4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education (http://www.accessscience.com/) Oligocene Article by: Haq, Bilal U. Division of Ocean Sciences, National Science Foundation, Arlington, Virginia. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097­8542.468000 (http://dx.doi.org/10.1036/1097­8542.468000) Content Subdivisions Tectonics, oceans, and climate Life Bibliography Additional Readings The third oldest of the seven geological epochs of the Cenozoic Era. It corresponds to an interval of geological time (and rocks deposited during that time) from the close of the Eocene Epoch to the beginning of the Miocene Epoch. The most recent geological time scales assign an age of 34 to 24 million years before present (MYBP) to the Oligocene Epoch. See also: Cenozoic (/content/cenozoic/118600); Eocene (/content/eocene/236400); Miocene (/content/miocene/427400) In his early subdivision of the Tertiary published in 1833, Charles Lyell established four stratigraphic units, from the oldest to the youngest: Eocene, Miocene, and Older and Younger Pliocene. Stratigraphers in the Netherlands and Germany, however, kept describing strata that they considered to be intermediate in position and characteristics between Eocene and Miocene. They concluded that these sediments represented a major marine transgression in northern Europe at the close of the Eocene Epoch. This eventually led the German stratigrapher E. Beyrich in 1854 to propose the Oligocene as an independent subdivision of the Tertiary based on a sequence of marine, brackish­ water, and nonmarine sediments of the Mainz Basin in Germany. He proposed a new Oligocene Epoch and constructed it out of the younger part of Eocene and the older part of Miocene epochs of the Lyellian subdivisions of Tertiary. An important event that characterizes the Oligocene Epoch was the development of extensive glaciation on the continent of Antarctica. Prior to that time, the world was largely ice­free through much of the Mesozoic and early Tertiary. A significant amount of ice is now known to have existed on the Antarctic continent since at least the beginning of the Oligocene, when the Earth was ushered into its most recent phase of ice­house conditions. This in turn created revolutions in the global climatic and hydrographic systems, with important repercussions for the marine and terrestrial biota. The changes include steepened latitudinal and vertical thermal gradients affecting major fluctuations in global climates, and the shift in the route of global dispersal of marine biota from an ancestral equatorial Tethys seaway, which had become severely restricted by Oligocene time, to the newly initiated circum­ Antarctic circulation. See also: Glacial history (/content/glacial­history/800690) http://www.accessscience.com/content/oligocene/468000 1/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education Subdivisions In modern time scales, this epoch is subdivided into two series, a Lower and an Upper Oligocene. Northern European Rupelian and Chattian stratigraphic stages are designated to be time­equivalent to the Early and Late Oligocene, respectively (Fig. 1). Worldwide, the epoch represents an overall regressive sequence when there was a drawdown of global sea level, with relatively deeper, marine facies in the Early Oligocene and shallower­water to nonmarine facies in the Late Oligocene. See also: Facies (geology) (/content/facies­geology/250000) Fig. 1 Oligocene stages and their temporal equivalents. The Rupelian Stage was proposed by A. Dumont in 1849, based on sedimentary strata in Belgium. The Tongrian Stage, also described from Belgium by Dumont in 1839, was originally regarded to be, in part, older than the Rupelian by Beyrich, who considered that the Tongrian and Rupelian together constituted his new epochal unit, the Oligocene. In 1983, K. Mayer­Eymar erected the Lattorfian Stage with a type locality in Saxony, Germany. Mayer­ Eymar considered the Lattorfian to be temporally equivalent to the Early Oligocene. Both the Tongrian and Lattorfian strata were later shown to range downward into the uppermost Eocene, and their use has largely been abandoned in favor of an expanded notion of the Rupelian. Because the Rupelian at its type section spans http://www.accessscience.com/content/oligocene/468000 2/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education somewhat less than the time interval included in the Early Oligocene, some French stratigraphers prefer the Stampian Stage to Rupelian. The Stampian was described by A. D'Orbigny in 1852 from marine and lacustrine beds at Etampes in France. The Chattian, elected by consensus as the standard stage for the Late Oligocene, was based on marine sand beds near Kassel, Germany, and proposed by T. Fuchs