The John Day Formation is a series of rock strata exposed in the Picture Gorge district of the

John Day River The John Day River is a tributary of the Columbia River, approximately long, in northeastern Oregon in the United States. It is known as the Mah-Hah River by the Cayuse people, the original inhabitants of the region. Undammed along its entire ...

basin and elsewhere in north-central

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

in the United States. The Picture Gorge exposure lies east of the Blue Mountain uplift, which cuts southwest–northeast through the Horse Heaven mining district northeast of

Madras Chennai (, ), formerly known as Madras ( the official name until 1996), is the capital city of Tamil Nadu, the southernmost Indian state. The largest city of the state in area and population, Chennai is located on the Coromandel Coast of th ...

. Aside from the Picture Gorge district, which defines the type, the formation is visible on the surface in two other areas: another exposure is in the Warm Springs district west of the uplift, between it and the

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, ...

, and the third is along the south side of the

Ochoco Mountains The Ochoco Mountains are a mountain range in central Oregon in the United States, located at the western end of the Blue Mountains. They were formed when Permian, Triassic, and Jurassic rocks were slowly uplifted by volcanic eruptions to form t ...

. All three exposures, consisting mainly of

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

aceous sediments and

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

rock rich in

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

, lie

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

between the older rocks of the Clarno Formation below and

Columbia River basalts The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt grou ...

above.

Stratigraphy

The strata, which vary in age from 39 million years to 18 million years, were formed mainly from ashfalls from volcanoes due to a series of calderas now linked to the Yellowstone hotspot. Some of the major layers within the group exposed in the Picture Gorge district are the Big Basin Formation and Bridge Creek Beds (35 to 32 million years), the Turtle Cove Formation (30 to 28 million years), the Picture Gorge Ignimbrite (28.7 million years), the Kimberly Formation (28 to 25 million years), and the Haystack Formation (25 to 18 million years). Located in the general vicinity of what became the Cascade Range, the John Day volcanoes emitted large volumes of ash and dust, much of which settled in the John Day basin. The rapid deposition of the ash preserved the remains of plants and animals living in the region. Some of the solidified ash and the fossils they contain are found in the

John Day Fossil Beds National Monument John Day Fossil Beds National Monument is a U.S. national monument in Wheeler and Grant counties in east-central Oregon. Located within the John Day River basin and managed by the National Park Service, the park is known for its well-preserved l ...

. Because ash and other debris fell during varied climatic and volcanic conditions and accumulated from many eruptions extending into the early

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

(about 20 million years ago), the sediment layers in the fossil beds vary in their chemical composition and color. The lowermost layer contains red ash, such as that exposed in the Painted Hills Unit of the national monument. The layer above it is mainly pea-green clay. On top of the pea-green layer are buff-colored layers.Orr, pp. 30–31

Paleontology

Fossils found in the John Day Formation include a wide variety of plants and more than 100 species of mammals, including dogs, cats,

oreodont Merycoidodontoidea, sometimes called "oreodonts" or "ruminating hogs", is an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were gener ...

s, horses, camels, and rodents. Among the notable plant fossils are ''

Metasequoia ''Metasequoia'', or dawn redwoods, is a genus of fast-growing deciduous trees, one of three species of conifers known as redwoods. The living species '' Metasequoia glyptostroboides'' is native to Lichuan county in Hubei province, China. Althou ...

'' (Dawn Redwood), a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

thought to have gone extinct worldwide until it was discovered alive in China in the early twentieth century. Among the paleobiota found in the formation is the ''

Daeodon ''Daeodon'' is an extinct genus of entelodont even-toed ungulates that inhabited North America about 23 to 20 million years ago during the latest Oligocene and earliest Miocene. The type species is ''Daeodon shoshonensis'', described by a ...

'', whose type species, ''Daeodon shoshonensis'', has been found in the formation.

Mammals

Apatotheres

Carnivorans

Eulipotyphlans

Metatherians

Rodents

Ungulates

Reptiles

Squamates

References

Bibliography

* Orr, Elizabeth L., and Orr, William N. (1999). ''Geology of Oregon'', 5th ed. Dubuque, Iowa: Kendall/Hunt Publishing Company. {{ISBN, 0-7872-6608-6. Geologic formations of Oregon Neogene geology of Oregon Eocene United States Oligocene North America Miocene United States Sandstone formations of the United States Tuff formations John Day Fossil Beds National Monument