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 ) , 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 ...

, northern

Chihuahua Chihuahua may refer to: Places *Chihuahua (state), a Mexican state **Chihuahua (dog), a breed of dog named after the state **Chihuahua cheese, a type of cheese originating in the state **Chihuahua City, the capital city of the state **Chihuahua Mun ...

, and far west

Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...

. 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 of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± ...

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

of 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 Pr ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

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

El Paso, Texas El Paso (; "the pass") is a city in and the seat of El Paso County in the western corner of the U.S. state of Texas. The 2020 population of the city from the U.S. Census Bureau was 678,815, making it the 23rd-largest city in the U.S., the ...

, 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, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient se ...

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 minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

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.Blatt ''et al.'' 1980, ...

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 The Muleros Formation is a geologic formation in New Mexico, which is particularly well exposed at Cerro de Cristo Rey near El Paso, Texas. It preserves fossils dating back to the early Cretaceous period. Descriptiom The formation consists of hi ...

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.

Fossils

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

Lima Lima ( ; ), originally founded as Ciudad de Los Reyes (City of The Kings) is the capital and the largest city of Peru. It is located in the valleys of the Chillón, Rímac and Lurín Rivers, in the desert zone of the central coastal part of t ...

wacoensis'' Roemer, ''Lima mexicana'' Bose, ''

Pecten Pecten or pectin may refer to: Pecten Biology * Pecten (biology), any comb like structure in animals * ''Pecten'' (bivalve), a genus of scallops * Pecten (company), a subsidiary of Sinopec * Pecten oculi, a structure in the bird retina whic ...

texanus'' var. ''elongatus'' Bose, ''Pecten subalpinus'' Bose, ''

Plicatula Plicatula is a genus of saltwater clams, marine bivalve molluscs, known commonly as kitten's paws or kittenpaws in the family Plicatulidae. MolluscaBase eds. (2022). MolluscaBase. Plicatula Lamarck, 1801. Accessed through: World Register of Ma ...

incongrua'' Conrad, ''

Ostrea ''Ostrea'' is a genus of edible oysters, marine bivalve mollusks in the family Ostreidae, the oysters. Fossil records This genus is very ancient. It is known in the fossil records from the Permian to the Quaternary (age range: from 259 to ...

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

Turritella ''Turritella'' is a genus of medium-sized sea snails with an Operculum (gastropod), operculum, marine (ocean), marine gastropod mollusc, mollusks in the family Turritellidae.Vos, C.; Gofas, S. (2013). Turritella Lamarck, 1799. Accessed through: W ...

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 a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefi ...

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan 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 end, ...

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

s (especially ''Cribratina texana''),

dinoflagellate The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates are ...

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 ...

worms,

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

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

ichnofossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

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

Ancorichnus ''Ancorichnus'' is an ichnogenus. Ancorichnus, is distinguished by structured mantle peripheral to a meniscate core. The mantle is not considered as a wall structure since it formed by the locomotive behaviour of the burrow producer. This ichnotax ...

'', ''

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 sedimen ...

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

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

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

Gordia ''Gordia'' is an ichnofossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fos ...

'', ''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 ro ...

'', ''

Skolithos ''Skolithos'' (formerly spelled ''Scolithus'' or ''Skolithus'') is a common trace fossil ichnogenus that is, or was originally, an approximately vertical cylindrical burrow. It is produced by a variety of organisms in shallow marine environment ...

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

Thalassinoides ''Thalassinoides'' is an ichnogenus of trace fossil used to refer to "dichotomously or T-branched boxworks, mazes and shafts, unlined and unornamented". Facies of ''Thalassinoides'' increased suddenly in abundance at the beginning of the Mesoz ...

'', ''

Treptichnus ''Treptichnus'' (formerly named ''Phycodes'', ''Manykodes'' by J. Dzik, and also known as ''Trichophycus''See e.gfossiilid.info: paleodiversity in Baltoscandia: Trichophycus pedum/ref>) is the preserved burrow of an animal. As such, it is regar ...

'', 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 vi ...

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 Oceanic anoxic events or anoxic events ( anoxia conditions) describe periods wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters. Although anoxic events have no ...

, 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