The Baca Formation is a

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 ...

in southern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

and

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a U.S. state, state in the Southwestern United States. It is the list of U.S. states and territories by area, 6th largest and the list of U.S. states and territories by population, 14 ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the

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

period.

Description

The formation consists of coarse conglomerate, red and white

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 ...

, and red

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...

. Its total thickness is , of which is conglomerate. The formation rests unconformably on the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

Mesaverde Group The Mesaverde Group is a Late Cretaceous stratigraphic group found in areas of Colorado, New Mexico, Utah, and Wyoming, in the Western United States. History The Mesaverde Formation was first described by W.H.Holmes in 1877 during the ...

and is overlain by the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

Spears Formation The Spears Group is a group of geologic formations exposed in and around the northeast Mogollon-Datil volcanic field of southwestern New Mexico. It has a radiometric age of 33 to 39 million years, corresponding to the Eocene to Oligocene epoch ...

. The formation was deposited in a basin that developed in Eocene time in eastern Arizona and west-central New Mexico. The basin was mostly a system of

braided streams A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, ''aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment l ...

subject to frequent flash floods, with meandering streams restricted to the easternmost part of the basin. Paleocurrent directions showed that stream flow was generally to the east-northeast. Shallow

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

lakes and a large, shallow closed lake in the eastern part of the basin were also present, though

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

beds are nearly absent due to extensive

river delta A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more ra ...

formation. Deposition ended with the eruption of the

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 ...

, which disrupted the Baca

drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic conditio ...

and buried the basin in volcaniclastic rock.

Fossils

The formation is relatively poor in

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

fossils, and only six of biostratigraphic significance have been found. These are the titanotheres '' Paleosyops'' and '' Manteoceras'', an

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poste ...

fossil trackway A fossil track or ichnite (Greek "''ιχνιον''" (''ichnion'') – a track, trace or footstep) is a fossilized footprint. This is a type of trace fossil. A fossil trackway is a sequence of fossil tracks left by a single organism. Over the yea ...

and '' Protoreodon'' fossil, and a small

mammalian Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as ''biota''. Zoo ...

at Mariano Mesa. The titanotheres are characteristic of the Bridgerian age while the trackways indicate a late Eocene age or younger.

History of investigation

The formation was first described by R.H. Wilpolt and his coinvestigators in 1946 and named for exposures in Baca Canyon in the

Bear Mountains The Bear Mountains lie just north of the village of Magdalena and north of the Magdalena Mountains in New Mexico. The Bear Mountains are located within the Magdalena Ranger District of the Cibola National Forest. There are two Inventoried R ...

Footnotes

References

* * * * {{cite journal , last1=Wilpolt , first1=R.H. , last2=MacAlpin , first2=A.J. , last3=Bates , first3=R.L. , last4=Vorbe , first4=George , year=1946 , title=Geologic map and stratigraphic sections of Paleozoic rocks of Joyita Hills, Los Pinos Mountains, and northern Chupadera Mesa, Valencia, Torrance, and Socorro Counties, New Mexico , journal=U.S. Geological Survey Oil and Gas Investigations Preliminary Map , volume=OM-61 , url=https://ngmdb.usgs.gov/Prodesc/proddesc_5422.htm , accessdate=31 July 2020 Geology of Arizona Paleogene formations of New Mexico

Description

Fossils

History of investigation

See also

Footnotes

References