Oligocene System (from the Gr. OXi'yos, few, and rcatvos, recent), in geology, the name given to the second division of the older Tertiary rocks, viz. those which occur above the Eocene and below the Miocene strata. These rocks were originally classed by Sir C. Lyell as " older Miocene," the term Oligocene being proposed by H. E. Beyrich in 1854 and again in 1858. Following A. de Lapparent, the Oligocene is here regarded as divisible into two stages, an upper one, the Etampian (from Etampes), equivalent to the Rupelian of A. Dumont (1849), and a lower one, the Sannoisian (from Sannois near Paris), equivalent to the Tongrian (from Tongris in Limburg) of Dumont (1859). This lower division is the Ligurian of some authors, and corresponds with the Lattorfian (Latdorf) of K. Mayer in north Germany; it is in part the equivalent of the older term Ludian of de Lapparent. It should be pointed out that several authors retain the Aquitanian stage (see Miocene) at the top of the Oligocene, but there are sufficiently good reasons for removing it to the younger system.
The Oligocene deposits are of fresh-water, brackish, marine and terrestrial origin; they include soft sands, sandstones, grits, marls, shales, limestones, conglomerates and lignites. The geographical aspect of Europe during this period is indicated on the accompanying map. Here and there, as in N. Germany, the sea gained ground that had been unoccupied by Eocene waters, but important changes, associated with the continuation of elevatory processes in the Pyrenees and Alps which had begun in the preceding period, were in progress, and a general relative uplifting took place which caused much of the Eocene sea floor to be occupied at this time by lake basins and lagoons. The movements, however, were not all of a negative character as regards the water areas, for oscillations were evidently frequent, and subsidence must .have been considerable in some regions to admit of the accumulation of the great thickness of material found deposited there. Perhaps the most striking change from Eocene topography in Europe is to be seen in the extension of the Oligocene sea over North Germany, whence it extended eastward through Poland and Russia to the AralCaspian region, communicating thence with Arctic waters by way of a Ural depression. The Asian extension of the central mediterranean sea appears to have begun to be limited. It was later in the period when the wide-spread emersion set in.
In Britain Oligocene formations are found only in the Hampshire Basin and the Isle of Wight; from the admixture of fresh-water, marine and estuarine deposits, E. Forbes named these the " Fluviomarine series." The following are the more important subdivisions, in descending order: The Hamstead (Hampstead) beds, marine at the top, with Ostrea callifera, Natica, &c., estuarine and freshwater below, with Unio, Viviparus and the remains of crocodiles, turtles and mammals. The Bembridge marls, fresh-water, estuarine and marine, resting upon the Bembridge limestone, with many fresh-water fossils such as Limnaea, Planorbis, Chara, large land snails, Amphidromus, Helix, Glandina, and many insects and plant leaves. The Osborne beds, marls, clays and limestones, with Unio, Limnaea, &c. The Headon beds (upper), fresh-water clays, marls and limestones (middle), brackish and marine, more sandy (lower), brackish and fresh-water clays, marls, tufaceous limestones and sandstones. The clays and sands of the Bovey Basin in Devonshire were formerly classed as Miocene, but they are now regarded by C. Reid as Eocene on the evidence of the plant remains, though there is still a possibility that they may be found to be of Oligocene age.
In France the best-known tract of Oligocene rocks rests in the Paris basin in close relationship with the underlying Eocene. These rocks include the first and second gypsum beds, the source of " plaster of Paris "; at Montmartre the first or upper bed is 20 metres in thickness, and some of the beds contain siliceous nodules (fusils) and numerous mammalian remains. Above the gypsum beds is the travertine of Champigny-sun-Marne, a series of blue and white marls (supra-gypseous marls), followed by the " glaises vents " or greenish marls. At the top of the lower Oligocene of this district is the lacustrine " calcaire de Brie " or middle travertine, which at Fertesous-Jouane is exploited for millstones; this is associated with the Fontainebleau limestone, which at Chateau-Landon and Souppes is sufficiently compact to form an important building stone, used in the Arc de Triomphe and other structures in Paris. The upper Oligocene of Paris begins with the marnes a huitres, followed by the brackish and fresh-water molasse of Etrechy, and a series of sandy beds, of which the best known are those of Fontainebleau, Etampes and Ormoy; in these occur the groups of calcite crystals, charged with sand, familiar in all mineral collections. Elsewhere in France similar mixed marine, fresh-water and brackish beds are found: in Aquitaine there are marine and lacustrine marls, limestones and molasse; marine beds occur at Biarritz; lacustrine and fresh-water marls and limestones with lignite appear in the sub-Pyrenees; in Provence there are brackish red clays, conglomerates and lignites, with limestones in the upper parts; and in Limagne there are mottled sands, arkoses; clays and fresh-water limestones. In the Jura region and on the borders of the central massif a peculiar group of deposits, the terrain siderolithique, is found in beds and in pockets in. Jurassic limestones. Sometimes this deposit consists of red clay (bolus) with nests of pisolitic iron, as in Jura and Franche-comte, Alsace, &c.;. occasionally, as in Bourgogne, Berry, the valley of the Aubois, Chatillon, it is made up of a breccia or conglomerate of Jurassic pebbles cemented with limonite and carbonate of lime or silica (an intimate mixture of marl and iron ore in these districts is called " castillard "). At Quercy the cementing material is phosphate of lime derived from the bones of mammals (Adapis, Necrolemur, Palaeotherium, Xiphodon, &c.), which are so numerous that it has been suggested that these animals must have been suffocated by gaseous emanations. Similar ferruginous deposits occur in South Germany.
