The

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

Bidahochi Formation lies at an elevation of about to at the southeast of the

Colorado Plateau The Colorado Plateau, also known as the Colorado Plateau Province, is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. This province covers an area of ...

; the deposits are from Hopi Lake (also called Bidahochi Lake), and the deposits extend southwards to the region at the north perimeter of the White Mountains of central-east

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

. Bidahochi Lake is thought to have been a single "large lake, or several shallow, and ephemeral ones." Various fossil types are found; also bird trackways. The Bidahochi Formation contains basalt flows that form an erosional protecting unit above the highly erodable

Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In Ne ...

. The Bidahochi Formation extends approximately 112 miles north–south, or north-northwest by south-southeast, a length approximately equivalent to today's

Great Salt Lake The Great Salt Lake is the largest saltwater lake in the Western Hemisphere and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah and has a substantial impact upon the local climate, particula ...

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

. The Bidahochi is possibly the only extensive nonvolcanic

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

formation of the Colorado Plateau. It has been studied by geologists for clues to conditions on the plateau during this time interval and to test hypotheses for the initial incision of the

Grand Canyon The Grand Canyon (, yuf-x-yav, Wi:kaʼi:la, , Southern Paiute language: Paxa’uipi, ) is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a m ...

Description

At its reference section near

Indian Wells, Arizona Indian Wells ( nv, ) is an Census Designated Place in Navajo County, Arizona, United States. Indian Wells Elementary School is located in the town, it serves 600 students. It is also the name of the local Navajo chapter of the Navajo Nation Cou ...

, the Bidahochi Formation is divided into three informal members. The lower member is thick and consists mostly of

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

and fine

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

with a basal

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

pebbles. The beds show distorted crossbedding and ripple marks, and plant fossils (impressions and petrified wood) are present. The middle volcanic member, which is only thick, consists of volcaniclastics, mostly

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

and

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

pebbles. The upper member is over thick, forms ledges and slopes, and is mostly

mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone, slate, and shale. Most of the particles of which the stone is composed are less than and are too ...

and

. The formation is exposed over an area of over in the Black Mesa and

San Juan Basin The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah a ...

s. In many locations, the formation includes

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

, which can appear at any level in the formation, but are most common in the middle informal member. The lava flows within the formation are typically found capping mesas. The contacts between the informal members do not define consistent stratigraphic positions, and the members are probably better interpreted as depositional

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

rather than stratigraphic units. The formation is underlain by various

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

formations, such as the

, and is typically the uppermost named unit in its outcrop area. The formation is likely middle

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 ...

to middle

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Fence Lake Formation. One of the lava flows within the formation has a radiometric age of 4.1 million years, and radiometric dating of other associated flows and ash beds gives the formation an age range of 15.5 to 4 million years. The formation is interpreted as mostly

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

, though eolian, lacustrine, and

spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a ...

deposits are also present, in addition to lava flows and ash beds. The lower member has been interpreted as a lacustrine facies and the upper member as a

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

facies. The lacustrine deposits suggests widespread lakes in the area, though geologists continue to debate whether there was a single large lake (Hopi Lake) or a series of smaller ephemeral lakes, or whether Hopi Lake was a shallow playa lake or a deep lake. However, the evidence from the

maars A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...

and

scoria cones A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...

erupted through the formation favors a shallow, possibly ephemeral, lake. The formation has been likened to the

Ogallala Formation The Ogallala Formation is a Miocene to early Pliocene geologic formation in the central High Plains of the western United States and the location of the Ogallala Aquifer. In Nebraska and South Dakota it is also classified as the Ogallala Group. N ...

of eastern New Mexico and west

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

in both age and depositional environment.

Hopi Lake and the Grand Canyon

The Bidahochi is possibly the only large non-volcanic geologic formation of Neogene age on the Colorado Plateau. This has made it of interest to geologists attempting to reconstruct conditions on the Colorado Plateau during the Neogene and for theories of the formation of the Grand Canyon. In some locations, the Bidahochi Formation preserves an erosional surface in the underlying Mesozoic bedrock (Hopi Buttes surface). Examination of clay minerals in the erosional surface using

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

indicate a lack of chemical

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...

. This in turn is an indication that the southern Colorado Plateau has been semiarid throughout most of the Neogene, even at times when the rest of western North America was relatively damp. The plateau may then have been a deep basin in the

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...

of the southern

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico in ...

and the Central Arizona Highlands. It has also been suggested that Hopi Lake played a role in the original incision of the Grand Canyon. According to this hypothesis, the lake filled to the point where it overtopped its basin to the west, resulting in rapid incision and formation of the canyon. However, the characteristics of maars and scoria cones in the Bidahochi suggest that Hopi Lake was quite shallow, and a deep lake would have filled very quickly. An alternate hypothesis is that the Colorado River originally joined a drainage to the north, the Bell River, and the western part of the canyon was flooded by Hualapai Lake, which backed up to the Bidahochi basin. Only after the opening of the

Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean that separates the Baja Ca ...

was the Colorado River integrated into a drainage to the south. Old delta deposits in the Bidahochi Formation at the mouths of paleocanyons are cited as evidence for this hypothesis.

History of investigation

The formation was first named by A.B. Regan in 1924, for outcrops throughout the

Hopi Buttes volcanic field Hopi Buttes volcanic field is a Monogenetic volcanic field, monogenetic volcanic field located on the Colorado Plateau mostly on the Navajo Reservation around the town of Dilkon, Arizona, Dilkon in northeastern Arizona north of Holbrook, Arizona, ...

. In 1954,

Charles Repenning Charles Albert Repenning (August 4, 1922 in Oak Park, Illinois – January 5, 2005 in Lakewood, Colorado) was an American paleontologist and zoologist noted for his work on shrews, fossil rodents, modern pinnipeds and their extinct relatives, ...

and James Irwin defined a reference section near Indian Wells, and divided the unit into informal lower, volcanic, and upper members. D.W. Love concluded that the members defined in the formation by earlier investigators were mere depositional facies and recommended their abandonment.

Footnotes

References

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External links