palms, figs, cinnamon flourished along with the cactus, magnolia, sequoia, cypress and ferns. The late Eocene flora of Europe was very similar to its descendant in modern Australasia.

See A. de Lapparent, Traite de geologie, vol. iii. (5th ed., 1906), which contains a good general account of the period, with numerous references to original papers. Also R. B. Newton, Systematic List of the Frederick E. Edwards Collection of British Oligocene and Eocene Mollusca in the British Museum (Natural History) (1891), pp. 299-325 G. D. Harris, "A Revision of our Lower Eocenes," Proc. Geologists' Assoc. x., 1887-1888; W. B. Clark, "Correlation Papers: Eocene" (1891), U.S. Geol. Survey Bull. No. 83. For more recent literature consult Geological Literature added to the Geological Society's Library, published annually by the society. (J. A. H.)


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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

Contents

English

Adjective

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Wikipedia has an article on:

Wikipedia Eocene (comparative more Eocene, superlative most Eocene)

Positive
Eocene

Comparative
more Eocene

Superlative
most Eocene

  1. (geology) of a geologic epoch within the Paleogene period from about 56 to 34 million years ago

Noun

Eocene

  1. (geology) the Eocene epoch

See also


Simple English

The Eocene is a geological epoch or stage, the second in the Tertiary. It lasted from about 55.8 million years to 34 million years, and was followed by the Oligocene epoch. During its 22 million years, modern mammals became prominent. Most modern mammalian orders, and many of the modern families are present by the end of the Eocene.

The Eocene, like the Palaeocene before it, had a climate much warmer than today. At the start of the Eocene the Palaeocene–Eocene Thermal Maximum was reached. This lasted for 100,000 years, and caused a large extinction event. At the end of the Eocene another extinction event occurred, this time probably caused by meteorite strikes in Siberia and Chesapeake Bay.

The land was heavily forested, with temperate forests into the polar regions, and the many herbivorous mammals were browsers, not grazers.


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