The U-Bar Formation is a
geologic formation in southwestern
New Mexico. 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 in ...
dating back to the
early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
period.
Description
The formation consists mostly of alternating beds of
limestone and
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
, but with massive upper
reef limestone beds (varying from in thickness) and some massive
calcarenite beds towards the middle of the formation. Its total thickness is . The massive reef limestones cap ridges that are steep on one side but dip gently on the other. The formation has gradational contacts with both the underlying
Hell-to-Finish Formation
The Hell-to-Finish Formation is a geologic formation in southwestern New Mexico. It preserves fossils dating back to the early Cretaceous period.
Description
The formation consists mostly of red to red-brown to gray or green shale, mudstone, ...
and the overlying
Mojado Formation.
Although fossils are abundant, they tend not to be age-specific. However, careful study of the fossils has yielded an age of
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
to middle
Albian.
Fossils
The lowest beds of the formation contain only small
pelecypods, but abundant oyster fossils are found further up in the formation. The alternating shale and limestone beds contain fossil
echinoids, the
foraminifer ''
Orbitolina'', the
gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
''
Lunatia
''Lunatia'' is a genus of predatory sea snails, marine gastropod mollusks in the family Naticidae, the moon snails.Bouchet, P. (2012). Lunatia. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxde ...
praegrandis'' Roemger, and small oysters. The upper part of the formation contains massive fossil
bioherm reefs. ''Orbitolina'' is also found above the reef beds, in the uppermost part of the formation.
History of investigation
The formation name was first used by Zeller in 1965. Drewes assigned the formation to the
Bisbee Group
The Bisbee Group is a geologic group in Arizona, Mexico, and New Mexico. It preserves fossils dating back to the early Cretaceous period.
Description
The group consists of sandstone, claystone, limestone, and conglomerate, with a maximum ...
in 1991.
See also
*
List of fossiliferous stratigraphic units in New Mexico
*
Paleontology in New Mexico
Paleontology in New Mexico refers to paleontological research occurring within or conducted by people from the U.S. state of New Mexico. The fossil record of New Mexico is exceptionally complete and spans almost the entire stratigraphic colu ...
Footnotes
References
*
*
* {{cite journal , last1=Zeller , first1=R.A. Jr. , last2=Alper , first2=A.M. , year=1965 , title=Geology of the Walnut Wells quadrangle, Hidalgo County, New Mexico , journal=New Mexico Bureau of Mines and Mineral Resources Bulletin , volume=84 , url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/84/B84.pdf , access-date=15 September 2020
Cretaceous formations of New Mexico