The Spears Group is a

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

s exposed in and around the northeast

Mogollon-Datil volcanic field The Mogollon-Datil volcanic field is a large () silicic volcanic field in western New Mexico (Mogollon Mountains-Datil, New Mexico). It is a part of an extensive Eocene to Oligocene volcanic event which includes the San Juan volcanic field in sou ...

of southwestern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

. It has a radiometric age of 33 to 39 million years, corresponding to the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided b ...

Description

The group is composed primarily of

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

beds, deposited in

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

s or

braided stream A braided river (also called braided channel or braided stream) consists of a network of river channels separated by small, often temporary, islands called '' braid bars'' or, in British English usage, '' aits'' or ''eyots''. Braided streams t ...

s, with minor

basaltic andesite Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Ameri ...

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

lava flows. It contains some lacustrine

delta Delta commonly refers to: * Delta (letter) (Δ or δ), the fourth letter of the Greek alphabet * D (NATO phonetic alphabet: "Delta"), the fourth letter in the Latin alphabet * River delta, at a river mouth * Delta Air Lines, a major US carrier ...

deposits at its base. The total thickness is as much as . The group grades below into the

Baca Formation The Baca Formation is a geologic formation in southern New Mexico and Arizona. It preserves fossils dating back to the Eocene period. Description The formation consists of coarse conglomerate, red and white sandstone, and red clay. Its total ...

and interfingers with

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s and ash flow sheets of the Mogollon Group and Datil Group. K-Ar and

fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiometr ...

suggest an age of 33 to 39 million years. The Dog Springs Formation shows striking

sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ...

indicating that the

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock flow down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

beds making up most of the formation slumped over distance scales of miles on the underlying Baca Formation beds. In some locations,

clastic dike A clastic dike is a seam of sedimentary material that fills an open fracture in and cuts across sedimentary rock strata or layering in other rock types. Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) ...

s derived from the Baca Formation penetrate the overlying Dog Springs Formation.

Formations

The group is divided into numerous formations. In descending stratigraphic order, these are: * South Crosby Peak Formation * Rincon Windmill Formation * Chavez Canyon Formation * Dog Springs Formation * Rubio Peak Formation * Palm Park Formation * Pueblo Creek Formation * Bell Top Formation * Rock Springs Formation In addition, a number of informal units have been described within the group.

History of investigation

The name was first used by W.H. Tonking in 1957 for the lowest beds of the Datil Volcanics (as then designated). G.R. Osburn and C.E. Chapin raised the unit to formation rank within the Datil Group and divided into members separated by ash flow sheets. In 1994, Steven M. Cather and coinvestigators raised the Spears Formation to group rank and its members to formation rank, redefining the group as all volcaniclastic apron sediments of the Mogollon-Datil volcanic field. They also described several informal formations within the group.

References

{{Reflist Geologic groups of New Mexico Paleogene formations of New Mexico Andesite groups