In the Alpine region the Oligocene rocks assume the character of the Flysch, a complex assemblage of manly and sandy shales and soft sandstones with calcareous cement (" macigno "). The Flysch phase of deposition had begun before the close of the preceding period, but the bulk of it belongs to the Oligocene, and is especially characteristic of the lower part. The Flysch may attain a very great thickness; in Dauphine it is said to be 2000 metres. Obscure plantlike impressions are common on certain horizons of this formation, and have received such names as Chondrites, Fucoids, Helminthoidea. The " gres de Taveyannaz " and " Wildflysch " of Lake Thun contain fragments of eruptive rocks. Marine beds occur at Barreme, Desert, Chambery, &c., and parallel with the normal Flysch in the higher Alps of Vaudois is a nummulitic limestone; both here and near Interlaken, in the marble of Ralligstocke, calcareous algae are abundant. Part of the " schistes des Grisons " (" Biindner Schiefer ") have been regarded as of Oligocene age. In the Leman region the " Flysch rouge " at the foot of the Dent du Midi belongs to the upper part of the Flysch formation.
In North Germany the lower Oligocene consists largely of sandy marls, often glauconitic; typical localities are Egeln near Magdeburg and Latdorf near Bernburg; at Samland the glauconitic sand contains nodules of amber, with insects, derived from Eocene strata. The upper Oligocene beds, which cover a wide area, comprise the Stettin sands and Septarian Clay or Rupelton, marine beds tending to merge laterally one into another. In the Mainz basin a petroleumbearing sandy marl is found at Pechelbronn and Lobsann in Alsace underlying a fresh-water limestone which is followed by the marine tt Meeressand " of Alzey. Lignites (Braunkohl) are widely spread in this region and appear at Latdorf, Leipzig, in Westphalia and Mecklenburg; at Halle is a variety called pyropissite, which is exploited at Weissenfels for the manufacture of paraffin.
Map of Europe in the early cart of the Oligocene Period In Belgium a sandy series (Wemmelian, Asschian, Henisian), mainly of brackish-water origin, is succeeded by the marine sands of Bergh (with the clay of Boom), which pass up through the inferior sands of Bolderberg into the Miocene. In Switzerland, beyond the limits of the Flysch, nearer the Alpine massif, is a belt of grits, limestones and clays in. an uncompacted condition, to which the name "molasse " is usually given; mixed with the molasse is an inconstant conglomeratic littoral formation, called Nagelfluh. The molasse occurs also in Bavaria, where it is several thousand feet thick and contains lignites. Oligocene deposits occur in the Carpathian region and Tirol; as Flysch and brackish and lacustrine beds with lignite in Klausenburg, lignites at Haring in Tirol. In the Spanish Pyrenees they are well developed; in the Apennines the scaly clays (" argille scagliose ") are of this age; while in Calabria they are represented by thick conglomerates and Flysch. Flysch appears also in Dalmatia and Istria (where it is called " tassello ") and in North Bosnia, where it contains marine limestones. Lignites are found at Sotzka and Styria, marine beds in the Balkan peninsula, glauconitic sands prevail in South Russia, Flysch with sands and grits in the Caucasus, while marine deposits also occupy the Aral-Caspian region and Armenia, and are to be traced into Persia. Oligocene rocks are known in North Africa, Algeria, Tunis and Egypt, with the silicified trees and basalt sheets north of the Fayum. In North America the rocks of this period have not been very clearly differentiated, but they may possibly be represented by the White river beds of S. Dakota, the white and blue marls of Jackson on the Mississippi, the " Jacksonian " white limestone of Alabama, the limestone of Ocala in Florida, certain lacustrine clays in the Uinta basin, and by the ribband shales with asphalt and petroleum in the coastal range of California. In South America and the Antilles upper Oligocene is found, and the lignite beds of Coronel and Lota in Chile and in the Straits of Magellan may be of this age; in Patagonia are the lower Oligocene marine beds (" Patagonian ") and beds with mammalian remains. In New Zealand the Oamaru series of J. Hutton is regarded as Oligocene; at its base are interstratified basic volcanic rocks.
A correlation of Oligocene strata is summarized in the following table: - in the Eocene seas (Coelopleurus, Echinolampus, Clypeaster, Scutella). Corals were abundant, and nummulites still continued till near the close of the period, but they were diminished in size.
REFERENCES. - " Geology of the Isle of Wight," Mem. Geol. Survey (2nd ed. 1889); A. von Koenen, Abhand. geol. Specialkart Preuss. x. (1889-1894); M. Vollest, Der Braunkohlenbergbaum (Halle, 1889); E. van den Brocek, " Materiaux pour l'etude de l'Oligocene beige," Bull. Soc. Belg. Geol. (1894); also the works of O. Heer, H. Filhol, G. Vasseur, H. F. Osborn, A. Gaudry, H. Douville, R. B. Newton, H. Dall, M. Cossmann, G. Lambert, &c., and the article FLYSCH. (J. A. H.)
