The Mesilla Valley Shale 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 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 ...

, northern Chihuahua, and far west

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

. 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

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the

early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

. The formation is particularly well exposed at Cerro de Cristo Rey, near

El Paso, Texas El Paso (; ; or ) is a city in and the county seat of El Paso County, Texas, United States. The 2020 United States census, 2020 population of the city from the United States Census Bureau, U.S. Census Bureau was 678,815, making it the List of ...

, where it is part of a thick sequence of uplifted Cretaceous beds that record transgressions (advances of the sea onto land) and regressions (retreats of the sea from the land) of the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...

during the mid-Cretaceous.

Description

The formation consists mostly of dark green to olive clay

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

with some thin

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. Although its permeabil ...

beds and highly fossiliferous calcareous beds. It lies

conformably Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

on the Muleros Formation and is conformably overlain by the

Mojado Formation The Mojado 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 sandstone and shale, with some limestone, and siltstone. It re ...

. Total thickness is . Lucas and his coinvestigators divide the formation into three informal members and interpret the lower two as a complete secondary deposition cycle and the upper as the base of the next deposition cycle. The formation was deposited on the upper to middle

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an islan ...

Fossils

The formation contains at least 20 genera and 23 species of fossils. These include the molluscs ''

Lima Lima ( ; ), founded in 1535 as the Ciudad de los Reyes (, Spanish for "City of Biblical Magi, Kings"), is the capital and largest city of Peru. It is located in the valleys of the Chillón River, Chillón, Rímac River, Rímac and Lurín Rive ...

wacoensis'' Roemer, ''Lima mexicana'' Bose, '' Pecten texanus'' var. ''elongatus'' Bose, ''Pecten subalpinus'' Bose, '' Plicatula incongrua'' Conrad, ''

Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records Although molecular studies suggest that ''Ostrea'' first appeared around the Eocene and originated no earlier than the Cre ...

quadriplicata'' (Shumard), ''Texigryphaea washitaensis''. '' Trigonia emery'' Conrad, ''Helicocryptus mexicanus'' Bose, and ''

Turritella ''Turritella'' is a genus of medium-sized sea snails with an operculum, marine gastropod mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: World Register of Marine Species at http://www. ...

granulata'' Sowerby var. ''cenomanensis'' d'Orbigny and the echinoderm '' Heteraster bravoensis'' (Bose). These are characteristic of the Albian. The formation also contains

ammonoid Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

foraminiferan Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of Rhizarian protists characterized by streaming granular ectoplasm for catching food and other uses; and commonl ...

s (especially ''Cribratina texana''),

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

serpulid The Serpulidae are a family of sessile, tube-building annelid worms in the class Polychaeta. The members of this family differ from other sabellid tube worms in that they have a specialized operculum that blocks the entrance of their tubes wh ...

worms,

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...

ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

s, calcareous algae, and some terrestrial plant fragments. The formation also contains invertebrate trace fossils (

ichnofossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

s), which are tracks or burrows left in the sediments. These include '' Ancorichnus'', ''

Arenicolites ''Arenicolites'' is a U-shaped ichnotaxon (trace fossil) dating from Ediacaran times onwards in South Australia. The trace shown by this fossil, is a pair of closely spaced circles on a bedding plane In geology, a bed is a layer of sediment, ...

'', ''Bergueria'', ''Bichordites'', ''Cardioichnus'', ''

Chondrites A chondrite is a stony (non-metallic) meteorite that has not been modified by either melting or planetary differentiation, differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar Syste ...

'', ''Cochlichnus'', ''Coprulus'', ''

Gordia ''Gordia'' is an ichnofossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which ar ...

'', ''Helicodromites'', ''Lockeia'', ''

Ophiomorpha ''Ophiomorpha'' is an ichnotaxon, usually interpreted as a burrow of an organism (specifically a crustacean) living in the near-shore environment. The burrow lining is more or less smooth on the inside, and densely to strongly mammalated or nod ...

'', ''Palaeophycus'', ''

Planolites ''Planolites'' is an ichnogenus found throughout the Ediacaran and the Phanerozoic that is made during the feeding process of worm-like animals. The traces are generally small, , unlined, and rarely branched, with fill that differs from the host ...

'', ''Protovirgularia'', ''

Rhizocorallium ''Rhizocorallium'' is an ichnogenus of burrow, the inclination of which is typically within 10° of the bedding planes of the sediment. These burrows can be very large, over a meter long in sediments that show good preservation, e.g. Jurassic r ...

'', ''

Skolithos ''Skolithos'' (formerly spelled ''Scolithus'' or ''Skolithus'') is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow with a distinct lining. It was produced globally by a variety of organ ...

'', ''Spongeliomorpha'', ''Taenidium'', ''

Thalassinoides ''Thalassinoides'' is an ichnogenus of trace fossil (fossil records of lifeforms' movement, rather than of the lifeforms themselves) used to refer to "dichotomously or T-branched boxworks, mazes and shafts, unlined and unornamented". Facies of ' ...

'', '' Treptichnus'', and a biofilm, ''Rugalichnus''. This diversity of trace fossils was preserved in storm deposits (

tempestite Tempestites are storm deposits that can be recognized throughout the geologic record. They are studied in the scientific disciplines of sedimentary geology and paleotempestology. The deposits derive their meaning from the word ''tempest'', a v ...

s) below the wave base. All are typical of the Albian, and the presence of ''Chondrites'' and other trace fossils characteristic of low oxygen conditions indicate that the trace fossils were left during an

ocean anoxic event An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have not happened for millions of years, the geologic ...

, dated to 100.6-100.2 Ma.

History of investigation

The beds making up the formation were first described by E. Bose in 1906 as subdivision 6 of his

stratigraphic section A stratigraphic section is a sequence of layers of rocks in the order they were deposited. It is based on the principle of original horizontality, which states that layers of sediment are originally deposited horizontally under the action of ...

. W.S. Strain assigned the name Mesilla Valley Shale to this subdivision in 1976.

Footnotes

References

* * * * {{cite journal , last1=Strain , first1=W.S. , year=1976 , title=New formation names in the Cretaceous at Cerro de Cristo Rey, Dona Ana County, New Mexico; Appendix 2 , journal=New Mexico Bureau of Mines and Mineral Resources Memoir , volume=31 , pages=77–82 , url=https://geoinfo.nmt.edu/publications/monographs/memoirs/downloads/31/Memoir-31.pdf , accessdate=5 August 2020 Shale formations of the United States Cretaceous formations of New Mexico

Description

Fossils

History of investigation

See also

Footnotes

References