Oligocene

OLIGOCENE (Gr. 6X17os, few, and KatvOs, recent), in geol ogy, the name given to the second period of the Tertiary era. The Oligocene system thus includes those strata which occur above the Eocene and below the Miocene. These rocks were originally classed by Sir Charles Lyell as Older Miocene, the term Oligocene being proposed by H. E. Beyrich in 1854 and again in 1858. The Oligocene is thus the upper division of the older Tertiary period or Palaeogene. (See TERTIARY, EOCENE.) Conditions During the Oligocene.—The Oligocene de posits are of fresh-water, brackish, marine and terrestrial origin; they include sands, soft sandstones, grits, marls, shales, lime stones, conglomerates and lignites. Here and there, as in northern Germany, the sea gained ground that had been unoccupied by Eocene waters, but important changes 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. Thus there is a general tendency for marine Oligocene deposits to occupy a more restricted area than the marine Eocene. Amongst the earliest of the Tertiary mountain chains to arise were the Pyrenees and the mountains of Provence (the Alpine chain properly speaking) which were already in existence at the end of the Eocene. Thus relatively unfolded Oligocene sediments are found on the flanks of belts of folded Eocene strata. In some areas the folding of the margins of Eocene gulfs resulted in the deepening of the centre of the trough and permitted huge thick nesses of Oligocene to be deposited. This, for example, is the case in the Burmese gulf where Oligocene sediments reach a thickness of over io,000 feet. As in the Eocene period, minor os cillations of level gave rise to cycles of sedimentation. Perhaps the most striking change from Eocene topography in Europe, shown on the accompanying map, is the extension of the Oligocene sea over north Germany, whence it extended eastward through Poland and Russia to the Aral-Caspian region, communicating thence with Arctic waters by way of a Ural depression. It was later in the period when the widespread emersion set in.

Life of the Period.

In its general features the life of the period resembles that of the Eocene (q.v.) ; foraminifera were abundant; nummulites were still numerous but of smaller size and are characteristically associated with lepidocyclines. Corals and bryozoa were abundant in the clear waters of southern seas but echinoids rather less numerous. Cephalopods were still fewer but gastropods and lamellibranchs assumed more and more their present-day importance, and the genera are those already noted in the Eocene. Amongst vertebrates, rays and dog-fish were the dominant marine fish ; tortoises were abundant and the genus Rana made its appearance. The most interesting feature of the land fauna is the great variety of mammals, especially from the interior of North America and the continental Oligocene of Europe and Asia. The mammals are the most important fossils for dis tinguishing subdivisions of the period. Whilst many of the mam mals were of mixed types, others are clearly representatives of modern groups and forerunners of existing genera. Thus it is pos sible to distinguish carnivores (Canidae and Felidae), insectivores, rodents, ruminants and camels. Forerunners of the rhinoceros, elephant and horse are all clearly separable. An account of the flora will be found in the article dealing with PALAEOBOTANY. The abundance of Chara, associated with such molluscan genera as Limnaea and Planorbis in fresh-water deposits is especially note worthy.

Oligocene Stratigraphy.

Neither the lower nor the upper limit of the Oligocene is well defined. As mentioned in the article on the Eocene (q.v.), the lower limit is usually taken as the top of the Bartonian stage (including the Ludian). The Aquitanian stage is classed sometimes with the Oligocene, sometimes with the Miocene. The first opinion is adopted by those who pay first attention to the mammalian faunas—the Aquitanian fauna is an impoverished representative of preceding faunas and is not marked by the appearance of new forms. On the other hand the Aquita nian deposits seem to mark the feeble beginning of the Miocene transgression.