The Todilto Formation 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 northern

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, Ker ...

and southeastern

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

. 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 Callovian stage of the middle

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

period.

Description

The formation consists of evaporites. It is divided into a lower calcareous

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 ...

(the Luciano Mesa Member) up to thick, and an upper

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...

bed (the Tonque Arroyo Member). Based on varve counts in the Luciano Mesa Member, the formation was laid down in a geologically brief period of time, likely in a salina (a coastal body of saline water) that was replenished both by rivers and by seepage or periodic flooding from the

Sundance Sea The Sundance Sea was an epeiric sea that existed in North America during the mid-to-late Jurassic Period of the Mesozoic Era. It was an arm of what is now the Arctic Ocean, and extended through what is now western Canada into the central western ...

. The varves show a 10 to 13 year periodicity that is interpreted as the solar sunspot cycle, showing that this cycle has existed for at least 160 million years. The contact with the underlying

Entrada Formation The Entrada Sandstone is a formation in the San Rafael Group found in the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona, and southeast Utah. Part of the Colorado Plateau, this formation was deposited during the Jurass ...

is very sharp and may indicate the Todilto Sea formed catastrophically from a breach in a barrier between the basin and the Sundance Sea. The presence of dasyclad algae in the Luciano Mesa Member indicates at least some marine flooding.

Fossils

Dasyclad algae have been found in the Luciano Mesa Member. Evidence has been found for stromatolites in the formation near Mesa Montañosa .

Economic resources

The formation is mined for gypsum in the northern

Albuquerque Basin The Albuquerque Basin (or Middle Rio Grande Basin) is a structural basin and ecoregion within the Rio Grande rift in central New Mexico. It contains the city of Albuquerque. Geologically, the Albuquerque Basin is a half-graben that slopes down to ...

, including along the La Bajada escarpment south of I-25, near San Felipe Pueblo, and at White Mesa near

San Ysidro San Ysidro (Spanish for " St. Isidore", ) is a district of the City of San Diego, immediately north of the Mexico–United States border. It neighbors Otay Mesa West to the north, Otay Mesa to the east, and Nestor and the Tijuana River Valley t ...

. It has also yielded uranium ore.

History of investigation

The formation was first described by H.E. Gregory in 1917 for exposures at Todilto Park in the

San Juan Basin The San Juan Basin is a geologic structural basin located near the Four Corners region of the Southwestern United States. The basin covers 7,500 square miles and resides in northwestern New Mexico, southwestern Colorado, and parts of Utah a ...

. He assigned it to his (now defunct) La Plata Group. It has sometimes been included in the Morrison Formation.

Footnotes

References