in 1894. Fuchs considered the Chattian to be younger than Rupelian and older than the Aquitanian Stage (which was later placed in the Lower Miocene). Later studies have shown that there is a temporal gap between the top of Chattian and the base of Aquitanian as defined at their stratotype sections. However, in stratigraphy it is normal practice to extend the formal concept of standard stages to include such gaps. Thus, to accommodate the gap between the uppermost Eocene stage of Priabonian and the Oligocene Rupelian, and the gap between Chattian and Aquitanian, the formal concepts of Rupelian and Aquitanian were extended downward by later stratigraphers to bridge the intervening lacunae. Other regional stages of the Oligocene include the Egerian, sometimes used in central and eastern European countries, which is in part equivalent to the Chattian, but may extend into the lower part of Miocene. In Russia the Caucasian Stage is considered to be an equivalent to the Egerian. Marine Oligocene in California is subdivided into the Refugian and Zemorrian stages, the former stage encompassing only the earliest Oligocene, and the latter spanning the bulk of the epoch. In the Gulf coast of the United States the Oligocene extends from the upper part of the Jacksonian Stage to Vicksburgian, Chickasawhayan, and lower part of Anahuacan stages. Temporally, the latter stage extends into the Miocene. Australian stratigraphers include the Willungian, Janjukian, and lower part of Longfordian stages in their Oligocene, and the New Zealanders now consider the epoch to span three local stages: Whaingaroan, Duntroonian, and Waitakian. Paleontologists who study mammal and other vertebrate fossils often use their own subdivisions to express the ages of terrestrial assemblages. Workers in Europe consider the Oligocene to span three subdivisions, Suevian, Arvernian, and lower part of Agenian. North American vertebrate paleontologists include Orellan, Whiteneyan, and the lower two­thirds of the Arikareean ages in their concept of the Oligocene. See also: Paleontology (/content/paleontology/484100); Stratigraphy (/content/stratigraphy/659000) Tectonics, oceans, and climate Radical changes occurred in the Oligocene Epoch that revolutionized global oceanographic and climatic conditions. The most prominent change was the shift of circumglobal circulation (and a major means of biotic dispersal) from the equatorial to the southern high­latitude regions. Both the restriction of the equatorial flow between the Indian and the Atlantic oceans through the ancestral Tethys seaway, and the opening of the oceanic gateway at Drake Passage occurred during the Oligocene. These changes had important repercussions that led to the entry of the Earth into predominantly ice­house conditions that continue to the present. The cooling of the polar regions in the Oligocene led to the accentuation of latitudinal and vertical thermal gradients and an increase in seasonality. This resulted in a fundamental shift in the bottom­water regime, from density­driven warm, saline bottom waters in the Cretaceous and Paleocene­Eocene, to cold bottom waters, largely driven by thermal contrast. The accentuated latitudinal thermal contrast also resulted in the expansion of the erosive activity of bottom waters. See also: Cretaceous (/content/cretaceous/167800) http://www.accessscience.com/content/oligocene/468000 3/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education The separation of Svalbard and Greenland in the latest Eocene and the breaching of the Rio Grande Rise in the South Atlantic in the earliest Oligocene set the stage for the crossing of the important climatic threshold near the Eocene­Oligocene boundary. It has been suggested that after the development of the connection between the Arctic and the Norwegian Sea that allowed the supply of cold deep water to the Atlantic and the southern high latitudes, more moisture became available around Antarctica. This, combined with the partial isolation of Antarctica, may have led to large­scale freezing at sea level and the initiation of the formation of Antarctic Bottom Water. There is evidence that the temperature of the bottom water dropped by 4–5°C (7–9°F) near this boundary, and the change from warm to cold bottom water may date back to this time. The development of the psychrosphere (deeper, cold layer of the ocean) manifests itself in the presence of widespread erosional hiatuses in the eastern Indian and southwestern Pacific oceans, where scouring by cold bottom waters has stripped away much of the older sediments. The vigorous activity of the deep water is also indicated by widespread drift sediments in the North Atlantic. Another important tectonic event that had major significance for the oceanic­climatic