Nacimiento Formation
Encyclopedia
The Nacimiento Formation is a sedimentary rock formation found in the San Juan Basin
of western New Mexico
(United States
) and named for the Nacimiento Mountains
. It is a heterogeneous nonmarine formation composed of shale
, siltstone
, and sandstone
, deposited in floodplain
, fluvial
and lacustrine
settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~64.5 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot. This unit interbeds with the underlying Ojo Alamo Formation
but is separated by an unconformity
from the overlying San Jose Formation.
Workers in the early 1900s divided the rocks of the Nacimiento Formation into two formations, the lower Puerco Formation and the upper Torrejon Formation. This was rejected on the grounds that there were no lithological
differences between the two, only differences in fossil
faunas, making determination of which formation was present in a given area impossible if fossils could not be found. The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan
and Torrejonian
North American Land Mammal Ages
.
The Nacimiento Formation is divided into several subunits known as members. In outcrops in southern areas of the formation, the Puercan fauna is found in the Arroyo Chijuillita Member, the Torrejonian fauna is found in the Ojo Encino Member, and the uppermost Escavada Member lacks age-diagnostic fossils. In northern outcrops, the two lower formations are indistinguishable, and are called the "main body". Above them are two more members, as yet unnamed except in a thesis. They preserve a younger, Tiffanian
fauna. The Puercan and Torrejonian faunas are further subdivided into several biostratigraphic zones
.
Many fossils are known from the Nacimiento Formation, although bone is often altered into phosphatic
concretion
s. Fossils belonging to a number of different organisms have been found here, including: plants (mostly dicotyledon
ous angiosperms), gastropods, freshwater
bivalves, cartilaginous fish
and bony fish
, salamanders, turtles, champsosaurs
, amphisbaenia
ns, lizards, snakes, crocodilians, birds, and a variety of archaic mammals. Mammalian groups represented include multituberculates, didelphid marsupial
s, insectivora
ns, plesiadapiforms, carnivora
ns, taeniodont
s, mesonychid
s, condylarth
s, and cimolesta
ns.
San Juan Basin
The San Juan Basin is a geologic structural basin in the Four Corners region of the Southwestern United States; its main portion covers around , encompassing much of northwestern New Mexico, southwest Colorado, and parts of Arizona and Utah....
of western New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
(United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
) and named for the Nacimiento Mountains
Nacimiento Mountains
The Sierra Nacimiento , or Nacimiento Mountains, are a mountain range in the northwestern part of the US state of New Mexico. They are just west of the more prominent Jemez Mountains near the town of Cuba, and are separated from them by the Río Guadalupe and the Río de Las Vacas...
. It is a heterogeneous nonmarine formation composed of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
, deposited in floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
, fluvial
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
and lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~64.5 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot. This unit interbeds with the underlying Ojo Alamo Formation
Ojo Alamo Formation
The Ojo Alamo Formation is a geologic formation spanning the Mesozoic/Cenozoic boundary. Dinosaur remains are among the fossils that have been recovered from the formation, though all dinosaur remains come from the lowest part of the formation, the Naashoibito member The Ojo Alamo Formation is a...
but is separated by an unconformity
Unconformity
An unconformity is a buried erosion 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 of time before deposition of the younger, but the term is used to describe...
from the overlying San Jose Formation.
Workers in the early 1900s divided the rocks of the Nacimiento Formation into two formations, the lower Puerco Formation and the upper Torrejon Formation. This was rejected on the grounds that there were no lithological
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
differences between the two, only differences in fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
faunas, making determination of which formation was present in a given area impossible if fossils could not be found. The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan
Puercan
The Puercan North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 65,000,000 to 63,300,000 years BP lasting . It is usually considered to be within the Paleocene...
and Torrejonian
Torrejonian
The Torrejonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 63,300,000 to 60,200,000 years BP lasting . It is usually considered to overlap the Selandian and Thanetian within the...
North American Land Mammal Ages
North American Land Mammal Ages
The North American Mammal Ages establishes a geologic timescale for prehistoric North American fauna beginning 66.5 Ma during the Paleocene and continuing through to the Late Pleistocene...
.
The Nacimiento Formation is divided into several subunits known as members. In outcrops in southern areas of the formation, the Puercan fauna is found in the Arroyo Chijuillita Member, the Torrejonian fauna is found in the Ojo Encino Member, and the uppermost Escavada Member lacks age-diagnostic fossils. In northern outcrops, the two lower formations are indistinguishable, and are called the "main body". Above them are two more members, as yet unnamed except in a thesis. They preserve a younger, Tiffanian
Tiffanian
The Tiffanian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 60,200,000 to 56,800,000 years BP lasting . It is usually considered to overlap the Selandian and Thanetian within the...
fauna. The Puercan and Torrejonian faunas are further subdivided into several biostratigraphic zones
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
.
Many fossils are known from the Nacimiento Formation, although bone is often altered into phosphatic
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...
s. Fossils belonging to a number of different organisms have been found here, including: plants (mostly dicotyledon
Dicotyledon
The dicotyledons, also known as dicots, are a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group...
ous angiosperms), gastropods, freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
bivalves, cartilaginous fish
Chondrichthyes
Chondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
and bony fish
Osteichthyes
Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
, salamanders, turtles, champsosaurs
Choristodera
Choristodera is an order of semi-aquatic diapsid reptiles which ranged from the Middle Jurassic, or possibly Late Triassic, to at least the early Miocene. Choristoderes have been found in North America, Asia, and Europe. The most common fossils are typically found from the Late Cretaceous to the...
, amphisbaenia
Amphisbaenia
The Amphisbaenia are a usually legless suborder of squamates closely related to lizards and snakes. As many species possess a pink body coloration and scales arranged in rings, they have a superficial resemblance to earthworms. They are very poorly understood, due to their burrowing lifestyle...
ns, lizards, snakes, crocodilians, birds, and a variety of archaic mammals. Mammalian groups represented include multituberculates, didelphid marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, insectivora
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
ns, plesiadapiforms, carnivora
Carnivora
The diverse order Carnivora |Latin]] carō "flesh", + vorāre "to devour") includes over 260 species of placental mammals. Its members are formally referred to as carnivorans, while the word "carnivore" can refer to any meat-eating animal...
ns, taeniodont
Taeniodont
The taeniodonts were an early group of mammals who lived from the Palaeocene to the Eocene. This group evolved quickly into highly specialized digging animals. Taeniodont species varied greatly in size, from rat-sized to species as large as a bear. Later species developed prominent front teeth and...
s, mesonychid
Mesonychid
Mesonychia are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls and to cetaceans...
s, condylarth
Condylarth
Condylarthra is an order of extinct placental mammals known primarily from the Paleocene and Eocene epochs. Condylarths are among the most characteristic Paleocene mammals and they illustrate the evolutionary level of the Paleocene mammal fauna....
s, and cimolesta
Cimolesta
Cimolesta is an extinct order of mammals. A few experts place the pangolins within Cimolesta, though most other experts prefer to place the pangolins within their own order, Pholidota....
ns.