The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the first period of the Cenozoic Era, the tenth period of the Phanerozoic and is divided into the Paleocene, Eocene, and Oligocene epochs. The earlier term Tertiary Period was used to define the time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use. Paleogene is often abbreviated "Pg", although the United States Geological Survey uses the abbreviation "" for the Paleogene on the Survey's geologic maps.
Much of the world's modern vertebrate diversity originated in a rapid surge of diversification in the early Paleogene, as survivors of the Cretaceous–Paleogene extinction event took advantage of empty ecological niches left behind by the extinction of the non-avian dinosaurs, pterosaurs, marine reptiles, and primitive fish groups. Mammals continued to diversify from relatively small, simple forms into a highly diverse group ranging from small-bodied forms to very large ones, radiating into multiple orders and colonizing the air and marine ecosystems by the Eocene. Birds, the only surviving group of dinosaurs, quickly diversified from the very few neognath and paleognath clades that survived the extinction event, also radiating into multiple orders, colonizing different ecosystems and achieving an extreme level of morphological diversity. Percomorph fish, the most diverse group of vertebrates today, first appeared near the end of the Cretaceous but saw a very rapid radiation into their modern order and family-level diversity during the Paleogene, achieving a diverse array of morphologies.
The Paleogene is marked by considerable changes in climate from the Paleocene–Eocene Thermal Maximum, through global cooling during the Eocene to the first appearance of permanent ice sheets in the Antarctic at the beginning of the Oligocene.
Paleocene
The Paleocene is the first series/epoch of the Paleogene and lasted from 66.0 Ma to 56.0 Ma. It is divided into three stages: the Danian 66.0 - 61.6 Ma; Selandian 61.6 - 59.2 Ma; and, Thanetian 59.2 - 56.0 Ma. The GSSP for the base of the Cenozoic, Paleogene and Paleocene is at Oued Djerfane, west of El Kef, Tunisia. It is marked by an iridium anomaly produced by an asteroid impact, and is associated with the Cretaceous–Paleogene extinction event. The boundary is defined as the rusty colored base of a 50 cm thick clay, which would have been deposited over only a few days. Similar layers are seen in marine and continental deposits worldwide. These layers include the iridium anomaly, microtektites, nickel-rich spinel crystals and shocked quartz, all indicators of a major extraterrestrial impact. The remains of the crater are found at Chicxulub on the Yucatan Peninsula in Mexico. The extinction of the non-avian dinosaurs, ammonites and dramatic changes in marine plankton and many other groups of organisms, are also used for correlation purposes.
Alpine–Himalayan orogeny
Alpine orogeny
The Alpine orogeny developed in response to the collision between the African and Eurasian plates during the closing of the Neotethys Ocean and the opening of the Central Atlantic Ocean. The result was a series of arcuate mountain ranges, from the Tell-Rif-Betic cordillera in the western Mediterranean through the Alps, Carpathians, Apennines, Dinarides and Hellenides to the Taurides in the east. Convergence between the Iberian and European plates led to the Pyrenean orogeny and, as Adria pushed northwards the Alps and Carpathian orogens began to develop.
Between about 40 and 30 Ma, subduction began along the western Mediterranean arc of the Tell, Rif, Betic and Apennine mountain chains. The rate of convergence was less than the subduction rate of the dense lithosphere of the western Mediterranean and roll-back of the subducting slab led to the arcuate structure of these mountain ranges.
Zagros Mountains
The Zagros mountain belt stretches for c. 2000 km from the eastern border of Iraq to the Makran coast in southern Iran. It formed as a result of the convergence and collision of the Arabian and Eurasian plates as the Neotethys Ocean closed and is composed sediments scraped from the descending Arabian Plate.
From the Late Cretaceous, a volcanic arc developed on the Eurasia margin as the Neotethys crust was subducted beneath it. A separate intra-oceanic subduction zone in the Neotethys resulted in the obduction of ocean crust onto the Arabian margin in the Late Cretaceous to Paleocene, with break-off of the subducted oceanic plate close to the Arabian margin occurring during the Eocene. To the south of this zone, the Himalaya are composed of metasedimentary rocks scraped off the now subducted Indian continental crust and mantle lithosphere as the collision progressed.
Debate about the amount of deformation seen in the geological record in the India–Eurasia collision zone versus the size of Greater India, the timing and nature of the collision relative to the decrease in plate velocity, and explanations for the unusually high velocity of the Indian plate have led to several models for Greater India: 1) A Late Cretaceous to early Paleocene subduction zone may have lain between India and Eurasia in the Neotethys, dividing the region into two plates, subduction was followed by collision of India with Eurasia in the middle Eocene. In this model Greater India would have been less than 900 km wide; 3) This model assigns older dates to parts of Greater India, which changes its paleogeographic position relative to Eurasia and creates a Greater India formed of extended continental crust 2000–3000 km wide.
South East Asia
The Alpine-Himalayan Orogenic Belt in Southeast Asia extends from the Himalayas in India through Myanmar (West Burma block) Sumatra, Java to West Sulawesi.
During the Late Cretaceous to Paleogene, the northward movement of the Indian plate led to the highly oblique subduction of the Neotethys along the edge of the West Burma block and the development of a major north-south transform fault along the margin of Southeast Asia to the south. Extension between North America and Eurasia, also in the early Eocene, led to the opening of the Eurasian Basin across the Arctic, which was linked to the Baffin Bay Ridge and Mid-Atlantic Ridge to the south via major strike slip faults.
