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 , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

, northern Chihuahua, and far west

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

. It preserves fossils 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 ± 0 ...

age of the early Cretaceous

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 county seat, seat of El Paso County, Texas, El Paso County in the western corner of the U.S. state of Texas. The 2020 population of the city from the United States Census Bureau, U.S. Census Bureau w ...

, 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 sea ...

during the mid-Cretaceous.

Description

The formation consists mostly of dark green to olive clay shale with some thin siltstone beds and highly fossiliferous calcareous beds. It lies conformably 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 ...

. 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 ...

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 This genus is very ancient. It is known in the fossil records from the Permian to the Quaternary (age range: from 259 to 0. ...

quadriplicata'' (Shumard), ''Texigryphaea washitaensis''. ''

Trigonia ''Trigonia'' is an extinct genus of saltwater clams, fossil marine bivalve mollusk in the family Trigoniidae. The fossil range of the genus spans the Paleozoic, Mesozoic and Paleocene of the Cenozoic, from 298 to 56 Ma. Description The gen ...

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.m ...

granulata'' Sowerby var. ''cenomanensis'' d'Orbigny and the echinoderm ''

Heteraster ''Heteraster'' is an extinct genus of sea urchins belonging to the family Toxasteridae. These slow-moving shallow infaunal deposit feeder-detritivores lived during the Cretaceous The Cretaceous ( ) is a geological period that lasted from abo ...

bravoensis'' (Bose). These are characteristic of the Albian. The formation also contains ammonoids,

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, w ...

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 an ...

s (especially ''Cribratina texana''), dinoflagellates, serpulid 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 ...

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, typi ...

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'', '' Arenicolites'', ''Bergueria'', ''Bichordites'', ''Cardioichnus'', '' Chondrites'', ''Cochlichnus'', ''Coprulus'', '' Gordia'', ''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'', ''Protovirgularia'', '' Rhizocorallium'', ''

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 environmen ...

'', ''Spongeliomorpha'', ''Taenidium'', '' Thalassinoides'', '' Treptichnus'', and a biofilm, ''Rugalichnus''. This diversity of trace fossils was preserved in storm deposits ( tempestites) 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, 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. 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