North America was more extensively submerged in the Miocene than in the preceding Oligocene epoch and underwent considerable crustal disturbances. The Atlantic and Gulf coasts were flooded about as extensively as in the Eocene epoch. Miocene rocks are found along the Atlantic as far N as Martha's Vineyard, but the series, everywhere thin, is thickest and least interrupted from New Jersey to Maryland. On the Gulf coast it extends from Florida westward to Texas. The Atlantic series is chiefly marls, clays, and sands, with diatomaceous earth; the Florida series, chiefly limestone (Florida having risen as an island in the late Oligocene); the Gulf series, limestone and clastic sediments.
On the Pacific coast, the Great Valley of California was submerged at the beginning of the Miocene. The deposition of the Vaqueros sandstone, clay, and conglomerate was followed by the formation of the oil-rich Monterey series, partly sandstone and shale but largely diatomaceous tufa. In mid-Miocene time there was extensive mountain building in this region; the Cascades and Coast Ranges were elevating, although the Rocky Mts. had by then eroded to low relief. This disturbance was accompanied by volcanic activity—the Columbia and Snake river plateaus consist of over 200,000 sq mi (520,000 sq km) of basaltic lava flows up to 10,000 ft (3,000 m) thick—and by the first known movement along the San Andreas fault zone, engendered by the collision of the North American continental plate with the Pacific Ocean plate (see plate tectonics).
Late in the Miocene a new, extensive submergence resulted in the deposition of the San Pablo shale and sandstone. The sediments of the California Miocene came chiefly from the Sierra Nevada and the Klamaths, which, through erosion, were peneplained by the close of the epoch. In the western interior of North America the Columbia River basalt plateau of Idaho, Washington, Oregon, N California, and N Nevada was formed by a great outpouring of lava, which continued in the succeeding Pliocene epoch.
During the Miocene most of N Europe was elevated, but marine waters covered E Spain, S France, Italy, and a depressed area extending through Hungary to a basin around Vienna. In addition to considerable mountain making, lagoons were formed at the base of the Carpathians and north of the Caucasus in the regions now occupied by the Romanian and Baku oil fields.
The mammalian life of the Miocene was marked by further stages in the development of the horse, by the multiplication and final extinction of the giant hogs, and by the appearance of the mastodons, raccoons, and weasels. Cats, camels, doglike carnivores, and rhinoceroses were common, and species of a great ape (Dryopithecus) inhabited S Europe, Asia, and Africa. In the Miocene a distinct cooling of the climate resulted in the reduction of forests and an increase in grassy plains.
As the earth cooled, it went from the Oligocene epoch through the Miocene and into the Pliocene. The Miocene boundaries are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Oligocene and the cooler Pliocene.
The plants and animals of the Miocene were fairly modern. Mammals and birds were well-established. Whales, seals, and kelp spread.
|Messinian||(7.246 – 5.332 mya)|
|Tortonian||(11.608 – 7.246 mya)|
|Serravallian||(13.65 – 11.608 mya)|
|Langhian||(15.97 – 13.65 mya)|
|Burdigalian||(20.43 – 15.97 mya)|
|Aquitanian||(23.03 – 20.43 mya)|
These subdivisions within the Miocene are defined by the relative abundance of different species of calcareous nanofossils (calcite platelets shed by brown single-celled algae) and foraminifera (single-celled protists with diagnostic shells). Two subdivisions each form the Early, Middle and Late Miocene.
Regionally, other systems are used. These ages often extend across the ICS epoch boundary into the Pliocene and Oligocene:
|Mitchellian||(10.5 – 5 mya); extends into the Early Pliocene|
|Bairnsdalian||(15 – 10.5 mya)|
|Balcombian||(15.5 – 15 mya)|
|Batesfordian||(16.5 – 15.5 mya)|
|Longfordian||(27.5 – 16.5 mya); includes much of the Late Oligocene|
|Delmontian||(7.5 – 2.9 mya); includes much of the Pliocene|
|Mohnian||(13.5 – 7.5 mya)|
|Luisian||(15.5 – 13.5 mya)|
|Relizian||(16.5 – 15.5 mya)|
|Saucesian||(22 – 16.5 mya)|
|Zemorrian||(33.5 – 22 mya); includes nearly all the Oligocene|
|Yuian||(9.5 – 3.6 mya); includes the Early Pliocene|
|Fujian||(11.1 – 9.5 mya)|
|Kaburan||(13.5 – 11.1 mya)|
|Tozawan||(15.97 – 13.5 mya)|
|Haranoyan||(18.2 – 15.97 mya)|
|Kapitean||(6 – 4.8 mya); extends into the Early Pliocene|
|Tongaporutuan||(10 – 6 mya)|
|Waiauan||(11.5 – 10 mya)|
|Lillburnian||(15 – 11.5 mya)|
|Cliffdenian||(16.5 – 15 mya)|
|Altonian||(17.5 – 16.5 mya)|
|Awamoan||(20 – 17.5 mya)|
|Hutchinsonian||(21 – 20 mya)|
|Otaian||(23.03 – 21 mya)|
|Hemphillian||(9 – 4.75 mya); includes much of the Early Pliocene|
|Clarendonian||(11.8 – 9 mya)|
|Barstovian||(15.5 – 11.8 mya)|
|Hemingfordian||(19 – 15.5 mya)|
|Arikareean||(30.5 – 19 mya); includes much of the Oligocene|
|Huayquerian||(9 – 5.4 mya); the Montehermosan barely extends into the Miocene|
|Chasicoan||(10 – 9 mya)|
|Mayoian||(12 – 10 mya)|
|Laventan||(13.8 – 12 mya)|
|Colloncurian||(15.5 – 12 mya)|
|Friasian||(16.3 – 15.5 mya)|
|Santacrucian||(17.5 – 16.3 mya)|
|Colhuehuapian||(21 – 17.5 mya)|
|Deseadan||(29 – 21 mya); includes much of the Oligocene|
Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula.
Mountain building took place in Western North America and Europe. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the American Great Plains and in Argentina.
India continued to collide with Asia, creating more mountain ranges. The Tethys Seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish-Arabian region between 19 and 12 mya. Subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene.
The global trend was one towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the Late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.
Recognizable crows, ducks, auks, grouses and owls appear in the Miocene. By the epoch's end, all or almost all modern bird families are believed to have been present; the few post-Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved instead of too equivocal in character. Marine birds reached their highest diversity ever in the course of this epoch.
Brown algae, called kelp, proliferate, supporting new species of sea life, including otters, fish and various invertebrates. The cetaceans diversified, and some modern genera appeared, such as the sperm whales. The pinnipeds, which appeared near the end of the Oligocene, became more aquatic.
Approximately 100 species of apes lived during this time. They occupied much of the Old World and ranged in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominoid clade, but molecular evidence indicates this ape lived from between 15 to 12 million years ago.