From the Eocene and into the early Oligocene, Greenland acted as an independent plate moving northwards and rotating anticlockwise. This led to compression across the Canadian Arctic Archipelago, Svalbard and northern Greenland resulting in the Eureka orogeny.|alt=Cliff face showing three layers of rock; the bottom layer is sedimentary rock; the middle layer forms columns, whilst the layer above is blocky in appearance.]]
The North Atlantic Igneous Province stretches across the Greenland and northwest European margins and is associated with the proto-Icelandic mantle plume, which rose beneath the Greenland lithosphere at c. 65 Ma.
The arrival of the proto-Iceland plume has been considered the driving mechanism for rifting in the North Atlantic. However, that rifting and initial seafloor spreading occurred prior to the arrival of the plume, large scale magmatism occurred at a distance to rifting, and that rifting propagated towards, rather than away from the plume, has led to the suggestion the plume and associated magmatism may have been a result, rather than a cause, of the plate tectonic forces that led to the propagation of rifting from the Central to the North Atlantic. With the Laramide uplift the Western Interior Seaway was divided and then retreated.
Over the Paleogene, changes in plate motion and episodes of regional slab shallowing and steepening resulted in variations in the magnitude of crustal shortening and amounts of magmatism along the length of the Andes.
Caribbean
The Caribbean plate is largely composed of oceanic crust of the Caribbean Large Igneous Province that formed during the Late Cretaceous.
During the Eocene (c. 45 Ma), subduction of the Farallon plate along the Central American subduction zone was (re)established.
By the Oligocene, the intra-oceanic Central American volcanic arc began to collide with northwestern South American. The resulting changes in stress between the Pacific and Philippine Sea plates initiated subduction along the Izu-Bonin-Mariana and Tonga-Kermadec arcs. leading to major strike-slip movements and the formation of the San Andreas Fault.
Antarctica
Slow seafloor spreading continued between Australia and East Antarctica. Shallow water channels probably developed south of Tasmania opening the Tasmanian Passage in the Eocene and deep ocean routes opening from the mid Oligocene. Rifting between the Antarctic Peninsula and the southern tip of South America formed the Drake Passage and opened the Southern Ocean also during this time, completing the breakup of Gondwana. The opening of these passages and the creation of the Southern Ocean established the Antarctic Circumpolar Current. Glaciers began to build across the Antarctica continent that now lay isolated in the south polar region and surrounded by cold ocean waters. These changes contributed to the fall in global temperatures and the beginning of icehouse conditions. To the west, in the early Oligocene, flood basalts erupted across Ethiopia, northeast Sudan and southwest Yemen as the Afar mantle plume began to impact the base of the African lithosphere. with only the brief interruption of the Latest Danian Event (c. 62.2 Ma) when global temperatures rose. There is no evidence for ice sheets at the poles during the Paleocene. According to a study published in 2018, from about 56 to 48 Ma, annual air temperatures over land and at mid-latitude averaged about 23–29 °C (± 4.7 °C). For comparison, this was 10 to 15 °C higher than the current annual mean temperatures in these areas. Fluctuating sea levels meant, during low stands, a land bridge formed across the Bering Straits between North America and Eurasia allowing the movement of land animals between the two continents. and the Eocene Thermal Maximum 3 (c. 53 Ma). The early Eocene warm conditions were brought to an end by the Azolla event. This change of climate at about 48.5 Ma, is believed to have been caused by a proliferation of aquatic ferns from the genus Azolla, resulting in the sequestering of large amounts of CO<sub>2</sub> from the atmosphere by the plants. From this time until about 34 Ma, there was a slow cooling trend known as the Middle-Late Eocene Cooling. and the global mean surface temperature continued to decrease gradually during the Rupelian.
Mammals began a rapid diversification during this period. After the Cretaceous–Paleogene extinction event, which saw the demise of the non-avian dinosaurs, mammals began to evolve from a few small and generalized forms into most of the modern varieties we see presently. Some of these mammals evolved into large forms that dominated the land, while others became capable of living in marine, specialized terrestrial, and airborne environments. Those that adapted to the oceans became modern cetaceans and sirenians, while those that adapted to trees became animals such as Sciuridaes or primates, the group to which humans belong.
Birds, extant dinosaurs which were already well established by the end of the Cretaceous, also experienced adaptive radiation as they took over the skies left empty by the now extinct pterosaurs. Some flightless birds such as penguins, ratites, and terror birds also filled niches left by the hesperornithes and other extinct dinosaurs.
Myctophids first appeared in the Late Palaeocene or Early Eocene, and during the Eocene and most of the Oligocene were restricted to shelf seas before expanding their range into the open ocean during the warm climatic interval at the end of the Oligocene.
Pronounced cooling in the Oligocene resulted in a massive floral shift, and many extant modern plants arose during this time. Grasses and herbs, such as Artemisia, began to proliferate, at the expense of tropical plants, which began to decrease. Conifer forests developed in mountainous areas. This cooling trend continued, with major fluctuation, until the end of the Pleistocene period. This evidence for this floral shift is found in the palynological record.
See also
References
External links
- Paleogene Microfossils: 180+ images of Foraminifera
- Paleogene (chronostratigraphy scale)