66-23.03 million years ago
Map of Earth during the Eocene
|Mean atmospheric content over period duration||c. 26 vol %|
(130 % of modern level)
|Mean atmospheric content over period duration||c. 500 ppm|
(2 times pre-industrial level)
|Mean surface temperature over period duration||c. 18 °C|
(4 °C above modern level)
The Paleogene ( PAL-ee-?-jeen, -ee-oh-, PAY-lee-, -lee-oh-; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the span of time now covered by the Paleogene and subsequent Neogene periods; despite no longer being recognised as a formal stratigraphic term, 'Tertiary' is still widely found in earth science literature and remains in informal use. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous-Paleogene extinction event that ended the preceding Cretaceous Period. The United States Geological Survey uses the abbreviation PE for the Paleogene, but the more commonly used abbreviation is PG with the PE being used for Paleocene.
This period consists of the Paleocene, Eocene, and Oligocene epochs. The end of the Paleocene (55.5/54.8 Mya) was marked by the Paleocene-Eocene Thermal Maximum, one of the most significant periods of global change during the Cenozoic, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals. The term 'Paleogene System' is applied to the rocks deposited during the 'Paleogene Period'.
The global climate during the Paleogene departed from the hot and humid conditions of the late Mesozoic era and began a cooling and drying trend which, despite having been periodically disrupted by warm periods such as the Paleocene-Eocene Thermal Maximum, persisted until the temperature began to rise again due to the end of the most recent glacial period of the current ice age. The trend was partly caused by the formation of the Antarctic Circumpolar Current, which significantly lowered oceanic water temperatures. A 2018 study estimated that during the early Palaeogene about 56-48 million years ago, annual air temperatures, over land and at mid-latitude, averaged about 23-29 °C (± 4.7 °C), which is 5-10 °C higher than most previous estimates. Or for comparison, it was 10 to 15 °C higher than current annual mean temperatures in these areas; the authors suggest that the current atmospheric carbon dioxide trajectory, if it continues, could establish these temperatures again.
During the Paleogene, the continents continued to drift closer to their current positions. India was in the process of colliding with Asia, forming the Himalayas. The Atlantic Ocean continued to widen by a few centimeters each year. Africa was moving north to meet with Europe and form the Mediterranean Sea, while South America was moving closer to North America (they would later connect via the Isthmus of Panama). Inland seas retreated from North America early in the period. Australia had also separated from Antarctica and was drifting toward Southeast Asia.
Mammals began a rapid diversification during this period. After the Cretaceous-Paleogene extinction event, which saw the demise of the non-avian dinosaurs, mammals transformed from a few small and generalized forms that began to evolve into most of the modern varieties we see today. Some of these mammals would evolve into large forms that would dominate the land, while others would become capable of living in marine, specialized terrestrial, and airborne environments. Those that took to the oceans became modern cetaceans, while those that took to the trees became primates, the group to which humans belong. Birds, 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.
Pronounced cooling in the Oligocene led to a massive floral shift and many extant modern plants arose during this time. Grasses and herbs such as Artemisia began to appear at the expense of tropical plants, which began to decline. Conifer forests developed in mountainous areas. This cooling trend continued, with major fluctuation, until the end of the Pleistocene. This evidence for this floral shift is found in the palynological record.
The Paleogene is notable in the context of offshore oil drilling, and especially in Gulf of Mexico oil exploration, where it is commonly referred to as the "Lower Tertiary". These rock formations represent the current cutting edge of deep-water oil discovery.
Lower Tertiary rock formations encountered in the Gulf of Mexico oil industry usually tend to be comparatively high temperature and high pressure reservoirs, often with high sand content (70%+) or under very thick evaporite sediment layers.
Lower Tertiary explorations include (partial list):