Miocene epoch (mī´əsēn), fourth epoch of the Tertiary period in the Cenozoic era of geologic time (see Geologic Timescale, table), lasting from around 24.6 to 5.1 million years ago.
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.